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.

SE blog 1

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

SE blog 3

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.

Changes in IBC from 2009 to 2012: Seismic Design

The transition from one building code to the next always begs the question, “how is the newer code different?” There are several changes between the 2009 IBC and 2012 IBC that will change the way designers approach seismic design. This blog post is a broad overview of some of the changes. Since it’s not practical to cover all the changes between the previous and new codes in detail in one post, the discussion will be mainly on 2012 IBC and the corresponding ASCE7-10 reference standard.

Seismic ground motion map

Seismic ground motion map

The seismic ground motion maps have been updated to match ASCE7-10. The titles of the maps in IBC were revised from “Maximum Considered Earthquake Ground Motion” to “Risk-Targeted Maximum Considered Earthquake (MCER) Ground Motion Response Accelerations” in order to reflect the titles in 2009 NEHRP and ASCE 7-10. As in previous editions, some areas will prove difficult to read due to the contour lines, so the USGS site and GPS coordinates are recommended (http://earthquake.usgs.gov). Additional information about changes made for 2009 NEHRP is available at www.nibs.org or www.bssconline.org.

The term “occupancy category” was replaced with “risk category” in the 2012 IBC for consistency with the term used in ASCE 7-10. This change was made because it was decided that the use of the word “occupancy” implied the category was directly tied to occupancy classifications in the code, while the word “risk” more accurately communicates that the category is based on acceptable risk of failure.

Seis-pic 2ASCE7-10 revised the way designers use the corresponding Drift amplification, Cd, and Overstrength factor, Ωo, of the Response modification factor, R.  In ASCE7-05, when there is a vertical combination of different R-values, the Cd, and Ωo cannot decrease as you go down each level of a building. In ASCE7-10 (12.2.3.1), the Cd and Ωo always correspond to the R-Value as you go down. The adjacent figure illustrates the new provision to use the corresponding Cd, and Ωo with the R-value at each level.

ASCE7-10 (12.3.4.1) added a clarification for out-of-plane anchorage forces where the redundancy factor, p = 1.0.  The intent of the redundancy factor was to ensure the vertical seismic-resisting system with insufficient redundancy had adequate strength. The design forces for out-of-plane wall loading are not redundancy requirements. ASCE7-10 (12.11.12) revised the out-of-plane wall anchorage force equation where the anchorage forces are reduced for shorter diaphragm spans.

Seis-pic 3Light-frame construction structures are no longer exempt from amplification of accidental torsion in ASCE7-10 (12.8.4.3).  There are many structures vulnerable to torsional effects including some “tuck under” parking buildings that are often light-frame structures. See posts  titled Soft-Story Retrofits and City of San Francisco Implements Soft-Story Retrofit Ordinance for more discussion of soft-story, light-frame buildings.

This is just a brief summary of changes related to seismic design found in the 2012 IBC.  What are other changes that will modify your approach to seismic design?

Use of Holdowns During Shearwall Assembly

When designing a shearwall according to the International Building Code (IBC), a holdown connector is used to resist the overturning moment due to lateral loading.  From a structural statics point of view, a shearwall without dead load or holdowns would have zero lateral-resisting capacity without any restraint to resist the overturning moment. Since the wall assembly still has the sill plate anchorage providing resistance to overturning, testing can measure the capacity of a wall assembly without holdowns.

We have performed multiple tests comparing the performance of a shearwall with and without holdowns.  Diagrams of the test setups are provided in Figure 1 below.

Figure 1: Shearwall test setups

Figure 1: Shearwall test setups

The top of wall was attached to the actuating ram using a steel channel and fastened to the double top plates with 3” SDS screws. The ram pushed and pulled the top of wall according to the CUREE test protocol.

No Holdown Wall:

A wall assembly without holdowns can only rely on the wood sill plate members, sill plate anchorage and sheathing to resist the overturning force.  The two limit states commonly observed in the test: 1) The sheathing fasteners prying the sill plate in cross-grain tension. (see Figure 2)  2) Fastener tearing the sheathing at the sill plate. (see Figure 3) Little damage was observed between the sheathing and end post along the height of the post.  Figure 4 is the load vs. displacement graph showing the peak load, 928 lbs., at relatively small displacement, 1.57”.

Sill plate split

Figure 2: Sill plate split

Figure 3: Fastener tearing through sheathing

Figure 3: Fastener tearing through sheathing

Performance of shearwall without holdowns

Figure 4: Graph showing performance of wall without holdowns

Wall With Holdown:

The change in restraining the end posts increases wall stiffness, capacity and ductility of the assembly.  The peak load was 2,907 lbs. at a displacement of 2.3”.  (see Figure 5)  The use of a holdown to restrain the post and engage additional sheathing fasteners minimized cross-grain tension on the sill plate compared with the test without holdowns. (see Figure 6) The increase in both strength and ductility comes from the additional number of fasteners engaged along the height of the post when the post is restrained. (see Figure 7) The assembly with holdowns was able to achieve approximately three times more strength compared with the same amount of material used without holdowns.  Ductility also increased substantially, which can be observed from illustrating the hysteresis curves of both tested assemblies for comparison. (see Figure 5)

Figure 5: Wall with holdowns compared to performance without holdowns

Figure 5: Wall with holdowns compared to performance without holdowns

Figure 6: Post restrained by holdown minimized cross grain tension on sill plate

Figure 6: Post restrained by holdown minimized cross grain tension on sill plate

Figure 7: Sheathing pull through along height of post

Figure 7: Sheathing pull through along height of post

 

 

 

 

 

 

 

 

 

The comparison between the two walls is based on a 4 foot wide by 8 foot tall configuration.  A wall with a different aspect ratio may change the performance, but walls with holdowns will achieve higher loads, and lower displacements, and more ductile performance.

What are your thoughts about shearwall assembly? Let us know in the comments below.

New Holdown Requirements for the IRC® and IBC® Portal Frame Bracing Method

The IRC® contains several different narrow bracing methods that are made up of portal frames. One method that is useful if you are using intermittent wall bracing is the Method PFH Portal Frame with Holdowns. This method relies on low-deflection holdown anchorage at the bottom, and substantial nailing at the overlap of the sheathing and the header at the top to prevent overturning of the narrow panel. An identical method for use as wall bracing is in the Conventional Construction section in Chapter 23 of the IBC®. These portal frames were first included in the 2006 IBC and IRC.

Method PFH- Portal Fram With Holdowns

Method PFH- Portal Fram With Holdowns

The method was originally tested with straps clamped to a steel test bed to simulate the embedded holdown straps. The straps were nailed to the wood with enough nails to mimic a 4,200 lb. strap anchor. The original test report is APA T2002-70. At that time, the Simpson Strong-Tie® STHD14 had a published allowable load in excess of 4,200 lbs. based on then-current Acceptance Criteria, so hardware was available to construct this frame throughout the country. However, in 2008, ICC Evaluation Service developed a new acceptance criteria for embedded connectors, AC398, Acceptance Criteria for Cast-in-place Cold-formed Steel Connectors in Concrete for Light-frame Construction. This was in response to the changes in ACI 318 for anchors in concrete. When re-tested and evaluated using the new Acceptance Criteria, the allowable load for STHD14 was reduced below 4,200 lbs. for use in buildings designed for Seismic Design Categories C through F.   The same thing happened to other manufacturers’ embedded holdown allowable loads. That made it impossible to properly construct this bracing method in those areas. In response to this, Simpson Strong-Tie worked with APA, the Engineered Wood Association, to design a new test that would determine if a lower capacity holdown could be used with this portal frame method.  APA performed the testing at their Tacoma, Washington testing lab. Since the initial testing of the portal frames with the 4,200 lb. holdown was performed using the outdated SEAOSC protocol with an older testing rig that used a stiff beam above the wall, both the old tests with a simulated 4,200 lb. holdown and new tests with a simulated 3,500 lb. holdown were rerun in accordance with the current ASTM E2126 test method using the CUREe protocol. The test was published as Test Report T2012L-24. The tests showed little to no effect of reducing the holdown from 4,200 lbs. to 3,500 lbs. allowable load. Here is one of the graphs of the backbone curves comparing the two assemblies for a 16-inch wide, 10-foot tall portal frame.

Comparison graph of two assemblies for a 16-inch wide, 10-foot tall portal frame.

Comparison graph of two assemblies for a 16-inch wide, 10-foot tall portal frame.

With the testing complete, APA prepared and submitted code changes to both the 2012 International Building Code® and 2012 International Residential Code®. The IBC proposal is S291-12, and can be found on page 605 of the 2012 Proposed Changes to the International Building Code – Structural. The IRC proposal is RB311-13, and can be found on page 613 of the 2013 Proposed Changes to the International Residential Code-Building. With support from Simpson Strong-Tie, both of the proposals were approved. So in the 2015 IRC, bracing method PFH will require an embedded strap-type holdown with a minimum capacity of 3,500 lbs. instead of 4,200 lbs. The same will hold true for the Alternate Braced Wall Panel Adjacent to a Door or Window Opening bracing method in the 2015 IBC. APA also re-tested the portal frames with only two sill plates instead of three. This will allow the use of a 5/8” by 8” Titen HD® anchor as a retrofit anchor bolt. What are your thoughts? Let us know in the comments below.

Know Your Code

I attended a CFSEI and Steel Framing Alliance webinar last week entitled Specifying Cold-Formed Steel: Finding and Avoiding Pitfalls in Structural General Notes and Architectural Specifications. The presenter was Don Allen, P.E., from DSi Engineering, LLC, and he focused on issues specifically related to design and specification of cold-formed steel (CFS) in contract documents.

The first post I ever wrote for this blog was But I Don’t Design Cold-Formed Steel… I talked about how limited my initial experience was with cold-formed steel and how I was forced to learn it on the job when projects required it. During the webinar, I winced a few times recalling my first CFS project when Don mentioned why you should not do certain things — and they were things I used to do.

1997 UBC

1997 UBC

Referencing the “most current edition” of a standard was something I remember doing in our general notes, and the webinar mentioned why it is important to verify that specified reference standards are correct for the governing building code for the project. I first designed under the 1994 Uniform Building Code, and then used the 1997 UBC for many years after that. The Uniform Building Code was almost self-contained in that it covered gravity, seismic, and wind load requirements in Chapter 16, and then each of the material chapters had most of the design requirements in the code.

A significant change in the International Building Codes has been removing many of the design requirements and simply referencing the appropriate design standards. Whereas the UBC had methods for calculating wind loads, the IBC simply refers you to ASCE 7 for wind loads. Similarly, Chapter 19 of the 1997 UBC had many pages of concrete design requirements. Now, the 2012 IBC has just a few pages referencing ACI 318 and then makes several amendments to it.

Continue reading

Do 50 Kip Wood Construction Connectors Exist?

The 2009 IBC Section 1604.4 states, “Load effects on structural members and their connections shall be determined by methods of structural analysis that take into account equilibrium, general stability, geometric compatibility and both short and long-term material properties.” This requirement applies to a 200 pound handrail connection as well as a 50,000 pound glulam connection.

Hanger installation at jobsite

50 kips is not a typical beam reaction in wood framed construction, but we’ve received some recent requests to design higher capacity hangers for use in wood podium decks for mixed-use structures. Although post-tensioned concrete is most commonly used for this application, the use of heavy timber for podium decks is driven by the benefits of wood: sustainability, saving construction time and money, architecturally attractive, long-term energy savings, light construction material, and performance in earthquakes.

Continue reading

CFS Framed Shear Walls – A Code History

CFS Framed House

In a previous blog post, I talked about the challenges engineers may face when designing cold-formed steel and some resources available. When designing a building to the current building code, it can be helpful for engineers to understand the history of the different code requirements. This week I will discuss the code development history of CFS framed shear walls.

Prior to the 1997 Uniform Building Code (UBC), there were limited code provisions for design of cold formed steel-framed shear walls. The 1994 UBC had seismic R-factors for light-framed walls, but little else with respect to design or detailing. Code provisions were introduced in the 1997 UBC that included:

Continue reading