C&C Wind Loads with a Canopy 1

[SteelPE]

I am currently designing a new store front for an existing building. The building contains a 10'-0" wide canopy along the front (code is ASCE 7-10). With regards to GCpi and canopies, would you use GCpi = +/-0.55 or +/-0.18. I would lean towards +/-0.55 as the wind is not allowed to flow over the top of the building as it is being trapped by the canopy.

 

[GC_Hopi]

Have you looked at the definition for roof overhangs. You might fall under that for determining wind loading.

 

[txeng91]

I believe the ASCE-7 says to use C&C wall pressures for soffits/canopies.

 

[SteelPE]

I am not looking to design the canopy itself, but rather the wall under the canopy, so 30.10 doesn't really help (from what I can tell, that section uses GCpi as well).

The question basically boils down to which GCpi is correct for the design of a wall under a canopy that is 15' tall x 10' wide (I believe +/- 0.55 is correct).

 

[EZBuilding]

Is the wall in question an enclosure wall for the building or a wall on the outside of the building supporting the canopy?

 

[MotorCity]

Flip the problem around and look at it as if you were designing the canopy as an open structure (open on 3 sides where the 4th side is your store front).

 

[JAE]

0.55 is for partially open structures. The presence of a canopy / overhang doesn't affect whether your main building supporting the canopy is closed (0.18) or partially open (0.55).
I think you are confusing two different things.

The internal pressures from wind are only affected by how quickly wind pressures can get ramped up INSIDE the main building due to its semi-enclosed condition....not whether there is a large overhang on the OUTSIDE of the building.

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[Ron247]

Partially open is easier for me to visualize as "mostly closed". This situation does not sound like it is mostly closed. The wording the code uses is kind of misleading. Partially enclosed is mostly open.

 

[SteelPE]

The way I think of a partially enclosed building is for the wind to enter the building, hit an obstruction and have no place to go.... effectively increasing the loads induced by wind. When I look at the continuous canopy on the side of the building I envision the wind hitting the front wall of the building and then trying to flow over the roof, but it's being blocked effectively increasing the loads induced by the wind.

So I am designing the wall directly under the canopy and I can see the wind loads being increased due to the presence of the canopy which is blocking the wind from flowing over the building.

Maybe my thought process is wrong with regards to this type of structure.

 

[Ron247]

Look at the code. .55 is Partially Enclosed (Mostly Open) while .18 is Partially Open (Mostly Enclosed)

You get the .55 when you have windward wall with a lot of openings, but your other walls do not have many openings to let the wind out. A lot of the calcs are based on the windward wall openings more than the other walls. So a mostly open windward wall is worse than a mostly enclosed windward wall.

 

[JAE]

SteelPE - I suppose you could use that concept to kick up the EXTERNAL windward load on the wall below the canopy (raw engineering judgement) and there's nothing to stop you from doing that.
But it isn't an effect due to the internal pressure of main building in my view.

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[kissymoose]

Not to say you should be calculating this as C&C loads on a roof overhang, but what's happening with roof overhangs and the adjacent wall is the same concept, wind hitting the wall and encountering a horizontal surface on its way at the top. Section 30.10 says the load on the soffit (overhang underside) is the same as the adjacent wall, calculated as if there wasn't an overhang. The referred wall calculation figures 30.4-1 and 30.6-1 make no mention of having to account for the overhang.

 

[EZBuilding]

I agree with the 0.18 assessment.

 

[structSU10]

I had run across this paper previously. Maybe it will clarify some things.