Simpson Strong-Tie® R&D engineers are always looking to make products even better and more cost effective, in ways that will improve life not only for homeowners, but also for Designers and builders. In the following post, Aram Khachadourian explains how the newly designed SawTooth™ point on our code-listed Strong-Drive® SDWS Timber screw makes driving faster and easier with no predrilling. The flat head also makes connector and sheathing placement a lot smoother. Continue Reading
Three years ago, we created this blog post based on a technical support question we often receive about allowable fastener loads for ledgers to wood framing over gypsum board. Given that this is still a frequent question and a relevant topic, we decided to revisit the post and update it.
Drywall. Wall board. Sheetrock. Sackett Board? A product called Sackett Board was invented in the 1890s, which was made by plastering within wool felt paper. United States Gypsum Corporation refined Sackett Board for several years until 1916, when they developed a new method of producing boards with a single layer of plaster and paper. This innovation was eventually branded SHEETROCK®. More details about the history of USG can be found here.
No matter what you call it, gypsum board is found in almost every type of construction. Architects use it for sound and fire ratings, while structural engineers need to account for its weight in our load calculations. A common technical support question we receive is for allowable fastener loads for ledgers to wood framing over gypsum board.
One method to evaluate a fastener spanning across gypsum board is to treat the gypsum material as an air gap. Technical Report 12, General Dowel Equations for Calculating Lateral Connection Values, is published by the American Wood Council.
TR12 has yield limit equations that allow a designer to account for a gap between the main member and side member of a connection. With a gap of zero (g=0), the TR12 equations provide the same results as the NDS yield limit equations.
The equations are fairly complex, but it should be intuitive that the calculated fastener capacity decreases with increasing gap. Engineers are often surprised to see a 40, 50, even 60% drop in fastener capacity with one layer of 5/8” gypsum board. So what else can you do?
Testing, of course! In So, What’s Behind a Screw’s Allowable Load? I discussed the methods used to load rate a proprietary fastener such as the Simpson Strong-Tie® Strong-Drive® SDS or SDW screws. To recap, ICC-ES Acceptance Criteria for Alternate Dowel Type Fasteners, AC233, allows you to calculate and do verification tests, or load rate based on testing alone. We develop our allowable loads primarily by testing, as the performance enhancing features and material optimizations in our fasteners are not addressed by NDS equations.
So to determine the performance of a fastener installed through gypsum board, we tested the fastener through gypsum board. This is easier to do if you happen to have a test lab with a lot of wood and fasteners in it. We did have to run down to the local hardware store to pick up gypsum board for the testing.
A full set of allowable loads for Strong-Drive SDWH and SDWS are available on strongtie.com. The information is given as single fastener shear values for engineered design, and also screw spacing tables for common ledger configurations. As much fun as writing spreadsheets to do the Technical Report 12 calculations is, having tabulated values based on testing is much easier.
In the fastener marketplace, Simpson Strong-Tie stands apart from the rest. Quality and reliability is our top priority.
Experiential learning — has it happened to you? Certainly it has, because experiential learning is learning derived from experience. It happens in everyday life, in engineering and in product development, too. For example, experience has taught us that after a product is launched, our customers will find applications for the product that were never expected or listed in the product brief. Also, experience has shown us that larger fasteners tend to be placed in applications that have greater structural and safety demands.
When the larger Deck-Drive™ DWP screws were manufactured, we decided that they should be marketed as “load-rated” screws because they were big enough to support physically large parts and would be expected to provide structural load resistance.
So what is a “load-rated” screw? To Simpson Strong-Tie, a load-rated screw is a threaded fastener that has controlled dimensions and physical properties, as well as validated connection properties. Load-rated fasteners are also subject to the same quality inspection that would occur if they were undergoing an evaluation report.
The products in the focus of this blog are Deck-Drive DWP Wood stainless-steel tapping screws. They are made from stainless steel (Types 305 and 316) and are particularly interesting because they have a box thread design feature. What is a box thread and what are its benefits? A box thread is a thread that is square rather than round. It is formed by rolling (not a trivial tooling challenge) like a standard thread. The box thread is preferred for some applications in part because of the low torque required to install the screw; that is, the installation demand is low relative to standard threads of the same pitch (number of threads per inch). You can easily see the box thread by looking from the point of the screw toward the head. The square corners of the box thread rotate at a defined angle, giving the threaded length a twisted appearance. The box thread is also used on the Timber-Hex SS screws. See Figure 1 for an illustration.
When we load rate a fastener, ICC-ES AC233 (Acceptance Criteria for Alternate Dowel-type Threaded Fasteners, 2015) is the guiding document. Essentially, we do everything that would be done if the product was going into an evaluation report. The testing uses representative products and is witnessed by a third party, and every test report is reviewed and stamped by a professional engineer. The DWP screws that are fully load rated are No. 12 and No. 14 that are three to six inches long. This means that we have evaluated by test the shear and tensile strengths, bending yield strength, head pull-through resistance, withdrawal resistance and certain logical lateral shear configurations of these models. The connection properties are developed in at least three species combinations of wood representing a range of specific gravities. Each cell in the connection load matrix is a reference allowable value based on a mean of at least 15 tests that is subject to a precision of five percent at a 75-percent confidence level. Table 1 is snipped from the prepublication spreadsheets.
While we were working on the No. 12 and No. 14 screws, we also realized that No. 10 DWP screws often require withdrawal loads because they are used in decks and docks to fasten the decking to the structural frame. You can see in Table 1 that the withdrawal loads were included for No. 10 DWP screws and the related properties, because uplift resistance is often engineered for those applications.
What is the test method for each property in the load table? See Table 2 for the test method used for each property and the related data for that property. The reference allowable shear loads shown in Table 1 represent more than 1,200 individual tests, and each test includes wood specific gravity, moisture content and continuous load-displacement data from start of test to past ultimate load.
Load rating screws is a big job, and it creates an elevated continuous quality-monitoring obligation. However, our experience has taught us that the engineering community needs information and reference properties that can be relied on when specifying, and thus working with load-rated screws makes it possible to put your stamp on a design with confidence.
We look forward to hearing from you about load-rated fasteners, because we learn from you every time you contact us.
For many years, builders have struggled with the awkward sole-plate-to-rim-board attachment. They often install a few nails and call it good, resulting in a connection with significantly less capacity than needed. This connection is critical to ensure that seismic and wind loads are adequately transferred to the lateral-force-resisting system. With screws becoming much more common in construction, we saw an opportunity to address this problem.
We offer a variety of structural wood screws that have shank diameters ranging from 0.135″ to 0.244″. They form our Strong-Drive® line of structural fasteners. The Simpson Strong-Tie® Strong-Drive SDWC Truss, SDWH Timber-Hex, SDWS Timber, SDWV Sole-to-Rim and SDS Heavy-Duty Connector structural wood screws as shown in Figure 1 can be used to attach sole plates to a rim board as shown in Figure 2. These screws provide structural integrity in the wall-to-floor connection.
The sole-to-rim connection is considered a dry service location. When the sole plate and the rim are both clean wood (not treated), then any of the screws can be used as long as they meet the design loads. However, if one or both members of the connection are treated with fire retardants or preservatives, then you must use the SDWS Timber screw, SDWH Timber-Hex screw or SDS Heavy-Duty Connector screw. The SDWS, SDWH and SDS screws all have corrosion-resistance ratings in their evaluation reports.
The Strong-Drive SDWV structural wood screw has the smallest diameter among these screws. The SDWV is 4″ long and has a 0.135″- diameter shank, and a large 0.400″-diameter ribbed-head with a deep six-lobe recess to provide clean countersinking. It is designed to be fast driving with very low torque. The Strong-Drive SDWS offers one of the larger diameters. It has a 0.220″-diameter shank and is offered in lengths of 4″, 5″ and 6″. It has a large 0.750″-diameter washer head which provides maximum bearing area. Longer screws allow designers to meet the minimum penetration requirement into a rim board, when the sole plate is a 3x or a double 2x member.
We have tested various combinations of sole plates, floor sheathing, and rim boards. Typical test assemblies were built and tested with two (2) Strong-Drive® screws spaced at either 3″ or 6″. Results were analyzed per ICC-ES AC233, “Acceptance Criteria for Alternate Dowel-type Threaded Fasteners.” The allowable loads listed in Table 1 are based on the average ultimate test load of at least 10 tests, divided by a safety factor of 5.0, and are rated per single fastener. The results of these tests can be found in the engineering letter L-F-SOLRMSCRW16.
The evaluated sole plates include southern pine (SP), Douglas fir-larch (DF), hem-fir (HF), and spruce-pine-fir (SPF) in single 2x, 3x or double 2x configurations. Floor sheathing thicknesses are allowed up to 1 1/8″ thick. Rim boards can be LVL or LSL structural composite lumber or DF, SP, HF or SPF sawn lumber. The load rating also assumes that the floor sheathing is fastened separately and per code.
See strongtie.com for evaluation report information if it is needed.
As a Designer, you can specify any of these Strong-Drive screws that fit your design requirements. Please visit our website and download L-F-SOLRMSCRW16 for more details.
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 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.
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.
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.
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.
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.
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.
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.
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.
If you need more information about the nails and screws recommended for use with Simpson Strong-Tie connectors, visit strongtie.comand 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.
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.
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:
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.
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.
Exposure Condition 3 with chemically treated wood and moisture. This test covers all the general construction applications.
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!
Designing built-up columns? Now there’s a way to mechanically laminate multiple 2x members to meet the specifications in the NDS. Simpson Strong-Tie evaluated Strong-Drive® SDW Truss-Ply screws for attaching multiple laminations with easier installation methods. With these screws, there’s no longer a need to nail from both sides of the column, or to use not-so-common 30d nails as specified in the NDS, or to pre-drill for bolts. Instead, installers can now install all the screws from one side of the built-up column, which provides time and cost savings.
Columns can be classified into solid columns, built-up columns and spaced columns. Solid columns are single members or individual members glued together to act as one solid member. Mechanically laminated built-up columns are formed by fastening two or more members with bolts, nails or screws. If built-up members have spacer blocks between the members, they create a spaced column. The design of built-up columns is different from the design of solid columns.
These three classes of columns have differing load capacities. The capacity of a built-up column can be expressed as a percentage of the strength of a solid column of the same dimensions and made with material of the same grade and species. The ratio of the built-up column compression capacity to that of a solid column is defined as efficiency (1). The efficiency of built-up columns is 1.0 in the strong axis and between 60 and 75 percent in the weak axis depending on the type of fastening. The loss in capacity in the weak axis compared to a solid column is due to the slip between the laminations.
Column failure is due to crushing, buckling, or a combination of both modes. Short columns more often experience crushing failure, and long columns tend to fail more often by buckling. According to the NDS, the efficiencies are generally higher in short and long columns than in intermediate columns. The NDS assigns nailed built-up intermediate columns a 60% efficiency and bolted built-up intermediate columns a 75% efficiency. Even though long and short columns would have higher efficiencies, all column lengths are assigned a single efficiency. Note that provisions in NDS 188.8.131.52 allow short columns to use full design values when designed as individual columns.
Whole-section engineered wood products are recommended for higher compression loads, although they can add cost.
Design of solid columns is addressed in Section 3.7 of the NDS. The main difference between solid column and built-up column capacity is in the calculation of Cp, the column stability factor. The column stability factor adjusts for the buckling effect on the column capacity. If the column is completely braced in all directions, then Cp can be taken as 1. For all other conditions, Cp should to be evaluated for both strong-axis and weak-axis bracing conditions. In solid columns, the column stability factor is calculated as follows:
In this calculation, le/d represents the larger of the ratios l1/d1 and l2/d2 as shown in Figure 1. The slenderness ratio of solid columns, le/d, shall not exceed 50. Higher slenderness ratios have a lower Cp factor, which means that a slender column can buckle more easily and has lower compression capacity than a similar column with a lower slenderness ratio. The same holds true for built-up columns.
Built-up columns fastened with nails or bolts are addressed in Sections 15.3.3 and 15.3.4 of the NDS. However, fastening built-up column members with screws is not addressed in the NDS. For built-up columns, the only difference in design compared to solid columns is the addition of Kf, a column stability coefficient, in the calculation of Cp. See Figure 2 for built-up column notation. For built-up columns, Cp is calculated as follows:
The Cp value is calculated for slenderness ratios based on l1/d1 and l2/d2, and the smaller Cp is used to calculate the adjusted compression design value parallel to grain. In the strong axis, Kf = 1, and the design is similar to solid columns. However, in the weak axis buckling is affected by the slip and load transfer that occurs at fasteners between the laminations, and the Kf factor changes with the type of fastener.
NDS Section 15.3.1 provides the limitations for built-up columns based on these design attributes:
Each lamination has a rectangular cross section and is at least 1-1/2” thick,
All laminations are of same depth and faces of adjacent laminations are in contact,
All laminations are full column length.
These limitations apply to laminations fastened with nails and bolts. In Simpson Strong-Tie design method they also apply to Strong-Drive SDW screws.
The spacing and end distance requirements for nails are covered in Section 184.108.40.206 of the NDS. The nails need to be driven from opposite sides of the column and need to penetrate at least ¾ of the thickness of last lamination. If all of the requirements are met, Kf of 0.6 can be used in the calculation of Cp, when l2/d2 is the limiting ratio for calculation of FcE. The NDS does not provide a table for built-up column capacities fastened with nails. The designer has to run through calculations and follow the provisions of NDS Section 15.3.3 to determine the capacity of a nailed built-up column.
Let’s figure out the nail length needed to connect 3 – 2xmembers. For a 3-ply member the nail length needs to be a minimum of 2 x 1.5inches +3/4 x 1.5 inches = 4.125 inches. Only 30d or higher nails are available in these lengths. Since these nails are not commonly used in the job site and do not fit the regular nail gun, installers may need to use a special nail gun. The NDS provides some typical nailing schedules that are shown here in Figure 3.
NDS Section 220.127.116.11 has end, edge distance and spacing requirements for bolts. Also a metal plate or washer is required between the wood and the bolt head and between the wood and the nut. The nuts should be tightened to ensure that the faces of adjacent laminations are in contact. Figure 4 is a detail showing the typical bolting schedules. If the requirements of the NDS Section 15.3.4 are met, Kf of 0.75 can be used in the calculation of Cp, when l2/d2 is the limiting ratio for calculation of FcE. The NDS does not provide built-up column capacities fastened with bolts. Again the designer has to determine these capacities by calculation.
New Option – Fastening with Simpson Strong-Tie® Strong-Drive® TRUSS –PLY SDW Screws
Simpson Strong-Tie® tested column assemblies as shown in the Figure 5 to determine Column Stability Coefficient, Kf, for SDW screws. The limitations of NDS Section 15.3.1 have been found to also apply to Strong-Drive Truss-Ply SDW screws. The spacing and end distance requirements for the screws are shown in Figure 5. One huge advantage of using SDW screws is the screws can be installed from one side of the column or from both sides of the column. Installation from one side or both sides affects the Kf factor used in the calculation of Cp. If the screws are installed from one side of the column, then Kf of 0.6 can be used in the calculation of Cp when l2/d2 is used to calculate FcE. If the screws are installed from both sides of the column, then Kf of 0.7 can be used in the calculation of Cp when l2/d2 is the limiting ratio for calculation of FcE.
Let’s work on a design example for built-up columns fastened with Strong-Drive® Truss-Ply® SDW screws:
Example: Calculate the capacity of a 3-2×6 built-up member attached with SDW screws with a) installation of screws from only one side b) Installation of screws from both sides of the column.
Column Type: Built-up
Column Length: 10 ft.
Bracing: Completely unbraced in both directions
Size if Column: 3 – 2 x 6
Wood Species: SPF
Load Duration Factor, CD: 1
Temperature Factor, Ct: 1
Wet Service Factor, CM: 1
Per Table 4.3.1 (table shown below) of NDS:
Fc’ = Fc x CD x CM x Ct x CF x Cp
Per Table 4A or 4B of NDS, Compression parallel to grain, Fc = 1150 psi
Emin = 510000 psi
Size Factor CF = 1.1
Fc* = Fc x CD x CM x Ct x CF 1265 psi
Now let’s calculate Cp in both directions:
Cp in Strong-Axis –
Fc* = reference compression design value parallel-to-grain multiplied by all applicable modification factors except Cp (see 2.3 of NDS)
FcE = 0.822 Emin‘/ (l1/d1)2
l1 = 120 in
d1 = 5.5 in
FcE = 881 psi
Kf = 1.0 for solid columns and for built-up columns where l1/d1 is used to calculate FcE and the built-up columns are either nailed or bolted
c = 0.8 for sawn lumber
Substituting the values above, Cp = 0.557
Cp in Weak-Axis –
Fc* = reference compression design value parallel-to-grain multiplied by all applicable modification factors except Cp (see 2.3 of NDS)
FcE = 0.822 Emin‘/ (l2/d2)2
l2 = 120 in
d2 = 4.5 in
FcE = 590 psi
Kf = 0.6 for built-up columns fastened with SDW screws from one side of column
Kf = 0.7 for built-up columns fastened with SDW screws from both sides of column
c = 0.8 for sawn lumber
Substituting the values above with Kf = 0.6, Cp = 0.246 (Screws installed from one side)
Substituting the values above with Kf = 0.7, Cp = 0.287 (Screws installed from both sides)
For screws installed from same side, minimum Cp = Minimum (0.557, 0.246) = 0.246
Column Capacity = Fc* x Cp x d1 x d2= 7690 lbs.
For screws installed from both sides, minimum Cp = Minimum (0.557, 0.287) = 0.287
Column Capacity = Fc* x Cp x d1 x d2= 8970 lbs.
To avoid these long calculation steps and to help the designer, Simpson Strong-Tie compiled a table with allowable compression capacities for built-up columns made with several typical combinations of No. 2 visually graded lumber and fastened with SDW screws. Now that you have a new and faster way of fastening multiple plies using SDW screws along with an easy to use design table, go ahead and design away!
If you have any questions or comments about fastening built-up columns with Simpson Strong-Tie fasteners, pass them along to us in the Engineering Department.
Malhotra, S.K and A.P Sukumar, A Simplified Procedure for Built-up Wood Compression members, St. John’s, Newfoundland, Annual Conference, Canadian Society for Civil Engineering, June 1-18, 1989.
Malhotra, S.K and D.B. Van Dyer, Rational Approach to the Design of Built-Up Timber Columns, Madison, WI, Forest Products Research Society (Forest Products Society), Wood Science, Vol. 9, No. 4: 174-186, 1977.
National Design Specification for Wood Construction (NDS), ANSI/AWC NDS-2012. 2012. American Wood Council, Leesburg, VA. 282 pp
In any given year, Simpson Strong-Tie fields several questions about the use of our connectors and fasteners with pressure-treated fire-retardant wood products. Most often asked is whether this application meets the building code requirements for Type III construction, and whether there is a legitimate concern about corrosion. While there haven’t been any specific discussions on this topic in the SE Blog, there have been related discussions surrounding sources of corrosion, such as: Corrosion: The Issues, Code Requirements, Research and Solutions, Corrosion in Coastal Environments, Deck Fasteners – Deck Board to Framing Attachments. This post will explore several resources that we hope will enable you to make an informed decision about which type of pressure-treated Fire-Retardant-Treated Wood (FRTW) to choose for use with steel fasteners and connectors.
One factor contributing to the frequency of these questions is the increased height of buildings now being constructed. With increased height, there is a requirement for increased fire rating. To meet the minimum fire rating for taller buildings, the building code requires noncombustible construction for the exterior walls. As an exception to using noncombustible construction, the 2015 International Building Code (IBC®) section 602.3 allows the use of fire-retardant wood framing complying with IBC section 2303.2. This allows the use of wood-framed construction where noncombustible materials would otherwise be required.
In the 2009 IBC, Section 2304.9.5, “Fasteners in preservative-treated and fire-retardant-treated wood,” was revised to include many subsections (2304.9.5.1 through 2304.9.5.4) dealing with these wood treatments in various types of environmental applications. Section 2304.9.5.3 addressed the use of FRTW in exterior applications or wet or damp locations, and 2304.9.5.4 addressed FRTW in interior applications. These sections carried over to the 2012 IBC, and were moved to Section 2304.10.5 in the 2015 IBC. FRTW is listed in various other sections within the code. For more information about FRTW within the code (e.g., strength adjustments, testing, wood structural panels, moisture content), the Western Wood Preservers Institute has a couple of documents to consult: 2009 IBC Document and 2013 CBC Document. They also have a number of different links to various wood associations.
As shown in Figure 1 below, fasteners (including nuts and washers) used with FRTW in exterior conditions or where the wood’s service condition may include wet or damp locations need to be hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. This section does permit other fasteners (excluding nails, wood screws, timber rivets and lag screws) to be mechanically galvanized in accordance with ASTM B 695, Class 55 at a minimum. As shown in Figure 2, fasteners (including nuts and washers) used with FRTW in interior conditions need to be in accordance with the manufacturer’s recommendations, or, if no recommendations are present, to comply with 2304.9.5.3.
In Type III construction where the exterior walls may be FRTW in accordance with 2012 IBC Section 602.3, one question that often comes up is whether the defined “exterior wall” should comply with Section 2304.9.5.3 or 2304.9.5.4. While there are many different views on this point, it is our opinion at Simpson Strong-Tie that Section 2304.9.5.4 would apply to the exterior walls. Since the exterior finishes of the building envelope are intended to protect the wood and components within its cavity from exterior elements such as rain or moisture, the inside of the wall would be dry.
There are many FRTW product choices on the market; take a look at the American Wood Council’s list of treaters. Unlike the preservative-treated wood industry, however, the FRTW industry involves proprietary formulations and retentions. As a result, Simpson Strong-Tie has not evaluated the FRTW products. In our current connector and fastener catalogs, C-C-2015 Wood Connector Construction and C-F-14 Fastening Systems, you will find a newly revised Corrosion Resistance Classifications chart, shown in Figure 3 below, which can be found on page 15 in each catalog. The FRTW classification has been added to the chart in the last column. The corrosion protection recommendations for FRTW in various environmental applications is set to medium or high, corresponding to a number of options for connectors and fasteners as shown in the Corrosion Resistance Recommendations chart, shown in Figure 4. These general guideline recommendations are set to these levels for two reasons: (1) there are unknown variations of chemicals commercially available on the market, and (2) Simpson Strong-Tie has not conducted testing of these treated wood components.
The information above is not the only information readily available. There are many different tests that can be done on FRTW, as noted in the Western Wood Preservers Institute’s document. One such test for corrosion is Military Specification MIL-1914E, which deals with lumber and plywood. Another is AWPA E12-08, Standard Method of Determining Corrosion of Metals in Contact with Treated Wood. Manufacturers of FRTW products who applied for and received an ICC-ES Evaluation Report must submit the results of testing for their specific chemicals in contact with various types of steel. ICC-ES Acceptance Criteria 66 (AC66), the Acceptance Criteria for Fire-Retardant-Treated Wood, requires applicants to submit information regarding the FRTW product in contact with metal. The result is a section published in each manufacturer’s evaluation report (typically Section 3.4) addressing the product use in contact with metal. Many published reports contain similar language, such as “The corrosion rate of aluminum, carbon steel, galvanized steel, copper or red brass in contact with wood is not increased by (name of manufacturer) fire-retardant treatment when the product is used as recommended by the manufacturer.” Structural engineers should check the architect’s specification on this type of material. Product evaluation reports should also be checked to ensure proper specification of hardware and fastener coatings to protect against corrosion. Each evaluation report also contains the applicable strength adjustment factors, which vary from one product to another.
Selecting the proper FRTW product for use in your building is crucial. There are many different options available. Be sure to select a product based on the published information and to communicate that information to the entire design team. Evaluation reports are a great source of information because the independently witnessed testing of manufacturers has been reviewed by the agency reviewing the report. Finally, understanding FRTW chemicals and their behavior when in contact with other building products will ensure expected performance of your structures.
What has been your experience with FRTW? What minimum recommendations do you provide in your construction documents?
This week’s post was written by Bob Leichti, Manager of Engineering for Fastening Systems. Prior to joining Simpson Strong-Tie in 2012, Bob was an Engineering Manager covering structural fasteners, hand tools, regulatory compliance and code reports for a major manufacturer of power tools and equipment. Prior to that, Bob was a Professor in the Department of Wood Science and Engineering at Oregon State University. He received his B.S. and M.S. from the University of Illinois, and his M.S. and Ph.D. from Auburn University.
Structures and connections can be designed either using Allowable Strength Design (ASD) method or Load and Resistance Factor Design (LRFD) method. In the ASD method, the allowable strength is calculated by dividing the nominal strength by a safety factor. In the LRFD method, the design strength is calculated by multiplying the nominal strength by the resistance factor. In design, the adjusted ASD design value is compared to a calculated load or stress. As long as the adjusted ASD design value exceeds the calculated load of stress, then the ASD design value is judged safe. In LRFD design, the nominal strength is equated to factored loads. If the factored strength is greater than the factored loads, then the design can be accepted. ASD is the more common method adopted in the professional world.
LRFD is relatively new to wood design. Prior to 2005, the National Design Specification for Wood Construction (NDS) was based on allowable stress design (ASD). In the 2005 edition, the American Wood Council incorporated Load and Resistance Factor Design (LRFD) into the NDS. To this day, most wood design in the US relies on ASD, but the use of LRFD is becoming more common. On the other hand, the steel design industry already uses the LRFD philosophy for design, and for that reason, design values for steel self-drilling tapping screws are offered in both ASD and LRFD.
The published design values for Simpson Strong-Tie wood fastener products are in ASD format and the allowable loads are generally shown at a load duration factor of CD = 1.0. The reference design loads listed shall be multiplied by all adjustment factors listed in Table 10.3.1 of NDS 2012 to determine adjusted design values. The load tables are listed in ASD format because ICC-ES acceptance criteria, such as AC233 (Alternate Dowel-type Threaded Fasteners) and AC120 (Wood Frame Horizontal Diaphragms, Vertical Shear Walls, and Braced Walls with Alternate Fasteners), that are used to qualify structural wood screws do not address the development of LRFD values for wood screws. However, one can establish the nominal strength values for fasteners from reference ASD design values for use in LRFD format by following the instructions of NDS (2012), Table 10.3.1. Reference design values shall be multiplied by the format conversion factor KF as specified in Table N1 of NDS 2012. Format conversion factors adjust reference ASD design values to LRFD reference resistances. They are also multiplied by the resistance factor, Φ as specified in Table N2 and Time Effect Factor, λ as specified in Table N3.
For e.g., the table below lists the ASD allowable shear loads for SDWS screw in Douglas Fir-Larch and Southern Pine Lumber:
For the SDWS22300DB screw with a wood side member thickness of 1.5 inches, the allowable shear load is 255 lbs. with a wood load duration factor of CD = 1.0. To convert this to an LRFD load, refer to table 10.3.1 of NDS 2012 and Appendix N, Tables N1, N2 and N3. Per Table 10.3.1 we need to multiply the reference load with format conversion factor KF, resistance factor, Φ and time effect factor, λ From the Table N1, the format conversion factor KF for connections is 2.16/Φ. From Table N2, for connections Φ=0.65. Let us assume a λ of 1.0 from Table N3. The LRFD load is calculated by multiplying the allowable shear load with the factors above.
LRFD load = Allowable shear load (at a load duration factor of CD = 1.0) x KF x Φ x λ
LRFD Load = 255 x (2.16/0.65) x 0.65 x 1.0 = 551 lbs.
For steel self-drilling, self-tapping screws, the omega and resistance factors used for calculating ASD and LRFD loads are based on American Iron and Steel Institute (AISI) standard S100. For Simpson Strong-Tie steel self-drilling, self-tapping screws the load tables are listed in both ASD and LRFD format.
If the screw connection capacities are calculated based on tests, the ASD values are calculated by dividing the tested nominal strength which is the average of the ultimate strength values from all the tests with the safety factor, Ω. For LRFD load, the tested nominal strength is multiplied by a resistance factor, Φ. When tests are performed for evaluating the connection capacities, the safety factor, Ω and the resistance factor, Φ are evaluated in accordance with Section F of AISI S100. Simpson Strong-Tie derives the LRFD values for steel self-drilling, self-tapping screws in LRFD format because this is part of ICC-ES AC118 (Tapping Screw Fasteners). See evaluation report ICC-ES ESR 3006 for examples of ASD and LRFD design values for the same fastener products.
If the screw capacities are determined based on the calculated nominal strength, the ASD loads and LRFD loads are determined based on Section E4 of AISI S100. For e.g., the table below lists the ASD loads and the LRFD loads based on calculations for #14 x 1” screw.
From the table, for 33 mil (20ga) steel to 33 mil (20ga) steel shear connection, the calculated nominal shear strength is 600 lbs.
Nominal Strength = 600 lbs.
From Section E4, the safety and resistance factors for connections are:
Now that you know the basics of ASD and LRFD, make sure you choose the one best suited for your specific material and construction application. If you have ideas for which of our products you would like to see in ASD and LRFD loads, be sure to let us know!
Engineered wood products have been used in wood-framed construction for many decades. Early forms of engineered wood include plywood as replacement for 1x wood sheathing and glu-laminated beams that could be fabricated in larger sizes with optimized material utilization. I-joists utilizing deep plywood webs and solid sawn lumber flanges solved the challenge of longer floor spans. Oriented strand board (OSB) eventually replaced plywood in the webs, while the innovation of laminated veneer lumber (LVL) became common in the flange material.
In addition to I-joists, structural composite lumber is widely used as a replacement for solid lumber. This could be for a number of reasons such as availability of longer lengths, straighter sections and higher strengths. Structural composite lumber (SCL) may be LVL, parallel strand lumber (PSL), laminated strand lumber (LSL) or oriented strand board (OSB).
Structural composite lumber has two faces. If the cross-section is rectangular, say 3½x5¼, the narrow face will show the edges of the SCL layers. In a square section, the face that shows the SCL layers is still referred to as the narrow face. Fasteners will have lower performance when they are installed in the narrow face of SCL. While this is not an issue for beams, Simpson Strong-Tie connectors such as post bases, column caps or holdowns may have reduced allowable loads when installed on the narrow face of SCL columns.
To support the use of Simpson Strong-Tie connectors installed on SCL post material, we have run many tests over the years. The reductions are published in the technical bulletins, T-SCLCLM13 (U.S. version) and T-C-SCLCLMCAN13 (Canada version). The reduction factors range from 0.45 to 1.0, and vary based on SCL material type – LSL, PSL, or LVL – and also by connector and fastener type.
It is important to understand the magnitude of the reductions. While narrow face installations may be unavoidable, engineers will need to specify the correct lumber and hardware combination to meet the design loads.