Pier/footing design

[dmoench01]

Hello,

This seems like a dumb question to me but I'm trying to minimize the reinforcing in this pier beam design. The government client has set the parameters of what you see in the attached section I've drawn. There will be a rigid frame column sitting on this beam and I've included the worst load combination for thrust at the top of this beam. I understand how to check stability against overturning. I understand that the beam is being twisted to the left thus producing tension in the right face etc. I typically design true piers to spread footings and this is a hybrid for me and want to make sure I'm attacking it correctly. There is no soils report but I'm assuming 2000psf allowable bearing. Also not sure how minimum steel requirements per ACI affect this structural element? So, in summary, how would you design this member?

Thanks,

Dwayne

 

[KootK]

There must be more to the story here. Is there a slab on grade that can resist the thrust? Maybe piles or footings in the foreground? What you've got in the sketch, on its own, does't look promising.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[manstrom]

Run a tie beam to the other end of the frame. Otherwise you'll end up with a monster of a footing. Run a few #6 bars with a healthy splice to counteract the thrust.

If designing the pier without a tie beam. It may act as both a column and a beam, adjust phi factors accordingly. Minimum steel will vary as well.

 

[dmoench01]

Hey guys,

This is the boiler plate section this U.S. State uses for all its salt/storage facilities. There are no left out elements, this is what they use. They have dozens of buildings like this and we are trying to break into the market there. I'm not holding out on you, this is what they use and my unfamiliarity with this type of foundation prompted the question.

 

[spats]

This must be a state where they never get snow loads, and winds are light. In other words, they have never had a situation where any significant thrust was developed. First thing is to worry about the stability of the system, not the reinforcing. Good luck getting this to work. Not a good idea to repeat someone else's mistake. Ask them to justify their detail with supporting calculations from a previous project.

 

[KootK]

Ok. What are the dimensions of the shed? What is the frame spacing? Are you sure that the frame isn't stiff enough to be viable without utilizing foundation thrust? I'm imagining something small here.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dmoench01]

This building is 120' wide and has a bay spacing of 20'. This is in snow country and has 90mph winds. They pushed this beam down 3' to frost.

 

[ztengguy]

A tie beam would be the best if possible. How much of the footing to the left and right of the column are you depending on to get your bearing to work?

 

[dmoench01]

I did a quick calculation on this footing utilizing about 8' of the length at each column and it is still unstable. I'm not comfortable using more than that because the beam is only 7'-6" tall, even though the building columns are 20' o.c. I guess what I'm hearing from you guys is this is a bad situation?

 

[ztengguy]

Yes. If you are comfortable using an 8' long footing, then determine how wide it needs to be to be stable. I dont see why you need a 4' wide footing all around this building, design a pad footing at the column, then perhaps a 18" wall around between the pad footings.

 

[WinelandV]

I just calculated out a no-tie footing for a 120' wide, 30' bay spacing building in the upper Midwest last week. Ended up with a 12' long (direction of thrust) x 7' wide x 3' thick block of concrete to resist the forces. Even with that much mass, I still had to include dead weight from the continuous footing for stability.

Going by that, I really see no way that the pier that you've shown can possibly hope to resist the imposed forces.

Also, the 16.2kips of thrust looks light for a 100' span with 20' bays - this, however, is just my opinion.

 

[dmoench01]

ztenguy - The shape of this footing has been dictated to us by a DOT department. I don't have the option to change the width of the footing, it was their requirement. The 4'x7'-6" footing/beam is continuous all the way around the building. It is their design.

 

[dcarr82775]

I could see someone (not me) pulling out all the stops and convincing themselves it could work if not for the 4'-6" grade step. Just looks like a bad idea, made worse with that step.

 

[dmoench01]

Winelandy- I guess I getting the picture that no one likes this footing. The reactions are accurate, we have Metal Building Software that calculates these forces and this is due to DL+CL+SL, the reduced for slope snow load is 14.3 psf.

 

[manstrom]

14.3 psf?

I doubt that shed is heated. Ct = ?

If it's government, they may just want it this way. Design a big ass footing.

 

[jayrod12]

I think everyone is leery because that 3 feet of backfill on the left side is doing nothing to help. By the time the footing were to move enough to engage the resistance of the soil it would allow for too much lateral movement at the top of the wall.

Given the loading and geometry shown (assuming the whole vertical reaction can be used to resist overturning) I figure you need approximately 8' of footing length to provide enough overturning resistance (and that doesn't include any safety factor). Are you comfortable saying that 8-10' of the footing length will contribute to overturning resistance before the top of footing moves laterally? I don't think I am.

 

[dcarr82775]

I would change the reinforcing in that massive beam a bit, but that 4.5'x7.5' beam can do a lot of good. I did some scratch calcs and sliding and overturning do not look as bad as I thought they would when I make use of that beam. But once I add in the soil loads from the grade step it blows up on me, and that doesn't account for any surcharge in the building.

 

[KootK]

For effective width, I'd be willing to go 15'. I still don't like it though. Same gut feeling as Jayrod.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dmoench01]

dcarr82775-

What I have is Movert=16200x7'-6"=121,500ft-lbs. disregarding soil pressure and surcharge loads like you stated.
Mresis=23,200 x 4'/2 + 4'x7'-6"x8'x150x4'/2=118,400 ft-lbs. (used 8' of beam)
F.S.=118400/121500=0.97

Thus factor of safety is not even 1.0 and I was shooting for 1.50.

 

[jayrod12]

And the surcharge loading due to the salt/sand being stored in the building is a considerable number. and the machinery to move it etc.

Ask the State's reviewing engineer's to seal it for you. And then demand calculations that back up their design.