Revisiting Stainless-Steel Nail Calculations . . . .

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.

Stay tuned!

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.


How Heat Treating Helps Concrete Anchoring Products Meet Tougher Load Demands

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.

Figure 1. Steel microstructures: (a) soft steel example; (b) heat treated steel example.2

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).

Figure 2. Fasteners entering a heat treating furnace.3

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.

Figure 3. Cross-section of surface hardened bar showing different hardness zones at the surface and in the interior.4

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.

Mechanical Anchors

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.

2 “Microstructure of the AISI 4340 Steel.” Digital Image. Research Gate, n.d. Web. 14 June 2017 https://www.researchgate.net.

3 “Heat Treat Furnace.” Digital Image. ThomasNet Web Solutions, n.d. 14 June 2017 http://www.morganohare.com/heat-treating.html.

4 “Macrographs Showing Case Depth of Steels.” Digital Image. Science and Education Publishing Co. Ltd, n.d. 14 June 2017 http://pubs.sciepub.com.

Design More with Our New Steel Deck Diaphragm Calculator App!

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:

  1. 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.
  2. 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!

 

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.

cfs-clip-attachment

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?

 

Soft-Story Retrofits Using the New Simpson Strong-Tie Retrofit Design Guide

Thousands of soft-story buildings up and down the West Coast require retrofits to prevent collapse in the event of a major earthquake. Whether the retrofits are mandated by a city ordinance (as in San Francisco, Berkeley and Los Angeles) or are undertaken as voluntary upgrades, the benefits of adding necessary bracing to strengthen the ground story are immense. Simpson Strong-Tie has taken the lead, with our new Soft-Story Retrofit Guide, to provide information that helps engineers find solutions to reinforce soft-story buildings against collapse. We are also providing information on the two methods that can be used for the analysis and design of these soft story retrofits.

soft-story-retrofit-guideAfter the initial information section of the guide, a two-page illustrated spread (pp. 14–15) shows various retrofit products that could be used to retrofit the soft-story structure with reference to the following pages. Three main lateral-force-resisting systems highlighted in this graphic are the Strong Frame® special moment frame (SMF), the new Strong-Wall® wood shearwall, and conventional plywood shearwalls. Individual retrofit components are also shown, such as connection plates and straps for lateral-load transfer, anchors for attachment to the foundation, fasteners and additional products such as the RPBZ retrofit post base and AC post caps for providing a positive connection.

soft-story-product-illustrationTurning the page, you come to the section describing in detail the many benefits of the Strong-Frame special moment frame (SMF) in a retrofit situation. The engineered performance of the SMF provides the additional strength and ductility that the building requires and can be fine-tuned by selecting various combinations of beams, columns, and Yield-Link® structural fuse sizes. A typical retrofit Strong Frame® SMF comes in three complete pieces allowing for the frame to be installed on the interior of the structure in tight quarters. The frame is simply installed using a 100% snug-tight field-bolted installation with no on-site welding or lateral-beam bracing required.

field-installation-beam-to-columnThe next lateral system we focus on is the Strong-Wall® shearwall and the new grade beam solutions offered to reduce the concrete footprint. The new Strong-Wall wood shearwall includes an improved front-access holdown and top-of-wall connection plates for easier installation. Both the Strong Frame SMF and the Strong-Wall wood shearwall have load-drift curves available for use with FEMA P-807. Site-built shearwalls can be installed using retrofit anchor bolts at the mudsill and new holdowns at the shearwall end posts.

strong-wall-wood-shearwall-pushover-curveIn the pages following the lateral systems, various products are shown with tabulated LRFD capacities, whereas ASD capacities are typically provided in the order literature for these products. Both ASD and LRFD capacities have been provided for products with new testing values such as the A35 and L90 angles installed with ⅝”-long SPAX screws into three different common floor sheathing materials, as well as for the new HSLQ heavy-shear transfer angle designed to transfer higher lateral forces directly from 4x blocking to the 4x nailer on the Strong-Frame SMF, even when a shim is used between the floor system and the frame. LRFD capacities are provided in this new Soft-Story Retrofit Guide specifically for use with the FEMA P-807 design methodology. This methodology specifies in section 6.5.1 that:

Load path elements should be designed to develop the full strength and the intended mechanism of the principal wall or frame elements. Therefore, to ensure reliability, appropriate strength reduction factors should be applied to the ultimate strengths of load path elements. Specific criteria may be derived from principles of capacity design or from other codes or standards, such as ASCE/SEI 41 or building code provisions involving the overstrength factor, Ωo.

FEMA P-807 bases the capacity of the retrofit elements on the peak strength. LRFD capacities are provided for various load-path connector products, which can be used to develop the full strength of the lateral-force-resisting element to satisfy this requirement.

typical-a35-hslq412-installationWrapping up, the guide focuses on the various free design tools and resources available for the evaluation, design and detailing of the soft-story structure retrofit. These tools include the Weak Story Tool with Simpson Strong-Tie® Strong Frame® Moment Frames, Design Tutorials for the WST for both San Francisco– and Los Angeles–style buildings, our Soft-Story Retrofit Training Course offering CEUs, Strong Frame Moment Frame Selector Software, Anchor Designer™ Software for ACI 318, ETAG and CSA, and tailored frame solutions using our free engineering services.

soft-story-documentsFor other information regarding soft-story retrofits, refer to previous blogs in “Soft-Story Retrofits,”  “City of San Francisco Implements Soft-Story Retrofit Ordinance,” and “Applying new FEMA P-807 Weak Story Tool to Soft-Story Retrofit.”

 

 

 

DoD-Compliant CFS Wall Framing Design

jeff-kreinkeThis week’s post comes from Jeff Kreinke, PE, SE, a structural Project Manager with Excel Engineering in Fond du Lac, WI. Jeff earned his Bachelor’s degree in Civil & Environmental Engineering from the University of Wisconsin-Madison. He has worked in Excel Engineering’s structural department for 15 years and specializes in cold-formed steel systems design. This specialization has included the design of dozens of “blast” resistant structures per the Unified Facilities Criteria standards. Jeff is licensed in 22 states as a Professional or Structural Engineer.

Back in the year 2000, the U.S. Department of Defense (DoD) was charged with incorporating antiterrorism protective features into the planning, design and execution of its facilities. The main document developed to meet this requirement is the Unified Facilities Criteria “DoD Minimum Antiterrorism Standards for Buildings” (UFC 4-010-01). The current version was published in October 2013. This document covers what is most commonly referred to as “blast” design.

All DoD inhabited buildings, billeting (military housing) and high-occupancy family housing projects are required to comply with this standard. The most common projects incorporating cold-formed steel framing are for the military branches (including National Guard and Reserve components). Notable exceptions are: low-occupancy buildings (11 occupants or fewer), transitional structures (with intended life cycles of five years or less), standalone retail establishments and parking structures. A complete list of excluded buildings types is located in chapter 1-9 of UFC 4-010-01.

General structural requirements for DoD projects are provided in the UFC 1-200-01 standard. The current model building code referenced is the 2012 IBC. Specific structural design criteria are also listed for all major military bases. UFC 4-010-01 specifically states that its requirements do not supersede the structural requirements of UFC 1-200-01. Basically, three lateral designs are required for all DoD projects: wind, seismic and blast.

The main design strategies employed by UFC 4-010-01 are to maximize standoff distance, prevent progressive collapse and minimize hazardous flying debris. Progressive collapse avoidance is addressed by UFC 4-023-03. The main criterion to note is that it is only required in structures of three or more stories in height.

standoff-distances-with-controlled-perimeter

UFC 4-010-01 tables B-1 and B-2 provide the conventional standoff distances and minimum standoff distances for a building based on its construction type and building category. The conventional standoff distance is where conventional construction may be used for building components other than windows and doors without a specific analysis of blast effects. The minimum standoff distance is the smallest permissible distance allowed regardless of any analysis or construction hardening.

standoff-distances-for-new-existing-buildings

conventional-construction-standoff-distances

To minimize hazardous flying debris, two basic components are addressed: typical wall framing and opening support framing. UFC 4-010-01 Table 2-3 provides the allowable height for cold-formed stud typical wall framing based on material, spacing, support conditions and supported weight.

conventional-construction-parameters

conventional-construction-parameters2

For both brick veneer and EIFS cladding, the maximum allowable height is 12″-0″.  Note that 50 ksi stud material is required to meet these requirements. There is no similar strength requirement for connections.

For framing that meets both the conventional standoff distance and allowable member height, NO further blast analysis is required. For framing that does not meet either the conventional standoff distance or allowable member height, additional blast analysis IS required. The support framing (head/sill/jambs) for windows and skylights ALWAYS requires additional blast analysis. Per section B-3.3.3 of UFC 4-010-01, the support framing for doors, glazed or solid, is not required to be analyzed for blast. Doors are designed to remain in their frames and not become hazards to building occupants. Sidelights and transoms that are included within the door assembly are also not required to be analyzed for blast. There are exclusions provided for unoccupied areas of buildings, such as exterior stairwells and exterior walkways. Personal experience with the U.S. Army Corps of Engineers Protective Design Center (USACoE PDC) has shown that unoccupied attic areas are also allowed to be excluded from the provisions.

Two “levels of protection” are provided for in the UFC 4-010-01. Only Low Level of Protection (LLOP) and Very Low Level of Protection (VLLOP) buildings are addressed. Low Level of Protection allows for moderate damage with collapse being unlikely and having the potential for serious but not fatal human injury. Very Low Level of Protection allows for heavy structural damage with progressive collapse unlikely and having serious human injury likely with potential fatalities. Any projects that require a higher Level of Protection designation must be referenced elsewhere. The project appropriate “Level of Protection” is provided in UFC 4-01-01 Tables B-1 and B-2.

UFC 4-010-01 allows for two basic analysis methods for performing blast design, static analysis and dynamic analysis. Static analysis can be performed with the aid of the Simpson Strong-Tie® CFS DesignerTM software, while dynamic analysis must be performed with the “Single-degree-of-freedom Blast Effects Design Spreadsheet” (SBEDS) obtained from the USACoE PDC. Generally, dynamic analysis will provide lighter members than static analysis.

Static Analysis Method

Static analysis of “punched” openings (framed with head and sill members and supported by jamb studs) is only allowed to be performed if the conventional standoff distance is met. Ribbon windows, aluminum curtainwalls, storefronts, etc., are required to be designed by the dynamic analysis method.  The “punched” window supporting structure is to be designed to account for the increased tributary area representing the area of the window and the walls above and below it.  These supporting elements must have moment and shear capacities greater than the calculated conventional wall capacities multiplied by the applicable tributary area increase.

illustration-of-tributary-width-valuesFor example, if the tributary area of the “punched” opening is three times the typical full-height stud spacing, three members are required for the jamb stud assembly. An alternate member with moment and shear capacities greater than or equal to three times the typical full-height members’ capacity can also be used.  UFC 4-01-01 section B-3.1.4.1 states that connection loads shall be determined based on the increase in member shear capacity. It makes a difference which members are chosen for the jamb stud assembly, as the connection must be designed for the shear capacity of that specific assembly. Per UFC 4-010-01 section B-3.1, the connectors themselves are designed using LRFD methods with a Load Factor of 1.0. The resistance factor for bending is allowed to be 1.0, while the resistance factors for other failure modes remain per the AISI code (Shear = 0.95). Per UFC 3-340-02 section 5-47, when LRFD values are not published for connectors, a value of 1.7 times the allowable strength is permitted. Published LRFD strengths already have the appropriate resistance factors incorporated. Simpson Strong-Tie publishes LRFD values for all of its connectors.

Dynamic Analysis Method

As mentioned above, dynamic analysis is performed with the SBEDS spreadsheet tool provided free of charge by the USACoE PDC. There is a simple approval process to undergo in order to receive a software key for the program. The general inputs for the program are fairly straightforward. There is a list of standard preset stud shapes that can be selected. The list is not comprehensive, but shape property inputs are provided for “user defined” members.

SBEDS General Input

SBEDS General Input

The SBEDS spreadsheet is formatted to analyze single-span wall framing. For analyzing opening support framing, the stud spacing and supported weight will have to be adjusted. Per PDC TR-10-02, the stud spacing is to be figured the same way as in the static analysis method. The spreadsheet analyzes the entire wall at one supported weight, so this will also have to be adjusted to account for the differing weights of the building cladding and glazing, providing an average support weight. For EIFS cladding, all weights are typically assumed to be the same (6 psf actual). For brick veneer (46 psf actual), the supported weight must be reduced to account for the lower weight of the glazing (6 psf actual). The wall span will need to be adjusted to the actual length of the head and sill members.

The blast parameters are input as “Charge Weight & Standoff.”  The appropriate project explosive weight is referenced from the UFC 4-010-02 standard. This publication is authorized only to U.S. Government agencies and their contractors. Approval must be obtained from the USACoE PDC to obtain this document. The standoff distance is entered as the project-specific standoff distance. The Incidence angle can be calculated as the arc tangent (ATAN) of the height to the center of the opening divided by the actual standoff distance. The remaining typical blast parameter inputs are shown below.

SBEDS Blast Parameters Input

SBEDS Blast Parameters Input

The typical response criteria inputs are shown below. Typically stud framing is required to be “connected top and bottom.” There is an option to select “Top Slip Track,” but personal experience has shown that this option does not pass the analysis. There are two options provided with the “Level of Protection” type. “Primary” would be selected for load-bearing projects, while “Secondary-NS (Non-Structural)” would be selected for curtainwall-type projects.

SBEDS Response Criteria Input

SBEDS Response Criteria Input

For the Solution Control inputs, the main check is to verify that the inputted time step matches the calculated recommended time step. The calculated value will change when various other inputs are changed. The “% of Critical Damping” can be entered as 5% for CFS framing, per the spreadsheet cell comment. The “Initial Velocity” is entered as zero, also per the spreadsheet cell comment.

SBEDS Solution Control Input

SBEDS Solution Control Input

Connections are designed to meet or exceed the “Peak Reactions Based on Ultimate Flexural Resistance” value given in the SBEDS output. Per UFC 4-010-01 section B-3.1, design is done per LRFD methodology with Load Factors equal to 1.0. The Resistance Factor for bending is allowed to be 1.0, while the Resistance Factors for other failure modes are per the AISI code (Shear = 0.95). Published LRFD strengths already have the appropriate resistance factors incorporated. The conservative assumption would be that shear controls the failure, and an increase in the LRFD Resistance Factor is not appropriate.

SBEDS Reactions Output

SBEDS Reactions Output

Per UFC 3-340-02, there are additional increases in connection strength that can be taken for dynamic blast design. The Strength Increase Factor (SIF, a.k.a. Static Increase Factor or Average Strength Factor) considers that yield stresses for all CFS materials provided are typically higher than the minimum yield stresses required by ASTM A446 (replaced by ASTM A653) steel. Per section 5-12.1, the SIF is listed as 1.21 for all CFS framing but only applies to the yield stress (Fy). Any calculations involving the ultimate stress (Fu) value are not allowed to be increased. The SIF also does not apply to fasteners (screws, bolts, PAFs, welds, etc.). The Dynamic Increase Factor (DIF) considers the strain-rate effects from rapid blast loading. Per section 5-34.2, this increase is listed as 1.10 for all CFS framing and per UFC 4-0-010-01 section 4-7, 1.05 for welds. Conservatively, it is assumed that the DIF equals 1.0 for all other fasteners (screws, bolts, PAFs, etc.).

Dynamic Connection Strength = (LRFD Strength)*(SIF)*(DIF)

For example, per Table 1 given below (reference Simpson Strong-Tie® engineering letter L-CF-CWCLRFD15), the SCB45.5 Bypass Slide Clip (3 screws to 16 ga. material) has a Dynamic Connection Strength = (2,025)*(1.0)*(1.10) = 2,227.5 lb., meeting the required reaction shown above.

scs-bypass-framing-slide-clip-connector

Again per UFC 3-340-02 section 5-47, when LRFD values are not published for connectors, a value of 1.7 times the allowable strength is permitted.

Dynamic Connection Strength = 1.7*(Allowable Strength)*(DIF)

Simpson Strong-Tie publishes LRFD values for all of its connectors.

References:

UFC 4-010-01:  February 9, 2012 (Change 1 – October 1, 2013)

UFC 3-340-02:  December 5, 2008 (Change 2 – September 1, 2014)

PDC-TR 06-01:  December 2012

PDC TR-10-02:  April 2012

https://pdc.usace.army.mil/

 

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!

speed7

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.

 

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.