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In the preparation of the model building codes, the load side of the equation comes mostly from the following three sources:

• 1997 National Earthquake Hazard Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 302 & 303), which are published by the Building Seismic Safety Council (BSSC). This document is essentially a mix of the requirements in the following two.
• ANSI/ASCE 7-98 Minimum Design Loads for Buildings and Other Structures, which is published by the American Society of Civil Engineers (ASCE).
• The Structural Engineers Association of California (SEAOC) Blue Book.

Thus, these documents -- particularly the NEHRP Provisions -- are the basis of loads, load factors and load combinations in the model building codes, the International Building Code and most other building-code documents.

One recent development in particular should be noted. ASD load combinations have been significantly modified in the 1998 revision of ASCE 7. The use of the 1/3 stress increase on the material side has been disallowed and replaced with a reciprocal 3/4 reduction factor on the load side. However, the application of the 3/4 reduction factor is strictly limited in ASCE 7-98. It can't be applied to dead load in any combination. It can't be applied to a transient load acting alone. And it can;t be applied to multiple transient loads if the reduced sum is less than one of the transients acting alone. After working all of the new written restrictions into equation form, it appears that there are now eleven distinct load combinations in ASD.

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