Foundation for PEMB with Heavy Moment and Uplift Reactions
Foundation for PEMB with Heavy Moment and Uplift Reactions
(OP)
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.
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.






RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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.
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
They frame width is 125'-0". This is just a portion of the entire building.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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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RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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"
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Yes, that is what I meant.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
There are some serviceability stiffness advantages to providing fixity in the base, so I wouldn't dismiss this as a poor design without a lot more information.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Mike McCann, PE, SE (WA)
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
As far as the loads that ajdg29 is trying to design for, as others have said, they look incorrect. I'd request the calculations and see if you can recreate them. One thing to do in the future is to specify that all supports to be pinned at the base.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
1) Gravity loads contribute a good deal to the reactions and;
2) I suspect that the reason for pursuing base fixity was to limit a) midspan roof deflection and b) lateral wind drift.
If you could talk your PEMB supplier into a little adventure, I wonder if post-tensioning the frame might be an elegant improvement here. While exotic looking, in this instance, post-tensioning would simply involve some extra, routine hardware and a couple of extra installation steps. There would be cost involved but, perhaps, less cost than some of the rather extreme foundation solutions that we've been contemplating.
As I see it, post-tensioning would have the following advantages:
1) You could essentially undo much of the gravity load deformations. This would be similar to load balancing in PT concrete.
2) You could stiffen the frame against lateral drift by making the rafter assembly stiffer. It would be closer to the stiffness that one would expect with a center line of columns.
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
I know someone who designed a university's basketball arena using a configuration very similar to what you posted, complete with the prestressing. Some 25 years later its still standing.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
1) From a material usage perspective, supporting loads with tension members is generally the most efficient way to go. No big surprise there.
2) #1 doesn't translate into overall economy until the scale of the structure gets pretty big. This would include stadia, long span bridges etc.
I've no doubt that, at some scale, a solution of this nature would start to make economic sense for portal frames. The trick is identifying that tipping point.
The solution to a really large portal frame seems simple enough in principle: commensurately large members that would supply the requisite stiffness. However, I suspect that there are limitations inherent in the shipping and fabrication processes that penalize the use of very large members. In such situations, it may be economical to do something like post-tensioning if it would keep the base frame looking a little more "typical". Maybe the PEMB guys can vet this supposition for me.
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
I'm not a PEMB guy but that makes sense to me. Otherwise indoor sports stadiums would be PEMB's, right? And I'm not referring to their practice facilities, I'm referring to the likes of the Astrodome, Superdome, Metrodome, etc.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
Jim,
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
One possibility if the building is not too long is to simply make it a braced frame in both directions and remove much of the moment resistance requirement. The frames could still use moment knees to assist, but the bulk of the restraint would be in the bracing over to the endwalls at each end. Of course the combination of bracing with the rigid knees would suggest a 3D analysis of the building.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
You've got me quite curious now. Any chance you'd want to name this arena? And thanks for the feedback on my wacky idea.
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.
RE: Foundation for PEMB with Heavy Moment and Uplift Reactions
http://hucc.hamptonu.edu/
Click on some of the pictures on the page for an expanded view. What's not shown in the pictures, of course, is the prestressing that occurred during construction. The middle of the trusses were jacked downward using cables thus kicking the columns outward in their slotted holes. When the jacking was complete the columns were bolted in place and the prestressing force released. Seems to have worked...