Comparative Analysis Steel.docx

LRFD and ASD loads are not directly comparable because they are used differently by the design codes. LRFD loads are generally compared to member or component STRENGTH whereas ASD loads are compared to member or component allowable values that are less than the full strength of the member or component.

Allowable Stress Design (ASD) method is based on the principle that stresses developed in the structural members should not exceed a certain fraction of elastic limit. This is old method of design which only considers elastic strength of material and hence limits the allowable stresses to a fraction of this limit (e.g. 40-50%). All loads are taken as service loads and no factor is applied to increase these services loads. The major drawback of this method is that it does not take into account the Plastic and Strain Hardening stages of material, hence, it becomes overly conservative in certain situations, while due to considering loads at service load values only, it produces unsafe results in other situations. Further serviceability limits are also not considered in ASD method, which may result in structures which although safe, do not fulfill there intended purpose. Load and Resistance Factor Design (LRFD) method, is based on the principle that strength (resistance) of various materials is scaled down by some factors while the applied loads are scaled up by some factors, and thereby the structural elements are designed using reduced strength and increased loads. The strength of materials considered for design is the ultimate strength, which results in utilization of elastic, plastic and strain hardening stages of material thereby giving economical and safe design consistently. The factors by which strength is reduced depends on the confidence of predictability of strength of the material. Hence, for example strength reduction factor for steel is less than that for concrete due to the fact that strength of steel can be much precisely predicted (as well as ensured) than that for concrete. Similarly, load factors are more for those loads which are highly unpredictable than loads which can be more accurately predicted. Thus load factor for dead load is less that that for live load or wind load as dead load will not vary as much as live or wind loads. Further LRFD method also considers serviceability limits like maximum allowable deflection, cracking etc. in addition to the strength design.

The first difference between ASD and LRFD, historically, has been that the old Allowable Stress Design compared actual and allowable stresses while LRFD compares required strength to actual strengths. The difference between looking at strengths vs. stresses does not present much of a problem since the difference is normally just multiplying or dividing both sides of the limit state inequalities by a section property, depending on which way you are going.

Load combination equations[edit] Allowable Strength Design[edit] For ASD, the required strength, Ra, is determined from the following load combinations (according to the AISC SCM, 13 ed.) and:[2] D+F D+H+F+L+T D + H + F + (Lr or S or R) D + H + F + 0.75(L + T) + 0.75(Lr or S or R) D + H + F ± (0.6W or 0.7E) D + H + F + (0.75W or 0.7E) + 0.75L + 0.75(Lr or S or R) 0.6D + 0.6W 0.6D ± 0.7E where:             

D = dead load, Di = weight of Ice, E = earthquake load, F = load due to fluids with well-defined pressures and maximum heights, Fa = flood load, H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials, L = live load due to occupancy, Lr = roof live load, S = snow load, R = nominal load due to initial rainwater or ice, exclusive of the ponding contribution, T = self straining load, W = wind load, Wi = wind on ice..

Special Provisions exist for accounting flood loads and atmospheric loads i.e. Di and W i Note that Allowable Strength Design is NOT equivalent to Allowable Stress Design, as governed by AISC 9th Edition. Allowable Strength Design still uses a strength, or ultimate level, design approach.

Load and Resistance Factor Design[edit] For LRFD, the required strength, Ru, is determined from the following factored load combinations: 1.4(D + F) 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R) 1.2D + 1.6(Lr or S or R) + (L or 0.8W) 1.2D + 1.0W + L + 0.5(Lr or S or R) 1.2D ± 1.0E + L + 0.2S + 0.9D + 1.6W + 1.6H 0.9D + 1.6 H ± (1.6W or 1.0E) where the letters for the loads are the same as for ASD. For the wind consideration, the ASCE allows a "position correction factor" which turns the coefficient of wind action to 1.36: 1.2D + 1.36W + .... the same above or 0.9D - 1.36W

The LRFD is certainly an improvement over the ASD. It takes better account of variability in both loading and resistance. LRFD is a reliability based design. Even before taking my reliability based design class, I always had my doubt in applying a single factor of safety for an output derived from inputs of varying reliability. The one disadvantage of LRFD is the lack of sufficient statistical data to select appropriate factors in certain specific cases, but still better than ASD.

ASD and LRFDMajor Differences •Load Combinations and load factors •ASD results are based on the stresses andLRFD results are based on the forces andmoments capacity •Static analysis is acceptable for ASD butnonlinear geometric analysis is required for LRFD •Beams and flexural members •C b computation