How to Pick a Connector Series – Selecting Fasteners

The parts won’t hold themselves up. They have to be fastened in place.

The previous blog in the How to Pick a Connector Series by Randy Shackelford, on “ Selecting a Joist Hanger,” covered the available Simpson Strong-Tie joist hanger options and how to pick a hanger for your design. This week’s blog focuses on the fasteners recommended for various wood connectors.

For straps, holdowns and other connectors, the first step is to specify a product that meets the load and corrosion resistance requirements. Then, specify fastening that is appropriate. The Wood Construction Connectors catalog, C-C-2015, offers fastener information for every Simpson Strong-Tie connector used in wood construction. If you specify the type and number of fasteners and install them as shown in the catalog, then your installation will get full design values. Many connectors are designed to be installed with either nails or Strong-Drive® SD Connector screws. Some products must be installed with Strong-Drive SDS Heavy-Duty Connector screws. Figure 1 is a snip from page 76 of catalog C-C-2015. Here the face-mount hanger table gives the size and number of nails to be installed in the header and the joist, and the table note defines the nail size terminology. Let’s take a look at the various fasteners used for Simpson Strong-Tie connectors of all varieties.

Figure 1. A snip from the face-mount hangers table showing the size and number of nails to be used in the header and joist. The footnote defines the nail sizes in the table.

Figure 1. A snip from the face-mount hangers table showing the size and number of nails to be used in the header and joist. The footnote defines the nail sizes in the table.

Figure 2 shows a scale view of almost all of the fasteners used with connectors. You can find this illustration in the Fastening Systems catalog, C-F-14, and the Wood Construction Connectors catalog, C-C-2015. However, we are continually designing, evaluating and adding new fasteners to use with our connectors. Check our website for the latest and greatest.

Figure 2. Fastener types and sizes specified for Simpson Strong-Tie connectors.

Figure 2. Fastener types and sizes specified for Simpson Strong-Tie connectors.

Keep in mind some generalities that are to be considered in every connector fastener specification.

  • Type and size – Be sure to specify the correct type of fastener and size; for nails, that means diameter and length.
  • Do not mix fasteners – Do not combine nails and screws in the same connector unless specifically allowed to do so in the load table.
  • Corrosion resistance – Consider environmental corrosion and galvanic corrosion. For environmental corrosion, specify fasteners that have corrosion resistance similar to the connector; for galvanic corrosion, the fasteners and connector should be galvanically compatible. Figure 3 shows the corrosion resistance recommendations for fasteners and connectors.
Figure 3. Corrosion resistance recommendations.

Figure 3. Corrosion resistance recommendations.

NAILS

Nail terminology is messy. In a recent Structure Magazine article (July 2016), the author made the point that nail specifications are frequently misinterpreted (or overlooked), and as a result the built system does not have the intended design capacity. In general construction vernacular, specification by penny size identifies only the length. For example, a “10d” specification could be interpreted to mean 10d common – 0.148″ x 3″, 10d box – 0.128″ x 3″, 10d sinker – 0.120″ x 2.875″, or the 10d x 2.5″ – 0.148″ x 2.5″. See NDS-12, Appendix L, Table L4 for the length, nail diameter and head diameter of Common, Box, and Sinker steel wire nails. What if the face-mount hanger needed 0.148”x3” nails to achieve full load, but the face-mount hanger was installed with 0.148″ x2.5″?  In this case, the nail substitution causes a reduction in load capacity of 15%. The load capacity losses would be even greater if 10d sinker or 10d box nails were used. The load adjustment factors for nail substitutions used with face- mount hangers and straight straps are shown in Table 3.

Simpson Strong-Tie nail terminology further complicates nail specification because, in Strong-Tie lingo, the penny reference is to diameter (not to length). This is further reason to write nail specifications in terms of diameter and length.

The best way to prevent mistakes is to specify nails by both length AND diameter.

There are two types of connector nails available, the Strong-Drive® SCNR Ring-Shank Connector nail and the Strong-Drive SCN Smooth-Shank Connector nail. SCN stands for Structural Connector Nails. R would refer to ring- shank nails. Currently most ring-shank connector nails are available in Type 316 stainless steel. Reasons for this are discussed here. The smooth-shank nails are made of carbon steel and either have a hot-dip galvanized (HDG) finish meeting the specifications of ASTM A153, Class D, or have a bright finish. Stainless-steel ring-shank nails are recommended for stainless-steel connectors. Use hot-dip galvanized nails with ZMAX® and HDG connectors. See Table 1 for the nail properties.

Table 1. Simpson Strong-Tie® connector nail terminology decoder. The penny size refers to diameter and “N” indicates a short nail.

Table 1. Simpson Strong-Tie® connector nail terminology decoder. The penny size refers to diameter and “N” indicates a short nail.

Simpson Strong-Tie connector nail specifications include common nails, sinker nails and short nails. Nails used in connectors should always have a full round head and meet the bending yield requirements of ASTM F1667, Table S1. Nails can be driven with a hammer or power-driven. Table 2 shows the Strong-Tie connector nails by catalog name, size and model number.

Table 2. Simpson Strong-Tie® Strong-Drive® SCN and SCNR Connector nails. HDG is hot-dip galvanized per ASTM A153, Class D; EG is electro-galvanized per ASTM B641, Class 1; SS is Type 316 stainless steel; “A” indicates ring-shank. These are collated for power-tool nailing in paper tape (PT).

Table 2. Simpson Strong-Tie® Strong-Drive® SCN and SCNR Connector nails. HDG is hot-dip galvanized per ASTM A153, Class D; EG is electro-galvanized per ASTM B641, Class 1; SS is Type 316 stainless steel; “A” indicates ring-shank. These are collated for power-tool nailing in paper tape (PT).

Remember that connector double-shear nailing should always use full-length common nails. Do not use shorter nails in double-shear conditions.

Table 3 is snipped from the Fastening Systems catalog, and it shows load adjustment factors for optional fasteners used in face-mount hangers and straps.

Table 3. From the Fastening Systems catalog, C-F-14. Load adjustment factors and footnotes.

Table 3. From the Fastening Systems catalog, C-F-14. Load adjustment factors and footnotes.

SD Screws

Figure 4. SD9112 CONNECTOR Screw.

Figure 4. SD9112 CONNECTOR Screw.

Almost 150 Simpson Strong-Tie connectors can be installed with Simpson Strong-Tie Strong-Drive® SD Connector screws (Figure 4). The shanks of the SD Connector screws are designed to match the fastener holes in Simpson Strong-tie connectors. The screw features, dimensions, strengths and allowable single-fastener properties are given in ICC-ES ESR-3046, and the SD screws have been qualified for use in engineered wood products. See ICC-ES ESR-3096 for code-approved connectors installed with SD screws.

SD screws can make connector and strap installation easier and can also provide some resistance that is needed beyond what might be offered by nails. Ease of installation is sometimes an issue in tight places where it might be much easier to use a screw-driving tool rather than a hammer or a power nailer. Some installations are improved by using screws instead of nails, especially where pulling away from the mounting member is a possible failure mode. For example, joist hangers for a deck need withdrawal resistance to help keep the deck tightly connected to the ledger.

SD screws are available in four sizes as shown in Table 4 below. These screws are mechanically galvanized per ASTM B695, Class 55, and have corrosion-resistance qualifications for use in chemically treated wood for Exposure Conditions 1 and 3 per ICC-ES AC257, which is the acceptance criterion for Corrosion-Resistant Fasteners and Evaluation of Corrosion Effects of Wood Treatment Chemicals. See ICC-ES ESR-3046 for corrosion resistance details. Visit SD Screws in Connectors for a complete list of connectors that can be installed with SD screws.

Table 4. SD Connector Screws.

Table 4. SD Connector Screws.

Here are a few specification and construction tips for SD screws:

  • SD10 screws replace 16d common and N16 nails in face-mount hangers and straps.
  • SD9 screws replace 8d and 10d common and 1-1/2″ size nails and 16d sinker nails (all nails 0.148″ and 0.131″ diameter) in face-mount hangers and straps.
  • When SD screws are to be an alternative to nails, specify and use only SD screws. Other types of screws shall not be substituted.
  • SD screws are required to be installed by turning. Do not drive them with a hammer or palm nailer!
  • SD screws and nails cannot be mixed in the same connector.

SDS Screws

Figure 5. Strong-Drive® SDS HEAVY-DUTY CONNECTOR Screw.

Figure 5. Strong-Drive® SDS HEAVY-DUTY CONNECTOR Screw.

The Simpson Strong-Tie Strong Drive® SDS Heavy-Duty Connector screws are 1/4″ screws with a hex washer head (Figure 5). They are available in nine lengths. Table 5 shows the available SDS screws. SDS Screws are available with a double-barrier coating or in Type 316 stainless steel. These screws can be installed with no predrilling and have been extensively tested in various applications. SDS screws can be used for both interior and exterior applications. See ICC-ES ESR-2236 for dimensions, mechanical properties and single-fastener allowable properties. As shown in the evaluation report, SDS screws are also qualified for use in chemically treated wood. See the evaluation report for particulars. SDS screws also have been qualified for use in engineered wood products.

Table 5. SDS Heavy-Duty Connector Screws.

Table 5. SDS Heavy-Duty Connector Screws.

If you need more information about the nails and screws recommended for use with Simpson Strong-Tie connectors, visit strongtie.com and see the appropriate catalog, flier or engineering letter. Remember, your choice of fasteners affects the load capacity of your connections.

Let us know if you have any comments on Simpson Strong-Tie fasteners for straps, holdowns and other connectors.

 

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).

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.