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

 

[SteelPE]

Remember that you need a FOS against uplift. As BA states, if you do not apply the equation properly you will end up with a FOS of 1.0. The FOS is built into the 0.6D + 1.0W load combination already. When applied properly you will end up with a 1/0.6 = 1.667 FOS.

 

[MisterBill]

The adjacent/Flanking foundation walls alongside the pier may provide additional dead load. So the wall footings, soil and selfweight of wall all contributed to the dead load too.

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.

 

[Lion06]

I have, in certain cases, backed off on the 0.6DL factor. I've gone up to 0.65 already when I've been in a pinch for an existing condition. I justified this by saying that 90% of the DL is real and known (from concrete walls and slabs) and was comfortable eating into the safety factor a little to save a whole lot of headache.

 

[msquared48]

This discussion reminds me of the designed knockpout panels for flood design, allowing a portion of the structure to fail by design so that the majority can stand.

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

 

[a2mfk]

I know where you are coming from Mike... And it usually works out that way from what I have seen and read about. I have only come across one PEMB frame failure related to wind, and saw it with my own two eyes on a forensic job or I would not believe it... It was a tornado, and it was a downward pressure failure. There was not much wind damage to this structure at all other than the buckling of a frame, ie, almost no cladding missing. Pretty crazy and rare I would think. And 50 feet away a ranch house had ZERO damage, tornadoes are amazing like that!

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....

 

[SteelPE]

I understand your point m^2 and designing for walls to fail under flood loading is something I have struggled with in the past.

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?

 

[JedClampett]

All of this talk of the siding failing is fine for the structure. But what about the contents and god forbid, occupants? The critical question is; Does the owner and his insurance carrier agree that all of his equipment, inventory, and interior furnishings are sacrificed in the event of a hurricane event? I hate to keep dwelling on this, but do the users of these buildings understand that they're likely to partially fail in a high wind, such that the contents are exposed? Or do they expect a building that protects them from the elements?
It's easy for us to discuss this in purely analytical terms but these buildings have real living people in them.

 

[a2mfk]

Jed- the average person doesn't care. Try and talk to a friend or relative about the quality of a structure such as their house, particularly if they are in the market to buy one. They think they are all the same, meets code and got a permit, right? They care more about the vaulted ceilings and granite counters...

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

 

[msquared48]

I understand the concern with the occupants, and that is the primary concern, of course. However, that does not change the fact that the sheeting connection is failing due to local overload, or other factors.

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

 

[a2mfk]

My opinion is no, because that is opening up a PEMB can of worms... All building codes can do is state minimum requirements, and one would hope PEMB engineers are designing the components and cladding for these. Its the nature of the beast that the components and cladding fail before the steel frames, and it seems to be a good thing.

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.