Engineers spend much of their career designing and detailing main structural members which are exposed to significant structural loads. An experienced engineer will often master this type of design and excel at detailing an efficient building system. However, these same savvy engineers are sometimes left scratching their heads when tasked with providing a clean and simple design for attaching components such as cabinets, shelves or handrails to interior finish. Simpson Strong-Tie’s versatile new WBAC Wood Backing Steel Connector provides the engineer with a fully tested design solution that efficiently and easily attaches wood backing members for heavy wall hangings.
Category: Cold-Form Steel Connectors
With more than 50 years of experience in product design, testing and manufacturing, Simpson Strong-Tie provides comprehensive structural solutions for cold-formed steel construction.
Beat Building Drift with the New DSSCB Drift Strut Slide Connector from Simpson Strong-Tie
Structural engineers concerned with building envelopes are always looking for better solutions that help isolate the cladding from the primary structure in conditions where large building drift is a concern. Simpson Strong-Tie has an answer with a unique and innovative solution, the new DSSCB (drift strut sliding clip bypass).
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Decrypting Cold-Formed Steel Connection Design
As published in STRUCTURE magazine, September 2016. Written by Randy Daudet, P.E., S.E., Product Manager at Simpson Strong-Tie. Re-posted with permission.
One of the world’s greatest unsolved mysteries of our time lies in a courtyard outside of the Central Intelligence Agency (CIA) headquarters in Langley, Virginia. It’s a sculpture called Kryptos, and although it’s been partially solved, it contains an inscription that has puzzled the most renowned cryptanalysts since being erected in 1990. Meanwhile, in another part of the DC Beltway about 15 miles to the southeast, another great mystery is being deciphered at the American and Iron Institute (AISI) headquarters.Continue Reading
Specifying Self-Drilling Screws: “Standard” vs. “Engineered”
In my past life as a Design Engineer, when specifying a screw the size of the screw was the key feature that I considered. In my mind, a #10 screw performed better than #8, and a #12 was better than #10 and all #10 screws were the same. But that is not always true. Just as a shoe size or a dress size may not be exactly the same for all brands, a screw of the same size from different manufacturers may perform differently. The head type, head design, thread design (fine, coarse, thread angle, pitch), thread type (like box threads, buttress threads, unified, square) and drill point type (like #1, #3, #5 drill point) can influence the performance of a screw. When innovatively designed, a #10 engineered screw can meet or exceed the performance of a #12 or #14 screw in loads and drill time and could result in cost savings. You can use fewer screws, which would mean labor savings. For example, our newly designed XU34B1016 screw, which is a #10 screw with 16 threads per inch, a hex washer head and a #1 drill point, that performs better than a #14 standard screw in lighter gauge steels.
What Are Self-Drilling Tapping Screws?
Self-drilling tapping screws, or self-drilling screws, as the name implies, drill their own hole, eliminating the need for predrilling, and form or cut internal mating threads. They are relatively fast to instal compared to bolts or welds. Unlike pins, they do not require a thick support material to be used. They can be used in very thin steel, such as 26 gauge, up to steel that is ½” thick. Self-drilling screws may be a perfect choice for most applications involving cold-formed steel (CFS). They are most commonly used for CFS connections: either attaching CFS to CFS, wood to CFS or CFS to wood. They are a logical choice when the other side of the connection or material is not accessible.
Most self-drilling screws are made of steel wire that meets the specification of ASTM A510 minimum grade 1018 material as specified in ASTM C1513 standard. Self-drilling screws are heat treated to case harden then so that they meet the hardness, ductility, torsional strength and drill drive requirements as specified in ASTM C1513 standard. ASTM C1513 refers to SAE J78 for the dimensional and performance requirements of self-drilling screws.
Screw Selection
While selecting the screw, you need to figure out the head type that works for the application. For example, a flat-head screw would be a good choice for wood-to-steel applications, but for steel-to-steel applications, a hex head or a pan head may be a better choice. Similarly, the length of the screw should be sufficient to fasten the members of the connection together. According to Section D1.3 of AISI S200, the screw should be at least equal in length to the total thickness of the material including gaps with a minimum of three exposed threads. The length of the drill point is another important feature to consider. It should be long enough to drill through the entire thickness of the material before engaging the threads. This is because thread forming occurs with fewer revolutions than the drilling process. if the drill point length is not long enough, the screw threads can engage the connection material and the screw can bind and break.
Some drill points also have “wings” to drill a hole in the material that is larger in diameter than the threaded shank. Screws with this kind of point are mainly used for wood-to-steel applications. The blog post by Jeff Ellis titled “Wings or No Wings” provides some useful insights for screws with wings when used in shearwall applications.
The Test Standards and Evaluation Criteria for Standard and Engineered Screws
Per Section D1 of AISI S200, screws used for steel-to-steel connections or sheathing-to-steel connections shall be in compliance with ASTM C1513 or an approved design or design standard.
For ASTM C1513–compliant screws (per AISI S100), Section E4 provides equations to calculate shear, pullout and pullover of screws used in steel-to-steel connections. It also provides safety and resistance factors for calculating allowable strength or design strength. These equations are based on the results of tests done worldwide and the many different types of screws used in the tests. As a result, these equations seem to have a great degree of conservatism.
As discussed earlier, many factors, such as the head type and washer diameter, thread profile, drill point type and length, installation torque and the installation method affect or influence the performance of a screw. In order to qualify the screws as ASTM C1513–compliant or better performing, manufacturers need to have their screws evaluated per Acceptance criteria for Tapping Screw Fasteners AC118 developed by International Code Council – Evaluation Service. The criteria have different requirements depending on whether the intention is to qualify as standard screws or proprietary screws. For proprietary screws, connection shear, pullout and pullover tests are performed in accordance with the AISI S905 test method. The shear strength and tensile strength of the screw itself are evaluated per test standard AISI S904. The safety and resistance factors are calculated in accordance with Section F of AISI S100. The pictures below are some test set-ups per AISI S905 and AISI S904 test procedures.
Another important consideration is corrosion resistance. AC118 has a requirement for testing the fasteners for corrosion resistance in accordance with ASTM B117 for a minimum of 12 hours. The screws tested shall not show any white rust after 3 hours or any red rust after 12 hours of the test. At the same time, it is important to keep in mind that hardened screws are prone to hydrogen embrittlement and are not recommended for exterior or wet condition applications. Also, these screws are not recommended for use with dissimilar metals. If self-drilling screws are to be used in exterior environments, the screws need to be selectively heat treated to keep the core and surface hardness in a range that reduces the susceptibility to hydrogen embrittlement. Other fastener options for exterior environments are stainless-steel screws.
This table shows are some of our screw offerings for CFS applications. Our stainless-screw options can be found in Fastening Systems Catalog (C-F-14) or at www.strongtie.com.
What are the screws that you most commonly specify? Share your screw preferences and your ideas on self-drilling screws in your comments below.
Don’t Buckle at the Knees: RCKW Testing
A previous blog post described how Simpson Strong-Tie tests and loadrates connectors used with cold-formed steel structural members per acceptance criteria ICC-ES AC261.
This week, I would like to describe how we test and determine engineering design values for RCKW, Rigid Connector Kneewall, in a CFS wall assembly and how the data can help designers perform engineering calculations accurately and efficiently.Continue Reading
Cold-Formed Steel Connectors
This blog has described how we load rate different products based on test standards, which are covered under various ICC-ES Acceptance Criteria, or ACs. The first was a post on wood connectors (AC13), then holdowns (AC155), threaded fasteners (AC233) and cast-in-place anchors for light-frame construction (AC398 and AC399). I realized today that I have never talked about how we test and load rate connectors for cold-formed steel.
But first, a confession – it has taken me many years to stop calling it “light-gauge steel.” When I started designing with cold-formed steel, I called it “light-gauge” because I had a binder of design information put together by the Light Gauge Steel Engineers Association. Advocates for CFS felt that “light-gauge” may make people think “weak” or “non-structural,” and that perception would limit the use of cold-formed steel in construction. So there was a deliberate effort to banish the word light-gauge and replace it with cold-formed steel, or CFS. I still slip every once in a while.
Connectors for light-gauge, ahem, I mean cold-formed steel members are covered under ICC-ES AC261 – Acceptance Criteria for Connectors Used with Cold-formed Steel Structural Members. The physical testing for cold-formed steel is similar to wood connectors. Build a setup representative of field conditions, apply load till failure and measure the load and deflection data. Both wood-to-wood and CFS connectors have a service limit state of 1/8” deflection.
Strength data for CFS connectors is analyzed much differently, however. Wood connectors generally use a safety factor of 3 on the lowest ultimate load (or average ultimate if six tests are run). We are often asked what the safety factor for CFS connectors is.
AISI S100 Chapter F details how to determine design strengths for tested CFS products. The design strength is the average test value, Rn, multiplied by an LRFD resistance factor, Φ, or divided by an ASD safety factor, Ω. Determining the resistance factor or corresponding safety factor is based on a statistical analysis dependent on several variables. This is similar in concept to how embedded concrete connectors tested to AC398 or AC399 are evaluated, which I discussed in this post.
I don’t want to get too deep into the Greek letters involved in the calculation. The factors that affect the allowable load calculation are type of member tested, variation in the test values, type of manufacturing, and number of samples tested. One factor that has a large impact on the calculation is the target reliability index, βo. In connector testing, this factor is 2.5 if the structural member (joist, stud, track, etc) fails and 3.5 if the connection fails. The net result is a higher safety factor for test values limited by the connection, and lower safety factors if the structural members governed the test load. Typical safety factors for CFS connectors are 1.8 to 2.0 where the failure mode is in the structural members and 2.2 to 2.9 for tests where the connection failed.
AC261 has a reduction factor, RS, which is used to adjust test values if your steel strength and/or steel thickness are over the specified minimum. CFS test setups often use different steel in the joist, header and the connector. Reductions are calculated based on the tested and specified strength and thickness for each member. The lowest reduction is used to adjust the test values.
One additional complexity in CFS testing is the multiple gauges of steel which must be evaluated. This requires more CFS test setups than a comparable wood connector would require. In the end, we have what we are really after. Design loads that specifiers can be confident in.
