What You Need to Know About Differences in Wind-Speed Reporting for Hurricanes

This week’s post was written by Darren Conrad, PE. Engineering Manager, Truss at Simpson Strong-Tie.

With Hurricane Irma wrapping up, the cleanup after Hurricane Harvey’s devastation underway in Houston and more big storms already churning in the Atlantic, it seems like a good time to discuss hurricanes and high wind. There is a great deal of good information out there to help us better understand hurricanes and their impact on people, structures and other property. To improve awareness of wind speeds and their measurement, this article will discuss a commonly misunderstood aspect of hurricane wind-speed reporting.

When a storm is approaching, you will hear meteorologists report wind speeds. They often refer to storm categories. These categories attempt to generalize expected damage to structures based on the wind speed of the storm. The wind speed for a given storm is a measure of the severity of the storm and the danger it poses to life and property. But how do meteorologists determine the wind speed that they are reporting? It seems so concrete and certain, but anyone who has been outside during a storm or windy day knows that wind isn’t constant at any one location over a period of time. It varies continuously in magnitude and direction over time. So how can something so variable be the subject of knowledge that is precise enough to be useful? How do we understand wind-speed measurements and make sure that when comparing them we are doing so in such a way that they are comparable? That is a great question.

The good news is that even though wind is variable, we have a commonly accepted way to measure wind speed and know something about a wind field or event that is occurring at a time and place. This is done by averaging measured wind speeds over specified lengths of time, or picking the highest average wind speed that occurs for a specified averaging interval from a longer period of time. A great resource for understanding how wind speeds are measured and reported can be seen here. From this explanation, it can be seen that a reported wind speed is meaningless without a specified averaging time. The shortest averaging intervals will yield the highest reported wind speeds. The longer averaging times will capture more peaks and lulls and yield lower reported wind speeds. The most common averaging intervals used to report wind speeds are three seconds, one minute and two minutes. Some countries even use a ten-minute averaging interval for reporting wind speeds. So the question arises, which average is correct? And the answer is, none of them and all of them. They are just different ways of looking at measured wind data. That is not very comforting, but one thing we can know is that none of them can be truly interpreted or compared without understanding this idea of averaging time. To make it more confusing, meteorologists and building codes do not use the same averaging interval when reporting or specifying wind speeds. This can lead to misunderstandings.

In general, you will hear meteorologists report sustained wind speeds when covering an approaching hurricane. They might also mix in some peak gusts, but for the most part they focus on sustained wind speeds. Sustained wind speeds for tropical cyclones use a 60-second averaging time. Sustained wind speed is also used by the Saffir-Simpson scale to roughly quantify the likely damage that the wind from a storm might cause typical buildings and other structures. There are criticisms of the accuracy of the Saffir-Simpson scale method, but it is widely used by the public to generalize about the severity of tropical cyclones; therefore, it is likely that the public might and commonly does attempt to compare reported sustained wind speeds to building-code-specified three-second-gust wind speeds to determine if their house or structure will withstand the storm. There is danger in making that comparison.

We need to be careful when comparing the reported sustained wind speed for a storm with the three-second-gust design wind speeds referenced in building codes and design standards. They are not the same and need to be converted before they can be compared for equivalence. After seeing the following example, one could easily see the possibility of the public or a public official comparing the sustained wind speeds reported by the weatherman to the wind speeds used by building codes and design standards and drawing conclusions that may underestimate the force and effect of the storm.

Let’s take a hypothetical situation where a building jurisdiction has adopted a wind speed of 130 mph three-second-gust design wind speed for structures built in that jurisdiction. There are various methods to convert wind speeds between different averaging times, and many factors that may need to be considered when doing that. One method for converting is the Durst method referenced in ASCE 7. Another more recent method recommended by the World Meteorological Organization provides a pretty straightforward conversion between sustained wind speed and three-second-gust wind speeds for near-surface applications. So for the sake of simplicity, we will use it for this example. If we convert a reported sustained wind speed of 130 mph to a three-second-gust average wind speed using this method, it equates to a three-second-gust wind speed for Off-Sea of 160 mph (Off-Sea is appropriate for an approaching hurricane). The adopted130 mph three-second-gust wind speed converts to 105 mph sustained wind speed. This difference could lead individuals in the path of the storm to underestimate its severity if they are not aware of the difference between averaging intervals for wind speeds. They could see the sustained wind speed of 130 mph being reported by the weather service when the storm is over open water and assume that their structure, or structures in their jurisdiction, will stand up fairly well. This would be a serious underestimate, since those structures would need to be designed to resist a 160 mph three-second-gust wind speed using ASCE 7 in order for that to be true. To say that a different way, one might think that their structure was designed for a Category 4 storm (130 mph sustained), when in fact it was actually designed for a Category 2 storm (105 mph sustained) using the Saffir-Simpson scale. Hurricane Irma at its maximum sustained wind speed of 185 mph would equate to a 227 mph three-second-gust wind speed using this conversion method. From a roof anchorage, lateral design and load path design perspective, the difference between 130 mph and 160 mph can be substantial, especially when the building is located on flat open terrain where Exposure C or Exposure D are appropriate assumptions for the design.

There is a lot more background and detail to this very complicated discussion, but the general point is to know your averaging times when comparing reported wind speeds, so as not to underestimate a storm’s force. If a storm is headed your way, hopefully you have already selected the proper hurricane tie for your structure; you have a well-defined and properly designed continuous load path; and you are protecting your exterior openings from windborne debris. Remember, the objective is not to protect the window or door product itself. Unless you are in the insurance business, you are preventing the breach of the opening to keep wind from pressurizing the structure, increasing loads on the structure and potentially causing catastrophic failure.

Know how to secure your structure against high winds, and be safe.

The Top 5 Helpful Tips for Using CFS Designer™ to Optimize Your Workflows

Back in April of last year, I had the opportunity to show how our new CFS Designer software  could help structural engineers “go lean” in their design process by eliminating repetitive tasks (while still meeting required design standards, of course!). Since then, I’ve had the opportunity to visit with hundreds of engineers in person to teach them about CFS Designer and how it can help them improve and optimize their workflows. As a power user of the software, I want to share my top tips for letting CFS Designer help you save the maximum amount of time.

Tip #1. You need to create only one design file for each project.
CFS Designer has to generate lots of code-compliant designs quickly, but that doesn’t mean you need to end up with dozens of unrecognizable file names on your desktop. The software includes a very handy WorkSpace area in the lower left-hand area of the screen that enables you to save all your wall, jamb, header, and general interaction designs in a single project space. This means that you will be saving only ONE file for each project, a feature that can save you a lot of confusion over time.

Figure 1. The orange box is highlighting the file name (which doubles as the Project Name on the summary reports), which shows up at the top of the WorkSpace area. In this example, I’ve added just one beam/stud model for the sake of simplicity.

Tip #2. Quickly duplicate similar wall sections or design types by right-clicking on the model name in the WorkSpace.
On cold-formed steel projects, there are often very similar wall sections or jambs that you’ll need to design. They may have slightly different parapet heights, different loading or different wall widths. Instead of starting from scratch and creating a new section every time, CFS Designer allows you to right-click on any existing design. The right-click action brings up a “Duplicate” pop-up which lets you create an identical model in your WorkSpace. You then have the ability to change the model name, make slight modifications, and then re-save your project to see it show up as a new model in the WorkSpace area.

Figure 2. Here’s where to right-click in order to get the “Duplicate” pop-up to appear.

Tip #3. Expand the “Member Forces” and “Connection Summary” sub-menus in the Beam Design module to get real-time updates of the reaction loads, member stresses and connection solutions.
A critical area of member design is the reaction points, because it doesn’t really matter whether your cold-formed steel member is adequately designed if the connection points don’t have a solution. Many engineers I met with thought they had to click on the “Summary Report” button every time they wanted to know the reaction forces, waiting anywhere from 10 to 15 seconds for the PDF file to load and then having to scroll through to find the correct section. Thankfully, there’s a much quicker way to view the reactions. CFS Designer instantly updates the reaction values on the design screen, but the onscreen menus that have this useful information need to be opened up first. Within the Beam Design module, click on the small down arrows to the left of “Member Forces” and “Connection Summary,” and that will expand these two useful sections and display the design information without your having to wait and generate the output. On a related note, another useful area to keep an eye on during design is the very bottom of the screen, where green text will let you know when your maximum member stress and web crippling check are compliant, red text will alert you if your member design is insufficient, and the deflection ratio limit is always displayed.

Figure 3. Here’s where to find the collapsed “Member Forces” and “Connection Summary” menus.

Figure 4. Click on the arrows to the left of the menu titles to see your important design information in detail.

Tip #4. Use the “WorkSpace Report” button for a one-click method of combining ALL the individual summary pages into a single PDF file.
After you’re done generating all your different models and saving them to your WorkSpace, you’re probably going to want to generate the output files you can print and add to your calculation package for submittal. One engineer I met with a couple of years ago told me that this was the most dreaded step because it meant she had to open each model, click on the “Summary Report” button, wait those 10–15 seconds for the PDF file to generate, and then print it out or save it. For large projects, this would need to occur 20–30 times – yikes! Thankfully, a huge part of the development of CFS Designer relies on feedback such as this to help Simpson Strong-Tie continuously improve the program’s functionality. The latest version of CFS Designer introduces a “WorkSpace Report” button, which takes a single click to create all of the summary reports for each model type, saved in a single PDF file.

Figure 5. Be sure to use the “WorkSpace Report” button to save yourself a ton of time generating all your printable output.

Tip #5. Use the onscreen tip pop-ups. Small gray question mark icons are strategically placed throughout CFS Designer to offer helpful tips and tricks for specific input boxes.
Structural engineers are expected to know a lot, but it isn’t always necessary to remember all the details if you know where to look them up. Because the information requested by some of the input boxes may not be completely self-evident, we built in some handy pop-up tips to help out. A small gray circle with a question mark inside makes its appearance next to input boxes. Hovering your mouse over one of these question marks will cause an info box to appear, letting you know what information is required, what code section to reference, or what design methodology is being used. I have found these pop-up tips to be immensely helpful, especially in conjunction with the program’s User’s Manual (located under the Help menu, at the top of the program).

Figure 6. I got this box to pop up by hovering over the question mark next to the “Load Modifiers” section of the Beam Input module. If you search for “Load Modifier” in the User’s Manual, it will direct you to the relevant AISI code section.

I’ve had fun sharing some of my top tips with everyone today, but there is a great opportunity coming up to learn even more about our CFS Designer software from one of the original developers of the software. Join me and Rob Madsen, P.E., Senior Project Engineer from Devco Engineering, for a one-hour live demo of the software and connection solutions. Rob has been described as one of the premier structural engineers in the cold-formed steel design arena, and he will walk you through detailed wall stud, jamb, header and stacked wall design examples using CFS Designer. I’ll be presenting on the innovative, tested and code-listed product solutions that Designers can use to save time in addressing the critical connection points in CFS design. We hope you can join us for the live demo, but if you have other commitments at that time, a recording of the webinar will be made available on our website for your viewing convenience. The course will also earn professional development hours (PDHs) and continuing education units (CEUs) for any folks who need credits to renew their professional licenses.

Bonus Tip: Sign up for our upcoming CFS Designer™ webinar on Thursday, September 28!

Further Reading

For additional information or articles of interest, check out these available resources:

    • AISIStandards – A free download of all the cold-formed steel framing standards adopted by the 2015 International Building Code.

 

    • CFSEI – The Cold-Formed Steel Engineering Institute, an incredibly useful technical and professional resource for Designers of cold-formed steel structures, with a huge library of technical notes.

 

 

 

Q&A About Advanced FRP Strengthening Design Principles

Our thoughts go out to everyone affected by Hurricane Harvey and this disaster in Texas. To help with relief efforts we are donating $50,000 to the American Red Cross Disaster Relief Fund. Employees at our Houston warehouse are safe and the employees from our McKinney branch will be doing as much as they can to help with relief efforts.

This week’s post was written by Griff Shapack, PE. FRP Design Engineer at Simpson Strong-Tie.

On July 25, 2017, Simpson Strong-Tie hosted the second interactive webinar in the Simpson Strong-Tie FRP Best Practices Series, “Advanced FRP Design Principles,” in which Kevin Davenport, P.E. – one of our Field Engineering Managers – and I discussed the best practices for fiber-reinforced polymer (FRP) strengthening design. The webinar examines the latest industry standards, proper use of material properties, and key governing limits when designing with FRP and discusses the assistance and support Simpson Strong-Tie Engineering Services offers from initial project assessment to installation. Watch the on-demand webinar and earn PDH and CEU credits here.

During the live webinar, we had the pleasure of taking questions from attendees during the Q&A session. Here is a curated selection of Q&A from that session:

While I see how you improve the flexural capacity of a beam, how do you increase its shear capacity to match new moment strength?

ACI 440.2R recommends checking the element for shear if FRP is used to increase flexural strength. U-wraps can be used to provide shear strengthening of a beam.

Are there any “pre-check” serviceability checks (deflection, vibration, etc.) similar to the ACI 440 strength check that you recommend when considering the use of FRP?

ACI 440.2R contains a few serviceability checks on the concrete, rebar and FRP that can be performed once you have designed a preliminary strengthening solution.

Are these strengthening limits for gravity loads only? What about for a seismic load combination?

Yes, the strengthening limits are just for gravity loading. Seismic loading does not require an existing capacity check as it is highly unlikely for the FRP to be damaged during a lateral event.

Did Simpson Strong-Tie perform load tests on FRP repaired timber piles?

We are currently testing our FRP products as applied to timber piles at West Virginia University. We have also implemented a full-scale testing program on damaged timber piles at our own lab using our FX-70® fiberglass jacket system.

Will any of your seminars cover FRP and CMU? Seismic applications?

Yes, these are topics we are considering for future webinars.

The 0.6 limit for compressive stress can be very limiting. Can this value be evaluated on a case-by-case basis? The Euro code allows higher limits on compressive stress?

Our designers will report this value, along with the section addressing this check from ACI 440.2R, to the EOR and discuss whether the EOR would like to proceed with the FRP strengthening on his or her project.

Which engineer (EOR or Delegated Engineer) is responsible for specifying the scope of special inspections?

We provide a draft FRP specification to the EOR to use in their final determination of the special inspection requirements for a project. It’s in the owner’s best interest to hire a qualified special inspection agency on an FRP installation project.

For complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/css or call your local Simpson Strong-Tie RPS specialist at (800) 999-5099.

Advanced FRP Design Principles

In this free webinar we will dive into some very important considerations including the latest industry standards, material properties and key governing limits when designing with FRP.


Why Fire-Rated Hangers Are Required in Type III Wood-Frame Buildings

One of the first mixed-use designs I worked on as a consulting structural engineer was a four-story wood-frame building over two levels of parking. Designing the main lateral-force-resisting system with plywood shearwalls was a challenge for this project that required new details to meet the high design loads. The high overturning forces were resisted using the Simpson Strong-Tie® Strong-Rod™ anchor tiedown system, which incorporates high-strength rods, bearing plates and shrinkage compensation devices.

At the time, these construction details using Strong-Rod systems and high- load shearwall diaphragms were new, innovative concepts. However, this method of construction rapidly became commonplace as intense demand for housing fueled the trend toward denser, mixed-use developments in downtown areas. I discussed the trend toward taller, denser developments in this post.

A more recent trend in wood-frame construction has been the shift to Type III wood-frame construction, which allows designs up to five stories. To help educate designers on some of the nuances of Type III wood-frame construction and provide guidance on meeting the associated code requirements, we reached out to Bruce Lindsey, the South Atlantic Regional Director for WoodWorks. Bruce wrote a two-part article entitled Fire Protection Considerations with Five-Story Wood-Frame BuildingsPart 1 and Part 2. This post will go into more detail on connecting the floor system to the two-hour fire-rated exterior walls and discuss our new DG series joist hangers that are specially designed for this application.

As a structural engineer, I was aware of fire requirements mostly because I needed to account for the weight of fire sprinklers, added layers of gypsum board, fire-proofing on steel, or concrete slab thickness in my design. While the increased loads can affect the vertical- and lateral-force-resisting systems, I seldom needed to change the details and connections in my designs.

The exterior walls in Type III wood-frame construction require fire-retardant-treated (FRT) lumber with two layers of gypsum board to provide a two-hour fire rating. There are many established fire-rated floor and wall assemblies available. The challenge, as discussed in Part 2 of Mr. Lindsey’s post, is detailing the intersections between the floor and wall systems. Connecting the floor framing to the exterior walls in Type III construction requires careful detailing to transfer the vertical loads without compromising the two-hour fire rating of the wall assembly.

Below is a summary of some of the possible fire wall connections as discussed in Mr. Lindsey’s previous blog posts.

A solid header on top of the wall that has adequate thickness to provide a two-hour rating through its charring capability. The cost and availability of solid rim board material should be considered.

A continuous 2x ledger or blocking to provide one hour of fire resistance. The second hour of resistance is provided by ceiling gypsum board. Some jurisdictions object to this detail over concerns about a fire starting within the floor cavity.

Some jurisdictions interpret the two-hour exterior wall requirement as applying only to the wall and not the floor. In such jurisdictions, designers can sometimes use standard platform framing in Type III construction.

A variation where the ledger can be installed over two layers of gypsum board. Simpson Strong-Tie has tested and published values for ledger connections over gypsum board using our SDWH and SDWC fasteners. The testing of these fasteners was discussed in our Spanning the Gap post from earlier this year.

In this detail, one hour of fire resistance is provided by a single layer of gypsum board running the full height of the wall with a hanger installed over the gypsum board. The second hour of resistance is provided by the ceiling gypsum board.

A variation of this detail is our DU/DHU series of drywall hangers that are installed over two layers of gypsum board. These were addressed in this post.

Designs using hangers or ledgers installed over gypsum board can create construction sequencing challenges. Since the gypsum board needs to be installed before the framing, the contractor will need to coordinate between the trades.

A new solution that eliminates sequencing issues for Type III construction is our series of DG/DGH/DGB fire wall hangers, which are designed to easily install on a two-hour wood stud fire wall. These top-flange hangers feature enough space to allow two layers of 5/8″ gypsum wall board to be slipped into place after the framing is complete.

These new fire wall hangers were tested in accordance with ICC-ES AC13 and ASTM D7147, which I discussed in How We Test – Part I: Wood Connectors. These standards do not explicitly detail how to test a hanger installed on a wood stud wall, so we collaborated closely with ICC Evaluation Services to develop a test setup that meets the intent of the standards.

All three of our new fire wall hangers have been tested according to ASTM E814 and received F (flame) and T (temperature) ratings for use on either or both sides of the fire wall. These ratings verify that the DG/DGH/DGB hangers do not reduce the two-hour fire wall assembly rating.

Our testing and load tables address installation of 2×4 or 2×6 stud walls constructed of Douglas fir (DF), southern pine (SP), spruce-pine-fir (SPF) or hem-fir (HF) lumber.

DG Hanger

DGH Hanger

DGB Hanger

Drywall Notch Detail

If you are working on a Type III wood-frame construction project, check out our Fire Wall Solutions page, which has product profiles with links to further information about the new DG hanger series, as well as our DU/DHU series of drywall hangers and fire wall fastener solutions using Strong-Drive® SDWS Timber screws.

Top 10 Changes to Structural Requirements in the 2018 IBC

This blog post will continue our series on the final results of the 2016 ICC Group B Code Change Hearings, and will focus on 10 major approved changes, of a structural nature, to the International Building Code (IBC).

  1. Adoption of ASCE 7-16
    • The IBC wind speed maps and seismic design maps have been updated.
    • A new section has been added to Chapter 16 to address tsunami loads.
    • Table 1607.1 has been revised to change the deck and balcony Live Loads to 1.5 times that of the occupancy served.
  2. New and Updated Reference Standards
    • 2015 IBC Standard ACI 530/ASCE 5/TMS 402-13 will be TMS402-16.
    • ACI 530.1/ASCE 6/TMS 602-13 will be TMS 602-16.
    • AISC 341-10 and 360-10 have both been updated to 2016 editions.
    • AISI S100-12 was updated to the 2016 edition.
    • AISI S220-11 and S230-07 were updated to the 2015 edition.
    • AISI S200, S210, S211, S212 and S214 have been combined into a new single standard, AISI S240-15.
    • AISI S213 was split into the new S240 and AISI S400-15.
    • ASCE 41-13 was updated to the 2017 edition.
    • The ICC 300 and ICC 400 were both updated from 2012 editions to 2017 editions.
    • ANSI/NC1.0-10 and ANSI/RD1.0-10 were all updated to 2017 editions.
  3. Section 1607.14.2 Added for Structural Stability of Fire Walls
    • This new section takes the 5 psf from NFPA 221, so designers will have consistent guidance on how to design fire walls for stability without having to buy another standard.
  4. Modifications of the IBC Special Inspections Approved
    • Section 1704.2.5 on special inspection of fabricated items has been clarified and streamlined.
    • The Exception to 1705.1.1 on special inspection of wood shear walls, shear panels and diaphragms was clarified to say that special inspections are not required when the specified spacing of fasteners at panel edges is more than 4 inches on center.
    • The special inspection requirements for structural steel seismic force-resisting systems and structural steel elements in seismic force-resisting systems were clarified by adding exceptions so that systems or elements not designed in accordance with AISC 341 would not have to be inspected using the requirements of that standard.
  5. Changes Pertaining to Storm Shelters
    • A new Section 1604.11 states that “Loads and load combinations on storm shelters shall be determined in accordance with ICC 500.”
    • An exception was added stating that when a storm shelter is added to a building, “the risk category for the normal occupancy of the building shall apply unless the storm shelter is a designated emergency shelter in accordance with Table 1604.5.”
    • Further clarification in Table 1604.5 states that the type of shelters designated as risk category IV are “Designated emergency shelters including earthquake or community storm shelters for use during and immediately after an event.”
  6. Changes to the IBC Conventional Construction Requirements in Chapter 23
    • The section on anchorage of foundation plates and sills to concrete or masonry foundations reorganized the requirements by Seismic Design Category (SDC) and added a new section on anchoring in SDC E. It also states that the anchor bolt must be in the middle third of the width of the plate and adds language to the sections on higher SDCs saying that if alternate anchor straps are used, they need to be spaced to provide equivalent anchorage to the specified 1/2″- or 5/8″-diameter bolts.
    • The second change permits single-member 2-by headers, to allow more space for insulation in a wall. 
  7. Modification to the Requirements for Nails and Staples in the IBC
    • ASTM F1667 Supplement One was adopted that specifies the method for testing nails for bending-yield strength and identifies a required minimum average bending moment for staples used for framing and sheathing connections.
    • Stainless-steel nails are required to meet ASTM F1667 and use Type 302, 304, 305 or 316 stainless steel, as necessary to achieve the corrosion resistance assumed in the code.
    • Staples used with preservative-treated wood or fire-retardant-treated wood are required to be stainless steel.
    • The new RSRS-01 nail was incorporated into TABLE 2304.10.1, the Fastening Schedule. The RSRS nail is a new roof sheathing ring shank nail designed to achieve higher withdrawal resistances, in order to meet the new higher component and cladding uplift forces of ASCE 7-16.
  8. Truss-Related Code Change
    • The information required on the truss design drawings was changed from “Metal connector plate type” to “Joint connection type” in recognition that not all trusses use metal connector plates.
  9. Code Change to Section 2304.12.2.2
    • A code change clarifies in which cases posts or columns will not be required to consist of naturally durable or preservative-treated wood. This change makes the requirements closer to the earlier ones, while maintaining consistency with the subsequent section on supporting members.
    • If a post or column is not naturally durable or preservative-treated, it will have to be supported by concrete piers or metal pedestals projecting at least 1″ above the slab or deck, such as Simpson Strong-Tie post bases that have a one-inch standoff.
  10. Code Change to IBC Appendix M
    • A code change from FEMA makes IBC Appendix M specific to refuge structures for vertical evacuation from tsunami, and the tsunami hazard mapping and structural design guidelines of ASCE 7-16 would be used rather than those in FEMA P-646.

Once the 2018 IBC is published in the fall, interested parties will have only a few months to develop code changes that will result in the 2021 I-Codes. Similar to this last cycle, code changes will be divided into two groups, Group A and Group B, and Group A code changes are due January 8, 2018. The schedule for the next cycle is already posted here.

What changes would you like to see for the 2021 codes?

The New Way to Connect with Strong Frame®

The April SE blog article, What Makes Strong Frame® Special Moment Frames So Special, explained the features and benefits of the Yield-Link® structural fuse design for the Strong Frame® special moment frame (SMF) connection. In this blog, I will be introducing the Yield-Link end-plate link (EPL) to the Strong Frame connection family.

What is the EPL?
The EPL connection (Figure 1) is the latest addition to the Strong Frame Strong Moment Frame (SMF) solution. The new EPL connection can accommodate a W8X beam which is approximately a 33% reduction in beam depth from a W12X beam. The frame is field bolted without the need for field welding which means a faster installation. The snug-tight bolt installation requirement means no special tools are required. The EPL SMF connection has the same benefit of not requiring any additional beam bracing as the T-Stub connection. The frame can be repaired after a large earthquake by replacing the Yield-Link connection. Since the shear tab bolts will be factory installed, installation time for the frame is reduced by 25% making the EPL connection one of the most straightforward connections to assemble.

Figure 1: New Yield-Link EPL connection

Why Did We Develop the EPL?
The development of the EPL came from strong interest and numerous requests to offer a solution with more head room for clearance of retrofit projects or enhancement for new construction using a shallower beam profile. The original T-stub link design has the shear tab welded to the column flange. The geometry of the shear tab meant that a W12X beam is required to accommodate the Yield-Link Flange. In Figure 2, you can see that a shallower beam profile will bring the Yield-Link flange closer to each other and limit the attachment of the shear tab. A new connection was needed.

Figure 2: Yield-Link flange interference with shear tab

Figure 3: 3 Bolt configuration with notched flange plate. (The 3rd bolt is on opposite side of beam.) The asymmetric layout produced uneven force distribution in the bolts.

How Did We Develop the EPL?
Multiple configurations were studied, including a notched flange plate with 3 bolts (Figure 3) to avoid interference with the shear tab connection to the column. In the end, a compact end plate link combining the shear tab and Yield-Link stem in a single connection was the final design. However, many questions loomed over the prototype. How will the single end plate design perform in a full scale test? Will the new configuration change the limit state? These questions needed to be studied prior to launching an expensive full-scale test program with multiple samples and configurations. Numerous Finite Element Analysis (FEA) models were studied and refined prior to full scale testing of a prototype. Modeling included ensuring the stem performs as a fuse (Figure 4) as discussed in the April blog and the integrity of the shear tab is maintained in the compact design. Figure 5 shows a graph comparing the analytical model to the actual full scale test. The full scale test with a complete beam and column assembly was performed to the requirements under AISC 341 Section K. The full scale test passed the requirements for the SMF classification as can be seen in Figure 6 for the specimen with 6-inch columns and 9-inch beam.

Figure 4: Equivalent Plastic Strain Plot of Yielding-Link Stem

Figure 5: Full Scale Test vs. Analytical model

Figure 6: Moment at Face of Column vs. Story Drift

Where Can I Get More Information?
The EPL is now recognized in the ICC-ES ESR-2802 code report as an SMF. EPL solutions are also offered in the Strong Frame Moment Frame Selector Software. Want to see how the new connection and member sizes can expand your design options? Visit www.strongtie.com to download the new Strong Frame Design Guide or contact your Simpson representative for more information.

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!

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.

Revisiting Spanning the Gap

Three years ago, we created this blog post based on a technical support question we often receive about allowable fastener loads for ledgers to wood framing over gypsum board. Given that this is still a frequent question and a relevant topic, we decided to revisit the post and update it.

Drywall. Wall board. Sheetrock. Sackett Board? A product called Sackett Board was invented in the 1890s, which was made by plastering within wool felt paper. United States Gypsum Corporation refined Sackett Board for several years until 1916, when they developed a new method of producing boards with a single layer of plaster and paper. This innovation was eventually branded SHEETROCK®. More details about the history of USG can be found here.

No matter what you call it, gypsum board is found in almost every type of construction. Architects use it for sound and fire ratings, while structural engineers need to account for its weight in our load calculations. A common technical support question we receive is for allowable fastener loads for ledgers to wood framing over gypsum board.

Ledger over Gypboard

Ledger over Gypboard

One method to evaluate a fastener spanning across gypsum board is to treat the gypsum material as an air gap. Technical Report 12, General Dowel Equations for Calculating Lateral Connection Values, is published by the American Wood Council.

Technical Report 12

Technical Report 12

TR12 has yield limit equations that allow a designer to account for a gap between the main member and side member of a connection. With a gap of zero (g=0), the TR12 equations provide the same results as the NDS yield limit equations.

Technical Report 12 Yield Limit Equations[1]

Technical Report 12 Yield Limit Equations

The equations are fairly complex, but it should be intuitive that the calculated fastener capacity decreases with increasing gap. Engineers are often surprised to see a 40, 50, even 60% drop in fastener capacity with one layer of 5/8” gypsum board. So what else can you do?

Testing, of course! In So, What’s Behind a Screw’s Allowable Load? I discussed the methods used to load rate a proprietary fastener such as the Simpson Strong-Tie® Strong-Drive® SDS or SDW screws. To recap, ICC-ES Acceptance Criteria for Alternate Dowel Type Fasteners, AC233, allows you to calculate and do verification tests, or load rate based on testing alone. We develop our allowable loads primarily by testing, as the performance enhancing features and material optimizations in our fasteners are not addressed by NDS equations.

So to determine the performance of a fastener installed through gypsum board, we tested the fastener through gypsum board. This is easier to do if you happen to have a test lab with a lot of wood and fasteners in it. We did have to run down to the local hardware store to pick up gypsum board for the testing.

SDWS Over 2 Layer Gypboard

SDWS Over 2 Layer Gypboard

SDWS-Over-2-Layer-Gypboard-Failure

SDWS Over 2 Layer Gypboard Failure

A full set of allowable loads for Strong-Drive SDWH and SDWS are available on strongtie.com. The information is given as single fastener shear values for engineered design, and also screw spacing tables for common ledger configurations. As much fun as writing spreadsheets to do the Technical Report 12 calculations is, having tabulated values based on testing is much easier.

Fastening Systems

In the fastener marketplace, Simpson Strong-Tie stands apart from the rest. Quality and reliability is our top priority.


Why You Should Specify Stainless-Steel Screw Anchors When Designing for Corrosive Environments

Figure 1. Spalled concrete below a concrete bridge.

I was driving under a concrete bridge one nice clear day in Chicago, and I happened to look up to see rusted rebar exposed below a concrete bridge. My beautiful wife, who is not a structural engineer, turned to me and asked, “What happened to that bridge?” I explained that there are many reasons why spalling occurs below a bridge. One common reason is the expansion of steel when it rusts or corrodes.

This week’s blog will briefly explain the corrosion process and why concrete spalls when the embedded metals corrode. Corrosion may be defined as the degradation of a material as a reaction to its environment.1. As described in our previous SE Blog post, “Corrosion: The Issues, Code Requirements, Research and Solutions” dated January 3, 2013, corrosion of metallic surfaces is an electrochemical process. Because of moisture evaporation, concrete is a porous material. Water and oxygen molecules enter the pores of the concrete, and an electrochemical process occurs with the carbon-steel bar. The iron in the steel is oxidized, which then produces rust. A buildup of rust products at the surface of the carbon-steel bar exerts an expansive force on the concrete. Based on the amount of oxidation, the rust products of steel can occupy more than six times the volume of the original steel.2 Over time, further rust occurs and surface cracks will form. Eventually spalling will occur, exposing the rusted carbon steel bar. (See figure 1.)

Figure 2. Stages of corrosion.

Just as with reinforcing bars below a concrete bridge, cracking and spalling can occur when a carbon-steel anchor is used adjacent to a concrete edge. Simpson Strong-Tie® has many anchorage products that can be used in these conditions to prevent cracking. One specific product is the new stainless-steel Titen HD® screw anchor. This new innovative screw anchor is made up of Type 316 stainless steel. As seen in Figure 3, Type 316 stainless steel has a high level of resistance. This makes the stainless-steel Titen HD an excellent choice when it comes to an anchorage solution in corrosive environments. These environments include wastewater treatment plants, exterior handrails, exterior ledger attachments, stadium seating, central utility plants, and kitchens just to name a few.

Figure 3. Simpson Strong-Tie level of corrosion by material/coating.

Unlike expansion anchors, screw anchors require the leading threads to cut into predrilled holes. This can be easily achieved with hardened carbon-steel cutting threads. Stainless steel is not hard enough to cut into concrete. The new innovative stainless-steel Titen HD solves the problem by brazing heat-treated carbon-steel cutting threads to the surface of the stainless-steel tips of the screw anchor. (See figure 4.) These carbon-steel threads are hard enough to cut grooves into the surface of a predrilled hole, allowing the anchor to be installed with ease. The volume of the carbon-steel cutting threads is less than 1% of the stainless steel, reducing the buildup of rust that eventually spalls the concrete edge. Other stainless-steel screw anchor manufacturers in the market have a bi-metal product that attaches a full carbon-steel tip. This bi-metal screw anchors contain up to 18% carbon steel. Such a large amount of carbon steel can expand up to six times its volume when it corrodes and can spall the concrete when used adjacent to an edge.

Figure 4. Carbon-steel cutting threads.

Figure 5. Graphic representation of spalling in concrete adjacent to an edge.

When designing an anchorage solution for your next job in a corrosive environment, the stainless-steel Titen HD will provide the best resistance for corrosion, and also give the ability to drive these anchors into the concrete with ease. More information about the product can be obtained by visiting strongtie.com/thdss.

  1. Corrosion Technology Laboratory (https://corrosion.ksc.nasa.gov/corr_fundamentals.htm).
  2. Galvanized Rebar (http://www.concreteconstruction.net/how-to/repair/galvanized-rebar_o).

Stainless-Steel Titen HD®

The Next Era of Stainless-Steel Screw Anchor For Concrete and Masonry.