ASCE 7-05 Effective Area for Parapet 1

[JoMoEIT]

I'm currently working on a project that consists of a PEMB as MFRS with our firm designing a brick and metal stud curtainwall system 17.33' tall. These studs are bypassing the PEMB system, being connected to a wind beam @ EL 15.17'. My question is in regards to both the design of the stud as well as the design of the connection. I'm contending that for design of the stud; parapet C&C pressure along with wall C&C pressure should be based on an effective area of the entire stud (17.33*17.33/3=100ft2) instead of the individual areas for individual pressures. This makes a significant difference in the deflection calculation and our office is in disagreement as to which way to move forward with this. Does anyone have any literature or insight addressing this situation?

Thanks for the help,
Brett

 

[MrHershey]

I would tend to agree with your application but don't really have anything to back it up. The application of ASCE 7 wind loads, especially components and cladding, isn't well-defined.

The reason for the different pressures at different effective wind areas is due to the statistical likelihood of a member seeing localized high pressures due to a small effective wind area. It's more likely that a small fastener will see 50+ psf wind loads over its whole area, for example, than for a roof joist to see 50+ psf wind loads over its whole area. Crossing between boundaries should not change your effective wind area for the entire member you are designing.

 

[JoMoEIT]

Thanks for the quick response Mark. I guess we'll go with what makes sense since the code isn't explicit.

 

[ron9876]

I would still be sure that the parapet worked with pressures based on its effective area. But for connection at the roof diaphragm I would use the full area.

 

[DST148]

Ref - Text book: Loads and Load Paths in Buildings by Narendra Taly and published by ICC.
The width of the effective wind area used to determine (G.Cp) need not be taken as less than
one-third the length of the area.
Some of the solved examples from the book:
Example 5.5 - Wall 17 feet high, 6" thick, R/W #4 @ 12".
Ae = 17 x 1 = 17 sft
Ae = 17 x 17/3 = 96 sft (controls)

Example 5.6 - Roof panel 2' x 4'
Ae = 2 * 4 = 8 sft (controls)
Ae = 4 * 4/3 = 5.33 sft

 

[wannabeSE]

Have you looked at the definition of "Effective Wind Area" in ASCE 7?

From ASCE 7-05 (7-10 is similar):
"EFFECTIVE WIND AREA, A: The area used to determine GCp. For component and cladding elements, the effective wind area in Figs. 6-11 through 6-17 and Fig. 6-19 is the span length multiplied by an effective width that need not be less than onethird the span length. For cladding fasteners, the effective wind area shall not be greater than the area that is tributary to an individual fastener."

 

[JoMoEIT]

Thanks for all the help guys. It's nice to get some insight from everyone on something like this. 12ga was looking a little excessive for this project but that cut the parapet pressures by about 40%. Hope everyone had a good 4th.

Brett