Introduction to the Site-Built Shearwall Designer Web Application

Written by Brandon Chi, Engineering Manager, Lateral Systems at Simpson Strong-Tie.

Wood shearwalls are typically used as a lateral-force-resisting system to counter the effects of lateral loads. Wood shearwalls need to be designed for shear forces (using sheathing and nailing), overturning (using holdowns), sliding (using anchorage to concrete) and drift, to list some of the main dangers.  The Simpson Site-Built Shearwall Designer (SBSD) web app is a quick and easy tool to design a wood shearwall based on demand load, wall geometry and design parameters.

The web application provides two options for generating an engineered shearwall solution: (1) Solid Walls; and (2) Walls with Opening using the force-transfer-around opening (FTAO) method. Both options generate solutions that offer different combinations of sheathing, nailing, holdowns, end studs and number/type of shear anchors. The app can generate a PDF output for each of the possible solutions. Design files can be saved and reused for future projects.

App Overview

Design Input: 

Figure 1 shows the input screens for the “Solid Walls” and “Walls with Opening” designs with common wall parameters that are applicable to both design options. The user interface uses quick drop-down menu and input fields for the designer to select the different options and parameters. Unless otherwise noted, all the input loads are to be nominal (un-factored) design loads. The application will apply load combinations to determine the maximum demand forces for the shearwall design.

Figure 1A. Application Design Criteria Input. – Solid Wall

Figure 1B. Application Design Criteria Input. – Walls with Opening

Figure 1C. Application Design Criteria Input. – Common Wall Input Parameters

Figure 2 shows the allowable stress design (ASD) load combinations used for calculating the demand loads for the different components of the wood shearwall (i.e., holdown, compression post, sheathing and nailing design, etc.).

Figure 2. Load Combinations.

In addition to the lateral loads (wind and seismic) applied at the top of the wall and the wall’s own weight, uniform loads on top of the wall and concentrated point loads at the end posts can also be modeled. (See Figure 3.)

Figure 3. Addition Loads on the Wall.

Embedded anchor or embedded strap holdowns can be modeled by the app. (See Figure 4.) For the embedded strap option, additional input parameters are required since they will affect the allowable load of the selected strap holdown.

Figure 4. Holdown Design Options.

The Designer has the option to include additional sources of vertical displacement for drift calculation. (See Figure 5.)

Figure 5. Other Sources of Vertical Displacement Options.

Design Calculations:

For hand-calculated design when the demand forces are determined, the holdown size and shear anchorage can be selected from tabulated values. Design for the sheathing/nailing and compression post is relatively straightforward as well; however, the shearwall drift calculation may take a bit more work. This is where the SBSD app comes in handy. Below are two sections on the shearwall drift and strap force calculations and assumptions used in the SBSD application. If you are interested, please contact Simpson Strong-Tie for other design assumptions used in designing the SBSD app.

Shearwall Deflection Calculations:

Equation 1 shows the shearwall deflection equation from the 2008 Edition of Wind & Seismic Special Design Provisions for Wind and Seismic (SDPWS).

The Δa value from the third term of the equation is the total vertical elongation of the wall holdown system from the applied shear in the shearwall. The third term accounts for the additional displacement from holdown displacement. For holdown deflection, the deflection value depends on the post size used with the holdown size. When hand-calculating shearwall drift, Designers may have to perform a couple of iterations to come to the final post and holdown size. The SBSD app accounts for the holdown displacement and the post size used for overturning force calculation.

For shearwall-with-opening deflection calculation, EQ-2 is used in the SBSD app.

The solid wall, ∆solid wall, term is calculated using EQ-1 above. For the window strip and wall pier deflection terms, the height “h” used in EQ-1 is taken as the height of the window opening. ∆a is the deflection from nail slip in the shearwall. For more information regarding shearwall deflection with opening, please refer to Example 1 in Volume 2 of the 2015 IBC SEAOC Structural/Seismic Design Manual.

Strap Force Calculations:

For the Wall with Opening design option, there are several methods (Drag Strut, Cantilever Beam, SEAOC/Tompson, Diekmann) to calculate the force transfer around the opening. In the SBSD app, the Diekmann technique is used to calculate the pier forces in the shearwall and the strap forces around the opening. When calculating the strap forces, the SBSD app assumes they are the same at the top and bottom of the opening. In addition, contribution of the gravity load only affects the overturning forces in the holdown and post design but not the wall pier forces or strap forces.

Design Output:

Once all design parameters are entered and calculated, a list of possible solutions (where available) will be shown. (See Figure 6.) Common parameters such as sheathing material and type, wood species, minimum lumber grade, etc., are shown first, followed by other design parameters. The user can filter the solutions by seismic drift or wind drift.

Figure 6. Onscreen Output.

The Designer can select the PDF button next to the desired solution to see a PDF design file on a separate screen. (See Figure 7.)  The PDF design file contains the detailed design criteria input by the Designer, calculated demand loads, shearwall material summary, and a design summary for holdown, sheathing, and compression post design. A detail summary for shearwall deflection is also shown, with each term of the shearwall deflection equation (EQ-1) separated. Shear anchorage and design assumption notes follow the design summary section. This PDF file can be saved and printed by the Designer.

Figure 7. Detailed PDF Output.

I hope you find the SBSD web app helpful for your day-to-day wood shearwall design needs. If you have any questions or comments, please leave them in the comments section below.

Why You Should Specify Stainless-Steel Screw Anchors When Designing for Corrosive Environments

Figure 1. Spalled concrete below a concrete bridge.

I was driving under a concrete bridge one nice clear day in Chicago, and I happened to look up to see rusted rebar exposed below a concrete bridge. My beautiful wife, who is not a structural engineer, turned to me and asked, “What happened to that bridge?” I explained that there are many reasons why spalling occurs below a bridge. One common reason is the expansion of steel when it rusts or corrodes.

This week’s blog will briefly explain the corrosion process and why concrete spalls when the embedded metals corrode. Corrosion may be defined as the degradation of a material as a reaction to its environment.1. As described in our previous SE Blog post, “Corrosion: The Issues, Code Requirements, Research and Solutions” dated January 3, 2013, corrosion of metallic surfaces is an electrochemical process. Because of moisture evaporation, concrete is a porous material. Water and oxygen molecules enter the pores of the concrete, and an electrochemical process occurs with the carbon-steel bar. The iron in the steel is oxidized, which then produces rust. A buildup of rust products at the surface of the carbon-steel bar exerts an expansive force on the concrete. Based on the amount of oxidation, the rust products of steel can occupy more than six times the volume of the original steel.2 Over time, further rust occurs and surface cracks will form. Eventually spalling will occur, exposing the rusted carbon steel bar. (See figure 1.)

Figure 2. Stages of corrosion.

Just as with reinforcing bars below a concrete bridge, cracking and spalling can occur when a carbon-steel anchor is used adjacent to a concrete edge. Simpson Strong-Tie® has many anchorage products that can be used in these conditions to prevent cracking. One specific product is the new stainless-steel Titen HD® screw anchor. This new innovative screw anchor is made up of Type 316 stainless steel. As seen in Figure 3, Type 316 stainless steel has a high level of resistance. This makes the stainless-steel Titen HD an excellent choice when it comes to an anchorage solution in corrosive environments. These environments include wastewater treatment plants, exterior handrails, exterior ledger attachments, stadium seating, central utility plants, and kitchens just to name a few.

Figure 3. Simpson Strong-Tie level of corrosion by material/coating.

Unlike expansion anchors, screw anchors require the leading threads to cut into predrilled holes. This can be easily achieved with hardened carbon-steel cutting threads. Stainless steel is not hard enough to cut into concrete. The new innovative stainless-steel Titen HD solves the problem by brazing heat-treated carbon-steel cutting threads to the surface of the stainless-steel tips of the screw anchor. (See figure 4.) These carbon-steel threads are hard enough to cut grooves into the surface of a predrilled hole, allowing the anchor to be installed with ease. The volume of the carbon-steel cutting threads is less than 1% of the stainless steel, reducing the buildup of rust that eventually spalls the concrete edge. Other stainless-steel screw anchor manufacturers in the market have a bi-metal product that attaches a full carbon-steel tip. This bi-metal screw anchors contain up to 18% carbon steel. Such a large amount of carbon steel can expand up to six times its volume when it corrodes and can spall the concrete when used adjacent to an edge.

Figure 4. Carbon-steel cutting threads.

Figure 5. Graphic representation of spalling in concrete adjacent to an edge.

When designing an anchorage solution for your next job in a corrosive environment, the stainless-steel Titen HD will provide the best resistance for corrosion, and also give the ability to drive these anchors into the concrete with ease. More information about the product can be obtained by visiting strongtie.com/thdss.

  1. Corrosion Technology Laboratory (https://corrosion.ksc.nasa.gov/corr_fundamentals.htm).
  2. Galvanized Rebar (http://www.concreteconstruction.net/how-to/repair/galvanized-rebar_o).

Stainless-Steel Titen HD®

The Next Era of Stainless-Steel Screw Anchor For Concrete and Masonry.


FAQs Regarding Strong-Rod Anchor Tiedown Systems (ATS) for Shearwall Overturning

How would a six-story light-frame wood building perform in a large earthquake? Back in 2009, Simpson Strong-Tie was a partner in the World’s Largest Earthquake Test, a collaboration of the NEESWood project, to answer that question. This was a full-scale test which subjected the building to 180% of the Northridge earthquake ground motions (approximately a M7.5). Within the building, Simpson Strong-Tie connectors and Strong-Frame SMF were used, with the Strong-Rod™ anchor tiedown system (ATS) serving as holdown for each shearwall.

The NEESWood building was designed under Performance-Based Design methodology, and the test was conducted as validation for the approach. Buildings of similar size to the NEESWood building are built to current codes using similar products. Mid-rise light-frame wood structures continue to be a popular form of construction in various densely populated cities across the country. As part of the lateral-force-resisting system, continuous rod systems are used as the holdown for the shearwall overturning restraints. Simpson Strong-Tie has been involved with continuous rod systems since the early 2000s when we launched the Strong-Rod anchor tiedown system.

Today, rod manufacturers design the continuous rod systems with design requirements (loading, geometry, etc.) Supporting documents (e.g., installation details, layouts, RFI/markups and calculations) are submitted for each unique project. Over the years, engineers have asked many questions related to the design of these systems. In this week’s blog, we will explore Frequently Asked Questions pertaining to Strong-Rod ATS systems used as shearwall overturning restraints (holdowns).

Is there a code report for the system?

The Strong-Rod ATS system is a series of rods (fully threaded rods and proprietary Strong-Rods), coupler nuts, bearing plates, nuts and shrinkage compensation devices (ATUD/TUD and RTUD).

The majority of these components are designed in accordance with the building code and reference standards (e.g., NDS, AISC). A project-specific calculation package is submitted for each job that addresses the evaluation of these elements. Therefore, these elements are not listed in evaluation reports.

Shrinkage compensation devices, on the other hand, are proprietary components which are not addressed by the building code or reference standards. Therefore, they are tested in accordance with ICC-ES acceptance criteria AC316 and are listed in ICC-ES ESR-2320. 

What is the material specification of the rods used above concrete?

The specified rod materials are shown in Table 1.

Table 1. ATS Rod Material Specifications

Can threaded rods or couplers be welded to steel beams?

Simpson Strong-Tie generally does not recommend this practice. Of the materials listed in Table 1, ASTM A307 material is the only specification that contains supplementary requirements for welding. When standard strength rod is supplied to the job, it is not guaranteed that this will be the material provided.

ASTM A449 and A193-B7 high-strength rods develop strength and ductility characteristics through controlled quenching and tempering treatments. Quenching is the rapid cooling of metal (usually by water or oil) to increase toughness and strength. This process often increases brittleness. Tempering is a controlled reheating of the metal which increases ductility after the quenching process. Precise timing in the application of temperature during the tempering process is critical to achieving a material with well-balanced mechanical properties. It is unlikely that field welding will satisfy the requirements of quenching and tempering.

Coupler nuts are generally fabricated from material exhibiting characteristics similar to high-strength rods. Thus, it is not recommended to weld coupler nuts to steel beams due to the potential for embrittlement.

Simpson Strong-Tie specifies a weldable cage which is fabricated from ASTM A36 material for such applications.

How do you calculate the Maximum ASD Tension Capacity provided in the job summary?

Simpson Strong-Tie provides a comprehensive design package for continuous rod systems used as holdowns for multi-story stacked shearwalls. The individual run calculations, as shown in Figure 1, provide the Maximum Tension Capacity, which correlates to the maximum force the system can deliver. Plan check often requests justification on how these values are derived at each level. These values are calculated, and the process explained below may be used on any Simpson Strong-Tie ATS Job Summary as justification.

Figure 1. Sample ATS Run Type SW9

The maximum tension capacity published within the Job Summary and the Installation Details is derived using the following procedure:

  • Step 1: Evaluate the top-most level. Compare the published capacities of the rod in tension, plate in bearing and the take-up device. The lowest of these three will govern and becomes the Maximum Tension Capacity for this level.
  • Step 2: Evaluate the next level down. (a) Sum the Maximum Tension Capacity from Step 1 and the published capacity of the take-up device from this level. (b) Sum the Maximum Tension Capacity from Step 1 and the published capacity of the plate in bearing from this level. (c) Compare derived values from (a) and (b) to the published capacity of rod in tension. The lowest of these three values will govern and becomes the Maximum Tension Capacity of this level.
  • Step 3: Repeat Step 2 as necessary until the bottom-most level is reached.

Applying this procedure to the sample run, SW9, will wield the following result:

  • Step 1: Evaluate capacities published at Level 4
    • Plate in bearing (PBRTUD5-6A) = 7.06 kips governs
    • Take-up device (RTUD6) = 20.83 kips
    • Rod in tension (ATS-R6) = 9.61 kips
      • The lowest value in Step 1 is the plate in bearing, hence 7.06 kips is the maximum load that can be delivered at Level 4 and is the Maximum Tension Capacity.
    • Step 2: Evaluate capacities at Level 3
      • Maximum Tension Capacity from Level 4 = 7.06 kips (See Step 1)
      • Maximum Tension Capacity from Level 4 + take-up device (ATS-ATUD9-2) = 7.06 + 12.79 = 19.85 kips
      • Maximum Tension Capacity from Level 4 + plate in bearing (PL9-3×5.5) = 7.06 + 10.03 = 17.09 kips
      • Rod in tension (ATS-R7) = 13.08 kips       governs
        • The lowest value in Step 2 is the rod in tension, hence 13.08 kips is the maximum load that can be delivered at Level 3 and is the Maximum Tension Capacity.
      • Step 3: Evaluate capacities at Level 2
        • Maximum Tension Capacity from Level 3 = 13.08 kips (See Step 2)
        • Maximum Tension Capacity from Level 3 + take-up device (ATS-ATUD9-2) = 13.08 + 15.56 = 28.64 kips
        • Maximum Tension Capacity from Level 3 + plate in bearing (PL9-3×5.5) = 13.08 + 10.03 = 23.11 kips
        • Rod in tension (ATS-R7) = 13.08 kips       governs
          • The lowest value in Step 3 is the rod in tension, hence 13.08 kips is the maximum load that can be delivered at Level 2 and is the Maximum Tension Capacity.
        • Step 4: Evaluate capacities at Level 1
          • Maximum Tension Capacity from Level 2 = 13.08 kips (See Step 3)
          • Maximum Tension Capacity from Level 2 + take-up device (ATS-ATUD14) = 13.08 + 24.39 = 37.47 kips
          • Maximum Tension Capacity from Level 2 + plate in bearing (PL14-3×8.5) = 13.08 + 13.98 = 27.05 kips       governs
          • Rod in tension (ATS-R11) = 32.30 kips
        • The lowest value in Step 4 is due to the plate in bearing, hence 27.05 kips is the maximum load that can be delivered at Level 1 and is the Maximum Tension Capacity.

In the System Deflection Summary page(s) of the Job Summary, is the Total System Deflection provided at Allowable or Strength levels?

Immediately following the individual run calculations in each job summary, Simpson Strong-Tie provides a summary of deflection of the rod system similar to what is shown in Figure 2. This breaks down the deformation of all components being considered. In the example below, the rod elongation and deflection of the take-up device are summed to provide the total deflection.

The calculated system deflection is presented at ASD level. See section below for how to use these system deflections for your drift calculation.

Figure 2. Sample System Deflection Check

What system deflection limit do you typically design to, and what does that include?

Unless otherwise specified on the plans or required by the building jurisdiction, Simpson Strong-Tie will design the continuous rod system to satisfy the deformation limits set forth in ICC-ES Acceptance Criteria (AC316). In some instances, the Designer may need a more restrictive deformation due to project specific conditions (e.g., tight building separations) and will require rod manufacturers to design for a lower deformation. Some jurisdictions (e.g., City of San Diego, City of San Francisco) may also have specific design requirements that continuous rod systems must conform to. The minimum recommended per-floor deformation limit set forth in AC316 is:

(Rod Elongation) + (Shrinkage Compensation Device Deflection) ≤  0.2” (ASD),

Or     (PDL/AE) + [ΔR + ΔA(PD/PA)] ≤ 0.2” (ASD)

Where:

PD = ASD demand cumulative tension load (kips)
L = length of the rod between restraints – i.e., floor-to-floor (in.)
A = net tensile area of the rod (in.2)
E = Young’s Modulus of Elasticity (29,000 ksi)
ΔR = seating increment of the shrinkage compensation device (as published in ICC-ES evaluation report)
ΔA = deflection of the shrinkage compensation device at the allowable load (as published in ICC-ES evaluation report)
PA = Allowable capacity (kips)

Should deformation limits be specified in the construction documents?

Simpson Strong-Tie strongly recommends this information be included in the construction documents. Along with the cumulative tension and compression forces, the required deformation limits for the holdown are important to ensure that rod manufacturers are designing the holdown to satisfy the desired shearwall performance.

 How do I use the system deformation limit?

The System Deflection is the total deformation of the holdown system from floor to floor (refer to the last two columns in Figure 2). This information represents the total ASD holdown deformation term, Δa, for each level and is to be used in the shearwall drift equation from the Special Design Provisions for Wind and Seismic (2015 SDPWS 4.3-1).

ASCE 12.8.6 requires that shearwall drift be calculated at strength level. Therefore, the information provided within the System Deflection Summary page needs to be converted from ASD to Strength Level. The conversion factors in Table 2 can be used to convert the ASD deformations to strength level. For discussions and methodology in converting bearing plate deformation to strength level, please refer to the WoodWorks Design Example of a Five-Story Wood Frame Structure over Podium Slab found here.  

Table 2. ATS Rod Deflection ASD to LRFD Conversion Factors

Can rod systems be used in Type III construction?

Yes! 2015 IBC §2303.2.5 requires that Fire Retardant-Treated Wood (FRTW) design values be adjusted based on the type of treatment used on the project. Adjustment factors vary for each FRTW manufacturer; refer to the ICC-ES evaluation report of the specified FRTW manufacturer for the unique adjustment values. Rod manufacturers need to know what treatment is being used so this information can be taken into consideration when designing compression posts and incremental bearing (bearing plates).

For more information and previous discussions on fire protection in mid-rise construction, see our previous posts:  Fire Protection Considerations with Five-Story Wood-Frame Buildings Part 1 and Part 2, and Connectors and Fasteners in Fire-Retardant-Treated Wood.

What are Simpson Strong-Tie’s guidelines for fire caulking material?

While there are many options for fire-rated caulking, these products can be used in conjunction with the Simpson Strong-Tie ATS system. Below is a list of considerations when selecting and specifying a material for use where the rods penetrate the top and sole plates:

  • The fire-rated caulking shall not be corrosive to metal when used in contact with ATS components.
  • Direct contact with shrinkage compensating devices (e.g., TUD, ATUD, RTUD) shall be avoided. Shrinkage compensating devices have moving components and may not function properly with debris interference.
  • Indirect contact with shrinkage compensating devices shall also be avoided. Shrinkage compensation accumulates up the building and therefore the largest shrinkage occurs at the top of the building. As such, when the building shrinks, remnants of the material may still be stuck to the threads of the rod and may be detrimental to the performance of some shrinkage compensating devices (e.g., an RTUD). It is recommended to detail the installation with shrinkage taken into consideration.
  • The fire-rated caulking should be pliable to accommodate wood shrinkage and the building moving down during this process.
  • The performance and the suitability of fire-rated caulking are outside the scope of Simpson Strong-Tie.

Why doesn’t your design include compression post design?

If the Engineer of Record has already specified compression posts to be used with a continuous rod system, Simpson Strong-Tie will not provide these on the holdown installation drawings. This is primarily done to prevent discrepancies between the specification in the contract documents and what is shown on the installation drawings.

 What is the maximum spacing between compression posts?

For platform-framed structures, the maximum spacing between compression posts is 9″. The large majority of Simpson Strong-Tie bearing plates will fit within the 9″ spacing requirement, eliminating the need for notching compression posts. In some framing conditions, such as balloon framing or a top chord bearing truss, the maximum spacing will be reduced to 6″. This is due to the limited amount of space between the top of the compression posts transferring uplift (via bearing) into the point of restraint (e.g., bearing plate) at the level above. To ensure this load path is complete, the posts need to be spaced closer.

What is the nailing schedule for the bridge block to the king studs?

Simpson Strong-Tie doesn’t recommend nailing the bridge block to the cripple as the bridge block member will shrink. Locking the bridge block in place may result in a gap forming between the bottom of the bridge block member and the top of the cripple studs, which is not accounted for in the Total System Deflection.

 Are there any published documents with design examples of continuous rod systems used in mid-rise construction?

There are two resources publicly available that provide discussion and examples. The first is a manual published by the Structural Engineers Association of California (SEAOC). Titled 2015 IBC SEAOC Structural/Seismic Design Manual Volume 2 – Examples for Light-Frame, Tilt-Up and Masonry Buildings, this document provides two examples  – one for a four-story wood hotel building, and the other for a three-story cold-formed steel apartment building on concrete podium deck.

Another useful resource is published by WoodWorks and is a design example of a five-story wood-frame structure over podium slab. This document can be found here.  

What questions do you have about the Strong-Rod ATS System? Leave them below.

SEAOSC Safer Cities Survey Results: How Are We Building Strength and Transparency in Our Communities?

Back in January, employees at Simpson were given the opportunity to learn more about the 401K retirement and investment plan. The big takeaways from my training session were a) save as much as you can as early as you can in life and b) use asset allocation to diversify your portfolio and avoid too much risk. Now, I’m not a big risk taker in general, so I dutifully picked a good blend of stocks and bonds with a range of low to high risk. It seems like a pretty sound strategy and it made me think of all the other ways I tend to minimize risk in my life. When I head to a restaurant, for example, I almost instinctively look for the county health grade sign in the window. When my husband and I went to go buy a new family car a couple years ago, I remember searching the National Highway Traffic Safety Administration (NHTSA) website for crash test ratings. Even when I’m doing something as mundane as having a snack, I will invariably flip over the Twinkie package to see just how many grams of fat are lurking inside (almost 5 per serving!). For all the rankings and information available to the general public for restaurants, cars and snacks, there isn’t much, if any, information to help us know if we’re minimizing our risk for one of the most common activities we do almost every day: walking into a building.

Risk level knob positioned on medium position, white background and orange light. 3D illustration concept for business security management.

 

Now before you accuse me of being overly dramatic about such a trivial activity, here’s some food for thought: research has shown that Americans spend approximately 90% of their day inside a building. That’s over 21 hours a day! Have you ever once thought to yourself, “I wonder if this building is safe? Would this building be able to withstand an earthquake or high wind event?” Or how about even taking a step back and asking, “Are there any buildings that are already known to be potentially vulnerable or unsafe, and has my city done anything to identify them?” Unfortunately, that kind of information about a city’s building stock is not usually readily available, but some in the community, including structural engineers, are working to change that.

Los Angeles skyline on a partly cloudy day with a row of palm trees in the foreground.

 

The charge is being led in California, a.k.a. Earthquake Country, where structural engineers are teaming up with cities to help identify buildings with known seismic vulnerabilities and provide input on seismic retrofit ordinances. Structural engineers have learned quite a bit about how buildings behave through observing building performance after major earthquakes, and building codes have been revised to address issues accordingly. However, according to the US Green Building Council, “…the annual replacement rate of buildings (the percent of the total building stock newly constructed or majorly renovated each year) has historically been about 2%, and during the economic recession and subsequent years, it’s been much lower.” This means that there are a lot of older buildings out there that have not been built to current building codes and were not designed with modern engineering knowledge.

Several cities in California have enacted mandatory seismic retrofit ordinances that require the strengthening of some types of known vulnerable buildings, but no state or nation-wide program currently exists. The Structural Engineers Association of Southern California (SEAOSC) recently decided to launch a study of which jurisdictions in the southern California region have started to take the steps necessary to enact critical building ordinances. According to SEAOSC President Jeff Ellis, S.E., “In order to develop an effective strategy to improve the safety and resilience of our communities, it is critical to benchmark building performance policies currently in place. For southern California, this benchmarking includes recognizing which building types are most vulnerable to collapse in earthquakes, and understanding whether or not there are programs in place to decrease risk and improve recovery time.” These results were presented in SEAOSC’s Safer Cities Survey, in partnership with the Dr. Lucy Jones Center for Science and Society and sponsored by Simpson Strong-Tie.

safer-cities-ca

This groundbreaking report is the first comprehensive look at what critical policies have been implemented in the region of the United States with the highest risk of earthquake damage. According to the Los Angeles Times, the survey “found that most local governments in the region have done nothing to mandate retrofits of important building types known to be at risk, such as concrete and wooden apartment buildings.”

The Safer Cities Survey highlights how the high population density of the SoCal region coupled with the numerous earthquake faults and aging buildings is an issue that needs to be addressed by all jurisdictions as soon as possible. An excerpt from the survey covers in detail why this issue is so important:

No building code is retroactive; a building is as strong as the building code that was in place when the building was built. When an earthquake in one location exposes a weakness in a type of building, the code is changed to prevent further construction of buildings with that weakness, but it does not make those buildings in other locations disappear. For example, in Los Angeles, the strongest earthquake shaking has only been experienced in the northern parts of the San Fernando Valley in 1971 and 1994 (Jones, 2015). In San Bernardino, a city near the intersection of the two most active faults in southern California where some of the strongest shaking is expected, the last time strong shaking was experienced was in 1899. Most buildings in southern California have only experienced relatively low levels of shaking and many hidden (and not so hidden) vulnerabilities await discovery in the next earthquake.

 The prevalence of the older, seismically vulnerable buildings varies across southern California. Some new communities, incorporated in the last twenty years, may have no vulnerable buildings at all. Much of Los Angeles County and the central areas of the other counties may have very old buildings in their original downtown that could be very dangerous in an earthquake, surrounded by other seismically vulnerable buildings constructed in the building booms of the 1950s and 1960s. Building codes do have provisions to require upgrading of the building structure when a building undergoes a significant alteration or when the use of it changes significantly (e.g., a warehouse gets converted to office or living space). Seismic upgrades can require changes to the fundamental structure of the building. Significantly for a city, many buildings never undergo a change that would trigger an upgrade. Consequently, known vulnerable buildings exist in many cities, waiting to kill or injure citizens, pose risks to neighboring buildings, and increase recovery time when a nearby earthquake strikes.

1994-northridge

The survey also serves as a valuable reference in being able to identify and understand what the known vulnerable buildings types are:

  1. Unreinforced masonry buildings: brick or masonry block buildings with no internal steel reinforcement — susceptible to collapse
  2. Wood-frame buildings with raised foundations: single-family homes not properly anchored to the foundation and/or built with a crawl space under the first floor — possible collapse of crawl space cripple walls or sliding off foundation
  3. Tilt-up concrete buildings: concrete walls connected to a wood roof — possible roof-to-wall connection failures leading to roof collapse
  4. Non-ductile reinforced concrete buildings: concrete buildings with insufficient steel reinforcement — susceptible to cracking and damage
  5. Soft first-story buildings: buildings with large openings in the first floor walls, typically for a garage — susceptible to collapse of the first story
  6. Pre-1994 steel moment frame buildings: steel frame buildings built before the 1994 Northridge earthquake with connections — susceptible to cracking leading to potential collapse

1933-earthquake-shot

Along with the comprehensive list of potentially dangerous buildings, the survey also offers key recommendations on how cities can directly address these hazards and reduce potential risks due to earthquakes. As a good starting point, the survey recommends having “…an active or planned program to assess the building inventory to gauge the number and locations of potentially vulnerable buildings…is one of the first steps in developing appropriate and prioritized risk mitigation and resilience strategies.

Economic costs can be substantial for businesses whose buildings have been affected by an earthquake. After a major seismic event, a structure needs to be cleared by the building department as safe before it can be reoccupied, and it will generally receive a green (safe), yellow (moderately damaged) or red (dangerous) tag.  A typical yellow-tagged building could take up to two months to be inspected, repaired and then cleared, meaning an enormous absence of income for businesses. The survey offers a strategy for getting businesses up and running quickly after an earthquake, in order to minimize such losses. The Safer Cities Survey recommends that cities adopt a “Back-to-Business” or “Building Re-Occupancy” program, which would “create partnerships between private parties and the City to allow rapid review of buildings in concert with City safety assessments…Back-to-Business programs…[allow] private parties to activate pre-qualified assessment teams, who became familiar with specific buildings to shorten evaluation time [and] support city inspections.

oes-inspectors-program

Basically, a program like this would allow a property owner to work with a structural engineer before an earthquake occurs. This way, the engineer is familiar with the building’s layout and potential risks, and can plan for addressing any potential damage. Having a program like this in place can dramatically shorten the recovery time for a business, from two months down to perhaps two weeks. Several cities have already adopted these types of programs, including San Francisco and Glendale, and it showed up as a component of Los Angeles’ Resilience by Design report.

Ultimately, the survey found that only a handful of cities have adopted any retrofit ordinance, but many cities indicated they were interested in learning more about how they could get started on the process. As a result, SEAOSC has launched a Safer Cities Advisory Program, which offers expert technical advice for any city looking to enact building retrofit ordinances and programs. This collaboration will hopefully help increase the momentum of strengthening southern California so that it can rebound more quickly from the next “Big One.”

We all want to minimize the risk in our lives, so let’s support our local structural engineering associations and building departments in exploring and enacting seismic building ordinances that benefit the entire community.

For additional information or articles of interest, please visit:

FRP Concrete Strengthening – Five Case Studies

Fiber-reinforced polymer (FRP) composite systems can be used to strengthen walls, slabs and other concrete or masonry members in buildings and other structures. The case studies below show ways in which Composite Strengthening Systems™ (CSS) provide valuable solutions for strengthening buildings and other structures for our customers.

Residential Project in San Francisco

The homeowner for this project wanted to repair some spalling concrete on his concrete piers and also wrap the piers with FRP. We worked with the contractor and homeowner to design a cost-effective solution. This was a successful project for all involved, since the alternative was to jacket the piers with costly and unsightly steel jackets.

residential-project-san-francisco

Materials: CSS-CUCF Carbon Fabric, CSS-ES Epoxy Saturant & Primer

School Project in Argentina

The goal of the project was to analyze a standard design of approximately 400 schools in Argentina that were built in the 1980s and to make recommendations to retrofit the structures to meet current seismic code requirements.  On analysis, it was found that columns were in need of shear reinforcement for the schools to meet the new seismic requirements.

Materials: CSS-UCF Carbon Fabric, CSS-CA Carbon FRP Anchors, CSS-ES Epoxy Saturant & Primer

Materials: CSS-UCF Carbon Fabric, CSS-CA Carbon FRP Anchors, CSS-ES Epoxy Saturant & Primer

Hospital Project in Butler, PA

The Engineer of Record on this project wanted to provide continuity across the slab construction joints, something which the existing rebar did not provide. We provided a design of Near-Surface-Mounted (NSM) laminates, which are installed in saw-cut grooves in the top of the concrete slab. This installation allows a flush finished surface, important for allowing the floor finishes to be installed on the slab.

Materials:CSS-CUCL Carbon Precured Laminate, CSS-EP Epoxy Paste & Filler

Materials: CSS-CUCL Carbon Precured Laminate, CSS-EP Epoxy Paste & Filler

Silo Project in Garden City, IA

The concrete silos on this project had spalling at the top portion, which caused a hazard at this site. After repairing the concrete, we provided a ring of carbon fabric to assist in keeping the top concrete of the silos solid for years to come.

Materials:CSS-CUCF Carbon Fabric, CSS-ES Epoxy Saturant & Primer

Materials: CSS-CUCF Carbon Fabric, CSS-ES Epoxy Saturant & Primer

Bridge Project in MN

MNDOT wanted to gain experience working with our CSS products on one of their bridges. We worked with their staff to design several types of strengthening solutions for bridge pier caps and columns. We then provided onsite installation training for the MNDOT maintenance staff to install the FRP products on the bridge.

Materials:CSS-CUCF Carbon Fabric CSS-CUGF E-glass Fabric CSS-ES Epoxy Saturant & Primer CSS-EP Epoxy Paste & Primer frp concrete strengthening

Materials: CSS-CUCF Carbon Fabric, CSS-CUGF E-glass Fabric, CSS-ES Epoxy Saturant & Primer, CSS-EP Epoxy Paste & Primer

We recognize that specifying Simpson Strong-Tie® Composite Strengthening Systems™ is unlike choosing any other product we offer. Leverage our expertise to help with your FRP strengthening designs. Our experienced technical representatives and licensed professional engineers provide complimentary design services and support – serving as your partner throughout the entire project cycle. Since no two buildings are alike, each project is optimally designed to the Designer’s individual specifications. Our pledge is to address your specific condition with a complete strengthening plan tailored to your needs, while minimizing downtime or loss of use, at the lowest possible installed cost.

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Your Partner During the Project Design Phase 

During the Designer’s initial evaluation or preparation of the construction documents, Simpson Strong-Tie can be contacted to help create the most cost-effective customized solution. These plans include detailed design calculations for each strengthening requirement and design drawings with all the necessary details to install the CSS system. Simpson Strong-Tie Engineering Services will work closely with the Design Engineer to provide all the necessary information required to design the system.

Why Use Our Design Services?

  • Assess feasibility studies to ensure suitable solutions to your application
  • Receive customized FRP strengthening solutions
  • Work with our trained contractor partners to provide rough-order-of-magnitude (ROM) budget estimates
  • Collaborate during the project design phase
  • Receive a full set of drawings and calculations to add to your submittal
  • Maintain the flexibility to provide the most cost-effective solution for your project
  • Gain trusted technical expertise in critical FRP design considerations

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Advanced FRP Design Principles

In this free webinar we will dive into some very important considerations including the latest industry standards, material properties and key governing limits when designing with FRP.


For complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/css or call your local Simpson Strong-Tie RPS specialist at (800) 999-5099.

 

Cold-Formed Steel Curtain-Wall Systems

In August 2012, Simpson Strong-Tie launched a comprehensive, innovative solution for curtain-wall framing. Our lead engineer for developing our line of connectors for curtain-wall construction explains the purpose of the curtain wall with the illustrations below.

steel-stud-framingFirst, curtain walls are not what you put up if you shared a room with your brother and sister when you were growing up. When I first learned about the use of cold-formed steel curtain walls, I laughed and thought: “Gosh, how useful this would be for someone growing up with 5 siblings in one bedroom!” I have always enjoyed the sense of humor that our engineers use to help explain technical topics.

Curtain walls can be described as exterior building walls with the primary purpose of protecting the interior building against the exterior weather and natural phenomena such as sun exposure, temperature changes, earthquakes, rain and wind.

To put it in structural terms, a curtain-wall system consists of non-load-bearing exterior walls that must still carry their own weight. Curtain walls are not part of the primary structural framing for the building, but they typically rely on the primary structural framing for support. Additionally, curtain walls receive wind and seismic loads and transfer these forces to the primary building structure.

Types of Curtain Walls

Glass and cladding curtain walls make up two basic types of curtain-wall systems. Glass curtain-wall systems are usually designed using aluminum-framed walls with in-fills of glass. The cladding curtain wall is a system with back-up framing that is covered in some type of cladding material. The cladding curtain-wall system is the type in which Simpson Strong-Tie products can be used.

mid-rise-buildings-1The back-up framing is the structural element of the curtain-wall system. It is typically constructed with cold-formed steel studs ranging from 31/2″ to 8″ deep, in 33 mil (20 ga.) to 97 mil (12 ga.) steel thicknesses. The framing studs are typically spaced at 16″or 24″ on center. There are many different types of cladding materials. They include, but are not limited to, exterior insulation finish systems (EIFS), glass-fiber-reinforced concrete (GFRC), bricks, metal panels and stone panels.

building-material-examplesDeflection

One essential function of the curtain wall is to allow for relative movement between the curtain-wall system and the main building structure. At first, it was not obvious to me why making this allowance was necessary, but our product development team creatively explained some of the reasons why this is an important must-have feature for curtain walls.

deflection-examplesFirst, the primary building will move up and down as it is loaded and unloaded by the live-load occupancy, similar to beam live-load deflections.

Second, the structure sways and has torsional displacement due to movement from lateral wind or seismic loads.

Third, concrete structures typically encounter creep and shrinkage, and there may be foundation differential settlement or soil compression from high-gravity loads.

Lastly, the temperature differential may cause the building elements to expand and contract, which, again, can result in relative movement between structural elements. This is similar to a bridge’s steel plate expansion joint system.

And if you are a curious designer like me, you probably wonder why the relative vertical moment is so significant in engineering design.

One key reason is to ensure that the curtain walls do not collect gravity loads from the building, so as to prevent overloading and possible failure of the stud framing. In addition, a well-designed curtain-wall system needs to retain the primary structural load path as intended by the building designer.

The other reason is to protect the cladding of the building. If you remember earlier, the cladding material may be marble, granite or natural stones that are often very expensive and heavy. In some cases, the cladding can be one of the most expensive systems in a building. And there are times when it’s much more cost-effective to design for relative movement than it is to over-design structural framing to address the stringent deflection requirements.

Construction Type

Bypass framing is a term that is often used in curtain-wall construction. In this system, the metal studs bypass the floor and hang off the outside edges of the building. You can see from the illustration how the studs run past, or bypass, the edge of the slab. In this case, the studs are supported vertically on the foundation at the bottom, and then run continuously past multiple floor levels.

Picture by Don Allen of Super Stud Building Products.

Picture by Don Allen of Super Stud Building Products.

In steel construction, concrete fill over metal deck is typically constructed with a heavy-gauge bent plate or structural angle. Connectors can attach directly to the steel angle or the web of an edge beam.

Simpson Strong-Tie SCB Bypass slide clip connections.

Simpson Strong-Tie SCB Bypass slide clip connections.

SSB Bypass Framing Slide-Clip Strut connections.

SSB Bypass Framing Slide-Clip Strut connections.

 

 

 

 

 

 

 

 

It may seem that this type of construction is too complex and requires great efforts to detail the many connections needed to hang the curtain wall off the outside of the building. So what are the compelling reasons to choose bypass framing construction?

Bypass framing can accommodate flexibility for the architect. In another words, the bypass configuration easily allows architects to create reveals, set-backs and other architectural features.  Plus, there are fewer joints to detail for movement when stud length can run continuously for several floors.  Another benefit is that the exterior finish can also be installed on a curtain-wall system with a tighter tolerance than the edge of the structure.

One other special bypass framing type is known as ribbon window or spandrel framing. Ribbon windows are a series of windows set side by side to form a continuous band horizontally across a façade. The vertical deflection for this type of bypass framing is typically accommodated at the window head. This type of bypass usually works well for panelized construction.

Another common curtain-wall system is infill framing, where the studs run from the top of one floor to the underside of the floor above. Sometimes it’s a challenge to attach bypass framing to the edge of thin concrete slabs. In the following illustration, deflection is designed at the top track of wall panels.

bypass-framing-in-actionIn Part 2 of this blog post series, I will provide more details about how we have innovated products to be used for this application, plus a more comprehensive post about the products we offer and how they are typically used.

In the meantime, you can check out our product offering. Our recent SC slide-clip and FC fixed-clip connectors are designed for high-seismic areas.

I would like to invite you to comment and provide feedback on this topic and tell us whether you’ve had any experience working with a Designer on a CFS curtain-wall project. If you are a Designer who specializes in this discipline, how are you designing curtain-wall systems for seismic forces?

 

Soft-Story Retrofits Using the New Simpson Strong-Tie Retrofit Design Guide

Thousands of soft-story buildings up and down the West Coast require retrofits to prevent collapse in the event of a major earthquake. Whether the retrofits are mandated by a city ordinance (as in San Francisco, Berkeley and Los Angeles) or are undertaken as voluntary upgrades, the benefits of adding necessary bracing to strengthen the ground story are immense. Simpson Strong-Tie has taken the lead, with our new Soft-Story Retrofit Guide, to provide information that helps engineers find solutions to reinforce soft-story buildings against collapse. We are also providing information on the two methods that can be used for the analysis and design of these soft story retrofits.

soft-story-retrofit-guideAfter the initial information section of the guide, a two-page illustrated spread (pp. 14–15) shows various retrofit products that could be used to retrofit the soft-story structure with reference to the following pages. Three main lateral-force-resisting systems highlighted in this graphic are the Strong Frame® special moment frame (SMF), the new Strong-Wall® wood shearwall, and conventional plywood shearwalls. Individual retrofit components are also shown, such as connection plates and straps for lateral-load transfer, anchors for attachment to the foundation, fasteners and additional products such as the RPBZ retrofit post base and AC post caps for providing a positive connection.

soft-story-product-illustrationTurning the page, you come to the section describing in detail the many benefits of the Strong-Frame special moment frame (SMF) in a retrofit situation. The engineered performance of the SMF provides the additional strength and ductility that the building requires and can be fine-tuned by selecting various combinations of beams, columns, and Yield-Link® structural fuse sizes. A typical retrofit Strong Frame® SMF comes in three complete pieces allowing for the frame to be installed on the interior of the structure in tight quarters. The frame is simply installed using a 100% snug-tight field-bolted installation with no on-site welding or lateral-beam bracing required.

field-installation-beam-to-columnThe next lateral system we focus on is the Strong-Wall® shearwall and the new grade beam solutions offered to reduce the concrete footprint. The new Strong-Wall wood shearwall includes an improved front-access holdown and top-of-wall connection plates for easier installation. Both the Strong Frame SMF and the Strong-Wall wood shearwall have load-drift curves available for use with FEMA P-807. Site-built shearwalls can be installed using retrofit anchor bolts at the mudsill and new holdowns at the shearwall end posts.

strong-wall-wood-shearwall-pushover-curveIn the pages following the lateral systems, various products are shown with tabulated LRFD capacities, whereas ASD capacities are typically provided in the order literature for these products. Both ASD and LRFD capacities have been provided for products with new testing values such as the A35 and L90 angles installed with ⅝”-long SPAX screws into three different common floor sheathing materials, as well as for the new HSLQ heavy-shear transfer angle designed to transfer higher lateral forces directly from 4x blocking to the 4x nailer on the Strong-Frame SMF, even when a shim is used between the floor system and the frame. LRFD capacities are provided in this new Soft-Story Retrofit Guide specifically for use with the FEMA P-807 design methodology. This methodology specifies in section 6.5.1 that:

Load path elements should be designed to develop the full strength and the intended mechanism of the principal wall or frame elements. Therefore, to ensure reliability, appropriate strength reduction factors should be applied to the ultimate strengths of load path elements. Specific criteria may be derived from principles of capacity design or from other codes or standards, such as ASCE/SEI 41 or building code provisions involving the overstrength factor, Ωo.

FEMA P-807 bases the capacity of the retrofit elements on the peak strength. LRFD capacities are provided for various load-path connector products, which can be used to develop the full strength of the lateral-force-resisting element to satisfy this requirement.

typical-a35-hslq412-installationWrapping up, the guide focuses on the various free design tools and resources available for the evaluation, design and detailing of the soft-story structure retrofit. These tools include the Weak Story Tool with Simpson Strong-Tie® Strong Frame® Moment Frames, Design Tutorials for the WST for both San Francisco– and Los Angeles–style buildings, our Soft-Story Retrofit Training Course offering CEUs, Strong Frame Moment Frame Selector Software, Anchor Designer™ Software for ACI 318, ETAG and CSA, and tailored frame solutions using our free engineering services.

soft-story-documentsFor other information regarding soft-story retrofits, refer to previous blogs in “Soft-Story Retrofits,”  “City of San Francisco Implements Soft-Story Retrofit Ordinance,” and “Applying new FEMA P-807 Weak Story Tool to Soft-Story Retrofit.”

 

 

 

Great ShakeOut Earthquake Drill 2016

The Great ShakeOut Earthquake Drill is an annual opportunity for people in homes, schools and organizations to practice what to do during earthquakes and improve their preparedness. In a post I wrote last October about the Great ShakeOut, I reminisced about the first earthquake I had to stop, drop and cover for – the Livermore earthquake in January, 1980. This year got me thinking about how our evacuation drills work.

At Simpson Strong-Tie, we use the annual Great ShakeOut drill to practice our building evacuation procedures. Evacuation drills are simple in concept – alarms go off and you exit the building. We have volunteer safety wardens in different departments who confirm that everyone actually leaves their offices. There are always a few people who want to stay inside and finish up a blog post. Once the building is empty and we have all met up in the designated meeting area, we do a roll call and wait for the all-clear to get back to work.

Several years ago the alarms went off. While waiting for the drill to end, we were concerned to see fire fighters arrive and rush into the building. Realizing this was not a drill, there were some tense moments of waiting. The fire chief and our president eventually walked out of the building and our president was yelling for one of our engineers. Turns out the engineer (who shall remain nameless) was cooking a chicken for lunch. Yes, a whole chicken. The chicken didn’t make it – I’m not sure what the guilty engineer had for lunch afterwards. At least we received extra evacuation practice that year. We aren’t allowed to cook whole chickens in the kitchen anymore.

Simpson Strong-Tie is helping increase awareness about earthquake safety and encouraging our customers to participate in the Great ShakeOut, which takes place next Thursday on October 20. It’s the largest earthquake drill in the world. More than 43 million people around the world have already registered on the site.

On October 20, from noon to 2:00 p.m. (PST), earthquake preparedness experts from the Washington Emergency Management Division and FEMA will join scientists with the Washington Department of Natural Resources and the Pacific Northwest Seismic Network for a Reddit Ask Me Anything – an online Q&A. Our very own Emory Montague will be answering questions. The public is invited to ask questions here. (Just remember that this thread opens the day before the event and not sooner.)

Emory Montague from Simpson Strong-Tie

Emory, ready to answer some seismic-related questions.

We’re also providing resources on how to retrofit homes and buildings, and have information for engineers here and for homeowners here.

Earthquake risk is not just a California issue. According to the USGS, structures in 42 of 50 states are at risk for seismic damage. As many of you know, we have done a considerable amount of earthquake research, and are committed to helping our customers build safer, stronger homes and buildings. We continue to conduct extensive testing at our state-of-the-art Tye Gilb lab in Stockton, California. We have also worked with the City of San Francisco to offer education and retrofit solutions to address their mandatory soft-story building retrofit ordinance and have created a section on our website to give building owners and engineers information to help them meet the requirements of the ordinance.

Last year, Tim Kaucher, our Southwestern regional Engineering Manager, wrote about the City of Los Angeles’s Seismic Safety Plan in this post. Since that time, the City of Los Angeles has put that plan into action by adopting mandatory retrofit ordinances for both soft-story buildings and non-ductile concrete buildings. Fortunately, California has not had a damaging earthquake for some time now. As a structural engineer, I find it encouraging to see government policy makers resist complacency and enact laws to promote public safety.

Participating in the Great ShakeOut Earthquake Drill is a small thing we can all do to make ourselves more prepared for an earthquake. If your office hasn’t signed up for the Great ShakeOut Earthquake Drill, we encourage you to visit shakeout.org and do so now.

Concrete Anchorage for ASD Designs

One of the first things I learned in school about using load combinations was that you had to pick either Load and Resistance Factor Design (LRFD)/Strength Design (SD) or Allowable Stress Design (ASD) for a building and stick with it, no mixing allowed! This worked for the most part since many material design standards were available in a dual format. So even though I may prefer to use LRFD for steel and ASD for wood, when a steel beam was needed at the bottom of a wood-framed building that was designed using ASD load combinations, the steel beam could easily be designed using the ASD loads that were already calculated for the wood framing above since AISC 360 is a dual- format material standard. And when the wood-framed building had to anchor to concrete, ASD anchor values were available in the IBC for cast-in-place anchors and from manufacturers for post-installed anchors in easy-to-use tables, even though ACI 318 was not a dual-format material standard. (Those were good times!)

Then along came ACI 318-02 and its introduction of Appendix D – Anchoring to Concrete, which requires the use of Strength Design. The 2003 IBC referenced Appendix D for Strength Design anchorage, but it also provided a table of ASD values for some cast-in-place headed anchors that did not resist earthquake loads or effects. This option to use ASD anchors for limited cases remained in the 2006, 2009 and 2012 codes. In the 2015 IBC, all references to the ASD anchor values have been removed, closing the book on the old way of designing anchors.

ICC-ES-equation-tensionSo what do you do now? Well, there is some guidance provided by ICC-ES for manufacturers to convert calculated SD capacities to ASD allowable load values. Since there is no conversion procedure stated in the IBC or referenced standards, designers may want to use this generally accepted method for converting anchor capacities designed using ACI 318. ICC-ES acceptance criteria for post-installed mechanical and adhesive anchors (AC193 and AC308) and cast-in-place steel connectors and proprietary bolts (AC398 and AC399) outline a procedure to convert LRFD capacities to ASD using a weighted average for the governing LRFD/SD load combination. So if the governing load combination for this anchor was 1.2D + 1.6L and the dead load was 1,000 pounds and the live load was 4,000, then the conversion factor would be (1.2)(0.2) + (1.6)(0.8) = 1.52 (keep in mind that the LRFD/SD capacity is divided by the conversion factor in the ICC-ES equation shown here for tension).

Right away, there are a few things that you may be thinking:

  1. What about load factors that may exist in ASD load combinations?
  2. It may just be easier to just recalculate my design loads using LRFD/SD combinations!
  3. The resulting allowable loads will vary based on the load type, or combination thereof.
  4. If the ACI 318 design strength is limited by the steel anchor, then the conversion will result in an allowable load that is different from the allowable load listed for the steel element in AISC 360.

Let’s take a look at these objections one by one.

Item 1: Since unfactored earthquake loads are determined at the ultimate level in the IBC, they have an LRFD/SD load factor of 1.0 and an ASD load factor less than 1.0, which is also true for wind loads in the 2012 and 2015 IBC (see graphic below). Using the LRFD/SD load factor of 1.0 obviously does not convert the capacity from LRFD to ASD so you must also account for ASD load factors when calculating the conversion factor. To do so, instead of just using the LRFD load factor, use the ratio of LRFD Factor over ASD Factor. So if the governing load combination for an anchor was 0.9D + 1.0E and the dead load was 1,000 pounds and the seismic load was 4,000, then the conversion factor would be (0.9)(0.2) + (1.0/0.7)(0.8) = 1.32.

ICC-ES-equations

Item 2: Even though the weighted average conversion requires you to go back and dissect the demand load into its various load types, often this can be simplified. ICC-ES acceptance criteria permit you to conservatively use the largest load factor. The most common application I run into is working with ASD-level tension loads for wood shearwall overturning that must be evaluated using SD-level capacities for the concrete anchorage. Since these loads almost always consist of wind or seismic loads, using the largest factor is not overly conservative. Depending on the direction in which you are converting the demand loads or resistance capacities, the adjustment factors are as shown in the figure below. Affected Simpson Strong-Tie products now have different allowable load tables for each load type. (For examples, see pp. 33-36 of our Wood Construction Connectors catalog for wind/seismic tables and pp. 28-30 of our Anchoring and Fastening Systems catalog for static/wind/seismic tables.)

IBC-ealier-later

Item 3: I am unsure whether there is any sound rationale for having allowable loads for an anchor resisting 10% dead load and 90% live load differ from those of an anchor that resists 20% dead load and 80% live load. Perhaps a reader could share some insight, but I just accept it as an expedience for constructing an ASD conversion method for a material design standard that was developed for SD methodology only.

Item 4: We have differing opinions within our engineering department on how to handle the steel strength component of the various SD failure modes listed in ACI 318. Some believe all SD failure modes in ACI 318 should be converted using the load factor conversion method. I side with others who believe that the ASD capacity of a steel element should be determined using AISC 360. So when converting SD anchor tension values for a headed anchor, I would apply the conversion factor to the concrete breakout and pullout failure modes from ACI 318, but use the ASD steel strength from AISC 360.

Finally, I wanted to point out that the seismic provisions in ACI 318, such as ductility and stretch length, must be considered when designing anchors and are not always apparent when simply converting to ASD. For this reason, I usually suggest converting ASD demand loads to SD levels so you can use our Anchor Designer™ software to check all of the ACI 318 provisions. But for some quick references, we now publish tabulated ASD values for our code-listed mechanical and adhesive anchors in our C-A-2016 catalog —  just be sure to read all of the footnotes!

Simpson Strong-Tie® Strong-Wall® Wood Shearwall – The Latest in Our Prefabricated Shearwall Panel Line Part 2

In last week’s blog post, we introduced the Simpson Strong-Tie® Strong-Wall® Wood Shearwall. Let’s now take a step back and understand how we evaluate a prefabricated shear panel to begin with.

First, we start with the International Building Code (IBC) or applicable state or regional building code. We would be directed to ASCE7 to determine wind and seismic design requirements as applicable. In particular, this would entail determination of the seismic design coefficients, including the response modification factor, R, overstrength factor, Ωo, and deflection amplification factor, Cd, for the applicable seismic-force-resisting system. Then back to the IBC for the applicable building material: Chapter 23 covers Wood. Here, we would be referred to AWC’s Special Design Provisions for Wind and Seismic (SDPWS) if we’re designing a lateral-force-resisting system to resist wind and seismic forces using traditional site-built methods.

Design Documents: IBC, ASCE7 and SDPWS

Design Documents: IBC, ASCE7 and SDPWS

These methods are tried and true and have been shown to perform very well in light-frame construction during wind or seismic events. But over the years, many people have come to enjoy things like lots of natural light in our homes, great rooms with tall ceilings and off-street secure parking.

prefab2Due to Shearwall aspect ratio limitations defined in SDPWS as well as the strength and stiffness limitations of these traditional materials – including wood structural panel sheathing, plywood siding and structural fiberboard sheathing, to name a few – we’re left looking for alternative solutions. Thankfully, the IBC has left room for the use of innovative solutions beyond what’s explicitly stated in the code. Section 104.11 of the 2015 IBC provides the following provision:

104.11 Alternative material, design and methods of construction and equipment

The provisions of this code are not intended to prevent the installation of any material or prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method, or work offered is, for the purpose intended, not less than the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety…

104.11.1 Research Reports. Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.

104.11.2 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code […] the building official shall have the authority to require tests as evidence of compliance…

Research Reports

The route we at Simpson Strong-Tie typically take is to obtain a research report from an approved source, i.e., the ICC Evaluation Service or the IAPMO Uniform Evaluation Service. Each of these evaluation service agencies publishes acceptance criteria that have gone through a public review process and contain evaluation procedures. The evaluation procedures might contain referenced codes and test methods, analysis procedures and requirements for compatibility with code-prescribed systems.

Prefabricated Panel Evaluation

Let’s once again take a step back and consider the function of our Strong-Wall® shearwalls. They’re prefabricated panels intended to provide lateral and vertical load-carrying capacity to a light-framed wood structure where traditional methods are not applicable or are insufficient. We need to provide a complete lateral load path, which ensures that the load continues through the top connection into the panel and then into the foundation through the bottom connection. To evaluate the panel’s ability to do what we’re asking of it, we use a combination of testing and calculations with considerations for concrete bearing, fastener shear, combined member loading, tension and shear anchorage, panel strength and stiffness, etc.

I could write a five-thousand-word feature story for the New York Times discussing the calculations in great detail, but let’s focus on the more exciting part – testing! Simpson Strong-Tie has several accredited facilities across the country where all of this testing takes place; click here for more info.

Testing Acceptance Criteria

Now to pull back the curtain a bit on the criteria we follow in our testing: We test our panels in accordance with the criteria provided in ICC-ES AC130 – Acceptance Criteria for Prefabricated Wood Shear Panels or ICC-ES AC322 – Acceptance Criteria for Prefabricated, Cold-Formed, Steel Lateral-Force-Resisting Vertical Assemblies, as applicable. These criteria reference the applicable ASTM Standard, ASTM E2126-11, which illustrates test set-up requirements and defines the loading protocol among other things. If you’re interested, the work done by the folks involved with the CUREE-Caltech Woodframe Project, which is the basis for the testing protocol we use today, makes for an excellent read. The CUREE protocol, as it’s known, is a displacement-controlled cyclic loading history that defines how to load a panel. A reference displacement, Δ, is determined from monotonic testing, and the cyclic loading protocol, which is a series of increasing displacements whose amplitudes are functions of Δ, is developed. I’ve provided a graphic depicting the protocol below.

CUREE Loading Protocol (Excerpt from ASTM E2126-11)

CUREE Loading Protocol (Excerpt from ASTM E2126-11)

When prefabricated shear panels are subjected to the loading protocol shown above, a load-displacement response is generated; we call this a hysteresis loop or curve.

Hysteresis Curve (Excerpt from ASTM E2126-11)

Hysteresis Curve (Excerpt from ASTM E2126-11)

We then use this curve to generate an average envelope (backbone) curve that will be used for analysis in accordance with the procedures defined in AC130 or AC322 as applicable.

Average Envelope Curve (Except from ASTM E2126-11)

Average Envelope Curve (Except from ASTM E2126-11)

Returning to the acceptance criteria, there are different points of interest on the average envelope curve depending upon whether we’re establishing allowable test-based values for wind-governed designs or for seismic-governed designs. I should also note that both wind and seismic designs consider both drift and strength limits when determining allowable design values.

Wind is fairly straightforward, so let’s start there. While the building code does not explicitly define a story drift limit for wind design, the acceptance criteria do. The allowable wind drift, Δwind, shall be taken as H/180, where H is the story height. The allowable ASD in-plane shear value, Vwind, is taken as the load corresponding to Δwind. I mentioned a strength limit as well; this is simply taken as the ultimate test load divided by a safety factor of 2.0.

Contrary to wind design, the building code does define a story drift limit for seismic design. ASCE7 Table 12.12-1 defines the allowable story drift, δx, as 0.025H for our purposes, where H is the story height. The strength design level response displacement, δxe, is now determined using ASCE7 Equation 12.8-15 as referenced in AC130 and AC322 as follows:

formula1

Where:

  • δxe = LRFD strength design level response displacement
  • δx = Allowable story drift = 0.025H for Risk Category I/II Buildings (ASCE7 Table 12.12-1)
  • Ie = Seismic importance factor = 1.0 for Risk Category I Buildings (ASCE7 Table 1.5-2)
  • Cd = Deflection amplification factor = 4.0 for bearing wall systems consisting of light-frame wood walls sheathed with wood structural panels rated for shear resistance (ASCE7 Table 12.2-1)

We then consider the shear load corresponding to the strength level response displacement, VLRFD, and multiply this value by 0.7 to determine the allowable ASD shear based on the seismic drift limit, VASD. Lastly, the seismic strength limit is taken as the ultimate test load divided by a safety factor of 2.5.

Compatibility with Code-Prescribed Methods

We’ve gone through the steps to evaluate the allowable design values for our panels, but we’re not done yet. AC130 and AC322 define a series of criteria to ensure that the seismic response is compatible with code-defined methods with respect to strength, ductility and deformation capacity. Once we verify that these compatibility parameters have been satisfied, we may then apply the response modification factor, R, overstrength factor, Ωo, and deflection amplification factor, Cd, defined in ASCE7 for bearing wall systems consisting of light-frame wood or cold-formed steel walls sheathed with wood structural panels or steel sheets. This enables the prefabricated shearwalls to be used in light-frame wood or cold-formed steel construction. I’ve very briefly covered an important topic in seismic compatibility, but there has been plenty published on the issue; I recommend perusing the article here for more details.

We’ve now followed the path from building code to acceptance criteria to evaluation report. More importantly, we understand why Strong-Wall® shearwall panels are required and the basics of how they’re evaluated. If there are items that you’d like to see covered in more detail or if you have questions, let us know in the comments below.