Footing pedestal design in Seismic Zones 2,3 & 4

[lahpe65]

Per UBC 1922.10.3 Structural plain concrete members are not permitted in Seismic Zones 2, 3 & 4.

UBC 1921.4.1 refers to "frame members". Is this refering to moment frame members only or does it include members of a braced frame structure?

Chapter 21 of ACI seems to only refer to moment frames.

I am designing a footing pedestal for a steel column of a braced frame structure and I want to be sure I follow the correct design procedure.

 

[HochwaltPE]

You should design the pedestal for the capacity of the braced frame as determined by the probable brace capacity. Assuming you do this, the pedestal can be designed conventionally because it will not require significant ductility.

 

[lahpe65]

Thank you for responding to my question.

Conventional design was my first inclination. However, based on conventional design the reinforcement ratio is less than .01 and UBC 1921.4.3.1 states that it shall not be less than .01 (That is if I fit into the criteria of that code, which leads back to my original question). Therefore, am I required to use a minimum of .01?

 

[HochwaltPE]

If the pedestal is significantly oversized due to geometry, I would be willing to let the reinforcing ratio go as low 0.005, treating this as an "architectural" column. The way PCA Column designs "architectural" columns by reducing the effective concrete area so that As/Aeffective = 0.01. (Rather than actually reducing the area, the software reduces f'c, which is a reasonable approximation.) If your pedestal is large enough, your Pu/Ag ratio may actually be low enough to classify the pedestal as a flexural member and use an reinforcing limits for flexural members.

Having said all that, I would also think about how much you're saving by sharpening your pencil this much.