This year the NASCC (North American Steel Construction Conference) will be in Baltimore, Maryland. The conference is the annual educational and networking event for the structural steel industry, which attracts attendees and exhibitors from all over the world. With more than 130 sessions this year, the conference will provide attendees the opportunity to learn the latest in research, design, technology and best practice in the steel industry. Continue Reading
This week’s post was written by Clifton Melcher, Senior Product Manager at 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). Continue Reading
I recently had the pleasure of presenting a webinar with Rob Madsen, PE, of Devco Engineering on our CFS Designer software, “Increase Productivity in Your Cold-Formed Steel Design Projects.” The webinar took place on September 28, and a recording is available online on our training website for anyone who wasn’t able to join us. Viewing the recording (and completing the associated test) qualifies for continuing education units and professional development hours. The webinar covers how to use the CFS Designer software to design complex loading conditions for beams, wall studs, walls with openings, and stacked walls using cold-formed steel studs, tracks, built-up sections, and even custom shapes. We received some excellent questions during the webinar, but due to time constraints were only able to answer a few during the live webinar. Rob and I did get a chance to answer all the questions in a Q&A document from which I’d like to share some excerpts. The complete Q&A webinar list can be accessed here, or through the online recording. Continue Reading
Back in April of last year, I had the opportunity to show how our new CFS Designer software could help structural engineers “go lean” in their design process by eliminating repetitive tasks (while still meeting required design standards, of course!). Since then, I’ve had the opportunity to visit with hundreds of engineers in person to teach them about CFS Designer and how it can help them improve and optimize their workflows. As a power user of the software, I want to share my top tips for letting CFS Designer help you save the maximum amount of time. Continue Reading
This week’s post was written by Bob Leichti, Manager of Engineering for Fastening Systems.
Those of you who have been following the Simpson Strong-Tie SE Blog for a while may recall our 2013 blog post on the withdrawal resistance of stainless-steel nails. There have been several developments relating to that subject since that blog was posted, and we want to help you catch up.
First, the National Design Specification for Wood Construction (NDS) was revised in 2015. In the 2015-NDS revision, a new chapter 10, Cross-Laminated Timber, was created, moving Dowel-Type Fasteners from Chapter 11 to Chapter 12. Every place in the original blog post where there is a snip of the NDS, you will find the same information in NDS-2015 Chapter 12. Did you know that you can download a free, view-only copy of the NDS from the American Wood Council at awc.org?
Second, after we published our blog post about stainless-steel nail withdrawal, a journal paper was published about withdrawal resistance of stainless-steel nails. This paper has all the nitty-gritty related to withdrawal resistance and bending yield strength for smooth-shank stainless-steel nails: Ramer, D.R. and Zelinka, S.L. (2015). “Withdrawal Strength and Bending Yield Strength of Stainless Steel Nails,” Journal of Structural Engineering, American Society of Civil Engineers, Vol. 141, no. 5, 7 pp. (DOI: 10:1061/ASCE)ST.1943-541X.0001088).
Third, the NDS has been through another revision cycle and will soon have a 2018 copyright date. The chapter on dowel-type fasteners has some significant revisions that we will discuss in a blog post when the NDS-2018 is published later this year. SPOILER ALERT: NDS-2018 has a new withdrawal function for smooth-shank stainless steel nails.
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 SteelConstruction 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.
Joel Houck is a senior R&D engineer for Simpson Strong-Tie’s Infrastructure-Commercial-Industrial (ICI) group based out of the new West Chicago, IL location. He has spent the last 17 years with Simpson developing new mechanical anchors and adhesive anchor components, as well as developing a lot of the lab equipment required to test these products. This experience has given him extensive knowledge and insight into the concrete anchor industry, especially when it comes to the proper function and performance of anchors. Joel is a professionally licensed mechanical engineer in the state of Illinois.
There’s a saying in Chicago, “If you don’t like the weather, just wait fifteen minutes.” That’s especially true in the spring, when temperatures can easily vary by over 50° from one day to the next. As the temperature plunges into the blustery 30s one evening following a sunny high in the 80s, I throw my jacket on over my T-shirt, and I’m reminded that large swings in temperature tend to bring about changes in behavior as well. This isn’t true just with people, but with many materials as well, and it brings to mind a thermal process called heat treating. This is a process that is used on some concrete anchoring products in order to make them stronger and more durable. You may have heard of this process without fully understanding what it is or why it’s useful. In this post, I will try to scratch the surface of the topic with a very basic overview of how heat treating is used to improve the performance of concrete anchors.
According to the ASM Handbook:Heat Treating, heat treatment is a process of heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties.1 In practical terms, metals (usually steel in the case of most concrete anchors) are heat treated in order to improve their properties in some way over their base condition. When steel wire is formed into the complex shapes of anchors during the manufacturing process, the steel needs to be soft and formable; however, it is often beneficial to the performance of the final anchor product to be much harder and stronger than the base steel from which it’s formed. That’s where heat treating comes into play. By heating and cooling soft steel in a controlled manner, changes are made to the crystal structure of the steel in order to improve mechanical properties such as hardness, toughness, strength or wear resistance. Although the steel undergoes very complex microstructural changes during the heat treatment process, the end result is fairly straightforward – the once soft steel becomes harder and stronger as dictated by the heat treating process. As concrete anchors become more and more complex in order to meet the needs of building codes and designers, heat treating is becoming a more common and necessary component of high-strength anchors.
Depending on the desired results, there are many different types of heat treating processes that can be considered. The type of heat treatment and the parameters that are used can be customized for the steel type and the specific anchor application. There are several different types of heat treatments that are typically used for anchors. Two of the most common types are through hardening (also called neutral hardening) and surface hardening (also called case hardening).
Through hardening changes the mechanical properties (hardness, strength, ductility, etc.) of the steel without affecting its chemical composition. In order to alter the microstructure of the steel, it is heated in a furnace to a very high temperature, and then rapidly cooled, usually by submerging it in a liquid quench medium such as water or oil. This process will generally result in a very hard, but brittle material, so a secondary operation, called tempering, is employed after quenching. To temper steel, it is reheated to a lower temperature and then cooled in order to remove the stresses and brittleness created during the original quenching operation. Through hardening is useful where increased strength and toughness are required and surface wear isn’t a big concern, such as in our Crimp Drive® and split-drive anchors, setting tools for drop-in type anchors, high-strength all-thread-rod for adhesive anchors, and gas- or powder actuated fasteners. In order to effectively through harden an anchor, moderate levels of hardening elements must be present in the base steel, usually in the form of carbon. As the carbon content in the steel increases, so does the ability to harden it. The chemical composition of the steel along with the specific heat treating parameters will determine the level of hardness, strength and toughness of the final parts.
Surface hardening changes the hardness of the steel at the surface of the part by modifying the chemical composition of the steel at its surface only. This is done by altering the atmosphere in the heat treating furnace in order to get alloying elements, usually carbon, to diffuse into the surface of the steel. The increased carbon content increases the hardenability of the steel at the surface, but it can’t penetrate deeply into the steel, so a thin case forms around the surface of the steel with higher strength and hardness than the interior of the part. This creates parts that have high ductility throughout most of the interior, but that also have hard, wear-resistant surfaces. This type of heat treatment is useful in heavy-duty anchors where components of the anchors are sliding against each other during the setting process. It’s also useful in screw anchors, where the steel threads need to be very hard and wear resistant in order to cut into the concrete, but the ductility of the anchor must be maintained in order to avoid brittle failures in service. Just as with through hardening, there are many variations of surface hardening used in anchors, depending on the specific application.
By using these two processes along with other heat treating processes, we are able to expand our ability to meet the higher demands placed on anchors in an industry that continues to evolve. As heat treating and steel chemistry continue to innovate, we will continue to use these developments to provide our customers with No-Equal concrete anchors that meet our high standard for performance and safety.
From complex infrastructure projects to do-it-yourself ventures, Simpson Strong-Tie offers a wide variety of anchoring products to meet virtually any need.
1 Lampman et al. (1997). ASM Handbook: Heat Treating. Materials Park, OH: ASM International.
People are always innovating new things! There are always new tools, software, apps or, more recently, digital assistants to help us organize our life! Here’s something I want to share with you. Recently my family bought Google Home, and both my boys (ages 8 and 5) are constantly exploring it and testing its capabilities: “Hey, Google, play this music” or “Hey, Google, what time is it?” or “Hey, Google, repeat ‘Nathan is bad.’” While Google Home helps them with the former requests, it simply says, “I am still learning,” in response to commands like “repeat ‘Nathan is bad.’” It’s funny to see them experiment and come up with creative ideas to use the tool. Many of us appreciate tools that help us be more organized or increase our efficiency or that are simply fun to use. Our new revised diaphragm calculator for designing metal decks is our attempt to help the engineering community get more done in less time.
So What Are the Updates and Revisions?
We have updated our design software to design per Canadian Standards like CSA136 and to design per Limit States Design. The app is so easy to use that you can design a steel deck diaphragm in minutes! The software designs steel decks for both shear and uplift forces acting on the deck and provides tables with diaphragm shear capacities for a given deck span using Simpson Strong-Tie deck fasteners that conform to Canadian codes and standards. These fasteners have an evaluation report, IAPMO UES ER-326, are recognized in SDI (Steel Deck Institute) DDM03 Appendix VII and IX and the CSSBI (Canadian Sheet Steel Building Institute) Design Manual and have FM approvals.
Overview of the App
When you open our diaphragm design software, Steel Deck Diaphragm Calculator, there is an option to “Select your Country.” You can choose to design for US standards, in which case you select the USA option, or you can select Canada Imperial or Canada Metric, which are new additions. The app has three sections: (1) Optimized Solutions, (2) Diaphragm Capacity Tables and (3) Other Diaphragm Tables. All three options are available for the USA option. The Optimized Solutions help you to design a deck for any given shear and uplift. You can refer to our previous blog, Design Examples for Steel Deck Diaphragm Calculator Web App, for some examples on how to design steel decks using the Optimized Solutions selection. Diaphragm Capacity Tables are available to the USA and both Canada selections. Other Diaphragm Tables is available only to the USA selection.
Metal Deck Diaphragm Design Using Limit States Design (LSD)
When you select Canada for the country, you will have the option to select Diaphragm Capacity Tables as shown in the screen shot below. You can generate diaphragm shear tables by entering:
Steel Deck Information: In this section, you select the type of the deck, the design method, the load type you would like the tables to be generated in and the deck thickness. You can enter uplift if you would like to design the deck for combined shear and tension, or leave the net uplift as zero if you are generating shear-only tables.
Quik Drive Fastener Information: In this section, you input information about the structural and side-lap fasteners.
Click the Calculate button to generate the tables.
A PDF copy of the tables can be generated in either English or French.
This easy-to-use design software can be used by the designers, specifiers or erectors to generate the tables required. More information about our X series of screws (including XL and XM), tools and the required industry approvals for designing the profiled deck diaphragms can be found on our website at strongtie.com.
Please try out the app and let us know your comments and feedback so we can continue to improve our software to better serve your needs!
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.
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.
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.
First, 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.
The 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.
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.
First, 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.
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.
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.
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.
In 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.
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?