Load combinations for gravity columns design

[compe_ad]

Hi all! Do we design the gravity columns using the load combinations that have seismic component in it? Aren't the gravity columns are designed to resist gravity loads only? Can you please make the concept clear for me?
I saw following load combinations being taken for the design of gravity columns. Does this make sense? In my opinion, gravity columns are to be designed for gravity load combination only. But it is important to check section 18.14 of ACI. Here we have to check column for seismic load cases in order to make sure column maintains its integrity during seismic event. This section of ACI triggers the detailing of column not the design of the column.

 

[jayrod12]

Even for gravity only columns, is there not vertical excitation in the seismic loads?

What about designing the columns to accommodate the anticipated seismic drift?

 

[Stenbrook]

When you do seismic analysis, there is both horizontal and vertical seismic forces you have to consider. Ev is vertical, Eh is horizontal. So assuming you aren't using your columns as any part of moment frame to resist lateral, you would have to at the very least resist the vertical seismic forces and include them in your load combinations that you have shown above. This vertical component is defined in ASCE 7.

 

[compe_ad]

Yes, columns are not the part of lateral load resisting system. So do you mean only the vertical component of seismic component should be considered for the design?

 

[jayrod12]

The vertical component of the seismic at a minimum, plus you must design the columns taking into account the potential lateral seismic drift, therefore increasing the second order effects on your columns.

 

[compe_ad]

We donot design the gravity columns for the forces induced by lateral seismic load, correct? But we have to ensure we provide the detailing as per section 18.14 of ACI so that columns maintain its integrity during seismic event.

 

[jayrod12]

Not directly for the lateral forces due to a seismic event. However, the gravity only columns must be capable of resisting their design loads at the seismic lateral displacement. Therefore there will be added second order effects to take into account during the design.