Carport Wind Loads

[SteelPE]

I am currently designing a carport for a client. The required wind loads are IBC2009 utilizing ASCE7-05. The dimensions of the carport LxWxH = 180'x24'x10'. The carport is open on three of the four sides with the back 180' wall of the carport a solid wall (area is to be used for storage).

A question has come up with regards to wind loads and if the roof pressures should be calculated as if the structure were an overhang. Since the building is partially enclosed and the structure is in a relatively high wind zone (115mph exp C) we are ending up with some pretty high numbers. However, this approach seems pretty conservative.

How would others treat the design of the roof with regards to wind? Would you conservatively consider the roof to be an overhang, or would you approach the problem a different way?

 

[jfmann]

For roof design......in ASCE 7-95, start with 6.5.13 Wind Loads On Open Buildings With Monolope, Pitched or Trough Roofs

To be most conservative, assume wall is breached, so that wind must be considered for open roof in both directions.

"Overhang" does not seem appropriate for open building with relatively large plan dimensions, especially compared to low height.


John F Mann, PE

http://www.structural101.com

 

[Ron]

Agree with jfmann....I've designed hundreds of free standing and attached canopies using exactly the approach jfmann mentioned. The only time I've used an overhang is for a narrow canopy attached to a building.

 

[KootK]

X3. I think that the overhang provisions assume a wind speed up adjacent to the wall element supporting the overhang. That would't be the case on your truly open structure.

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.

 

[jfmann]

Reference in prior post was intended to be ASCE 7-05 (not 95....that was now a while ago!)

More recent edition (ASCE 7-10) has similar provisions........though wind design is now provided in several chapters towards back of code.
See 27.4.3 and Figure 27.4.7.

Interestingly, your condition does not satisfy code requirements (ASCE 7-05 & ASCE 7-10) for "Open" building or "Partially Enclosed" building and therefore would have to be considered "Enclosed" building as the default (per code); see Definitions. This is where "engineering judgment" must be used!

John F Mann, PE

http://www.structural101.com

 

[SteelPE]

While I understand the thought process, I'm not sure I have fully conveyed the question (or maybe I am not fully understanding the response). Attached you will find a few sketches that were provided by the architect. Here you can see that there is a solid wall along the backside of the carport. In running through the calculations for a partially enclosed structure when the wind acts against the broad side of the structure I get the following:

Ao=1800 ft2
Ag=1800 ft2
Aoi=320 ft2
Agi= 5720ft2

Ao/Ag=1.0 (however each wall is not 80% open as one is 100% closed)
Ao/Aoi = 5.625>1.1
Aoi/Agi = 0.056<0.2

Hence the building is partially enclosed in this direction. Is this not correct?

 

[jfmann]

Although not clearly explained in code provisions.......calculations should be performed for each primary wind direction, separately.

Then, for wind parallel to long wall......Ao is full area of (missing) endwall. Although this may sound silly......consider one endwall consisting only of a very narrow solid strip (vertically, but perhaps horizontally also), such that open area is 98% (or 92% or similar).

I submit these code provisions are not intended for such condition......see definition for "Building Or Other Structure, Regular Shaped".......then refer to conditions for use of code provisions (6.4.1.1 & 6.4.1.2 in ASCE 7-05).

Essentially, your structure as configured is not really a "building" as defined by wind-load provisions.......such that you must use engineering judgment .........as suggested in previous posts.

Agi must include area of the 3 open (missing) walls as well as area of roof.......which results in much greater value.
Similarly.....Aoi must include area of "openings" in each wall (not including wall subject to positive wind pressure) and roof

Since the 80% open condition is not satisfied for long wall, the "building" does not satisfy requirements to be considered Open (even if it were considered to be a "building", which it does not).

Especially for wind direction parallel to long wall..... it looks like provisions as noted in previous posts should be used.


John F Mann, PE

http://www.structural101.com