Wind Exposure Categories 2

[Swish72]

My coworkers and I have been having several long and involved discussions on the Wind Exposure Categories described in the 2006 IRC. Several questions have arisen out of what we feel are "grey areas". We understand that the code refers to ASCE 7 under certain circumstances, but these questions strictly involve the IRC.

Table R301.2(1) is the Climatic and Geographic Design Criteria table which should be established by the local jurisdiction. There is a column for Wind Speed but not for Exposure Category, however the referenced note "d" states: "The jurisdiciton shall fill in this part of the table with the wind speed... Wind exposure category shall be determined on a site-specific basis in accordance..." It is implied that the governing municipality decides the appropriate exposure category. Question #1) Does the EOR have the right to challenge that decision?

Let's say a subdivision under construction would definitively be categorized as exposure B on three sides; east, south and west. The category from the north would be C. Question #2) Would you design all aspects of the MWFRS to resist a C type exposure?

Suppose you're engineering these residences from north to south. Question #3A) Assuming the first row of houses has been designed for a C exposure, is there a point in this development where the residences can be designed for a B exposure? Question #3B) Where is that breaking point located?

Let's say the builder decides to construct their first house in the very middle of this subdivision which, when ALL the houses are constructed, would definitively be a B exposure. Question #4) Is it permissible to design that house as a B exposure even though it currently is not?

We've got our own interpretations that sometimes conflict, but we'd like to hear from you. Thanks in advance for your responses.

 

[CJJS]

You are actually getting paid to do a lateral design for a residence? That is a rarity in my region.

 

[msquared48]

You mean you do them for free???

Mike McCann
McCann Engineering

 

[CJJS]

Ha ha. It's rare that anyone pays to have it done. Residences don't require a professional engineer to design. If it is done, usually only gravity loads are looked at.

 

[ctcray]

Some questions to ask yourself:

Is doing 3 directions for B and 1 direction for C cost effective? Might this house be rotated 90 degrees from it's original footprint on the lot, requiring a redesign? Might the same house be built elsewhere in the neighborhood where the exposures will be different from the first house? Are you going to save the builder that much money by doing some parts for C and others for B in the same house?

Since the house is being "engineered" it's probably already going to cost more to build than a prescriptively built house. How much more is doing the entire structure as Exposure C gonna cost? (probably not a lot)

 

[Swish72]

ctcray...Apparently you've missed the point of this post entirely. The questions are purely theoretical in nature designed to ignite a discussion. How we actually design the structure is not part of the issue at hand. I challenge you and everyone else to provide a thoughtful and appropriate response.

 

[ctcray]

Question #1) Does the EOR have the right to challenge that decision?

YES.

Let's say a subdivision under construction would definitively be categorized as exposure B on three sides; east, south and west. The category from the north would be C. Question #2) Would you design all aspects of the MWFRS to resist a C type exposure?

YES. especially after considering the questions i posed above.

Question #3A) Assuming the first row of houses has been designed for a C exposure, is there a point in this development where the residences can be designed for a B exposure?

Probably, but why bother.

Question #3B) Where is that breaking point located?

See answer to #3a

Let's say the builder decides to construct their first house in the very middle of this subdivision which, when ALL the houses are constructed, would definitively be a B exposure. Question #4) Is it permissible to design that house as a B exposure even though it currently is not?

NO. Due to the downturn in the housing market, the builder stops building any new homes in the neighborhood and leaves the houses in the middle designed for B left exposed to C.

Sorry my first response wasn't thoughtful enough.

 

[Ken5]

1.) Can EOR challenge the Bldg Official(s)? Yes

2.) Say a house is in middle of development and has initial C exposure to north. When development is completed then B exposure in all directions. If there is a "reasonable degree of certainty" (as the deposing attorney will say) that the development will be completed shortly, then exposure B all directions could be considered. But Building Official has final say.

3. As a fenestration supplier to upscale housing projects, we sometimes pay careful attention to exposure category mix. Say back of house faces a large lake (exp C) while other directions are exp B. A large fenestration assembly facing lake has exp C positive and exp B negative component & cladding wind loads. That's important if IRC, Figure R301.1(7), wall corner area (5) applies.

Good discussion ctcray!

 

[rday]

Swish72

1) Yes, but don't expect to prevail easily. You are asking the AHJ to allow you to violate adopted law.

2) Yes, but I like to keep things simple.

3A) Probably, but one is depending on construction sequencing of a sub-division for load reduction. I don't get warm fuzzies from it.

3B) Again, we need construction sequencing and maybe a wind tunnel. Again, no warm fuzzies.

4) NO. I would never count on a plan that may never come to fruition to reduce loads on a building I am designing today. It may be years before the subdivision is built out. What if your house gets built and a month later the wind storm comes along. Will your house, built to a lesser standard, survive? I wouldn't gamble my seal on it.