Welcome to the Structural Engineering Blog

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Welcome to our Structural Engineering Blog! I’m Paul McEntee, Engineering R&D Manager at Simpson Strong-Tie. We’ll cover a variety of structural engineering topics here that I hope interest you and help with your projects and work. Social media is “uncharted territory” for a lot of us (me included!), but we here at Simpson Strong-Tie think this is a good way to connect and even start useful discussions among our peers in a way that’s easy to use and doesn’t take up too much of your time. Continue reading

Multi-Ply Beam Load Transfer

Larger beams are often built up out of smaller 2x or 1¾” members. This can be done for several different reasons: for the convenience of handling smaller members on the jobsite, or because solid 4x, 6x or glulam material is not readily available, or for reasons of cost. Engineered wood such as laminated veneer lumber (LVL) is often used for its high load capacity and multiple 1¾” plies are built up to get the required capacity for the application.

8-Ply LVL Beam in HHGU14 Test

8-Ply LVL Beam in HHGU14 Test

When a built-up beam is loaded concentrically as in the test setup shown, fastening the members is not critical since that giant steel plate will load each ply of the beam. In the field, built-up beams or girders commonly support joists or beams framing into their side. The built-up members must be connected to transfer load from the loaded ply into the other plies.

SDW - Uniform Allowable Loads

Allowable Uniform Loads and Spacing Requirements

SDW - Assembly Types and Spacing Requirements

Page 303 of our Fastening Systems catalog, C-F-14 provides allowable uniform load tables for side-loaded multi-ply assemblies using LVL, PSL or LSL material. The calculation for the allowable load applied to the outside ply of a multi-ply beam is:

Screen Shot 2016-04-28 at 2.07.29 PM

While uniform loads are very common, Designers often request additional information to design multi-ply beam connections to transfer concentrated loads. Simpson Strong-Tie has created a new engineering letter, L-F-SDWMLTPLY16, which complements the information in the Fastening Systems catalog by providing allowable loads in a single fastener format. Designers can use the information to calculate the number of fasteners required for a given point load.

Simpson Strong-Tie® Strong-Drive® SDW EWP-Ply Screw – Allowable Loads for Side-Loaded Multi-Ply Assemblies per Screw

Simpson Strong-Tie® Strong-Drive® SDW EWP-Ply Screw – Allowable Loads for Side-Loaded Multi-Ply Assemblies per Screw

In order to ensure load transfer, the SDW screws need to be located relatively close to the connection. At first glance, it may appear challenging to fit enough fasteners while meeting the non-staggered row-spacing requirements. However, we have found that most loads can be managed by taking advantage of the ⅝” stagger allowance.

SDW – Maximum Fastener Spacing from Point Load

SDW – Maximum Fastener Spacing from Point Load

If you are curious what happened in that HHGU14 test, the screws pulled out of the header with a load slightly exceeding 101,000 pounds. Failure photo 2 shows a close-up of the pullout failure. The tested load was very close to the maximum calculated capacity for the SDS screws in the connector, so it was a great test result. What are your thoughts? Let us know in the comments below.

HHGU14 Test Failure 1 HHGU14 Test Failure 2

Coating Evaluation for Fasteners – Code-Approved and Alternative Coatings

Who likes red rust? No one I know! How do we avoid corroding of fasteners? Corrosion can be controlled or eliminated by providing a corrosion-resistant base metal or a protective finish or coating that is capable of withstanding the exposure environment. When fasteners get corroded, they not only look bad from outside but can also lose their load capacity. To ensure continued fastener performance, we have to control for corrosion. This blog focuses on evaluating the corrosion resistance of the fasteners.

What does the building code specify?

For use in preservative-treated wood, the IBC-2015 specifies fasteners that are hot-dipped galvanized, stainless steel, silicon bronze or copper. Section 2304.10.5.1 of IBC-2015 (Figure 1) covers fastener and connector requirements for preservative-treated wood (chemically treated wood). While chemically treated wood is part of the corrosion hazard, it is not the whole corrosion hazard. Weather exposure, airborne chemicals and other environmental conditions contribute to the corrosion hazard for metal hardware. In addition, the main issue with the code-referenced requirements for fasteners and connectors used with preservative-treated wood is that not all preservative treatments deliver the same corrosion hazard and not all fasteners can be hot-dip galvanized.

Figure 1: Section 2304.10.5.1 IBC-2015.

Figure 1: Section 2304.10.5.1 IBC-2015.

What if we want to use an alternative base material or coating for fasteners?

How do we evaluate the corrosion resistance of the alternative material or coating? The codes do not provide test methods to evaluate alternate materials and coatings. However, the International Code Council–Evaluation Service (ICC-ES) developed acceptance criteria to evaluate alternative coatings that are not code recognized for use in different environments. The purpose of acceptance criteria ICC-ES AC257, Acceptance Criteria for Corrosion-Resistant Fasteners and Evaluation of Corrosion Effects of Wood Treatment Chemicals, is twofold: (1) to establish requirements for evaluating the corrosion resistance of fasteners that are exposed to wood-treatment chemicals, weather and salt corrosion in coastal areas; and (2) to evaluate the corrosion effects of wood-treatment chemicals. In this blog post, we will concentrate on the evaluation of corrosion resistance of fasteners. The criteria provide a protocol to evaluate the corrosion resistance of fasteners where hot-dip galvanized fasteners serve as a performance benchmark. The fasteners evaluated by these criteria are nails or screws that are exposed directly to wood-treatment chemicals and that may be exposed to one or more corrosion accelerators like high humidity, elevated temperatures, high moisture or salt exposure.

The fasteners may be evaluated for any of the four exposure conditions:

  1. Exposure Condition 1 with high humidity. This test can be used to evaluate fasteners that could be exposed to high humidity. Typical applications that fall under this category are treated wood in dry-use applications.
  2. Exposure Condition 2 with untreated wood and salt water. This test can be used to evaluate fasteners that are above ground but exposed to coastal salt exposure.
  3. Exposure Condition 3 with chemically treated wood and moisture. This test covers all the general construction applications.
  4. Exposure Condition 4 with chemically treated wood and salt water. Typical applications include coastal construction applications.

Depending on the exposure condition being used for fastener evaluation, the fasteners are installed in wood that could be either chemically treated or untreated. Then the wood and the fasteners are placed in the chamber and artificially exposed to the evaluation environment. Two types of test procedures are to be completed for exposure condition 2 through 4. The purpose of these tests is not to predict the corrosion resistance of the coatings being evaluated, but to compare them to fasteners with the benchmark coating (ASTM A153, Class D) in side-by-side exposure to the accelerated corrosion environment.

ASTM B117 Continuous Salt-Spray Test

ASTM B117 is a continuous salt-spray test. For Exposure Condition 3, distilled water is used instead of salt water. The fasteners are continuously exposed to either moisture or salt spray in this test, and the test is run for about 1,440 hours after which the fasteners are evaluated for corrosion. This is an accelerated corrosion test that exposes the fasteners to a corrosive attack so the corrosion resistance of the coatings can be compared to a benchmark coating (hot-dip galvanized).

ASTM G85, Annex A5

The second test is ASTM G85, Annex A5 which is a cyclic test with alternate wet and dry cycles. The cycles are 1-hour dry-off and 1-hour fog alternatively. This is a cyclic accelerated corrosion test and relates more closely to real long-term exposure. This test is more representative of the actual environment than the continuous salt-spray test.  As in the ASTM B117 test, the fasteners along with the wood are exposed to 1,440 hours, after which the corrosion on the fasteners is evaluated and compared to fasteners with the benchmark coating.

Test Method and Evaluation

The test process involves installing 10 benchmark fasteners along with 10 fasteners for each alternative coating being evaluated. The fasteners are arranged in the wood with a spacing of 12 times the fastener diameter between the fasteners. A kerf cut is provided in the wood between the fasteners to isolate the fasteners as shown in Figure 2 and to ensure elevated moisture content in the wood surrounding the fastener shank. The moisture and retention levels of the wood are measured, and the fasteners are then installed in the chamber as shown in Figure 3 and exposed to the designated condition. The test is run for the period specified, after which the fasteners are removed, cleaned and compared to the benchmark for corrosion evaluation. Figure 4 shows the wood and fastener heads after 1,440 hours (60 days). The heads and shanks of the fasteners are visually graded for corrosion in accordance with ASTM D610. If the alternate coating performs equivalent to or better than the benchmark coating — that is, if the corrosion is no greater than in the benchmark — then the coating has passed the test and can be used as an alternative to the code-approved coating. Figure 5 shows the benchmark and alternative fasteners that are removed from the chamber after 1,440 hours.

As you can see, the alternative coatings have to go through extended and rigorous testing and evaluation as part of the approval process before being specified for any of the fasteners. Some alternative coatings provide even better corrosion resistance than the code recognized options. Sometimes, also, the thickness of these alternative coatings may be smaller than the thick coating required for hot-dip galvanized parts. Some of our coatings, such as the Double-Barrier coating, the Quik Guard® coating and the ASTM B695 Class 55 Mechanically Galvanized have gone through this rigorous testing and have been approved for use in preservative-treated wood in the AC257 Exposure Conditions 1 and 3. In addition, these coatings have been qualified for use with chemical retentions that are typical of AWPA Use Category 4A – General Ground Contact. No salt is found in AC257 Exposure Conditions 1 and 3. Please refer to our Fastener Systems Catalog, C-F-14, pages 13–15 for corrosion recommendations and pages 16–17 for additional information on coatings.

What do you look for specifically in a fastener? Do you have a preference for a certain coating type or color? Let us know in the comments below!

Figure 2: Fasteners with different coatings along with the benchmark, installed in wood and separated by kerf cuts.

Figure 2: Fasteners with different coatings along with the benchmark, installed in wood and separated by kerf cuts.

Figure 3: Fasteners and wood pieces installed in the chamber.

Figure 3: Fasteners and wood pieces installed in the chamber.

Figure 4: Snap shot of fasteners in ASTM B117 chamber after 1,440 hours.

Figure 4: Snap shot of fasteners in ASTM B117 chamber after 1,440 hours.

Figure 5: Fasteners after 1,440 hours of exposure, removed from the wood, cleaned and compared to benchmark. Coating 1 – Benchmark (Hot- dip Galvanized) and Coating 2 (Alternative coating).

Figure 5: Fasteners after 1,440 hours of exposure, removed from the wood, cleaned and compared to benchmark. Coating 1 – Benchmark (Hot- dip Galvanized) and Coating 2 (Alternative coating).

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

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

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

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

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

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

Figure 1. Sample building elevation with section marks.

Figure 1. Sample building elevation with section marks.

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

Figure 2. Sample building wall sections.

Figure 2. Sample building wall sections.

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

Figure 3. Sample calculation of wall stud loading diagram.

Figure 3. Sample calculation of wall stud loading diagram.

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

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

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

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

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

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

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

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

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

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

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

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

speed7

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

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

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

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

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

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

 

Simultaneous Loading on Hurricane Ties

“Structures are connections held together by members” (Hardy Cross)

I heard this quote recently during a presentation at the Midwest Wood Solutions Fair. I had to write it down for future reference because of course, all of us here at Simpson Strong-Tie are pretty passionate about connections. I figured it wouldn’t take too long before I’d find an opportunity to use it. So when I started to write this blog post about the proper selection of a truss-to-wall connection, I knew I had found my opportunity – how fitting this quote is!

There are plenty of photos of damage wrought by past hurricanes to prove that the connection between the roof and the structure is a critical detail. In a previous blog  post, I wrote about whose responsibility it is to specify a truss-to-wall connection (hint: it’s not the truss Designer’s).  This blog post is going to focus on the proper specification of a truss-to-wall connection, the methods for evaluating those connections under combined loading and a little background on those methods (i.e., the fun stuff for engineers).

hurricane1

Take a quick look at a truss design drawing, and you will see a reaction summary that specifies the downward reaction, uplift and a horizontal reaction (if applicable) at each bearing location. Some people are tempted to look only at the uplift reaction, go to a catalog or web app, and find the lowest-cost hurricane tie with a capacity that meets or barely exceeds the uplift reaction.

hurricane2

However, if uplift was the only loading that needed to be resisted by a hurricane tie, why would we publish all those F1 and F2 allowable loads in our catalog?

hurricane3

Of course, many of you know that those F1 and F2 allowable loads are used to resist the lateral loads acting on the end and side walls of the building, which are in addition to the uplift forces.  Therefore, it is not adequate to select a hurricane tie based on uplift reactions alone.

Excerpt from BCSI (2015 Version)

Excerpt from BCSI (2015 Version)

Where does one get the lateral loads parallel and perpendicular to the plate which must be resisted by the truss-to-wall connection? Definitely not from the truss design drawing! Unless otherwise noted, the horizontal reaction on a truss design should not be confused with a lateral reaction due to the wind acting on the walls – it is simply a horizontal reaction due to the wind load (or a drag load) being applied to the truss profile. It is also important to note that any truss-to-wall connection specified on a truss design drawing was most likely selected based on the uplift reaction alone. There may even be a note that says the connection is for “uplift only” and does not consider lateral loads. In this case, unless additional consideration is made for the lateral loads, the use of that connector alone would be inadequate.

Say, for example, that the uplift and lateral/shear load requirements for a truss-to-wall connection are as follows:

Uplift = 795 lb.

Shear (parallel-to-wall) = 185 lb.  (F1)

Lateral (perp-to-wall) = 135 lb.  (F2)
Based on those demand loads, will an H10A work?

hurricane5

An initial look at the H10A’s allowable loads suggests it might be adequate. However, when these loads are entered into the Connector-Selector, no H10A solution is found.

Combined Uplift, F1 and F2 Loads

Combined Uplift, F1 and F2 Loads

Why? Because Connector-Selector is evaluating the connector for simultaneous loading in more than one direction using a traditional linear interaction equation approach as specified in our catalog:

hurricane7

If the shear and lateral forces were to be resisted by another means, such that the H10A only had to resist the 795 lb. of uplift, then it would be an adequate connector for the job. For example, the F1 load might be resisted with blocking and RBC clips, and the F2 loads might be resisted with toe-nails that are used to attach the truss to the wall prior to the installation of the H10A connectors. However, if all three loads need to be resisted by the same connector, then the H10A is not adequate according to the linear interaction equation.

Uplift Only

Uplift Only

Some might question how valid this method of evaluation is – Is it necessary? Is it adequate? How do we know? And that is where the interesting information comes in. Several years ago, Simpson Strong-Tie partnered with Clemson University on an experimental study with the following primary objectives:

1. To verify the perceived notion that the capacity of the connector is reduced when loaded in more than one direction and that the linear interaction equation is conservative in acknowledging this combined load effect.

2. To propose an alternative, more efficient method if possible.

Three types of metal connectors were selected for this study – the H2.5A, H10, and the META20 strap – based on their different characteristics and ability to represent general classes of connectors. The connectors were subjected to uni-axial, bi-axial and tri-axial loads and the normalized capacities of the connectors were plotted along with different interaction/design surfaces.

These interaction plots were used to visualize and parameterize the combined load effect on the capacity of the connectors. The three different interaction plots that were examined were the traditional linear relationship, a quadratic interaction surface and a cuboid design space.

Tri-axial Test Frame

Tri-axial Test Frame

Interaction plot for tri-axial loads on a cuboid design space

Interaction plot for tri-axial loads on a cuboid design space

The results?  Not only was the use of the linear interaction equation justified by this study, but a new, more efficient cuboid design surface was also identified. It provides twice the usable design space of the surface currently used for tri-axial loading and still provides for a safe design (and for the bi-axial case, it is even more conservative than the linear equation). This alternative method is given in our catalog as follows:

hurricane11

Now we can go back to the H10A and re-evaluate it using this alternative method:

hurricane12

As it turns out, the H10A does have adequate capacity to resist the simultaneous uplift, shear and lateral loads in this example. This just goes to show that the alternative method is definitely worth utilizing, whenever possible, especially when a connector fails the linear equation.

For more information about the study, see Evaluation of Three Typical Roof Framing-to-Top Plate/Concrete Simpson Strong-Tie Metal Connectors under Combined Loading.

What is your preferred method for resisting the combined shear, lateral and uplift forces acting on the truss-to-wall connections? Let us know in the comments below!

Fine Homebuilding Video Series: How to Build a Deck

We’re partnering with folks at Fine Homebuilding on a video series on how to build a deck that is code compliant and that highlights the critical connections of a deck. This series is called Ultimate Deck Build 2016. The video series comprises five videos that walk professionals through the recent code changes for the key connections of a deck.

The series features David Finkenbinder, P.E., a branch engineer for Simpson Strong-Tie who is passionate about deck codes and safety. He offers information on load resistance and the hardware that professionals can use at the crucial connections to make a deck code compliant. “This was a great opportunity to collaborate with the team at Fine Homebuilding, to communicate the connections on a typical residential deck and the role that they serve to develop a strong deck structure,” said David. “These same connections would also likely be common in similar details created by an Engineer, when designing a deck per the International Building Code (IBC).”

Screen Shot 2016-03-30 at 3.49.17 PM

The videos are being released every Wednesday during the month of March and feature the following deck connections:

  • Ledger Connection: This is the primary connection between a deck and a house. David tells the Fine Homebuilding team about various code- compliant options for attaching a deck ledger to a home.
  • Beam and Support Posts: David explains how connectors at this critical point can prevent uplift and resist lateral and downward forces. He also discusses footing sizes and post-installation anchor solutions.
  • Joists: This video reviews proper joist hanger installation and the benefits of installing hurricane ties between the joists and the beams. David goes into common joist hanger misinstallations, such as using the wrong fasteners or using a joist hanger at the end of a ledger.
  • Guardrail Posts: David reviews the different ways that you can attach a guardrail post so as to resist an outward horizontal load.
  • Stairs: David explains code-compliant options for attaching stringers to a deck frame.

Make sure to watch the series and let us know what you think. For more information, Fine Homebuilding has created an article titled “Critical Deck Connections.”

(Please note: this article is member-only/subscription content, so to read it you’ll need to either subscribe online or pick up the April/May issue of Fine Homebuilding.)

Screen Shot 2016-03-30 at 4.06.33 PM

Corrosion Resistance Classification

This week we’re blogging about corrosion, and we’re not talking about rusting of the soul — we’re talking about oxidation of steel.

In 2014, we reviewed our corrosion protection recommendations for new catalog publications. In doing so, we realized that we could facilitate selection of hardware and fasteners if our Corrosion Resistance Classifications for treated wood were linked to common design conditions described in the codes. We made some revisions to our Corrosion Resistance Classifications during that exercise. This blog post talks about those changes and some current related activity in the wood treatment industry.

The common design conditions for corrosion-resistant wood construction include the wood materials with associated treatments and the environmental corrosion agents. The American Wood Protection Association (AWPA), which is an ANSI-accredited consensus standards organization, publishes the code-referenced standard, AWPA U1-15 Use Category System: User Specifications for Treated Wood.

When you specify treated wood, this is the standard that defines the appropriate treatment chemicals and chemical retentions depending on the exposure condition and bio-hazard, which the AWPA has summarized into a Use Category (UC) system. Figure 1 is a clip from the AWPA web site that gives a glimpse at the UC system. As the UC rating increases from UC1 to UC5, the chemical retention increases because the bio-hazard is increasing. Corrosion hazards are directly related to the combination of treatment chemical, treatment chemical retention and use environment.

Figure 1. Summary of AWPA Use Category System (http://www.awpa.com/standards/U1excerpt.pdf)

Figure 1. Summary of AWPA Use Category System http://www.awpa.com/standards/U1excerpt.pdf

The AWPA UC system does not include environmental corrosion agents. As a result, we had to separately integrate those with treatment chemical effects as we developed the corrosion resistance classifications.

Finally, one more evaluation system had to be addressed: the exposure conditions of ICC-ES AC257 — Corrosion Resistant Fasteners and Evaluation of Corrosion Effects of Wood Treatment Chemicals. In the end, we developed Corrosion Resistance Classifications that considered the AWPA Use Categories, environmental corrosion agents and the ICC-ES AC257 exposure conditions.

Some of you may be thinking that we have not mentioned another aspect of corrosion — galvanic corrosion. Galvanic corrosion results when metals with dissimilar electrical potentials are placed in contact in the presence of an electrolyte (water). We’ll take up galvanic corrosion in a subsequent blog post.

Our basic Corrosion Resistance Classification table is shown in Figure 2.

Figure 2. Corrosion Resistance Classifications (from C-F-2014, p. 15, or strongtie.com)

Figure 2. Corrosion Resistance Classifications (from C-F-2014, p. 15, or strongtie.com)

The ratings shown in the table — Low, Medium, High and Severe — refer to the corrosion resistance of Simpson Strong-Tie coating systems and base metals. An example of a coating system that is rated “Low” is paint or electro-galvanized zinc. An example of a material rated for “Severe” corrosion conditions is Type 316 stainless steel.

To use the Corrosion Resistance Classifications table, find the Environment, then move to the correct column in the Material to Be Fastened section; identify a rating. Then look in the companion table labeled “Corrosion Resistance Recommendations” to identify a coating or base metal that is appropriate for your project. Be sure to read the table notes to the Corrosion Resistance Classifications for exceptions and limitations. We implemented this system to simplify product selection. Let’s take a look at each aspect that contributes to the Corrosion Resistance Classifications table.

Environment                                                                                      

The environment captures the moisture, atmospheric conditions and other elements that affect corrosion rate. “Dry Service” usually means an interior space with low moisture content or dampness. No liquid water is present in this sort of environment. The absence of moisture limits the electrochemical reaction needed to produce what we see as corrosion. “Wet Service” usually means exterior exposure and involves liquid water as direct exposure or condensation and wood moisture contents that can exceed air-dry conditions and may be temporary or persist for prolonged periods. We incorporated environmental agents with the “Elevated Service and Ocean/Waterfront” conditions. These environmental agents include fumes, acid rain, airborne salinity, etc. The “Uncertain Environment” was included for the Designer who does not know the corrosive conditions in service.

Material Being Fastened

Here we distinguish between clean materials and wood treated with chemicals — wood preservatives or fire-retardant chemicals (FRT). Untreated softwoods used for framing are generally not significantly corrosive. This does not include cedars and redwood, which are a special case. Cedars tend to be corrosive and particularly prone to staining when fastened with carbon steel hardware and fasteners. As a result, our recommendations for untreated softwoods are generally a function of the environment — moisture, weather exposure and corrosion agents such as salt spray, sulfur or fertilizer fumes and acid rain are all examples.

Some treatment chemicals do not significantly increase the corrosion hazard. These are the SBX-DOT treatment chemicals (inorganic boron and borate treatments). These are not typically used in exterior environments or for high-moisture conditions. The preservatives are not chemically bound to the wood and they can leach out under exposure to liquid moisture, which would leave the wood unprotected.  The corrosion hazard attendant to these chemicals is similar to that of untreated wood and the codes permit the use of bare carbon steel in contact with wood treated with these chemicals (IBC2015, Section 2304.10.5.1 and IRC2015, Section R317.3.1 (exception 3)).

Most of the waterborne chemicals in common use contribute to an elevated corrosion hazard. Some of the common wood treatment chemicals include formulations of alkaline copper quaternary (ACQ), copper azole (CA), ammoniacal copper zinc arsenate (ACZA) and micronized copper azole (MCA). The AWPA UC system defines the exposure conditions for each Use Category as well as the chemical retention required to prevent a decay failure. The MCA formulations are alternatives to those specified in the code-referenced standard through the evaluation report process and are not standardized by the AWPA. The evaluation report process for wood preservatives requires the submission of evidence in compliance with ICC-ES AC326 — Proprietary Wood Preservative Systems Common Requirements for Treatment Process, Test Methods and Performance.

We realize that UC4A is a general-use ground-contact condition, and further, it is the maximum necessary specification for treated wood in many building applications. The Simpson Strong-Tie Corrosion Resistance Classifications recognize that the corrosion hazard of treatment chemical retentions for UC4A in Wet Service is a “Medium” corrosion condition (with the exception of ACZA, which is rated “High” in Wet Service). This means that carbon steel products with sufficient corrosion resistance (e.g., ZMAX, double barrier coating, etc.) can be used in these conditions assuming no other corrosion-causing agents are present.

On the other hand, the moisture conditions and treatment chemical retentions are elevated in UC4B and UC4C, and there is also a potential for salt exposure, which further escalates the corrosion hazard. In these conditions, stainless steel is generally recommended for connectors and fasteners as the best material for mitigating the corrosion risk.

The last column in the Corrosion Resistance Classifications table is devoted to FRT wood. Fire-retardant treatment chemicals are proprietary and are deemed to meet the requirements of the codes through the evaluation report process (ICC-ES AC66 — Acceptance Criteria for Fire-Retardant-Treated Wood). We cannot evaluate the corrosion resistance of hardware to all of the FRT formulations.  However, we have reviewed most of the FRT evaluation reports for corrosion information. The corrosion effects of FRT chemicals, like preservative treatment chemicals, are minimized in dry-service conditions because the electrochemical reaction cannot progress or is slowed without an electrolyte. The Corrosion Resistance Classifications reflect that information. The Designer should always follow the FRT evaluation reports in addition to considering our recommendations.

It is important to note that the Corrosion Resistance Classifications are not associated with specific applications. Rather, the ratings are based on the integrated effects of the environment and the wood treatment where the chemical retentions given in the AWPA Use Category system play an important role in the ratings. This makes it relatively straightforward to select hardware that is adequate for a design environment.

Changes to the AWPA U1 Standard and Effects on Corrosion Resistance Classifications

As noted here and in the online JLC article, wood preservative chemicals can achieve compliance with the codes by either of two methods:

  1. The product is a generic product (e.g., ACQ-D or CA-B) and is listed in the AWPA U1 standard; or
  2. The product has an evaluation report obtained by submitting evidence in accordance with ICC-ES AC326 — Proprietary Wood Preservative Systems Common Requirements for Treatment Process, Test Methods and Performance.

You may be aware that the AWPA is revising its code-referenced standard, AWPA U1-15, Use Category System: User Specification for Treated Wood. The consensus process is ongoing and is not complete. However, AWPA member chemical companies (Viance, Koppers, and Arch) have placed information in the market.  In parallel with the AWPA, ICC-ES has modified AC326 to reflect the changes ongoing in the AWPA U1 standard. Simpson Strong-Tie has been in contact with the AWPA, other industry associations and industry professionals to understand the potential effects on metal hardware of the AWPA U1 and ICC-ES AC326 revisions.

The proposed revisions to the AWPA U1 standard modify the definitions for UC3A, UC3B, UC4A, UC4B and UC4C. The most important revisions are to UC3B and UC4A. The new definition for applications in UC3B suggests that beams and joists in decks and docks may have bio-hazards that exceed the UC3B assumptions, while the new UC4A definition will include above-ground applications with ground- contact hazards. The revised AWPA U1 standard will be published in the May–June 2016 time frame; AWPA U1-16 will be included in the 2018 codes.

The revision to ICC-ES AC326 also modifies the definitions for UC3B, UC4A and UC4B. ICC-ES AC326 has an implementation date of July 2016, which will cause some changes to specifications this summer.  Micronized copper azole (MCA) formulations are the most common treatment chemicals that will be affected by this action.

Revisions to the Use Category definitions are being driven by two issues:

  1. Wood treated for UC3 is sometimes used in near-ground applications where the bio-hazard is more like UC4.
  2. Under-treatment compromises the margin of safety to bio-hazards, which can lead to decay failures.

Rather than revisit the retention specifications in AWPA U1 standard, the AWPA is modifying the definitions for the Use Categories that are involved, and that language has been carried into ICC-ES AC326 to ensure that the two systems are consistent with each other. The result of changes to the Use Category definitions will likely cause some specifications to change from UC3B to UC4A or from UC4A to UC4B. The main effects will likely be to specifications in eastern and southern states, where there may be more chemical in the wood to meet retention specifications.

The Simpson Strong-Tie Corrosion Resistance Classifications make specific reference to the corrosive levels of environmental conditions and the chemical treatment and retentions of the AWPA Use Categories, not to applications. As a result, the AWPA U1 revisions and the parallel changes to ICC-ES AC326 will not necessitate a change in our corrosion recommendations, because the chemical retentions for each Use Category have not changed. However, your hardware specifications could change for typical applications depending on the Use Category of the treated wood in your project. Our information suggests that this issue is still not settled within the industry, and we will pass along information as we learn it.

Simpson Strong-Tie is currently preparing new catalogs for the coming year and will be updating the corrosion information in those publications and our website. We’re interested in your experience with our Corrosion Resistance Classifications and whether you have suggestions for how we might make the content more useful to you.

Habitat for Humanity Introduces Habitat Strong Program

You’re probably already familiar with Habitat for Humanity, a nonprofit builder of simple, decent and affordable homes for low-income families around the world. According to builderonline.com, they were the 15th-largest builder in the country in 2015 when ranked by number of closings. Simpson Strong-Tie has been an official national partner with Habitat for Humanity since 2007, making contributions of cash and products exceeding $2.5 million in that time, and Simpson Strong-Tie employees have spent hundreds of hours building homes and training local Habitat affiliates.

Habitat for Humanity Home

We know from working on Habitat houses that they tend to be well built. There were newspaper articles about Habitat houses performing better than neighboring houses in Hurricane Andrew. In an effort to better benefit the homeowners they serve, Habitat has recently started a formal program to build even better, code-plus homes that could stand up to local hazards and document the methods used during construction. The name of this new program is Habitat Strong. Simpson Strong-Tie is proud to be a major sponsor of the program.

Habitat Strong actually began as a pilot project funded by Travelers Insurance that built 20 disaster-resistant homes in Alabama, Mississippi, New York and Connecticut. The success of that project convinced Habitat of the importance of building stronger, more resilient homes in all parts of the country. Starting from those regional hurricane-inspired efforts, the Habitat Strong program is now being used by more than 48 affiliates throughout the country, as shown on this map.

Habitat for Humanity Habitat Strong affiliate map.

According to Habitat for Humanity, “The Habitat Strong program is designed to promote the building of homes that are more durable, resilient, and physically stronger. The need for stronger homes has become increasingly apparent, and through Habitat Strong’s fortified codes-plus building practices, we are able to strengthen homes’ building envelopes, which enable[s] them to better withstand natural disasters in every region of the country. This program was developed specifically for the Habitat model to be affordable and volunteer-friendly, while offering benefits to partner families that will last for years to come. Based on these principles, we believe that building homes Habitat Strong is the right thing to do!”

Habitat for Humanity has established a set of construction standards for Habitat Strong that are based on the Insurance Institute for Business & Home Safety® (IBHS) FORTIFIED Home™ program. The FORTIFIED program is a scientifically developed, systems-based incremental approach for creating stronger, safer homes. There are three levels of FORTIFIED Home™ designations: Bronze, Silver and Gold. Each level builds upon measures at the preceding level to increase the disaster resistance of the home. You can take a look at the FORTIFIED Home standards on the IBHS website at www.disastersafety.org.

There are now three separate sets of FORTIFIED Home™ standards: Hurricane, High Wind & Hail, and High Wind. In general, the three levels consist of the following:

Bronze:

  • Strengthen roof deck fastening by using 8d ring-shank nails in a closer-than-normal nailing pattern.
  • Apply a secondary water barrier to the roof deck so there will still be protection from water damage even if the roof covering is blown off.
  • Install a roof covering that is rated for high winds and, if appropriate, hail forces.
  • Prune nearby trees to prevent damage to the home during a wind event.

Silver:

  • Complete all requirements for Bronze.
  • Brace gable ends over 4′ tall and ensure they are sheathed with a minimum thickness of wood structural panel.
  • Anchor wood frame chimneys to the roof structure.
  • Anchor attached structures, such as porches and carports, from the roof to the foundation.

Gold:

  • Complete all requirements for Silver.
  • Provide a continuous load path for wind forces from the roof to the foundation. In a normal 115-mph wind zone, the load path is to be designed for at least 140 mph.
  • Provide a garage door that is rated for high winds.

Habitat for Humanity is recommending to their affiliates that homes built in coastal areas be built to the IBHS Gold standard for hurricanes, and those built in inland areas be built at a minimum to the Bronze or Silver standards for high winds. The Habitat homes that meet the Bronze or Silver standards will be certified as Habitat Strong. Habitat homes that are built to the Gold standard will be certified as Habitat Strong+.

Simpson Strong-Tie is proud to be assisting Habitat for Humanity with Habitat Strong. In January, we hosted a training for Texas affiliates that was offered by Habitat and IBHS staff at our Houston training facility. We also donated connectors for a demonstration home at Michigan State University that we helped design.

If you would like more information on Habitat Strong, contact HabitatStrong@habitat.org. To learn how you can help Habitat for Humanity, visit www.habitat.org/getinv/volunteer.

Are you aware of any other programs for strengthening affordable housing? Let us know in the comments below.

 

 

 

Introducing Our New Website

We know many of you visit our website on a regular basis for product and technical information and to use our software, calculator tools and other web apps. If you haven’t visited strongtie.com recently, it has a new look and several new features, including enhanced search and browsing and a mobile-friendly design. Here are some of the new features and site improvements:

  • Update-to-date product information: If there is a new code report, catalog or product you will be able to find that information on our new website first. It has the latest product and technical information while retaining the same features and information you expect.
  • Enhanced search and browsing: You can now search for our products based on specific product attributes. Our enhanced search capabilities allow you to explore our collection of products by applying filters so you can quickly and easily browse and find the products that you are looking for.
  • Mobile-friendly: Our new site has a responsive design that allows you to view the site in any format. From large desktops to mobile devices, you can view our site in the office or while on the go.
  • Enhanced Visuals: We have added new and improved photographs, illustrations and graphics so that you can see our products in greater detail.

We hope the new website better serves your design and technical needs. If you have any suggestions, comments or feedback, please email us at web@strongtie.com or leave a comment below.

str-eng-blog-new-site-v2

Installation Errors – They Happen

A few years ago, we did a post on creative uses of our products. Most of the uses shown were artistic, or functional do-it-yourself projects, with one odd car spoiler modification. This week, I was reviewing some slides in a presentation that I give a few times a year regarding product installation errors. I call them misinstallations, but I’m not sure that’s a word. I thought I’d share a few of the more instructional ones. Most of the photos were curated by our northwestern region training manager, Olga Psomostithis – thanks Olga!

Double Shear Hangers

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Double shear hangers require joist fasteners that are long enough to penetrate through the hanger, through the joist and into the header. The joist nails help transfer load from the joist into the header, resulting in higher allowable loads.

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The installation shown has had the double shear tabs bent back, and nails installed straight into the joist. Since the joist nails do not penetrate the header, this would result in a reduced capacity.

Holdowns

I’m including the trailer hitch installation because it makes me laugh no matter how many times I see it.

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A very common question we get about holdowns is related to posts being offset too far from the anchor bolt (or is the anchor too far from the post?). In the installation shown below, the holdown is not flush with the post as the anchor bolt is offset about 1 inch. For small offsets up to about 1½”, a common solution is to raise the holdown off the sill plate and extend the anchor bolt with a coupler and bend it so there is a small (1:12) slope to it.

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The holdown test standard, ICC-ES AC155, which is discussed in this post, requires that holdowns are tested raised off the test bed, which you can see in the photo below. Holdowns may be raised up to 18” above the top of concrete without a reduction in load provided that the additional elongation of the anchor rod is accounted for.

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I like this photo because the installer put on the nail stops to protect the pipes. It is good to remember that plumbing happens when laying out a structural system.

Oh boy, does it happen.

Oh boy, does it happen.

install9STHD Holdowns

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The photo above is not a misinstallation, but something that can happen. Embedded strap-style holdowns are cost-effective solutions for shearwall overturning or wind uplift. It is permitted to bend the straps to horizontal and back to vertical one cycle. If spalls form, they should be evaluated for reduced loads. Any portion of the strap left exposed should be protected against corrosion.

Hanger Gaps

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Gaps can occur between trusses and supporting girders for a variety of reasons. For standard hanger tests, a 1/8″ gap is required between the joist and header per ASTM D7147. A resource for evaluating conditions with larger gaps is our technical bulletin Allowable Loads for Joist Hangers with Gaps. The technical bulletin has load data for a variety of hangers with gaps up to 3/8″, as well as recommended repairs for larger gaps. Our HTU product series comprises truss hangers specifically engineered to allow gaps up to ½”.

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After going through a design project and carefully selecting the members and details of construction, it can be frustrating as an engineer to get that phone call from the general contractor or building inspector informing you that something is not right with the construction. Understanding some of the resources available to address installation errors can help solve these problems more quickly, and get you back to designing the next project.

Impact Community Resilience as a USRC Member and Certified Rater

The U.S. Resiliency Council (USRC) recently launched its Building Rating System for earthquake hazards. The Rating System assigns a score of from one to five stars for three building performance measures: Safety, Damage (repair cost) and Recovery (time to regain basic function).

US Resiliency Council header

This first-of-its-kind building performance rating is based on decades of earthquake engineering research and observations of earthquake damage and recovery. It will become an important component of future sustainable and resilient community goals. The USRC will expand its building performance ratings to include other natural hazards such as hurricanes, tornadoes and floods in the coming years.

Building damaged by natural disaster

With the USRC rating system, users will receive reliable and consistent information about a building’s expected performance during an earthquake and the estimated speed of its recovery afterwards. They can use this information to help them make decisions about purchasing or leasing buildings in which they live, work or invest, or about financing or insuring these buildings. The USRC Rating System also allows businesses and communities to plan and prepare for disasters by giving them data on the likely performance of their building stock. With the support of its Sustaining Members, the USRC will play an important role in long-term strategic capital and disaster recovery planning for communities and businesses. With the USRC Rating System, owners can specify the desired level of performance for their important facilities, to ensure that they not only survive, but also continue operations after a disaster in accordance with their expectations.

The steps to obtain a USRC Verified or Transaction Rating are as follows.

  1. Select Rating type The building owner or building jurisdiction determines the desired USRC Rating: Transaction or Verified.
  2. Select Certified Rating ProfessionalThe building owner selects and contracts with a USRC Certified Rating Professional (CRP) to complete a seismic evaluation of the building. Owners can search for CRPs and see what the requirements are for individuals to be USRC-certified at www.usrc-portal.org.
  3. Perform detailed evaluation and determine preliminary Rating The CRP performs a seismic engineering evaluation of the subject building using one of the USRC-approved evaluation methodologies, which include ASCE 41 and FEMA P-58, and translates their findings into a three-dimensional rating using the USRC translation matrix. A simplified version of the translation matrix for an ASCE 31/41 assessment is shown below:USRC translation matrix
  4. Submit evaluation and Rating to USRC The CRP’s evaluation report, proposed Rating and application fee are submitted to the USRC along with a request for either a Transaction or a Verified Rating.
  5. USRC performs review and issues Rating The USRC reviews the submission for completeness. The USRC will then either issue a Transaction Rating certificate, or one of its USRC Certified Rating Reviewers will perform a technical review before issuing a Verified Rating certificate.

Becoming a USRC Certified Rating Professional or Reviewer:

The minimum requirements for becoming a USRC Certified Rating Professional include an educational background in structural engineering, five years of relevant building evaluation experience as a licensed Professional Engineer and professional references.

The minimum requirements for becoming a USRC Certified Rating Reviewer includes either holding a Structural Engineering license followed by five years of relevant experience, or a Professional Engineering License followed by 10 years of relevant experience.

Details of the application process and other requirements for certification are provided on both the USRC website, www.usrc.org, and the USRC portal, www.usrc-portal.org. The cost to become a USRC Certified Rating Professional or Reviewer is $600 for individuals with a $100 annual renewal fee. Individual and corporate members have discounts on certification.

Building Inspector Looking At New Property

USRC Membership

The U.S. Resiliency Council is a growing 501(c)(3) nonprofit organization with the vision of a world in which building performance in earthquakes and other natural hazards is better understood by building owners, tenants, financial institutions and communities. Corporations, organizations and individuals who are stakeholders in the built environment and who have a passion for improving the resiliency of our nation, have the opportunity to support the USRC through sustaining memberships. With the help of its sustaining members, the USRC will encourage:

  • Increasing market demand for better-performing buildings
  • Fostering collaboration among diverse stakeholders and technical experts
  • Promoting integrity, stability, consistency and transparency of rating systems
  • Educating and advocating for safe buildings and a better public understanding of building performance

USRC Membership List

USRCs members include many of the largest and most respected professional A/E firms and engineering professional societies in the country. Membership is open to all companies, individuals, communities and other stakeholders in the built environment. Information on joining the USRC can be found at the USRC website www.usrc.org.

What contributions from engineers are necessary to help create more resilient communities? Let us know in the comments below.