A Few Words Regarding Load Combinations in SEL |
  
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A Few Words Regarding Load Combinations in SEL |
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First, you will notice that ENERCALC SEL now includes prepopulated load combination sets for IBC 2012 and for ASCE 7-10 in acknowledgment of the fact that many States have already adopted the newest codes and standards and others are sure to follow.
Next, you will see that the standard load combination sets that are delivered with the program have been separated into sets for IBC versions and sets for ASCE 7 versions. This has been done to better acknowledge the subtle differences that exist between the IBC load combinations and the ASCE 7 load combinations.
One important difference to note is that the ASCE 7 Strength Design load combinations always apply a load factor of 0.2 on Snow Load when combined with Seismic Load, whereas the IBC load combinations apply a factor of either 0.2 or 0.7 on Snow Load when combined with Seismic Load, depending upon whether the roof is of a configuration that does or does not shed snow.
Another subtle difference creeps into the Allowable Stress Design combinations with regard to the percentage of Snow Load to use in combination with Seismic. The IBC load combinations require a factor of 0.75 on Snow Load in combination with Seismic Load when the flat roof Snow Load exceeds 30 psf, but they do not require the use of any Snow Load in combination with Seismic when the flat roof snow load is less than 30 psf. This is in contrast to the ASCE 7 Allowable Stress Design combinations which require a factor of 0.75 on Snow Load in combination with Seismic regardless of the magnitude of the snow load.
It would seem that the load combinations stipulated in IBC would govern over those set forth in ASCE 7, because IBC is generally the Governing Building Code, which then refers to ASCE 7 as a referenced standard for specific items (such as special load combinations incorporating seismic load effects including overstrength factor). However, the issue gets murkier on two accounts:
1. Not all jurisdictions adopt IBC as their Governing Building Code. So in jurisdictions that have a different Governing Building Code, it is possible that ASCE 7 load combinations could be referenced.
2. IBC identifies ACI 318 as a referenced standard. ACI 318 includes its own load combinations (which happen to match ASCE 7 load combinations). IBC does not specifically amend or omit that portion of ACI 318. So in a sense, there is a bit of a contradiction requiring some judgment on the part of the designer.
Given the multitude of possibilities, it was decided that we could best serve the user base by providing all of the basic load combination set templates that are likely to be needed. Of course, there is always the ability to edit those templates or create new ones for project-specific needs or to suit office standards.
