Pre-Engineered Rigid Frame on Tall Pier

[trekkie]

I have a condition where the pre-engineered building we have is on a sloped grade but the architect wants the rigid frames to be the same size so at one end the pier comes up to grade and then as the grade slopes down the top of pier stays at the same elevation until the last rigid frame is sitting on a 5'-6" high pier (4'6" above grade and 1'-0" below.

The concern I have is that by just looking at the pure statics of it, the reactions in the hairpin ties going into the slab would appear to be significantly increased for this condition due to the lever action.

To clarify I have a 50 kip horizontal reaction at the top of the pier, then 4'-6" down to my slab, then another foot down to my 2ft thick footing.

Am I over thinking this or will the reaction in the hairpins be much larger like I'm thinking in this case? Any suggestions to lower it?

 

[jayrod12]

Much larger if you intend on resolving all of the overturning in the slab and none in the footing.

Elsewise, you could design the footing and pier to resist all of the overturning (likely an unfeasible footing size) and the slab technically wouldn't need to provide any lateral support.

 

[DaveAtkins]

If the horizontal reactions are directly resisted by the hairpins, why would the forces in the slab not be equal and opposite?

DaveAtkins

 

[jayrod12]

The forces in the slab are equal and opposite. I understood his question as, if my pre-eng frame column sits on a pier 4'-6" above my slab is the force going into the slab higher than when the pre-eng frame column sits on a pier at the top of slab.

As I noted, it depends on how you are resolving the lateral forces, assuming a pin connection for your footing then yes the load into your slab would increase.

 

[trekkie]

Here is a sketch of what I was talking about. Now that I've sketched it out I think I just need tie rods connecting the foundation to the opposite side instead of going into the slab assuming I can get the overturning to work out. Thanks for your replies.

 

[CBSE]

Can you run the frame down to the slab like you typically would with simple metal buildings, and then encase the frame in concrete to the 4'6" elevation the architect wants? That might be your easiest solution.

 

[spats]

I would tell the architect to get real, or you're going to have to buttress the heck out of it, and add a huge footing.

 

[msquared48]

You could install a grade beam flush with the top of the slab and bottom of the footing, or deeper, to develop the moment, tying the grade beam to the column at the opposite end of the frame.

Mike McCann, PE, SE (WA)

 

[mtu1972]

Couple of observations: 1) your numbers are not right, unless you have added in a 1.5 x's Factor of Safety in; and 2) I would model to the center of the footing (as a minimum) and add a fixed base there. This greatly reduces the lateral forces. You will still end up with a large footing. See no way around that.

gjc

 

[SteelPE]

Why are you using hairpins with this system? Like others, I tell my client to run the columns down to top of slab or better yet in this instance, top of footing. Otherwise, I would just design a huge footing and be done with it. If I had 50kips in a column at SOG level I would more than likely use tie rods across the building as the hairpins are trying to do too much.

 

[darthsoilsguy2]

it seems like your free-body diagram is off to me. shouldn't you be balancing the overturning forces at the bottom-left point on the base of the footing. there must be some very significant down forces to generate a 50 kip outward horizontal force (maybe snowloading condition). those downforces would resist overturning moment with a lever arm of the 1/2 the footing. not saying it does all the work but it certainly is a big part.