Pre-Engineered Metal Canopy Question

[VAStrEngr]

I have been tasked with providing the contract drawings for a pre-engineered metal canopy. I say "tasked" because as a proud structural engineer I DESPISE the term pre-engineered and prefer to use my intelligence to just design it myself. This is the problem with working for non-structural engineers who immediately run to a catalogue anytime they actually have to design something.

Anyway, it's not my name on the letterhead and it's a tough job market so here I am. The problem I am facing is that on top of performance specifying the canopy itself, I have been told to also performance specify the foundations. I have worked with a few pre-engineered structures in the past and have always assumed responsibility for the design of the foundations. It was my understanding that this was the common way of doing things in the pre-engineered world...the design engineer designs the foundations, provides a footprint/basic framing information, and provides loads for the structure.

I was hoping to find some references proving/disproving this or at least hear from other guys out there if this is the way you typically do it or not. Is it common to pass off all design responsibility including the foundations for a pre-engineered structure to the contractor/supplier? What resources can I use to argue the point that the design engineer should/should not be responsible for the foundations?

Thanks in advance!

 

[a2mfk]

I have never heard of any pre-engineered metal building supplier doing anything but metal building design (and they don't want to even if you ask them). They are not even the EOR, that would be you, who would design the foundations and provide the performance criteria. You would also design the anchor bolts into the foundations, and the concrete SOG.

I believe this is industry standard.

And I wouldn't be too tough on PEMBs, they have a time and a place, though I do not like it when they are crowbarred into building designs where they don't belong. I imagine with this tight market PEMBs are getting a lot more consideration. But when it comes to warehouses, light industrial, open canopies, etc. sometimes they are by far the most economical because of their proprietary shapes, especially if they use all metal cladding and roofing.

 

[VAStrEngr]

Good feedback, I appreciate it.

I was being a little facetious in my references to the PEMB guys. They most definately have a place and a purpose and as you mentioned will usually be the cheaper alternative for the correct application.

I'm guilt of venting a little of the frustration from some of my colleagues who assume that since it is pre-engineered then it somehow is exempt from the same design requirements we as the structural EOR use (ASCE 7 for instance) which they consider to be overly conservative and unnecessary.

 

[a2mfk]

Oh, I feel your pain. PEMB projects can go very bad and give you tons of headaches. You have to be very prudent with the architect, owner, and all of your notes and specs and don't let them push you around. And like everything, not all PEMB are the same... My chief complaint with PEMB projects is usually the only thing anyone cares about is cost, and you are viewed as an obstacle in the way of their super cheap building.

 

[Ron]

There's a difference between PEMB's and pre-engineered metal canopies. Foundations are almost never included with PEMB's, but not so with canopies. I have done over 700 canopy designs and many, if not most, included the foundation design. There's a reason for this.

When a structural engineer of record designs a foundation, he will ask the manufacturer for reactions from which to design. He will assume either a flat footing or a pedestal/pier design. That can be a problem with some canopy manufacturers who provide rigid frame bents with moments developed at the column to footing connection.

For light frame canopies (aluminum for instance), the columns are often embedded in the concrete footing. To do this requires knowing the depth of embedment for fixity and developing the footing for relatively large moments and potential for loss of contact between the soil and the footing for a portion of the footing when subjected to wind loads. There's nothing unusual about this, it's just that the connection between the column and the footing is a bit unconventional and not something that a typical design engineer will see day to day. Further, in order to prevent overturning, the adjacent walkway often has to be incorporated in the design of the foundation to develop adequate resisting moment.

In many of the canopy frames that I've done, but did not do the footing design as well, I've seen compatibility issues between the provided foundation and the provided canopy sections...so it is usually just easier to provide the footing design along with the canopy frame design. In every case, I provide full calculations so that the SEOR can check the frame and the footings for compliance with his/her intent.

 

[southard2]

I do a lot of school projects. I have always had the canopies and their foundations delegated to a specialty engineer. For one thing I'm pretty sure if I did the analysis everyone would once again complain about how large my footings are. Plus as the last poster mentioned their is the question of embedment length. The other reason I won't design the foundations is that I don't want to touch those things with a thousand foot pole. They are like airplane wings just waiting to take off. The calculations I've seen for the canopies seem a bit over simplified to me. I think most of them are probably under designed. Then again they are just canopies. But because of the cheap nature of it all I'd rather have all the engineering stay with one person. Basically I treat them as a separate structure entirely and thus I see no reason for them not to be pre-engineered.

With regard to metal buildings I've been doing a lot of them lately and I don't like them. Way way too much liability and no one understands that the slab is a structural element. I also don't like how close the anchor bolts are placed to the edge. I could go on but I think the risk to reward ratio is way bad.

But I have certain clients I have to keep happy.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[VAStrEngr]

Interesting perspectives. I am curious though in this case which is the wiser alternative. To match similar structures, it will be a steel canopy. It also isn't a standard framing sequence and column spacing has to vary in order to avoid equipment.

Should I contact a supplier and get reactions for their design knowing that another supplier would be different? Is it better just to list the loads and leave all of it to the supplier? Not quite sure how to strike a balance between my responsibilities as the SE but leave enough flexibility for the supplier (whoever it might eventually be).

 

[Ron]

VAStrEng..

If you give the loads, then you take some of the liability for the canopy design. Are you sure you want to do that if you don't design the whole thing?

If you give the parameters, the engineer for the canopy supplier has to meet those as a delegated function. I would suggest that you transmit those directly to the identified engineer.

As the Engineer of Record, you set the parameters and require that the canopy supplier meet YOUR requirements for engineering information in addition to the shop drawings. Have them provide a full calculation set and if you can't follow the calcs provided, then have them clarify. If it doesn't seem like it meets your requirements or if you still have questions, make them do it again.

If you want to do the foundation design, then make sure they give you all the parameters of their design, not just some reactions. With a steel canopy, there is usually no developed moment at the footing connection, but obviously you still have to be concerned about overturning moment.

Due to the relatively low loading and reactions, they will likely want to use epoxy or expansion anchor bolts, rather than embedded bolts (unless they will provide a plywood or steel template, I would go with the epoxy or expansion bolts...otherwise they won't fit).