Pre-Eng Building Foundation Question. 1

[CABaker]

I'm working on a pre-eng building foundation where the columns will rest on piers 8' about the finished floor. The footings are founded at 5'6 below the finished floor. The span of each building frame is 145' and the shear loads are very high.

One option I seen used is to tie the pier to the slab or to the opposite column pier to take the shear load, inducing a kickback underneath the footing to be taken in friction. Any remaining moment at the base of the footing can be treated like an eccentrically loaded spread footing.

My question is how would I split up the reaction forces here. How much of the load is taken by moment under the base of the footing and how much by the shear tie-friction couple?

Has anyone run into a problem like this before?

 

[msquared48]

The couple to resist the moment would be between the tension tie to the slab and the resultant of the passive pressure diagram. I would not rely on the friction between the footing and the soil mass. That way, by design, no moment is put to the footing.

If you want to, you can assign an arbitrary percentage of the moment to the footing, but the footing would have to rotate to develop this moment. Considering the other mechanism, and the depth of burial you are providing, I do not see this happening.

Mike McCann
MMC Engineering

 

[CABaker]

Taking the passive pressure into account is not good practice in cases like this. The deflection required to develop the passive pressure force would be in the order of inches. This would mean a deflection at the top of the pier of 3+ inches.

If this were a retaining wall, you'd of course be be correct.

Using the tie and friction load is standard practice in these building types, but typically the columns rest close to the slab-on-grade and the shear load can be easily taken by a tie between columns and I would worry about the friction load at the base.

I think this could end up with no ties and huge footings.

 

[dik]

The least costly method is to use ties in the slab... Transferring the horizontal shear into the footings will be very costly.

Dik

 

[msquared48]

dik:

Taking the shear to the slab is no problem here. The problem comes with the 8' high concrete piers (above the slab level) on which the mainframe sits causing an overturning moment to the foundation, whatever it is.

What dirtywaterdog says, may have to be if the deflection is too great to develop the lateral resistive kick.

Do you have a geotech for the project you could throw this question to? i.e., how much would the soil have to move to develop the thrust needed? You might ve able to restrict this lateral movement with a lateral compressive tie beam at the 5'-6" below the slab level too.

Mike McCann
MMC Engineering

 

[dik]

Any chance you can increase the frame by 8'? Will reduce thrust. Else, it's a geotekkie thing and you will likely have to extend your 8' high piers to a foundation wall to resist by passive earth pressure.

Might be that ties at the 8' level will be OK, except for the tallest employees <G>

Dik

 

[hokie66]

That's clever.

 

[jike]

I would also recommend not placing the columns at the pier height. Extend the columns to the floor and encase them in a pier if required.

Otherwise, the pier will deflect too much and will have a tendency to reduce the strength of the rigid frame. You could compensate for gthis by figuring the amount that the frame is weakened and then specify additional load to compensate for this weakening effect. I would much rather extend the columns to the base.

 

[msquared48]

I agree with Jike.

Mike McCann
MMC Engineering