The U.S. Resiliency Council (USRC) recently launched its Building Rating System for earthquake hazards. The Rating System assigns a score of from one to five stars for three building performance measures: Safety, Damage (repair cost) and Recovery (time to regain basic function).
This first-of-its-kind building performance rating is based on decades of earthquake engineering research and observations of earthquake damage and recovery. It will become an important component of future sustainable and resilient community goals. The USRC will expand its building performance ratings to include other natural hazards such as hurricanes, tornadoes and floods in the coming years.
With the USRC rating system, users will receive reliable and consistent information about a building’s expected performance during an earthquake and the estimated speed of its recovery afterwards. They can use this information to help them make decisions about purchasing or leasing buildings in which they live, work or invest, or about financing or insuring these buildings. The USRC Rating System also allows businesses and communities to plan and prepare for disasters by giving them data on the likely performance of their building stock. With the support of its Sustaining Members, the USRC will play an important role in long-term strategic capital and disaster recovery planning for communities and businesses. With the USRC Rating System, owners can specify the desired level of performance for their important facilities, to ensure that they not only survive, but also continue operations after a disaster in accordance with their expectations.
The steps to obtain a USRC Verified or Transaction Rating are as follows.
Select Rating type – The building owner or building jurisdiction determines the desired USRC Rating: Transaction or Verified.
Select Certified Rating Professional – The building owner selects and contracts with a USRC Certified Rating Professional (CRP) to complete a seismic evaluation of the building. Owners can search for CRPs and see what the requirements are for individuals to be USRC-certified at www.usrc-portal.org.
Perform detailed evaluation and determine preliminary Rating – The CRP performs a seismic engineering evaluation of the subject building using one of the USRC-approved evaluation methodologies, which include ASCE 41 and FEMA P-58, and translates their findings into a three-dimensional rating using the USRC translation matrix. A simplified version of the translation matrix for an ASCE 31/41 assessment is shown below:
Submit evaluation and Rating to USRC – The CRP’s evaluation report, proposed Rating and application fee are submitted to the USRC along with a request for either a Transaction or a Verified Rating.
USRC performs review and issues Rating – The USRC reviews the submission for completeness. The USRC will then either issue a Transaction Rating certificate, or one of its USRC Certified Rating Reviewers will perform a technical review before issuing a Verified Rating certificate.
Becoming a USRC Certified Rating Professional or Reviewer:
The minimum requirements for becoming a USRC Certified Rating Professional include an educational background in structural engineering, five years of relevant building evaluation experience as a licensed Professional Engineer and professional references.
The minimum requirements for becoming a USRC Certified Rating Reviewer includes either holding a Structural Engineering license followed by five years of relevant experience, or a Professional Engineering License followed by 10 years of relevant experience.
Details of the application process and other requirements for certification are provided on both the USRC website, www.usrc.org,and the USRC portal, www.usrc-portal.org. The cost to become a USRC Certified Rating Professional or Reviewer is $600 for individuals with a $100 annual renewal fee. Individual and corporate members have discounts on certification.
The U.S. Resiliency Council is a growing 501(c)(3) nonprofit organization with the vision of a world in which building performance in earthquakes and other natural hazards is better understood by building owners, tenants, financial institutions and communities. Corporations, organizations and individuals who are stakeholders in the built environment and who have a passion for improving the resiliency of our nation, have the opportunity to support the USRC through sustaining memberships. With the help of its sustaining members, the USRC will encourage:
Increasing market demand for better-performing buildings
Fostering collaboration among diverse stakeholders and technical experts
Promoting integrity, stability, consistency and transparency of rating systems
Educating and advocating for safe buildings and a better public understanding of building performance
USRCs members include many of the largest and most respected professional A/E firms and engineering professional societies in the country. Membership is open to all companies, individuals, communities and other stakeholders in the built environment. Information on joining the USRC can be found at the USRC website www.usrc.org.
What contributions from engineers are necessary to help create more resilient communities? Let us know in the comments below.
Over the weekend, I had the pleasure of watching my daughter in her cheer competition. I was amazed at all the intricate detail they had to remember and practice. The entire team had to move in sync to create a routine filed with jumps, tumbles, flyers and kicks. This attention to detail reminded me of the new ratcheting take-up device (RTUD) that Simpson Strong-Tie has just developed to accommodate 5/8″ and ¾” diameter rods. The synchronized movement of the internal inserts allows the rod to move smoothly through the device as it ratchets. The new RTUDs are cost effective and allow unlimited movement to mitigate wood shrinkage in a multi-story wood- framed building. When designing such a building, the Designer needs to consider the effect of shrinkage and how to properly mitigate it.
Shrinkage is natural in a wood member. As moisture reaches its equilibrium in a built environment, the volume of a wood member decreases. The decrease in moisture causes a wood-framed building to shrink.
The IBC allows construction of light-framed buildings up to 5 and 6 stories in the United States and Canada respectively. Based on the type of floor framing system, the incremental shrinkage can be up to ¼” or more per floor. In a 5-story building, that can add up to 1-¼” or more and possibly double that when construction settlement is included.
The Simpson Strong-Tie Wood Shrinkage Calculator is a perfect tool to determine the total shrinkage your building can experience.
In order to accommodate the shrinkage that occurs in a multi-story wood-framed building, Simpson Strong-Tie offers several shrinkage compensating devices. These devices have been tested per ICC-ES Acceptance Criteria 316 (AC316) and are listed under ICC-ES ESR-2320 (currently being updated for the new RTUD5, RTUD6, and ATUD9-3).
AC316 limits the rod elongation and device displacement to 0.2 inches between restraints in shearwalls. This deflection limit is to be used in calculating the total lateral drift of a light-framed wood shearwall.
The 0.2-inch allowable limit prescribed in AC316 is important to a shearwall’s structural ability to transfer the necessary lateral loads through the structure below to the foundation level. This limit assures that the structural integrity of the nails and sill plates used to transfer the lateral loads through the shearwalls is not compromised during a seismic or wind event. Testing has shown that sill plates can crack when excessive deformation is observed in a shearwalls. Nails have also been observed to pull out during testing. Additional information on this can be found here.
In AC316, 3 types of devices are listed.
Compression-Controlled Shrinkage Compensating Device (CCSCD): This type of device is controlled by compression loading, where the rod passes uninterrupted through the device. Simpson Strong-Tie has several screw-type take-up devices, such as the Aluminum Take-Up Device (ATUD) and the Steel Take-Up Device (TUD), of this type.
Tension-Controlled Shrinkage Compensating Device (TCSCD): This type of device is controlled by tension loading, where the rod is attached or engaged by the device and allows the rod to ratchet through as the wood shrinks. The Simpson Strong-Tie Ratcheting Take-Up Device (RTUD) is of this type.
Tension-controlled Shrinkage Compensating Coupling Device (TCSCCD): This type of device is controlled by tension loading that connects rods or anchors together. The Simpson Strong-Tie Coupling Take-Up Device (CTUD) is of this type.
Each device type has unique features that are important in achieving the best performance for different conditions and loads. The following table is a summary of each device.
The most cost-effective Simpson Strong-Tie shrinkage compensation device is the RTUD. This device has the smallest number of components and allows the rod unlimited travel through the device. It is ideal at the top level of a rod system run or where small rod diameters are used. Simpson Strong-Tie RTUDs can now accommodate 5/8″ (RTUD5) and ¾” (RTUD6) diameter rods.
How do you choose the best device for your projects? A Designer will have to consider the following during their design.
Rod Tension (Overturning) Check:
Rods at each level designed to meet the cumulative overturning tension force per level
Standard and high-strength steel rods designed not to exceed tensile capacity as defined in AISC specification
Standard threaded rod based on 36 / 58 ksi (Fy/Fu)
High-strength Strong-Rod based on 92 / 120 ksi (Fy/Fu
H150 Strong-Rod based on 130 / 150 ksi (Fy/Fu)
Rod elongation (see below)
Bearing Plate Check
Bearing plates designed to transfer incremental overturning force per level into the rod
Bearing stress on wood member limited in accordance with the NDS to provide proper bearing capacity and limit wood crushing
Bearing plate thickness has been sized to limit plate bending in order to provide full bearing on wood member
Shrinkage Take-Up Device Check
Shrinkage take-up device is selected to accommodate estimated wood shrinkage to eliminate gaps in the system load path
Load capacity of the take-up device compared with incremental overturning force to ensure that load is transferred into rod
Shrinkage compensation device deflection is included in system displacement
System deformation is an integral design component impacting the selection of rods, bearing plates and shrinkage take-up devices
Rod elongation plus take-up device displacement is limited to a maximum of 0.2″ per level or as further limited by the requirements of the engineer or jurisdiction
Total system deformation reported for use in Δa term (total vertical elongation of wall anchorage system per NDS equation) when calculating shearwall deflection
Both seating increment (ΔR) and deflection at allowable load (ΔA) are included in the overall system movement. These are listed in the evaluation report ICC-ES ESR-2320 for take-up devices
Optional Compression Post Design
Compression post design can be performed upon request along with the Strong-Rod System
Compression post design limited to buckling or bearing perpendicular to grain on wood plate
Anchorage design tools are available
Anchorage design information conforms to AC 318 anchorage provisions and Simpson Strong-Tie testing
In order to properly design a continuous rod tie-down system for your shearwall overturning restraint, all of the factors listed above will need to be taken into consideration.
A Designer can also contact Simpson Strong-Tie by going to www.strongtie.com/srs and filling out the online “Contact Us” page to have Simpson Strong-Tie design the continuous rod tie-down system for you. This design service does not cost you a dime. A few items will be required from the Designer in order for Simpson Strong-Tie to create a cost-effective rod run (it is recommended that on the Designer specify these in the construction documents):
There is a maximum system displacement of 0.2″ per level, which includes rod elongation and shrinkage compensation device deflection. Some jurisdictions may impose a smaller deflection limit.
Bearing plates and shrinkage compensation devices are required at every level.
Cumulative and incremental forces must be listed at each level in Allowable Stress Design (ASD) force levels.
Construction documents must include drawings and calculations proving that design requirements have been met. These drawings and calculations should be submitted to the Designer for review and the Authority Having Jurisdiction for approval.
More information can be obtained from our website at www.strongtie.com/srs, where a new design guide for the U.S., F-L-SRS15, and a new catalog for Canada, C-L-SRSCAN16, are available for download.
This week is the 10th anniversary of Hurricane Katrina, and we have all seen articles on the lessons learned from the storm. Engineers learn something new from every storm. However, I think that Hurricane Katrina just gave us some very strong reminders of things we already knew.
Hurricane Katrina reminded us that hurricanes are flood events as well as high-wind events. And I don’t mean the flooding in New Orleans. No, I mean the flooding along the Gulf Coast from Louisiana to Florida.
I witnessed the complete devastation of the Mississippi Gulf Coast from Waveland to Biloxi. Structures within the first few (and often many) blocks from the beach were simply flattened by water. Fortunately, these areas are coming back, but the structures being built there now bear little resemblance to the homes that graced the beach 10 years ago.
I remember my father-in-law having his new house built on the coast in Waveland more than 20 years ago. As a young engineer, I gave it the once over and noted that the builder had connected the roof framing to the top plate, but little else. I made some recommendations, such as continuing the connections down throughout the rest of the house to the foundation. The builder followed my suggestions and then presented my father-in-law with the bill “for your son-in-law the inspector.” He was happy to pay it. Nevertheless, although the house was wind resistant, it could not stand up to the rushing waters from Hurricane Katrina.
Katrina reminds us that the only way to get away from floods, other than not building near the water, is to elevate structures above them. Due to flood regulations, new houses along the Gulf Coast are now elevated high in the air, in the hope of avoiding flooding from future storms. Simpson Strong-Tie is proud to have developed some products during the last few years that make it easier to build structures elevated on pilings.
Hurricane Katrina reminds us of the value of building codes. After the storm, the LSU Hurricane Center conducted a number of simulation studies on the effect of a direct, Katrina-like storm on the states of Louisiana, Mississippi and Alabama. The simulations were run on the existing stock of buildings, and then run again on the same stock of buildings, assuming that certain features that result from modern building codes were present. These features included shutters or impact-resistant windows, enhanced nailing of the roof deck to the roof framing, framing connected together with hurricane clips and straps to achieve a continuous load path. In addition, in the Louisiana study, a secondary water barrier over the joints in the roof sheathing was added.
The studies found that the decrease in wind damage from the simulated storms was astounding. In Louisiana, the study showed a 79% reduction in economic losses due to wind. In Alabama, the study revealed a 72% reduction in economic losses due to wind. The Gulf states seem to have received the message loud and clear. In the years following Hurricane Katrina, Louisiana adopted a statewide building code and Mississippi adopted a uniform building code for the four counties along the coast. Recently, Alabama has also adopted a statewide residential and energy code. But in general, building codes are still quite varied in coastal states. This report from the Insurance Institute for Business and Home Safety evaluates the effectiveness of building codes in coastal states.
Finally, Hurricane Katrina reminds those of us who do damage surveys that you need to know what you are getting into before you go. As soon as the storm hit and we saw the scope of the damage, four members of the Simpson Strong-Tie Engineering Department in our McKinney, Texas, office decided we needed to go see the damage first-hand before any repairs were made. So two days after the storm struck, off we went to Jackson, Mississippi. There, we rented two vans stocked up with food, water and fuel. Unfortunately, the fuel and the food/water ended up in separate vans. Before long, we were separated in traffic and could not communicate due to loss of cell signal.
Our team spent two days viewing the damage first-hand along the Louisiana and Mississippi coast, but spent a lot of time our last day trying to find some fuel so we could make it back to Jackson. I remember spending the night in a hotel without power full of storm victims, and then months later receiving the bill and being charged for a movie!
What do you remember from Hurricane Katrina? Let us know in the comments below.
This week’s post comes from Scott Fischer who is an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2006, Scott has worked on cast-in-place connectors to concrete. He helped develop the current testing criteria for cast-in-place concrete products and also performed the testing and code report requirements needed for these product lines. Prior to joining Simpson Strong-Tie, Scott worked for nine years as a consulting engineer. His experience includes the design and analysis of concrete structures, including post-tensioned slab design, concrete lateral systems and foundations. Scott is a licensed professional engineer in the state of California and received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo.
How often do you get the opportunity to high five a co-worker in the office? Maybe it’s when you just worked through a really complex calculation, or finally figured out that tough detail. Whatever it might be, there are times when we should raise a hand and celebrate the hard work that we do. So when we recently relaunched the Simpson Strong-Tie Strong-Rod™ Systems website, which includes a link to our new Shallow Podium Anchorage Solutions, there were a few high fives going around the office. With that in mind, we want to share the latest developments and continue our anchorage-to-concrete blog discussions that began in May 2012, continued with a March 2014 post referencing the Structure magazine article on anchor testing, and more recently one discussing our release of anchor reinforcement solutions for Steel Strong-Wall® shearwall to grade beams.
Anchor Reinforcement Testing and Research Program
Simpson Strong-Tie has been studying cast-in-place tension anchorage and anchor reinforcement concepts extensively over the past several years. Designing an anchor solution in a thin concrete slab for a high anchor demand load while meeting the ductility provisions of ACI 318-11, D.220.127.116.11 is extremely challenging. A strong industry-need for a safe, logical, yet economical design solution, led to a cooperative research program between Structural Engineers Association of Northern California (SEAONC) members and Simpson Strong-Tie. Testing was initiated by a Special Project Initiative grant from SEAONC to members Andy Fennell, P.E. (principal at SCL) and Gary Mochizuki, S.E. (principal at Structural Solutions at the time, now with Simpson Strong-Tie). We completed the program with continued involvement of SEAONC members. This is the second time we have partnered with SEAONC on non-proprietary anchor bolt testing. The first partnership, on sill plate anchor bolts in shear, resulted in successful code change provisions (led by SEAONC) restoring the capacity of these connections to pre-ACI 318 Appendix D values.
This current research program has focused on non-proprietary anchor reinforcement detailing that increases the nominal breakout capacity of concrete slabs. The anchor design satisfies the seismic ductility requirements of ACI 318-11 Appendix D and also significantly increases design capacity for wind applications. The project goal was to provide a design solution for the industry with independently witnessed proof testing of the anchor reinforcement detailing and application of ACI 318-11 Appendix D design procedures. (Note: Appendix D is moved into a new Chapter 17 in ACI 318-14.)
Significant Test Findings and Design Concepts
Anchoring to relatively thin concrete slabs introduces many unique challenges, so testing was bound to reveal some unique findings. The goal was to increase concrete breakout capacities and also satisfy the ACI 318 anchor ductility requirements with anchor reinforcement detailing. Here are some of the significant findings:
Relatively thin concrete slabs do not allow the placement of anchor reinforcement to drag the load down into a larger mass of concrete as shown in RD.5.2.9. Modified anchor reinforcement was required (Figure 2). The required area of anchor reinforcement is based on D.18.104.22.168(a) where the required area of anchor reinforcement exceeds the anchor steel strength, or 1.2Nsa < (nAsfy x 0.707). The 0.707 is for the 45 degree slope of the bars. The proof testing showed the horizontal leg development outside the cone and continuity through the cone adequately developed the anchor reinforcement.
ACI 318-11, D.4.2.1 states that when anchor reinforcement is provided, calculation of concrete breakout strength is not required. You know we love load path discussions, so where does the load go once it gets into the anchor reinforcement? The tests indicated that when the anchor reinforcement is provided, the concrete breakout area increases. This limit state is an extended breakout area past the anchor reinforcement bends that will form when that reinforcement is properly quantified and configured. The extended breakout is similar to multiple anchors loading the slab at each bottom bend of the anchor reinforcement. We have applied this concept to the calculations to evaluate extended breakout past the anchor reinforcement bends.
Let’s follow the load path some more. Once the anchor is connected to the slab, what is the slab bending capacity? The testing showed that to achieve the anchorage capacity, the slab must have an adequate amount of flexural reinforcement with anchorage forces corresponding to ACI 318-11, Section D.22.214.171.124 applied to the slab. Guidance from this section says to apply the anchor tension loads obtained from either design load combinations that include E, with E increased by Omega, or 1.2 x Nominal steel strength of the anchor (Nsa). If the anchors are not oversized, designing for 1.2Nsa should be the most economical solution. For wind applications, the slab Designer should consider the project specified design loads.
A vertical concrete block shear forming at the anchor bearing plate is possible if the anchor embedment is shallow and the anchor reinforcement is working to resist the initial anchor bolt breakout area. Our testing showed that this block shear is separate from Appendix D Pullout and is dependent on embedment depth, perimeter of the bearing surface and concrete strength.
For anchors with shallow embedment that have a double nut and washer, the concrete breakout can begin from the top nut, thereby reducing the effective embedment depth. To address this, we eliminated the top nut from our specified anchor assembly kit to insure the breakout begins from the top of the fixed-in-place plate washer.
The testing and modeling also allowed us to re-examine the appropriate bearing area for the plate washer, Abrg. The flat top surface of a nut is typically circular due to the chamfer at the points, so the resulting bearing area of the plate washer is circular extending out the thickness of the plate from the flat-to-flat dimension of the nut. For near-edge conditions, the side-face blowout capacity can be the controlling limit state and where the plate washer bearing area becomes more important.
Edge testing with anchor reinforcement details showed that the breakout area will spread out and begin from the anchor reinforcement bends, like the mid-slab. In a mid-slab condition, the breakout slope follows the 1.5:1 or 35 degree slope used in Appendix D. However for the near edge, we found that 1.5 x effective embedment (hef) from anchor reinforcement bends only holds true parallel to the edge. Due to eccentricities, the breakout angle from the anchor reinforcement bends into the slab (perpendicular to the edge) is steeper and a steeper 45 degree slope should be used.
The testing showed that even though we had cracks intersecting the anchor as it was loaded, the capacities exceeded uncracked assumptions. This is likely due to the flexural reinforcement running through the breakout cone providing continuity across the cracks. We’re still studying the effect of the flexural reinforcement, so for now we recommend assuming cracked concrete and providing a minimum of 4 – #5 flexural bars each way at anchor locations that require anchor reinforcement. Your slab design may require more flexural bars or they may already be there to meet other slab design requirements.
What Solutions Are Now Being Offered and How Do I Get Them?
With our newly launched Strong-Rod Systems webpages, you now go to the Shallow Podium Anchor link to find anchorage solutions. On the website, you will find anchor reinforcement detail drawings and design load tables with slab design recommendations. We’ll also be adding sample calculations, a guideline for selecting your anchor solutions, 3-D anchor reinforcement graphics and guidelines for addressing your condition if your installation is outside the scope of the current solutions that we offer. The anchor reinforcement is non-proprietary and is fabricated by the rebar supplier, but the configuration and placement is described in the details. In addition, you will see detailed information about the Simpson Strong-Tie Shallow Anchor Rod and Anchor Bolt Locator that is specified as a kit in the load tables. Note the absence of the top nut for reasons described above.
How Do We Specify It and Use It?
So now you know what is on the website but how do you put all these pieces together and apply them to a specific design? As a design professional, you will drive the bus on applying these details and design tables onto your drawings. Similar to specifying Strong-Wall® shearwalls or Strong Frame® moment frames, it just takes a little upfront coordination on your drawings. Typically, you’ll start with a slab key plan that shows the anchor bolt locations. You will need to know the design uplift forces from the light-frame structure above the slab and some basic project variables like specified concrete strength, slab thickness(es) and whether the structure is in high seismic or wind-controlled areas. Now you can choose the necessary design tables from the website by clicking on the individual design tables tab. Your key variables will help you select your specific table based on slab thickness, concrete strength, near edge, etc., and of course wind or seismic.
Once you have selected your design table, just match your project demand loads or your project-specified anchor bolt with the tabulated ASD or LRFD capacity to select the appropriate shallow anchor callout and reference detail that you can identify on the key plan. The detail callout from the table will send you to sheet SA1 where you will find the anchor reinforcement details and Shallow Anchor Kit recommended for your condition. As mentioned previously, the anchor reinforcement would be fabricated and bent by the rebar guys, but would follow these details. You can download the details shown on sheet SA1, place them on your construction documents and then coordinate them with your plans or schedules similar to how you might provide a shear wall schedule. The footnotes that accompany the tables provide important slab design information and other design and installation recommendations. You’ll soon be able to download the sample design calculations, use them as a tool to help follow the design procedure of the recommended details and submit with your project.
What if My Situation Does Not Fit the Details on the Website?
Though a great number of installations will be covered by these details and tables, there will be conditions that currently cannot be addressed with anchor reinforcement solutions. Installs that may be outside of the current scope could include: a demand uplift that’s too large, a slab that’s too thin, lower concrete strength, corner installs, double wood frame shear walls or two close anchors in tension. To address these conditions, we suggest alternatives like slight adjustment or reconfiguration of the shear walls, thickening slab edges, adding downturn concrete beams or extending the anchorage from above down into a cast-in-place wall or further extended down into the footing at ground level.
The joint SEAONC-Simpson Strong-Tie testing project has shown that anchor reinforcement details can greatly increase the breakout strength of concrete to support cast-in-place anchor bolts in concrete slabs. The testing also showed that the design provisions in ACI 318-11 Appendix D can be rationally applied to these anchor reinforcement details. The testing and Appendix D calculation approach are the basis of the details, load tables, graphics and application guides that can be found on our new Strong-Rod Systems webpages. We’re updating these pages as we create more content, so check back frequently. We look forward to working with you on your anchorage installation challenges and hope that some of these solutions will help you with projects you are working on today. How about a high five?
What do you think about these new anchorage solutions? Let us know by posting a comment below.
This week was our new employee Sales and Product Orientation class. It reminded me of the post A Little Fun with Testing where we broke a bowling ball. Although breaking stuff is fun, my second favorite part of the class is teaching about the importance of a continuous load path. I think it is really the most important thing a Structural Engineer does. If we don’t pay attention to the loads, where they occur and create a path so they can get where they need to go, a building may not stand up. This week, we also released some new tools and information for our new Strong-Rod™ Systems, which are used to complete the load path for multi-story wood-framed shearwall overturning restraint and roof uplift restraint.
Two Load Paths
All wood-framed buildings need to be designed to resist shearwall overturning and roof-uplift forces. To transfer these tensile forces through the load path, connectors (hurricane ties, straps and holdowns) have been the traditional answer. Simpson Strong-Tie offers a few options there. With the growth in multi-story wood-framed structures, where the code requires shrinkage to be addressed and overturning and uplift forces are typically higher, rod systems have become an increasingly popular load restraint solution. Our Anchor Tiedown System (ATS) for shearwall overturning restraint has been around for many years. A new Strong-Rod Systems Design Guide and revamped web pages provide information on new design options, components and configurations.
The guide and website focus more on the unique design considerations for rod systems, how you should specify the system and highlight the design services that we provide. They also provide more detail and design information for our relatively new Uplift Restraint System (URS) for roofs. Connectors are a common choice for transferring the net roof uplift forces from wind events down the structure. Although in some high-wind areas, rod systems are preferred.
I’ll touch on some of the design considerations for these types of systems below, but back to the load path. For shearwall overturning restraint using holdowns, the load path is fairly simple. Once the lateral load is in the shearwall, the sheathing and nailing lifts up on the post. The holdown connects to the post, holding it down and transferring the forces to the foundation or level below. A continuous rod tiedown system follows a little different path. The sheathing and nailing lifts up on the boundary posts and the posts push up on the framing above until the load is resisted in bearing by a bearing plate. The load is then transferred into the rod and down to the foundation. There has been a lot of testing and research on the effects of skipping restraint locations where a bearing plate restraint is installed at every other floor or only at the top level. Doing that will change the load path because the load has to continue to travel up until a restraint holds it down. It also negatively impacts the stiffness and drift of the shearwall stack, not to mention increases project cost because the boundary posts, rod and bearing must be sized to transfer the cumulative overturning forces from each level.
Wood Shrinkage, Take-up Devices and Displacement Limits
Shrinkage is not just a Seinfeld episode cult classic. It is also something that designers need to consider when designing wood structures. IBC Section 2304.3.3 requires that designers evaluate the impact of wood shrinkage on the building structure when bearing walls support more than two floors and a roof. The effects of wood shrinkage can impact many things in the structure from finishes to MEP systems to the continuous rod system. As the wood members lose moisture, the wood shrinks and the building settles. This can cause gaps at the bearing plate locations of continuous rod systems because the continuous steel rod doesn’t shrink. That is where the magic of take-up devices comes in. They allow the building to shrink but keep gaps from forming by filling the gap (expanding devices – can be screw style or ratcheting), ratcheting down the rod (ratcheting devices), or making the rod shrink as much as the wood (contracting coupling device).
In addition to keeping the rod system tight to insure the intended performance, it is important to consider the movement associated with the rod system when under wind or earthquake loading. The IBC requires shearwall displacements to be within story drift limits in moderate to high seismic regions. We highlighted some of the changes coming for the evaluation of shearwall deflection in the previous post discussing the New Treatment of Shear Wall Aspect Ratios in the 2015 SDPWS. For continuous rod systems, there are some additional limits. ICC-ES AC316Acceptance Criteria for Shrinkage Compensating Devices requires designs to limit displacement between restraints to 0.20 inches (including rod elongation and device displacement) for shearwall restraint. The movement of the take-up device plays a big part in meeting this requirement and the rod diameter required. Screw-style devices have the lowest total movement. Ratcheting devices are appropriate in many cases as well such as the upper levels where loads are lower, but may require larger rod diameters to meet the displacement limit.
ICC-ES AC391Acceptance Criteria for Continuous Rod Tie-down Runs and Continuous Rod Tie-down Systems Used to Resist Wind Uplift covers continuous rod systems for roof uplift restraint. The displacement limit for the Continuous Rod Tie-down Run (just the rod system components) is limited to 0.18 inches of rod elongation for the total length of rod. The Strong-Rod URS evaluates the Continuous Rod Tie-down System (the whole load path). Displacement limits for the system are L/240 for the top plate bending and 0.25 inches total deflection at the top plate between tie-down runs (including top plate bending, rod elongation, wood bearing deformation and take-up device displacement). The differences between the rod run and rod system analysis as well as other design considerations are explained in more detail in the design guide and on our website.
I always end my continuous load path presentation during orientation class with the same questions and if they were paying attention I get the response I want.
“What is the most important thing a Structural Engineer does?”
“Designs a continuous load path for the building!”
“What does Simpson Strong-Tie do?”
“Provides product and system solutions to help engineers do their job!”
Take a look at the new Strong-Rod Systems tools and information and let us know how we can help you with your next multi-story wood-framed project.
What related blog topics would you like to discuss? Let us know in the comments below.
There certainly seems to be increased awareness of the potential for damage and injury from tornadoes these days. Recent information published by the Federal Emergency Management Agency (FEMA) and the Federal Alliance for Safe Homes (FLASH) help explain that. This increased awareness has led to a growing interest in tornado shelters for protection of life and property.
This FEMA graphic shows that most areas of the United States have been affected by a tornado at some point since 1996, and many have been affected by one or more strong tornadoes (EF3 or greater).
Living in North Texas near the Simpson Strong-Tie manufacturing plant in McKinney, Texas, I know all too well the sinking feeling of hearing the tornado sirens and turning on the TV to find you are under a tornado watch. FLASH recently published a graphic developed by the National Weather Service that shows the large number of U.S. counties that have been under a tornado watch between 2003-2014, and the high number of warnings that some counties experienced.
Other than moving to an area that has fewer tornadoes, one of the best ways to protect your family and at least have more peace of mind during tornado season is to have a tornado shelter or safe room. These structures are designed and tested to resist the highest winds that meteorologists and engineers believe occur at ground level during a tornado and the debris that is contained in tornado winds.
Tornado shelters can be either pre-fabricated and installed by a specialty shelter manufacturer, or can be site-built from a designed plan or pre-engineered plan. A good source for information on pre-fabricated shelters is the National Storm Shelter Association, a self-policing organization that has strict requirements for the design, testing and installation of its members’ shelters.
FEMA publishes a document, P-320, Taking Shelter from the Storm, that provides good information on safe rooms in general, as well as several pre-engineered plans for tornado safe rooms.
To highlight the different types of safe rooms covered by FEMA P-320, FEMA, FLASH and the Portland Cement Association (PCA) sponsored an exhibit at January’s International Builder’s Show. The exhibit was called the “Home Safe Home Tornado Saferoom Showcase.” It featured six different types of saferooms that builders could incorporate into the homes they build. Simpson Strong-Tie and the American Wood Council collaborated to build a wood frame with steel sheathing safe room meeting the FEMA P-320 plans. Other safe rooms shown at the exhibit included pre-cast concrete and pre-manufactured steel shelters manufactured by NSSA members, and reinforced CMU, ICF cast-in-place concrete and aluminum formed cast-in-place concrete built to FEMA P-320 plans.
Simpson Strong-Tie staff in McKinney, Texas, constructed the wood frame/steel sheathing safe room in panels and shipped it to the show. It was built from locally sourced lumber, readily available fasteners and connectors and sheets of 16 ga. steel (which we happen to keep here at the factory). It had cut-away sheathing at the corners to show the three layers of sheathing needed. Our message to builders was that this type of shelter would be the easiest for their framers to build on their sites.
The sponsors of the exhibit took advantage of the variety of safe rooms in one place to film a video series, “Which Tornado Safe Room is Right for You?” The videos are posted at the FLASH StrongHomes channel on YouTube. The series provides comparative information on cast-in-place, concrete block masonry, insulated concrete forms, precast concrete and wood-frame safe rooms, with the goal of helping consumers to better understand their tornado safe room options.
“Today’s marketplace offers an unprecedented range of high-performing, affordable options to save lives and preserve peace of mind for the millions of families in the path of severe weather,” said FLASH President and CEO Leslie Chapman-Henderson. “These videos will help families understand their options for a properly built safe room that will deliver life safety when it counts.”
FLASH released the videos earlier this month as part America’s PrepareAthon!, a grassroots campaign to increase community emergency preparedness and resilience through hazard-specific drills, group discussions and exercises. The overall goal of the program is to get individuals to understand which disasters could happen in their community, know what to do to be safe and mitigate damage from those disasters, take action to increase their preparedness, and go one step farther by participating in resilience planning for their community. Currently, the program focuses on preparing for the disasters of tornadoes, hurricanes, floods, wildfires, earthquakes and winter storms.
Do you know what the risk of disasters is in your community? If you are subject to tornado risk, would you like to build your own safe room, have one built to pre-engineered plans or buy one from a reputable manufacturer? Let us know in the comments below.
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.
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:
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.
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.
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.
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:
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:
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.
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 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.”
Happy New Year! 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:
The mission of FLASH is “Strengthening homes and safeguarding families from disasters of all kind.” FLASH recently celebrated its 15th year, and Simpson Strong-Tie has been right there with them for most of those years.
Creating the StormStruck® Experience
Perhaps the biggest outcome of our work with FLASH is our partnership in “StormStruck: A Tale of Two Homes®, located at INNOVENTIONS in Epcot® at the Walt Disney World Resort. StormStruck is a fun, interactive 4D experience that teaches visitors the steps they can take to protect their homes and families during severe weather. Visitors experience a storm and its effects on the “house” they are sitting in, and then decide how to best rebuild to resist the next storm. StormStruck has been seen by almost 800,000 people in the past year, and more than four million since opening in 2008. If you visit EPCOT®, be sure to head over to INNOVENTIONS East to see our exhibit.Continue Reading
[Simpson Strong-Tie note: Shane Vilasineekul is the Simpson Strong-Tie Engineering Manager for the Northeast U.S. and one of our guest bloggers for the Structural Engineering Blog. For more on Shane, see his bio here.]
The end of this month will mark the one year anniversary of Superstorm Sandy hitting the coastlines of New Jersey, New York, and surrounding states. A lot of construction has taken place in the last 12 months, but most of the rebuilding will occur over the next few years. The boardwalks were a high priority because of their effect on tourism, which is so vital to the local economies, and most of them have been completed (see my previous post about rebuilding after Sandy here). Now the focus has shifted to repairing, raising, and rebuilding homes.
I am writing this while sitting in the Newark airport, headed home after presenting one of our workshops on high wind design. The workshop was held at a hotel in Manahawkin, New Jersey that happened to be used last year by residents displaced by the storm, including some of the architects and engineers in attendance this morning. After talking to a few of them at the breaks, it sounded like they are struggling with the current state of building provisions, which were quickly put in place to ensure rebuilt properties are more resilient, including new flood elevations and renewed focus on code compliance.