Steel Design Load Safety Factors

[auxblade]

I'm a young Eng studying steel design for my PE exam. I'm very familar with LRFD method of live/dead load factors and their combinations. When using AISD method and tables are these same factors applicable. I have came across many different approaches. Is a 1.7(dead+live) factor too "safe"? Any literature on the topic would be appreciated.

Thanks

Auxblade

 

[SlideRuleEra]

You don't use factors in ASD. The AISC Manual of Steel Construction (ASD), Ninth Edition has the information that you are looking for in Section 5. However since you are familar with LRFD it may seem to be presented in an unusual way. For example, for a compact section the allowable stress is 66% of the yield strength. In general terms ASD and LRFD are more or less the "reciprocal" of each other. When using ASD you are knowingly designing members at less than their maximum capacity (66% of maximum in the above example). With either system you wind up with a "safety margin", you just get their in different ways. Hope this helps and good luck on the exam.

 

[IFRs]

Well stated, SlideRuleEra.

I would say that when using the ASD method, you determine the actual stresses using the known loads and geometry of the structure combined with section properties of the members. No factors are applied to the loads. Dead load is 100% of the dead load, concentrated live loads are 100% of the concentrated live loads, etc. Load cases are determined using combinations of dead, live, seismic, external pressure, wind, etc. In some cases the load case summations involve factors but they are not to be confused with LRFD factors. Stresses like axial and bending are combined using formulas.

Allowable stresses are then either found in tables or calculated based on material properties and section geometry.

A comparison of "actual" versus "allowable" for the various load cases completes the analysis.

I hope that did not muddy the waters too much!!