Cantilever Canopy - Deflection

[bpstruct]

Have a canopy (cantilever style) with fabric covering supported by custom trusses (fabricated of pipe). Those trusses are supported by pipe columns. I have been asked to do an analysis of the original drawings. The details show members that are substantially undersized. After increasing as required for strength, I'm still running into pretty big deflection problems at the cantilever free end. Even after doubling the cantilever length as allowed by IBC, I pull a total deflection from my model that exceeds that value by about 2 inches. If I place horizontal supports at the tops of the pipe columns (thus eliminating lateral movement of the columns), my vertical deflection at the cantilever end is below the limit. Should I be adding the height of the column to the cantilever length of truss for determining the allowable deflection since it is technically all cantilevering off of the foundation?

 

[hokie66]

Deflection is deflection, whatever contributes. But deflection is a serviceability issue, and thus requires judgment to determine acceptability or not. What serviceability issue does deflection of the canopy create? If it is just visual perception, that might be solved by cambering. If it is drainage/ponding or the like, then additional stiffness may be required.

 

[bpstruct]

Really just a visual issue. All of the members are already curved so there is no risk of ponding. I'm comfortable with it after the modifications I've made for strength. Problem is the reviewer....He questions every single judgment call that is made. Several pieces to the project. All of it has been a nightmare.

 

[Ron]

Agree with hokie66...if deflection is a result of both the column bending and beam, then you have two choices...ignore or stiffen.

 

[paddingtongreen]

Camber out the DL deflection.
What happens when the wind blows? you could end up with more upward lead than downward.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msquared48]

Are the columns pinned at the top or is there any fixity?

If the top and bottom chords of the trusses frame in at different levels on the column, then, depending on the connection, there could be some fixity that could induce a double curvature in the column, reducing the lateral deflection.

A sketch could help.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[csd72]

As others have said, I would probably tend to accept the deflection.

If it is deflection due to snow drift then this probably will not actually fully occur more than once or twice in its lifespan.

What is the consequence? It is a fabric structure and psychologically people expect this type of thing to deflect a bit more than a rigid building.

Better that it deflect too much than the other way around i.e. it would be worse if it was to discourage ponding towards the building.

This situation with US building departments has gotten out of hand, they should be focused on items relevant to life safety only and not fiddly pedantic points that are none of their busines and should only be a matter between a qualified engineer and their client.

 

[bpstruct]

Thanks for all the responses.