Solar Carport/Canopy - Seismic System Selection

[Bwnarrow3]

Hi all,

I’m currently designing a solar carport/canopy to be installed in a parking lot in California. SDC D. The structure consists of a single row of HSS steel columns supporting WF “girders” (moment connections) with wide flange “purlins” spanning between (pinned connections). I have currently selected “Special Cantilever Column System” as the seismic system. Do you think that would be appropriate?

I’m also somewhat confused about detailing requirements. Minimum width to thickness ratios (AISC 341) are requiring my members to be a lot more “compact” than I would otherwise select. Could anyone shed some light on this? Thinking intuitively, it would seem to me that “highly ductile members” would favor less compact shapes to allow the member to behave in a more ductile manner.

Any help is appreciated. Thank you.

 

[driftLimiter]

Yes I think special cantilever columns is an appropriate selection. Watch out for foundation requirements on that system.

Regarding compactness it seems like you have reversed the thinking. Compact sections have a more ductile failure mode because their strengths are not limited by local instability. This is important for reversible loading due to seismic because the assumptions used to determine the R factor are based on a specific type of hysteresis response.

 

[Bwnarrow3]

Thank you kindly for your response

 

[sandman21]

The issue with special cantilever column systems is the bracing of the columns and unbraced length of the column.

 

[Bwnarrow3]

Sandman, can you elaborate? For a 13' tall cantilever column structure, how are braces intended to be configured? I am using an HSS column, and the entire column will technically be unbraced (fixed at the bottom). Not sure what I'm missing here.

 

[sandman21]

ASIC 341 E6.Ab stability bracing of columns, columns shall be braced to satisfy section D1.2a. The plastic hinge for the SCCS should be at the fixed base connection. You need to brace the column such that the unbraced length is less than section D1.2a. You have restraint at the bottom and likely at the top but if you have an unbraced length of say 8' you would need to add in an element to support the column.

 

[KootK]

I agree with the bracing requirement but feel that it can usually be surmounted with a judicious choice of column section to obviate the need for bracing between the ends. Below is my spitballing the allowable height for a 6" standard pipe.

 

[KootK]

...HSS steel columns supporting WF “girders” (moment connections)...

Are the girders moment connected to the columns? If so, you would seem to have the option of using moment frames for your LFRS rather than cantilevered columns.

 

[Bwnarrow3]

All, just closing the loop on this, I ended up using an ordinary steel cantilever column system selection, based on superscript “l” in the table in ASCE 7. This removed any seismic detailing considerations. Thanks for your help!