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The Efficient Use of Outrigger & Belt Truss in Tall Buildings
Submitted By: Neeraj Agrawal
The outrigger and belt truss system is one of the lateral loads resisting system in which the external columns are tied to the central core wall with very stiff outriggers and belt truss at one or more levels. The belt truss tied the peripheral column of building while the outriggers engage them with main or central shear wall. The aim of this method is to reduce obstructed space compared to the conventional method. The floor space is usually free of columns and is between the core and the external columns, thus increasing the functional efficiency of the building. Exterior columns restrained the core wall from free rotation through outrigger arms. Outrigger and belt trusses, connect planar vertical trusses and exterior frame columns. Outrigger system can lead to very efficient use of structural materials by mobilizing the axial strength and stiffness of exterior columns.
The Concept Of Outrigger
The great sailing ships of the past and present use outriggers to help resist the wind forces in their sails. Like the ship, the core in the tall building can be related to the mast of the ship, the outrigger acting like the spreaders and the exterior columns like the stays or shroud of the ship.
The Use Of Outriggers In High-rise Buildings To Control The Forces
The incorporation of an outrigger which connects the two elements together provides a stiffer component which act together to resist the overturning forces. When an outrigger-braced building deflects under wind or seismic load, the outrigger which connects to the core wall and the exterior columns, makes the whole system to act as a unit in resisting the lateral load.
Types Of Outrigger Truss System
On the basis of connectivity of core to exterior columns, this system may be divided as in two types :
- Conventional Outrigger Concept
- Virtual Outrigger Concept
Conventional Outrigger Concept
In the conventional outrigger concept, the outrigger trusses or girders are connected directly to shear walls or braced frames at the core and to columns located outboard of the core. Typically (but not necessarily), the columns are at the outer edges of the building.
Virtual Outrigger Concept
In the “virtual” outrigger, the same transfer of overturning from the core to elements outboard of the core is achieved, but without a direct connection between the outrigger trusses and the core. The basic idea behind the virtual outrigger concept is to use floor diaphragms, which are typically very stiff and strong in their own plane.
Belt Trusses As Virtual Outriggers
The use of belt trusses as virtual outriggers avoids many of the problems associated with the use of conventional outriggers. The principle is the same as when belt trusses are used as virtual outriggers. Some fraction of the moment in the core is converted into a horizontal couple in the floors at the top and the bottom of the basement.
Advantages Of Use Of Belt Trusses As Virtual Outriggers1.There are no truss diagonals extending from the core to the exterior of the building.2.The need to locate outrigger columns where they can be conveniently engaged by trusses extending from the core is eliminated.3. The complicated truss-to-core connection is eliminated4. Differential shortening or settlement between the core and the outboard columns does not affect the virtual outrigger system since the floor diaphragms, though stiff in their own plane, are very flexible in the vertical, out-of-plane direction.
Factors Affecting The Effectiveness Of Outrigger System1.The stiffness and location of the outrigger truss system.2.The stiffness and location of the Belt truss system.3. Geometry of the tall building.4. Stiffness of the central core.5. Floor-to-floor height of the tall building.
Advantages of using Outrigger & Belt truss system1.There are no trusses in the space between the core and the building exterior.2. There are fewer constraints on the location of exterior columns. The need to locate large exterior columns where they can be directly engaged by outrigger trusses extending from the core is eliminated.3. All exterior columns (not just certain designated outrigger columns) participate in resisting overturning moment.4.The difficult connection of the outrigger trusses to the core is eliminated.5. Core overturning moments can be reduced through the reverse moment applied to the core at each outrigger connection.6. Exterior framing can consist of simple beam and column framing without the need for rigid-frame-type connection, thus reducing the overall cost.7. Reduction or elimination of uplift and net tension forces without the column and foundation system.
Plaza Rakyat Tower
The 77-story Plaza Rakyat office tower in Kuala Lumpur, Malaysia, uses a concrete shear core, a concrete perimeter frame, exterior concrete belt walls at two levels, and a conventional outrigger system at the roof as the building’s lateral load-resisting system
Jin Mao Tower
The 88-story Jin Mao Office, Hotel, observation, retail tower is in Shanghai, China. The tower is built around an octagon-shaped concrete shear wall core surrounded by 8 exterior composite super columns and 8 exterior steel columns. Three sets of 8 two-story high outrigger trusses connect the columns to the core at six of the floors to provide additional support
Conclusions On Tall Buildings Using Outrigger & Belt Truss
1.There are no trusses in the space between the core and the building exterior.
2. There are fewer constraints on the location of exterior columns. The need to locate large exterior columns where they can be directly engaged by outrigger trusses extending from the core is eliminated.3. All exterior columns (not just certain designated outrigger columns) participate in resisting overturning moment.4. The difficult connection of the outrigger trusses to the core is eliminated.5. Complications caused by differential shortening of the core and the outrigger columns are avoided.March 19, 2014
Presentation on Prefabricated Structures & Prefabrication - Concept, Components & Advantages
- by Priyaranjan Behera
Concept of prefabrication?
Prefabrication is the method of construction which includes assembling components of a structure in a manufacturing or production site, transporting complete assemblies or partial assemblies
to the site where the structure is to be located. It is combination of good design with modern high performance components and quality controlled manufacturing procedures. This work is carried out in two stages, manufacturing of components in a place other than final location and their erection in position.
Prefabricated sections are produced in large quantities in a factory and then shipped to various construction sites. This procedure may allow work to continue despite poor weather conditions and should reduce any waste in time and material at the site. Precast concrete units are cast and hardened before being used for construction. Sometimes builders cast components at the building site and hoist them into place after they harden. This technique permits the speedy erection of structures.
The Prefabrication as defined will be done in two stages
- Manufacturing at factory condition and erection of
- components at the required location.
This requires certain stages of preparation. They are Casting, Curing, Transportation and erection.
Casting: - Precast components are casted with controlled cement concrete in moulds of required shape and sizes. The vibrator is used to vibrate concrete and this removes any honeycombing inside the components.Curing: - After 24 hours of casting, the casted components are released from the mould and transported to curing tanks. Certain special components like railway sleepers where high strength is required are steam cured.
Transportation and erection: - After complete curing is done the components are transported to the site with heavy trucks and erection will be done using cranes with skilled labour force.
Prefab Building Components
Some of the prefabricated components are as explained below
Columns: - A column is a vertical member carrying the beam and floor loadings to the foundation. It is a compression member and therefore the column connection is required to be proper. The main principle involved in making column connections is to ensure continuity and this can be achieved by a variety of methods.
Beams: - Beams can vary in their complexity of design and reinforcement from the very simple beam formed over an isolated opening to the more common encountered in frames where the beams transfer their loadings to the column. Methods of connecting beams and columns are
- A precasting concrete haunch is cast on to the column with a locating dowel or stud bolt to fix the beam.
- A projecting metal corbel is fixed to the column and the beam is bolted to the corbel.
- Column and beam reinforcement, generally in the form of hooks, are left exposed. The two members are hooked together and covered with insitu concrete to complete the joint. This is as shown in the figure.
Waffle unit for flooring / roofing: - These are suitable for roofs / floors spanning in two directions. They are laid in a grid pattern. These units are cast in moulds. The saving achieved is not much. Also Shuttering are complicated and costly. Time consumption for construction is less
Advantages And Disadvantages
- Saving in cost, material, time & manpower.
- Shuttering and scaffolding is not necessary.
- Installation of building services and finishes can be done immediately.
- Independent of weather condition.
- Components produced at close supervision .so quality is good
- Clean and dry work at site.
- Possibility of alterations and reuse
- Correct shape and dimensions and sharp edges are maintained.
- Very thin sections can be entirely precast with precision.
- Handling and transportation may cause breakages of members during the transit and extra provision is to be made.
- Difficulty in connecting precast units so as to produce same effect as monolithic. This leads to non-monolithic construction.
- They are to be exactly placed in position, otherwise the loads coming on them are likely to get changed and the member may be affected.
- High transport cost
- Need of erection equipment
- Skilled labour and supervision is required.
The prefab components and prefab structures eliminate space and time over conventional constructions. Although prefabrication is employed to a large extent in a wide variety of countries, in India, construction industry, in spite of its expansion continues to adopt same conventional methods. The root of the economic problem is the fact that the majority of the households do not have the confidence needed to construct prefab houses. These structures are easy to erect as it is light material. This type of prefab buildings were constructed in earthquake prone areas of Lature and Gujarat.March 18, 2014
Repair Techniques for Cracks in Concrete - Crack Repair
-submitted by Shereef MI , - K.S.R. College Of Engineering
Various types of Repair Methodologies
- Muting and sealing
- Resin injection
- Dry packing
- Polymer impregnation
- Vacuum impregnation
- Autogenous healing
- Flexible sealing
- Drilling and plugging
Stitching of Concrete
In this technique, the crack is bridged with U-shaped metal units called stitching dogs before being repaired with a rigid resin material. A non- shrink grout or an epoxy resin based adhesive should be used to anchor the legs of the dogs. Stitching is suitable when tensile strength must be re established across major cracks. Stitching dogs should be of variable length and orientation.
Benefits Of Cracked Stitching
- Quick, simple, effective and permanent.
- The grout combination provides an excellent bond within the substrate.
- Masonry remains flexible enough to accommodate natural building movement.
- Non-disruptive structural stabilization with no additional stress
Muting And Sealing
This is the simplest and most common method of crack repair. It can be executed with relatively unskilled labor and can be used to seal both fine pattern cracks and larger isolated cracks. This involves enlarging the crack along its exposed face and sealing it with crack fillers. Care should be taken to ensure that the entire crack is routed and sealed.
Epoxy resins are usually selected for crack injection because of their high mechanical strength and resistance to most chemical environments encountered by concrete. Epoxies are rigid and not suitable for active cracks. This method is used to restore structural soundness of members where cracks are dormant or can be prevented from further movements.
Underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure. Foundation underpinning is a means of transferring loads to deeper soils or bedrock.
Purpose Of Underpinning
- To obtain additional foundation capacity
- To modify the existing foundation system
- To create new foundations through which the existing load may be wholly or partially transferred into deeper soil
- To arrest the excessive settlement
- To improve the future performance of the existing foundations
When Underpinning Is Required?
- Construction of a new project with deeper foundation adjacent to an existing building.
- Change in the use of structure
- The properties of the soil supporting the foundation may have changed or was mischaracterized during planning.
- To support a structure which is sinking or tilting due to ground subsidence or instability of the super structure
Methods Used For Underpinning
- Pit Underpinning
- Push Piers System
- Helical Pier System
- Pile Underpinning
- Other Methods
- Chemical Grouting
- Micro fine Grouting
- Micro piles
Fire Damage Repairs
- Timber structures may be repaired with new timbers or composites of steel and timber members.
- Steel structures are normally repaired with steel.
- Both concrete and masonry structural elements are frequently repaired with fiber reinforced polymers (FRP).
- Concrete structures are occasionally repaired with shortcrete
Repair of leakage structureMarch 8, 2014
Techniques for Rehabilitation & Repair Of Structures
-submitted by Shereef MI , - K.S.R. College Of Engineering
The success of repair activity depends on the identification of the root cause of the deterioration of the concrete structures. If this cause is properly identified, satisfactory repairs can be done for the improvement of strength and durability, thus extending the life of the structure, is not difficult to achieve. General procedure in the repair of distressed concrete structure:
- Support the structural members properly as required.
- Remove all cracked, spalled and loose concrete.
- Clean the exposed concrete surfaces and steel reinforcement.
- Provide additional reinforcing bars, if the loss in reinforcement is more than 10%
- Apply shortcreting/polymer concrete for patch repair work and grouting for porous/honeycombed concrete.
- Apply protective coatings over the exposed/repaired surface.
Guniting is mechanically applied material consisting of cement, aggregates and water. The cement and sand are batched and mixed in the usual way and conveyed through a hose pipe with the help of compressed air. A separate pipe line brings water under pressure and the water and cement aggregate mix are passed through and intimately mixed in a special manifold and then projected at high velocity to the surface being repaired.
In good quality work, a density around 2100kg/m3 is achieved. For effective guniting, the nozzle should be kept at 60cm to 150cm from the work normal to the surface. Before guniting is applied, the old concrete surface is prepared properly, all the cracks treated and the new reinforcement fixed in position. Cracks wider than about 0.5 mm should be cut out and filled with hand-applied mortar or with gunite.
Shortcrete is defined as “ mortar or concrete pneumatically projected at high speed onto a surface”.Types of shorcrete:
- Dry mix: Dry cement, sand and coarse aggregate are premixed with only sufficient water to reduce dusting.
- Wet mix: The cement, sand and coarse aggregate are mixed with water and the resulting concrete is then pumped to the nozzle where compressed air propels the wet mixture onto the surface.
Shortcrete has been used to repair :
- canal and spillway linings and walls
- the faces of dams, tunnel linings
- highway bridges and tunnels
- deteriorating natural rock walls and
- earthen slopes
- to thicken and strengthen existing concrete surfaces
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March 6, 2014
Interface between Structural & Geotechnical consultants by Bhavin ShahBrief details of the video :Evening Lecture Series of IAStructE, Gujarat (www.iaseguj.org), dated : 29-Nov-14As India is likely to experience huge infrastructure growth in near future, more and more structures would be required to build which would have higher complexities and to be constructed within the shortest time. In such scenario, interfacing between structural engineer & geotechnical engineer is going to be very vital to arrive at the correct foundation design parameters for safety and optimisation. We will discuss few such interface related issues in Part-1 of the lecture.The lecture highlights few of the important points related to interface between structural & geotechnical consultant for safety & optimisation of the structure.
SPEAKER: Mr. Bhavin Shah
Mr. Shah has 14+ years of experience in design of different types of structures for diversified industries such as Oil & Gas, Power, Industrial structures, Commercial, Residential, etc. in India & abroad. Mr. Shah is Associate Vice President (Structures) at VMS. He has also worked in past with L&T Chiyoda Limited & Adani Infra (I) Ltd.January 19, 2015
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