Exposure Categories in Rural Settings

[phamENG]

As I sit here in my home office and watch a logging company clear cut the several hundred acres next to my semi-rural home - converting my house's wind exposure from B to C in the process - I'm curious: how is everyone classifying wind exposure for rural houses in the woods?

I was trained to use what's there at the time of the design, citing the idea that we can predict everything that will happen to the house or the surroundings. That feels a tad hollow now. Granted, a lot of clear cutting operations result in developments that will maintain that exposure B. Mine won't (we don't have utilities down this road and the 15 year utility extension plan doesn't include my area, and everything here is classified as wetlands now anyway so development costs are too high without higher density neighborhoods or one offs on pre-existing lots), and it seems quite plausible that many others in rural settings won't. If somebody owns a large tract of land and wants to build a house on it, sure, but I'm starting to wonder if accounting for the trees is a bad idea for smaller lots in wooded, rural settings.

(Note: I'm in the Mid-Atlantic, and a large number of our forests are pine or have a decent quantity of pine in them, so they don't lose their foliage en masse.)

Any thoughts?

 

[DaveAtkins]

You can only design for what you know at the time of design. If you know there is a good chance the site will become Exposure C within the design life of the building, then design for Exposure C. Otherwise, design for Exposure B.

I think a lot of engineers assume Exposure C the vast majority of the time, unless they know the site is Exposure B or Exposure D.

DaveAtkins

 

[phamENG]

Thanks, Dave.

If you know there is a good chance the site will become Exposure C within the design life of the building, then design for Exposure C.

That's the crux of it. Unless you're designing next to a nature preserve (or similar protected space where large scale logging is prohibited), do you think it's reasonable to assume that the trees won't be there?

(I always research sites prior to design to verify exposure category. But ASCE 7 allows "wooded areas" in the Surface Roughness B classification.

 

[EngineeringEric]

pham, just make it all exposure D. and give it some additional capacity Plus, by the time that 50-year or 100-yr storm comes all those trees will be back right? you just need to worry for the 7-year return now

If I was your projects engineer, i would have likely done exp-B by the sounds of it

 

[phamENG]

Ha. Yeah. That'll work...right up until I give up wondering why my clients don't call me back.

And I agree - that's what I would have and have done several times in the past. Being suddenly faced with what may well be a relatively rare occurrence that could impact me personally has me questioning it, though.

 

[JStephen]

Well, it could be surrounded by subdivisions and shopping malls, they all get blown away or burned up in a nuclear attack or the next asteroid, and then you have the same problem, Exp. C again.
Keep in mind all those categories and wind speeds are based on probabilities, and that exposure category is subject to certain probabilities as well.

 

[phamENG]

Right. Because a nuclear holocaust that spares a single house is as likely as somebody selling the timber rights on the next parcel. I appreciate your attempt to contribute to the discussion, and my guess is that you're trying to say that you don't find it reasonable, but it's a bit of a stretch to get that out of your statement. A simple "no" would have sufficed, but I can relate to penchant for hyperbole.

The wind speeds are based on probabilities. All of our design work is based on statistics and probabilities. Probabilities of the expected load occurring. Probabilities of the material or workmanship being less than we expected. And it all gets boiled down to standard design loads and capacities. Are the odds slim? Sure. But they're high enough that we have a separate design category for them.

My question isn't "will the max wind speed happen", it's a question to judge the state of practice and what other engineers are doing. I was taught one way. Perhaps everyone was. Perhaps I'm the only one. Just trying to see where everyone is on a subject that, until today, I had taken for granted.

 

[EngineeringEric]

to expand on my little comment, i know my firm does exp B for non-water front almost always in our related areas. Even when in a forest. Never thought of when the forest goes away. The firms we compete (they all are a lot of initials, you know them all!) with also often use Exp B on their projects unless they are waterfront/oceanfront. We sometimes do their shops for contractors so we see their drawings on a regular basis.

I think we can defend it by saying either they will grow trees back, or you are getting a new subdivision near you or a new shopping center... something that will break up the wind. it won't be flat land for long a statistically significant length of time. And shoot, even if they left some trees, that 115 mph storm will just make those pines into projectiles!!!!

 

[phamENG]

Thanks, Eric. I had the same impression, but it's nice to get confirmation from somebody else in my market.

 

[canwesteng]

I don't think clear cutting a forest is all that likely - for most cottages etc, I think the forest is part of the appeal then residents would push back pretty heavily again clear cutting the whole forest. I guess this maybe only relevant in Canada where it's all crown land, but I'm sure there is some kind of process for getting approval before clear cutting a forest. At least the optics are better clear cutting forest where no one is living.

 

[JLNJ]

I have always seen the problem as the difference between B and C being too great. There is a temptation to use B (I’m looking at YOU, PEMB people). The actual definition of B is pretty tight but people want to use it anyway because of the cost savings.

On the other hand, C (used to be “open grasslands”) seems too penal. It seems to me that most sites are somewhere in between but there is no “in between” exposure category.

If phamEng specifies my proposed B++ exposure he might worry less about some adjacent trees being clear cut in the future.

 

[phamENG]

canwesteng - yeah, that doesn't really work around here. Very much a "my land my rules" sort of place. I think there may be an environmental review, but they'll be wrapping up a 100+ acre clear cut here in the next couple of weeks and there was no hearing, no notification to the neighbors, etc. A hunt club owns the land and the members didn't even know until the week the loggers arrived. Same thing happened with the 45+ acres on the other side of me 4 years ago. (I don't have such a large plot - I'm one of a string of 1-3 acre parcels tucked in between the big ones.)

JLNJ - B++...I like it. There's a procedure in the commentary of ASCE 7 to do just what you're saying. You can take into account the quantity and disbursement of surface roughness variations and come up with a more accurate adjustment factor, but it's pretty onerous on the engineering side for a single family residential fee.

 

[DaveAtkins]

I also like JLNJ's comment. It seems like ASCE could allow the engineer to interpolate between the Exposure B value for Kh (or Kz) and the Exposure C value.

DaveAtkins

 

[RWW0002]

As far as planning for the clear cutting goes, I agree with the commenters above. However, the use of exposure B for most all non-waterfront properties does not seem to be in line with ASCE-7. In rural areas (especially with any degree of farming), I find myself in Exposure C more often than not. The "Less than 1500 ft. from open field" requirements gets me quite often.

Just wondering, how many of you would apply Exposure C to all buildings in the following picture (even in the middle of the subdivisions)??

 

[DaveAtkins]

It would depend on which surface roughness predominates within the 1500'. It looks like Exposure B might be appropriate in the upper left corner of the photo.

DaveAtkins

 

[Aesur]

This topic is very interesting for me as I have been trying to teach engineers over the years that many times the buildings they are designing as Exposure B are actually Exposure C. The most recent version of ASCE7 Commentary has finally made it easier to defend my argument that open areas force projects to be C many times, and the size of open areas doesn't have to be all that large, a small parking lot can sometimes do it depending on it's location in reference to the building.

As for forests, I have always wondered the same thing, but was taught that you design for what is there at the present as you cannot predict the future use of land. I believe that the assumption is that they are going to only cut it down to develop it. If you live in a region where forest lands are used for Timber, I would be more likely to design as C knowing there is a good change it will be cut down in the future. Luckily I live in such an area where B is rare, except for the few isolated locations in the city where there are no "open areas" around, so we don't have to spend much time picking between the two classifications.

 

[RWW0002]

@ Dave - I agree that the foreground in the photo should be exposure C with the possibility that the background may have areas of B, however I routinely run across areas such as this and see designers using predominantly Exposure B. Truss designers, PEMB engineers almost always use Exposure B and will argue with EOR/Owner that anything more stringent is wasting the owner's money (undercutting the EOR in the process in many cases).

I also see other designers acting as EOR who are much quicker than I am to give a site Exposure B to sites that boarder field or open areas justified based on common practice of the past. ASCE 7 is pretty explicit on the issue, in both the code and the commentary, yet the practice still prevails.

While we have not seen widespread wind-related issues in my area in recent years, I feel like designers should be a bit more careful with using correct wind exposure, especially now that wind speeds have been lowered for a large majority of the interior of the country (ASCE7-16).

 

[RWW0002]

I meant to mention by the way that the image above is directly from the ASCE-7 commentary for Exposure C (based on the caption, all buildings in the photo are Exposure C).

Sorry to hijack the thread a bit.

Relating to the original topic, I will be more likely to be a bit less conservative on exposure category and future exposure potential when using the new ASCE7-16 windspeeds for sites in the interior of the country..

 

[phamENG]

RWW0002 - no worries. Hijack away. This was just a 'hmmm....' moment and I wanted to start a discussion/take a poll. Thanks for taking part. I thought I recognized the picture.

I live on an exurban/rural border - nothing but electricity and cable on my road, and spots nearby that didn't even have cable until a couple years ago, but 20 minutes one direction is the CBD of one of the largest urban regions in Virginia and 20 minutes the other way is the absolute middle of nowhere. So I deal with all exposure categories regularly. We have B for in town, and C for the swamps, flats, and farms, and D on the coasts.

I think I'm going to draw a line at the utility boundaries. Where water/wastewater infrastructure exists, selling timber and then developing the land is the profitable choice. Where it isn't, development is limited to too-sparse arrangements and so replanting is likely - which leaves you with 7-15 years before you get back to anything like what B is looking for. I know I can't design for every eventuality, but I do think it's important to consider the most likely circumstances the structure will face during its reasonable design life. With this, I'll be in line with the SOC for B, and perhaps a little conservative where I opt for C in the middle of a forest.

 

[RWW0002]

Your approach makes sense to me. Good luck fighting the fight if a delegated design is involved though..

I practice in several rural areas or small town bordering rural areas and push for exposure C whenever I can. As far as future exposure in these areas it is more likely to go from C to B than vise-versa.

I posted the photo without the caption intact because I feel like there are plenty of engineers that would give exposure B to the buildings in the foreground and it sounds like it is common practice in many areas. In the past I think many engineers justify that the exposure category is likely to become B soon into the life of the structure as the area develops. My point is that with the drop in wind speed we are loosing some built in "fluff" in wind loads and I feel like some extra diligence in exposure category is merited for wind design post ASCE7-16.

Here is the photo, caption included.