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.

 

[dcarr82775]

I love it, but you crazy Koot!

 

[Archie264]

Kookt,

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.

 

[KootK]

Yeah, there's definitely a little cray-cray embodied in my proposal, no doubt about that. I was reading up on my Jorg Schlaich for inspiration over the weekend. Two takeaways:

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.

 

[Archie264]

>>>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'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.

 

[jimstructures]

Just for my interest would you give us some more information on the building you are trying to design the foundation for. I entered a trial building to see what numbers I got back; 125' wide x 150' long x 83' eave height wit a 1:12 roof pitch and 30' bays. I got no where near the numbers you reaction sheet is giving. I didn't fix the column bases however. That shouldn't make any difference in your gravity only loads. I wonder if this building has long bay type framing? If I make the bays something like 60' in length then I would start to approach your numbers. I'm just wondering about the physical layout of the building. Your manufacturer is a very reputable company so I don't doubt the reality of your numbers but I would really like to understand the dimensions of you building. I chose to site my practice building in the Houston area to get the 100 mile per hour wind loads from ASCE 7-05. I am assuming either just at the edge of hurricane zones or essential facilities and/or high occupancy facilities.

Jim,

 

[ajh1]

I suspect this building has a very severe lateral deflection restriction. I designed a similar (slightly smaller) building years ago that had a stacker crane in it with very severe lateral deflection limitations. I ended up having to do a moment base condition to keep some control over the steel sizes. When requested by the builder I investigated the possibility of a pinned base ala this discussion and my deflections went sky high. Even putting in something on the order of 10 times as much steel did not pull the deflections down to an acceptable level for the constraints.
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.

 

[KootK]

I know someone who designed a university's basketball arena using a configuration very similar to what you posted, complete with the prestressing.

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.

 

[Archie264]

Here you go.

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...