# 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, K_{d} (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*

## 2 thoughts on “Why the Lateral Loads Factors Differs in ASD Load Combinations ?”

## [Part 2] Step by Step Analysis Procedure of Seismic Loads Based on IBC2012/ASCE7-10 | Struczone

(September 2, 2016 - 10:09 PM)[…] Why the Lateral Loads Factors Differs in ASD Load Combinations ? […]

## Combining Horizontal and Vertical Earthquake Effects | Struczone

(May 18, 2016 - 3:25 PM)[…] Why the Lateral Loads Factors Differs in ASD Load Combinations ? How do Earthquakes Affect Reinforced Concrete Buildings ? How do Columns in RC Buildings Resist Earthquakes? How do Beams in RC Buildings Resist Earthquake ? […]