# Short Answer Questions / Objective Civil Engineering Questions

## In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure.

In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

Segmental Construction Method Span-by-span

Vincent T. H. CHU

## Why is the span length ratio of end span/approach span to its neighboring inner spans usually about 0.75?

From aesthetic point of view, an odd number of spans with a decrease in length in the direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the arrangement of irregular span lengths is not recommended because it gives a feeling of uneasiness.

From structural point of view, for a multi-span bridge with equal span length, the sagging moment at the mid-span of the end span/approach span is largest. In order to reduce this moment, the span length of end span/approach span is designed to be 0.75 of inner spans. However, this ratio should not be less than 0.40 because of the effect of uplifting at the end span/approach span support.

Note: End span refers to the last span in a continuous bridge while approach span refers top the first span of a
bridge.

Bursting Reinforcement

Vincent T. H. CHU

## What are the three major types of reinforcement used in prestressing?

(i) Spalling reinforcement
Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.

(ii) Equilibrium reinforcement
Equilibrium reinforcement is required where there are several anchorages in which prestressing loads are applied sequentially.

(iii) Bursting Reinforcement
Tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces.

Bursting Reinforcement

Vincent T. H. CHU

## What is the advantage of sliding bearings over roller bearings?

In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee. However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.

Sliding Bearing

Vincent T. H. CHU

## Are diaphragms necessary in the design of concrete box girder bridges?

Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult. In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

Vertical Cross Diaphragm

Vincent T. H. CHU

### What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?

Basically, piers constructed monolithically with the bridge deck are advantageous in the following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long and slender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing replacement. On the other hand, in maintenance aspect substantial cost and time savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.

(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers.

Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.

Prestressed I girders

Vincent T. H. CHU

### In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25L segment is constructed in the first phase of construction?

Basically, there are mainly three reasons for this arrangement:

(i) The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed structure.

(ii) The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span is uniformly loaded) in which the moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the structure.

(iii) In case of a prestressed bridge, prestressing work has to be carried out after the construction of first segment of the bridge. If the prestressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the prestressing location at 1.25L from the first pier where there is relatively more space to accommodate prestressing works.

Prestressed I girders

Note: Span-by-span construction means that it is constructed from one span to another until its completion.

Vincent T. H. CHU

## In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?

To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.

The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.

Vincent T. H. CHU

## How to determine the size of elastomeric bearings?

For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design size of elastomeric bearings are based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.

Laminated Elastomeric Bearings

The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

Vincent T. H. CHU

## In the design of elastomeric bearings, why are steel plates inserted inside the bearings?

For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.

However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.

Vincent T. H. CHU

## In prestressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?

If wires/strands are stressed individually inside the same duct, then those stressed strand/wires will bear against those unstressed ones and trap them. Therefore, the friction of the trapped wires is high and is undesirable.

Vincent T. H. CHU

## Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?

In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth. Improvement in drainage details and application of painting and coating to bridges help to solve this problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.

## What are the advantages of assigning the central pier and the abutment as fixed piers?

Bearing Articulation refer to the arrangement, configuration selection of bridge bearings inorder to facilitate force transfer from the superstructure to substructure by achieving predefined degree of freedom (Displacements or Rotations) at the support locations/bearings. Bearing articulation has a major impact on the design of substructure of bridges.

1. For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
2. For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.

Symbols for common bearing types (from Table 1 of BS EN 1337-1)

Typical Bearing articulation of a curved bridge

Thank you Vincent T. H. CHU

## What are the advantages of assigning the central pier and the abutment as fixed piers?

Bearing Articulation refer to the arrangement, configuration selection of bridge bearings inorder to facilitate force transfer from the superstructure to substructure by achieving predefined degree of freedom (Displacements or Rotations) at the support locations/bearings. Bearing articulation has a major impact on the design of substructure of bridges.

1. For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
2. For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.

Symbols for common bearing types (from Table 1 of BS EN 1337-1)

Typical Bearing articulation of a curved bridge

Thank you Vincent T. H. CHU

## In bridge widening projects, the method of stitching is normally employed for connecting existing deck to the new deck. What are the problems associated with this method in terms of shrinkage of concrete?

In the method of stitching, it is a normal practice to construct the widening part of the bridge at first and let it stay undisturbed for several months. After that, concreting will then be carried out for the stitch between the existing deck and the new deck. In this way, the dead load of the widened part of bridge is supported by itself and loads arising from the newly constructed deck will not be transferred to the existing deck which is not designed to take up these extra loads.

One of the main concerns is the effect of stress induced by shrinkage of newly widened part of the bridge on the existing bridge. To address this problem, the widened part of the bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing bridge so that shrinkage of the new bridge will take place within this period and the effect of shrinkage stress exerted on the new bridge is minimized.

The bridge was widened by the stitching of an in-situ parapet.

Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches. To solve this problem, rapid hardening cement is used for the stitching concrete so as to shorten the time of setting of concrete. Moreover, the stitching work is designed to be carried out at nights of least traffic (Saturday night) and the existing bridge may even be closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed.

Sometimes, longitudinal joints are used in connecting new bridge segments to existing bridges. The main problem associated with this design is the safety concern of vehicles. The change of frictional coefficients of bridge deck and longitudinal joints when vehicles change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of longitudinal joints in bridges is quite difficult.

Note: Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.

by Vincent T. H. CHU

## In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

### Introduction

The incremental launching method is one of the highly mechanised erection methods used in bridge construction. The method consists of manufacturing the superstructure of a bridge by sections in a prefabrication area behind one of the abutments; each new unit is concreted directly against the preceding one and after it has hardened the resultant structure is moved forward by the length of one unit. This principle has already been used for many years in the construction of steel bridges. This is hardly surprising, in view of the equal strength of steel in tension and compression since, provided the design is suitable, the alternating stresses which occur when the bridge is slid forwards can be accepted without difficulty. This is not so with concrete, which can withstand only small tensile stresses without damage. Special measures are therefore necessary to enable concrete bridges to be slid forward by steps; the skilful use of prestressing is the most important of these measures.

Incrementally launched bridge construction Itz Valley Bridge near Coburg

#### In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

1. During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
2. The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central prestress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively. Later when the whole superstructure is completely launched, continuity prestressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central prestress.
3. For very long span bridge, temporary piers are provided to limit the cantilever moment.

## What is “preset” during installation of bridge bearings?

“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the following components: length of bearing + 2 x irreversible movement + 2 x reversible movement. Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + 2 x reversible movement. In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.

Laminated Elastomeric Bearings

Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.

## Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.

Jacking in prestressing

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.

## What is batch in Civil Engineering from the topic Cement?

Batch is defined as quantity of concrete mixed in one cycle of operations of a batch mixer, or the quantity discharged during 1 min from a continuous mixer, or the quantity of concrete conveyed ready-mixed in a vehicle.

## What is free water/cement ratio in Civil Engineering from the topic Concrete?

Free Water/Cement Ratio is defined as ratio of the mass of free water (that is, excluding the water absorbed by the aggregate to reach a saturated surface dry condition) to the mass of cement in a concrete mix.

• ###### What is cement content in Civil Engineering from the topic Cement?

Cement Content is defined as mass of cement contained in a cubic metre of fresh, fully compacted concrete, expressed in kg/m3.

## What is ready-mixed concrete in Civil Engineering from the topic Concrete?

Ready-Mixed Concrete is defined as concrete mixed in a stationary mixer or in a truck-mixer and supplied in the fresh condition to the purchaser, either at the site or into the purchaser’s vehicles.

• ###### What is cement in Civil Engineering from the topic Cement?

Cement is defined as hydraulic binder that sets and hardens by chemical interaction with water and is capable of doing so under water

• ###### What is cement type in Civil Engineering from the topic Cement?

Cement Type is defined as classification of a cement on its main constituents.

• ###### What is certified average alkali content in Civil Engineering from the topic Cement?

Certified Average Alkali Content is defined as the average of 25 consecutive determinations of equivalent alkali content carried out on samples each of which is representative of a day’s production.

## What is characteristic strength in Civil Engineering from the topic Concrete?

Characteristic Strength is defined as that value of strength below which 5 percentage of the population of all possible strength measurements of the specified concrete are expected to fall.

## What is declared mean alkali content in Civil Engineering from the topic Cement?

Declared Mean Alkali Content is defined as the mean alkali content, expressed as the sodium oxide equivalent, which will not be exceeded without prior notice from the manufacturer. This is the certified alkali content plus a margin that reflects the manufacturer’s variability of production.

## What is standard strength class (of cement) in Civil Engineering from the topic Concrete?

Standard Strength Class (Of Cement) is defined as classification of a cement based on its compressive strength at 28 days measured on mortar prisms in accordance with advised code.

## What is guaranteed alkali limit in Civil Engineering from the topic Cement?

Guaranteed Alkali Limit is defined as the alkali limit, expressed as the sodium oxide equivalent, which the constituent material supplier guarantees will not be exceeded by any test result, on any spot sample.

## In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure.

In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

Segmental Construction Method Span-by-span

Vincent T. H. CHU

## What is prescribed mix in Civil Engineering from the topic Concrete?

Prescribed Mix is defined as mix for which the purchaser specifies the proportions of the constituents and is responsible for ensuring that these proportions produce a concrete with the required performance.

## What is designated mix in Civil Engineering from the topic Concrete?

Designated Mix is defined as mix produced in accordance with the specification given in Section 5 of BS 5328-2:1997 and requiring the producer to hold current product conformity certification based on product testing and surveillance coupled with approval of the producer’s quality system.

## What is standard mix in Civil Engineering from the topic Concrete?

Standard Mix is defined as mix selected from the restricted list given in Section 4 of BS 5328-2:1997 and made with a restricted range of materials.

## What is designed mix in Civil Engineering from the topic Concrete?

Designed Mix is defined as mix for which the purchaser is responsible for specifying the required performance and the producer is responsible for selecting the mix proportions to produce the specified performance.

## What is heavyweight concrete in Civil Engineering from the topic Concrete?

Heavyweight Concrete is defined as hardened concrete having an oven-dried density greater than 2600 kg/m3.

## What is lightweight concrete in Civil Engineering from the topic Concrete?

Lightweight Concrete is defined as hardened concrete having an oven-dried density not greater than 2000 kg/m3.

## What is grade (of concrete) in Civil Engineering from the topic Concrete?

Grade (Of Concrete) is defined as numerical value of 28 day characteristic strength, expressed in N/mm2.

## What is density of fresh concrete in Civil Engineering from the topic Concrete?

Density Of Fresh Concrete is defined as mass of quantity of compacted fresh concrete divided by its volume, expressed in kg/m3.

## What is normal-weight concrete in Civil Engineering from the topic Concrete?

Normal-Weight Concrete is defined as hardened concrete having an oven-dried density greater than 2000 kg/m3 but not exceeding 2600 kg/m3.

## In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure.

In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

Segmental Construction Method Span-by-span

Vincent T. H. CHU

## What are the three major types of reinforcement used in prestressing?

(i) Spalling reinforcement
Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.

(ii) Equilibrium reinforcement
Equilibrium reinforcement is required where there are several anchorages in which prestressing loads are applied sequentially.

(iii) Bursting Reinforcement
Tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces.

Bursting Reinforcement

Vincent T. H. CHU

## What is the advantage of sliding bearings over roller bearings?

In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee. However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.

Sliding Bearing

Vincent T. H. CHU

## Are diaphragms necessary in the design of concrete box girder bridges?

Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult. In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

Vertical Cross Diaphragm

Vincent T. H. CHU

### What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?

Basically, piers constructed monolithically with the bridge deck are advantageous in the following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long and slender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing replacement. On the other hand, in maintenance aspect substantial cost and time savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.

(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers.

Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.

Prestressed I girders

Vincent T. H. CHU

## What is the Limiting serviceability deflections in Structural members like Beams and Slabs as per BS: 8110?

To prevent noticeable sag in the structural members, serviceability deflections shall be restricted to L/250. Where L is the span of the member or Length in case of cantilevers (BS8110-2:1985).

What is the effective depth d

## What are the permissible stresses in Tension Steel and Concrete as per BS8110 while checking a section for crack widths?

Permissible stress in Steel and concrete is defined as following.

Stress in steel is limited to 0.8 x fy (BS8007:1987 Appendix B2 and Cl. 3.8.3, BS8110-2:1985)

Stress in concrete is limited to 0.45 x fcu (BS8007:1987 Appendix B2)

Permissible Stress

## What is the effective depth in beams or slabs? How is it calculated?

Effective depth is the distance between extreme compression fiber to the centroid of tension reinforcement in a section under flexure. If h is the total depth of section, c the clear cover to outer reinforcement, d” and d’ be the diameters of tension bar and shear links respectively, the effective depth of section for flexure design of the section can be calculated as h – c – d’ – (d”/2), which will be the C.G of tension reinforcement.

What is the effective depth d

### In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25L segment is constructed in the first phase of construction?

Basically, there are mainly three reasons for this arrangement:

(i) The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed structure.

(ii) The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span is uniformly loaded) in which the moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the structure.

(iii) In case of a prestressed bridge, prestressing work has to be carried out after the construction of first segment of the bridge. If the prestressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the prestressing location at 1.25L from the first pier where there is relatively more space to accommodate prestressing works.

Prestressed I girders

Note: Span-by-span construction means that it is constructed from one span to another until its completion.

Vincent T. H. CHU

## In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?

To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.

The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.

Vincent T. H. CHU

## How to determine the size of elastomeric bearings?

For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design size of elastomeric bearings are based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.

Laminated Elastomeric Bearings

The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

Vincent T. H. CHU

## In the design of elastomeric bearings, why are steel plates inserted inside the bearings?

For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.

However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.

Vincent T. H. CHU

## In prestressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?

If wires/strands are stressed individually inside the same duct, then those stressed strand/wires will bear against those unstressed ones and trap them. Therefore, the friction of the trapped wires is high and is undesirable.

Vincent T. H. CHU

## Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?

In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth. Improvement in drainage details and application of painting and coating to bridges help to solve this problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.

## What are the advantages of assigning the central pier and the abutment as fixed piers?

Bearing Articulation refer to the arrangement, configuration selection of bridge bearings inorder to facilitate force transfer from the superstructure to substructure by achieving predefined degree of freedom (Displacements or Rotations) at the support locations/bearings. Bearing articulation has a major impact on the design of substructure of bridges.

1. For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
2. For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.

Symbols for common bearing types (from Table 1 of BS EN 1337-1)

Typical Bearing articulation of a curved bridge

Thank you Vincent T. H. CHU

## What are the advantages of assigning the central pier and the abutment as fixed piers?

Bearing Articulation refer to the arrangement, configuration selection of bridge bearings inorder to facilitate force transfer from the superstructure to substructure by achieving predefined degree of freedom (Displacements or Rotations) at the support locations/bearings. Bearing articulation has a major impact on the design of substructure of bridges.

1. For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
2. For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.

Symbols for common bearing types (from Table 1 of BS EN 1337-1)

Typical Bearing articulation of a curved bridge

Thank you Vincent T. H. CHU

## In bridge widening projects, the method of stitching is normally employed for connecting existing deck to the new deck. What are the problems associated with this method in terms of shrinkage of concrete?

In the method of stitching, it is a normal practice to construct the widening part of the bridge at first and let it stay undisturbed for several months. After that, concreting will then be carried out for the stitch between the existing deck and the new deck. In this way, the dead load of the widened part of bridge is supported by itself and loads arising from the newly constructed deck will not be transferred to the existing deck which is not designed to take up these extra loads.

One of the main concerns is the effect of stress induced by shrinkage of newly widened part of the bridge on the existing bridge. To address this problem, the widened part of the bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing bridge so that shrinkage of the new bridge will take place within this period and the effect of shrinkage stress exerted on the new bridge is minimized.

The bridge was widened by the stitching of an in-situ parapet.

Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches. To solve this problem, rapid hardening cement is used for the stitching concrete so as to shorten the time of setting of concrete. Moreover, the stitching work is designed to be carried out at nights of least traffic (Saturday night) and the existing bridge may even be closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed.

Sometimes, longitudinal joints are used in connecting new bridge segments to existing bridges. The main problem associated with this design is the safety concern of vehicles. The change of frictional coefficients of bridge deck and longitudinal joints when vehicles change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of longitudinal joints in bridges is quite difficult.

Note: Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.

by Vincent T. H. CHU

## In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

### Introduction

The incremental launching method is one of the highly mechanised erection methods used in bridge construction. The method consists of manufacturing the superstructure of a bridge by sections in a prefabrication area behind one of the abutments; each new unit is concreted directly against the preceding one and after it has hardened the resultant structure is moved forward by the length of one unit. This principle has already been used for many years in the construction of steel bridges. This is hardly surprising, in view of the equal strength of steel in tension and compression since, provided the design is suitable, the alternating stresses which occur when the bridge is slid forwards can be accepted without difficulty. This is not so with concrete, which can withstand only small tensile stresses without damage. Special measures are therefore necessary to enable concrete bridges to be slid forward by steps; the skilful use of prestressing is the most important of these measures.

Incrementally launched bridge construction Itz Valley Bridge near Coburg

#### In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

1. During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
2. The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central prestress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively. Later when the whole superstructure is completely launched, continuity prestressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central prestress.
3. For very long span bridge, temporary piers are provided to limit the cantilever moment.

## What is “preset” during installation of bridge bearings?

“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the following components: length of bearing + 2 x irreversible movement + 2 x reversible movement. Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + 2 x reversible movement. In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.

Laminated Elastomeric Bearings

Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.

## Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.

Jacking in prestressing

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.

• ###### What is the minimum grade of concrete to be used for RCC as per IS 456?

minimum grade of concrete to  be used for RCC as per IS 456 is M20 (page  1 IS 456)

## What is purchaser in Civil Engineering from the topic Concrete?

Purchaser is defined as person or authority entering a contract to buy concrete.

## In the design of a simply supported skew bridge, which direction of reinforcement should be provided?

In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure.

In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

Segmental Construction Method Span-by-span

Vincent T. H. CHU

## Why is the span length ratio of end span/approach span to its neighboring inner spans usually about 0.75?

From aesthetic point of view, an odd number of spans with a decrease in length in the direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the arrangement of irregular span lengths is not recommended because it gives a feeling of uneasiness.

From structural point of view, for a multi-span bridge with equal span length, the sagging moment at the mid-span of the end span/approach span is largest. In order to reduce this moment, the span length of end span/approach span is designed to be 0.75 of inner spans. However, this ratio should not be less than 0.40 because of the effect of uplifting at the end span/approach span support.

Note: End span refers to the last span in a continuous bridge while approach span refers top the first span of a
bridge.

Bursting Reinforcement

Vincent T. H. CHU

## What are the three major types of reinforcement used in prestressing?

(i) Spalling reinforcement
Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.

(ii) Equilibrium reinforcement
Equilibrium reinforcement is required where there are several anchorages in which prestressing loads are applied sequentially.

(iii) Bursting Reinforcement
Tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces.

Bursting Reinforcement

Vincent T. H. CHU

## What is the advantage of sliding bearings over roller bearings?

In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee. However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.

Sliding Bearing

Vincent T. H. CHU

## Are diaphragms necessary in the design of concrete box girder bridges?

Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult. In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

Vertical Cross Diaphragm

Vincent T. H. CHU

### What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?

Basically, piers constructed monolithically with the bridge deck are advantageous in the following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long and slender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing replacement. On the other hand, in maintenance aspect substantial cost and time savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.

(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers.

Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.

Prestressed I girders

Vincent T. H. CHU

## What is the Limiting serviceability deflections in Structural members like Beams and Slabs as per BS: 8110?

To prevent noticeable sag in the structural members, serviceability deflections shall be restricted to L/250. Where L is the span of the member or Length in case of cantilevers (BS8110-2:1985).

What is the effective depth d

## What are the permissible stresses in Tension Steel and Concrete as per BS8110 while checking a section for crack widths?

Permissible stress in Steel and concrete is defined as following.

Stress in steel is limited to 0.8 x fy (BS8007:1987 Appendix B2 and Cl. 3.8.3, BS8110-2:1985)

Stress in concrete is limited to 0.45 x fcu (BS8007:1987 Appendix B2)

Permissible Stress

## What is the effective depth in beams or slabs? How is it calculated?

Effective depth is the distance between extreme compression fiber to the centroid of tension reinforcement in a section under flexure. If h is the total depth of section, c the clear cover to outer reinforcement, d” and d’ be the diameters of tension bar and shear links respectively, the effective depth of section for flexure design of the section can be calculated as h – c – d’ – (d”/2), which will be the C.G of tension reinforcement.

What is the effective depth d

### In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25L segment is constructed in the first phase of construction?

Basically, there are mainly three reasons for this arrangement:

(i) The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed structure.

(ii) The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span is uniformly loaded) in which the moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the structure.

(iii) In case of a prestressed bridge, prestressing work has to be carried out after the construction of first segment of the bridge. If the prestressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the prestressing location at 1.25L from the first pier where there is relatively more space to accommodate prestressing works.

Prestressed I girders

Note: Span-by-span construction means that it is constructed from one span to another until its completion.

Vincent T. H. CHU

## In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?

To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.

The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.

Vincent T. H. CHU

## How to determine the size of elastomeric bearings?

For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design size of elastomeric bearings are based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.

Laminated Elastomeric Bearings

The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

Vincent T. H. CHU

## In the design of elastomeric bearings, why are steel plates inserted inside the bearings?

For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.

However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.

Vincent T. H. CHU

## In prestressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?

If wires/strands are stressed individually inside the same duct, then those stressed strand/wires will bear against those unstressed ones and trap them. Therefore, the friction of the trapped wires is high and is undesirable.

Vincent T. H. CHU

## Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?

In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth. Improvement in drainage details and application of painting and coating to bridges help to solve this problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.

## What are the advantages of assigning the central pier and the abutment as fixed piers?

Bearing Articulation refer to the arrangement, configuration selection of bridge bearings inorder to facilitate force transfer from the superstructure to substructure by achieving predefined degree of freedom (Displacements or Rotations) at the support locations/bearings. Bearing articulation has a major impact on the design of substructure of bridges.

1. For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
2. For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.

Symbols for common bearing types (from Table 1 of BS EN 1337-1)

Typical Bearing articulation of a curved bridge

Thank you Vincent T. H. CHU

## What are the advantages of assigning the central pier and the abutment as fixed piers?

Bearing Articulation refer to the arrangement, configuration selection of bridge bearings inorder to facilitate force transfer from the superstructure to substructure by achieving predefined degree of freedom (Displacements or Rotations) at the support locations/bearings. Bearing articulation has a major impact on the design of substructure of bridges.

1. For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.
2. For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.

Symbols for common bearing types (from Table 1 of BS EN 1337-1)

Typical Bearing articulation of a curved bridge

Thank you Vincent T. H. CHU

## In bridge widening projects, the method of stitching is normally employed for connecting existing deck to the new deck. What are the problems associated with this method in terms of shrinkage of concrete?

In the method of stitching, it is a normal practice to construct the widening part of the bridge at first and let it stay undisturbed for several months. After that, concreting will then be carried out for the stitch between the existing deck and the new deck. In this way, the dead load of the widened part of bridge is supported by itself and loads arising from the newly constructed deck will not be transferred to the existing deck which is not designed to take up these extra loads.

One of the main concerns is the effect of stress induced by shrinkage of newly widened part of the bridge on the existing bridge. To address this problem, the widened part of the bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing bridge so that shrinkage of the new bridge will take place within this period and the effect of shrinkage stress exerted on the new bridge is minimized.

The bridge was widened by the stitching of an in-situ parapet.

Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches. To solve this problem, rapid hardening cement is used for the stitching concrete so as to shorten the time of setting of concrete. Moreover, the stitching work is designed to be carried out at nights of least traffic (Saturday night) and the existing bridge may even be closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed.

Sometimes, longitudinal joints are used in connecting new bridge segments to existing bridges. The main problem associated with this design is the safety concern of vehicles. The change of frictional coefficients of bridge deck and longitudinal joints when vehicles change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of longitudinal joints in bridges is quite difficult.

Note: Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.

by Vincent T. H. CHU

## In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

### Introduction

The incremental launching method is one of the highly mechanised erection methods used in bridge construction. The method consists of manufacturing the superstructure of a bridge by sections in a prefabrication area behind one of the abutments; each new unit is concreted directly against the preceding one and after it has hardened the resultant structure is moved forward by the length of one unit. This principle has already been used for many years in the construction of steel bridges. This is hardly surprising, in view of the equal strength of steel in tension and compression since, provided the design is suitable, the alternating stresses which occur when the bridge is slid forwards can be accepted without difficulty. This is not so with concrete, which can withstand only small tensile stresses without damage. Special measures are therefore necessary to enable concrete bridges to be slid forward by steps; the skilful use of prestressing is the most important of these measures.

Incrementally launched bridge construction Itz Valley Bridge near Coburg

#### In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?

1. During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
2. The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central prestress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively. Later when the whole superstructure is completely launched, continuity prestressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central prestress.
3. For very long span bridge, temporary piers are provided to limit the cantilever moment.

## What is “preset” during installation of bridge bearings?

“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the following components: length of bearing + 2 x irreversible movement + 2 x reversible movement. Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + 2 x reversible movement. In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.

Laminated Elastomeric Bearings

Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.

## Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.

Jacking in prestressing

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.

### What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?

Basically, piers constructed monolithically with the bridge deck are advantageous in the following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long and slender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing replacement. On the other hand, in maintenance aspect substantial cost and time savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.

(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers.

Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.

Prestressed I girders

Vincent T. H. CHU

### In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25L segment is constructed in the first phase of construction?

Basically, there are mainly three reasons for this arrangement:

(i) The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed structure.

(ii) The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span is uniformly loaded) in which the moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the structure.

(iii) In case of a prestressed bridge, prestressing work has to be carried out after the construction of first segment of the bridge. If the prestressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the prestressing location at 1.25L from the first pier where there is relatively more space to accommodate prestressing works.

Prestressed I girders

Note: Span-by-span construction means that it is constructed from one span to another until its completion.

Vincent T. H. CHU

## In a curved prestressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?

To determine the orientation of guided bearings, one should understand the movement of curved region of a prestressed bridge. Movement of prestress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pretress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.

The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.

Vincent T. H. CHU

## How to determine the size of elastomeric bearings?

For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design size of elastomeric bearings are based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.

Laminated Elastomeric Bearings

The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

Vincent T. H. CHU

## Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.

Jacking in prestressing

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.

• ###### How to convert k Newton (kN) to kilograms (kg)?

How to convert kilo Newton (kN) to kilograms (kg)?

Multiply by 101.94 kg or approx. 100 kg.

Using formula(f=ma)

1N = (1 / 9.81) kg

therefore,

1kN = 1000 x (1/9.81) kg

• ###### How to convert kilo Newton (kN) to kilograms (kg)?

How to convert kilo Newton (kN) to kilograms (kg)?

Multiply by 101.94 kg or approx. 100 kg.

Using formula(f=ma)

1N = (1 / 9.81) kg

therefore,

1kN = 1000 x (1/9.81) kg

### Mechanics

• ###### How to convert k Newton (kN) to kilograms (kg)?

How to convert kilo Newton (kN) to kilograms (kg)?

Multiply by 101.94 kg or approx. 100 kg.

Using formula(f=ma)

1N = (1 / 9.81) kg

therefore,

1kN = 1000 x (1/9.81) kg

• ###### How to convert kilo Newton (kN) to kilograms (kg)?

How to convert kilo Newton (kN) to kilograms (kg)?

Multiply by 101.94 kg or approx. 100 kg.

Using formula(f=ma)

1N = (1 / 9.81) kg

therefore,

1kN = 1000 x (1/9.81) kg

## What are the different classifications of Soil Water?

Classification of Soil Water – Heaney; Crown 1995

There are mainly four types of Soil water

Structural water or Molecular bounded water – It cannot be removed by oven drying.

Gravitational water – The water in the soil due to the movement of water under gravitational forces. It moves into. through and out of soil due to gravity.

Hygroscopic water – Water which is absorbed from the atmosphere and which is held tightly by the soil particles, and which is unavailable for plants to absorb is called hygroscopic water. It is the water lost from an air-dry soil when it is heated to 105ºC.

Capillary Water – Water that is retained in the soil along with hygroscopic moisture and water vapor, after the gravitational water has drained off. Capillary water is held by surface tension as a film of moisture on the surface of soil particles and peds. It is retained as minute bodies of water filling part of the pore space between particles.

## What is the difference between normal bolts and high friction grip bolts?

High friction grip bolts are commonly used in structural steelwork. They normally consist of high tensile strength bolts and nuts with washers. The bolts are tightened to a shank tension so that the transverse load across the joint is resisted by the friction between the plated rather than the bolt shank’s shear strength.

Friction Grip Bolts

## What is the function of washers when using bolts?

The purpose of installing washers in a typical bolting system is to distribute the loads under bolt heads and nuts by providing a larger area under stress. Otherwise, the bearing stress of bolts may exceed the bearing strength of the connecting materials and this leads to loss of preload of bolts and creeping of materials. Alternatively, flanged fasteners instead of using washers could be adopted to achieve the same purpose.

Different types of steel bolt washers

### What is the difference between fasteners, bolts and screws?

Fastener is a general term to describe something which is used as a restraint for holding things together or attaching to other things. The main physical distinction between screws and bolts is that screws are entirely full of threads while bolts contain shanks without threads. However, a better interpretation of the differences between the two is that bolts are always fitted with nuts. On the contrary, screws are normally used with tapped holes.

Screw

Bolt and Nuts

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