FOS for Overturning with Testing

[RDR89]

Hi all, a colleague of mine completed a "pull test" today to determine the pull-out load for a product we are planning to use as a small equipment foundation. My question is - what factor of safety (if any) should be used to determine the appropriate load limits, particularly for stability? My initial thought is that 1.5 is industry practice for overturning, but when I look at the IBC equations that we would use in analysis in the absence of testing data, the equations for both LRFD and ASD inherently have a "safety factor" of 1 (0.9D+1.0W or 0.6D+0.6W). My colleague is inclined to use the actual pull-out value, which I would equate to an ultimate load value and I am not comfortable with that, but I don't know what a good rule of thumb safety factor would be. Any thoughts?

 

[human909]

For a novel piece of equipment. You should do multiple pull out tests multiple times in multiple states. If the substrate is earth then you'll get massive variability and you'll need to adjust you ultimate design capacity accordingly.

 

[phamENG]

Did you look at section 1708 of the IBC? That might be a good place to start.

 

[canwesteng]

ASCE/IBC load case equations consider ULS wind, not specificied. The factor of safety is included in the wind load by selecting the correct return period.

 

[RDR89]

@Canwesteng that’s my understanding that the load factors are inherent to the load combination provided the correct wind map is used, corresponding to the correct risk category. Is that what you are referring to?

If yes, then the “FOS” in the load combinations works out to be 1 for 0.6D+0.6W (using the ASD combo for a stability check). Forgive my misunderstanding if this is not what you were saying.

My colleague is going to “translate” the failure load per the test into a corresponding design wind load/speed to use for the product and my hesitation is that 1. The test is not completely reflective of a true equipment subject to wind load scenario, 2. Using that failure load as the design load leaves no room for error which inevitably happens in construction and 3. As human909 pointed out, multiple tests should be performed to get an idea but only one was performed. They are planning to specify the backfill for product use, but I think there is still a large room for error.

I am not trying to be unnecessarily conservative but I think a safety factor should be applied. I just don’t have a gauge whether that is being overly conservative and what a good safety factor would be. I also know I will get pushback on it.

 

[JAE]

The 0.6D + 0.6W combo is not a 1:1 safety factor.

You have:
0.6D - service dead load with a safety factor of 1/0.6 = 1.667 applied
0.6W - Ultimate wind load adjusted down to a service load wind.

So comparing service dead to service wind gets you a SF = 1.667.

For overturning, technically SF against OT is for structures where gravity is resisting lateral non-gravity induced loads and the structure is free to "topple" over.
With an anchored connection at the base, you would simply check the anchors for the relevant load combinations for strength. Overturning would be checked at the base of any foundation on the supporting soils.

 

[RDR89]

JAE I see what you’re saying. There are a number of threads on here discussing the safety factor so I apologize for the redundant question and my misunderstanding. It is a gravity resisting structure, so overturning is being checked at the base. Apologies if that wasn’t made clear.

Out of curiosity, do folks use the 0.9D+1.0W combo or the 0.6D+0.6W combo for checking stability?

 

[Aesur]

I'm confused, are you checking for overturning or pullout, I would think both have different levels of Safety Factors. For instance it is common to use a SF=5 for fasteners, I forget where this came from, but I believe it was a military document that became an industry standards, although I believe some companies are going with lower SF's now. As PhamEng pointed out, 1708 of the IBC is In-Situ load testing, however there are particular requirements for duration of loading etc. 1709.3.1 may be more applicable as you mention this may be used in multiple locations, which leads me to believe it's maybe not In-Situ and more Preconstruction load testing. In this case the SF is 2.5.

It would be more helpful to understand your load test procedure as pullout and overturning are not the same thing, for overturning I would expect uplift on one side of the equipment and pressure on the other creating a more accurate test. Note, you could come up with an overturning moment with uplift only on one side, but I haven't seen it done this way, for this you would take your uplift force and divide by 2 to create the couple reactions simulating pressure on one side and uplift on the opposite, so your capacity would be Uplift Load/2/2.5 SF.

 

[ads0221]

Whichever one you pick stick with it for consistency. We use ASD service load combinations to check stability of foundations.

 

[RDR89]

@Aesur we're doing overturning not pull-out. Sorry, wrong verbiage.