Caisson foundations and ties for Pre-Engineered metal building
Caisson foundations and ties for Pre-Engineered metal building
(OP)
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.
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.






RE: Caisson foundations and ties for Pre-Engineered metal building
RE: Caisson foundations and ties for Pre-Engineered metal building
RE: Caisson foundations and ties for Pre-Engineered metal building
Creep could still be a factor over time with the caissons and hairpins will help mitigate that.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Caisson foundations and ties for Pre-Engineered metal building
But no matter your foundation system, I agree with Mike.
RE: Caisson foundations and ties for Pre-Engineered metal building
1. No slab situation, such as a riding arena, and
2. Lateral loads so high that hairpins cannot take the load.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Caisson foundations and ties for Pre-Engineered metal building
The "grade beams extending 4 feet below grade", do seem like an overkill here.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Caisson foundations and ties for Pre-Engineered metal building
Brad
RE: Caisson foundations and ties for Pre-Engineered metal building
The grade beams carry only very light exterior wall loads, and are extended to 4 feet below grade for frost protection. I am relying on the fill for the lateral load resistance, but using a relatively low allowable.
RE: Caisson foundations and ties for Pre-Engineered metal building
RE: Caisson foundations and ties for Pre-Engineered metal building
BA
RE: Caisson foundations and ties for Pre-Engineered metal building
Brad
RE: Caisson foundations and ties for Pre-Engineered metal building
To think that hairpins from the foundation into the floor slab satisfies the support condition assumed by the PEMB designer can be risky in MHO.
I am investigating a foundation failure under a PEMB frame. In the course of my investigation I modeled the PEMB frame, which is assumed to have pinned connections at the bottoms of the columns. I applied the full design snow load + the dead load and recorded the maximum moments in the frame. Then, I released one of the column's X-direction constraint (introduced a roller). Surprisingly, the column deflected horizontally only a little over 3/8" (50 ft span). But even more surprising was the increase in the positive moment in the roof beam - to 140% of the previous value.
The interior floor slab section connected to the foundation may offer enough drag resistance to the columns horizontal reaction. Or it may not. Then consider the amount of floor slab saw-cutting done to counteract slab shrinkage cracking - is there really continuity across the width of the building? In my situation, 3/8" horizontal movement of the column is within the realm of jobsite construction tolerances and lack of quality control. But ask yourself - can the roof frame safely sustain a 40% increase in positive moment?
If you're comfortable that the frame can sustain the moment increase caused by an outward movement at the bottom of the frame's column(s), then why worry about tying the foundation to the floor slab, or introducing a tie rod under the floor slab?
Guaranteed to elicit some interesting testimoney should a failure occur and the PEMB manufacturer wants to shed some liability.
Ralph
Structures Consulting
Northeast USA
RE: Caisson foundations and ties for Pre-Engineered metal building
I do not find it surprising that you would find a 40% increase in positive moment if you substitute a roller support for a hinge support. But the fact remains that a roller support is not realistic when the foundations are tied together by a tie beam or a tie slab.
BA
RE: Caisson foundations and ties for Pre-Engineered metal building
RE: Caisson foundations and ties for Pre-Engineered metal building
If you get too much spread and moment, increase the section of the tension tie.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Caisson foundations and ties for Pre-Engineered metal building
I cannot agree that hairpin bars tied to a slab is more like a roller than a pin, but there is some strain in the tie, so it is somewhere between hinge and roller. If the piles are designed to resist the horizontal force without ties, there will also be lateral strain at the PEMB baseplates.
If the PEMB designer expects the foundation to resist horizontal forces without strain, he is making an unrealistic assumption.
BA
RE: Caisson foundations and ties for Pre-Engineered metal building
BA - You are correct about the PEMB designer having perhaps unrealistic expectations of the foundation restraint condition for his columns. But then, they seem to consistently toss that issue to others - their responsibility ends at the bottom of the columns' base plates.
I really wish a PEMB designer would toss in his/her 2 cents. Both from their personal feelings and from the company's position. It would be beneficial to those who do design the foundations to have this insight.
Remember, the 3/8" movement was from a very quick approximation of the building's frame. I would really likt to have the members' fabrication drawings to do a better analysis, but the PEMB folks seem to be reluctant to share.
Ralph
Structures Consulting
Northeast USA
RE: Caisson foundations and ties for Pre-Engineered metal building
If the foundation designer uses a tension tie either below the slab or within the slab, the unit strain e in the tie is E/σ where σ is the stress at working load. For σ = 20,000 psi, e = 0.00069 and the total strain for a length of 50' is 0.41" which exceeds 3/8" by ten percent.
I cannot believe that substituting a roller for one hinge would result in a lateral deflection as low as 3/8". That would be one heck of a stiff frame.
BA
RE: Caisson foundations and ties for Pre-Engineered metal building
RE: Caisson foundations and ties for Pre-Engineered metal building
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
RE: Caisson foundations and ties for Pre-Engineered metal building
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
RE: Caisson foundations and ties for Pre-Engineered metal building
RE: Caisson foundations and ties for Pre-Engineered metal building
Brad
RE: Caisson foundations and ties for Pre-Engineered metal building
Hokie, By fixed I meant fixed in the X direction. I should have been clearer. The OP stated:
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
RE: Caisson foundations and ties for Pre-Engineered metal building
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)