wind exposure
wind exposure
(OP)
What's the correct way to determine the wind exposure category of a building? The building code seems vague (big surprise). Other engineers in my office usually just design the building for exposure B. But I don't see anyone going out to the building site with a tape measure and walking 2600' in all directions (who determines the prevailing upwind direction? Do I need to hire a meterologist now???)checking that there are "numerous closely spaced obstructions having the size of single-family dwellings or larger." Also, what does numerous mean specifically??? I think it's easy to differentiate between exposure A and D. But how do you typically differentiate between B and C. I think C would be a safer choice unless you actually go to the site or get some aerial satellite photos (which I have never seen anyone do). But then you start getting complaints from the owner that the building is overdesigned. What do you think?






RE: wind exposure
I always get aerial photos of the site I'm designing on... its easy, get on GoogleEarth or FlashEarth. Comparing those photos with the photos in the commentary I've gotten a good feel for exposure categories.
If in doubt call the local building authority (which you should be doing up front anyway).
RE: wind exposure
Also keep in mind that some cities call for Exposure C regardless. In that case, it doesn't matter if you're in a downtown city center or not.
RE: wind exposure
Please don't be like some engineers and just spec C cause it's the more conservative easy approach. Spec what you feel is justified for the site (or required by the building department). I have seen projects where going from B to C has caused many thousands of dollars in cost increase for the structure.
RE: wind exposure
I agree with CFSeng regarding not being conservative.
I did a factory where we did not have the time to visit the site, so we assumed an exposed condition.
The client had already had a preliminary design (based on the actual site conditions) and there was a big issue on the increased cost.
Now with these aerial photographs on the web, you have no excuse.
RE: wind exposure
One option would be to use B as a default, unless it can be shown otherwise. The other would be to use C as a default, unless shown otherwise. For the types of projects that I've worked on, I would call the B route reckless as opposed to calling the C route conservative.
One last note: Look at the photos in the ASCE 7 commentary. The 3rd Exposure B photo (on page 251 in the 1998 version) even says that some of the buildings in the picture should be designed for Exposure C because of the clearing adjacent to the building. That seems like a pretty strong argument against the "default B" route.
RE: wind exposure
I do agree with CFSeng, we need not be conservative.
RE: wind exposure
Category 1 - same as Exposure D
Category 2 - same as Exposure C
Category 3 - same as Exposure B
Category 4 - City centres with high buildings
I often interpolate to category 2.5
RE: wind exposure
Very interesting thread. I am not conversant with ASCE 7 but as the thread is about buildings and exposure categories I just wondered whether the design should consider possible future situations where adjacent buildings may be demolished thereby changing the exposure. Surely it would be more applicable to consider exposure C (if this is more onerous) than category B in this instance. Buildings are not there forever!!!
RE: wind exposure
This is what we used to do for factories and commercial buildings in Australia.
Tomfh - unlike the Australian code, ASCE7 does not have different wind load factors for different wind directions. This is probabbly due to the prevalence of hurricanes and tornadoes in many regions.
RE: wind exposure
I must confess I rarely use anything but Md = 1.0
RE: wind exposure
Summarized per commentary in ASCE 7-05
exposure B's Kz corresponds to lower limit of the range of z0, whereas exposure C and D correspond to the typical value of z0.
The reason for the difference is that exposure B, which is applicable to suburban areas, often contains open patches, such as highways, parking lots, and playing fields... Using the lower limit of z0 helps account for this
so the demolition of a nearby building should not change its exposure from B to C
RE: wind exposure
I typically have designed buildings for the same exposure on all sides even where there is open water (Exposure D) on only one side.
RE: wind exposure
RE: wind exposure
Regarding future demolition, I would not expect one or two buildings to make that much of a difference, over a realtive short time period of a couple of years.
Don Phillips
http://worthingtonengineering.com
RE: wind exposure
That's correct. Mind you under service wind speeds water surfaces may be Exposure D, but that is just nit-picking.
RE: wind exposure