pre-engineered metal building 1

[recentgrad]

Howcome the horizontal component of reaction at base of pre-engineered metal building is significantly high that we need hair pins?

 

[jike]

Pre-engineered buildings are usually built as "rigid frames" which produce an outward kick at the base of the exterior columns due to vertical gravity loading. Wind or seismic forces will add to this lateral thrust.

 

[14159]

One reason it's such a problem is that there is very little column dead load. This causes the footing sliding resistance to be next-to-nothing, so the choice is to either use hairpins (or some other idea) or have a very large footing to provide more dead load.

These large thrusts mentioned by jike also are often offset vertically from the footing far enough to cause the footing overturning ratio to be a problem. Using some kind of reinforcement in the slab resists these forces at a more favorable elevation.

Many people will not use hairpins or other reinforcement because of the argument that somebody will eventually cut it. It's a valid concern, but I have a hard time getting too worked up over it. The actual likely load is nowhere near the design live load except in heavy snow areas. There's also the unscientific observation that I've never heard of a metal building failing because the footing slid, causing a roller instead of a pin boundary condition.

BTW, the correct 2005 terminologiy is Metal Building System -- there's not much pre-engineered about these things nowadays.

14159

 

[archeng59]

14159, I've inspected metal buildings (office spaces or small commercial buildings) after high wind loadings (tornados in Oklahoma) and found cracks in the floor slabs in the area of rigid frame bases. if memory serves me correctly, most if not all of the floor slabs were not reinforced with wire mesh or rebar but had the fibermesh instead. I assumed the hairpins were not installed and associated the cracks with the horizontal thrust of the frame. In buildings with reinforcing in the slab which likely included hairpins, I have not found those cracks in the slabs. The buildings didn't fall down, but the slab was damaged.

 

[JAE]

archeng59 - many times the foundation system is designed without any lateral tie to the slab or opposite foundation. But in those cases, the actual footing and pedestal atop the footing are designed to resist the lateral force from the column via the size of footing - with due regard for the unbalanced stress on the supporting soil.

This is usually done in the smaller buildings but can conceivably be done for larger - just costs more. This may explain those times when you saw cracks without reinforcing ties in the slab.

But the lateral forces must be resisted in some fashion.

 

[archeng59]

you're correct JAE. I always use a reinforced slab and hairpins. there is more than one way to take care of it, for sure.

 

[rlewistx]

I always use rebar in a PEMB foundation slab, not WWF. I want to make sure there are good bars to take the load. I also make sure there are not construction joints perpendicular to th rigid frames. I always detail construction joints parallel to the frames.

For larger thrust loads I use a tie beam instead of counting on the slab reinforcing. Tie beams have to have mechanical splices instead of lap splices per ACI tension members.

 

[prblmfxr]

I've designed alot of Pre-Eng Metal Bldg fdns, and am very curious on any research or forensic evidence on actual horizontal loads transferred from bottom of column to top of pier due to gravity loads to the rigid frames. I've always designed according to the conventional "theory" and, as expected, haven't experienced any problems. It sure would be nice if someone could prove that we can design these footings with alot less fuss, i.e. little or no horizontal ties or moment-resisting foundations. Any input appreciated.

 

[rlewistx]

The amount of movement to "engage" the foundation to resist the thrust is usually more than you would want. Also, if the slab and foundation are poured monolithically, this movement will cause cracking in the slab.

My opinion is the slab tie is a stiffer connection and more reliable than trying to get resistance out of a foundation, unless you have something like a drilled pier foundation.