ASCE 7-16 Wind Exposure B Vs. C

[Robcat71]

Hi All,

I am working on a school project that has some athletic fields around the perimeter of the building site, but for the most part is surrounded by housing developments and wooded areas. You can get an idea of the distances/scale from the football field and track shown on the attached photo. The red markups are where the new additions and structural work for the project are located on site. I understand that the default is to use the worst case scenario, but there will be a significant impact on the project using the higher wind pressures for exposure C and I'm trying to avoid it. I Personally feel comfortable calling this exposure B, but there are some conflicting opinions within our office.

Thanks in advance for your expertise!

 

[Luceid]

There are some great photos in the ASCE 7-16 commentary (Figure C26.7-5(a), for example) and some of them seem relatively similar to the scenario you have. I'm no expert in wind however, so I'll cede to others comment on their experience.

 

[flight7]

As long as you're surrounded by houses and woods for 2600 feet from the project, I would feel comfortable with Exposure B. It's unlikely that extensive housing will revert back to clear fields (exposure C) within the lifetime of your building.

 

[phamENG]

For C&C, code requires you to use worst case. For MWFRS, code requires you to use worst case OR develop direction dependent wind loads. So if one of those fields qualifies as exposure C, you have to consider C from that direction.

This is not a subjective thing. ASCE 7 lays out distances and terrain descriptions really clearly. The commentary even gives you methods of estimating factors for mixed terrain types. If the numbers say it's C, then it's C.

I'm gong to go back a little bit on the not subjective statement. The patch of trees next to the school - what's the deal with that? Clearing of those trees - and not building a development - can easily turn it from B to C. And I think we do have a responsibility to consider that in some areas where periodic clear cutting of forests is done to harvest timber, leaving the buildings on the edge of these developments and previously wooded areas exposed to significantly higher wind loads for 5-15 out of every 20-30 years (depending on the height of the house and the species of the tree).

 

[Aesur]

As others have said the commentary does a great job in ASCE7-16 explaining how to actually calculate if it's B or C. One local jurisdiction actually requires this calculation if you use B. What you want to look at is open areas, I believe you may find that you are C for 3 quadrants. Also consider that there are height requirements, sometimes a school may be taller than surrounding development and therefore may not meet the requirements of B, this one I rarely have dealt with, so don't specifically recall much about that.