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. www.iccsafe.org

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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.

CDC Campaign On Preparing for a Zombie Apocalypse

CDC Campaign On Preparing for a Zombie Apocalypse

CDC Zombie Campaign for Emergency Preparedness

CDC Zombie Campaign for Emergency Preparedness

One of the highlights of the conference was a viewing of the Center for Disease Control’s unique Zombie Apocalypse campaign.  The idea is that preparing for disaster is very much like preparing for a zombie attack.  While it was fun, it was also educational because it tied into FLASH’s mitigation methods too.

A very interesting panel on the first day of the conference dealt with how members of each generation handle things differently.  A panel of Baby Boomers and Millennials (Generation Y) highlighted the fact that different generations had different ways of doing the same thing.

See the link below for an interesting summary of Baby Boomers, Gen X and Gen Y:

Generations Overview copy

One highlight of the first afternoon of the conference was the RenaissanceRe Challenge.  The challenge involved two teams of university students who had a chance to present their idea for projects to help Florida citizens confront wind events. The winning team of the challenge would receive a $20,000 scholarship.  The team from Florida International University presented their idea for “Aerodynamic Intelligent Mitigation.”  This consisted of a retrofittable element that is placed around the edges of a roof that has been proven to reduce uplift forces on the roof.  Meanwhile, the team from the University of Florida, the Miti-Gators, countered with their idea for a cell phone “app” that would evaluate the wind resistance of a house.  The idea would be that any prospective homebuyer could evaluate a home they were considering purchasing to see how wind resistant it would be compared to other homes.  This is important because in Florida, 70% of homes were built before the Florida Building Code was adopted.  In the end, the Miti-Gators won the scholarship.

Being a building-code nerd, the definite highlight of the second day was the panel discussion about building codes. Here are some of my favorite key takeaways:

Codes:

  • The building code is the first opportunity to ensure that buildings are built properly.  Mitigation is the second opportunity.
  • Legislators need to mandate adoption, consumers need to demand use, enforcers need to ask for adequate funding, and builders need to understand.
  • So little attention is given to building departments and enforcers, more attention is given to the fire department and local police.
  • Building codes originated due to disasters.  We need to change the purpose of codes from safety to resiliency.  We need to worry about what will happen to the building stock for years to come, not just the first cost today.
  • The insurance industry has found that where codes are scientifically sound, consistently enforced, and implemented across communities, the return in investment in building codes is large.
  • Building codes are one of the cornerstones of effective mitigation.  There are not many other places where you can make a change that will have an effect in perpetuity.
  • Building codes are an expression of society.
  • Building code adoption is the battle between making buildings safe and the increased costs to the builder and developer.  The builder does not get the long-term benefits that the owner does.

Politics:

  • The 800 pound gorilla is public policy:  The risks people face are being masked by pandering politicians.  Public policy interferes with the message that people need to understand their risk.  The way to help people understand their risk is through the marketplace setting rates for insurance.
  • A distorted market deprives consumers of proper pricing signals that can encourage dangerous behavior.

Preparedness:

  • Preparedness will not become a part of society until it is profitable
  • The difference between natural hazards and natural disasters is that nature causes the hazard, and human behavior causes the disasters.
  • Education of the public is the answer.  Owners must be taught to overcome the thought that “This natural disaster MIGHT happen, but I know that if I spend the money to prepare now it WILL cost me, so I’ll take the risk.”
  • People need to understand that “Disasters don’t just happen to the other guy.”

Finally, in the conclusion, “Building codes work; they save lives; stay the course; we’re making a difference.”

I couldn’t have said it better my self.

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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