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 includes prepopulated load combination sets for IBC and for ASCE 7.
Next, you'll notice that the standard load combination sets provided with the program are categorized into sets for the IBC and sets for ASCE 7. This categorization reflects the nuanced differences between the IBC and ASCE 7 load combinations which existed prior to IBC 2024 and ASCE 7-22.
For example, 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.
Users should understand that the selected load combination set is a distinct setting, separate from the Governing Code. These two are only superficially related, often sharing similar names but not functionality. ENERCALC does not automatically select a load combination set based on the chosen Governing Code because there is no direct correlation between the two. For example, users might create a custom load combination set like “ABCCoLoads” as an office standard. Therefore, even though some preloaded load combination sets in the program may reference the IBC version they were developed from, this is merely a naming convention. It remains crucial for users to choose the load combination set that best suits their specific calculation needs.
