Hydrogen Embrittlement in High-Strength Steels

The use of high-strength steels in anchors and fasteners is not unusual in the construction world. For example, high-strength threaded rods are often used to reduce the number of mild-steel rods that would be required to meet a design load for an adhesive anchor. High-strength steel is essential to the function of some fasteners, such as self-drilling and self-tapping screws.

However, for anchors and fasteners, there are limits to what can be accomplished by increasing steel strengths due to a phenomenon known as hydrogen embrittlement. Hydrogen embrittlement is well known by anchor and fastener manufacturers, but it is not widely known by structural engineers. The purpose of this blog post is to provide an overview of this important subject and some insight into one reason why steels used in construction anchors and fasteners have upper strength limits.

What is hydrogen embrittlement?

Hydrogen embrittlement is a significant permanent loss of strength that can occur in some steels when hydrogen atoms are present in the steel and stress is applied. Embrittlement occurs as hydrogen atoms migrate to the region of highest stress and cause microcracks. When a crack forms, the hydrogen then migrates to the tip of the crack (Figure 1) and causes continued crack growth until the effective fastener cross-section is so reduced that the remaining cross-section is overloaded and the fastener fails. Figures 2, 3 and 4 show a failure plane and coarse granular morphology and intergranular cracks (dark lines in Figures 3 and 4). These failures occur suddenly, without warning, after the fastener or rod has been loaded for a period of time.

Figure 1 – Conceptualized migration of hydrogen to the crack tip causing further cracking.

Figure 1 – Conceptualized migration of hydrogen to the crack tip causing further cracking.

Fig. 2

Fig. 2

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Fig. 3

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Figure 2,3,4 – Scanning electron microscope images of intergranular cracking in a steel screw due to hydrogen embrittlement.

All three of the aforementioned conditions must be present for hydrogen embrittlement to occur:

  • A steel that is susceptible to hydrogen embrittlement
  • Atomic hydrogen (H+ ions, not H2 gas)
  • Stress, such as from tightening or applied loads

If an application involves these, then there is a possibility of hydrogen embrittlement and fastener failure. The possibility of and time to failure depend on the degree of each of these conditions. Therefore, the level of concern should depend on the degree to which the Designer expects all three of these conditions to be present in the application.


What steels are sensitive to hydrogen embrittlement?

A fastener’s susceptibility to hydrogen embrittlement increases with elevated tensile strength or hardness. It is generally accepted that good-quality fasteners with an actual (not specified) Rockwell C scale hardness of less than 38 (tensile strength of less than approximately 170,000 psi) are not ordinarily susceptible to hydrogen embrittlement. Fastener manufacturers often establish lower hardness limits as an extra margin of safety for variability that may occur in production.

When are hydrogen atoms (H+) present?

Hydrogen atoms can be introduced into a fastener from manufacturing or from the service environment.

Sources of hydrogen from manufacturing:

Hydrogen can be present in the steel-making process, but the amount present in good-quality steel is below the level that causes problems. The most common way that hydrogen is introduced during anchor and fastener manufacturing is from the cleaning and coating processes. These processes often utilize acids that produce hydrogen. Compounding this problem is that many popular coatings like zinc plating (ASTM B633) and hot-dip galvanizing (ASTM A153) create a barrier around the fastener that does not allow hydrogen to easily diffuse out of the fastener. Some coatings, like mechanical galvanizing (ASTM B695), do have sufficient porosity that hydrogen can diffuse through the coating at room temperature.

Manufacturers most commonly manage internal hydrogen sources by minimizing cleaning time and by baking plated fasteners after coating for a number of hours to help diffuse hydrogen through the coating and out of the fastener. In some cases, mechanical cleaning and alkaline cleaning are used to prevent hydrogen from being introduced.

In cases where internal hydrogen is not properly managed in the manufacturing process, failure usually occurs quickly, within 48 hours of fastener installation.

External sources of hydrogen:

Hydrogen can also be introduced from the service environment. Zinc coatings galvanically protect steel from corrosion in damp or wet service environments. However, this process results in an electric current being passed through the water (H2O) to produce hydrogen (H+) and hydroxide (OH) ions as shown in Figure 5. Since this process requires corrosion to generate the hydrogen, failures from externally generated hydrogen usually take much longer to occur than when internal hydrogen is the cause. Steel failure from externally generated hydrogen can take anywhere from weeks to years.

Figure 5 – Production of hydrogen (H+) from the galvanic protection of steel by a zinc coating

Figure 5 – Production of hydrogen (H+) from the galvanic protection of steel by a zinc coating

How much stress is too much stress?

Fortunately, for products that require very high-strength steels, hydrogen embrittlement risk diminishes, even for sensitive steels, as applied loads are reduced. For such products there are a number of complex tests that fastener manufacturers utilize in development and quality control to verify that the risk of hydrogen embrittlement is controlled for the intended application at the rated loads.

Closing thoughts

As illustrated in the discussion above, hydrogen embrittlement can be a serious concern for high-strength anchors and fasteners. A Designer should exercise great care to guard against the sudden brittle steel failure that this phenomenon can produce. There are a number of good practices to follow in this regard:

  • Do not utilize exotic high-strength steels with actual tensile strengths greater than 170,000 psi (Rockwell C-scale hardness of 38) without carefully considering hydrogen embrittlement risk. Note that actual tensile strength may be higher than specified tensile strengths.
  • Ensure that quality sources of high-strength fasteners are specified. High-quality manufacturers have design and manufacturing practices in place to guard against hydrogen embrittlement.
  • Use discretion in selecting very high-strength fasteners for corrosive applications since these conditions often produce hydrogen through the galvanic process.

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?

Design Examples for Steel Deck Diaphragm Calculator Web App

This week’s blog post was written by Neelima Tapata, R&D Engineer for Fastening Systems. She works in the development, testing and code approval of fasteners. She joined Simpson Strong-Tie in 2011, bringing 10 years of design experience in multi- and single-family residential structures in cold-formed steel and wood, curtain wall framing design, steel structures and concrete design. Neelima earned her bachelor’s degree in Civil Engineering from J.N.T.U in India and M.S. in Civil Engineering with a focus on Structural Engineering from Lamar University. She is a registered Professional Engineer in the State of California.

Like most engineers, you are probably often working against tight deadlines,  on multiple projects and within short delivery times. If you have ever wished for a design tool that would make your work easier, we have an app for that. It’s a simple, quick and easy-to-use tool called the “Steel Deck Diaphragm Calculator” for designing steel deck diaphragms. This tool is so user friendly you can start using it in minutes without spending hours in training. This app can be found on our website, and you don’t need to install anything.

The Steel Deck Diaphragm Calculator has two parts to it: “Optimized Solutions” and “Diaphragm Capacity Tables.” Optimized Solutions is a Designer’s tool and it offers optimized design solutions based on cost and labor for a given shear and uplift. The app provides multiple solutions starting with the lowest cost option using different Simpson Strong-Tie® structural and side-lap fasteners. Calculations can be generated for any of the solutions and a submittal package can be created with the code reports, Factory Mutual Approval reports, fastener information, corrosion information, available fliers, and SDI DDM03 Appendix VII and Appendix IX that includes Simpson Strong-Tie fasteners. Currently, this tool can be used for designing with only Simpson Strong-Tie fasteners. We will be including weld options in this calculator very soon. Stay tuned!

The Diaphragm Capacity Tables calculator can be used to develop a table of diaphragm capacities based on the effects of combined shear and tension.

steeldeck1

When “Optimized Solutions” is selected, the following input is requested:

Step 1: Building Information   ̶   Enter general information about the project, like the project name, the length and width of the building to be designed along with spacing between the support members such as joist spacing, is entered.

Step 2: Steel Deck Information   ̶   Select the type of the steel deck along with the fill type. You can select the panel width from the options or select “Any panel width” option for the program to design the panel width. Choose the deck thickness or select the “Optimize” option for the program to design the optimum deck thickness. You also have an option of editing the steel deck properties to accommodate proprietary decks that are within the limitations of SDI DDM03 Section 1.2. Select the joist steel (support) thickness that the deck material will be attached to. For some fasteners, the shear strength of the fastener is dependent on this support thickness.

Step 3: Load Information   ̶  Enter the shear and uplift demand and select the load type as either “wind” or “seismic” and the design method as “ASD” or “LRFD.”

Step 4: Fastener Information   ̶  This is the last step of input before designing. In the fastener information section, you have the option to choose a structural and side-lap fastener or let the program design the most cost-effective structural and side-lap options. This can be done by checking the “Provide optimized solutions” option. The default options in the program are usually the best choice. However, you can change or modify as needed for your project. You can also set the side-lap fastener range or leave it to the default of 0 to 12 fasteners.

Now let’s work on an example:

Design a roof deck for a length of L = 500 ft. and a width b = 300 ft. The roof deck is a WR (wide rib) type panel, with a panel width of 36″.  The roof deck is supported by joists that are ¼” thick and spaced at 5 ft. on center. Design the diaphragm for wind loading using Allowable Stress Design method. The diaphragm should be designed for a diaphragm shear of 1200 plf. and a net uplift of 30 psf. The steel deck is ASTM A653 SS Grade 33 deck with Fu = 45 ksi.

This information is entered in the web app, as seen below.

steeldeck2

After inputting all the information, click on the Calculate button. You will see the five best solutions sorted by lowest cost and least amount of labor. Then click on the Submittal Generator button. Upon pressing this button, a new column called “Solution” is added with an option button for each solution. You can select any of the solutions. Below the Submittal Generator button, you can select various Code Reports and Approvals and Notes and Information selections that you want included in the submittal. After selecting these items, click on the Generate Submittal button. Now a pdf package will be generated with all of your selections.

steeldeck3

Below is the screen shot of the first page containing Table of Contents from the PDF copy generated. The PDF copy contains the solutions generated by the program, then the detailed calculations for the solution that is selected. In this case, as you can see in the screen shot above, detailed calculations for solution #1 are included with XLQ114T1224 structural screws; XU34S1016 side-lap screws; 36/9 structural pattern and with (10) side-lap fasteners; diaphragm shear strength of 1205 plf. and diaphragm shear stiffness of 91.786 kip/in. The detailed calculations are followed by IAPMO UES ER-326 code report and FM Approval report #3050714.

steeldeck4

Below is another example of a roof deck to be designed for multiple zones.

Design a roof diaphragm that will be zoned into three different areas. Zoning is a good way to optimize the economy of the roof diaphragm. Below are the required diaphragm shears and uplift in the three zones.

Zone 1: Diaphragm shear = 1200 plf.; Net uplift = 30 psf.; Length and width of zone 1 = 300 ft. x 200 ft.
              Joist spacing = 5 ft.

Zone 2: Diaphragm shear = 1400 plf.; Net uplift = 0 psf.; Length and width of zone 2 = 500 ft. x 200 ft.
              Joist spacing = 5.5 ft.

Zone 3: Diaphragm shear = 1000 plf.; Net uplift = 25 psf.; Length and width of zone 3 = 300 ft. x 200 ft.
              Joist spacing = 4.75 ft.

Refer to the example above for all other information not given.

To design for multiple zones first select the Multi-Zone Input button, which is below the Fastener Information section as shown below:

steeldeck5

When you click on the Multi-Zone Input button, you can see a toggle button appearing above a few selections as shown below. The default for the toggle button is globalbutton, which means that this selection is same for all the zones. You can click on the toggle button to change to zonebutton. Then the selection below changes to a label and reads Zone Variable. After all the selections that need to be zone variables are selected, click the Add Zone button. Keep adding zones as needed. A maximum of five zones can be added. After creating the zones, add the information for each zone and click the Calculate button.

steeldeck6

When the Calculate button is clicked, the results for each zone are listed. The five best solutions are listed for each of the zones as shown below.

steeldeck7

Similar to previous example, select the Generate Submittal button to select the solutions to be included in the submittal generator. Select one solution for each zone and then check the items like the code reports or notes to be included in the submittal. Click Generate Submittal to create the submittal package.

See the screen shot below for the steps.

steeldeck8

Now that you know how easy it is to design using our web app, use this app for your future projects. We welcome your feedback on features you find useful as well as your input on how we could make this program more useful to suit your needs. Let us know in the comments below.

 

Connectors and Fasteners in Fire-Retardant-Treated Wood

In any given year, Simpson Strong-Tie fields several questions about the use of our connectors and fasteners with pressure-treated fire-retardant wood products. Most often asked is whether this application meets the building code requirements for Type III construction, and whether there is a legitimate concern about corrosion. While there haven’t been any specific discussions on this topic in the SE Blog, there have been related discussions surrounding sources of corrosion, such as: Corrosion: The Issues, Code Requirements, Research and Solutions, Corrosion in Coastal Environments, Deck Fasteners – Deck Board to Framing Attachments. This post will explore several resources that we hope will enable you to make an informed decision about which type of pressure-treated Fire-Retardant-Treated Wood (FRTW) to choose for use with steel fasteners and connectors.

One factor contributing to the frequency of these questions is the increased height of buildings now being constructed. With increased height, there is a requirement for increased fire rating. To meet the minimum fire rating for taller buildings, the building code requires noncombustible construction for the exterior walls. As an exception to using noncombustible construction, the 2015 International Building Code (IBC®) section 602.3 allows the use of fire-retardant wood framing complying with IBC section 2303.2. This allows the use of wood-framed construction where noncombustible materials would otherwise be required.

In the 2009 IBC, Section 2304.9.5, “Fasteners in preservative-treated and fire-retardant-treated wood,” was revised to include many subsections (2304.9.5.1 through 2304.9.5.4) dealing with these wood treatments in various types of environmental applications. Section 2304.9.5.3 addressed the use of FRTW in exterior applications or wet or damp locations, and 2304.9.5.4 addressed FRTW in interior applications. These sections carried over to the 2012 IBC, and were moved to Section 2304.10.5 in the 2015 IBC. FRTW is listed in various other sections within the code. For more information about FRTW within the code (e.g., strength adjustments, testing, wood structural panels, moisture content), the Western Wood Preservers Institute has a couple of documents to consult: 2009 IBC Document and 2013 CBC Document. They also have a number of different links to various wood associations.

As shown in Figure 1 below, fasteners (including nuts and washers) used with FRTW in exterior conditions or where the wood’s service condition may include wet or damp locations need to be hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. This section does permit other fasteners (excluding nails, wood screws, timber rivets and lag screws) to be mechanically galvanized in accordance with ASTM B 695, Class 55 at a minimum. As shown in Figure 2, fasteners (including nuts and washers) used with FRTW in interior conditions need to be in accordance with the manufacturer’s recommendations, or, if no recommendations are present, to comply with 2304.9.5.3.

Figure 1:  Section 2304.9.5.3 of the 2012 IBC (Source ICC)

Figure 1: Section 2304.9.5.3 of the 2012 IBC (Source ICC)

Figure 2:  Section 2304.9.5.4 of the 2012 IBC (Source ICC)

Figure 2: Section 2304.9.5.4 of the 2012 IBC (Source ICC)

In Type III construction where the exterior walls may be FRTW in accordance with 2012 IBC Section 602.3, one question that often comes up is whether the defined “exterior wall” should comply with Section 2304.9.5.3 or 2304.9.5.4. While there are many different views on this point, it is our opinion at Simpson Strong-Tie that Section 2304.9.5.4 would apply to the exterior walls. Since the exterior finishes of the building envelope are intended to protect the wood and components within its cavity from exterior elements such as rain or moisture, the inside of the wall would be dry.

There are many FRTW product choices on the market; take a look at the American Wood Council’s list of treaters. Unlike the preservative-treated wood industry, however, the FRTW industry involves proprietary formulations and retentions. As a result, Simpson Strong-Tie has not evaluated the FRTW products. In our current connector and fastener catalogs, C-C-2015 Wood Connector Construction and C-F-14 Fastening Systems, you will find a newly revised Corrosion Resistance Classifications chart, shown in Figure 3 below, which can be found on page 15 in each catalog. The FRTW classification has been added to the chart in the last column. The corrosion protection recommendations for FRTW in various environmental applications is set to medium or high, corresponding to a number of options for connectors and fasteners as shown in the Corrosion Resistance Recommendations chart, shown in Figure 4. These general guideline recommendations are set to these levels for two reasons: (1) there are unknown variations of chemicals commercially available on the market, and (2) Simpson Strong-Tie has not conducted testing of these treated wood components.

Figure 3: Simpson Strong-Tie Corrosion Resistance Classifications Chart

Figure 3: Simpson Strong-Tie Corrosion Resistance Classifications Chart

Figure 4: Simpson Strong-Tie Corrosion Resistance Recommendations Chart

Figure 4: Simpson Strong-Tie Corrosion Resistance Recommendations Chart

The information above is not the only information readily available. There are many different tests that can be done on FRTW, as noted in the Western Wood Preservers Institute’s document. One such test for corrosion is Military Specification MIL-1914E, which deals with lumber and plywood. Another is AWPA E12-08, Standard Method of Determining Corrosion of Metals in Contact with Treated Wood. Manufacturers of FRTW products who applied for and received an ICC-ES Evaluation Report must submit the results of testing for their specific chemicals in contact with various types of steel. ICC-ES Acceptance Criteria 66 (AC66), the Acceptance Criteria for Fire-Retardant-Treated Wood, requires applicants to submit information regarding the FRTW product in contact with metal. The result is a section published in each manufacturer’s evaluation report (typically Section 3.4) addressing the product use in contact with metal. Many published reports contain similar language, such as “The corrosion rate of aluminum, carbon steel, galvanized steel, copper or red brass in contact with wood is not increased by (name of manufacturer) fire-retardant treatment when the product is used as recommended by the manufacturer.” Structural engineers should check the architect’s specification on this type of material. Product evaluation reports should also be checked to ensure proper specification of hardware and fastener coatings to protect against corrosion. Each evaluation report also contains the applicable strength adjustment factors, which vary from one product to another.

Selecting the proper FRTW product for use in your building is crucial. There are many different options available. Be sure to select a product based on the published information and to communicate that information to the entire design team. Evaluation reports are a great source of information because the independently witnessed testing of manufacturers has been reviewed by the agency reviewing the report. Finally, understanding FRTW chemicals and their behavior when in contact with other building products will ensure expected performance of your structures.

What has been your experience with FRTW? What minimum recommendations do you provide in your construction documents?

 

 

Our Latest Online Resource: Steel Deck Diaphragm Calculator

Although Simpson Strong-Tie is best known for our structural products: engineered structural connectors, lateral systems, fasteners and fastening systems, anchoring products and most recently, concrete repair, protection and strengthening (RPS) systems, we are continually developing new and exciting software solutions. As we’ve discussed in prior blog posts, Simpson Strong-Tie has numerous software programs and web and mobile apps available for download or online use at www.strongtie.com/software. Today, I’d like to review our recently launched web app, the Steel Deck Diaphragm Calculator. The calculator is accessible from any web browser and doesn’t require downloading or installing special software.

While the method of designing and specifying a steel deck and its attachment can vary by region, most designers are familiar with the Steel Deck Institute (SDI) and its Diaphragm Design Manual, 3rd Edition (DDM03). DDM03 presents diaphragm shear strength and stiffness equations for various steel deck profiles and commonly used attachment types (welds, power-actuated fasteners, or screws). The calculations can be quite tedious, so the SDI has developed numerous tables using these equations and placed them at the back of DDM03 for easy reference.

Typical diaphragm shear table from SDI’s DDM03. Image credit: SDI.

Typical diaphragm shear table from SDI’s DDM03. Image credit: SDI.

Since the tables in DDM03 are based solely on the fasteners and deck profiles included, determining diaphragm capacities utilizing any other proprietary fastener or deck profile fall on the designer or the proprietary product’s manufacturer. Enter Simpson Strong-Tie.

Our Steel Deck Diaphragm Calculator enables users to produce custom diaphragm tables similar to those in DDM03, generate detailed calculations using SDI equations based on project-specific inputs, as well as optimize deck fastening systems to ensure the most cost-effective design is utilized. The calculator incorporates our X-series steel decking screws, including the recently launched Strong-Drive® XL Large-Head Metal Screw, which has one of the highest capacities in the industry and in most cases, can be used as a 1-for-1 replacement of pins or 5/8 diameter puddle welds. (For additional information comparing Simpson Strong-Tie X-series and XL screws to pins or welds, review F-Q- STLDECK14.)

Decide whether to optimize a design or generate diaphragm tables.

Decide whether to optimize a design or generate diaphragm tables.

The app can be used with minimal required input to generate tables and project-specific calculations. A more detailed analysis can be performed by inputting parameters for up to five unique zones, including overall dimensions, diaphragm shear, joist spacing, uplift and more.

Input detailed information for up to 5 different zones on the same project.

Input detailed information for up to 5 different zones on the same project.

One unfortunate aspect of many web apps is that your work is typically lost once you close your web browser. I’m happy to report that the folks here in our app development group have added the ability to save and upload project files. The calculator also provides a clean PDF printout of your results while giving you the option to generate a submittal package with supporting documentation, such as code reports, product approvals and installation recommendations.

Generate a submittal that includes all calculations and necessary supporting documentation.

Generate a submittal that includes all calculations and necessary supporting documentation.

Try the revised Steel Deck Diaphragm Calculator yourself and let us know what you think. We always appreciate the feedback!

Wide Flange Beams in Light Frame Construction

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

Beam below joists

Beam below joists

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

Beam flush with ceiling

Beam flush with ceiling

Framing hung off beam

Framing hung off beam

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

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

Hanger Install

Hanger Install

Nailer Table

Nailer Table

Installers may also wish to connect the hangers using powder-actuated fasteners in lieu of welding. Allowable loads for several of our top flange hangers are addressed in an engineering letter, ITS, MIT, LBV, and BA Hangers Installed on a Steel Header with Powder-Actuated Fasteners.

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

BA, MIT and ITS Hanger Tests

BA, MIT and ITS Hanger Tests

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

Wings or No Wings?

Guest blogger Jeff Ellis, engineering manager

Guest blogger Jeff Ellis, engineering manager

While the title of this blog post might remind you of the tasty turkey dinner you enjoyed on Thanksgiving, it’s actually a question regarding a shear wall component’s effect on performance. What type of fastener do you use to attach wood structural panel sheathing to cold-formed steel (CFS) framing, and what is the effect on a shear wall assembly?

Wood Structural Panel Sheathed CFS Framed Shear Walls.( Image credit: Don Allen, DSi Engineering)

Wood Structural Panel Sheathed CFS Framed Shear Walls.( Image credit: Don Allen, DSi Engineering)

Structural sheathing is most commonly attached to CFS framing with self-piercing or self-drilling tapping screws, power driven pins, and adhesives.

The AISI North American Standard for Cold-Formed Steel Framing – Lateral Design standard (S213) specifies using either #8 or #10 self-tapping screws (depending on the assembly) that comply with ASTM C1513, and have a minimum head diameter of 0.285” or 0.333”, respectively.

It’s worth noting that you cannot verify ASTM C1513 compliance by simple inspection. While screw dimensions are easy to measure, other features such as hardness, ductility, torsional strength, drill drive, and thread tapping cannot be evaluated in the field or by visual inspection. It’s prudent that a Designer and jurisdiction expect a screw manufacturer to validate its product’s compliance with ASTM C1513. This can be done through test reports by an accredited test lab and evaluation data, or by an evaluation report published by an ANSI-accredited product certification entity such as ICC-ES or IAPMO UES. Continue reading

Call for Papers: 22nd International Conference on CFS Structures

Guest blogger Jeff Ellis, engineering manager

Guest blogger Jeff Ellis, engineering manager

The Wei-Wen Yu Center for Cold-Formed Steel Structures has issued a Call for Papers for the 22nd International Conference on Cold-Formed Steel (CFS) Structures, to be held Nov. 5-6 in St. Louis, MO. The goal of the conference is to enable sharing of state-of-the-art information pertaining to CFS design.

Both engineering researchers and practitioners have provided valuable contributions to the conference. Past proceedings are available online.

International ConferenceResearchers and practitioners are encouraged to submit abstracts for consideration by the conference steering committee. Application-oriented topics highlighting innovations in CFS applications are strongly encouraged. The deadline is Dec. 31. Abstracts may be submitted by e-mail to ccfss@mst.edu.

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

Simpson Strong-Tie Literature at Your Fingertips

A couple of years ago, my brother-in-law asked if I could stop by the swim club where he is a board member. He was overseeing a construction project to upgrade the buildings and patio covers, which involved dry-rot repairs and the addition of Simpson Strong-Tie® connectors to create a continuous load path. He wanted me to meet with the contractor and make some suggestions for alternate connectors. The as-built conditions didn’t work for the specified connectors at a few locations, and there were some spots where he thought the connectors were “ugly.” I’m probably in the minority on this, but I think shiny galvanized steel connectors are just beautiful. So the “ugly” comment stung a little bit.

Once I got over my hurt feelings, I grabbed my Wood Construction Connectors catalog, a Deck Connection and Fastening Guide, and a few other fliers and technical bulletins that I thought might be helpful and drove across town to meet them. With literature in hand, we were able to come up with ways to work around the more difficult areas, and also select some more aesthetically pleasing architectural connectors at prominent locations. I thought we were done, and then the contractor had a few more questions on anchoring that I needed an Anchoring and Fastening Systems Catalog to look up some information on – and I didn’t have one! I managed to muddle through with my smartphone and find the information online, but couldn’t help but think that there had to be a better way to access design information when you are out of the office.

ipad overviewThe better way has arrived in the latest version of the Simpson Strong-Tie® Literature Library mobile app. It was just launched this month and is much more comprehensive than the first version. There are several new features that I wanted to highlight for you.

Continue reading