I am getting a failure for a concrete pier but there is no explanation of what the problem is other than the red X on the LRFD load combinations tab. What is not working correctly here? The bearing pressure is all that I really care about and that checks out okay. I can jack up the concrete strength and the soil capacity but neither of those help.
Hi @WilsonEngineers, thanks again for posting here. I can’t reproduce your calculations as I don’t see what is the loading on the pier, but I suspect this is related to uplift. If you hover on “FAIL” at the top or on the “X” in the “LRFD Load Combinations” section, you can see more information about the error - check the image below. If the below is your case, please double check the loading and make sure you don’t have an uplift condition and it should be ok. Cheers!
Deleting the wind loads did solve the problem, but how can this be addressed going forward? This is a covered porch with wind on the rafters so the calc looks complete to any reviewer.
But it is not necessary to trace wind loads down to the piers for such a simple structure. I’d be shot by my clients if I started upsizing pier footings to prevent porches from blowing away.
@WilsonEngineers @lucas It’s pretty rare to get a porch or deck that won’t have net uplift since they are “lighter.” The uplift on the pier usually causes the error since the calculator doesn’t address any inherent soil skin friction that would resist the uplift. I do a simple calculation outside of clearcalcs with the surface area of the pier that is embedded within the soil times a skin friction value to come up with a capacity of uplift that the pier would resist. You would need an appropriate post base to adequately transfer the uplift.
@PG218 I won’t disagree with the theory on that, but it’s just not a thing that we are worrying about here in northern Pennsylvania. Sure, there could be situations where I would go there, but not in this case. There was also something else going on with this calc. I tried increasing the footings to 5ftx5ft and the calc still failed. The logical solution for me is to just void any lines in the calc that show wind.
I was especially curious why LRFD load combinations failed but ASD did not. It is not a simple math/combination explanation from what I can tell.
@WilsonEngineers @lucas I could be way off but you may be dealing with a small amount of uplift where the ASD combination of 0.6D+0.6W is positive but the LRFD combination of 0.9D+1.0W is negative. Did your loads in the unfactored and factored section increase when you increased the size of the pier? The self-weight appears to be applied after the load combination step is completed so that the weight of the pier itself doesn’t help you offset the uplift within the factored loads (which it would of course would). The self-weight is used to determine the bearing pressure. A quick second way to check is to turn the self-weight off in applied loads and review the unfactored and factored loads, there is no change for me.
I don’t even know at this point…
I deleted the wind loads and then later re-added them back in without the issue repeating.
I’m going to write this one off as a glitch in the matrix and move on. I’ve done several similar deck/pier calcs with load tracing and this is the one and only time I have seen this error.
