×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Those are some serious loads. Fun.

Quote (ajdg29)

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.

Quote (ajdg29)

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Are you absolutely positive that these are ASD reactions?

Mike McCann, PE, SE (WA)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Quote (JAE)

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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

Mike McCann, PE, SE (WA)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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

Mike McCann, PE, SE (WA)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
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"

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Perhaps the PEMB engineer needs more mainframe lines to lessen the reactions. Is this in a high wind area? Seismic should not control for a PEMB unless it is supporting concrete tilt up walls.

Mike McCann, PE, SE (WA)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
Wind speed is 100mph ASD (2009 IBC). Seismic Sds = 0.099. The moments are due to wind per the loading sheet I attached a few posts up.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Designing rigid frames for 125' spans involves some tradeoffs. You can save a lot of steel by providing fixity at the base, but with extra cost in the footings. I haven't done "PEMB" building footings for buildings of this magnitude, but purposely designed industrial buildings, particularly with cranes, often have heavy bases like this and deep footings.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
So maybe design a grade beam as a deep concrete beam to transfer the moment to deep piles to resolve the force couple?

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Or have the PEMB pin the base... Now there's a thought...

Mike McCann, PE, SE (WA)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Yes, but the 200k uplift will still require a huge foundation to resolve, so pinning the base may not have a large effect as hoped.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

ajdg29,
Yes, that is what I meant.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

ajdf29, refer back to the article jeffandmike linked above. Fixing the base makes the building cheaper for the metal building manufacturer, but makes the foundation cost go up significantly, as you're experiencing. FWIW, I would send this back and make them pin the base and add in more bents if necessary.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

When the engineer doesn't have complete control of the structural design, this sort of thing happens. We don't know much about the use of this building, or how it is being contracted. If the PEMB manufacturer has just been contracted to provide a building to cover space, and this is the most economical design to meet the criteria, then the footing designer is stuck with dealing with it.

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

Did you tell the owner that the PEMB manufacturer is costing him a ton of money in concrete with his design? If not, I would not go too deep into this design beyond some prelim calcs to get a magnitude of the foundation that one might need. The owner is bound to ask how much. This is a case of a supplier robbing Peter to pay Paul. I would expect that adding a few tons of steel would be far more cost effective than trying to deal with those base forces.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

I would agree...

Mike McCann, PE, SE (WA)


RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

I believe it is unrealistic to expect a rigid response of a fdn for such a large moment unless one goes to extreme lengths(mat fdn) to resist it...it does not take much rotation of the fdn to transfer this moment back into the steel frames which are not designed to handle it.....as others suggested, the optimum solution is the have them design the steel frames to take this moment and have pinned bases......it looks like the specs for these PEMBs should be tightened up to require the PEMB suppliers to design the steel frames to have pinned bases.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

I totally agree with:

Quote (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. )

You PEMB guys might think it's all clever to take no responsibility for the "blob of concrete" but the people who do, like me, are passing the word that a) it's not reasonable and b) you're not meeting us halfway by not giving us reasonable loads, load combinations, bolt spacings or edge distances. And the people who pay for these are noticing.
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

A lot of people wanting pinned bases without much information on the building, except that the frames span 125'. We don't even know how high the building is, or much about the applied loads. But if the distance between the bolts is 5'-6", the column is big, maybe 60" deep. (Sorry, on rereading, the building is 82' high at the knee). Not your everyday PEMB, I would have thought.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

82' high and 125' span on 3000 psf soil. I see a deep foundation system in the owners future.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

(OP)
Ok, going back over the reactions given in what I posted earlier (18:29), using the correct signage, I get the maximum moment using ASD combination 16-14 (0.6D+W) as 1893k-ft. Still, this is a huge moment to account for, and I'm coming up with a grade beam of 7'W x 5'D x 35'L to meet the allowable bearing pressure of 3,990PSF.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

What about a shorter gradebeam that is supported on deep foundations. Turning the moment into axial forces (both up and down).

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

This will be a little off topic but what the hey. When I look at your frame geometry and the supplied reactions, a couple of things jump out at me:

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

Just realized that I've got my PT rods in backwards in the columns. Please think of them mirrored about a vertical axis.

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

More gooder.

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 love it, but you crazy Koot!

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

>>>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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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,

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

Quote (Archie264)

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.

RE: Foundation for PEMB with Heavy Moment and Uplift Reactions

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...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources