Wind load at podium (below high-rise tower)

[jdshiv]

Does anyone have suggestions about how to use Method 2 to determine C&C wind load at the roof of a podium?

Assume 60' tall podium on two sides of a 600' tall office tower, roof area of podium roughly equal to tower area. There is a debeate in our office about what mean roof height to use for podium loads.

My argument is that it should be OK to use the mean height of the podium, loosely justified by Figure 6-12 in ASCE 7-02 for stepped roofs, even though this figure technically doesn't apply for H>60'. But there is another argument that you might need to use the total high-rise height in all cases where "qh" is specified in the Method 2 equations; the rationale could be to account for increased loads on the podium due to downdraft of wind from the tower.

Ultimately a wind tunnel test will likely be required since Method 2 may not be valid for such an irregularity in elevation. But we want to use Method 2 to estimate loads for preliminary glazing design.

Last year there was a post about whether a rooftop penthouse height should be included in the mean roof height, but the final opinions seemed mixed:

http://www.eng-tips.com/viewthread.cfm?qid=197553&page=1

 

[jdshiv]

clarification:
There were two related previous threads, the one in my original post was a slightly different question about a gym that pops up above a sprawling low roof.

The recent thread about roof parapet & penthouse height was:

http://www.eng-tips.com/viewthread.cfm?qid=224229&page=3

 

[civilperson]

Podium is miss used as I understand the post. Podium in architecture is a foundation wall. In common usage, a lectern or pulpit.

 

[jdshiv]

civilperson,
FYI -- "Podium" in architectural/structural vernacular refers to the base of a tall building, generally wider than the tower footprint and much shorter. Try a google search for "skyscraper podium" to see examples.

 

[frv]

civilperson-

jdshiv is correct.

 

[WillisV]

1. Definitely do a wind tunnel.

2. For prelminary estimates use qh based on height of the tower itself. On recent projects of similiar stature I have seen the uplift pressures on the podium equal or slightly exceed those at the top of the tower itself.

 

[jdshiv]

WillisV,
Thanks for your response. I'm glad I asked the question, since my initial gut feeling turned out to be questionable.

I'll definitely use the main tower height to estimate podium wind loads, and will make sure to insist on wind tunnel testing before final design.