IBC Wind Load Criteria

[BrentL]

I work at a consulting firm in south Mississippi where, due to the sudden demand, we are just starting to get involved in some residential construction.

The International Building Code (R301.2.1.1) specifies that in areas where the basic wind speed exceends 110 mph (in our area it is 150 mph) construction shall be designed in accordance with one of five referenced standards, one of which is the Southern Building Code's Standard for Hurricane Resistant Residential Construction (SSTD 10).

Many of the tables in SSTD 10 show differing requirements based on wind speed. The wind speeds given are 90, 100, and 110 mph. Since the IBC is telling me to design in accordance with SSTD 10, I read this to mean that I use the wind speed given by the table in SSTD 10, which is 100 mph for my location. Otherwise, IBC is telling those of us who live in areas with winds over 110 mph to use a code that only allows for wind loads up to 110 mph.

Some clarification would be appreciated.

 

[eng02002]

Why don't you use ASCE 7. To me, it is more straight forward.

 

[JAE]

Does the IBC apply to your residential? In most cases, you would use the IRC.

 

[BrentL]

I typically use ASCE7 for determining all my loads. But there are certain requirements like anchor bolt spacing, etc. that are directly specified by code.

JAE, I was actually referring to the IRC when I mentioned the IBC. Sorry for the confusion!

 

[JAE]

I believe that the SSTD 10 you refer to is using the Fastest Mile Wind (which in the older codes went up to 110 mph at the bottom tip of Florida.

The IRC uses the 3 second gust speed which for Mississippi runs between 140 and 150 on the coast.

Table 1609.3.1 shows how the two relate. 110 FM wind is equivalent to 130 3-sec gust. If you are 150 mph (3 sec gust), the FM wind is 130 mph. If the SSTD 10 doesn't go up to 130 mph, then perhaps you need another standard to work from. I'm not that familiar with SSTD 10 so can't help beyond this.

 

[mjl23]

I don't think it's a question whether or not to use ASCE7. ASCE7 is for determining loading.

My understanding, as JAE eludes to, is that Table 1609.3.1 was added to correlate the windspeeds in order to use the prescriptive methods like SSTD. SSTD, WFCM, and the other manuals have prescriptive guidelines for acceptable designs. Some of the prescriptive methods have limitations on design speeds so you may not be able to use them (see the notes in the code for the exceptions when the prescriptive methods can be used).

So back to your specific question, the code is not telling you that you must use SSTD. The code is giving you an OPTION to use SSTD. It sounds like the wind loads exceed the exception limits for the use of SSTD in your case.