Vertical Wind Pressures on a Cantilevered Canopy using ASCE 7-05 1

[kmc23]

I'm designing a flat, cantilevered steel canopy at a height of 18' on a building that has a roof mean height of 67'. It's relatively small...cantilevers out 11.5' and is only 25' wide with WT's at 5'o.c. The overall building length is 200'.

What is the correct procedure to find the vertical wind pressures (up and down) using ASCE 7-05? Do I use the C&C equation in section 6.5.12.4.2, since my roof mean height is over 60'? Or is it considered a roof overhang per section 6.5.11.4? I don't believe that it's an open structure, since it's attached to the building. What do you think?

Thanks for your response!

 

[Ron]

kmc23...good question. I've done lots of canopy design and have run into different interpretations by different engineers of record when they review my calculations.

In most cases I would consider it an open structure or an overhang. The roof section of the canopy is not over 60', so you don't have to consider that.

If it is truly attached to the building, then I would consider it an overhang. If it has an independent structure and is only incidentally attached, I would consider it an open structure.

I had one SEOR interpret a similar canopy condition as being partially enclosed. I didn't agree and finally got him to see the point (though his first explanation of his reasoning seemed to have some merit) about it just being a C&C analysis.

 

[Teguci]

Definitely an overhang

MWFRS
Cp = 0.8 on the underside plus 0.9 (or other as determined from roof angle - consider both directions) for the top of the roof

p = qh * G * Cp - Direction of wind is up only GCp = 1.445

CaC per 6.5.11.4
Fig 11B; GCp = 1.4 for flat roof
Fig 11C; GCp = 2.5 at corner else 2.2 up to 27 deg roof
Fig 11D; GCp = 1.8 up to 45 deg roof

 

[ron9876]

Overhang with C&C loading based on whatever the overhang pressures would be for your building height, etc. Trib areas not large enough for MWFRS.

That would be for wind perpendicular to the building face. If there is very much area exposed when cosidering wind parallel to the building face that is when it really gets fuzzy.

 

[slickdeals]

I have used Figure 6-18A (MWFRS) and 6-19A (C&C) in the past assuming a blockage of more than 50%.

 

[SAIL3]

Rather than nit-pick it to death as it seems the codes nowadays are indicating, I would probably use GCp= +/-2.2 on entire canaopy and move on.
What might concern me more is the snow load(if you are in a snow region) and the possibility of snow sliding off the main bldg roof.

 

[kmc23]

Thanks to all of you for your help. It looks as if there are multiple ways to look at it. In my original design I used GCp = 2.8 from Figure 6-17 (zone 3). That seams conservative and it works for the desired architectural WT profile, but wasn't sure if that approach was correct. It'll be interesting to see if the plan checker has any comments.

@Ron - The WT cantilevers are only connected to the building with a moment connection to the first floor beams.

@Teguchi - The Cp = 0.8 is from 6.5.11.4.1, correct? Is the 0.9 from fig 6-6 for Normal to Ridge for theta<10, h/L <0.5? And are you suggesting that I need to add the overhang value of GCp = 1.445 to the C&C GCp to get a combined GCp = 2.845? Or are they separate?

@Ron9876 - There's not much exposed in the parallel direction.

@slickdeals - Are you saying that it's an open structure, but that the building causes the > 50% blockage on the back half of the canopy?

@Sails3 - No snow load. I think the last time it snowed here was in 1972!

 

[Teguci]

GCp = 0.85 (0.8 + 0.9) = 1.445

0.9 assumes roof is flat in one direction.

 

[slickdeals]

@kmc23: Yes

 

[a2mfk]

I have heard interesting arguments for different pressures being used for this situation, but like Ron have always used overhang pressures. It is the most conservative also and has made me sleep well at night.