Welcome to the Structural Engineering Blog

Featured

Welcome to our Structural Engineering Blog! I’m Paul McEntee, Engineering R&D Manager at Simpson Strong-Tie. We’ll cover a variety of structural engineering topics here that I hope interest you and help with your projects and work. Social media is “uncharted territory” for a lot of us (me included!), but we here at Simpson Strong-Tie think this is a good way to connect and even start useful discussions among our peers in a way that’s easy to use and doesn’t take up too much of your time. Continue reading

Strong-Wall Bracing Selector: Bridging the Gap between Engineered Design and Prescriptive Construction

Asking a structural engineer to design wall bracing under the IRC® can be like asking a French pastry chef to bake a cake using Betty Crocker’s Cookbook. The temptation is to toss out the prescriptive IRC recipe and design the house using ASCE 7 loads and the AWC SDPWS shear wall provisions per the IBC®. But if only a portion of the house needs to be engineered, there may be an easier option.

The prescriptive IRC states an IBC engineered design “is permitted for all buildings and structures and parts thereof” but the design must be “compatible with the performance of the conventional framed system.” But how exactly does an IRC braced wall panel perform? The code doesn’t come right out and tell us, but there are two bracing methods that are essentially shear walls masquerading as braced wall panels: Method ABW and Method BV-WSP. Backing into their allowable loads gives us the key to determining equivalence and eliminates the need to develop lateral forces.

But before you can bust out the slide rule and start crunching numbers, you need to figure out how much bracing the prescriptive code requires. We developed our Wall-Bracing-Length Calculator in 2010 to help designers do just that. And last month, we launched our Strong-Wall® Bracing Selector tool to make it easier to specify equivalent solutions for tricky situations.

Strong-Wall Bracing Selector

Strong-Wall Bracing Selector

Wall-Bracing-Length Calculator

Wall-Bracing-Length Calculator

You can export the required lengths (and project information) from the Wall-Bracing-Length Calculator directly into the Strong-Wall Bracing Selector or you can manually enter in the required lengths. The selector app will provide a list of Strong-Wall panels that have an equivalent length, evaluate their anchorage loads and return a list of pre-engineered anchor solutions for a variety of foundation types.

If you’re familiar with our Strong-Wall Prescriptive Design Guide (T-SWPDG10), the selector automates this 84-page document in just a few steps. One big upgrade is the ability to select a solution to meet the exact amount of bracing that is required. If you needed 2.8-ft. of wall bracing, you have to round up to the tabulated 4-ft. solutions if you are using the guide, but now you can select a wall solution that is equivalent to 2.8-ft., which might mean a smaller wall width or better anchor options. You also have the ability to save the selector file for later modifications, create a PDF of the job-specific output, or email the PDF directly from the program.

Strong-Wall Prescriptive Design Guide

Strong-Wall Prescriptive Design Guide

So next time you get asked to “design” some wall bracing, see if our Wall-Bracing-Length Calculator and Strong-Wall Bracing Selector might save you some time. There is a tutorial and a design example on the Bracing Selector web page, but it’s very easy to use so you may just want to dive right in. I should also point out that the Strong-Wall SB panels have not yet been implemented into the program, but bracing information for them is available on strongtie.com in posted letters for wind (L-L-SWSBWBRCE14), seismic (L-L-SWSBSBRCE14), and seismic with masonry veneer (L-L-SWSBVBRCE14).

Let us know what you think of this new tool in the comments below.

 

Deck Fasteners – Deck Board to Framing Attachments

When you’re building a deck, it’s important to know the types of fasteners you need to use with the various materials that are available. On this week’s post we explore some deck fastener applications as well as offer suggestions on how to avoid a few common problems. We will address two generic types of deck boards fastened to wood framing: preservative-treated wood and composite decking.

Preservative-Treated Wood Decking

With preservative treated wood, it pays to know the board treatment. Wood is treated with a water-borne treatment chemical (typically micronized copper azole these days) and then it is either sent out wet or it is kiln dried. Wet-treated wood can have a moisture content (MC) greater than 30%. Wood that is subsequently kiln dried to remove excess moisture after the treatment process is labeled Kiln-Dried After Treatment (KDAT) and has a MC of about 15%. Wood deck boards with preservative treatments will be labeled as such regardless of their moisture condition.

The moisture condition of the deck boards determines how best to fasten and space your deck boards. Wet wood will shrink in width and thickness after installation. As a result, you should install these boards butted tight so that gaps will emerge after they dry in place. On the other hand, KDAT wood or wood that is dry should be installed with 1/8” gaps between boards so there is a slight gap after the boards get wet and swell due to rain, ice and snow. Some manufacturers suggest using an 8d common nail for spacing when installing KDAT decking, as seen in the figure below.

Deck Shrinkage and Swelling

Deck Shrinkage and Swelling

Shrinking and swelling of any installed deck board can cause the deck fastener to bend back and forth with the MC cycling. This causes many deck fasteners to break because of the fatigue loading, which can be exacerbated by the brittle steel used in most deck screws.

To combat this problem, Simpson Strong-Tie developed the DSV Wood screw. This screw is specifically designed with increased ductility to handle the bending induced by deck board movement. It is available in a variety of lengths with threads optimized to prevent jacking between the deck board and the framing, ensuring a snug long-lasting connection.

DSV wood screw

DSV wood screw

Composite deck boards are made from a mixture of wood fiber and plastic or are entirely “plastic.” Wood plastic composites and plastic materials exhibit thermal expansion, so they expand and contract in thickness, width and length as a function of temperature and solar heating. Consequently, they typically require a special screw designed for composite decking. Screws for this application will often utilize a two-thread design. The lower thread drives the screw into the framing while the upper thread pulls the loosened composite material back into the hole and holds the deck board tight to the joist. Composite screws also have a cap-style head that covers any residual material left around the screw body and leaves a clean finish. Ductility is important to these screws too.

Given the wide variety of composite deck producers, we designed a screw that works well with all of them. The DCU screw works in all types of composite decking fastened to wood framing.

A note about cellular PVC deck boards: the manufacturers’ recommendation of stainless-steel screws restricts the use of many deck board fasteners. Be sure to read and follow the decking material fastener requirements. Simpson Strong-Tie has a broad offering of painted stainless-steel deck screws available to match PVC deck boards. Find the proper match for your board here.

For other deck fastener applications, including decking fastened to steel framing, and information about other deck fasteners available, see our website product page.

Are there other applications that you want to know about that we didn’t share here? Let us know in the comments below. As always, call us in the Engineering Department if you have questions.

 

 

Newest Connector to Satisfy Code

“Does Simpson Strong-Tie write the building code?”

If you work at Simpson Strong-Tie, you get asked this question from time to time when you’re in the field. Over the years, I’ve heard it dozens of times, and because the answer is obviously “no,” it makes you wonder why this belief persists with so many people in the industry. Well, here is my theory: We develop and test products for new code provisions faster than it takes states to adopt the newest codes. So a designer, contractor or building official will often hear about a new Simpson Strong-Tie product or tested application that fills a need before their state building code even defines what that need is. Here are some recent examples:

  • The FWAZ foundation anchor released in 2007 for a 2006 IRC provision that addresses soil pressure loads on basement walls
  • Strong-Drive® SDS screw testing for deck ledgers published in 2008 as alternates to bolts and lags that weren’t prescribed in the IRC until the 2009 edition
  • The DTT2 deck tension tie released in 2009 is used for a 2009 IRC provision that addresses lateral loads on decks
  • BPS ½ -6 bearing plate released in 2011 to address new provisions for shear wall bearing plates in the 2008 SDPWS, which is referenced in the 2009 and 2012 IBC

The latest example is the DTT1Z deck tension tie. Two of our engineers, Randy Shackelford and David Finkenbinder, attended the ICC hearings that resulted in the new 2015 IRC. As soon as a new provision was passed to provide an alternate 750-pound deck lateral load connection (submitted by Washington Assoc. of Building Officials, not Simpson Strong-Tie) we began working on a connector designed to do the job. After several months of R&D, field trials and new tooling, our presses began to stamp out the first production run of the DTT1Z to meet the 2015 IRC provision on December 30, 2014.

DTT1Z Production Run

DTT1Z Production Run

2015 IRC Detail

2015 IRC Detail

The IRC detail shows an ideal condition where the bottom of the deck joist lines up with the wall plates in the house. We tested this application, but we also wanted to support variations that may come up in the field. The results of this testing appear in our T-C-DECKLAT15 technical bulletin. We also tested the DTT1Z with our Strong-Drive® SDWH Timber-Hex HDG screw and our Titen HD® concrete screw anchor so it can be used in a variety of applications, including prescriptive wall bracing and (very) light shear walls. Many of these applications are covered in the code report (ER130) that was completed just this past week.

2015 IRC Test Setup

Test setup: 2015 IRC detail

Joist Scab Test Setup

Test setup: Blocking attached to side of joist

Joist Blocking Test Setup

Test setup: Blocking running between joists

 

 

 

 

 

 

 

If you are interested in reading more about the new IRC deck provisions, Randy wrote about them in his Code Corner column in the current Structural Report and David wrote about them in this blog last August.

In case you are wondering how I respond when asked if we write the code, lately I have been answering it with another question: No, but do you know who is responsible for writing the code? My answer to this is “all of us.” If you don’t like what is in there now, work with an association that represents your interests (NCSEA for a lot of us) to submit a code-change proposal, or even submit one yourself. There is no guarantee it will get in, but if it involves a connection, I can guarantee we will get working on it right away!

Let us know if you see a need for a new connection product. If you already have a product idea and would like to work with us to develop it, you can more-formally submit it here.

Steel Strong-Wall® Footings Just got a Little Slimmer!

While 54 inches is a good height and will get you on most amusement park rides, what about this dimension for the width of a footing? We did some tests recently — actually a lot of tests — that answered that question.

Steel Strong-Wall® narrow panels are great for resisting high seismic or wind loads, but due to their narrow widths, their resulting anchor uplift forces can be rather hefty, requiring very large pad footings. How large? For Seismic Design Categories C-F, the largest cracked concrete solution per ACI318-11 Appendix D has a width of 54 inches and an effective embedment depth of 18 inches in order to ensure the anchor remains ductile. The overall length of this footing, as seen in Figure 1, can be up to 132 inches. While purely code driven, these solutions have historically presented challenges in the field. Most concrete contractors have to dig footings this size by hand. This often leads to discussions with their engineers about finding a better solution.

Figure 1:  Slab-on-Grade Installation (Traditional Solution)

Figure 1: Slab-on-Grade Installation (Traditional Solution)

Simpson Strong-Tie has been studying cast-in-place anchorage extensively in recent years. Our research has been featured in a couple of blog posts: The Anchorage to Concrete Challenge – How Do You Meet It? and Podium Anchorage – Structure Magazine. Concrete podium slab anchorage was a multi-year test program that started with grant funding from the Structural Engineers Associations of Northern California for initial concept testing at Scientific Construction Laboratories Inc. and wrapped up with full-scale detailed testing completed at the Simpson Strong-Tie Tye Gilb Laboratory in Stockton, California. This joint venture studied the performance of anchorage reinforcement into thin podium deck slabs (10-14 inch) to resist the high overturning forces of continuous rod systems on 4-5-story mid-rise construction. The testing confirmed the need to comply with Appendix D requirements to prevent plastic hinging at anchor locations. Be on the lookout for an SE Blog post on that topic in the near future. Armed with what we learned, we decided to develop tested anchor reinforcing solutions for the Steel Strong-Wall.

The newly developed anchor reinforcement solutions for grade beams are calculated in accordance with ACI318 Appendix D and tested to validate performance. Anchor reinforcement isn’t a new concept, as it’s been in ACI318 for some time. Essentially, anchor reinforcements transfer load from the anchor bolt to the reinforcing, which restrains the breakout cone from occurring. For the new grade beam details, the additional ties near the anchor are designed to resist the load from the anchor and are developed into the grade beam. The new details offer solutions with widths as narrow as 18 inches when anchor reinforcement is used.

Two details have been developed: one for the larger panels (SSW18, SSW21, SSW24) as shown in Detail 1/SSW1.1, and one for the smaller panels (SSW12, SSW15) as shown in Detail 2/SSW1.1. The difference between the two is the number of anchor reinforcement ties specified in Detail 3/SSW1.1. For SSW18, SSW21 and SSW24 panels (Detail 1/SSW1.1), the total number of reinforcement per anchor is specified. Due to their smaller sizes, the anchor reinforcement ties specified in Detail 2/SSW1.1 for the SSW12 and SSW15 panels are the total required per panel.

Detail 1/SSW1.1

Detail 1/SSW1.1

Detail 2/SSW1.1

Detail 2/SSW1.1

Detail 3/SSW1.1

Detail 3/SSW1.1

Validation Testing

From the concrete podium deck anchorage test program, we discovered that the flexural and shear capacity of the slab is critical to anchor performance and must be designed to exceed the demands created by the attached structure. For grade beams, this also holds true. In wind-load applications, this demand includes the factored demand from the Steel Strong-Wall. In seismic applications, our testing and analysis showed that achieving the anchor performance expected by Appendix D design methodologies requires the concrete member design strength to resist the amplified anchor design demand from Appendix D Section D.3.3.4.3.

Validation testing was conducted to evaluate this concept. The test program consisted of a number of specimens with different configurations, including:

  • Closed tie anchor reinforcement
  • Non-closed tie u-stirrup anchor reinforcement
  • Control specimen without anchor reinforcement

Flexural and shear reinforcement were designed to resist Appendix D amplified anchorage forces and were compared to test beams designed for non-amplified strength level forces. The results of the testing are shown in Figure 2. In the higher Seismic Design Categories (C-F), the anchor assembly must be designed to satisfy Section D.3.3.4.3 in ACI318-11 Appendix D. In accordance with D.3.3.4.3 (a), the concrete breakout strength needs to be greater than 1.2 times the nominal steel strength of the anchor, 1.2NSA. This requires a concrete breakout strength of 87 kips for a Steel Strong-Wall that uses a 1-inch high-strength anchor.

Figure 2: Steel Strong-Wall Grade Beam Testing

Figure 2: Steel Strong-Wall Grade Beam Testing

Grade beams without the anchor reinforcement detail and with flexural and shear reinforcement designed to the Appendix D amplified anchorage forces performed similar to those with closed-tie anchor reinforcement and flexural and shear reinforcements designed to the non-amplified strength level forces. Both, however, came up short of the necessary forces required by Section D.3.3.4.3 (a). From Test V852, we discovered that even though the flexural and shear reinforcement were designed with the amplified forces, the non-closed tie u-stirrups did not ensure the intended performance. From observation, the u-stirrups do not provide adequate confinement of the concrete and tend to open up under loading conditions, resulting in splitting of the beam at the top as can be seen in the photo.

Test V852: Non-Closed U-Stirrups

Test V852: Non-Closed U-Stirrups

Tests W785 and W841 resulted in the best performance. Both test specimens contained flexural and shear reinforcement designed for the amplified forces, as well as closed-ties. Two configurations were tested to study their performance — two piece closed-tie anchor reinforcement in W785 and a single piece closed-tie anchor reinforcement in Test W841. As seen in Figure 2, their performance was very similar, and met the requirements of Section D.3.3.4.3 (a). The closed-ties helped confine the concrete near the top of the beam, allowing the assembly to reach the expected performance load (See the photo below). It’s important to indicate the following specifics in the New Grade Beam Anchor Reinforcement Details:

  • Anchor Reinforcement is #4 closed-ties
  • SSWAB embedment depth is 16″ +/- ½” (as shown in Detail 3/SSW1.1). This is to ensure there is enough development length of the anchor reinforcement on both sides of the theoretical breakout surface as required by ACI318-11 D.5.2.9.
  • The minimum distances from the anchor bolt plate washer to top and bottom of closed tie reinforcement are 13 inches and 5 inches respectively to ensure proper development above and below the concrete breakout cone (refer to Detail 3/SSW1.1).
  • The spacing between the two vertical legs of the anchor reinforcement tie must be 10 inches apart. While this may differ from your shear reinforcement elsewhere in the grade beam, it ensures the reinforcement is located close enough to the anchor and adequate development length is provided.
  • Flexural reinforcement (top and bottom) and shear reinforcement (ties throughout the grade beam length) are per the designer. Simpson Strong-Tie has provided information in Detail 3/SSW1.1 for the applicable minimum LRFD Applied Design Seismic Moment (See Figure 3) to make sure the grade beam design will at least resist the applied anchor forces. Project design loads not related to the Strong-Wall panel also should be considered and could control the grade beam design.
Closed-Tie Anchor Reinforcement

Closed-Tie Anchor Reinforcement

Figure 3: LRFD Applied Design Seismic Moment

Figure 3: LRFD Applied Design Seismic Moment

Simpson Strong-Tie is interested in hearing your thoughts on the new details. What is your opinion? How have the new details been received on your job sites?

Testing Fasteners for Deck Ledger Connections

This week’s blog post was written by Aram Khachadourian, R&D Engineer for Fastening Systems. Since joining Simpson Strong-Tie 14 years ago, he has designed and tested holdowns, hangers, truss connectors and anchor bolts. He has drafted numerous acceptance criteria as well as quality standards. His current focus is the development, testing and code approval of structural fasteners. Prior to his work at Simpson Strong-Tie, he spent his time designing steel buildings including strip malls, wineries and airplane hangars. Aram graduated from the University of California at Davis with a Civil Engineering degree, and is a registered professional engineer in California.

As we approach the beginning of spring, homeowners across the country are starting to turn their thoughts to the backyard and making plans to add a new deck for summer enjoyment.

As a contractor, designer, or homeowner, you want to know that this new deck will have the structural integrity to stand firm for many years and remain safe for everybody who will use it. While there are many aspects to building a safe, strong deck, today we are focusing on the attachment of the deck ledger to the structure.

Prior to 2009, numerous catastrophic deck failures attributed to improper deck ledger attachments demonstrated the need for building code guidance. A calculated solution was overly conservative because the sheathing layer, typically present between the deck ledger and the structure’s band joist, was considered to be a gap in the connection. A prescriptive approach to deck ledger attachments was finally introduced in the 2009 International Residential Code (IRC). Table R502.2.2.1 provided fastener spacings for ½”-diameter lag screws and bolts. These values were based on testing conducted by researchers at Virginia Tech and Washington State University.

The tests included a variety of band joist types, with pressure-treated Hem-Fir as the deck ledger material. The deck ledger was tested at high moisture content to represent a wet, worst-case field condition. The test assembly had a load bar spanning two joists that were attached to the deck ledger with joist hangers. The ledger was attached through the sheathing to the rim board. Only the rim board was supported by the test frame. The average ultimate load was divided by a factor-of-safety of 3 and then further divided by the load duration coefficient of 1.6 to achieve an allowable load. These values were then applied to a deck live load of 40 psf plus a deck dead load of 10 psf to derive allowable fastener on-center spacings for various joist spans.

When Simpson Strong-Tie began to rate fasteners for ledger connections, we used a similar method of testing and analysis. However, we incorporated a few changes. One of the changes we implemented was a symmetric test set-up.  The original test assembly had a ledger on one end of the joists and a support member as the boundary condition on the other. We put a ledger at each end of the joists so stiffness differences in the supports would not affect the test results. We also chose a larger factor-of-safety of 3.2 (instead of 3.0) to maintain consistency with calculation of fastener allowable loads in other applications. In order to provide our customers with a broader range of construction options, we tested many typical rim board and ledger materials, and we ran tests with single and double ledgers. You can see an example of a typical test set up here:

A typical testing set up

Testing deck ledger connections.

We have tested many Simpson Strong-Tie® Strong-Drive® fasteners for ledger applications including the SDWS Timber screw (SDWS22DB), SDWH Timber-Hex SS screw (SDWH-SS), SDWH Timber-Hex screw (SDWH19DB), and SDS Heavy-Duty Connector screw (SDS). We also have information regarding ledgers attached to studs and ledgers fastened over gypsum board. You can find all of this information in our latest fastener catalog.

One final construction tip – deck ledgers can fail due to cross-grain tension. This occurs when the joist hangers are attached to the deck ledger near the bottom of the ledger, but the fasteners holding the ledger to the building are near the top of the ledger. To prevent cross-grain tension failure, place the joist hangers so at least half of the ledger fasteners are below the joist hanger line.

Take a look through the various ledger options in our fastener catalog, and if we don’t address your condition, let us know. As always, call us in the Engineering Department if you have questions.

Please share your feedback in the comments area below.

Part II: Tensile Performance of Simpson Strong-Tie® SET-XP® Adhesive in Reinforced Brick – Test Results

Guest blogger Jason Oakley, field engineer

Guest blogger Jason Oakley, field engineer

This week’s blog post is written by Jason Oakley. Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in Structural Engineering and earned his MBA from Cal State Fullerton in 2013. He is a field engineer for Simpson Strong-Tie who has specialized in anchor systems for more than 12 years. He also covers concrete repair and Fiber-Reinforced Polymer (FRP) systems. His territory includes Southern California, Hawaii and Guam.

This post is the second of a two-part series on the results of research on anchorage in reinforced brick. The research was done to shed light on what tensile values can be expected for adhesive anchors. In last week’s post, we covered the test set-up. This week, we’re taking a look at our results and findings.

To briefly recap the test set up, it was conducted in September 2014, at an office building in Burbank, Calif. Slated for demolition, this building provided an opportunity for Simpson Strong-Tie to install and test 1/2-inch diameter anchors using Simpson Strong-Tie® SET-XP® anchoring adhesive in both the face and end of the 8-1/2 inch wide reinforced brick wall. A 12-ton rated pull rig at the face and end of the wall was used to pull test the anchors to failure.

Table 1 shows the results for both face and end of wall anchors. Each data set was limited to testing three anchors of the same diameter and embedment depth. The coefficient of variation (COV) showed that the spread of the data was fairly narrow (11% maximum) for the face of wall anchors, but much higher for the end of wall anchors (24%). There are a couple of things worth noting here.

Table 1 - Tensile Results of 1/2" Diameter Threaded Rods in Reinforced Brick

Table 1 – Tensile Results of 1/2″ Diameter Threaded Rods in Reinforced Brick

Anchors 4, 5 and 6 showed that reinforced brick is capable of achieving significant capacity for anchors embedded past the grouted portion of the wall to a depth of six inches. The threaded rods were a mix of F1554 Gr. 36 (newer specification) and A307 Gr. C (older specification – likely the anchors that failed at 14,000 lbs.), which might explain the observed variation in capacity for anchors 4, 5 and 6. At what point breakout would have been achieved if higher tensile strength steel had been used is unknown but it can be estimated. What is clear is a significant reduction – probably around 60% (relative to an estimated breakout capacity of around 17,000 lbs. for an anchor embedded six inches deep far away from an edge) – can be expected for near-edge conditions, despite the presence of two #4 bars running along the edge of the wall at the window. A near-edge failure is shown in Figure 6.

Figure 6 – Anchor 13 near edge at window (anchor 14 and 15 similar)

Figure 6 – Anchor 13 near edge at window (anchor 14 and 15 similar)

At a reduced embedment depth of 4-1/2 inches, Table 1 showed that anchor location (anchors 7 through 12) had little effect on performance whether anchors were installed in the middle of the brick or in the head joint mortar. The failure modes were largely a combination of breakout and pullout as shown in Figure 7 and 8.

Figure 7 – Failures of anchors 7 through 12 (white arrows point to anchor center)

Figure 7 – Failures of anchors 7 through 12 (white arrows point to anchor center)

Figure 8 – Anchor 10 failure at face of wall (anchors 7, 8, 9, 11 and 12 similar)

Figure 8 – Anchor 10 failure at face of wall (anchors 7, 8, 9, 11 and 12 similar)

The end of wall anchor results shown in Table 1 revealed a significant reduction in adhesive tensile capacity and greater variation (COV) relative to face of wall results. Two possible contributing factors for such a high COV could be:

1) The bond strength between the grout and surrounding brick wythes is variable, and

2) The size and quantity of the voids present in the grout is probably inconsistent along the height of the wall – some areas are better than others – leading to further variation of the test results.

Figure 9 shows evidence of a slip plane failure for anchors 1, 2 and 3. Looking at the brick top and bottom surface, referred to as the bed, a scored surface can be seen running perpendicular to the length of the brick (and hence the wall surface) as shown in Figure 10. Perhaps the intent of scores is to help improve the bond strength between the brick and mortar. But this assumed benefit is limited to the bed line. The face and side of the brick are smooth. Consequently, the bond strength between the grout and brick is low enough, combined with lack of grout confinement between the two wythes, to have an appreciable effect on the anchor ultimate tensile capacity.

Figure 9 – Anchor 1, 2, and 3 failures at end of wall (1/2 inch x 6” emb.)

Figure 9 – Anchor 1, 2, and 3 failures at end of wall (1/2 inch x 6” emb.)

Figure 10 – Reinforced brick bed profile

Figure 10 – Reinforced brick bed profile

To summarize, this test program discovered that the tensile performance of 1/2-inch adhesive anchors in the face of the wall can be substantial for cases where anchors are located far enough away from a free edge. Performance is similar for anchors placed in the center of the brick or in the mortar joint, suggesting it doesn’t matter where the anchors are placed on the wall (obviously this isn’t true for anchors near a free edge). Special precautions should be taken especially for anchors located near an edge where small intermittent voids may exist in the grout. Anchor installation should ensure that sufficient quantity of adhesive has been injected into the hole. Figure 11 proves that this is possible. However, screen tubes should be considered if large voids are present, although large voids are expected to be rare in reinforced brick. End of wall anchorage applications should be designed carefully especially if significant tensile capacity is a design requirement.

Figure 11 – Anchor 2 end of wall voids filled with SET-XP® adhesive

Figure 11 – Anchor 2 end of wall voids filled with SET-XP® adhesive

Determining what the allowable load should be can be a little tricky. ICC-ES AC 58, the criteria used for adhesive anchors in masonry base material, lists several safety factors depending on whether creep and/or seismic tests have been performed. Conducting creep and seismic tests on an outdated building material like reinforced brick would be difficult because replicating 60- year-old construction accurately in the laboratory will probably be difficult. Reinforced brick has been largely replaced by grout-filled CMU as the preferred masonry building material — at least in Southern California. What safety factor should be used that would permit seismic loading of anchors in a relatively antiquated building material like reinforced brick is debatable.  Perhaps AC 60, the criteria used for assessing adhesive anchor performance in unreinforced masonry elements (URM), would serve as the best guide. It requires a minimum safety factor of five against failure and limits adhesive anchors to resisting earthquake loads only. But AC 60 also requires that the average ultimate load used not exceed an axial displacement of 1/8″ and limits the allowable load to no more than 1,200 lbs.

Despite the obvious structural dissimilarity between URM and reinforced brick and additional AC 60 requirements, Table 2 shows what the allowable loads would look like for the results of this test program if a safety factor of five was chosen. These loads are based on a wall of unknown material properties (compressive strength, tensile strength and bond, etc.) for a specific building, and may not apply to other reinforced brick buildings.

Table 2 – Allowable loads of 1/2-inch diameter threaded rods in reinforced brick using AC58

Table 2 – Allowable loads of 1/2-inch diameter threaded rods in reinforced brick using AC58

Many factors were not investigated in this test program, such as shear, creep, the simulated seismic test, just to name a few. While the evidence so far suggests that an adhesive anchor in reinforced brick performs similarly to grout-filled CMU, more testing would be necessary to substantiate this claim fully. What is very clear is the tensile tests performed on the 60-year-old Burbank office building showed that reinforced brick is a material capable of resisting appreciable anchorage forces. Of course, while a major effort is made by manufacturers to provide engineers with lab tested “code values” for design use, it can’t be ignored that the material properties of any structural element can be variable. Additional factors such as material deterioration, workmanship, etc., can all have an effect on anchorage capacity. This means that it’s never a bad idea to assess anchor performance through site-specific pull tests if gauging strength accurately is important to the anchor system design.

What have your experiences been with reinforced brick? Have you called for pull tests in this material? What were the results? Please feel free to share your experiences in the comments below.

 

Part I: Tensile Performance of Simpson Strong-Tie® SET-XP Adhesive in Reinforced Brick: Test Set Up

Guest blogger Jason Oakley, field engineer

Guest blogger Jason Oakley, field engineer

This week’s blog post is written by Jason Oakley. Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in Structural Engineering and earned his MBA from Cal State Fullerton in 2013. He is a field engineer for Simpson Strong-Tie who has specialized in anchor systems for more than 12 years. He also covers concrete repair and Fiber-Reinforced Polymer (FRP) systems. His territory includes Southern California, Hawaii and Guam.

 

This post is Part I of a two-part series. In this post, we’ll cover the test set-up and next week in Part II, we’ll take a look at our results and findings.

More than half a century ago, reinforced brick was a fairly common construction material used in buildings located in Southern California and probably elsewhere in the U.S. Reinforced brick can be found in schools, universities, and office buildings that still stand today. This material should not be confused with unreinforced brick masonry (URM) that is also composed of bricks but is structurally inferior to reinforced brick. Engineers are often called to look at existing reinforced brick structures to recommend retrofit schemes that, for example, might strengthen the out-of-plane wall anchorage between the roof (or floor) and wall to improve building performance during an earthquake. Yet, limited or no information exists on the performance of adhesive anchors in this base material. This series of posts shares the results of research on anchorage in reinforced brick in hopes of shedding light on what tensile values can be expected for adhesive anchors, including any important findings encountered during installation and testing.

Reinforced brick sample

Reinforced brick sample

In September 2014, one wall of an office building in Burbank, CA, was slated for demolition. This presented an opportunity for Simpson Strong-Tie to install and test 1/2-inch diameter anchors using Simpson Strong-Tie® SET-XP® anchoring adhesive in both the face and end of the 8-1/2 inch wide reinforced brick wall. The building is shown in Figure 1 and the wall cross section is shown in Figure 2. The bricks measured 3 inches wide by 3-1/2 inches tall by 11-1/2 inches long and the drawings required that the bricks conform to ASTM C62-50, a standard that still exists today. According to the drawings, the walls were reinforced with #4 vertical bars spaced 24 inches on center. Mortar was specified as “1 part plastic cement and 3 parts sand.” The grout used to fill the 2-1/2 inch gap between the two brick wythes is identical to the mortar except “add sufficient water to pour.” The engineer’s drawings specified two #4 bars running parallel to the edge at all wall openings including windows. Although the actual material properties of the mortar, grout, brick, and bond between these components are unknown, the results and findings of this research should serve as a reasonable but rough indicator as to the material quality and workmanship of the wall. Anchor identification numbers and locations are shown in Figures 3 and 4.

Figure 1 – Reinforced brick building

Figure 1 – Reinforced brick building

Figure 2 – Reinforced brick section

Figure 2 – Reinforced brick section

Figure 3 – Anchor identification at inside face of wall (anchor diameter ½”)

Figure 3 – Anchor identification at inside face of wall (anchor diameter ½”)

Figure 4 – Anchor identification at end of wall (anchor diameter ½”)

Figure 4 – Anchor identification at end of wall (anchor diameter ½”)

While the brick base material was mostly solid, in some cases it was necessary to inject more adhesive in the hole due to the presence of small intermittent voids in the grout that were doubtlessly air pockets trapped during the grouting process. To resolve this problem, enough adhesive was injected such that excess adhesive could be observed coming out of the hole during insertion of the ½ inch diameter all-thread rod. This condition was limited to anchors located near the window edge (anchors 13, 14 and 15) and the end of wall (anchors 1, 2 and 3). The base material was solid at all other locations. No screen tubes were used for any holes.

Figure 5 shows a 12-ton rated pull rig at the face and end of the wall used to pull test the anchors to failure. The pull rig reaction bridge has a clearance of 12 inches between supports to allow breakout as a possible failure mode. Using a reaction bridge extension increases the clear span to 18 inches. ASTM 488 requires a free span clearance of four times the embedment depth. This standard was not followed because exceeding the flexural bending capacity of the wall was a concern. In most cases a minimum clear span of at least three times the anchor embedment depth was met.

Figure 5 – Pull rig without (left) and with (right) reaction bridge extension

Figure 5 – Pull rig without (left) and with (right) reaction bridge extension

With the testing parameters in place, next week I’ll share the results of the tests.

More Fun with Testing

A couple of years back, I did a blog post with a video of a bowling ball exploding. It’s a fun test to show guests who visit our connector lab. Of course, we also do a joist hanger or holdown test to demonstrate a real test used to load rate our products. The problem is some of our tests just aren’t too exciting to the general population. It’s a bit anticlimactic when the wood slowly crushes or the fasteners withdraw until the test specimen just can’t take any load. But bowling balls explode, and explode fast!

In the last couple of months, our connector test lab ran a number of built-up post compression tests. We were looking for data to compare the performance of built-up posts whose members were fastened with connectors (nails, screws, or bolts) to posts that were glued together.

Southern Pine Built Up Setup

Southern Pine Built-up Setup

 

Southern Pine Built Up Failure

Southern Pine Built-up Failure

 

Spruce-Pine-Fir 2x6 Built Up Post

Spruce-Pine-Fir 2×6 Built-up Post

 

Spruce-Pine-Fir 2x4 Built Up

Spruce-Pine-Fir 2×4 Built-up Post

Our test presses have compression capacities ranging from 100 kips to 200 kips. While we have tested some really heavy connectors, most of our tests are under 50 kips ultimate load. The built-up post testing was exciting to watch as loads got as high as 180 kips and had some very dramatic failures. More fun than the bowling balls, but a little more difficult to contain the explosions.

I have no numbers to share from this testing, as design procedures exist in the code for built-up posts. A few non-technical things we learned from doing this built-up post testing include:

  • Short posts can take a lot of load
  • Regular wood glue requires careful application to get good bond over the full area of a board
  • We haven’t mastered glue application
  • Posts can explode
  • Heavy steel plates go flying when posts explode

Not scientific, but fun to watch. The videos were captured on an iPhone by R&D Lab Testing Technician Steve Ziagos. Steve also blogs about Do-It-Yourself projects on our DIY Done Right blog. Enjoy the video.

 

 

New Treatment of Shear Wall Aspect Ratios in the 2015 SDPWS

This post was co-written by Simpson Engineer Randy Shackelford and AWC Engineer Phil Line.

The 2015 International Building Code references a newly updated 2015 Edition of the American Wood Council Special Design Provisions for Wind and Seismic standard (SDPWS). The updated SDPWS contains new provisions for design of high aspect ratio shear walls. For wood structural panels shear walls, the term high aspect ratio is considered to apply to walls with an aspect ratio greater than 2:1.

Figure 1. SDPWS 2008 Table 4.3.4 and illustration of shear wall with height, h and length, bs

In the 2015 SDPWS, reduction factors for high aspect ratio shear walls are no longer contained in the footnotes to Table 4.3.4 (See Figure 2). Instead, these factors are included in new provisions accounting for the reduced strength and stiffness of high aspect ratio shear walls.

Figure 2.  SDPWS 2015 Table 4.3.4

Figure 2. SDPWS 2015 Table 4.3.4

Deflection Compatibility – Calculation Method

New Section 4.3.3.4.1 states that “Shear distribution to individual shear walls in a shear wall line shall provide the same calculated deflection, δsw, in each shear wall.” Using this equal deflection calculation method for distribution of shear, the unit shear assigned to each shear wall within a shear wall line varies based on its stiffness relative to that of the other shear walls in the shear wall line. Thus, a shear wall having relatively low stiffness, as is the case of a high aspect ratio shear wall within a shear wall line containing a longer shear wall, is assigned a reduced unit shear (see Figure 3).

Figure 3. Illustration of deflection compatibility

 

In addition, Section 4.3.4.2 contains a new aspect ratio factor, 1.25 – 0.125h/bs, that specifically accounts for the reduced unit shear capacity of high aspect ratio shear walls. The strength reduction varies linearly from 1.00 for a 2:1 aspect ratio shear wall to 0.81 for a 3.5:1 aspect ratio shear wall. Notably, this strength reduction applies for shear walls resisting either seismic forces or wind forces. For both wind and seismic, the controlling unit shear capacity is the smaller of the values from strength criteria of 4.3.4.2 or deflection compatibility criteria or 4.3.3.4.1.

Deflection Compatibility – 2bs/h Adjustment Factor Method

The 2bs/h factor, previously addressed by footnote 1 of Table 4.3.4, is now an alternative to the equal deflection calculation method of 4.3.3.4.1 and applies to shear walls resisting either wind or seismic forces. This adjustment factor method allows the designer to distribute shear in proportion to shear wall strength provided that shear walls with high aspect ratio have strength adjusted by the 2bs/h factor. The strength reduction varies linearly from 1.00 for 2:1 aspect ratio shear walls to 0.57 for 3.5:1 aspect ratio shear walls. This adjustment factor method provides roughly similar designs to the equal deflection calculation method for a shear wall line comprised of a 1:1 aspect ratio wall segment in combination with a high aspect ratio shear wall segment.

In prior editions of SDPWS, a common misunderstanding was that the 2bs/h factor represented an actual reduction in unit shear capacity for high aspect ratio shear walls as opposed to a reduction factor to account for stiffness compatibility. The actual reduction in unit shear capacity of high aspect ratio shear walls is represented by the factor, 1.25 – 0.125h/bs, as noted previously. The 2bs/h factor is the more severe of the two factors and is not applied simultaneously with the 1.25-0.125h/bs factor.

What are the major implications for design?

  • For seismic design, the 2bs/h factor method continues unchanged, but is presented as an alternative to the equal deflection method in 4.3.3.4.1 for providing deflection compatibility. The equal deflection calculation method can produce both more and less efficient designs that may result from the 2bs/h factor method depending on the relative stiffness of shear walls in the wall line. For example, design unit shear for shear wall lines comprised entirely of 3.5:1 aspect ratio shear walls can be as much as 40% greater (i.e. 0.81/0.57=1.42) than prior editions if not limited by seismic drift criteria.
  • For wind design, high aspect ratio shear wall factors apply for the first time. For shear walls with 3.5:1 aspect ratio, unit shear capacity is reduced to not more than 81% of that used in prior editions. The actual reduction will vary by actual method used to account for deflection compatibility.
  • The equal deflection calculation method is sensitive to many factors in the shear wall deflection calculation including hold-down slip, sheathing type and nailing, and framing moisture content. The familiar 2bs/h factor method for deflection compatibility is less sensitive to factors that affect shear wall deflection calculations and in many cases will produce more efficient designs.

As the 2015 International Building Code is adopted in various jurisdictions, designers will need to be aware of these new requirements for design of high aspect ratio shear walls. The 2015 SDPWS also contains other important revisions that designers should pay attention to. The American Wood Council provides a read-only version of the standard on their website that is available free of charge.

Please contribute your thoughts to these new requirements in the comments below.

What Factors Contribute To A “Resilient” Community?

The world has seen many increasingly catastrophic natural disasters in the past decade, including Hurricane Katrina (Category 3) striking New Orleans in 2005, 2010’s 7.0 magnitude Haiti and 8.8 magnitude Chili earthquakes, the 9.0 magnitude Japan earthquake along with the Christchurch earthquake (6.3 magnitude) in 2011, the tornado outbreak in 2011 which included an EF4 striking Tuscaloosa, AL and a multiple-vortex EF5 striking Joplin, MO. We also saw Category 2 Hurricane Sandy, the largest Atlantic hurricane on record in 2012 and the EF5 tornado striking Moore, Oklahoma in 2013.

New Orleans was approximately $2 billion ahead of Nashville in real gross domestic product in 2002, but suffered an $80 billion loss due to Hurricane Katrina. With economic factors such as business interruption, business loss and population loss, New Orleans fell significantly behind Nashville by approximately $105 billion in real gross domestic product from 2005 to 2012 as shown in Figure 1.

Economic growth chart for New Orleans versus Nashville

Figure 1: Economic repercussions: New Orleans vs. Nashville economic growth from 2002 to 2012 (Courtesy of Dr. Lucy Jones, USGS)

A June 2014 article in Engineering News-Record noted, “Economists predict it will take some $35 billion and 50 to 100 years for New Zealand to recover from the February 2011 Canterbury earthquake, which killed 185 people and devastated Christchurch, the nation’s third-largest .” (See Figure 2)

New Zealand earthquake soft story building collapse

Figure 2: 2011 New Zealand earthquake soft story building collapse (Courtesy of Dr. Andy Buchanan)

In 2008, the USGS forecasted a 99% probability that a 6.7 magnitude or greater earthquake would occur in California. An earthquake scenario was developed for the Southern California ShakeOut explaining the effects of a 7.8 magnitude earthquake on Southern California caused by a rupture of the southern portion of the San Andreas Fault. The scenario was developed by Dr. Lucy Jones of the USGS and a group of more than 300 scientists. It estimated approximately 1,800 deaths, 50,000 injuries and $213 billion of economic losses.

The economic losses included approximately $48 billion due to shaking damage, $65 billion due to fire damage, $96 billion due to business interruption costs and $4 billion due to traffic delays.

With this kind of devastation, building owners, building occupants, builders and designers are looking to better understand the performance expected from buildings built to minimum code requirements, and what the costs are of building to the minimum or above the minimum before and after a disaster.

After an earthquake, survivors often say they thought their building was built to code and wonder why it was so damaged or had to be demolished. Many don’t realize that building to the code minimum in earthquake country means there will be significant damage to the building and that it may need to be razed, as the cost to repair is too high. Christchurch is an example of this (see Figure 3).

2011 Christchurch CBD earthquake impact

Figure 3: 2011 Christchurch CBD earthquake impact (Courtesy of Dr. Ron Mayes, USRC)

Another consideration of the effects of a natural disaster is the interaction with the built environment. While it would seem that each building owner is responsible for the building(s) they own, their buildings’ performance in a natural disaster can adversely affect adjacent buildings, infrastructure and citizens, thereby greatly affecting the performance and recovery of neighbors and the community overall. Additionally, since natural resources are stressed and energy costs are increasing, most communities are making efforts to reduce their use with various sustainability or green initiatives. Buildings represent a significant amount of materials and energy. It’s been said that the most “green” building is the one already built versus one having to be re-built after a significant event.

These issues have led to discussion about the “resiliency” of a community. Webster’s Dictionary defines “resiliency” as “. . .able to become strong, healthy, or successful again after something bad happens” or “. . .able to return to an original shape after being pulled, stretched, pressed, bent, etc.

There are tools that consumers already use to understand the quality and risk associated with a product or service, such as consumer report ratings for various products from cars to appliances, car crash test ratings and the restaurant grading system. To offer a similar information tool for buildings, a new non-profit organization called the United States Resiliency Council (USRC) was formed. The goal of the USRC is to serve as a credible unbiased tool for local governments, building owners, lenders, insurance providers and occupants by providing information on the quality and risk associated with a building after a natural disaster. Simpson Strong-Tie is a Founding Member of the USRC along with 63 other companies and organizations such as ATC, EERI, NCSEA, SEAOC.

The USRC vision is “. . .a world in which building performance in disasters such as earthquakes, hurricanes, tornadoes, floods and blast are more widely understood” and its mission is “. . .to be the administrative vehicle for implementing rating systems for buildings subject to natural and manmade disasters, and to educate the building industry and the general public about these risks.” Keys to the consistency and credibility of their building rating system includes certifying engineers to perform ratings and requiring a technical audit of the ratings by certified reviewers.

The rating process begins with a building evaluation by a USRC certified engineer using the Tier 1 and 2 check list procedure of ASCE 41-13, “Seismic Evaluation of Existing Buildings,” which describes a three-tiered process for seismic evaluation of existing buildings to either the Life Safety or Immediate Occupancy Performance Level. Alternately, the certified engineer may use FEMA P-58, “Seismic Performance Assessment of Buildings,” which expresses analysis results in terms of deaths, dollars and down time. Then the certified engineer converts the findings from ASCE 41 or FEMA P-58 to a USRC rating. The USRC earthquake hazard rating system describes building performance using three dimensions: Safety, Repair Cost, and Time to Regain Basic Function. Within each dimension, there are five thresholds of performance, each represented by a star as shown in Figure 4.

USRC earthquake hazard building rating system of three dimensions with five thresholds of performance

Figure 4: USRC earthquake hazard building rating system of three dimensions with five thresholds of performance (Courtesy of USRC)

A three star rating means loss of life is unlikely, the building repair cost will likely be less than 20% and the time to regain basic function will likely be within weeks to months. Typical buildings built to the code minimum would likely receive a three star rating.

As discussed in a previous blog post, Los Angeles Mayor Garcetti formed a Seismic Safety Task Force led by Dr. Lucy Jones which developed the “Resilience by Design” report. The report contains recommended strategies to identify and seismically strengthen vulnerable existing buildings, water infrastructure and communication framework. It included a voluntary earthquake hazard building rating using the USRC system. Los Angeles plans to lead by example by having city-owned buildings rated to better understand the quality and needs of their building stock. Importantly, the report also offered incentive recommendations such as waiving permit fees and a five-year exemption from business tax for those businesses moving into retrofitted buildings to “. . .help ensure the successful implementation of the recommendations.”

The San Francisco Community Action Plan for Seismic Safety (CAPSS) Earthquake Safety Implementation Program (ESIP) listed 50 tasks to be implemented over 30 years including a Mandatory Soft-Story Retrofit Program. This program was signed into law in the spring of 2013 as we have covered in a previous blog post.

Other cities are looking into similar strengthening strategies as L.A. and S.F. Hopefully, individuals, building owners, occupants, financiers, insurance organizations, other organizations and government officials will work together to determine the vulnerabilities in their built environment and develop strategies to address them. This will better ensure that communities not only survive coming natural disasters, but also are able to recover more quickly.

What should be the measures of a resilient community? Which organizations or efforts are working to educate and improve your community resiliency? Let us know in the comments below.