Changes to 2012 IBC for Wind Design

The Greek philosopher Heraclitusis credited with saying “The only thing that is constant is change.”

If that applies to building codes, then it applies doubly to wind design using the 2012 International Building Code® (IBC).

The wind load requirements in Section 1609 of the IBC are based on ASCE 7 and refer to this document for most design information. In the 2012 IBC, the referenced version of ASCE 7 changed from the 2005 edition to the 2010 edition. In ASCE 7-10, the wind design requirements have been completely revised, including a complete design philosophy change.

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Wind design has changed from an allowable strength-based philosophy with a load factor of 1 in the ASD load combination to an ultimate strength design philosophy with a load factor of 1 in the strength design load combination. This means wind design has a similar basis as seismic design. So the new load combinations for wind look like this:

Strength Design: 0.9D + 1.0W
Allowable Stress Design: 0.6D + 0.6W

Because of the change in load factor and philosophy, the basic wind speed map had to be altered. In the past, one map was provided and the design return period was increased for certain occupancies by multiplying the load by an importance factor. In ASCE 7-10 there are three maps provided so now an importance factor is no longer needed. The return period of the map depends on the risk to human life, health and welfare that would result from the failure of that type of building. This was previously called the Occupancy Category, but it is now called the Risk Category.

Risk Category III and IV buildings use a basic wind speed map based on a 1,700-year return period. Risk Category II buildings use a basic wind speed map based on a 700-year return period. And Risk Category I buildings use a basic wind speed map based on a 300-year return period. Because of the higher return period, the mapped design wind speed will be much higher than when using previous maps. However, with the lower load factors, actual design loads will be the same or in many areas lower due to other changes in the way the map was developed.

wind map
Excerpted from the 2015 International Residential Code; Copyright 2014. Washington D.C.: International Code Council. Reproduced with permission. All rights reserved.

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Another change to ASCE 7-10 for wind design is that Exposure D is no longer excluded from hurricane prone regions; so buildings exposed to large bodies of water in hurricane prone regions will have to be designed for Exposure D.

Because of the change in wind speeds, there is a change in the definitions of windborne debris regions. Due to the different wind speed design maps, the windborne debris region will be different depending on the Risk Category of the building being built. The windborne debris region is now defined as areas within hurricane-prone regions that are either within 1 mile of the coastal mean high water line where the ultimate design wind speed is 130 mph or greater; or any areas where the ultimate design wind speed is 140 mph or greater; or Hawaii. Risk Category II buildings and structures and Risk Category III buildings and structures (except health care facilities), use the 700-year Risk Category II map to define wind speeds for the purpose of determining windborne debris regions. Risk Category IV buildings and structures and Risk Category III health care facilities use the 1700-year return Category III/IV wind speed map to define wind speeds for the purpose of determining windborne debris regions.

Finally, a new simplified method for determining wind loading on ENCLOSED SIMPLE DIAPHRAGM BUILDINGS WITH h ≤ 160 ft has been added to ASCE 7-10. This is different from the simplified all heights method in the IBC, so it will be interesting to see which method becomes more widely used. Which method do you prefer? Let us know in the comments below.

Educated in a FLASH, Part 2

This week’s blog was written by Branch Engineer Randy Shackelford, P.E., who has been an engineer for the Simpson Strong-Tie Southeast Region since 1994. He is an active member of several influential committees, including the AISI Committee on Framing Standards, the American Wood Council Wood Design Standards Committee, and the Federal Alliance for Safe Homes Technical Advisory Committee. He is vice-president and member of the Board of Directors of the National Storm Shelter Association. Randy has been a guest speaker at numerous outside seminars and workshops as a connector and high wind expert. Here is Randy’s post:

In my last blog post, I gave an overview of FLASH, the Federal Alliance for Safe Homes, and how Simpson Strong-Tie partners with them. Last November, FLASH held their Annual Conference.  The theme of this past meeting was “15 Years of Stronger Homes and Safer Families,” and it was one of their best conferences yet.Continue Reading

Designing Steel Roof Deck Attachment for Combined Tension and Shear

In last week’s post I made a reference to California’s golden sunshine. Californians may have to deal with wildfires, earthquakes, and wearing sunscreen year round, but we generally don’t have high wind to worry about. In a previous blog post, Roof Deck Design Considerations for High Wind Events, I discussed some of the general challenges in designing for wind uplift. This week, I wanted to get a little more specific and discuss the standards applicable to steel roof deck attachments.

Historically, procedures for design of structural connections that utilize fasteners recognize the importance of combined loading effects on structural performance. Whether bolts or screws are the connector choice, the metallic structure of steel alloys includes transverse slip planes of relatively low resistance. This results in yield criteria (function that defines the onset of yielding or fracture) that are often limited by load-deformation performance along transverse planes.

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