Canopy Wind Loading

[rgerk]

I'm working on an aluminum sidewalk canopy that is 7ft wide x 102ft long. The building is approximately 122ft x 116ft and the canopy runs parallel with the 116ft side. The canopy is supported by the structure on one side and column supported on the other. The eave height of the building is 18ft and the eave of the canopy is 11ft 6 in.

In determining the wind load requirements, I generally look at these as open, however, the h/L ration is 1.64 for this example which precludes me from using the open wind loads.

V=140mph, Exp C, Cat III. I used the C&C loads to analyze the roof deck with p=160psf. No problem for the deck. The beams and columns can't support this load and I don't feel C&C apply to them anyway.

Question is, which loading would you guys use for the MWFRS on the beams and columns. I've looked at using the overhang pressures, but I'm not sure they even apply. Any other thoughts?

Thanks for thinking with me.

Ryan

 

[Ron]

The Open Structure condition still applies. You mention h/L but did you mean L/B? If so, consider the worst case condition since your L/B might not work within the table.

Use MWFRS for the structure itself, unless it transfers load to the primary structure....if that's the case use C&C loading.

I don't typically apply overhang pressures on a canopy of this width, though you could.

 

[southard2]

I think you should use components and cladding wind loads for the columns and beams unless you can show that during the wind load case the component in question is loaded from two surfaces.

If the sides were cladded the columns would have to surfaces loaded. The column top and the lateral wind load on the side of the columns.

Items that should be components and cladding based are roof deck, steel joists, brick veneer, etc...

Columns typically only take a load in one direction so these are usually controlled by components and cladding for the uplift case. Keep in mind you can use the MFWRS if the tributary area is greater than 800 square feet. Also remember that often with larger areas the components and cladding minus 0.6 DL isn't much different than the MFWRS loads anyway. It just depends on your scenario.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[rgerk]

Ron,
I meant h/L. Tables in ASCE 7-05 for MWFRS and C&C both have limits of 0.25<h/L<1.0 for Open Structures. That's where I'm a little lost as how to apply the Open Structure loads. My h/L ratio is 1.64 which exceeds the 1.0. Do you know why the open structure loads are limited to h/L<1.0?

I would agree that using the overhang loads doesn't seem right, but using the enclosed or partially enclosed roof loads don't seem right either.

southard2,
I'm a little confused with your post. The columns and perimeter beams are part of the MWFRS and therefore I don't believe qualify for C&C loading. My main confusion with the code is that Open Structure Wind seams to be the logical loading, however, the case doesn't apply due to my high h/L ratio of 1.64.

Any other thoughts? Thanks for both of your replies.

 

[a2mfk]

Overhang or not an overhang, its a discussion I've heard throughout the years, especially in Florida. I was hoping ASCE/FBC would have cleared this up by now...

Why is this not the same as an overhang? Because of the difference in height of the two roofs? Its mounted to the building on one side, and on the other side it is supported by posts. If it was cantilevered out from the building would it be an overhang?

I have always treated them (and I'm not alone), perhaps conservatively, as overhangs. Think of how the wind hits the canopy, it goes over top of the roof and underneath. The wind underneath hits the wall and then is forced up to cause upward pressure on the ceiling/underside of the roof (same pressure as the wall according to FBC). So you have surface loading on both sides of the canopy roof, just like an overhang. I know a 10-15 ft wide canopy adjacent to a building is not EXACTLY the same is a 3ft overhang, but its somewhere in between probably?

Perhaps the step in the two roofs reduces the pressure on the canopy upper surface, but I have not seen that cited/explained in a reliable source.

Modeling it as a free-standing open canopy (like a gas station or park structure) would result in very low pressures compared to an overhang, which I am not convinced is correct. I would like to know if there is a source out there treating this specific situation...

 

[slickdeals]

Andrew,
I am dealing with a similar issue. I think there is a difference between an overhang on a roof and an entry canopy.

When the wind hits an entrance canopy, it separates at the eave. Wind blowing below the canopy will stagnate at the wall and cause an upward pressure. However, wind also stagnates at the top of the canopy where it hits the wall and will produce a downward force (again it is all dependent on the slope of the canopy).

The effect of these will result in a pressure that is less drastic than that of an overhang which will purely see only uplift (Negative at top and stagnation from bottom).

Hope I explained it right. Others can chime in.

 

[a2mfk]

Slick,
Why won't the wind go up the top surface of the canopy roof and then up the building wall? Not quite seeing it causing a downward pressure. For discussion sake say its 4:12. Not that you are wrong... I guess this is nowhere in ASCE yet? I don't have a copy handy.

 

[slickdeals]

It is not in ASCE 7 explicitly, but take a look at Figure 6-12 (ASCE 7-05).

Look at Note 1.
It states that "....Positive values of GCp equal to those of walls shall in Figure 6-11A shall apply on the cross-hatched areas shown in Figure 6-12"

Think of it as an upside down overhang. Just like in the soffit loading discussion where you apply the positive wall pressure as an upward pressure on the overhang.

 

[a2mfk]

Thanks Slick, I will research that.