ASCE 7 overturning 1

[canwesteng]

When doing work south of the border, I've always used a factor of safety of 1.5 for overturning. The ASCE 7-16 load combination is 0.9D+1.0W for LSD (or 0.6D+0.6W for ASD). Am I allowed to satisfy these load cases with no additional factor of safety, i.e. is the load factor built into the calculation of wind loads? For comparison, the overturning load case in Canada is 0.9D+1.4W, which works out to a FOS of just over 1.5.

 

[phamENG]

A few of the "old timers" in my last office used to insist on meeting a factor of safety = 2 for uplift/overturning in the foundation. I've never found such a requirement in modern codes, though, and I think it was just a hold over from what they learned early in their careers.

In general, I don't apply an additional factor to the ASCE 7 load combinations. I use them as is. I'm curious, what factors are applied to your wind loading for other conditions? In ASCE 7-05 and earlier (when wind forces were calculated at a much lower return period), it was 0.9D+1.6W for LRFD (what we call LSD down here). Maybe it's a dissimilarity in the way the wind loads themselves are calculated but the different factors bring them to a comparable reliability rating?

 

[canwesteng]

I'm probably doing the opposite of dating myself here, but I've never used anything older than ASCE 7-10, with the 1.0W wind factor. I've always given myself a 1.5 factor of as a feel good factor, but on this job I'd benefit quite a bit from the extra 1.5. Wind loads are calculated entirely differently in Canada, really only the end pressures can be comparable.

 

[phamENG]

I only used 7-05 because my jurisdiction is usually at least one code revision behind (we just finished the year long transition to IBC2015...).

How do the pressures compare? If you calculate it for a simple square building with ASCE 7-10 and the Canadian code (with which I have zero experience), what's the difference, percentage-wise?

 

[canwesteng]

Just ball parking, they are pretty similar, although this is compared to 7-16, which is maybe 20-30% lighter than 7-10 (except C+C which seems to be higher in this version).

EDIT: Guess I should caveat this, most of the reduction in wind load for this job is because we're up in the hills and there is now an elevation factor. Probably most jobs done to 7-16 won't be more than 10% lighter for wind loads.

 

[phamENG]

Well I don't know how accurate it is, but the Jacabus website has a "Primary Structural Action" wind load calculator per the NBC 2010. It gives options for a 50 or 10 year return period. In a 50 year return period, ASCE 7-10 (and I believe 7-16 as well) has a 7% probability of exceedance, or a 700 year mean recurrence interval. If yours is lower than 7% in 50 years (or lower than a 700 year MRI), then I think that would account for the variation in the factors.

 

[dauwerda]

Take a look at this previous thread on this topic (particularly JoshPlumSE's post):

https://www.eng-tips.com/viewthread.cfm?qid=450974

 

[dauwerda]

Additional information is also provided in the IBC, section 1605.1.1:

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. Where the load combinations specified in Section 1605.2 are used, strength reduction factors applicable to soil resistance shall be provided by a registered design professional. The stability of retaining walls shall be verified in accordance with Section 1807.2.3.

where 1605.2 are strength design (LRFD) load combinations and 1605.3 are ASD load combinations.

 

[JoelTXCive]

I thought that overturning was a geometric check performed using nominal loads. A comparable item would be bearing capacity or overturning & sliding on a retaining wall.

You setup the geometry of your structure so that it will not overturn (or slide) with a safety factor. This could be 1.3 or 1.5 depending on project specific items.

After your geometry is good, you can go into regular steel, wood, or concrete design using LRFD or whatever your design methodology you want.

The key thing is that these are two separate processes: a geometric design of the overall structure, and a strength design of individual members.

 

[JoshPlumSE]

This has been discussed a number of times here on Eng-Tips, but my searches aren't finding the threads I remember. My Google-Fu must be rusty.... Therefore,, I'll re-state my case:

The 1.5 safety factor with a load combo of 0.9*DL is akin to a 1.0 safety factor with a load combo of 0.6 * DL.

1.5 safety factor >= 0.9*(DL stabilizing moment) / 1.0 overturning moment ==> True safety factor = 1.5 / 0.9 = 1.67

1.0 safety factor >= 0.6*(DL stabilizing moment) / 1.0 overturning moment ==> True safety factor = 1.0 / 0.6 = 1.67​

Same safety factor vs overturning as always....

I've seen safety factors of 2.0 (or even 3.0) frequently. This is usually a project level requirement for cases where the overturning demand is due to regular loading (i.e. retaining walls or eccentric gravity loads), not due to wind or seismic.

 

[canwesteng]

So actually, I can get away with using 0.6D + 0.6W for FOS=1 if I wanted to, although LFRD suggest 0.9D + 1.0W, so ~1.1. I'm actually quite concerned with respect to retaining walls designed using ASD though, it seems as though there is no factor of safety at all in ASD cases.

 

[canwesteng]

JP - in the new codes there is no 0.6D+1.0W, it is 0.6+0.6.

 

[canwesteng]

Joel - that logic doesn't quite work out. Say you are designing for ASD with 0.6D+0.6W for max uplift, and you find no tension so do not size anchors for it. Now going to the foundation, you use 1D+0.6W, and the foundation is seeing tension that the anchors could never transmit to it?

 

[dauwerda]

I'm actually quite concerned with respect to retaining walls designed using ASD though, it seems as though there is no factor of safety at all in ASD cases.

Per the previous excerpt from the IBC that I posted above, retaining walls need to be designed for stability per section 1807.2.3 of the 2018 IBC, which states:

1807.2.3 Safety Factor

Retaining walls shall be designed to resist the lateral action of soil to produce sliding and overturning with a minimum safety factor of 1.5 in each case. The load combinations of Section 1605 shall not apply to this requirement. Instead, design shall be based on 0.7 times nominal earthquake loads, 1.0 times other nominal loads, and investigation with one or more of the variable loads set to zero. The safety factor against lateral sliding shall be taken as the available soil resistance at the base of the retaining wall foundation divided by the net lateral force applied to the retaining wall.

Exception: Where earthquake loads are included, the minimum safety factor for retaining wall sliding and overturning shall be 1.1.

 

[canwesteng]

Whoops... sorry, I was to focused on footings that I missed the blurb on retaining walls. I guess in any case where a live load might be causing overturning it would be prudent to refer to that section to guide the ASD safety factor. I suppose that reinforces the idea that the load factor for wind is not required as the probability of exceedance of ASCE 7-16 wind loads is sufficiently low.

 

[phamENG]

canwesteng - when it comes to soil/structure interaction (such as overturning from a retaining wall or bearing pressure), I use geotechnical recommendations and not necessarily the load combinations in ASCE 7. I'll use the ASCE 7 combos to calculate internal structural load effects (bending moment in the wall, for instance) to detail the reinforcement, but the proportions of the wall (toe length, key, etc) will be based on a geotechnical factor of safety applicable to the check being performed.

Due to the change in MRI for calculated winds from 7-05 to 7-10 and later, the old 0.6D+1.0W is approximately equal to 0.6D+0.6W in the newer codes.