Pre-manufactured Metal Building Foundation Design
Pre-manufactured Metal Building Foundation Design
(OP)
We are part of a design-build project for a large retail store/warehouse facility. The column reactions where provided to us to design the foundations. There were some rather large uplift forces which caused us to have rather large footings. The contractor who was awarded the job has done other stores similar to this and says he has never had footings this large. We explained to him the need for the additional weight to counteract the uplift. He called up previous engineers who designed the foundations for him to see what they had done. These engineers replied that they ignore uplift because when the buildings fail in a wind storm they never lift up because the siding and roof coverings blow off first.
I was a little shocked at this, but at the same time they may have a point.
Any thoughts on this?
I was a little shocked at this, but at the same time they may have a point.
Any thoughts on this?
EIT






RE: Pre-manufactured Metal Building Foundation Design
RE: Pre-manufactured Metal Building Foundation Design
I have never heard of anyone doing this. Not designing for the reactions provided by the metal building mfr seems link a law suite waiting to happen.
Recently, in the NE, numerous metal buildings failed due to heavy snow loads. I only heard of one having problems because of the foundation (I think someone posted questions about it on the forum)
There are some things you can do to reduce the size of your metal building foundations. Things like installing hairpins and ties but these only help with the lateral forces at the base of the column and do nothing to reduce uplift.
RE: Pre-manufactured Metal Building Foundation Design
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Plymouth Tube
RE: Pre-manufactured Metal Building Foundation Design
I can not accept this reasoning. If the siding and roof covering blow away first in a wind storm which is within ASCE7 code calculated wind designation, then, it means the building is not designed correctly.
I also met this problem before. My design theory is that: 1. the floor slab will be sitting on top of the center column footing, which will bring a lot of slab weight on center column footer. 2. On the wall column footer, it is rigidly connected with foundation concrete stemwall, therefore, a lot of concrete wall weight will be added on wall column footer. By this way the footing size can be reduced and still has enough stablizing weight to resist uplift.
RE: Pre-manufactured Metal Building Foundation Design
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Tony Krempin, PE
TopKnot Engineering
RE: Pre-manufactured Metal Building Foundation Design
I do some of that type of work and have been told that other engineers ignore the 0.6D + W load case. All I can say is that they aren't designed in accordance with the code and my design complies with the code requirements.
RE: Pre-manufactured Metal Building Foundation Design
EIT
RE: Pre-manufactured Metal Building Foundation Design
I have designed the foundations for about 10 PEMB's in the last few months. I always design for uplift and I am in a hurricane prone region. Most of the time I use Belled Piers.
When Ike blowed over the top of us, I saw plenty of PEMP's that did not fly apart. I can imagine how much uplift is on the foundation is some of those winds.
I find it nice that you have the loadings to design by up front. Most of the time I have to run an analysis just so I can approximate the foundation sizes for permit drawings. Later on after I get the PEMP shops, adjust if required.
I would bet the contractor you are speaking of has already bid the job. Otherwise, why should they care if your footings are a little larger.
Stick to your guns.
RE: Pre-manufactured Metal Building Foundation Design
Not that I would do the following, but could the contractor be saying that the other engineer used the full concrete weight and not 60% of it? The rational could be that the weight of the concrete is more or less known. For example if the PEMB gave a .6DL+WL reaction of 10kips, the other engineer would use 69ft3 of concrete (assuming no other resistance for simplicity).
Agian, I would not do this, but it could be how someone might be able to greatly reduce the size.
RE: Pre-manufactured Metal Building Foundation Design
RE: Pre-manufactured Metal Building Foundation Design
I would also suggest using the internal slab to help in the HD (there is another thread on this that I plan on replying to soon that you may wish to read), and also get the geotech to provide Cu values for uplift don't just guess as this can be conservative. If you have internal pressure causing the roof to have large uplifts these can also be used to help in HD situations.
ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
RE: Pre-manufactured Metal Building Foundation Design
"LESS SUBSTRUCTURE COST BECAUSE OUR SIDING WILL FAIL BEFORE THE FOUNDATIONS ARE LOADED"
And the truly amazing thing is that you're pouring concrete that is incredibly cheap. Very little formwork, shoring, labor and pumping.
What drives engineers to devalue our work so? I'm all for a tight design, but it has to meet code.
RE: Pre-manufactured Metal Building Foundation Design
And I would not be surprised if SrVaro's theory isn't dead on, I know some engineers ignore the 0.6D when it comes to the concrete weight. This discussion has been beat to death in previous threads.
My response when I am asked about another engineer's design is that I would have to see the plans, run the calcs, and talk to the engineer before I could comment. I wish all engineers would extend this courtesy. Otherwise it is apples to oranges, even if they are similar structures.
Like others said- offer to review their drawings and reactions for free ;) And be ready to turn them into the board for negligence.
It won't be the last time you will get questioned on PEMB foundation design. Happened to me early on in my career in a design-build where they way underestimated the concrete. I said I was just following the code, which at the time I think was 1.5 FS, and showed them my calcs as simple as they were. Their stupid response was "I guess other engineers interpret code differently", and I said there is not much to interpret there, it is very clear and straightforward. Agreeing with the concept is another issue.
Do you know if anyone installs helical piers or perhaps bell piers as was mentioned? These may give you a large uplift resistance while using a lot less concrete. They are not that expensive in my area (Florida) installed and may be quite economical, especially on a large project. Since I have been exposed to them more, I am waiting to have a project with a lot of large footings due to uplift and maybe explore them as an option...
RE: Pre-manufactured Metal Building Foundation Design
As just a note, I went to Miami after Hurricane Andrew and so no metal buildings that lifted up, all of which lost the siding or roof panels.
RE: Pre-manufactured Metal Building Foundation Design
If footings are to be used, one way to reduce footing size is to go deeper in the ground, thus benefiting from the weight of soil above.
In any case, I agree with those who say the uplift forces cannot be ignored.
BA
RE: Pre-manufactured Metal Building Foundation Design
I have sized the footings without the 0.6 applied to the concrete weight as I agree with the argument that the weight of the concrete is 145 pcf and is not over/under estimated (to my knowledge) but we'll see what the boss says. Also we have done some work with a helical pier contractors which may become an option but currently they intend to pour large/thick (some 4.5' thick) concrete pad footings. I'm I allowed to use soil/concrete skin friction or is it even worth it?
EIT
RE: Pre-manufactured Metal Building Foundation Design
Skin friction could be used but the value should be conservative and should also be multiplied by 0.6.
BA
RE: Pre-manufactured Metal Building Foundation Design
other engineers, so you may actually understand what their full
conception is.
There is a valid possibility that the contractor "misses" some points of their reasoning due to poor engineering skill or.....
RE: Pre-manufactured Metal Building Foundation Design
EIT
RE: Pre-manufactured Metal Building Foundation Design
RE: Pre-manufactured Metal Building Foundation Design
But if it is an isolated interior pier then you may have to lower the underside of footing elevation to get the footing to a resonable size.
RE: Pre-manufactured Metal Building Foundation Design
RE: Pre-manufactured Metal Building Foundation Design
That being said, and has been stated before, I have seen a lot of sidewall and roof sheeting fly by in the hurricane damage videos, but the only PEMB steel structure I have ever seen fail was a gas station canopy that toppled over. Uplift failure? Maybe, maybe not.
Question being, what's wrong with designing specific structural components to fail in the condition of an overload so that the whole structure does not fail? I realize that this was not the original engineer's thinking, and I do disagree with it. I do not like the idea of dealing with flying sheet metal either, but it seems to happpen regardless in hurricane winds.
Mike McCann
MMC Engineering
RE: Pre-manufactured Metal Building Foundation Design
And thought into windborne debris has been given, especially in roofs. After Andrew they found neighboring buildings to gravel roofs literally shot blasted. Now if you want to do a gravel roof in S FL or anywhere in Florida the code requirements are so tough that nobody does them anymore. Ron probably knows way more about this than I do, but that has been my limited experience with gravel roofs. And I haven't seen a new one in FL, ever.
PEMB and wood framed structures in my experience and understanding in wind loading usually begin to fail in pieces- a window or door blows in creating a partially enclosed condition, greatly increasing the pressures on the components. Maybe a few pieces of roof sheathing at a corner or ridge will fail, or a truss or two, and the same thing happens. Then you either have enough wall and roof sections missing that the overall lateral wind loading is reduced (an OPEN structure) and the structure is saved, or, well, not- global failure...
Or with a PEMB you will have pieces of metal decking or siding, light gage steel framing, etc fail and then wind can get into the structure and usually cause enough other cladding to fail where you quickly have a more or less OPEN structure, so that your moment frames are not seeing the max design load. Thus we see very few frame failures, bolt pull-out failures at base plates, or the Loch Ness of them all- the spread footing being pulled out of the ground failure. Structures like these tend to fail in enough pieces and parts, whether intended to do so or not, that a global failure is pretty rare.
Pretty off topic at this point, sorry....
RE: Pre-manufactured Metal Building Foundation Design
However, with the flood loading argument, the space that floods is suppose to be unoccupied space. So when the walls fail, you don't hurt anyone. However, I'm not quite sure you could do this with a wind failure. PEMB are used for everything from gymnasiums to office buildings. How could you guarantee these buildings would be unoccupied at the time of failure?
RE: Pre-manufactured Metal Building Foundation Design
It's easy for us to discuss this in purely analytical terms but these buildings have real living people in them.
RE: Pre-manufactured Metal Building Foundation Design
But if you design an entire building to code, say, v=100mph, that wind speed may very well may be exceeded at some point by a tornado (close call or direct hit) or a hurricane that did not meet the current code criteria. So it is quite interesting and good to know what may begin to fail first, and how the overall structure will react as the wind hits 120, 130, 140 mph....
From my direct hurricane experience in the Orlando area in 2004, your best bet is in a new commercial building away from trees. Trees did most of the damage!
I like my current office building. Built in the early 1970s like a bunker, single story 8' tall CMU walls inside and down the central hallway, roof is flat and some type of precast panel. Shear walls galore and probably enough dead load to resist the uplift. There are a few interior offices with no windows and right on a bearing wall. Plan on being there if the big one comes along... That and I can keep working until the last second, JK :)
RE: Pre-manufactured Metal Building Foundation Design
As a further point of discussion, should all PEMB's be designed for both the open and enclosed conditions due to these cladding and roofing issues?
Mike McCann
MMC Engineering
RE: Pre-manufactured Metal Building Foundation Design
Also, it appears from this and many other discussions that most SEs are of the opinion that for the most part there is little "extra factor of safety" in a PEMB structure. Knowing what I know, if I am the owner, I may have the PEMB give me there 130mph design in my 110mph zone. What do you suppose that would cost, 3-5% more? Sounds pretty cheap compared to the hassle and deductibles of an insurance claim. Funny that a local steel building garage maker gives you their South Florida 150mph garage at one set cost. Cheaper for them to beef up their structure a little here and there then to have 100-150mph covered. Many window and door manufacturers do this also I believe.
You get what you pay for. If you want a bare bones metal structure that meets code by the skin of your teeth, then that is what you get with most PEMBs... You could make that argument with other systems, especially say residential wood framing ie roof trusses, connections, windows, doors, garage doors. You could make a lot of improvement in the most vulnerable areas for a small cost of the total structure, but it is not REQUIRED.
One way to get buildings built better other than forcing them through building codes is the insurance industry, and we have seen this already a little bit here in Florida. Carriers are required to offer discounts if your house meets certain criteria that makes it less vulnerable to wind damage. But most insurance companies see buildings as pieces of paper and many insure sight-unseen, to the point of making me laugh out loud. I see it every day. I built better forts as a kid then some of the insured structures I review.