Wood construction connectors Archives

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.

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.

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.

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.

Wide Flange Beams in Light Frame Construction

How did that beam get so big? This is what I had to ask myself when I finished sizing and detailing a steel beam that was supposed to fit within the floor joist depth for a flush ceiling. We were removing an unreinforced masonry bearing wall and installing a new wide flange beam to support the existing floor joists as part of a seismic retrofit and remodel. Since the floor joists spliced over the existing bearing wall, it would have been much easier to simply install a new beam below the joists.

The architect did not want the beam installed below the framing, as it would protrude too much. Steel design offers multiple wide flange sections that will work for a given loading. For this particular design, I could use a W24x55, a W16x67 or a W14x90. Each has about the same strength (section modulus, S_xx) and stiffness (moment of inertia, I_xx). Without constraints, you would select the lightest section that works. Space limitations that require a shallower beam result in increased beam weight (and cost).

I proposed two solutions for installing the beam in the floor space and hanging the joists off a nailer. One option allowed the steel beam to extend below the floor joists, while the other used a heavier, shallower beam to fit within the space. The owner wanted a flat ceiling and did not mind the added cost for the beam, which weighed about 60% more than the optimum beam size.

Regardless of space constraints for the design of a steel beam, structural engineers need to specify an appropriate hanger for connecting to the steel beam. Simpson Strong-Tie has many suitable top flange hangers. Most common are hangers that are attached to a wood nailer. Many top flange hangers may also be welded to the beam. Not every nailer solution is rated for uplift, so choose a hanger that meets your requirements. Uplift for welded hangers is addressed in a Simpson Strong-Tie® technical bulletin, T-C-WELDUPLFT2.

Installers may also wish to connect the hangers using powder-actuated fasteners in lieu of welding. Allowable loads for several of our top flange hangers are addressed in current catalog.

Of course, as with all of our hanger loads, we created those loads by running a lot of tests.

What are your thoughts on beam selection and installation? Let us know in the comments below.

How to Safely Select Nail Substitutions for Connectors

A few days ago, I was speaking to a customer about an application using nail substitutions for a joist hanger installation. Her questions come up often, so I thought I would dedicate a blog post to some of the resources available that cover the use of different nails in connectors.

Designers and builders often wish to use different fasteners than the catalog specifies. The application could require short nails that don’t penetrate through the back of a ledger or they want to use screws or sinker nails for easier installation. The Wood Connectors Catalog provides multiple options for alternate nailing for face mount hangers and straight straps on page 24.

The load adjustments for alternate fasteners cover substitutions from a common diameter of 16d to a 10d, or a 10d to an 8d. Multiple different replacement lengths are also covered, with reduction factors ranging from 0.64 to 1.0.

It is important to remember that double shear hangers require 3” minimum joist nails. Short nails installed at an angle in double shear hangers will not have adequate penetration into the header.

Pneumatic nail guns used for connector installation are commonly referred to as positive placement nail guns. These tools either have a nose piece that locates connector hole, or the nail itself protrudes from the tool so that the installer can line the nail up with the hole. Most positive placement tools do not accept nails longer than 2½”, so framers using these tools will want to use 1½” or 2½” nails. To accommodate installers using pneumatic nails, we have a technical bulletin T-PNUEMATIC. This bulletin provides adjustment factors for many of our most common embedded holdowns, post caps and bases, hangers and twist straps.

The question of nail size also comes up when attaching hangers to rim board, which can range from 1” to 1¾”. The adjustment factors in C-2013 don’t necessarily apply with rim board, since the material may be thinner the length of the nails used. We also have a technical bulletin for that application – T-RIMBDHGR.

Several of the reduction factors are the same as those in the catalog. Testing of hangers with 10dx1½ nails on 1” OSB or 1¼” LVL did not do as well, however. We observed that once the nails withdrew a little bit under load, they quickly lost capacity. For that reason, we recommend full length 10d or 16d nails on those materials.

Understanding that alternate fasteners are available for many connectors can help you pick the right fastener for you application. When you specify a connector, it is important to also specify the fasteners you require to achieve your design load.

What are your thoughts? Visit the blog and leave a comment!

Why Your NDS Nail Calcs Could Be Wrong. . .And What You Can Do About It

This week’s post was written by Bob Leichti, Manager of Engineering for Fastening Systems. Prior to joining Simpson Strong-Tie in 2012, Bob was an Engineering Manager covering structural fasteners, hand tools, regulatory compliance and code reports for a major manufacturer of power tools and equipment. Prior to that, Bob was a Professor in the Department of Wood Science and Engineering at Oregon State University. He received his B.S. and M.S. from the University of Illinois, and his M.S. and Ph.D. from Auburn University.

When test results don’t make sense, we start by eliminating causes of the problem. When our withdrawal test values came up low, we checked the load cell calibration, the specific gravity of the wood, the nail dimensions, even the units – everything was correct. So why were the nail withdrawal values so low? More wood, more nails, more tests – same results. Ultimately, we concluded that the withdrawal resistance of stainless-steel, smooth-shank nails is not well described by the withdrawal function in the 2012 NDS, section 11.2.3, equation 11.2-3.Continue Reading

So, What’s Behind A Screw’s Allowable Load?

This is Part 2 of a four-part series I’ll be doing on how connectors, fasteners, anchors and cold-formed steel systems are load rated. Read Part 1 and Part 1A.
These loads just can’t be right! Occasionally, I get this statement from engineers. This happens when they have been specifying commodity fasteners based on NDS load values and they get their first look at our higher screw values. Then the call comes in. They want to talk to someone to confirm what they are seeing is correct. I assure them the loads are right and give them this brief overview of how we got here:
Our first structural screw, the Simpson Strong-Tie(R) Strong-Drive(R) SDS, was originally load rated by plugging the bending yield strength and diameter into the NDS yield limit equations and using the load value from the governing failure mode. As later editions of the NDS modified the calculations and we did more testing, we found that the tested ultimate load of the SDS screw could be as much as ten times greater than the allowable load generated from the NDS equations.
Continue Reading

How We Test – Part I: Wood Connectors

If you have followed some of my earlier blog posts, you know I am passionate about testing. In my post, Testing – Then and Now, I said, “There simply is no substitute for a physical test.” Something I haven’t discussed in much detail, however, are some of the complexities involved in a good test.

LUS2 10-2 Uplift Test SetupLUS2 10-2 Uplift Test Failure — LUS2 10-2 Uplift Test Setup LUS2 10-2 Uplift Test Failure

For wood connector testing, we follow ASTM D7147-21 Testing and Establishing Allowable Loads of Joist Hangers. The actual testing is relatively straight forward – build at least three setups, test them, measure the deflection and ultimate loads.

It actually was that simple years ago, but modern test standards have more requirements than just breaking the part. First off, the steel used in the connector is important. To prevent overestimating a connector’s performance, ASTM D7147 has limits on the strength and thickness of steel used for testing relative to the specified material.

ASTM Steel Reduction Factor

The test standard acknowledges that actual steel strength will exceed the specified strength and the 3/2.5 term in the reduction factor gives you 20% leeway to exceed the specified minimums and not take reductions in allowable loads. Locating production parts made with the right strength steel requires a little bit of searching and we need to do a base metal thickness test on production parts. Prototype parts are somewhat easier because we can hand pick the steel used for making them.

Once we have the parts made with the correct steel in the test lab, we need the right wood. Two properties that affect hanger performance are specific gravity and moisture content of the wood. Moisture content is simply a measure of how wet the lumber is, and the test standards require load reductions if testing is done on wood at less than 11% moisture content. We test the moisture content and specific gravity of every board we receive prior to building test setups. All the good wood is stored in the conditioning room.

Specific gravity is a measure of how dense the wood is – denser wood usually means better test performance. Similar to steel, the test standard requires reductions if your tested specific gravity exceeds the specified. Unlike steel, there is no 20% fudge factor. Once a test is run, specific gravity samples are taken, numbered, measured, and then put in the oven to dry. With testing complete, we can finish the test report and do the calculations to load rate the product. It is a lot of work to create one allowable load for a table in a catalog or a flier – and I still love testing!

– Paul

What are your thoughts? Visit the blog and leave a comment!