Freestanding Walkway Canopy Sidesway

[Description]

I have an aluminum freestanding canopy which is designed with cantilevered columns in caisson foundations. I have no fixity in my beam/column connections and solely rely on the cantilever action for bending. If no nearby structures would cause an issue for sidesway deflection due to wind loading; what would a reasonable recommended maximum deflection be?

Sizing columns for strength I can make a relatively small section work which will result in about 2 1/2" of sidesway on an 11' tall column.

 

[jayrod12]

H/500. At a minimum. That's a huge amount of deflection you're getting with your current column spec. In theory you could go down to H/400 but the people using the walkway may not enjoy any sidesway.

 

[Description]

Just to be clear I'm talking about an exterior walkway rain cover canopy. Not a walkable surface; H/500 seems quite excessive for that purpose. My post may have been unclear.

Thanks

 

[MotorCity]

Assuming the structure can take it, I would use H/100 max, but also consider that the public tends to get concerned when they observe or feel movement in a structure. 2.5" is too much IMO.

 

[Description]

That's the number I've considered, although given some buildings are designed at H/100 for MWFRS loads fairly regularly I've wondered about backing down my requirement (lateral drift on the structure is from C&C lateral wind on column/beam/fascia faces only).

 

[Hokie93]

For drift, I would determine the wind pressures based on a 10-year MRI wind speed and then check the lateral deflection against H/300. That would result in a 0.88" limit if you take the span as "2L" for a cantilever.

 

[hokie66]

Hokie93, why would you take the span as 2L? The deflection he is looking for is the tip of the cantilever.

I think 2.5" is too much, so maybe an inch. If your deflection is that much, the rain will be sideways, so the people using the walkway won't be worrying about a bit of deflection.

Just curious, why is the canopy so high? The higher it is, the less protection it affords from rain.

 

[KootK]

I don't know that there needs to be a hard limit in this situation unless you're worried about bumping into something nearby. My concerns would be:

1) Including P-delta effects in the column design if there will be heavy snow combined with slender columns.
2) Giving some account to foundation flexibility as that's really all that you've got here.
3) Keeping your period within a sensible range so that you don't get some terrible resonance vibration thing going on in the wind.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

I agree with hokie66...limit to about an inch. Other than that, no criteria. I have done similar on the hundreds of these I've designed in high wind areas.

 

[Hokie93]

Hokie66: Span-to-depth ratios like H/300 are intended for simply supported members so it would be appropriate to take the member span (say, H) as 2H when checking deflection or drift of a cantilever member. So for a cantilever, the H/300 drift limit is really H/150. The span to consider for a cantilever member is explicitly included in the International Building Code. It is footnote (i) in Table 1604.3 of IBC 2012.

 

[Jerehmy]

What's the estimated lateral displacement angle of rotation at the top of the caisson?