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Seismic Design Requirements for a Concrete Pier Supporting a steel braced frame or moment frame.

Philip.screw

Philip.screw

Structural
Joined
Jan 4, 2024
Messages
10
Location
US
My firm is working on several projects where we have steel braced frame or moment frame on top of concrete piers. In one of our projects, we have a steel braced frame (OCBF) on a 6ft tall pier connecting it to the footings. I am in a high seismic region as well.

I am reviewing ACI318-19 to see what the design and reinforcement requirements for these piers are and I'm having trouble finding the right section. Chapter 18.13 does not seem to have much guidance for piers as a foundation element. I could design them with the pile requirements, but that does not feel correct as a spread footing is my foundation.

I talked to some of my co-workers and we think they should either be designed as a concrete special moment frame column. Our reasoning is we think the piers are considered members not designated as part of the SFRS. Section 18.14.3.2 (b) says "(b) Columns shall satisfy 18.7.4.1 and 18.7.6." which are the section for long. and transverse reinforcement of a SMF column.

Is our thought process reasonable? Or is there another section of ACI that other engineers use for their piers with seismic design and forces.
 
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You did not provide alot of info here. The no. of storeys and if the connection of steel column to pier is rigid or pinned are important points.
If i were in your shoes , i would go considering the no. of storeys and connection type,
- Assume the pier is relatively rigid if compared with steel column and design the 6 ft pedestal as short column and conn. des. and detailing as per ACI 318 chapter 17.
- other option is , I would analize the total system and assuming vertical combination , i would follow ASCE 12.2.3.1.
You may consider to provide more details to get better responds.
 
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It's an interesting though that hasn't generated as much discussion as I would have hoped. Up north, we are designing pedestals as concrete not detailed for seismic resistance but bumping up forces by something analogous to the overstrength factor. I generally steer clear of AISC 341 anytime possible for steel, but generally have just designed pedestals without any special detailing.
 
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Wondering if this counts as a vertical discontinuity/irregularity of the LFRS according to ASCE? Would have to check and see if that requires you to do anything additional.
 
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HTURKAK said:
You did not provide alot of info here. The no. of storeys and if the connection of steel column to pier is rigid or pinned are important points.
If i were in your shoes , i would go considering the no. of storeys and connection type,
- Assume the pier is relatively rigid if compared with steel column and design the 6 ft pedestal as short column and conn. des. and detailing as per ACI 318 chapter 17.
- other option is , I would analize the total system and assuming vertical combination , i would follow ASCE 12.2.3.1.
You may consider to provide more details to get better responds.
Click to expand...
I apologize for the late reply.

Let's say I have a 2-story steel ordinary concentric braced frame with a pinned column bases, or a steel special moment frame with fixed column bases.

In both cases I am relying on the pier to transfer my seismic forces into the foundation (spread footings).
 
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Philip.screw said:
In both cases I am relying on the pier to transfer my seismic forces into the foundation (spread footings).
Click to expand...
True . The best modelling would be different for each case. I 'm at a very different time zone and i saw your post after 10 -12 hours .

Some of them,
- Assume the supports of the steel structure in this case top of the piers at ground level and perform the analysis and design the piers for the reactions,
- Assume an imaginary storey with 6 ft height and having RC columns and follow the vertical combination rule ASCE 12.2.3.1.
- Other option , assume a rigid basement ( there should be perimeter wall also ) with the RC columns and flexible upper portion and follow the rules of Two stage analysis 12.2.3.2 Two-Stage Analysis Procedure.

You are the designer and you can use your judgement to choose the modelling type which best suits your case.
 
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