The Cold-Formed Steel Construction Catalog is HOT off the press!

The SE Blog is taking some time off for the 4th of July holiday this week. However, we’ve just released the 2017 edition of our Connectors for Cold-Formed Steel Construction catalog – order a hard copy to be mailed to your office or download a PDF copy and start using it today!

Connectors For Cold-Formed Steel Construction

The C-CF-2017 is a 308-page catalog including specifications, load tables and installation illustrations for our cold-formed steel connectors and clips, helping you easily specify and install in commercial curtain-wall, mid-rise and residential construction.


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. The mystery, structural behavior of cold-formed steel (CFS) clip angles, has puzzled engineers since the great George Winter helped AISI publish its first Specification in 1946. In particular, engineers have struggled with how thin-plate buckling behavior influences CFS clip angle strength under shear and compression loads. Additionally, there has been considerable debate within the AISI Specification Committee concerning anchor pull-over strength of CFS clip angles subject to tension.

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The primary problem has been the lack of test data to explain clip angle structural behavior. Even with modern Finite Element Analysis (FEA) tools, without test data to help establish initial deformations and boundary conditions, FEA models have proven inaccurate. Fortunately, joint funding provided by AISI, the Steel Framing Industry Association (SFIA), and the Steel Stud Manufactures Association (SSMA) has provided the much-needed testing that has culminated in AISI Research Report RP15-2, Load Bearing Clip Angle Design, that summarizes phase one of a multi-year research study. The report summarizes the structural behavior and preliminary design provisions for CFS load bearing clip angles and is based on testing that was carried out in 2014 and 2015 under the direction of Cheng Yu, Ph.D. at the University of North Texas. Yu’s team performed 33 tests for shear, 36 tests for compression, and 38 tests for pull-over due to tension. Clip angles ranged in thickness from 33 mils (20 ga.) to 97 mils (12 ga.), with leg dimensions that are common to the CFS framing industry. All of the test set-ups were designed so that clip angle failure would preclude fastener failure.

For shear, it was found that clips with smaller aspect ratios (L/B < 0.8) failed due to local buckling, while clips with larger aspect ratios failed due to lateral-torsional buckling. Shear test results were compared to the AISC Design Manual for coped beam flanges, but no correlation was found. Instead, a solution based on the Direct Strength Method (DSM) was employed that utilized FEA to develop a buckling coefficient for the standard critical elastic plate-buckling equation. Simplified methods were also developed to limit shear deformations to 1/8 inch. For compression, it was found that flexural buckling was the primary failure mode. Test results were compared to the gusset plate design provisions of AISI S214, North American Standard for Cold-Formed Steel Framing – Truss Design, and the axial compression member design provisions and web crippling design provisions of AISI S100, North American Specification for the Design of Cold-Formed Steel Structural Members, but no good agreement was found. Therefore, an alternate solution was developed that utilized column theory in conjunction with a Whitmore Section approach that yielded good agreement with test results. It was further found that using a buckling coefficient of 0.9 in the critical elastic buckling stress equation will produce conservative results. Finally, for pull-over due to tension, it was found that clip angle specimens exhibited significant deformation before pulling over the fastener heads (essentially the clip turns into a strap before pull-over occurs). However, regardless of this behavior, tested pull-over strength results were essentially half of AISI S100 pull-over equation E4.4.2-1.

Thanks to AISI Research Report RP15-2, there is a clearer understanding of the CFS clip angle structural behavior mysteries that have puzzled engineers for many years. However, just as the CIA’s Kryptos remains only partially solved, some aspects of clip angle behavior remain a mystery. For instance, how are the test results influenced by the fastener pattern? All of the test data to date has used a single line of symmetrically placed screws. This is something that does not occur for many practical CFS framing situations and will need additional research. Another glaring research hole is the load versus deflection behavior of clip angles under tension. As briefly mentioned above, the existing pull-over testing has demonstrated that excessive deflections can be expected before pull-over actually occurs. Obviously, most practical situations will dictate a deflection limit of something like 1/8 inch or 1/4 inch, but today we don’t have the test data to develop a solution. Fortunately, AISI in conjunction with its CFS industry partners continues to fund research on CFS clip angle behavior that will answer these questions, and possibly many more.

Cold-Formed Steel Curtain-Wall Systems

In August 2012, Simpson Strong-Tie launched a comprehensive, innovative solution for curtain-wall framing. Our lead engineer for developing our line of connectors for curtain-wall construction explains the purpose of the curtain wall with the illustrations below.

steel-stud-framingFirst, curtain walls are not what you put up if you shared a room with your brother and sister when you were growing up. When I first learned about the use of cold-formed steel curtain walls, I laughed and thought: “Gosh, how useful this would be for someone growing up with 5 siblings in one bedroom!” I have always enjoyed the sense of humor that our engineers use to help explain technical topics.

Curtain walls can be described as exterior building walls with the primary purpose of protecting the interior building against the exterior weather and natural phenomena such as sun exposure, temperature changes, earthquakes, rain and wind.

To put it in structural terms, a curtain-wall system consists of non-load-bearing exterior walls that must still carry their own weight. Curtain walls are not part of the primary structural framing for the building, but they typically rely on the primary structural framing for support. Additionally, curtain walls receive wind and seismic loads and transfer these forces to the primary building structure.

Types of Curtain Walls

Glass and cladding curtain walls make up two basic types of curtain-wall systems. Glass curtain-wall systems are usually designed using aluminum-framed walls with in-fills of glass. The cladding curtain wall is a system with back-up framing that is covered in some type of cladding material. The cladding curtain-wall system is the type in which Simpson Strong-Tie products can be used.

mid-rise-buildings-1The back-up framing is the structural element of the curtain-wall system. It is typically constructed with cold-formed steel studs ranging from 31/2″ to 8″ deep, in 33 mil (20 ga.) to 97 mil (12 ga.) steel thicknesses. The framing studs are typically spaced at 16″or 24″ on center. There are many different types of cladding materials. They include, but are not limited to, exterior insulation finish systems (EIFS), glass-fiber-reinforced concrete (GFRC), bricks, metal panels and stone panels.

building-material-examplesDeflection

One essential function of the curtain wall is to allow for relative movement between the curtain-wall system and the main building structure. At first, it was not obvious to me why making this allowance was necessary, but our product development team creatively explained some of the reasons why this is an important must-have feature for curtain walls.

deflection-examplesFirst, the primary building will move up and down as it is loaded and unloaded by the live-load occupancy, similar to beam live-load deflections.

Second, the structure sways and has torsional displacement due to movement from lateral wind or seismic loads.

Third, concrete structures typically encounter creep and shrinkage, and there may be foundation differential settlement or soil compression from high-gravity loads.

Lastly, the temperature differential may cause the building elements to expand and contract, which, again, can result in relative movement between structural elements. This is similar to a bridge’s steel plate expansion joint system.

And if you are a curious designer like me, you probably wonder why the relative vertical moment is so significant in engineering design.

One key reason is to ensure that the curtain walls do not collect gravity loads from the building, so as to prevent overloading and possible failure of the stud framing. In addition, a well-designed curtain-wall system needs to retain the primary structural load path as intended by the building designer.

The other reason is to protect the cladding of the building. If you remember earlier, the cladding material may be marble, granite or natural stones that are often very expensive and heavy. In some cases, the cladding can be one of the most expensive systems in a building. And there are times when it’s much more cost-effective to design for relative movement than it is to over-design structural framing to address the stringent deflection requirements.

Construction Type

Bypass framing is a term that is often used in curtain-wall construction. In this system, the metal studs bypass the floor and hang off the outside edges of the building. You can see from the illustration how the studs run past, or bypass, the edge of the slab. In this case, the studs are supported vertically on the foundation at the bottom, and then run continuously past multiple floor levels.

Picture by Don Allen of Super Stud Building Products.

Picture by Don Allen of Super Stud Building Products.

In steel construction, concrete fill over metal deck is typically constructed with a heavy-gauge bent plate or structural angle. Connectors can attach directly to the steel angle or the web of an edge beam.

Simpson Strong-Tie SCB Bypass slide clip connections.

Simpson Strong-Tie SCB Bypass slide clip connections.

SSB Bypass Framing Slide-Clip Strut connections.

SSB Bypass Framing Slide-Clip Strut connections.

 

 

 

 

 

 

 

 

It may seem that this type of construction is too complex and requires great efforts to detail the many connections needed to hang the curtain wall off the outside of the building. So what are the compelling reasons to choose bypass framing construction?

Bypass framing can accommodate flexibility for the architect. In another words, the bypass configuration easily allows architects to create reveals, set-backs and other architectural features.  Plus, there are fewer joints to detail for movement when stud length can run continuously for several floors.  Another benefit is that the exterior finish can also be installed on a curtain-wall system with a tighter tolerance than the edge of the structure.

One other special bypass framing type is known as ribbon window or spandrel framing. Ribbon windows are a series of windows set side by side to form a continuous band horizontally across a façade. The vertical deflection for this type of bypass framing is typically accommodated at the window head. This type of bypass usually works well for panelized construction.

Another common curtain-wall system is infill framing, where the studs run from the top of one floor to the underside of the floor above. Sometimes it’s a challenge to attach bypass framing to the edge of thin concrete slabs. In the following illustration, deflection is designed at the top track of wall panels.

bypass-framing-in-actionIn Part 2 of this blog post series, I will provide more details about how we have innovated products to be used for this application, plus a more comprehensive post about the products we offer and how they are typically used.

In the meantime, you can check out our product offering. Our recent SC slide-clip and FC fixed-clip connectors are designed for high-seismic areas.

I would like to invite you to comment and provide feedback on this topic and tell us whether you’ve had any experience working with a Designer on a CFS curtain-wall project. If you are a Designer who specializes in this discipline, how are you designing curtain-wall systems for seismic forces?

 

Get There Quicker! How CFS Designer Can Help Speed Up Your Design Process

Did you know that Simpson Strong-Tie is celebrating its 60th birthday this year? We started out with one punch press and the ability to bend light-gauge steel. Then, one Sunday evening in the summer of 1956, Barclay Simpson’s doorbell rang and a request for our first joist hanger led us into the wood connector business. Since then, we’ve continued to grow that business by focusing on our engineering, research and development efforts. Some might say that nowadays we’re an engineering company that also happens to manufacture products, as evidenced by our focus on developing technology tools over the past few years such as web calculators, an updated website and design software. Our focus on technology, however, is really another aspect of our continued commitment to excellence in manufacturing and our application of the tenets of lean manufacturing.

Many of you may already be familiar with the idea of lean manufacturing made famous by Toyota in the early 2000s, along with the principles of continual improvement and respect for people. The concept of continual improvement is based on the idea that you can always make small changes to improve your processes and products. Although they were established in a manufacturing setting, these ideals ring very true for engineering as well; eliminate steps in your design process that don’t add any value to the final project and always be on the lookout for tools or techniques that can speed up your process. Thinking lean isn’t about cutting corners to get your result faster, it’s about mindfully getting rid of the steps that aren’t helping you and finding better ways of doing everyday tasks.

As structural engineers, we can find ourselves working on a variety of projects that lead us to perform repetitive calculations to check different conditions, such as varying parapet heights on the exterior of a building, or we may find ourselves working with an unfamiliar material, such as light-gauge or cold-formed steel (CFS), where we have to take some time away from design to review reference materials such as AISI S200-12 North American Standard for Cold-Formed Steel Framing. Wouldn’t it be great if there were a design tool that could help you complete your light-gauge projects more quickly, in complete compliance with current building codes?

It turns out that Simpson Strong-Tie offers a design tool called CFS Designer™ to help structural engineers improve their project design flow. This program gives engineers the ability to design light-gauge stud and track members with complex beam loading and span conditions according to building code specifications. What does that actually mean, though? Allow me to illustrate with an example of a design project.

Let’s say you’re designing a building and part of your scope is the exterior wall framing, or “skin” of the building. You probably get sent some architectural plans that look something like this:

Figure 1. Sample building elevation with section marks.

Figure 1. Sample building elevation with section marks.

The architectural elevations will have wall section marks indicated for different framing situations. Two sample wall sections are shown in Figure 2.

Figure 2. Sample building wall sections.

Figure 2. Sample building wall sections.

This building has several different wall section types that include door and window locations, varying parapet heights, diverse finish materials that need to meet different deflection criteria, and different connection points back to the base building. The traditional design calculation that you would need to run for one wall section might begin with a loading diagram similar to Figure 3 below.

Figure 3. Sample calculation of wall stud loading diagram.

Figure 3. Sample calculation of wall stud loading diagram.

Once you have your loading diagram generated, you would need to use reference load tables or a computer analysis program to solve for the axial and moment demands, the reactions at the pinned supports, and the member deflections. 

After you determine the demand loads, you would then need to select a CFS member with sufficient properties, and you may need to iterate a few times to find a solution that meets the load and deflection parameters. After you’ve selected a member with the right width, gauge and steel strength, you’ll need to select an angle clip that can handle the demand loads, as well as fasteners to connect the clip to the CFS stud and to the base building. You would also need to also check the member design to ensure that it complies with bridging or bracing requirements per AISI. Then, after all that, you’d have to repeat the process again for all of the wall section types for your project.

Figure 4. Hmm, CFS design would sure be a lot easier if buildings were just huge windowless boxes…

Figure 4. Hmm, CFS design would sure be a lot easier if buildings were just huge windowless boxes…

Just writing out that whole process took some time, and you can imagine that actually running the calculations takes quite a bit longer. I think we can all agree that the design process we’ve outlined is time-consuming, and here’s where using CFS Designer™ to streamline your design process can really help.

CFS Designer is a structural engineering design program that can automate many of the manual steps that are required in the design process. It has an easy-to-understand graphical user interface that allows you to input your project parameters within a variety of design modules from walls and beams, jambs and headers, X-brace walls, shearwalls, floor joists, and roof rafters. The program also enables the design of single stud or track members, built-up box-sections, back-to-back sections, and nested stud or track sections. Figure 5 shows an example of how you would input the same stud we looked at before into the program.

Figure 5. CFS Designer™ user interface for wall stud design.

Figure 5. CFS Designer™ user interface for wall stud design.

The program will generate the loading diagrams and complete calculation package for all of these different situations. And along with checking the member properties and deflection limits, CFS Designer will also check bridging and bracing requirements and provide connector solutions for the studs using tested and code-listed Simpson Strong-Tie products. Figure 6 shows an example of the summary output you would receive.

Figure 6. The comprehensive summary output page that covers the complete member design down to the bracing and connection solutions.

Figure 6. The comprehensive summary output page that covers the complete member design down to the bracing and connection solutions.

One unique part of the output is toward the center of the second page, under the heading “Simpson Strong-Tie Connectors.” This section summarizes the tension and compression loads at each reaction point and then shows a connector solution (such as the SCB45.5) along with the number of screws to the stud and the number of #12 sheet-metal screws to anchor back to the base building. Simpson Strong-Tie has developed and tested a full array of connectors specifically for CFS curtain-wall construction as well as for interior tenant improvement framing, which allows designers to select a connection clip straight out of a catalog without needing to calculate their own designs per the code. It’s just another way we’re helping you to get a little leaner!

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Figure 7. A typical SCB/MSCB bypass framing slide-clip connector showing directional loading along with the table of allowable connector loads.

Figure 7. A typical SCB/MSCB bypass framing slide-clip connector showing directional loading along with the table of allowable connector loads.

The last part of the output shown in Figure 6 is titled “Simpson Strong-Tie Wall Stud Bridging Connectors.” It checks the bridging and bracing requirements per AISI S100 and selects a SUBH bridging connector, an innovative bridging solution developed by Simpson Strong-Tie that snaps into place and achieves design loads while only requiring one #10 screw to connect for 75% of applications.

Figure 8. A close-up of the SUBH installed (left) and a wall of studs with bridging installed using the LSUBH clips (right).

Figure 8. A close-up of the SUBH installed (left) and a wall of studs with bridging installed using the LSUBH clips (right).

You can download a free trial of CFS Designer™ and give it a test drive to see how much time it can save you on a design project. The trial version has almost full functionality, with the exception of not being able to print the output sheets. You can see purchasing information online, and you should always feel free to contact your local Simpson Strong-Tie engineering department with any questions you may have. I hope you are able to take advantage of this great tool to further improve your everyday design processes. We will be sure to keep you updated on our latest technology tools that help speed up the design process.  If you’re using CFS Designer, we’d like to hear your thoughts about the program. Please share them in the comments below.

 

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.

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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.

screws2

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.

screws3 screws4 screws5

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.

screw6a

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

hienprofileThis week’s post comes from Hien Nguyen, one of our R&D engineers at the Simpson Strong-Tie Home Office in Pleasanton, CA. Hien has worked in new product development for 17 years on a variety of products. While she still does a few connector projects for wood, her skills and passion for cold-formed steel construction have allowed her to become our expert in CFS product development. Before joining Simpson Strong-Tie, Hien worked as a consulting engineer doing building design. She has a bachelor of science in Civil Engineering from UC Davis, and is a California Licensed Civil Engineer. Here is Hien’s post:

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.

The RCKW was developed to provide optimal rotational resistance at the base of exterior kneewalls, parapets, handrail and guardrail systems as well as interior partial-height walls.

RCKW connectors were tested in CFS wall assemblies for 33 mil, 43 mil, and 54 mil steel thicknesses and in stud members with depths from 3½ to 8 inches. RCKW connectors with stiffeners, RCKWS, were also tested in CFS wall assemblies for 43 mil, 54 mil, and 68 mil stud thicknesses.

rckw1

The wall assembly is built using CFS stud framing, bottom and top tracks simulating the kneewall application in the field. The RCKW connectors are fastened to a stud using self-drilling screws and an anchor to the test bed foundation. The horizontal load (P) is applied to the CFS wall assembly at a height (hwall) of 38 inches. The instruments are also placed at the same height as the applied load to measure wall deflection.  The load and deflection data are recorded concurrently until the wall assembly fails.

The allowable moment, MASD, is determined by multiplying PASD, the allowable horizontal load, by hwall, wall height (MASD = PASD * hwall).

PASD is calculated from peak load or nominal load, PNominal, divided by Ω, a safety factor per AISI 100 Chapter F. The blog post on Cold-Formed Steel Connectors discusses safety factors for CFS testing.

Similarly, the allowable rotational angle, θASD, is also determined by wall deflection at allowable load, ∆ASD, divided by hwallASD = ∆ASD /  hwall).

So the assembly rotational stiffness, β, is calculated by MASD, divided by θASD (β = MASD / θASD).

The typical test performance curve for moment versus rotational angle is concave down and increasing as shown in the blue color curve. As a result, the rotational stiffness for RCKW is established by the secant stiffness, which is a red color straight line from zero to the allowable moment as shown below.

rckw2

The rotational stiffness captures connector deflection, stud deflection and fastener slip in various stud thicknesses. Whereas when the connectors are tested in a steel jig fixture, the rotational stiffness includes connector deflection only and not the fastener and stud deflection behaviors. The photos below are examples of member failures which include stud buckling, bottom track tearing, and screws tilting and bearing. These failure modes are reflected in our tabulated loads because of our assembly testing.

rckw34

Designers might wonder why the rotational stiffness is so important and how significant it is in Engineering Design. The IBC 2012 Building Code, Section 1604.3 indicates that structural systems and members shall be designed to have adequate stiffness to limit deflections and lateral drift. Table 1604.3 also provides deflection limits for various construction applications to which the Engineer must adhere.

For example, one of many common applications in CFS construction is the exterior kneewall system below a large window opening subject to the lateral pressure load. This kneewall system must not only be designed to provide moment strength to avoid the hinging failure at the base, but it must also be designed for deflection limits to prevent excess lateral drift that could result in cracking from various types of finish materials.

Since we performed comprehensive testing of full assemblies, engineers do not need to add stud deflection and fastener slip to the calculation. This saves time and eliminates guesswork with their specifications in a common 38 inch kneewall height.

Furthermore, we analyzed the test data to determine connector rotational stiffness, βc, which includes connector deflection, fastener slips, but not the stud deflection.  Connector rotational stiffness allows engineers to perform deflection calculations for assemblies of any height.  Design examples are available in the RCKW Kneewall Connectors flier.

Simpson Strong-Tie recognizes the complexity of performing hand calculations to accurately determine the anchorage reactions for the RCKW connectors. This post on Statics and Testing described how we established loads for our CFS SJC products through testing. We have also provided anchor reaction loads for connectors at allowable moments so engineers could skip this step in the calculations. We measure the anchor reactions by connecting the calibrated blue load cells with the threaded rod that anchors the RCKW connector. The load cell measures the tension forces in the rod directly.

rckw5

Connector strength and stiffness are critical for RCKW products where calculation or interpolation cannot capture the true performance accuracy the same way that testing would. For this reason, we have tabulated values for various stud member depths and thicknesses. Like Paul, I am amazed at the number of tests that go into this product. Ultimately, we can provide complete Engineer Design values that our specifiers can trust in determining adequate strength and stiffness to meet the code requirement.

 

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.

AC261 Joist Test 1

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 Safety Factor

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.

AC261 Joist Test 2

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.

Strength Reduction Factor

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.

RCKW Kneewall Setup

RCKW Kneewall Failure

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.

RCKW Load Table

24th Short Course on CFS Structures:
October 27-29 in St. Louis

Simpson Strong-Tie is sponsoring the 24th Short Course on Cold-Formed Steel Structures hosted by the Wei-Wen Yu Center for Cold-Formed Steel Structures (CCFSS). The course will be held on October 27-29, 2015 at the Drury Plaza Hotel at the Arch in St. Louis, MO.

This three-day course is for engineers who have limited or no experience designing with cold-formed steel (CFS), as well as those with experience who would like to expand their knowledge of cold-formed steel structural design. Lectures will be given by industry-recognized experts Roger LaBoube, Ph.D., P.E., and Sutton Stephens, Ph.D., P.E., S.E. The course is based on the 2012 AISI North American Specification for the Design of Cold-Formed Steel Structural Members and the 2012 North American Standards for Cold-Formed Steel Framing. Dr. Wei-Wen Yu’s book Cold-Formed Steel Design (4th Edition) will be a reference text.

The course will address such topics as design of wall studs, floor joists, purlins, girts, decks and panels. It is eligible for 2.4 Continuing Education Units (CEUs). Advance registration is requested by October 10, 2015. For more information and to register, click here.

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, except the new AISI S240 standard Section B1.2.2.3 clarifies that for curtain wall studs it is permissible to have sheathing on one side and discrete bracing for the other flange not spaced further than 8 feet on center. 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. . AISI S100 is silent regarding combined loading, but S211 provides guidance. S211 requires that, 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?

Midrise of Steel

Guest blogger Jeff Ellis, engineering manager

Guest blogger Jeff Ellis, engineering manager

The number of midrise structures constructed using light-frame cold-formed steel (CFS) certainly seems to be increasing each year. As with any material, there are benefits and challenges, especially in areas of moderate to high seismic risk. This post will discuss these as well as potential solutions.

Light-frame CFS midrise construction often uses ledger floor framing primarily to facilitate the load transfer detailing at the floor, tension anchorage (tie-downs or hold-downs) and compression chord studs or posts designed to resist the amplified seismic overturning loads. CFS framing is typically thin and singly symmetric.

Various CFS Construction Floor Framing Methods

Various CFS Construction Floor Framing Methods

 

 

 

 

 

 

Amplified Seismic Load

The AISI Lateral Design standard (AISI S213-07/S1-09) Section C5.1.2 requires that the nominal strength of uplift (tension) anchorage and the compression chord studs for shear walls resist the lesser of (1) the amplified seismic load or (2) the maximum load the system can deliver when the response modification coefficient, R, greater than 3. The amplified seismic load is defined as the load determined using the ASCE 7 seismic load combinations with the overstrength factor, Wo, which may be taken as 2.5 for CFS framed shear wall systems with flexible diaphragms.

Typically, the maximum the system can deliver to the uplift anchorage or chord studs is taken as the forces determined using the nominal shear strength of the shear wall assembly tabulated in the seismic shear wall table in S213 multiplied by 1.3. The S213 commentary accounts for the tabulated loads being based on Sequential Phased Displacement (SPD) rather than CUREE cyclic protocol and the degraded backbone curve. See the Structure magazine article that discusses the design of CFS framed lateral force-resisting systems.

Continuous Rod Tie-Down Systems

Light-framed CFS over three stories often use continuous rod tie-down systems rather than cold-formed steel hold-downs to resist shear wall overturning forces as they offer increased load capacity. Neglecting the dead load contribution, the amplified seismic load requirement for CFS shear walls using an R greater than 3 results in an 80% increase in the load used to size the continuous rod tie-down system compared to design level loads. For shear walls using an R greater than 3, it is important to note on the design drawings whether the uplift loads shown are ASD, LRFD, amplified ASD or amplified LRFD so the appropriate tie-down system may be designed.

Continuous Rod Tie-Down System Resisting Shear Wall Overturning Forces

Continuous Rod Tie-Down System Resisting Shear Wall Overturning Forces

Continuous rod tie-down systems are designed not only for strength, but also checked to ensure they do not deflect too much to cause the top of shear wall drift to exceed the code limit or to exceed the 0.20” vertical story deflection limit required by some jurisdictions and ICC-ES AC316. Take-up devices are used in CFS framed structures to take-up construction and settlement gaps that may occur.  AISI S200 Section C3.4.4 states that a gap of up to 1/8” might occur between the end of wall framing and the track. The vertical elongation of the continuous rod tie-down system includes rod elongation (PL/AE) and the take-up device deflection due to the seating increment and the deflection under load.

In addition, coordination is important in using continuous rod tie-down systems in CFS structures because the walls are often prefabricated offsite. An example is the consideration of the appropriate detail for the steel bearing plate installed at the floor sheathing in the story above to resist the uplift (tension) force from the story below.

One possible detail is to install the bearing plate in the bottom CFS track under all the CFS chord studs, but it’s important to ensure the bottom track flanges are deep enough to screw them to the stud flanges as the bearing plate can have a thickness of 1 ½” or more and typical tracks use 1 ¼” flanges. It is also important to ensure that the bearing plate width fits in the track. Another possible detail is to install the bearing plate under the CFS track under all the CFS chord studs.  However, then it must be cut into the floor sheathing and may cause the bottom track to be raised at the bearing plate. For this detail, the floor shear transfer must be detailed through the ledger into the CFS framing.

Continuous Rod Tie-Down System Steel Bearing Plate Coordination Issues

Continuous Rod Tie-Down System Steel Bearing Plate Coordination Issues

Concrete Tension Anchorage

The concrete tension anchorage is designed according to ACI 318 Appendix D using the continuous steel rod material and size in accordance with S213 to have the nominal strength to resist the lesser of the amplified seismic force or the maximum load the system can deliver. ACI 318-11 Section D.3.3.4.3 offers four force limits for design of concrete tension anchorage design in Seismic Design Category C through F:

(1)   The concrete nominal tension anchorage strength shall be greater than 1.2 times the ductile steel rod nominal tension anchorage strength

(2) The anchorage design strength shall be greater than the maximum tension force that can be delivered by a yielding attachment;

(3) The anchorage design strength shall be greater than the maximum tension force that can be delivered by a non-yielding attachment; and

(4) The anchorage design strength shall be greater than the amplified seismic force.

Typically either option (1) or (4) is used where (1) would lead to less concrete required than (4) if the bolt is efficiently sized while (4) would be required for such conditions as a vertical irregularity.  See the concrete anchorage and podium anchorage SE Blog posts for more details.

ACI 318-11 Section D.3.3.4.3 Anchorage Design Options

ACI 318-11 Section D.3.3.4.3 Anchorage Design Options

CFS Wall Stud Bracing

CFS studs are typically thin and singly symmetric and thus require bracing. AISI S211 (Wall Stud Design Standard) permits two types of bracing design that cannot be combined; sheathing based or steel based. There are limits on the stud axial strength when using sheathing braced design. It’s important to identify on the drawings that the sheathing braces the studs and another load combination must be used for the stud design.

2012 IBC Section 2211.4 requires stud bracing to be designed using either AISI S100 (North American Specification) or S211 (Wall Stud Design Standard). S100-07 Section D3.3 required nominal brace strength is to be 1% of the stud’s nominal compressive axial strength, but S100-12 Section D3.3 changes this to the required brace strength is to be 1% of the stud’s required compressive axial strength (demand load). In addition, D3.3 requires a certain stiffness for each brace. AISI S211 required brace strength is to be 2% of each stud’s required compressive axial strength for axially loaded studs and, for combined bending and axial loads, be designed for the combined brace force per S100 Section D3.2.2 and 2% of the stud’s required compressive axial strength.

There are two primary types of steel stud bracing systems: bridging and strap bracing. U-channel bridging extends through the stud punchouts and is attached to the stud with a clip, of which there are various solutions such as this post on Wall Stud Bridging.  Bridging bracing requires coordination with the building elements in the stud bay. It installs on one side of the wall, and does not bump out the wall sheathing. It also requires periodic anchorage to distribute the cumulative bracing loads to the structure for axially loaded studs often using strongback studs and does not require periodic anchorage for laterally loaded studs since the system is in equilibrium as the torsion in the stud is resisted by the U-channel bending.

Flat strap bracing is installed on either side of the wall and at locations other than the stud punchout. It bumps out the sheathing and requires periodic anchorage to distribute the cumulative bracing loads to the structure for axially and laterally loaded studs.

Beam

Strap and Block Bracing

Strap and Block Stud Bracing Anchored Periodically to Structure Using Strongbacks

Beam2

Bridging and Clip Bracing Anchored Periodically to Structure Using Strongbacks

Bridging and Clip Bracing Anchored Periodically to Structure Using Strongbacks

Bridging and Clip Bracing Anchored Periodically to Structure Using Diagonal Strap Bracing

Bridging and Clip Bracing Anchored Periodically to Structure Using Diagonal Strap Bracing

Light-frame cold-formed steel construction has been used successfully for many projects, but there are challenges  that must be addressed to ensure code compliance and desired performance. Some beneficial resources for designing CFS structures are the SEAOC 2012 IBC Structural/Seismic Design Manual Volumes 1 and 2 and the Cold-Formed Steel Engineers Institute’s (CFSEI) website where you can find technical notes and design guides.

What have been some of your observations or challenges in designing cold-formed steel midrise structures?