Why the Lateral Loads Factors Differs in ASD Load Combinations ?
The switch to strength-level earthquake forces was first made in the 1997 Uniform Building Code (UBC). Before then, we had service-level earthquake forces in our model codes and, back then, the load factor on service-level E was 1.4. When strength-level earthquake forces were first introduced, the strength-level E in ASD load combinations was divided by the load factor of 1.4, to bring it down to service-level. 1/1.4 was rounded off to 0.7.
In ASCE 7 editions prior to 1995 and in all codes based on those editions, the load factor on W was 1.3. It was relatively low because it accounted for the directionality of wind – the fact that wind seldom, if ever, strikes along the most critical direction of a building. In ASCE 7-95 an explicit directionality factor, Kd (which is equal to 0.85 for all buildings), was introduced in the definition of the velocity pressure, q, and, therefore, the wind pressure, p.
The decision was made to divide the 1.3 load factor on the wind effect, W, by the directionality factor of 0.85, to keep the factored wind effects more or less the same as before.
The resulting 1.3/0.85 = 1.53 was rounded up to 1.6, rather than down to 1.5.
With the introduction of strength-level W in ASCE 7-10 and the 2012 IBC, W in ASD load combinations needs to be divided now by the load factor of 1.6, to bring it down to service level. 1/1.6 has been rounded to 0.6.
Credit to: S.K. Ghosh Associates