ASCE 7-05 wind load on open structure

[MotorCity]

I have an open structure 65' tall and 22' x 30' in plan. I am trying to determine the pressure coefficient Cn using method 2. My h/L is either 65/22 or 65/30 which are both greater than 1. If this is the case, it appears I cannot use Fig 6-18 to determine Cn because 0.25<h/L<1.0 is not satisfied.

How do you obtain the pressure coefficients for an open structure if your h/L falls outside these limits?

 

[ToadJones]

What type of roof?

 

[ToadJones]

Looks like the code dropped the ball on this one. I couldn't even find an explanation in the commentary.

 

[MotorCity]

It is a 1.5" metal roof deck (no concrete topping or insulation).

I'm not sure if this issue is covered in errata. Maybe I am missing something.

 

[smb4050]

No help, just thoughts.

Read the limitations on for Method 2 in 6.5.1 & 2. Is it a flexible structure, and does it qualify for using this method? The wind loads may be closer to that of a bridge than building structure.

In ASCE 7-05 commentary section C6.5.6 for the figures you reference on pg 299 does talk about where the coefficients came from, but in section C6.5.13 seems to contradict this information? Perhaps the answer of why your condition isn't addressed in ASCE is the coefficients are based on wind tunnel test results limited to those parameters.

 

[paddingtongreen]

I don't have the up-to-date code, but this would have been a tower in the versions I used.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[woodman88]

If your structure does not meet the 0.25<h/L<1.0 requirement for the table would not this make it a unusual shape? Per ASCE/SEI 7-05 section 6.5.2 "...shall be designed using recognized literature documenting such wind load effects or shall use the wind tunnel procedure..."

Garth Dreger PE
AZ Phoenix area

 

[Ron]

In the commentary.....

Figs. 6-8, 6-18, 6-19, 6-21, and 6-22. With the exception of Fig.6.22, the pressure and force coefficient values in these tables are unchanged from ANSI A58.1-1972 and 1982, and ASCE 7-88 and 7-93. The coefficients specified in these tables are based on windtunnel tests conducted under conditions of uniform flow and low turbulence, and their validity in turbulent boundary-layer flows has yet to be completely established. [highlight]Additional pressure coefficients for conditions not specified herein may be found in two references [Refs. C6-96, C6-97][/highlight].

Those two references are....

[Ref. C6-96] ASCE. (1961). “Wind forces on structures.” Trans. ASCE, 126(2), 1124–1198.
[Ref. C6-97] (1956). Normen fur die Belastungsannahmen, die Inbetriebnahme und die Uberwachung der Bauten, SIA Technische Normen Nr 160, Zurich, Switzerland.

It's unfortunate that the code references documents not readily available. The first one can probably still be purchased through ASCE, but I wouldn't count on getting a copy of the second one!

The commentary also references Australian Standard AS1170.2-2000....maybe some of our friends from Oz can shed a little light on that standard. Hokie66/Rowing Engineer...where are ya?

 

[rowingengineer]

Ron,
Thanks for calling us from the other forum, I have attached the draft code of the AS1170, since it is a publicly available document there is no copy right issues ect with posting.

The area of this code that your are interested in are unchanged from the AS1170.2002, so it is a good reference for this problem. For the co-eff you want pages 33 and 34, however I think you would need to review the whole document, as I have no idea what the differences are between the code loadings.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rowingengineer]

here is the link to show it is free

http://infostore.saiglobal.com/store/Details.aspx?productID=1141115

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it