Preliminary Reactions for Pre-Engineered Metal Building 1

[rlewistx]

I've spent a good amount of time searching through the forums and have not found a thread addressing this.

For many years I had a great relationship with a PEMB supplier rep. He would give me preliminary column reactions for all my buildings in exchange for having the opportunity to bid on the projects. Sadly, he retired and I have no one to take his place. So now I am in the predicament of how to determine preliminary reactions for foundation design. I've tried other manufacturer's, but nobody wants to help unless there is a written contract to purchase the building.

I've looked into the MBS software, but it costs almost $2K per month and does much more than I need. How are other structural engineers calculating preliminary reactions for PEMB? What software do you use? I have access to lots of structural software programs, but I haven't found one that lends itself to estimating column reactions using the low-rise wind building loads that PEMB building use.

Appreciate any insight you can give on this.

 

[JoelTXCive]

I know that Alexander Newman's book has preliminary loading values in it.

https://a.co/d/bGujDoj

Link

I think that Newman got his values from a Nucor design manual though. I can't find it in my files, but I'll dig around.

 

[JedClampett]

Anecdote: Once I got loads from a PEMB Supplier Rep. and I used them for my preliminary design. His company won the project. When we got the submittal, the reps loads, now mine, were considerably low. So, the anchor bolts needed to be lengthened, foundations needed to be deepened, all very embarrassing. We asked the supplier and he gleefully told us we made a mistake on our preliminary loads. Of course, I responded that I got the loads from your rep. The rep shrank into the size of an ant, and we went on. The end conclusion was it was still my fault.

So I stopped getting loads from reps. Heck, even the good ones are hard to find. I do wind uplift, ignore dead loads and double them. Maybe I double them again. I put weaselly notes on the drawings and won't approve diddly (concrete, AB's etc.) until I get the official loads from the supplier. If I get squawks from the contractor about delays, I tell them to design their own anchors.

My point is, I'm tired of making compromises on these throw away buildings. I'm going to be super conservative since I'm going to look bad if the anchorage gets bigger. Don't refine the loads. Have you ever got a compliment on your efficient design for the anchor bolts? "You saved us $12!"

 

[Aesur]

In my area 90% of the time we are giving the preliminary reactions, however I believe there is normally some kind of contract for the PEMB engineering at the least with the supplier. Our local installer then shops multiple manufacturers and provides us prelim reactions. We actually have a few jurisdictions locally now that require final reactions and sealed PEMB drawings and calculations be submitted with the construction documents. When we have preliminary reactions depending on the source of said reactions and the building layout we will factor the loads up by 10 to 25% for foundation and anchor embedment designs. Once we have the final (after permits many times) we will compare and if the final reactions are less than the reactions we used we call it good, if they are substantially different we will notify the client that there may be savings in concrete if they pay us to redesign using the final reactions. If the reactions are higher then we will send a new proposal to update for the higher reactions. We have standard verbiage in the contracts relating to this and it's well documented on the plans - I have luckily never had a scenario where the final reactions were higher.

In the rare case where we don't have prelim reactions, we will use RISA to model a few bays, figure out bracing forces and then factor the loads up for the foundation design, however this is fairly rare (once every 10 designs or so). We always ask if they have prelim or final reactions before pricing, if not then we price higher knowing we will have to do some modeling ourselves.

I'm not familiar with your market, however I would start by talking a local installer and not fabricators/suppliers.

 

[JStephen]

JedClampett- I recall a former coworker complaining of similar issues. He could take a half-day to work out a more efficient layout, save the company $2,000, and nobody noticed. But if it was $12 extra, well then, somebody has screwed up!

 

[DoubleStud]

Why would you need the reactions now? I dont get it. All you are doing is designing the foundation and the anchor bolts? Why do you need it now?

 

[EngDM]

Why would you need the reactions now? I dont get it. All you are doing is designing the foundation and the anchor bolts? Why do you need it now?

How do you propose to design a foundation and anchor bolts without knowing the load that is being imposed on them? The pre-eng reactions typically aren't easily calculable to the extent required for a stamped drawing.

rlewistx - to answer your question, go heavy. I typically calculate all my factored reactions (LSD) and increase them by 10-15%. Once you receive stamped reactions you can feel free to modify your design to suit. If they ask why so heavy, tell them you need signed and stamped pre-eng reactions to justify going lighter.

 

[JStructsteel]

Why do you need prelim reactions? Does a bigger footing mean more fee or something? Wait until the project is designed and you get final reactions.

I wouldn't design anything without final reactions (signed and sealed drawings), since once a contractor has a size in his head (usually first one he gets) anything bigger will be a shit show.

 

[EngDM]

Why do you need prelim reactions? Does a bigger footing mean more fee or something? Wait until the project is designed and you get final reactions.

The problem where I am located is the increasing lead times for pre-eng buildings. The owners and project coordinators want everything done so that when pre-eng finally comes thru, it can go straight to construction. At least by sizing heavy, if the cost is approved then lowering the design doesn't negatively impact the project.

But it's a tricky task to ensure that pre-eng won't require more.

 

[driftLimiter]

I am in the process of designing a foundation system for a PEMB project that must go out to bid. Consequently we have to design the foundation system for reactions which are not yet fully specified.

-So I come up with preliminary frame lines based on experience ~20ft on center.
-Then since we are the EOR I know what the building loading requirements are for wind and seismic.
PEMBS fit nicely into a box shape and are pretty easy to run lateral loading for based on ASCE 7 so this is what we have done.

-Then I make a simple 2d model of a typical frame line and approximate the tapered frame girder sizes based on past similar projects if I can.
You must realize that the frame you select may distribute forces differently depending on the section sizes, but the base reactions can largely be determined by statics.

-Then I get my frame reactions, compare with other PEMBS we have on record of similar size and loading. Using judgement to determine if my reactions are appropriate.

-And finally for bidding purposes we multiplied all the reactions by a factor of like 1.4. We designed the footings and foundation system, anchorage reinforcement, for these amplified loads.

The thought is that when we get PEMB reactions we should be guessing higher for the most part and our Bid design will have more than enough capacity.

I don't think you need a PEMB software to do this. Their secret sauce is automation, special design tricks related to plate-girders, using every exception under the sun to minimize loads.

 

[JedClampett]

driftLimiter, I think you have it right. But one more trick that the supplier might pull is to space and locate the AB's in the worst possible way to get an acceptable capacity from them. Remember they won't do anything involving concrete.
[ul]
[li]Edge distance-If you get the absolute minimum per ACI, you're lucky. One way I use to avoid this is to show the girts on the outside of the columns. That adds 8 inches to the edge distance and will probably get you enough to get a decent capacity. Also, using hairpins helps.[/li]
[li]Spacing-It's amazing that the spacing needs to be so small (4 diameters, just the minimum ACI allows). Maybe it helps the pinned analysis assumption. But usually, the group will work, once you get the edge distance ironed out.[/li]
[/ul]
For a bid project, you might be able to defer the anchorage design back to the contractor. When the PEMB supplier refuses to do it (and they will), tell the contractor they need to hire someone to do anchorage. "It's in the contract." I'm working a lot of CMAR (the worst of all worlds by the way) and design build projects where the contractor can strong arm me into doing the anchorage calculations.

 

[driftLimiter]

Im a big fan on anchor reinforcement for these projects. 90% of PEMB baseplates and anchor configs I see are the same so pretty safe to assume as you suggest, the worst case scenario.

I almost always use a hairpin tie centered in the slab on grade to resist the outward shear kick, of the frame. Vertical anchorage I count only on the pad foundation for that calculation. I have done anchor reinforcement for this case as well but on PEMB I usually can get it to work just embedding the A.B.s in the footing.

 

[Aesur]

We try to use hairpins as @driftLimiter mentions when we can, but many times they are limited in capacity to less than 10 kips of thrust, so we end up with tie beams for the more complex or larger structures.

For vertical reactions, in my experience uplift almost always controls; I built a spreadsheet years ago that works for many larger manufacturers that designs for both pressure and uplift but accounts for 0.6D of the footing weight (90 pcf for concrete) and the soil above the footing with a footing buried 3' to top most of the time. Ideally the anchors would be embedded into the footing, but most contractors in our area hate that and therefore we have to hook bars in the pedestal and lap to the anchors, using embedded plates when needed. A trick here is with am embed plate you have uplift forces as bearing on the concrete and then develop the bars from the footing into the pedestal using Ldh for less depth. Ties are also used many times.

 

[EngDM]

driftLimiter, I think you have it right. But one more trick that the supplier might pull is to space and locate the AB's in the worst possible way to get an acceptable capacity from them. Remember they won't do anything involving concrete.
Edge distance-If you get the absolute minimum per ACI, you're lucky. One way I use to avoid this is to show the girts on the outside of the columns. That adds 8 inches to the edge distance and will probably get you enough to get a decent capacity. Also, using hairpins helps.
Spacing-It's amazing that the spacing needs to be so small (4 diameters, just the minimum ACI allows). Maybe it helps the pinned analysis assumption. But usually, the group will work, once you get the edge distance ironed out.
For a bid project, you might be able to defer the anchorage design back to the contractor. When the PEMB supplier refuses to do it (and they will), tell the contractor they need to hire someone to do anchorage. "It's in the contract." I'm working a lot of CMAR (the worst of all worlds by the way) and design build projects where the contractor can strong arm me into doing the anchorage calculations.

What I have done to get around this is to provide a box note on the foundation drawings that pilasters and anchor bolts are subject to change upon receiving stamped pre-engineered shop drawings. In my jurisdiction the city accepts these types of changes as a design continuation. That way you can deal with their anchors later.

I'm with driftLimiter, we always provide hairpins or dowels to anchor around the pre-eng anchor bolts. What we have done is provided thickened edges at highly loaded locations to engage more of the slab and reinforce the crap out of it.

 

[ntattose]

I have a great resource for PEMB reactions.How can we get into contact?