Caisson foundations and ties for Pre-Engineered metal building

[Jc67roch]

I am designing the foundations for a pre-engineered metal building. Due to fill and other soil conditions, we are founding it on 3 foot diameter caissions drilled 12 feet or so to bedrock. Grade beams span between the caissons. Piers for the building columns are to be constructed on top of the caissons and integral with the grade beams.

I have calculated that the caissons should have sufficient lateral load capacity to carry the loads from the building base plates, as well as the vertical loads. The column foundation piers should transfer these loads to the caisson tops. Do I still need to utilize hairpin bars in the floor slab, or cross ties between opposing column bases, to negate the lateral loads? Is it ok to rely on the lateral load capacity of the caissons as calculated? Additionally, it seems the grade beams extending 4 feet below grade also will offer a lot of excess lateral load capacity (safety factor) from passive resistance of the soil against them.

 

[RHTPE]

My real point is that the PEMB designers provide loads applied to the foundation. Do they assume perfectly pinned connections at the bottoms of the columns? Or do they anticipate some outward movement at the bottoms of the columns, which in turn increases the positive moment in the roof beam?

It's my opinion that to assume no outward movement at the column bases is unrealistic in the real world.

Ralph
Structures Consulting
Northeast USA

 

[BAretired]

RHTPE,

I agree, but it is a matter of scale. If the tie elongation is only a small fraction of the horizontal deflection of a roller support, then neglecting it is not introducing significant error. If your figure of 3/8" for roller deflection is correct, then the error would be very significant.

BA

 

[hokie66]

I agree that pins should be assumed to displace, and I think that 3/8" or about 10mm is in the right order for a 50' span. That is 18ksi in a tie across the building, assuming no slippage in any connections. So perhaps the PEMB designer should be given a maximum displacement of the base. Zero displacement is not practically achievable. I still think that you should check the 40% increase, Ralph...are you sure your support conditions were inputted correctly? In the other thread, you said the difference was from "fixed" to roller.

 

[Brad805]

Ralph, I am also suspicious about the small deflection increase as well as the moment increase caused by a 3/8" movement. I changed the supports in one simple frame out of curiosity and my deflections increased to 0.38ft. Did you try a forced displacement to model the foundation deviation? I usually find that 1/4" is a reasonable maximum to use for a pile foundation.

Brad

 

[RHTPE]

Hokie, By fixed I meant fixed in the X direction. I should have been clearer. The OP stated:

I have calculated that the caissons should have sufficient lateral load capacity to carry the loads from the building base plates, as well as the vertical loads. The column foundation piers should transfer these loads to the caisson tops.

My only point in adding comments to this thread was to try to elicit some input from someone who designs PEMB frames. I would, and I'm sure the OP would, like to know how much lateral movement a typical frame can accommodate before it becomes a design issue. I think we all agree that no matter how it's approached, there will be some degree of lateral displacement of the column bases.

As far as my quick analysis goes, it was a very rough approximation using the geometry of the building I'm investigating. First run was with the column bases pinned (both fixed in the X & Y directions, but free to rotate). Next run was with the X-direction displacement released on one column base. Now if the PEMB manufacturer would only release the fabrication drawing(s) for the frame ...


Ralph
Structures Consulting
Northeast USA

 

[TopKnot]

I have not seen any PEMB manufacturers that address deflection at the base. I've been tempted to force one to do so, but I think they are a relatively simple folk. Realistically, there are many things that affect deflection at the base.

1. Axial deflection of tie beam or rotation of spread footing.
2. Temperature axial deflection of tie beam and/or steel girder.
3. Bolt slip against over-sized holes.
4. Allowable tolerance in bolt placement.
5. Slight relaxation at bolted joints. (when there are no flange splice plates with slip critical bolts, the end plate bolt have to introduce some flexibility.
6. Variances in distance across the steel frame from manufacturing tolerances.

There are so many factors that affect movement at the supposedly pinned joints, the only thing you can guarantee is that it will most certainly deflect relative to the assumed distance. I assume there's a little moment re-distribution happening when a yield stress is significantly exceeded. These frames have worked well over a long time, so I say don't over-think it.

By the way, for your structural models, use springs of varying stiffness. Really, for any model that has a lot of fixed boundary conditions, you should always substitute springs for pinned connections to see how a little base deflection affects member forces. An inch or two here or there can really re-distribute forces in a frame. Just my opinion.

--Tony Krempin (TKE Engineering)