Pre-manufactured Metal Building Foundation Design 1

[RFreund]

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?

EIT

 

[dcarr82775]

The bits and pieces are not supposed to blow off in a windstorm are they?

 

[SteelPE]

Thoughts.....Yeah run.

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.

 

[EdStainless]

What if someone decides to build one of these in hurricane country and actually follows the tie/brace rules? I'll bet that it would stay together long enough to generate a lot of uplift.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[chrislaope]

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.

 

[TopKnot]

Be sure to consider the added weight of the soil cone that would be lifted out if it was to fail (provided the anchors bolts and reinforcing are adequately designed for the load.)

_________________________
Tony Krempin, PE
TopKnot Engineering

 

[ron9876]

In my opinion that is negligence. It is a tough position to be in when they say that "they have been building these building since they were a child and have never seen anything close to this" but you have to meet the requirements of the code. Like chrislaope said there are ways to reduce the size of footings.

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.

 

[RFreund]

Yeah, suspected that this would be the response and I now question who "the other" engineers actually are. We are sticking with the foundations that we have. Code is code for a reason. Just wanted to see if this idea had any merit/popularity.

EIT

 

[fancypants]

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.

 

[SrVaro]

I agree with all. I would never design a foundation without considering uplift and using the required load combos.

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.

 

[hokie66]

Tell the contractor that if he will give you the other engineers' names, you will report them to the board. He won't bother you anymore.

 

[rowingengineer]

Agree with Hokie66,

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.”

 

[JedClampett]

I think if the PEMB industry was smart, they could use this to market their product:
"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.

 

[a2mfk]

Agree with everyone. What a ridiculous thing to say (the other engineers), though the GC may be making this up and bluffing. Also, I think they are actually somewhat correct in their theory of how the structure may fail in a wind event. Unfortunately I do not see a PEMB holding together to the point where a footing is pulled out of the ground like a weed, but that doesn't mean you don't design the foundations correctly and to code.

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...

 

[bylar]

I use to do many metal building foundations and I always provided enough dead weight to resist the uplift provided by the manufacture. You can use some of the slab weight if the slab is designed for being uplifted. I eventually lost all of the contractors that I worked for because they got engineering that greatly reduced the size of the footings.

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.

 

[BAretired]

In my area, drilled concrete piles offer both axial and pullout resistance and they were the most economical system when I was practicing. Nowadays, screw piles seem to be gaining favour as they offer similar benefits for less cost.

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

 

[RFreund]

Thanks guys for the replies.

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

 

[BAretired]

Whatever your boss says, it is fundamentally wrong to omit the 0.6 factor for dead load. To do so means you have a factor of safety of 1.0 against uplift.

Skin friction could be used but the value should be conservative and should also be multiplied by 0.6.

BA

 

[vthomidis]

I think you should also try to get in contact directly with the
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.....

 

[RFreund]

BA - Good point, I have sized them both ways so we'll see what happens.



EIT