Overturning and Sliding FoS

[chippy502]

Ok, so looking through this forum and others there seems to be a big debate with people on both sides whether a 1.5 factor of safety needs to be applied to the 0.6DL + WL load case. From my research I have come to the conclusion you do not double them up, with the below linked article stating so in the most easy to understand terminology.

https://risa.com/news/why-dont-i-need-a-1-5-safety-factor-for-overturning-and-sliding-of-footings/

My question comes in wondering what about when 0.6DL + WL is not your controlling condition?? I have a column that comes down at an angle, so I am finding that my max horizontal force is actually with max gravity and not wind. So do I take the ASCE load case (DL + 3/4LL + 3/4SL) and compare that to 1.5? Or do I go back to the old formulas where everything had a factor of 1 and run that versus the 1.5? More importantly, is there anything anywhere that states this that I could fall back on should it be questioned during peer review? I am fairly sure there is nothing specifically stated in the codes, so I would think it would have to be an article or something in a structural journal/publication.

 

[BadgerPE]

I think in this case, a 1.5 FOS should be used in conjunction with your load combination listed. The LC 0.6D + 1.0W has been adjusted to build the FOS in while DL + 3/4LL + 3/4SL has not for overturning stability checks.

 

[DaveAtkins]

This is a really good question. My first response is to disagree with BadgerPE, because the load combinations are what we design for. Then again...I agree there is not a 1.5 FS built into all of the combinations.

DaveAtkins

 

[wannabeSE]

I have only received soils reports with allowable soil properties. Like bearing pressure, I assume the geotech used an appropriate factor of safety for the lateral earth pressures and friction coefficient.

 

[chippy502]

@wannabeSE - The soils report will provide allowable bearing pressures, lateral pressures, and friction coefficients. Although some of those values are used in the checks, the overturning and sliding checks are completely separate from what the soils engineer does.


Additionally, I have performed some more research and think I have my answer. Looking at section 1605.1.1 in the 2009/2012 IBC it states:

"Regardless of which load combinations are used to design for strength, where overall structure stability (such as stability against overturning, sliding, or buoyancy)is being verified use of the load combinations specified in Section 1605.2 or 1605.3 shall be permitted."

No mention of a 1.5 factor unless you are specifically looking at retaining walls covered in 1807.2.3. So using ASD load combos and you are good as long as you meet a FoS of 1.

 

[Hokie93]

I agree with BadgerPE. It seems prudent, and good engineering practice, to utilize a factor of safety against sliding and 1.5 is the generally accepted value. If utilizing allowable stress design, we incorporate factors of safety for all other strength limit states, why should sliding be any different?

 

[pete600]

In my opinion, this should be based on your local code. Where I work we use a FOS of 1.5 for sliding and overturning (min required by code) and then Calculate strength levels loads for design of the structure and then use the applicable ASD/LFRD factors to design the structural components.