BASE PLATE & PEDESTAL

[gotlboys]

I have designed a steel column carrying axial and moment loads. These loads were used to design the base plate and anchor bolts. Steel Column: W14x68; base plate: 700mm x 700mm, 45mm thick; R.C. pedestal 700mm x 700mm, h = 2.0m from N.G.L..

Will the moment load be transferred to pedestal and then to footing pad?
My preliminary analysis is the moment will not go to the pedestal and footing or the use of steel plate and anchor bolts terminates the moment at the top face of pedestal. Therefore, I will design the pedestal and footing without moments (axial load only).

Have I come up with appropriate analysis?

Your help is highly appreciated.

 

[KootK]

If I understand your situation correctly, I believe that the moments should indeed be considered in the design of the pedestal and footing. Why do you feel that the moment disappears at the anchorage connection?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[gotlboys]

As always, glad to have your reply Kootk.

It is a three-storey steel apartment building. I used wL^2/8 to calculate moments in steel girders which produces maximum moment at the mid-span and zero at the support. I did that to not have moments on the columns except from seismic. I knew the girders would be bigger than following a 'continuous beam' mechanism but I also knew I was going to save a bit in column and footing sizes.

However, I can't sleep well with this doubtful analysis I made:
1. DL and LL moments are zero, thus only seismic moment and axial loads are used to design base plate;
2. With limited reference, I somehow assumed that since the pedestal is large in size that it will force a hinge mechanism to take place between the base plate and the pedestal. I seemed to believe that the base plate will just yield (bend) on compression side and anchor bolts be pulled out on the opposite side as moment increases and thus the pedestal remains carrying no moments.

I stand corrected if the analysis has been inappropriate.

 

[SteynvW]

base plate will just yield (bend) on compression side and anchor bolts be pulled out on the opposite side

It seems to me that you are describing your failure, what about before the bar yields or the
anchor bolts don't pull out, it means the forces are still there and transferring the moment to the pedestal.

 

[KootK]

I agree with SteynvW's assessment.

I somehow assumed that since the pedestal is large in size that it will force a hinge mechanism

It may indeed force a plastic hinge mechanism. However, that means the the moment in the pier and footing is the plastic hinge moment, not zero moment.

and anchor bolts be pulled out on the opposite side as moment increases

I believe that you need to create a stable plastic hinge at this connection. As such, you'd wan a ductile yield mechanism. The anchor bolts shouldn't be allowed to pull out of the concrete.

I used wL^2/8 to calculate moments in steel girders which produces maximum moment at the mid-span and zero at the support. I did that to not have moments on the columns except from seismic.

I don't love this strategy in a high seismic environment as it may lead to underestimates of the moment in the column and base connections under seismic action. What is the lateral system here? Bracing? Moment frames?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sdz]

It depends on whether your base plate is designed and detailed to be flexible or rigid. From the size and thickness I'd say it was rigid and so you will get moments transferred to the pedestal.
What you assume in your design has to be carried through to detailing, and vice versa.

 

[msquared48]

I agree with Koot and the rest. You are going to have to either modify the connection to take the moment, or rerun your whole analysis for the structure using a pin connection there.

Mike McCann, PE, SE (WA)

 

[Vickna]

you will need to convert the moment at the base of the Steel column into upward and downward forces by dividing it with lever arm "e". and apply this to your pedestal and footing design

if Ms is the moment at the base of your steel column then Pc & Pt are the respective upward and downward forces
at the base after dividing Ms with e.

= Ms / e = Pc = Pt

where, e = distance from outer Anchor bolt to inner steel flange.

 

[gotlboys]

Kootk, it is moment frame. I really appreciate your advice that adopting my analysis may underestimate column moments during high seismic action.

Vickna, thanks for the advice.
I would like to confirm whether converting moment to axial load for pedestal and footing designs may alter the "end condition" at the interface between base plate and pedestal; fixed- to pinned-end.

 

[P205]

Vickna:

I believe the calculation of the Pc/Pt is wrong.
It should be Ms/e, where e is the distance between center to center of the anchor rods. Then, taking the Pt*d, where d is the distance from center of anchor rod to flange of beam, gives the moment in the baseplate.