ASCE 7-10 Canopy Wind Loads

[SteelPE]

I am in the process of designing a small free-standing canopy (8 ft x 35 ft 8’-0” high). Design code is IBC 2015/ASCE7-10. The canopy has a few vertical columns and horizontal beams but no siding (there is roofing). Roof slope is 1:12. I am trying to calculate the story shear for the roof for wind loads (seismic is straight forward). V= 117 mph risk category I.

From equation 27.4-3 my lateral loads are extremely small (due to the slight slope) so I believe I need to design in accordance with section 27.4.7 (using 16 psf minimum). What value do you use for Af? Am I to use the projection of the roof and structural elements only…. or am I suppose to use the whole area of the structure (8ft x 35ft x 16psf = 4,480#)? The latter seems a bit harsh.

 

[KootK]

This has been discussed at length here in the past if you can scare up the right thread. My recollections:

1) Some other countries, such as AU, have specific requirements for this.

2) Some countries's codes give consideration to the roughness of the surface as it would affect drag.

3) Personally, wherever roof slope is less than 15 degrees, I just calc wind shear as if it was 15 degrees.

 

[GC_Hopi]

It is good to remember that the codes are minimum standards. If your canopy is close to vehicle traffic you could consider a vehicle barrier load to simulate the structure being struck by a vehicle. It is also useful to consider the construction condition and if the roof elements may be used for lifeline anchorage then apply that load laterally at the roof line.

 

[CURVEB]

You might also consider the potential that someone will decide to add some type of cladding to it in the future. If it didn't have a major impact on the sizes, I would probably check it for wind on the projected area. If it bumps everything up significantly, maybe back it down a bit but KootK's recommendation #3 seems like a reasonable lower-bound.

Don't forget to consider P-Delta if your lateral system is something very flexible, especially if you have high snow loads. That may drive your member sizes up.

 

[SteelPE]

I'm not sure my main question has been answered.

If I have an open canopy that is 8' tall x 35' long with 12" deep beams, 1:12 pitch and 6-6" wide columns (3 along each side) what would you use for Af for this open structure?

Option #1 = 8'x35' = 280 square feet
Option #2 = 8'x35'x1/2 = 140 square feet
Option #3 = 35'x(1'+8") + 8'x.5'x3 =71 square feet

I added option #2 because option #1 seems incredibly punitive for a single story building, and normally 1/2 of the wind load does not end up in MWFRS in single story buildings. In option #3 the 8" is to account for the roof slope.

Adding cladding in the future is not my issue. It would be up to the person adding cladding to check if the structure was capable of such a change.

 

[GC_Hopi]

Per ASCE 7-10 you should be using Table 27.2-1 and paragraph 27.4.3 then default to paragraph 27.4.7 if necessary. And Af is defined in 26.3. I think the point being made was to use engineering judgement and if the wind loading is so low then maybe other types of lateral loading is controlling

 

[MotorCity]

For pipe racks, lattice towers and similar structures with a relatively low surface area, there is a force coefficient that is used to artificially increase the nominal projected area of a structural element. It usually depends on the size and shape of the structural element, accounts for shielding effects, etc. Even under the worst case, this coefficient does not exceed 2. It is intended to account for an increase in surface area due to things like ice buildup, adding on small fixtures, etc. So obviously if someone comes back 10 years later and decides to close in this canopy all bets are off. There is a publication by ASCE on petrochemical facilities that goes into great detail on calculating these force coefficients and ASCE 7 has a similar provision for wind on tanks, chimneys etc (they use the coefficient Cf).

 

[StrucDesignPE]

Just a thought, but have you considered looking at the profile of the roof framing and vertical rise as an inverted parapet or fascia per Section 27.4.3 in the last paragraph? This specifically refers to open buildings with monoslope roofs and a pitch between 0 and 5 degrees.

 

[SteelPE]

I have no fascia panels on this project. So that section does not apply:

In the end, I went with option #3 and when I ended up with 600# in my frame I increased it to 1,000#. The drift and stresses ended up being very small, so I imagine it could handle quite a bit more lateral load if necessary.

I am not in charge of designing the foundations for this project, so since my reactions are going to be viewed by someone else I didn't want to go off the rails with being conservative with the design.

 

[JLNJ]

I would probably apply a full load to each beam individually with no aerodynamic shade.

My first pass would be: three rows of 12" beams spanning 35', 20psf (total, windward plus leeward) on the projected area of each beam. So 3 x 35'/2 x 20 psf = 1050#. This is similar to your answer.