wind exposure

[broncosfan]

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?

 

[theonlynamenottaken]

Read the commentary from ASCE 7. C6.5.6 states, "A recent study has estimated that the majority of buildings (perhaps as much as 60% to 80%) have an exposure category corresponding to Exposure B". This commentary section also has photos of typical B, C & D exposure sites.

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).

 

[nutte]

The photos in the commentary are great in determining the exposure category. We typically use C, unless we're sure that something less is appropriate.

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.

 

[CFSEng]

I do what theonlynamenottaken says, use Google Earth. It's free and usually has pretty good pictures of the areas. They even have a measure tool so you can draw radial lines.

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.

 

[csd72]

Alternatively multimap.com has a photograph function that can be useful, I find this easier to use than google earth.

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.

 

[nutte]

I take issue with the "perhaps up to 60% to 80% should be B" comment found in ASCE 7. This may be true for buildings that are presently built, but I don't think you can extrapolate that to all new construction. Often new buildings are built in areas that are being developed, so they may be rural, desolate, void of surrounding buildings. I wouldn't be surprised at all if 60% to 80% of all buildings also are located in the 200 most populous cities in the country, which would place them near other buildings, impacting the exposure category.

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.

 

[gekay]

Its slways right to have a justification of our own for our buildings - taking due care in that justification. We can better use photos, google earth, or anything, and decide with concern about what they may reflect some years later, i.e. after the proposed building has become "old".

I do agree with CFSeng, we need not be conservative.

 

[Tomfh]

I agree that studying the sample photos and comparing to aerial photos is the way to go. I use the Australian wind code and we have categories 1-4 but the idea is the same.

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

 

[DSB123]

Hi All,
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!!!

 

[csd72]

A secondary question on this, using ASCE7, do people design to different categories in different directions?

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.

 

[Tomfh]

Tomfh - unlike the Australian code, ASCE7 does not have different wind load factors for different wind directions.

I must confess I rarely use anything but Md = 1.0

 

[CFSEng]

DSB123 - Keep in mind exposure B does account for some open areas, i.e. building nearby being demolished.

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

 

[jike]

I wouldn't think that the demolition of one building would make any difference but several buildings definitely could like urban renewal of a blighted area or perhaps where a hurricane wipes out most every building around except a few. Of course a hurricane most probably has tested those buildings to their ultimate limit.

I typically have designed buildings for the same exposure on all sides even where there is open water (Exposure D) on only one side.

 

[Robert216]

Our firm has always used the rational that an Exposure D will never exist along the coastline because in a high wind event, the waves generated from the wind will serve as obstructions. Any thoughts on this are appreciated.

 

[DonPhillips]

In central Ohio, this subject came up in a monthly building official's meeting about two years ago. A large subdivision is designed in a rural environment. The first house is exposure C, and perhaps the first few houses are C, but when is the first house designed to B? For our neck of the woods, the cost is not very high to design to C, since most trusses are designed to C anyway. The arguement comes in regarding when those houses will be built - if at all. So until those houses exist, exposure C. We have not had much pushback from the builder's.

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

 

[Tomfh]

Our firm has always used the rational that an Exposure D will never exist along the coastline because in a high wind event, the waves generated from the wind will serve as obstructions. Any thoughts on this are appreciated.

That's correct. Mind you under service wind speeds water surfaces may be Exposure D, but that is just nit-picking.

 

[FSS]

It also depends on what code you follow. Florida code has Exp B as default. ASCE 7 says Exp C is default. Florida also has exceptions to allow for Exp B to be used when area is being developed after originally being clear cut.