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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

BRACE FOR IMPACT! Bracing Design for Cold-Formed Steel Studs

While consideration of bracing is important for any structural element, this is especially true for thin, singly symmetric cold-formed steel (CFS) framing members such as wall studs. Without proper consideration of bracing, excessive buckling or even failure could occur. Bracing is required to resist buckling due to axial or out-of-plane lateral loads or a combination of the two.

There are two methods for bracing CFS studs as prescribed by the American Iron and Steel Institute (AISI) Committee on Framing Standards (COFS) S211 “North American Standard for Cold-Formed Steel Framing – Wall Stud Design” Section B1. One is sheathing braced design and the other is steel braced design.

Sheathing braced design has limitations, but it is a cost effective method of bracing studs since sheathing is typically attached to wall studs. This design method is based on an assumption that the sheathing connections to the stud are the bracing points and so it’s limited by the strength of the sheathing fastener to stud connection. Due to this limitation, the Designer has to use a steel braced design for most practical situations. AISI S211 prescribes a maximum nominal stud axial load for gypsum board sheathing with fasteners spaced no more than 12 inches on center. AISI S211 Section B1 and the Commentary discuss the design method and assumptions and demonstrate how to determine the sheathing bracing strength.

CFS Curtain Wall Stud Steel Clip and Bridging Bracing

CFS Curtain Wall Stud Steel Clip and Bridging Bracing

Sheathing braced design requires that identical sheathing is used on each side of the wall stud. The wall stud is connected to the top and bottom tracks or supporting members to provide lateral and torsional support and the construction drawings should note that the sheathing is a structural element. When the sheathing on either side is not identical, the Designer must assume the weaker of the two sheathings is attached to each side. In addition, the Designer is required to design the wall studs without the sheathing for the load combination 1.2D + (0.5L or 0.2S) + 0.2W as a consideration for construction loads of removed or ineffective sheathing. The Designer should neglect the rotational restraint of the sheathing when determining the wall stud flexural strength and is limited by the AISI S100 Section C5.1 interaction equations for designing a wall stud under combined axial and flexural loading.

Steel braced design may use the design methodology shown in AISI S211 or in AISI Committee on Specifications (COS) S100 “North American Specification for the Design of Cold-Formed Steel Structural Members.”

AISI S211 Table B1-1 Maximum Axial Nominal Load Limited by Gypsum Sheathing-to-Wall Stud Connection Capacity

AISI S211 Table B1-1 Maximum Axial Nominal Load Limited by Gypsum Sheathing-to-Wall Stud Connection Capacity

Steel braced design is typically either non-proprietary or proprietary “clip and bridging” bracing, or “flat strap and blocking” bracing periodically spaced along the height of the wall stud.

CFS Wall Stud Steel U-Channel Bridging Bracing

CFS Wall Stud Steel U-Channel Bridging Bracing

CFS Wall Stud Steel Flat-Strap Bracing and Blocking Bracing

CFS Wall Stud Steel Flat-Strap Bracing and Blocking Bracing

Proprietary wall bracing and wall stud design solutions can expedite design with load and stiffness tables and software as well as offer efficient, tested and code-listed solutions such as Simpson Strong-Tie wall stud bridging connectors.

Simpson Strong-Tie Bridging Connectors

Simpson Strong-Tie Bridging Connectors

Steel braced design is a more practical bracing method for several reasons. First, during construction, wall studs go unsheathed for many months, but are subjected to significant construction loads.This is especially true for load-bearing, mid-rise structures. Second, some sheathing products, including gypsum wallboard, can be easily damaged and rendered ineffective if subjected to water or moisture. Third, much higher bracing loads can be achieved using mechanical bracing. IBC Section 2211.4 permits Designers to design steel bracing for axially loaded studs using AISI S100 or S211. However, S100-07 requires the brace to be designed to resist not only 1% of the stud nominal axial compressive strength (S100-12 changes this to 1% of the required compressive axial strength), but also requires a certain brace stiffness. S211 requires the Designer to design the bracing for 2% of the stud design compression force, and it does not have a stiffness requirement. For combined loading, the Designer designs for the combined brace force determined using S100 Section D3.2.1 for the flexural load in the stud and either S100 or S211 for the axial load. In addition, the bracing force for stud bracing is accumulative as stated by S211 Commentary section B3. As a result, the periodic anchorage of the bracing to the structure such as strongbacks or diagonal strap bracing is required.

CFS Wall Stud Diagonal Strap Steel Bracing Anchorage

CFS Wall Stud Diagonal Strap Steel Bracing Anchorage

Some benefits and challenges of steel clip and bridging bracing include:

  • Proprietary solutions, such as the Simpson Strong-Tie SUBH bridging connector, can significantly reduce installed cost since many situations require only one screw at each connection.
  • Unlike strap bracing, u-channel bracing can be installed from one side of the wall.
  • U-channel bracing does not create build-up that can make drywall finishing more difficult.
  • Extra coordination may be required to ensure that u-channel bridging does not interfere with plumbing and electrical services that run vertically in the stud bay.
  • Bracing for axial loaded studs requires periodic anchorage to the structure, such as using strongbacks or diagonal strap bracing.
  • Bracing of laterally loaded studs does not require periodic anchorage since the system is in equilibrium as torsion in the stud is resisted by bridging (e.g., U-channel) bending.

Some benefits and challenges of steel flat strap and blocking bracing include:

  • May be installed at other locations than stud punchout.
  • Required to be installed on both sides of wall.
  • Bumps out sheathing.
  • Bracing for axial loaded studs requires periodic anchorage to structure, such as using strongbacks or diagonal strap bracing (same load direction in stud flanges).
  • Bracing for laterally loaded studs requires design of periodic blocking or periodic anchorage to the structure (opposite load direction in stud flanges).

There are several good examples Designers may reference when designing CFS wall stud bracing. They include AISI D110 Cold-Formed Steel Framing Design Guide that may be purchased from www.cfsei.org, SEAOC Structural/Seismic Design Manual Volume 2 Example 3 that may be purchased from www.seaoc.org, and the Simpson Strong-Tie wall stud steel bracing design example on page 60 of the C-CFS-15 CFS catalog.

AISI S110 Cold-Formed Steel Framing Design Guide

AISI S110 Cold-Formed Steel Framing Design Guide

SEAOC 2012 IBC Structural/Seismic Design Manual Volume 2

SEAOC 2012 IBC Structural/Seismic Design Manual Volume 2

Cold-formed steel framing is a versatile construction material, but Designers need to carefully consider the bracing requirements of the AISI specification and wall stud design standard. What cold-formed steel wall bracing challenges have you encountered and what were your solutions?

Report Back from Nepal – Assessing Seismic Damage from April/May Earthquakes

 Dr. H. Kit Miyamoto assessing damage in Nepal.

Dr. H. Kit Miyamoto assessing structural damage in Nepal.

This week’s post comes from Dr. H. Kit Miyamoto, S.E. Kit is CEO of Miyamoto International, a structural engineering firm and president of the nonprofit, Miyamoto Global Disaster Relief. He also is a California Seismic Safety Commissioner.

As soon as news spread that 7.8-magnitude and 7.3-magnitude earthquakes struck Nepal in April and May of this year, earthquake structural engineering experts from our firm, Miyamoto International, hopped on planes from three countries to offer assistance. We do this in hopes that our expertise and technical advice might help stricken communities recover; help them to build better and ultimately help save lives.

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Dr. H. Kit Miyamoto on site. Evidence of massive damage is present in the background.

While structural engineers are not first responders, we are well equipped to assess whether it is safe for people to return to homes, businesses, schools and critical-services buildings. We also can help people understand why some buildings stand while others collapse. This information is essential. It is the only way to protect people from future tragedy.

On touch down in Nepal, we found the airport filled with frightened people leaving the country. It is always a bit sobering to see people leaving while you head in.  We struck out for the hotel we could only hope was still standing. Once there, we found the building to be structurally sound, although uneasy guests opted to sleep in the courtyard, leaving us, the structural engineers, the only ones sleeping inside.

I expected the devastation in Kathmandu to be much greater than it was – we all did. Yet because the epicenter was about 50 miles from the capital, the quake’s power was partially dissipated by distance and the Kathmandu Valley’s soft river soil, which likely saved many lives and structures.

Nepal’s Minister of Education asked our team to examine some of the schools in remote areas.  We drove into a village to a horrible find: a large, three-story school reduced to a rubble pile of brick, concrete and broken desks. What we found were columns made of bricks without any reinforcement. This is something I saw in schools in Sichuan, China in 2008, where poor construction practices left tens of thousands of school children vulnerable to disaster. And yet, I have to say that Nepal was lucky. Although more than 7,000 of the country’s schools were severely damaged or destroyed, the quake hit midday on a Saturday when students were not in school. Had the quake hit in the middle of a school day, tens of thousands of students would have died.

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A look inside the collapsed school.

Driving out beyond the city, we found rural areas in the central and western regions particularly devastated, with entire villages destroyed and further isolated by road damage, closures, rugged terrain and the threat of landslides. As is the case in much of the world, unreinforced masonry construction presented the biggest problem. In rural Nepal – where traditional homes are made of stone, mud and wood – we found up to 90 percent of the structures destroyed. Whole villages were gone.

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Absolute devastation of a rural village.

Around the world, the experts involved in construction – from the engineer and contractor to the building inspector – have to invest in constructing safe buildings. No corruption. No excuses. Build as if your children are attending that school or living in those homes and we will begin to have seismically resilient cities.

Many of the newer high-rise buildings in Kathmandu have also exhibited crippling damage and we have assessed more than 30 of these to date. Even when building codes are adhered to, a big gap exists between what code provides and what society expects. Even in places like Los Angeles and San Francisco, people don’t understand that. At one meeting in Kathmandu, luxury condo owners were stunned and angry to learn that the building they bought into met standards, but was still too heavily damaged to occupy. These buildings are not usable now. The financial loss is enormous.

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An aerial view of the earthquake damage. 

We have to understand that people don’t have to die in earthquakes. Earthquakes don’t kill people; buildings kill people. Or more precisely, poorly constructed buildings. This is tragic and avoidable because we know how to design seismically strong buildings. When an earthquake strikes, our disaster response efforts include “knowledge transfer,” during which we train engineers, masons and contractors on simple seismic techniques that save lives and help communities “build back better.” Our profession has disaster-resilient solutions for new construction and retrofits.

At Miyamoto International, our mission is to save lives and positively impact economies through our work. In China, 169,000 lives were lost, including tens of thousands of students in 7,000 classrooms. In 2010 in Haiti, more than 200,000 people died. In Nepal, the earthquake killed more than 8,600 people. It doesn’t have to be this way.  Building seismically resilient cities is possible. It is achievable. We can save lives.

Florida Product Approvals Made Simple

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This year, the new 5th Edition of the Florida Building Code was released and is now in effect statewide. First printed in 2002, the Florida Building Code was developed as part of Florida’s response to the destruction caused by Hurricane Andrew and other hurricanes in the state.

Another component, which I would like to take a closer look at in today’s post, is a separate Florida Product Approval system designed to be a single source for approval of construction products for manufacturers, Designers and code enforcers. This single system streamlines the previous approach of different procedures for product approval in different jurisdictions. While statewide approval is not required, many jurisdictions, manufacturers and specifiers prefer using the statewide system to the alternative, which is called local product approval. To ensure uniformity of the state system, Florida law compels local jurisdictions to accept state-approved products without requiring further testing and evaluation of other evidence, as long as the product is being used consistent with the conditions of its approval.

The rules of the Florida Product Approval system are in Florida Rule 61G20-3. Here is some basic information about Florida Product Approval.

The Florida Product Approval system is only available for “approval of products and systems, which comprise the building envelope and structural frame, for compliance with the structural requirements of the Florida Building Code.” So users will only find certain types of products approved there. However, if you work in areas where design for wind resistance is required, the Florida system can be a gold mine of information for tested, rated and evaluated products. Not only will you find products like Simpson Strong-Tie connectors with our ICC-ES and IAPMO UES evaluation reports, but thousands of other tested and rated windows, doors, shutters, roof covering materials and other products that don’t typically get evaluation reports from national entities. The specific categories of products covered under the Florida system are exterior doors, impact protective systems, panel walls, roofing, shutters, skylights, structural components and windows.

To protect consumers, a recent law passed in Florida states that a product may not be advertised, sold or marketed as offering protection from hurricanes, windstorms or wind-borne debris unless it has either State Product Approval or local product approval. Selling unapproved products in this way is considered a violation of the Florida Deceptive and Unfair Trade Practices Act.

Once a manufacturer understands the process for achieving a statewide approval, it is not difficult to achieve, but it can be expensive. The manufacturer must apply on the State of Florida Building Code Information System (BCIS) website at www.floridabuilding.org. To prove compliance with the code, the manufacturer must upload either a test report, a product certification from an approved certification entity, an evaluation report from a Florida Professional Engineer or Architect, or an evaluation report from an approved evaluation entity (ICC-ES, IAPMU UES, or Miami-Dade County Product Control). Then, the manufacturer must hire an independent validator to review the application to ensure it complies with the Product Approval Rule and that there are no clerical errors. Finally, once the validation is complete, staff from the Department of Business and Professional Regulation reviews the application. Depending on the method used to indicate code compliance, the application may be approved at that time or it may have to go through additional review by the Florida Building Commission.

Here are several ways to find out if a product is approved.

  1. For Simpson Strong-Tie products, we maintain a page on www.strongtie.com that lists our Florida Product Approvals.
  2. The Florida Department of Business and Professional Regulation maintains a page where users can search Product Approvals by categories such as manufacturer, category of product, product name, or other attributes such as impact resistance or design pressure.
  3. A third-party group we work with has created a website called www.ApprovalZoom.com that lists various product evaluations and product approvals. In addition to listing Florida Product Approvals, they also list ICC-ES evaluation reports, Miami-Dade County Notices of Acceptance, Texas Department of Insurance Approvals, Los Angeles Department of Building Safety Approvals, AAMA certifications and Keystone certifications among others.
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Florida Department of Business and Professional Regulation Product Approvals search

The process for searching for approved products on the Florida BCIS is fairly simple.

  1. Go to www.floridabuilding.org
  2. On the menu on the left side of the page, click on Product Approval. Or, click this link to go directly to the search page.
  3. On the Product Approval Menu, click on Find a Product or Application. Note that at this location you can also search for approved organizations such as certification agencies, evaluation entities, quality assurance entities, testing laboratories and validation entities.
  4. Ensure the proper Code Version is shown. The current 2014 Florida Code is based on the 2012 International Codes.
  5. At this point, several options can be searched. You can search for all approvals by a specific product manufacturer or a certain type of building component by searching Category and Subcategory, or if searching for a specific product, by entering the manufacturer’s name and the product name.
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Select the option highlighted in red

I hope you find the information contained in the Florida Product Approval system useful. Do you have other needs to find approved products?

Kids + Structural Engineering = The Tech Challenge!

Our future is in good hands.

Our future is in good hands.

This week’s post comes from Marlou Rodriguez who is an R&D Engineer at our home office. Prior to joining Simpson Strong-Tie, Marlou worked as a consulting engineer. His experience includes commercial, multi-family residential, curtain wall systems and the design of seismic bracing for non-structural components. Marlou is a licensed professional Civil and Structural Engineer in California, and too many other states to list. He received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo. Here is Marlou’s post.

I recently had the amazing opportunity to volunteer as a judge at The Tech Museum of Innovation’s The Tech Challenge 2015 in San Jose, Calif. My role involved evaluating projects designed by teams of students in grades 4-12 whose challenge was to build an earthquake-safe structure.

The museum’s annual Tech Challenge is a great event that excites young minds by introducing kids ages 8-18 to the science and engineering design process with a hands-on project based on solving a real-world problem. This year’s challenge was to build a scaled structure that supports live load and is earthquake safe. Simpson Strong-Tie was a sponsor of this year’s event and I was excited to be invited as a judge.

While The Tech Challenge is always focused on solving a real-world problem, no one could have anticipated how real this one would become. On the first day, April 25, I woke up to the horrifying news that a 7.8 magnitude earthquake had devastated Nepal. Thousands of lives were affected by this devastating earthquake. On that morning, this terrible tragedy thousands of miles away really highlighted how important this design challenge really is.

Teams with their structures.

Teams with their structures.

The Tech Challenge spans two days and is divided into three categories: elementary, middle and high school. The judging process consists of two phases. The first phase – a Pre-Performance Interview – gives students a chance to be creative in presenting their teams and designs. They discuss their roles within their team, describe how they chose their design and materials, and explain their method for solving problems and challenges. The second phase places their structures on a test rig and simulates three earthquake movements to test stability. Each structure was judged on its ability to:

  • Stay standing during all three seismic events
  • Return back to its original position
  • Perform with the least amount of drift or the horizontal movement at the top most part of the structure.

As an engineer, I have spent 20 years designing structures to withstand earthquakes. But when I was in elementary school years ago, my thoughts were focused on which parking lot with new curbs, banks and rails or empty pools I could skateboard in. These kids are spending their weekends thinking of how to come up with a system to vertically support a high-rise building and ways to laterally support the building while dissipating the seismic energy induced by the testing rig.

It was amazing to see the ideas the children had in their designs. There were structures with fixed bases, some with innovative base isolation systems and even a few with mass dampers attached to the top of the structure. The lateral systems chosen by the children consisted of moment frames, braced frames and solid core systems – closely resembling the systems used in most buildings today.

The design rules included:

  • Plan dimension of the building was limited to 16” square, while the base of the building could not exceed 20” square
  • Structure height could not exceed six feet
  • Floor-to-ceiling height had to be a minimum of 5 inches
  • Gravity weight of the structure could not exceed 7 lbs.

In addition, there were some size and length limitations for the supporting materials, based on grade levels, and an additional live load was added to the structure by using bolts that were inserted into drilled holes. Not only did the teams have to adhere to the rules, but they also had to calculate the area of living space within their structure. All of these rules, calculations and how they overcame the challenges had to be documented in a detailed journal.

A design from an elementary school team.

A design from an elementary school team.

One design that stuck out to me was developed by a team of elementary school children. I had the pleasure of conducting their pre-performance interview. They had a typical rectangular building with an interior compression member made of stacked plastic PVC pipes. The lateral system comprised of some tension wires that were attached to the top of the building at the interior PVC column. The tension wires were angled as they went through the floors and finally attached to the four corners at the base. The central PVC column bear on the base but was not directly attached to the base. This was a form of base isolation. The four tension wires attached to the four corners of the building were turned toward the central PVC column and attached via a spring. The spring acted as a way for the central column to return to its original position. The design was very interesting and had some innovative features built into it. I could only imagine how it would have performed in the device performance phase of the event, since I wasn’t able to observe that part.

Structure designed by the elementary school team.

Structure designed by the elementary school team.

Close up of the base isolation of the team’s structure.

Close up of the base isolation of the team’s structure.

Earthquakes occur all over the world. These natural occurring events profoundly affect and change people’s lives. Although there are a lot of buildings that withstand earthquakes, there are still a lot of failures of existing buildings. Structural engineers learn from these failures and develop building codes and innovative products to resist future earthquakes. These future scientists, engineers and innovators that I had a pleasure of meeting are truly amazing kids. What struck me was how well these children were able to document their thought process and how they developed their final design. It makes me believe the future of this world is in great hands. I can’t wait to come back next year to judge another challenge.

Thanks for reading our blog – have a nice holiday weekend!

 

Anchor Reinforcement for Concrete Podium Slabs

Guest blogger Scott Fischer, R&D Engineer

Guest blogger Scott Fischer, R&D Engineer

This week’s post comes from Scott Fischer who is an R&D Engineer at our home office. Since joining Simpson Strong-Tie in 2006, Scott has worked on cast-in-place connectors to concrete. He helped develop the current testing criteria for cast-in-place concrete products and also performed the testing and code report requirements needed for these product lines. Prior to joining Simpson Strong-Tie, Scott worked for nine years as a consulting engineer. His experience includes the design and analysis of concrete structures, including post-tensioned slab design, concrete lateral systems and foundations. Scott is a licensed professional engineer in the state of California and received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo.

How often do you get the opportunity to high five a co-worker in the office? Maybe it’s when you just worked through a really complex calculation, or finally figured out that tough detail. Whatever it might be, there are times when we should raise a hand and celebrate the hard work that we do. So when we recently relaunched the Simpson Strong-Tie Strong-Rod™ Systems website, which includes a link to our new Shallow Podium Anchorage Solutions, there were a few high fives going around the office. With that in mind, we want to share the latest developments and continue our anchorage-to-concrete blog discussions that began in May 2012, continued with a March 2014 post referencing the Structure magazine article on anchor testing, and more recently one discussing our release of anchor reinforcement solutions for Steel Strong-Wall® shearwall to grade beams.

Anchor Reinforcement Testing and Research Program

Simpson Strong-Tie has been studying cast-in-place tension anchorage and anchor reinforcement concepts extensively over the past several years. Designing an anchor solution in a thin concrete slab for a high anchor demand load while meeting the ductility provisions of ACI 318-11, D.3.3.4.3 is extremely challenging. A strong industry-need for a safe, logical, yet economical design solution, led to a cooperative research program between Structural Engineers Association of Northern California (SEAONC) members and Simpson Strong-Tie. Testing was initiated by a Special Project Initiative grant from SEAONC to members Andy Fennell, P.E. (principal at SCL) and Gary Mochizuki, S.E. (principal at Structural Solutions at the time, now with Simpson Strong-Tie). We completed the program with continued involvement of SEAONC members. This is the second time we have partnered with SEAONC on non-proprietary anchor bolt testing. The first partnership, on sill plate anchor bolts in shear, resulted in successful code change provisions (led by SEAONC) restoring the capacity of these connections to pre-ACI 318 Appendix D values.

This current research program has focused on non-proprietary anchor reinforcement detailing that increases the nominal breakout capacity of concrete slabs. The anchor design satisfies the seismic ductility requirements of ACI 318-11 Appendix D and also significantly increases design capacity for wind applications. The project goal was to provide a design solution for the industry with independently witnessed proof testing of the anchor reinforcement detailing and application of ACI 318-11 Appendix D design procedures. (Note: Appendix D is moved into a new Chapter 17 in ACI 318-14.)

Test picture of anchor reinforcement

Figure 1: Test picture of anchor reinforcement

Anchor reinforcement detail at mid-slab

Figure 2: Anchor reinforcement detail at mid-slab

Significant Test Findings and Design Concepts

Anchoring to relatively thin concrete slabs introduces many unique challenges, so testing was bound to reveal some unique findings. The goal was to increase concrete breakout capacities and also satisfy the ACI 318 anchor ductility requirements with anchor reinforcement detailing. Here are some of the significant findings:

  • Relatively thin concrete slabs do not allow the placement of anchor reinforcement to drag the load down into a larger mass of concrete as shown in RD.5.2.9. Modified anchor reinforcement was required (Figure 2). The required area of anchor reinforcement is based on D.3.3.4.3(a) where the required area of anchor reinforcement exceeds the anchor steel strength, or 1.2Nsa < (nAsfy x 0.707). The 0.707 is for the 45 degree slope of the bars. The proof testing showed the horizontal leg development outside the cone and continuity through the cone adequately developed the anchor reinforcement.
  • ACI 318-11, D.4.2.1 states that when anchor reinforcement is provided, calculation of concrete breakout strength is not required. You know we love load path discussions, so where does the load go once it gets into the anchor reinforcement? The tests indicated that when the anchor reinforcement is provided, the concrete breakout area increases. This limit state is an extended breakout area past the anchor reinforcement bends that will form when that reinforcement is properly quantified and configured. The extended breakout is similar to multiple anchors loading the slab at each bottom bend of the anchor reinforcement. We have applied this concept to the calculations to evaluate extended breakout past the anchor reinforcement bends.
Initial breakout surface from head of anchor

Figure 3: Initial breakout surface from head of anchor

Breakout surface from bends in anchor reinforcement

Figure 4: Breakout surface from bends in anchor reinforcement

  • Let’s follow the load path some more. Once the anchor is connected to the slab, what is the slab bending capacity? The testing showed that to achieve the anchorage capacity, the slab must have an adequate amount of flexural reinforcement with anchorage forces corresponding to ACI 318-11, Section D.3.3.4.3 applied to the slab. Guidance from this section says to apply the anchor tension loads obtained from either design load combinations that include E, with E increased by Omega, or 1.2 x Nominal steel strength of the anchor (Nsa). If the anchors are not oversized, designing for 1.2Nsa should be the most economical solution. For wind applications, the slab Designer should consider the project specified design loads.
  • A vertical concrete block shear forming at the anchor bearing plate is possible if the anchor embedment is shallow and the anchor reinforcement is working to resist the initial anchor bolt breakout area. Our testing showed that this block shear is separate from Appendix D Pullout and is dependent on embedment depth, perimeter of the bearing surface and concrete strength.
  • For anchors with shallow embedment that have a double nut and washer, the concrete breakout can begin from the top nut, thereby reducing the effective embedment depth. To address this, we eliminated the top nut from our specified anchor assembly kit to insure the breakout begins from the top of the fixed-in-place plate washer.
  • The testing and modeling also allowed us to re-examine the appropriate bearing area for the plate washer, Abrg. The flat top surface of a nut is typically circular due to the chamfer at the points, so the resulting bearing area of the plate washer is circular extending out the thickness of the plate from the flat-to-flat dimension of the nut. For near-edge conditions, the side-face blowout capacity can be the controlling limit state and where the plate washer bearing area becomes more important.
  • Edge testing with anchor reinforcement details showed that the breakout area will spread out and begin from the anchor reinforcement bends, like the mid-slab. In a mid-slab condition, the breakout slope follows the 1.5:1 or 35 degree slope used in Appendix D. However for the near edge, we found that 1.5 x effective embedment (hef) from anchor reinforcement bends only holds true parallel to the edge. Due to eccentricities, the breakout angle from the anchor reinforcement bends into the slab (perpendicular to the edge) is steeper and a steeper 45 degree slope should be used.
Edge test with anchor reinforcement

Figure 5: Edge test with anchor reinforcement

Edge anchor reinforcement configuration

Figure 6: Edge anchor reinforcement configuration

  • The testing showed that even though we had cracks intersecting the anchor as it was loaded, the capacities exceeded uncracked assumptions. This is likely due to the flexural reinforcement running through the breakout cone providing continuity across the cracks. We’re still studying the effect of the flexural reinforcement, so for now we recommend assuming cracked concrete and providing a minimum of 4 – #5 flexural bars each way at anchor locations that require anchor reinforcement. Your slab design may require more flexural bars or they may already be there to meet other slab design requirements.

What Solutions Are Now Being Offered and How Do I Get Them?

With our newly launched Strong-Rod Systems webpages, you now go to the Shallow Podium Anchor link to find anchorage solutions. On the website, you will find anchor reinforcement detail drawings and design load tables with slab design recommendations. We’ll also be adding sample calculations, a guideline for selecting your anchor solutions, 3-D anchor reinforcement graphics and guidelines for addressing your condition if your installation is outside the scope of the current solutions that we offer. The anchor reinforcement is non-proprietary and is fabricated by the rebar supplier, but the configuration and placement is described in the details. In addition, you will see detailed information about the Simpson Strong-Tie Shallow Anchor Rod and Anchor Bolt Locator that is specified as a kit in the load tables. Note the absence of the top nut for reasons described above.

Shallow Anchor Kit

Figure 7: Shallow Anchor Kit

How Do We Specify It and Use It?

So now you know what is on the website but how do you put all these pieces together and apply them to a specific design? As a design professional, you will drive the bus on applying these details and design tables onto your drawings. Similar to specifying Strong-Wall® shearwalls or Strong Frame® moment frames, it just takes a little upfront coordination on your drawings. Typically, you’ll start with a slab key plan that shows the anchor bolt locations. You will need to know the design uplift forces from the light-frame structure above the slab and some basic project variables like specified concrete strength, slab thickness(es) and whether the structure is in high seismic or wind-controlled areas. Now you can choose the necessary design tables from the website by clicking on the individual design tables tab. Your key variables will help you select your specific table based on slab thickness, concrete strength, near edge, etc., and of course wind or seismic.

Sample Shallow Anchor design table

Figure 8: Sample Shallow Anchor design table

Once you have selected your design table, just match your project demand loads or your project-specified anchor bolt with the tabulated ASD or LRFD capacity to select the appropriate shallow anchor callout and reference detail that you can identify on the key plan. The detail callout from the table will send you to sheet SA1 where you will find the anchor reinforcement details and Shallow Anchor Kit recommended for your condition. As mentioned previously, the anchor reinforcement would be fabricated and bent by the rebar guys, but would follow these details. You can download the details shown on sheet SA1, place them on your construction documents and then coordinate them with your plans or schedules similar to how you might provide a shear wall schedule. The footnotes that accompany the tables provide important slab design information and other design and installation recommendations. You’ll soon be able to download the sample design calculations, use them as a tool to help follow the design procedure of the recommended details and submit with your project.

What if My Situation Does Not Fit the Details on the Website?

Though a great number of installations will be covered by these details and tables, there will be conditions that currently cannot be addressed with anchor reinforcement solutions. Installs that may be outside of the current scope could include: a demand uplift that’s too large, a slab that’s too thin, lower concrete strength, corner installs, double wood frame shear walls or two close anchors in tension. To address these conditions, we suggest alternatives like slight adjustment or reconfiguration of the shear walls, thickening slab edges, adding downturn concrete beams or extending the anchorage from above down into a cast-in-place wall or further extended down into the footing at ground level.

The joint SEAONC-Simpson Strong-Tie testing project has shown that anchor reinforcement details can greatly increase the breakout strength of concrete to support cast-in-place anchor bolts in concrete slabs. The testing also showed that the design provisions in ACI 318-11 Appendix D can be rationally applied to these anchor reinforcement details. The testing and Appendix D calculation approach are the basis of the details, load tables, graphics and application guides that can be found on our new Strong-Rod Systems webpages. We’re updating these pages as we create more content, so check back frequently. We look forward to working with you on your anchorage installation challenges and hope that some of these solutions will help you with projects you are working on today. How about a high five?

What do you think about these new anchorage solutions? Let us know by posting a comment below.

Design Examples for Steel Deck Diaphragm Calculator Web App

This week’s blog post was written by Neelima Tapata, R&D Engineer for Fastening Systems. She works in the development, testing and code approval of fasteners. She joined Simpson Strong-Tie in 2011, bringing 10 years of design experience in multi- and single-family residential structures in cold-formed steel and wood, curtain wall framing design, steel structures and concrete design. Neelima earned her bachelor’s degree in Civil Engineering from J.N.T.U in India and M.S. in Civil Engineering with a focus on Structural Engineering from Lamar University. She is a registered Professional Engineer in the State of California.

Like most engineers, you are probably often working against tight deadlines,  on multiple projects and within short delivery times. If you have ever wished for a design tool that would make your work easier, we have an app for that. It’s a simple, quick and easy-to-use tool called the “Steel Deck Diaphragm Calculator” for designing steel deck diaphragms. This tool is so user friendly you can start using it in minutes without spending hours in training. This app can be found on our website, and you don’t need to install anything.

The Steel Deck Diaphragm Calculator has two parts to it: “Optimized Solutions” and “Diaphragm Capacity Tables.” Optimized Solutions is a Designer’s tool and it offers optimized design solutions based on cost and labor for a given shear and uplift. The app provides multiple solutions starting with the lowest cost option using different Simpson Strong-Tie® structural and side-lap fasteners. Calculations can be generated for any of the solutions and a submittal package can be created with the code reports, Factory Mutual Approval reports, fastener information, corrosion information, available fliers, and SDI DDM03 Appendix VII and Appendix IX that includes Simpson Strong-Tie fasteners. Currently, this tool can be used for designing with only Simpson Strong-Tie fasteners. We will be including weld options in this calculator very soon. Stay tuned!

The Diaphragm Capacity Tables calculator can be used to develop a table of diaphragm capacities based on the effects of combined shear and tension.

steeldeck1

When “Optimized Solutions” is selected, the following input is requested:

Step 1: Building Information   ̶   Enter general information about the project, like the project name, the length and width of the building to be designed along with spacing between the support members such as joist spacing, is entered.

Step 2: Steel Deck Information   ̶   Select the type of the steel deck along with the fill type. You can select the panel width from the options or select “Any panel width” option for the program to design the panel width. Choose the deck thickness or select the “Optimize” option for the program to design the optimum deck thickness. You also have an option of editing the steel deck properties to accommodate proprietary decks that are within the limitations of SDI DDM03 Section 1.2. Select the joist steel (support) thickness that the deck material will be attached to. For some fasteners, the shear strength of the fastener is dependent on this support thickness.

Step 3: Load Information   ̶  Enter the shear and uplift demand and select the load type as either “wind” or “seismic” and the design method as “ASD” or “LRFD.”

Step 4: Fastener Information   ̶  This is the last step of input before designing. In the fastener information section, you have the option to choose a structural and side-lap fastener or let the program design the most cost-effective structural and side-lap options. This can be done by checking the “Provide optimized solutions” option. The default options in the program are usually the best choice. However, you can change or modify as needed for your project. You can also set the side-lap fastener range or leave it to the default of 0 to 12 fasteners.

Now let’s work on an example:

Design a roof deck for a length of L = 500 ft. and a width b = 300 ft. The roof deck is a WR (wide rib) type panel, with a panel width of 36″.  The roof deck is supported by joists that are ¼” thick and spaced at 5 ft. on center. Design the diaphragm for wind loading using Allowable Stress Design method. The diaphragm should be designed for a diaphragm shear of 1200 plf. and a net uplift of 30 psf. The steel deck is ASTM A653 SS Grade 33 deck with Fu = 45 ksi.

This information is entered in the web app, as seen below.

steeldeck2

After inputting all the information, click on the Calculate button. You will see the five best solutions sorted by lowest cost and least amount of labor. Then click on the Submittal Generator button. Upon pressing this button, a new column called “Solution” is added with an option button for each solution. You can select any of the solutions. Below the Submittal Generator button, you can select various Code Reports and Approvals and Notes and Information selections that you want included in the submittal. After selecting these items, click on the Generate Submittal button. Now a pdf package will be generated with all of your selections.

steeldeck3

Below is the screen shot of the first page containing Table of Contents from the PDF copy generated. The PDF copy contains the solutions generated by the program, then the detailed calculations for the solution that is selected. In this case, as you can see in the screen shot above, detailed calculations for solution #1 are included with XLQ114T1224 structural screws; XU34S1016 side-lap screws; 36/9 structural pattern and with (10) side-lap fasteners; diaphragm shear strength of 1205 plf. and diaphragm shear stiffness of 91.786 kip/in. The detailed calculations are followed by IAPMO UES ER-326 code report and FM Approval report #3050714.

steeldeck4

Below is another example of a roof deck to be designed for multiple zones.

Design a roof diaphragm that will be zoned into three different areas. Zoning is a good way to optimize the economy of the roof diaphragm. Below are the required diaphragm shears and uplift in the three zones.

Zone 1: Diaphragm shear = 1200 plf.; Net uplift = 30 psf.; Length and width of zone 1 = 300 ft. x 200 ft.
              Joist spacing = 5 ft.

Zone 2: Diaphragm shear = 1400 plf.; Net uplift = 0 psf.; Length and width of zone 2 = 500 ft. x 200 ft.
              Joist spacing = 5.5 ft.

Zone 3: Diaphragm shear = 1000 plf.; Net uplift = 25 psf.; Length and width of zone 3 = 300 ft. x 200 ft.
              Joist spacing = 4.75 ft.

Refer to the example above for all other information not given.

To design for multiple zones first select the Multi-Zone Input button, which is below the Fastener Information section as shown below:

steeldeck5

When you click on the Multi-Zone Input button, you can see a toggle button appearing above a few selections as shown below. The default for the toggle button is globalbutton, which means that this selection is same for all the zones. You can click on the toggle button to change to zonebutton. Then the selection below changes to a label and reads Zone Variable. After all the selections that need to be zone variables are selected, click the Add Zone button. Keep adding zones as needed. A maximum of five zones can be added. After creating the zones, add the information for each zone and click the Calculate button.

steeldeck6

When the Calculate button is clicked, the results for each zone are listed. The five best solutions are listed for each of the zones as shown below.

steeldeck7

Similar to previous example, select the Generate Submittal button to select the solutions to be included in the submittal generator. Select one solution for each zone and then check the items like the code reports or notes to be included in the submittal. Click Generate Submittal to create the submittal package.

See the screen shot below for the steps.

steeldeck8

Now that you know how easy it is to design using our web app, use this app for your future projects. We welcome your feedback on features you find useful as well as your input on how we could make this program more useful to suit your needs. Let us know in the comments below.

 

The Never-Ending Search for Truss Repair Information

Truss repair is one of the most frequently asked about truss topics. Not surprisingly, when we asked for suggested truss topics in a truss blog earlier this year, truss repair made the list. Because the summer months bring about a peak in new construction – and plenty of truss repairs to go along with it – the beginning of June is the perfect time to visit this topic.

From trusses that get dropped or cut/drilled/notched at the jobsite, to homeowners who want to modify their existing trusses to add a skylight or create attic space to fire-damaged trusses, a multitude of scenarios fall under the broad topic of truss repair. Today’s post focuses on various references and resources that can provide some assistance. But first it helps to break down the broad “truss repair” topic into more manageable-sized categories.

New Construction vs. Recent Construction vs. Old Construction

By far, the easiest type of truss repair is new construction, when the trusses either haven’t been installed yet or are still in the process of being installed. Whether the repair is relatively simple (e.g. a broken web) or a little more complicated (e.g. the trusses need to be stubbed), the beauty of new truss construction is that the truss manufacturer – and truss Designer – can be contacted and help with the repair. The truss Designer can easily open up the truss designs in the truss design software, quickly evaluate the trusses for the appropriate field conditions and issue a repair.

A good reference related to truss repairs for new truss construction is the Building Component Safety Information (BCSI) booklet jointly produced by SBCA and TPI. Section B5 of the BCSI booklet, which is also available as a stand-alone summary sheet, covers Truss Damage, Jobsite Modifications & Installation Errors. This field-guide document describes the steps to take when a truss at the jobsite is damaged, altered or improperly installed, common repair techniques, and the information to provide to the truss manufacturer when a truss is damaged, which will assist in the repair process.

bcsi

The next easiest truss type to repair is recent construction, where the trusses were constructed recently enough that:  a) the truss plates are easy to identify, and b) the truss design drawings may even still be available. In these cases, design professionals other than the original truss Designer may be contacted to repair the trusses. For some types of repairs, the design professional can work off the truss design drawing to design the repair. Other times it might be necessary to model and analyze the truss using structural design software; alternatively, a truss manufacturer can be contacted to model the truss in their truss design software for a fee.

Often, the design professional wants to know the design values for the truss plates that were used to construct the truss. If there are truss design drawings available, they will indicate which truss plates were used in the design, and then the truss plate manufacturer can be contacted for more information. It is also easy to search for the truss plate code reports online (for instance, check icc-es.org). If no truss design drawing is available, there is still a way to identify the truss plates. Currently, there are only five major truss plate manufacturers in the United States, and they are listed on the Truss Plate Institute website. That makes identification of the truss plates used in recently constructed trusses easier because all of the current manufacturers’ plates will have markings that are described in their code reports. (Note that there are also a couple of truss manufacturers in the U.S. that manufacture their own truss plates.)

AS-20 Truss Plate (ESR-2762)

AS-20 Truss Plate (ESR-2762)

Finally, the most challenging type of trusses for truss repairs are those found in older buildings. Design professionals involved in these types of repair often aren’t sure where to start.  Truss design drawings are often not available, and the act of trying to identify the truss plate manufacturer is challenging at best, unsuccessful at worst. As a point of reference, there were 14 truss plate manufacturers that were TPI members in 1987 (see image below), and only one of those companies is still in the current list of five companies. Therefore, the truss plates found in a truss built around 1987 will be difficult to identify. One option is to contact TPI and see if they can point you in the right direction.

TPI Member Listing from a 1987 Publication

TPI member listing from a 1987 publication

Simple vs. Complex Repairs

Another way to break down truss repairs is to divide them into easy and challenging repairs. People often ask for “standard” truss repair details. Unfortunately, standard details only address the simplest types of repair; and those usually aren’t the types of repair that are asked about. Details simply cannot cover the wide range of truss configurations and every type of repair situation.

Sample Repair Detail for a Simple Repair

Sample repair detail for a simple repair

With the exception of simple repairs, most truss repairs rely heavily on the judgment and experience of the design professional doing the repair. And because there are not entire textbooks devoted to truss repair (that I am aware of, anyway), Designers must pull from a variety of resources, both to learn more about truss repair and to design the repair. For repairs using plywood or OSB gussets, the APA Panel Design Specification is a must-have reference. Some people prefer to use dimension lumber scabs for their repairs, whenever possible, simply because they are more familiar with dimension lumber (and the NDS) than they are with Plywood/OSB or the APA Panel Design Specification.

Next, the fasteners for the repair must be selected and the allowable loads determined. For nail design values, I am a big fan of the American Wood Council’s Connection Calculator, which provides allowable nail shear values for just about any combination of main and side members that you can think of, including OSB and plywood side members – particularly handy for truss repairs. For more complex repairs, and especially repairs involving higher forces, an excellent fastener choice is a structural wood screw such as our Strong-Drive® SDS  or SDW screws. When I worked in the R&D department at Simpson Strong-Tie, a frequently asked question was whether we had double-shear values for our SDS screws. The questions always seemed to come from Designers who wanted them for truss repairs. Fortunately, we do have double-shear values for our SDS screws.. You can find them on page 319 of our Fastening catalog.

Page 319 of the Fastening Systems Catalog (C-F-14)

Page 319 of the Fastening Systems Catalog (C-F-14)

The Strong-Drive SDW screw was developed after the SDS screw, and while there are currently no double-shear values for the SDW, it is still another good option for repairs.

Fire-Damaged Trusses and Truss Collapses

These situations are in a category by themselves because they go beyond even the most complex repairs involving a major modification to the truss. The biggest difference is that the latter case involves mostly known facts and perhaps some conservative assumptions, whereas damage due to fire or collapse includes many unknowns. Most of the truss Designers I have spoken to about truss damage due to fire or truss collapse often recommend replacement of the trusses rather than repair because it is usually too difficult to quantify the damage to the lumber and/or joints. In fires, there can be “hidden” damage due to the sustained high temperatures, while the truss appears to have no visible damage. Likewise, in a truss collapse, not only may there be too many breaks in the trusses involved in the collapse, but there may also be trusses that suffered severe stresses during the collapse and have damage that is not visible. To attempt a repair in either of these cases often requires an inspection at the jobsite, and the result may still end up being replacement of some or all of the trusses. Therefore, the cost of a full-blown inspection should be weighed against the cost of replacing the trusses.

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The Structural Building Components Association website has a page with information pertaining to fire issues. It includes a couple of documents related to fire damage that are worth checking out.

Beyond the Blog: Where to Get More Truss Repair Information

The best bet for getting practical design information related to truss repairs is to keep an eye out for short courses, workshops or seminars. ASCE has hosted a Truss Repair Seminar (Evaluating Damage and Repairing Metal Plate Connected Wood Trusses) in the past and may very well offer something like it again. Virginia Tech recently hosted a short course on Advanced Design Topics in Wood Construction Engineering, which included a section on Wood Truss Repair Design Techniques.

What other references or resources for truss repair do you use?  Are there any upcoming truss repair courses that you know of?  Please let us know in the comments below!

Continuous Rod Restraint Systems for Multi-Story Wood Structures

This week was our new employee Sales and Product Orientation class. It reminded me of the post A Little Fun with Testing where we broke a bowling ball. Although breaking stuff is fun, my second favorite part of the class is teaching about the importance of a continuous load path. I think it is really the most important thing a Structural Engineer does. If we don’t pay attention to the loads, where they occur and create a path so they can get where they need to go, a building may not stand up. This week, we also released some new tools and information for our new Strong-Rod™ Systems, which are used to complete the load path for multi-story wood-framed shearwall overturning restraint and roof uplift restraint.

Two Load Paths

All wood-framed buildings need to be designed to resist shearwall overturning and roof-uplift forces. To transfer these tensile forces through the load path, connectors (hurricane ties, straps and holdowns) have been the traditional answer. Simpson Strong-Tie offers a few options there.  With the growth in multi-story wood-framed structures, where the code requires shrinkage to be addressed and overturning and uplift forces are typically higher, rod systems have become an increasingly popular load restraint solution. Our Anchor Tiedown System (ATS) for shearwall overturning restraint has been around for many years. A new Strong-Rod Systems Design Guide and revamped web pages provide information on new design options, components and configurations.

Strong-Rod Systems Seismic and Wind Restraint Systems Guide

Strong-Rod Systems Seismic and Wind Restraint Systems Guide

The guide and website focus more on the unique design considerations for rod systems, how you should specify the system and highlight the design services that we provide. They also provide more detail and design information for our relatively new Uplift Restraint System (URS) for roofs. Connectors are a common choice for transferring the net roof uplift forces from wind events down the structure. Although in some high-wind areas, rod systems are preferred.

ATS and URS Continuous Rod Tiedown Systems

ATS and URS Continuous Rod Tiedown Systems

I’ll touch on some of the design considerations for these types of systems below, but back to the load path. For shearwall overturning restraint using holdowns, the load path is fairly simple. Once the lateral load is in the shearwall, the sheathing and nailing lifts up on the post. The holdown connects to the post, holding it down and transferring the forces to the foundation or level below. A continuous rod tiedown system follows a little different path. The sheathing and nailing lifts up on the boundary posts and the posts push up on the framing above until the load is resisted in bearing by a bearing plate. The load is then transferred into the rod and down to the foundation. There has been a lot of testing and research on the effects of skipping restraint locations where a bearing plate restraint is installed at every other floor or only at the top level.  Doing that will change the load path because the load has to continue to travel up until a restraint holds it down. It also negatively impacts the stiffness and drift of the shearwall stack, not to mention increases project cost because the boundary posts, rod and bearing must be sized to transfer the cumulative overturning forces from each level.

ATS load path

ATS load path

Wood Shrinkage, Take-up Devices and Displacement Limits

Shrinkage is not just a Seinfeld episode cult classic. It is also something that designers need to consider when designing wood structures. IBC Section 2304.3.3 requires that designers evaluate the impact of wood shrinkage on the building structure when bearing walls support more than two floors and a roof. The effects of wood shrinkage can impact many things in the structure from finishes to MEP systems to the continuous rod system. As the wood members lose moisture, the wood shrinks and the building settles. This can cause gaps at the bearing plate locations of continuous rod systems because the continuous steel rod doesn’t shrink. That is where the magic of take-up devices comes in. They allow the building to shrink but keep gaps from forming by filling the gap (expanding devices – can be screw style or ratcheting), ratcheting down the rod (ratcheting devices), or making the rod shrink as much as the wood (contracting coupling device).

 

In addition to keeping the rod system tight to insure the intended performance, it is important to consider the movement associated with the rod system when under wind or earthquake loading. The IBC requires shearwall displacements to be within story drift limits in moderate to high seismic regions. We highlighted some of the changes coming for the evaluation of shearwall deflection in the previous post discussing the New Treatment of Shear Wall Aspect Ratios in the 2015 SDPWS. For continuous rod systems, there are some additional limits. ICC-ES AC316 Acceptance Criteria for Shrinkage Compensating Devices requires designs to limit displacement between restraints to 0.20 inches (including rod elongation and device displacement) for shearwall restraint. The movement of the take-up device plays a big part in meeting this requirement and the rod diameter required. Screw-style devices have the lowest total movement. Ratcheting devices are appropriate in many cases as well such as the upper levels where loads are lower, but may require larger rod diameters to meet the displacement limit.

figure5ICC-ES AC391 Acceptance Criteria for Continuous Rod Tie-down Runs and Continuous Rod Tie-down Systems Used to Resist Wind Uplift covers continuous rod systems for roof uplift restraint. The displacement limit for the Continuous Rod Tie-down Run (just the rod system components) is limited to 0.18 inches of rod elongation for the total length of rod. The Strong-Rod URS evaluates the Continuous Rod Tie-down System (the whole load path). Displacement limits for the system are L/240 for the top plate bending and 0.25 inches total deflection at the top plate between tie-down runs (including top plate bending, rod elongation, wood bearing deformation and take-up device displacement). The differences between the rod run and rod system analysis as well as other design considerations are explained in more detail in the design guide and on our website.

I always end my continuous load path presentation during orientation class with the same questions and if they were paying attention I get the response I want.

“What is the most important thing a Structural Engineer does?”

“Designs a continuous load path for the building!”

“What does Simpson Strong-Tie do?”

“Provides product and system solutions to help  engineers do their job!”

Take a look at the new Strong-Rod Systems tools and information and let us know how we can help you with your next multi-story wood-framed project.

What related blog topics would you like to discuss? Let us know in the comments below.

Holdown Anchorage Solutions

A common question we get from specifiers is “What anchor do I use with each holdown?” Prior to the adoption of ACI 318 Appendix D, this was somewhat simple to do. We had a very small table near the holdown section of our catalog that listed which SSTB anchor worked with each holdown.

The good old days! (Don’t use this today.)

The good old days! (Don’t use this today.)

During the good old days, anchor bolts had one capacity and concrete wasn’t cracked. ACI 318 Appendix D gives us reduced capacities in many situations, different design loads for seismic or wind and reductions for cracked concrete. These changes have combined to make anchor bolt design more challenging than it was under the 1997 Uniform Building Code.

This blog has had several posts related to holdowns. So, What’s Behind a Structural Connector’s Allowable Load? (Holdown Edition) explained how holdowns are tested and load rated in accordance with ICC-ES Acceptance Criteria. Damon Ho did a post, Use of Holdowns During Shearwall Assembly, which discussed the performance differences of shearwalls with and without holdowns, and Shane Vilasineekul did a Wood Shearwall Design Example. So I won’t get in to how to pick a holdown.

Once you have determined your uplift requirements and selected a post size and holdown, it is necessary to provide an anchor to the foundation. To help Designers select an anchor that works for a given holdown, we have created different tables that provide anchorage solutions for Simpson Strong-Tie holdowns.

SSTB Anchor Bolt

SSTB Anchor Bolt

SB Anchor Bolt

SB Anchor Bolt

There is one Engineering letter that addresses slab-on-grade foundations and another version that covers stemwall foundations. The tables are separated by wood species (DF/SP and SPF/HF) to give the most economical anchor design for each post material. The preferred anchor solutions are SSTB or SB anchors, as these proprietary anchor bolts are tested and will require the least amount of concrete. When SSTB or SB anchors do not have adequate capacity, we have tabulated solutions for the PAB anchors, which are pre-assembled anchors that are calculated in accordance with ACI 318 Appendix D.

Slab On Grade Anchorage Solutions

Slab On Grade Anchorage Solutions

Stemwall Anchorage Solutions

Stemwall Anchorage Solutions

The solutions in the letters are designed to match the capacity of the holdowns, which allows the contractor to select an anchor bolt if the engineer doesn’t specify one. They are primarily used by engineers who don’t want to design an anchor or select one from our catalog tables. We received some feedback from customers who were frustrated that some of our heavier holdowns required such a large footing for the PAB anchors, whereas a slightly smaller holdown worked with an SB or SSTB anchor in a standard 12″ footing with a 1½” pop out.

6-in Popout FEA Model

6-in Popout FEA Model

6-in Popout Physical Testing

6-in Popout Physical Testing

To achieve smaller footings using our SB1x30 anchor bolts, we reviewed our original testing and created finite element (FEA) models to determine what modifications to the slab-on-grade foundation details would meet our target loads. Of course, we ran physical tests to confirm the FEA models. With a 6″ pop out, we were able to achieve design loads for HD12, HDU14 and HHDQ14.

HD12, HDU14 & HHDQ14 Solutions

HD12, HDU14 and HHDQ14 Solutions

The revised footing solutions for the heavier holdowns require less excavation and less concrete than the previous Appendix D calculated solutions, reducing costs on the installation.

What has been your experience with holdown anchorage? Tell us in the comments below.

An Introduction to Helical Wall and Stitching Ties

This week’s post is written by Kevin Davenport, who works as a Field Engineer with Simpson Strong-Tie. Kevin is responsible for providing technical support on Simpson Strong-Tie products for Infrastructure, Commercial and Industrial market segments within his Southeastern territory. He is a registered professional engineer in Georgia and received his B.S. (’97) and M.S. (’98) from Clemson University. Kevin is a member of ICRI, ACI and various local chapters of SEA. 

What do you do when brickwork is in bad condition? Depending on what state the brickwork is in, a tear-down may be called for. However, often brickwork can be restored and strengthened using helical ties such as Simpson Strong-Tie® Heli-Tie™ wall ties and stitching ties. This post introduces these two types of helical ties, which might be just what you need for your next brick restoration project.

What is a helical tie? 

A helical tie is made by twisting a metal profile into the shape of a helix. The design of the Simpson Strong-Tie Heli-Tie wall tie also  incorporates a large core diameter in order to provide higher torsional capacity. The benefit of this feature is less axial deflection due to a propensity for normal helix shape to “uncoil” under tension load. Since helical ties are typically used in building façades, they are generally made from stainless steel in order provide the necessary corrosion resistance.  Helical ties can be used to retrofit and stabilize brickwork in two common applications: 1) Wall anchor applications, and 2) Stitching tie applications.

heli tie

What are common helical wall tie applications?

Application #1: Anchoring building façades to structural members

In a wall anchor application, the helical tie is used to stabilize the façade by transferring out of plane façade forces through the anchor into the backup material. The need for this type of reinforcement arises when pre-existing wall anchors were never installed, were inadequately spaced or have corroded away over time. Helical ties are an economical solution that can be installed directly through a brick façade into various backup materials such as solid concrete, CMU block, and even wood or metal studs.

Image 1

Image 1: Installing a helical tie.

A pilot hole is drilled through the existing brick wall and any air gap into the backup material. Then the helical tie is placed in an installation tool and driven into the pre-drilled hole. As it is driven, the fins of the helical tie tap into both the masonry and backup material and provide an expansion-free connection that will withstand tension and compression loads. Some helical wall ties, like the Heli-Tie, use an installation tool that countersinks the tie below the surface of the brickwork. This allows the hole to be patched and concealed with a color matching material.  Thereby, helical anchors allow the repair to be both efficient and inconspicuous when completed.

Image 2Image 3Image 4

There are presently no specific U.S. design standards for the use and qualification of helical wall ties. However, a rational calculation of required spacing given the demand load can be easily calculated using ACI 530 (Building Code Requirement and Specifications for Masonry Structures), Section 6.2, and test data with an appropriate factor of safety. In addition, many Designers also follow the detailing practice for prescriptive anchored veneer in Section 6.2.2.5.6 that prescribes the following:

  1. At least one anchor for each 3.5 ft2 of wall area, and
  2. A maximum anchor spacing of 32″ horizontal and 25″ vertical, and
  3. Around openings larger than 16″ in either dimension: Additional perimeter anchors at maximum 36″ spacing within 12″ of the opening.

Prescriptive anchors also require bed joints to be at least twice the thickness of the embedded anchor. However, this provision is not relevant for helical anchors since they are installed into a drilled hole, rather than embedded into a wet mortar joint.

Application #2: Stabilizing multiple-wythe brick walls

In this application, the wall tie is used to attach wythes of brick to one another in an effort to stabilize the wall. By intermittently alternating installation angles (0 o, 45 o, 0o, -45 o, etc.) the tie promotes more monolithic behavior of the wall.

Image 5

What are common helical stitching tie applications?

Unlike wall anchor applications, in a stitching tie application the helical tie is used to stabilize brickwork by transferring in- and out-of-plane shear and bending forces across an existing crack. Stitching ties are placed in the plane of the wall within the horizontal bed joint.

The existing bed joint is routed out deep enough to recess the helical tie and cleaned out. Then, the recess is filled about 2/3 deep with a repair mortar (such as Simpson Strong-Tie® FX-263 Rapid Hardening Vertical/Overhead Repair Mortar). The helical stitching tie is then pressed into the mortar, followed by a trowelling with encapsulating grout. The installation provides an inconspicuous repair and preserves the appearance of the structure.

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For red brick, we recommend placing stitching ties  at a minimum vertical spacing of 12″ and  extending the ties at least 20″ on either side of the crack.

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Helical ties are not something that you see on every jobsite, however, they can provide a fast and cost-effective solution for brickwork rehabilitation. Hopefully this post provided you some background about them and an insight into our Heli-Tie product offering. The recent launch of our Repair Protection Strengthening Systems product line complements the Heli-Tie with a wide array of other repair products.

Do you have any past experience with helical wall ties, or questions? Please share in the comments below.