Mean Roof Height used for Components and Claddding -ASCE 7-05 -SImplified Procedure 2

[186skier]

When designing a window wall of 12'-0" at the base of the building for components and cladding -Simplified per ASCE 7-05, would I use Lambda = 1.21 for H<15', or would I use Lambda = 1.29 for H= 20'. (Page 40 of the ASCE 7-05 FIG 6-2). The actual mean roof height is 18'-0"(+-)It seems like I could use the lower bound since the element is below the mean height.

 

[DaveAtkins]

I have always used the mean roof height to determine the wind load for EVERY component in the building.

DaveAtkins

 

[chorner26]

But should that apply if the building is say 5 stories and or 60'-0" in height because the difference is significant from the 1st level to the 5th level. Is it acceptable to use the floor height at each level?

 

[Archie264]

I have always used the mean roof height to determine the wind load for EVERY component in the building.

I love it! There comes a point where spending a lot of time and energy to massage things to within a gnat's whisker just isn't worth it. Especially since it results in considerably simplifying the calculations that follow.

 

[SAIL3]

It would be more beneficial, going forward, for the engineer to put the effort into trying to understand the underlying reason for this requirement rather than trying to satisfy the code verbatim.

 

[chorner26]

Actually no, the purpose of this question is an interpretation of the code, as I (186skier is chorner26) am running into a DSA plan checker that is requiring me to design per the AAMA Manual for aluminum extrusions. It is was my own engineering judgment, the answer would be to use the lower bound and understand that the element would not fail due to wind. But posting one here is try to gain some knowledge from the engineering community of what the actual code is saying.

 

[JedClampett]

I hate to be "that guy", but if the code says to use mean roof height and the component is 12'-0" above the ground, I still use the mean roof height.
Are you really saving that much material by fine tuning?

 

[wannabeSE]

I agree with Jed and want to add one thing. To my knowledge, the mean roof height is always used for external pressure on leeward walls.

 

[Archie264]

It would be more beneficial, going forward, for the engineer to put the effort into trying to understand the underlying reason for this requirement rather than trying to satisfy the code verbatim.

A corollary is that it would be beneficial if the researchers put effort into understanding why practitioners are finding ways around their requirements.

Simply calculating G per Sections 26.9.4 and 26.9.5 of ASCE7-10 is enough to rival ACI 318 Appendix D for complexity. And that's just but one step of many in getting to the design pressure. It makes me wonder if the researchers are engaged in an arms race. Or complexity race, rather.

 

[SAIL3]

I agree with Archie...the current codes are becoming so complex and opaque that the underlying engineering principles are lost and forcing the practicing engineer to ignore or make erroneous interpretations of the intent of the codes.All the more reason that one should have knowledge of the underlying engineering principles involved so one can safely ignore or work-around the time-consumming methods presented in the codes.

 

[dcarr82775]

Go through the calculation. Take your C&C wall pressure using a height of 12ft. Take your C&C suctions using the mean roof height as you would for any leeward wind. The suction will control, and thus the mean roof height.

 

[BUGGAR]

SAIL3 you nailed it!

 

[woodman88]

Which code?

Per the ASCE 7-05 & 7-10
"MEAN ROOF HEIGHT, h: The average of the roof eave height and the height to the highest point on the roof surface, except that, for roof angles of less than or equal to 10[sup]0[/sup], the mean roof height shall be the roof eave height."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[186skier]

Woodman88 and carr82775 ,
Thanks for the help.

I actually found the mean roof height in the front glossary (ASCE 7-05), but also decided to jump to the analytical procedure (Method 2) 0.00256KdKzKtIV^2I with +- GPF-GCPI. Also deducted 10 % per footnote #4. Worked like a charm.

 

[XR250]

My guess is that the suction loads have more to do with the speed of the air coming over the top of the building and not being influenced as much by the friction of the ground causing a pressure gradient. Hence, using the mean roof height for suction.