Foundation Stability

[junkbird]

When the IBC changed the load combinations that have reduced dead load (0.9DL to 0.6DL +WL or EQ) they also changed the foundation stability factor proportionally from 1.5 to 1.0. This seems to assume that these particular load combinations always govern when it comes to foundation stability, which is not always the case. There are situations where gravity live loads are destabilizing to the foundation, therefore leading to a situation where other load combinations that don’t have a reduced DL govern the stability of the foundation. This creates a design discontinuity between the older and newer load combinations. For example, the LC with 1.0DL + 1.0LL would yield the same overturning moment and sliding forces for the new and old load combinations. If the LL is destabilizing to an extent which this LC governs, the new load combinations have virtually no safety factor, i.e., 1.0, whereas the old LCs would have a safety factor 1.5 requiring a larger foundation. This seems to have been an oversight.

Does anyone have any comments about this apparent discontinuity in design safety?

 

[JoshPlumSE]

As much as it tries to be, the code can never be a cookbook where you always follow the recipe and you end up with a perfect product. No, you will always have to exercise engineering judgment. This is just a case where an experienced engineer will look at the INTENT of the code and manually provide a higher safety factor for that applied loading. Or, he/she will create a custom load combination to account for it....similar to reducing the dead load factor by 0.6. Something like Stabilizing dead load *0.6 + De-stabilizing dead load*1.0.

 

[junkbird]

Ya, that's what i've done in the past. It's frustrating when older supposedly "more experienced" engineers beat me over the head for being "too conservative" for compensating for this particular situation. I explain the it to them, but they don't seem to understand; and don't seem to want to. Mostly these are management guys that have been an "engineer" for 30 years but haven't actually designed anything for 20 years.