This week’s post was written by Darren Conrad, PE. Engineering Manager, Truss at Simpson Strong-Tie.
With Hurricane Irma wrapping up, the cleanup after Hurricane Harvey’s devastation underway in Houston and more big storms already churning in the Atlantic, it seems like a good time to discuss hurricanes and high wind. There is a great deal of good information out there to help us better understand hurricanes and their impact on people, structures and other property. To improve awareness of wind speeds and their measurement, this article will discuss a commonly misunderstood aspect of hurricane wind-speed reporting. Continue Reading
He huffed, and he puffed, and he blew the roof sheathing off! That’s not the way kids’ tale goes, but the dangers high winds pose to roof sheathing are very real. Once the roof sheathing is gone, the structure is open and its contents are exposed to the elements and much more vulnerable to wind or water damage. It is a storyline that we meet all too often in the news.
About two years ago, the ASTM subcommittee on Driven and Other Fasteners (F16.05), addressed fastening for roof sheathing in high-wind areas by adding a special nail to ASTM F1667-17 – Standard Specification for Driven Fasteners: Nails, Spikes and Staples. The Roof Sheathing Ring-Shank Nail was added to the standard as Table 46. Figure 1 illustrates the nail and lists its geometrical specifications. This is a family of five ring-shank nails that can be made from carbon steel or stainless steel (300 series). Specific features of these nails are the ring pitch (number of rings per inch), the ring diameter over the shank, the length of deformed shank and the head diameter. Also, note B specifies that the nails shall comply with the supplementary requirement of Table S1.1, which tabulates bending yield strength. In this diameter class, the minimum bending yield strength allowed is 100 ksi.
The IBHS (Insurance Institute for Business and Home Safety) discusses roof deck fastening in its Builders Guide that describes the “FORTIFIED for Safer Living” structures. The IBHS FORTIFIED program offers solutions that reduce building vulnerability to severe thunderstorms, hurricanes and tornadoes. Keeping the roof sheathing on the structure is critical to maintaining a safe enclosure and minimizing damage, and roof sheathing ring-shank nails can be part of the solution. As Figure 2 from IBHS (2008) shows, every wood-frame structure has wind vulnerability.
More importantly for the wood-frame engineering community, the Roof Sheathing Ring-Shank Nails are being included in the next revision of the AWC National Design Specification for Wood Construction (NDS-2018), which is a reference document to both the International Building Code and the International Residential Code. You will be able to use the same NDS-2018, chapter 12 withdrawal equation to calculate the withdrawal resistance for Roof Sheathing Ring-Shank Nails and Post Frame Ring-Shank nails. The calculated withdrawal will be based on the length of deformed shank embedded in the framing member. Also, Designers need to consider the risk of nail head pull-through when fastening roof sheathing with ring-shank nails. If the pull-through for roof sheathing ring-shank nails is not published, you will be able to use the new pull-through equation in the NDS-2018 to estimate that resistance. Simpson Strong-Tie has some stainless-steel products that meet the requirements for Roof Sheathing Ring-Shank Nails. These will be especially important to those in coastal high-wind areas. Table 1 shows some of the Simpson Strong-Tie nails that can be used as roof sheathing ring-shank nails. These nails meet the geometry and bending yield strength requirements given in ASTM F1667. See the Fastening Systems catalog C-F-2017 for nails in Type 316 stainless steel that also comply with the standard.
Improve your disaster resilience and withstand extreme winds by fastening the sheathing with roof sheathing ring-shank nails. You can find Roof Sheathing Ring-Shank nails in ASTM F1667, Table 46, and you will see them in the AWC NDS-2018, which will be available at the end of the year. Let us know your preferred fastening practices for roof sheathing.
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.
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 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.).
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.)
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.
The Designer has the option to include additional sources of vertical displacement for drift calculation. (See Figure 5.)
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.
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.
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.
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.
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.
It seems that each major hurricane tends to teach those of us in the construction industry some lesson. With Hurricane Andrew, the lessons were the importance of protection from windborne debris, and the importance of proper construction of overhangs on gable ends.
There are two main areas where gable ends can fail. One is a failure of the hinge at the connection between the top plate of the wall and the gable end framing, if the gable end is not balloon-framed with continuous studs. This is now addressed in the International Residential Code. Since 2009, Section R602.3 has required that “Studs shall be continuous from support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.”
The other common wind-related failure at gable ends is uplift of the roof decking at the overhang. This can be from two causes: inadequate nailing of the sheathing to supporting framing, or inadequate connections of the framing at the rake edge that supports the roof. As far as this author can tell, this area of light construction is not covered in the International Residential Code for wood framing, but it is covered for cold-formed steel framing, where Section R804.3.2.1.2 contains requirements for “Rake overhangs.” The two methods shown are the cantilever outlooker (Option 1) and the ladder outlooker (Option 2).
In the photo above, it appears that the cantilevered outlooker method was used, and that there was a failure of the outlooker connections at the gable end and the first full truss. If you look closely, the end nails from the full-height truss that were in the end of the outlookers can be seen in a couple of places.
If a truss roof is used with this method, the gable truss is manufactured 3½” shorter than the others. Then a 2×4 outlooker is placed over the dropped gable, and butted into the side of the adjacent full-height truss. Then the barge or fly rafter is attached to the end of the cantilevered outlooker. At the overhang, wind can cause uplift on both the bottom and top surface. The uplift at the end of the outlooker imparts an uplift force at the gable truss, which must be resisted by a tension connection such as a hurricane tie, and a downward force at the connection to the full-height truss.
The other method commonly used to support the sheathing and the barge rafter is the ladder method. With this technique, lookout blocks are used to connect the barge or fly rafter back to the gable framing. One way this can be constructed is as a full ladder, with parallel fly rafter and ledger with block framing in between. Either this assembly can be constructed on the ground and then raised and fastened in place, or it can be built in place at the overhang. Or there are also examples where a ledger is not used, and the block framing is just connected directly to the top chord of the gable truss or gable rafter. This method is less wind-resistant, and in literature is limited to a 12″ overhang.
If the gable overhang is to resist wind loads properly, it must either be designed, or constructed in accordance with some pre-engineered prescriptive detail. Figure 4 shown above was originally published in a Simpson Strong-Tie Technical Bulletin, the High Wind Framing Connection Guide. But this Guide is no longer published. As shown earlier in Figure 2, there are some prescriptive details in the IRC for cold-formed steel construction. These are limited to an overhang length of 12″ and apply for up to 139 miles-per-hour ultimate wind speed. For wood-framed construction, comparable details are contained in the American Wood Council Wood Frame Construction Manual. For the cantilevered outlooker method, connection design loads are published for various wind speeds. Cantilevered outlookers are permitted to extend out up to 24 inches, while the ladder outlookers are only permitted to extend out 12 inches. See below for excerpted figures and tables from the Wood Frame Construction Manual, courtesy of the American Wood Council.
In addition to the framing design, the connection of the roof decking at this location is critical. If you’re building to traditional construction methods, with 6″ nail spacing at panel edges and 12″ nail spacing at interior supports, the close nail spacing ends up at the nonstructural outer member, while the nailing at the actual roof edge over the gable is only 12″ on center. As shown in the details above, newer documents do indicate the importance of spacing the nails over the gable end at the closest spacing, both because these are subject to the highest withdrawal loads and because this is the edge of the diaphragm for transfer of lateral loads.
The Journal of Light Construction has a discussion of the unbraced gable end overhang on one of their Forums.
The Florida Division of Emergency Management provides some information on wind resistance of gable overhangs and some possible means of retrofitting them here.
Have you seen or designed with different methods for framing gable overhangs?
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.
So 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:
What about load factors that may exist in ASD load combinations?
It may just be easier to just recalculate my design loads using LRFD/SD combinations!
The resulting allowable loads will vary based on the load type, or combination thereof.
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.
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.)
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!
When it comes to wood-frame construction, hurricane ties are among the most commonly specified connectors. They play a critical role in a structure’s continuous load path and may be used in a variety of applications, like attaching roof framing members to the supporting wall top plate(s), or tying wall top or bottom plates to the studs. They are most commonly used to resist uplift forces, but depending on regional design and construction practices, hurricane ties may also resist lateral loads that act in- or out-of-plane in relation to the wall.
Simpson Strong-Tie manufactures approximately 20 different models of hurricane ties, not counting twist straps, other clips, or the new fully-threaded SDWC screws often used in the same applications. This assortment of models raises the question, “How do you select the right one?”
In this post, we’ll outline some of the key elements to consider when selecting a hurricane tie for your project.
Let’s start with the obvious one. If your building’s roof trusses have an uplift of 600 lb. at each end, don’t select a hurricane tie with a published capacity of less than 600 lb. It’s also important to consider combined loading if you plan to use the tie to resist both uplift and lateral loads. When the connector is resisting lateral loads, its capacity to resist uplift is reduced. I won’t go into too much detail on this topic since it was covered in a recent blog post, but in lieu of the traditional unity equation shown in Figure 1, certain Simpson Strong-Tie connectors (hurricane ties included) are permitted to use the alternative approach outlined in Figure 2.
What if the tabulated loads in the catalog for a single connector just aren’t enough? Use multiple connectors! An important note on using multiple connectors, though: Using four hurricane ties doesn’t always mean you’ll get 4x the load. Check out the recently updated F-C-HWRCAG16 High Wind-Resistant Construction Application Guide for allowable loads using multiple connectors and for guidance on the proper placement of connectors so as to avoid potential overlap or fastener interference.
While the majority of the hurricane ties that Simpson Strong-Tie offers are one-sided (such as the H2.5A), some are designed so the truss or rafter fits inside a “U” shape design to allow for fastening from both sides (such as the H1). If using the latter, make sure the width of the truss or rafter is suitable for the width of the opening in the hurricane tie – don’t select an H1 for a 2-ply roof truss.
Additionally, the height of the hurricane tie and the wood members being attached should be compatible. For example, an H2.5A would not be compatible with a roof truss configured with only a nominal 2×4 bottom chord over the plate since the two upper nail holes in the H2.5A will miss the 2×4 bottom chord (see Figure 4). This is actually such a common mis-installation that we specifically tested this scenario and have developed an engineering letter on it (note the greatly reduced capacity). In this case the ideal choice would be the H2.5T, which has been specifically designed for a 2×4 truss bottom chord.
It’s also essential to pay close attention to the diameter and length of the fasteners specified in the Simpson Strong-Tie literature. While many hurricane ties have been evaluated with 8d x 1½” nails for compatibility with nominal 2x roof framing, some require the use of a longer, 8d common (2½” long) nail and others require a larger-diameter 10d nail.
When specifying products for a continuous load path, it’s a good idea to select connectors that all use the same size nail to avoid improper installations on the job. It’s much easier if the installer doesn’t need to worry about which size nail he currently has loaded in his pneumatic nailer.
Do your roof and wall framing members line up? If so, creating a continuous load path can be made simpler by using a single hurricane tie to fasten the roof framing to studs. The H2A, H7Z, and H10S are some of the connectors designed to do just that. If your framing doesn’t align, though, you can use two connectors to complete the load path. For simplification and to reduce potential mix-ups in the field, consider selecting the same hurricane tie for your roof framing-to-top-plate and top plate-to-stud connections, like the H2.5A.
Besides the added benefit of fewer connectors to install, using a single hurricane tie from your roof framing to your wall studs can eliminate top-plate roll, a topic discussed at length in one of our technical bulletins.
Some additional factors that may influence your selection of a hurricane tie are:
Environmental factors and corrosion should be considered when selecting any product. Nearly every hurricane tie is available in ZMAX®, our heavier zinc galvanized coating, and several are available in Type 316 stainless steel. A full list of products available in ZMAX or stainless steel may be found on our website. On a related note, be sure to use a fastener with a finish similar to that of the hurricane tie in order to avoid galvanic corrosion caused by contact between dissimilar metals.
When retrofitting an existing structure, local jurisdiction requirements will also influence your decision on which hurricane tie to use. As an example, the state of Florida has very specific requirements for roof retrofitting, which we outline in a technical bulletin, and they specifically mention the roof-to-wall connection. Be sure to check with your local city, county or state for specific requirements before you decide to retrofit.
Availability of wind insurance discounts in your area could also affect your decision on which type of hurricane tie to use on your home. Your insurance company may provide a greater discount on your annual premium for ties that wrap over the top of your roof framing and are installed with a certain minimum quantity of nails. Check with your insurance provider for additional information and requirements.
Although this is a lot to take in, hopefully it makes choosing the right hurricane tie easier for you on your next project. Are there any other items you consider in your design that weren’t mentioned above? Let us know in the comments below.
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.
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.
Due 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…
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.
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.
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.
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:
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.
The ICC says that “Building Safety Month is a public awareness campaign to help individuals, families and businesses understand what it takes to create safe and sustainable structures. The campaign reinforces the need for adoption of modern, model building codes, a strong and efficient system of code enforcement and a well-trained, professional workforce to maintain the system.” Building Safety Month has a different focus each week for four weeks. Week One is “Building Solutions for All Ages.” Week Two is “The Science Behind the Codes.” Week Three is “Learn from the Past, Build for Tomorrow.” Finally, Week Four is “Building Codes, A Smart Investment.” Simpson Strong-Tie is proud to be a major sponsor of Week Three of Building Safety Month.
National Hurricane Preparedness Week is recognized each year to raise awareness of the threat posed to Americans by hurricanes. A Presidential Proclamation urged Americans to visit www.Ready.gov and www.Hurricanes.gov/prepare to learn ways to prepare for dangerous hurricanes before they strike. Each day of the week has a different theme. The themes are: ⦁ Determine your risk; develop an evacuation plan ⦁ Secure an insurance check-up; assemble disaster supplies ⦁ Strengthen your home ⦁ Identify your trusted sources of information for a hurricane event ⦁ Complete your written hurricane plan.
This week also marks the NOAA Hurricane Awareness Tour, where NOAA hurricane experts will fly with two of their hurricane research aircraft to five Gulf Coast Cities. Members of the public are invited to come tour the planes and meet the Hurricane Center staff along with representatives of partner agencies. The goal of the tour is to raise awareness about the importance of preparing for the upcoming hurricane season. The aircraft on the tour are an Air Force WC-130J and a NOAA G-IV. These “hurricane hunters” are flown in and around hurricanes to gather data that aids in forecasting the future of the storm. As with Hurricane Preparedness Week, each day of the tour features a different theme. Simpson Strong-Tie is pleased to be a sponsor for Thursday, when the theme is Strengthen Your Home. Representatives from Simpson Strong-Tie will be attending the event on Thursday to help educate homeowners on ways to make their homes safer.
Finally, this week is the official kickoff of a new hurricane resilience initiative, HurricaneStrong. Organized by FLASH, the Federal Alliance for Safe Homes and in partnership with FEMA, NOAA and other partners, the program aims to increase safety and reduce economic losses through collaboration with the most recognized public and private organizations in the disaster safety movement. HurricaneStrong is intended to become an annual effort, with activities starting prior to hurricane season and continuing through the end of the hurricane season on November 30. To learn more, visit www.hurricanestrong.org.
Experts consider these public education efforts to be more important every year, as it becomes longer since landfall of a major hurricane and as more and more people move to coastal areas. The public complacency bred from a lull in major storms has even been given a name: Hurricane Amnesia.
All these efforts may be coming at a good time, assuming one of the hurricane season forecasts is correct. A forecast from North Carolina State predicts an above-average Atlantic Basin hurricane season. On the other hand, forecasters at the Department of Atmospheric Science at Colorado State University are predicting an approximately average year.
Are you prepared for the natural hazards to which your geographic area is vulnerable? If not, do you know where to get the information you need?
This week’s post comes from Caleb Knudson, an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2005, he has been involved with engineered wood products and has more recently focused his efforts on our line of prefabricated Strong-Wall Shearwall panels. Caleb earned both his Bachelor’s and Master’s degrees in Civil Engineering with an emphasis on Structures from Washington State University. Upon completion of his graduate work, which focused on the performance of bolted timber connections, Caleb began his career at Simpson and is a licensed professional engineer in the state of California.
Some contractors and framers have large hands, which can pose a challenge for them when they’re trying to install the holdown nuts used to attach our Strong-Wall® SB (SWSB) Shearwall product to the foundation. Couple that challenge with the fact that anchorage attachment can only be achieved from the edges of the SWSB panel, and variable site-built framing conditions can limit access depending upon the installation sequence. To alleviate anchorage accessibility issues, we’ve required a gap between the existing adjacent framing and SWSB panel equal to the width of a 2x stud to provide access so the holdown nut can be tightened. Even so, try telling a framer an inch and a half is plenty of room in which to install the nut!
While the SWSB is a fantastic product with many great features and benefits from its field adjustability to its versatility with different applications and some of the highest allowable values in the industry, the installation challenges were real.
Back to the Drawing Board
Our goal was to develop a new holdown for the SWSB that would allow for face access of the anchor bolts, making the panel compatible with any framing condition, while maintaining equivalent performance. All we needed to do is cut a large hole in each face of the holdown without compromising strength or stiffness — piece of cake, right? Well, that’s exactly what we did. In the process, we addressed the needs of the architect, the engineer and the builder — and for bonus points, anchorage inspection is now much easier, which should make the building official happy too.
Introducing the Simpson Strong-Tie® Strong-Wall® Wood Shearwall
Simpson Strong-Tie® has just launched the Strong-Wall® Wood Shearwall (WSW) panel, which replaces the SWSB. The new panel provides the same features and benefits, and addresses the same applications as the SWSB; however, now it also features face-access holdowns distinguished by their Simpson Strong-Tie orange color.
We’ve also updated the top connection, which now provides two options based on installer preference. The standard installation uses the two shear plates shipped with the panel which are installed on each side of the panel by means of nails. As an alternative, the builder can install a single shear plate from either side of the panel using a combination of Strong-Drive® SD Connector screws and Strong-Drive® SDS Heavy-Duty Connector screws.
Allowable In-Plane Lateral Shear Loads
I mentioned that one of our primary development requirements was to meet the existing allowable design values of the SWSB. Not only did we meet our target values, but we exceeded them by as much as 25% for standard and balloon framing application panels and up to 50% for portal application panels. I’ve included a table below showing the most commonly specified standard and portal application SWSB models and how the allowable wind and seismic shear values compare to those of the corresponding WSW model.
Grade-Beam Anchorage Solutions
I’d be remiss if I didn’t point out the grade-beam anchorage solutions we’ve developed for use with the Strong-Wall Wood Shearwall. The solutions have been calculated to conform to ACI 318-14, and testing at the Simpson Strong-Tie Tyrell Gilb Research Laboratory confirmed the need to comply with ACI 318 requirements to prevent plastic hinging at anchor locations for seismic loading. The testing consisted of 1) control specimens without anchor reinforcement, 2) specimens with closed-tie anchor reinforcement, and 3) specimens with non-closed u-stirrups. Flexural and shear reinforcement were designed to resist amplified anchorage forces and compared to test beams designed for non-amplified strength-level forces.
Significant Findings from Testing
We found that grade-beam flexural and shear capacity is critical to anchor performance and must be designed to exceed the demands created by the attached structure. In wind load applications, this includes the factored demand from the WSW. In seismic applications, testing and analysis have shown that in order to achieve the anchor performance expected by ACI 318 Anchorage design methodologies, the concrete member design strength needs to resist the amplified anchor design demand from ACI 318-14 Section 184.108.40.206. To help Designers achieve this, Simpson Strong-Tie recommends applying the seismic design moment listed below at the WSW location.
We also found that closed-tie anchor reinforcement is critical to maintain the integrity of the reinforced core where the anchor is located. Testing with u-stirrups that did not include complete closed ties showed premature splitting failure of the grade beam. In a previous blog post, we discussed our grade-beam test program in much greater detail as it applies to our Steel Strong-Wall panels.
Strong-Wall® Wood Shearwall
To support the Strong-Wall Wood Shearwall, Simpson Strong-Tie has published a 52-page catalog with design information and installation details. We’ve also received code listing from ICC-ES; the evaluation report may be found here. Now that you’re all familiar with the WSW, be sure to check out next week’s blog post where we’ll cover the basics of prefabricated shear panel testing and evaluation. In addition, to help Designers understand all of the development and testing as well as design examples using prefabricated shearwalls, Simpson Strong-Tie will be offering a Prefabricated Wood Shearwall Webinar on June 21, 2016, covering:
The different types of prefabricated shearwalls and why they were developed.
The engineering and testing behind prefabricated shearwalls.
Best practices and design examples for designing to withstand seismic and wind events.
Code reports on shearwall applications.
Introduction of the latest Simpson Strong-Tie prefabricated shearwall.
Last but not least, we always appreciate hearing from you, whether you’re an engineer specifying our panels or in the field handling the installation. If there are applications that we haven’t addressed or additional resources that would be beneficial, please let us know in the comments below.
“Structures are connections held together by members” (Hardy Cross)
I heard this quote recently during a presentation at the Midwest Wood Solutions Fair. I had to write it down for future reference because of course, all of us here at Simpson Strong-Tie are pretty passionate about connections. I figured it wouldn’t take too long before I’d find an opportunity to use it. So when I started to write this blog post about the proper selection of a truss-to-wall connection, I knew I had found my opportunity – how fitting this quote is!
There are plenty of photos of damage wrought by past hurricanes to prove that the connection between the roof and the structure is a critical detail. In a previous blog post, I wrote about whose responsibility it is to specify a truss-to-wall connection (hint: it’s not the truss Designer’s). This blog post is going to focus on the proper specification of a truss-to-wall connection, the methods for evaluating those connections under combined loading and a little background on those methods (i.e., the fun stuff for engineers).
Take a quick look at a truss design drawing, and you will see a reaction summary that specifies the downward reaction, uplift and a horizontal reaction (if applicable) at each bearing location. Some people are tempted to look only at the uplift reaction, go to a catalog or web app, and find the lowest-cost hurricane tie with a capacity that meets or barely exceeds the uplift reaction.
However, if uplift was the only loading that needed to be resisted by a hurricane tie, why would we publish all those F1 and F2 allowable loads in our catalog?
Of course, many of you know that those F1 and F2 allowable loads are used to resist the lateral loads acting on the end and side walls of the building, which are in addition to the uplift forces. Therefore, it is not adequate to select a hurricane tie based on uplift reactions alone.
Where does one get the lateral loads parallel and perpendicular to the plate which must be resisted by the truss-to-wall connection? Definitely not from the truss design drawing! Unless otherwise noted, the horizontal reaction on a truss design should not be confused with a lateral reaction due to the wind acting on the walls – it is simply a horizontal reaction due to the wind load (or a drag load) being applied to the truss profile. It is also important to note that any truss-to-wall connection specified on a truss design drawing was most likely selected based on the uplift reaction alone. There may even be a note that says the connection is for “uplift only” and does not consider lateral loads. In this case, unless additional consideration is made for the lateral loads, the use of that connector alone would be inadequate.
Say, for example, that the uplift and lateral/shear load requirements for a truss-to-wall connection are as follows:
Uplift = 795 lb.
Shear (parallel-to-wall) = 185 lb. (F1)
Lateral (perp-to-wall) = 135 lb. (F2) Based on those demand loads, will an H10A work?
An initial look at the H10A’s allowable loads suggests it might be adequate. However, when these loads are entered into the Connector-Selector, no H10A solution is found.
Why? Because Connector-Selector is evaluating the connector for simultaneous loading in more than one direction using a traditional linear interaction equation approach as specified in our catalog:
If the shear and lateral forces were to be resisted by another means, such that the H10A only had to resist the 795 lb. of uplift, then it would be an adequate connector for the job. For example, the F1 load might be resisted with blocking and RBC clips, and the F2 loads might be resisted with toe-nails that are used to attach the truss to the wall prior to the installation of the H10A connectors. However, if all three loads need to be resisted by the same connector, then the H10A is not adequate according to the linear interaction equation.
Some might question how valid this method of evaluation is – Is it necessary? Is it adequate? How do we know? And that is where the interesting information comes in. Several years ago, Simpson Strong-Tie partnered with Clemson University on an experimental study with the following primary objectives:
1. To verify the perceived notion that the capacity of the connector is reduced when loaded in more than one direction and that the linear interaction equation is conservative in acknowledging this combined load effect.
2. To propose an alternative, more efficient method if possible.
Three types of metal connectors were selected for this study – the H2.5A, H10, and the META20 strap – based on their different characteristics and ability to represent general classes of connectors. The connectors were subjected to uni-axial, bi-axial and tri-axial loads and the normalized capacities of the connectors were plotted along with different interaction/design surfaces.
These interaction plots were used to visualize and parameterize the combined load effect on the capacity of the connectors. The three different interaction plots that were examined were the traditional linear relationship, a quadratic interaction surface and a cuboid design space.
The results? Not only was the use of the linear interaction equation justified by this study, but a new, more efficient cuboid design surface was also identified. It provides twice the usable design space of the surface currently used for tri-axial loading and still provides for a safe design (and for the bi-axial case, it is even more conservative than the linear equation). This alternative method is given in our catalog as follows:
Now we can go back to the H10A and re-evaluate it using this alternative method:
As it turns out, the H10A does have adequate capacity to resist the simultaneous uplift, shear and lateral loads in this example. This just goes to show that the alternative method is definitely worth utilizing, whenever possible, especially when a connector fails the linear equation.
For more information about the study, see Evaluation of Three Typical Roof Framing-to-Top Plate/Concrete Simpson Strong-Tie Metal Connectors under Combined Loading.
What is your preferred method for resisting the combined shear, lateral and uplift forces acting on the truss-to-wall connections? Let us know in the comments below!