ASCE 7-10 Load Combinations

[Deadblow]

Hello,

I am working on the foundation designs for a pre-engineered metal building that was designed according to the 2015 IBC and ASCE 7-10. This is my first foundation design using the 2015 IBC and the "new" wind loads. I am accustom to the 2009 IBC. I feel like this topic has been beat to death, but I still don't understand which load combination set, strength design (ASCE 7-10 2.3) or allowable stress design (ASCE 7-10 2.4), I should be using throughout my foundation design. The PEMB manufacturer has provided all of the reactions that come out of the column base plates. These reactions are noted to be "un-factored". The wind load reactions are at a "strength" value with a load factor of 1.0.

My design to check soil bearing pressures. I always believed that I check the service level applied soil bearing against the "allowable" soil bearing pressure. I think I should use the load combinations found in the allowable stress design (ASCE 7-10 2.4) to check this. For me, the governing load combination is "D+S" which results in bearing pressure of (11.7kips + 18.0kips)/36sq.ft = 0.825ksf < 2.0ksf therefore ok. Anybody disagree with this?

My design to check uplift on the footing. Per the notes from the PEMB manufacturer, I figured I should be using the load combinations found in the strength design (ASCE 7-10 2.3). For me, the governing load combination is "0.9D+1.0W" which results in uplift of (0.9x11.7kips + -13.5kips) = -2.97kips. Therefore my footing, and soil overburden need to weigh at least (2.97kip/0.9) = 3.3kips. Anybody disagree with this?

What if I use the allowable stress design load combinations to check the uplift on the footing? For me, the governing load combination is "0.6D+0.6W" which results in uplift of (0.6x11.7kips + 0.6(-13.5kips)) = -1.08kips. Therefore my footing, and soil overburden need to weigh at least (1.08kip/0.6) = 1.8kips. So which is it, 3.3kips or 1.8kips, and why? I used to just check uplift using "0.6D+W" per the allowable stress design load combinations of the 2009 IBC. For what it's worth, the software program that I use to double check my foundation designs is using "0.6D+0.6W", which is an allowable stress load combination, for both the soil bearing check and the uplift check. I don't understand why I end up with two different required footing sizes.

Which set of load combinations should I use for the design of the anchor bolts, ASD or LRFD?

The design of the concrete and reinforcement in the footing needs to be designed for the LRFD load combination, correct? It seems to me there is a lot of mixing load combination sets occurring. Is this correct?

Thanks for any explanations!

EIT

 

[JAE]

Agree with your general view that for soil bearing checks (footing size determination) you would use service level loads since you probably have, from the geotech, been provided with an "allowable" soil bearing value. This should never be used with factored loads.

Disagree with your suggestion that for uplift on the footing you use factored combinations. The applicable uplift check we use is 0.6D +/- 0.6W where W is the factored wind load, reduced by the 0.6 factor to convert it to a service load wind. The 0.6 factor on dead load includes the traditional 1.5 factor of safety against uplift.

The wind you get from the PEMB manufacturer may be either a factored wind or a service wind - better make sure you know for sure which.

Anchor bolts can be either ASD or LRFD depending on which you choose. We typically use LRFD since we utilize Hilti's Profis software for these checks and they use a factored approach.

Concrete/reinforcement typically uses LRFD but you could, in theory, use ASD - but not typically used.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Deadblow]

JAE,

Thank you for your response. That makes sense to me. So the 0.6D +/- 0.6W is in essence the 0.6D + W load combination from the 2009 IBC that I am used to using just now "W" has changed and so does the load combo.

EIT

 

[RPMG]

It is incorrect to use LRFD for footing uplift, and the discrepancy between the two is thus: 0.6W < W/1.5

 

[Deadblow]

Ok, thanks RPMG! It just seems weird to me to use one set of load combinations to calculate the uplift on my footing (ASD) of 1.08kips and then use LRFD load combinations to again calculate the same tension that is to resisted by my anchor bolts which is now calculated to be 2.97kips when in actuality they are going to be the same magnitude. I think this is what my design process will look like. Please correct me if I am wrong.

EIT

 

[daski]

I concur with JAE and RPMG although I'm not sure what the W/1.5 represents.
The concrete reinforcing should be designed to the Strength (LRFD) load combos. Allowable strength design of concrete is not done anymore. It was in ACI 318-95 as Appendix A, but was subsequently removed from ACI 318 in one of the later editions. It is definitely not in ACI 318-14, which is what you should be using.
Another question you can ask yourself is if in the uplift case the dead load of the footing should also be multiplied by 0.6. My opinion is "no", although I like to put a safety factor on the resisting load, i.e., Resisting load > S.F. x Uplift Load. If S.F. = 1.5, it kind of amounts to the same thing as reducing the footing weight by 0.6.


Mark Daski

 

[ALEXZ0622]

There's no doubt to use ASD load combo to check soil bearing, and LRFD load combo for member design.
For uplift, I would say it's a tricky thing. I would recommend this:

1. ASD load combo 0.6D + 0.6W simply means: under service wind, your structure has 1.5 safety factor against uplift. And service ONLY.

2. LRFD load combo 0.9D + 1.0W means: under even ultimate wind, your structure's footings will stick to the ground.

These two combo check different things, since 1.5 service does not have any relation with ultimate results, but you can make your own engineering judgement, if service level check is adequate/comfortable or not depends on your office's policy. However, LRFD load combo will always generate worse results.

P.S. 1.5 safety factor is coming from 0.6D. 1.0D minus 10% over-estimate self-weight, then 0.9D/1.5 = 0.6D. You will never get 0.6D in real life unless your project is not on earth.