Foundation for PEMB with Heavy Moment and Uplift Reactions

[ajdg29]

I am currently working on foundation design for a PEMB in Texas. There are three frames that have heavy moment reactions along with the reactions in the X & Y planes. This moment is roughly 2,500k-ft when looking at ASD load combinations for 2009IBC. Shear is roughly 80k, maximum downward load is 276k and uplift is 200k.

My question is what is typically done for a situation like this. As of right now, I was planning on extending the leg/base plate and providing a buried pad footing to support the vertical load, with a deep grade beam running over the top to take out the moment. I would weld rebar to the column to take out the force couple cause by the moment into the grade beam.

Is this this best way? The PEMB engineer said they typically just provide a connection to the finished floor level. The base plate they are providing at these connections are built up and are about 5'-6" wide between bolts to take out the moment with the 1-3/4" diameter 105ksi bolts. I'm just not sure that is the best way to resolve the high loads. Any direction would be appreciated. Thanks.

 

[jeffandmike]

Not a lot of time to respond. Hoping this helps a bit:

http://www.structuremag.org/?p=342

 

[KootK]

Those are some serious loads. Fun.

The PEMB engineer said they typically just provide a connection to the finished floor level.

Of course. You might be able to deal with your shear this way but I don't see how it would help with your moment and uplift.

As of right now, I was planning on extending the leg/base plate and providing a buried pad footing to support the vertical load, with a deep grade beam running over the top to take out the moment. I would weld rebar to the column to take out the force couple cause by the moment into the grade beam.

This would be excellent structurally. Given that the moments would no longer travel through the base plates, you could probably get rid of the fancy chair stuff. The main issue that I see here is cost. If I understand correctly, this would mean running some large grade beams across the entire width of your building. Another route would be large pier and a footing designed for overturning. With your loads, that could get pretty ugly too.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tbone73]

Can you put it back on the PEMB engineer to design for the moments in the frames themselves as opposed to transferring the moments to the foundation? I've designed several foundations for PEMBs and haven't seen them try to transfer moments at least not this magnitude.

 

[ajdg29]

I was planning on just a grade beam centered on that specific column, extending in the direction of the moment. The frame bay width is 125'-0", which is why they are saying the loads are so high. So I was planning on making it wide enough and long enough to support the moment given the allowable bearing pressure.

 

[ajdg29]

I agree tbone73. I've done one other foundation design, but there were no moments to resolve. Just lateral, downward, and uplift. I checked with them on the units and everything, and they said that it's definitely 2500k-ft.

 

[msquared48]

Are you absolutely positive that these are ASD reactions?

Mike McCann, PE, SE (WA)

 

[ajdg29]

Well, the loads are given by load case, to then be used in appropriate load combinations according to the PEMB.

 

[msquared48]

Then they may still be factored reactions and can be selectively reduced for footing sizing soil based on allowable soil bearing. Do they show the equations used?

Mike McCann, PE, SE (WA)

 

[ajdg29]

No, they just give the loading per type (dead, collateral, roof live, snow, wind, and seismic). See attached.

 

[jimstructures]

Not to beat a dead horse, Are you absolutely sure those moment reactions are given in kip-feet and not in kip-inches. Butler Manufacturing Company would normally give the column moment reaction (if it exists) kip-inches or inch-kips.

One more thing to check, when you say "the frame bay width is 125'-0", do you mean the building width is 125' or the building is made up of bays which are 125' in length. the second option would mean that you have 125' long roof structural (plus overlaps at the connections) which would be a pretty unusual pre-engineered building layout.

Jim

 

[ajdg29]

Yes, that's what I thought as well jimstructures. But they confirmed that it's "definitely" in kip-ft. See attached in my previous post.

They frame width is 125'-0". This is just a portion of the entire building.

 

[JAE]

I think most PEMB reactions are always service level. Never ever seen them as factored.

I also think there is fast approaching a need for the PEMB companies to get their &*#% together as they still design only for the bolt tension/shear and baseplate requirements with typically very small bolt spacings and expect the EOR to somehow magically meet Appendix D of ACI 318.

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[KootK]

I also think there is fast approaching a need for the PEMB companies to get their &*#% together as they still design only for the bolt tension/shear and baseplate requirements with typically very small bolt spacings and expect the EOR to somehow magically meet Appendix D of ACI 318.

Amen brother! I recently contacted the head of engineering for a dominant PEMB supplier in my area (10+ yrs experience). Not having done one in my current locale, I asked him what was typical for foundation strategy. He sent me two example plans which literally showed nothing at all below the level of the base plates. Liability/scope stuff.. sure. Then I called and asked "so what do you typically see for foundations for these types of projects?" The answer? "Usually the columns are anchored to a blob of concrete of some sort". Seriously. And that's pretty close to verbatim.

No offence to ajh and jimstructures who clearly know their craft and have been helping out around here for years.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Did you do a rough hand check on the dead plus live load values to verify the result.

Mike McCann, PE, SE (WA)

 

[ajdg29]

No, I don't. Just have the results. Guess it's looking like I may need to start getting further information from them to track this through. I'm not even getting a realistic grade beam size to work to take out this moment. Have 3,000 psf allowable bearing with 1/3 increase allowed for short term.

 

[hokie66]

When I have needed that degree of base fixity, I have always used a deep footing system. I would try a pair of bored piles.

 

[Wallache]

I would make sure the manufacturer is using the correct code as well. IBC 2012 wind loads with IBC 2009 load combos might explain these massive loads.

 

[msquared48]

That gets back to the LRFD/ASD mix...

Mike McCann, PE, SE (WA)

 

[ajdg29]

I am verifying with them now to make sure of the following:

1. All loads shown are service level to be used in either ASD or LRFD combinations
2. The units for the moment reactions is "k-ft" and not "k-in"