Wind on Elevated Walkways

[buck1017]

I found a previous forum from 2014 that discussed wind on elevated structures, but I would like to re-open the topic. I am working on an elevated enclosed walkway with a rectangular cross section, and I am trying to determine the correct uplift forces. The walkway is elevated about 16 feet above grade. With it being open underneath, there will obviously be airflow both above and below the walkway. The previous forum seemed to suggest that the wind on the roof and the soffit underneath would both be going away from the structure effectively reducing the overall uplift on the walkway. But the more I look at this, the more I disagree because of Bernoulli. Bernoulli's principle basically says that as wind speeds up, its pressure reduces, and vice versa. Slower speed, higher pressure. As I apply that to my situation, because of the drag against the earth, the wind speed should be considerably less at 0'-0" than it is at 30'-0" where the roof of the walkway is located, resulting in a net upward force. Does anyone have any experience in such a situation or can point me toward a possible wind tunnel study that was performed? This seems like something that someone has to have done a study on. I just can't find anything. Thanks in advance for your help.

Brandon Buchner, PE

 

[cpiedra23]

sorry if this is something you've already looked at, but using ASCE 7-10 wind design chapters (Chapter 26 and so on) would provide some info. i'm not sure of any elevated structure sections, but it'd be the place to start?

 

[buck1017]

The only provision from the ASCE that I would consider using in such an application is an elevated sign, but that isn't right either because there are internal pressures in a walkway, plus a roof and soffit that will be loaded, so I end up just using regular building design. I just don't think that that is going to give accurate results. What I'd REALLY like is if someone did a wind tunnel test.

 

[SAIL3]

I would expect the net vertical wind load to be negligable and could be neglected....anyway, wind loads are approx values and would not justify designing to a knats-eyebrow in this case....however, the OP is correct that a boundary layer exists @ grade up to, say, 30' but the total height of the walkway enclosure is probably at most 10'....

 

[zcp]

ASCE 7 open structure will give you a net uplift that should put you in the ballpark. Chapter 27 in 7-10 (see Fig 27.4-4). Would probably use obstructed wind flow and use it as a net on the whole thing.

ZCP

http://www.phoenix-engineer.com

 

[canwesteng]

Is this a walkway like a bridge or an enclosed walkway like a gallery? If it's the latter, the structural commentaries for the national building code in canada have guidance on these, and the vertical wind load is quite small.