How is Working Stress Method (ASD) different from Limit state method (LRFD or LFD)? Assumptions, Advantages and Comparisons
Limit state method (LRFD or LFD)
“A limit state is a condition beyond which a structural system or a structural component ceases to fulfill the function for which it is designed”
Various limit states are
Strength limit states: With respect to strength in shear, flexure, torsion, fatigue, bearing, settlement, bond or combined effects.
Serviceability limit states: With respect to deflection & cracking. The appearance, durability and performance of the structure must not be affected by deflection & cracking, buckling, stability
Special limit states: Damage or collapse in extreme earthquakes. Structural effects of fire, explosions, or vehicular collisions.
Design Assumptions and advantages of Limit state method
- Partial safety factor for material (γm) for yield and ultimate stress.
- Working loads are factored (increased) as per partial safely factor (γf) causing Limit State of strength.
- The design strength is calculated dividing the characteristic strength further by the partial safety factor for the material (γm), where γm depends on the material and the limit state being considered.
- Post buckling and post yielding plays important role in estimating capacity of structural elements at Limit State.
- Deformations are evaluated at working loads.
- This is based on the behavior of structure at different limit states ensuring adequate safety against each limitation.
- Tensile strength carried by of concrete is zero.
- The stress block represents in a more realistic manner when the structure is at the collapsing stage (limit state of collapse) subjected to design loads.
- Does not obey Hook`s law
- It consider the variability not only in resistance but also in the effects of load. Concept of separate partial safety factors of loads of different combinations in the two limit state methods.
- Concept of separate partial safety factors of materials depending on their quality control during preparation. Thus, γm for concrete is 1.5 and the same for steel is 1.15. This is more logical than one arbitrary value in the name of safety factor.
- A structure designed by employing limit state method of collapse and checked for other limit states will ensure the strength and stability requirements at the collapse under the design loads and also deflection and cracking at the limit state of serviceability. This will help to achieve the structure with acceptable probabilities that the structure will not become unfit for the use for which it is intended.
Working stress method
- Since the specifications set limit on the stresses, it became working stress method
- Factor of safely for yield stress, allowable stresses are less than ‘fy’.
- Pure elastic approach for analysis of structures under working loads.
- Yielding or buckling never occurs at working loads
- This method is based on the condition that the stresses caused by service loads without load factors are not to exceed the allowable stresses which are taken as a fraction of the ultimate stresses of the materials, fc’ for concrete and fy for steel.
- It deals only with elastic behavior of member perfectly elastic at all stages of loading; Stress-strain relations obey Hooks law (linear)
- Tensile stresses are taken by steel only.
- The Modular ratio is well defined value independent of time which is not true
- Results in larger compression steel percentage
- Working stress method does not give reasonable measure of strength, which is more fundamental measure of resistance than is allowable stress.
- Another drawback in working stress method is that safety is applied only to stress level. Loads are considered to be deterministic (without variation). For example, Permissible bending & direct compression are as fraction of crushing strength.