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Thrust at base of rigid frames 2

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rittz

Structural
Joined
Dec 30, 2007
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Location
CA
Does anyone know of a pre-eng manufacturer that puts out printout
which gives an idea of what the max allowable lateral deflection of the base of?
a rigid frame should be?
 
Lateral displacement should be minimised... and the value and the horizontal force varies depending on the loading and geometry of the PEMB... There are frame equations that can give you a bit of a guess about the magnitude of the forces.

Dik
 
The maximum horizontal reactions are usually given by the manufacturers. I expect they are usually calculated on the assumption of a hinged base which means that no lateral deflection has been considered. It is not possible to provide zero deflection, but as dik says, it should be minimized. Steel ties buried in the floor slab are the usual way to provide a horizontal reaction, so the amount of lateral deflection is equal to the strain in the ties. Some manufacturers may take this into account in the frame design.

Any lateral deflection at the base will decrease the maximum horizontal force but will increase moments elsewhere.

BA
 
I mean when a thrust rod is installed from one site of the frame
to the other. We can obviously calculate the thrust and rod size
from frame reactions given by the manufacturer. And we can
calculate the elongation under load. The rod must be sized to minimize
the elongation. My question was to learn how much the various
frames can tolerate.
 
They should be able to tolerate a strain corresponding to a stress in the order of 20,000psi in the ties. That would be a unit strain of approximately 0.0007 or about 1" per 100' of span.

BA
 
If you are talking about the pre-engineering building, I would say the pre-engineering building vendor always assumes the column base is PIN with zero deflection.

In the reality you have some deflection, that would cause some lateral load and member force deviating from the result getting from model using ideal PIN assumption, but for most of the time PIN base assumption shall be OK.

Ideally you shall calculate your base stiffness with spring in 3 directions, kx, ky, and kz, and pass these foundation stiffness to the building vendor so they can plug in their model and work out a more accurate result for you.

When you work out the foundation stiffness, you shall consider the concrete pile or pedestal’s cracking EI, with around 0.5EI to be more realistic, and apply the pile group facto as applicable. Just roughly estimate the lateral load (wind or seismic) at each column base and workout the deflection to get the stiffness = P/delta. You can safely assume these stiffness k are linear in a small deflection range.

I would say none of the pre-engineering building vendors I worked with before asked for the foundation stiffness and all of them just assume all column bases are ideally PIN.
I worked with Krupp on many heavy mining machinery and equipment and Krupp always asked the foundation stiffness. With the foundation stiffness plug in you will get the accurate support reaction and frame member forces.

anchor bolt design per ACI 318-11 crane beam design
 
When I designed PEMBs it was always a rigid reaction base assumption. But now that I do foundations for them I have been concerned about this same thing (tolerance for base movement). The method I am the most comfortable with is a continuous tie beam with mechanical splices in the rebar.
 
You can always go to the source and ask the PEB Engineer. It would be best to provide a value and ask the PEB Engineer to check for that value. Make sure you specify the deflections at each base if not the same.
Have you considered passive soil resistance against the grade beams in lieu of tie-rods?
 
I have considered passive soil resistance against the grade beams in lieu of tie-rods But never had the nerve to rely on it. Possoble future hanges to exterior grading pattern scare me. LOL
 
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