Simpson Strong-Tie Archives

Truss Repair Information: The Never-Ending Search

Truss repair is one of the most frequently asked about truss topics. Not surprisingly, when we asked for suggested truss topics in a truss blog earlier this year, truss repair information made the list. Because the summer months bring about a peak in new construction – and plenty of truss repairs to go along with it – the beginning of June is the perfect time to visit this topic.

From trusses that get dropped or cut/drilled/notched at the jobsite, to homeowners who want to modify their existing trusses to add a skylight or create attic space to fire-damaged trusses, a multitude of scenarios fall under the broad topic of truss repair. Today’s post focuses on various references and resources that can provide some assistance. But first it helps to break down the broad “truss repair” topic into more manageable-sized categories.

New Construction vs. Recent Construction vs. Old Construction

By far, the easiest type of truss repair is new construction, when the trusses either haven’t been installed yet or are still in the process of being installed. Whether the repair is relatively simple (e.g. a broken web) or a little more complicated (e.g. the trusses need to be stubbed), the beauty of new truss construction is that the truss manufacturer – and truss Designer – can be contacted and help with the repair. The truss Designer can easily open up the truss designs in the truss design software, quickly evaluate the trusses for the appropriate field conditions and issue a repair.

A good reference related to truss repairs for new truss construction is the Building Component Safety Information (BCSI) booklet jointly produced by SBCA and TPI. Section B5 of the BCSI booklet, which is also available as a stand-alone summary sheet, covers Truss Damage, Jobsite Modifications & Installation Errors. This field-guide document describes the steps to take when a truss at the jobsite is damaged, altered or improperly installed, common repair techniques, and the information to provide to the truss manufacturer when a truss is damaged, which will assist in the repair process.

The next easiest truss type to repair is recent construction, where the trusses were constructed recently enough that: a) the truss plates are easy to identify, and b) the truss design drawings may even still be available. In these cases, design professionals other than the original truss Designer may be contacted to repair the trusses. For some types of repairs, the design professional can work off the truss design drawing to design the repair. Other times it might be necessary to model and analyze the truss using structural design software; alternatively, a truss manufacturer can be contacted to model the truss in their truss design software for a fee.

Often, the design professional wants to know the design values for the truss plates that were used to construct the truss. If there are truss design drawings available, they will indicate which truss plates were used in the design, and then the truss plate manufacturer can be contacted for more information. It is also easy to search for the truss plate code reports online (for instance, check icc-es.org). If no truss design drawing is available, there is still a way to identify the truss plates. Currently, there are only five major truss plate manufacturers in the United States, and they are listed on the Truss Plate Institute website. That makes identification of the truss plates used in recently constructed trusses easier because all of the current manufacturers’ plates will have markings that are described in their code reports. (Note that there are also a couple of truss manufacturers in the U.S. that manufacture their own truss plates.)

Finally, the most challenging type of trusses for truss repairs are those found in older buildings. Design professionals involved in these types of repair often aren’t sure where to start. Truss design drawings are often not available, and the act of trying to identify the truss plate manufacturer is challenging at best, unsuccessful at worst. As a point of reference, there were 14 truss plate manufacturers that were TPI members in 1987 (see image below), and only one of those companies is still in the current list of five companies. Therefore, the truss plates found in a truss built around 1987 will be difficult to identify. One option is to contact TPI and see if they can point you in the right direction.

TPI Member Listing from a 1987 Publication — TPI member listing from a 1987 publication

Simple vs. Complex Repairs

Another way to break down truss repairs is to divide them into easy and challenging repairs. People often ask for “standard” truss repair details. Unfortunately, standard details only address the simplest types of repair; and those usually aren’t the types of repair that are asked about. Details simply cannot cover the wide range of truss configurations and every type of repair situation.

Sample Repair Detail for a Simple Repair — Sample repair detail for a simple repair

With the exception of simple repairs, most truss repairs rely heavily on the judgment and experience of the design professional doing the repair. And because there are not entire textbooks devoted to truss repair (that I am aware of, anyway), Designers must pull from a variety of resources, both to learn more about truss repair and to design the repair. For repairs using plywood or OSB gussets, the APA Panel Design Specification is a must-have reference. Some people prefer to use dimension lumber scabs for their repairs, whenever possible, simply because they are more familiar with dimension lumber (and the NDS) than they are with Plywood/OSB or the APA Panel Design Specification.

Next, the fasteners for the repair must be selected and the allowable loads determined. For nail design values, I am a big fan of the American Wood Council’s Connection Calculator, which provides allowable nail shear values for just about any combination of main and side members that you can think of, including OSB and plywood side members – particularly handy for truss repairs. For more complex repairs, and especially repairs involving higher forces, an excellent fastener choice is a structural wood screw such as our Strong-Drive® SDS or SDW screws. When I worked in the R&D department at Simpson Strong-Tie, a frequently asked question was whether we had double-shear values for our SDS screws. The questions always seemed to come from Designers who wanted them for truss repairs. Fortunately, we do have double-shear values for our SDS screws.. You can find them on page 319 of our Fastening catalog.

Page 319 of the Fastening Systems Catalog (C-F-14)

The Strong-Drive SDW screw was developed after the SDS screw, and while there are currently no double-shear values for the SDW, it is still another good option for repairs.

Fire-Damaged Trusses and Truss Collapses

These situations are in a category by themselves because they go beyond even the most complex repairs involving a major modification to the truss. The biggest difference is that the latter case involves mostly known facts and perhaps some conservative assumptions, whereas damage due to fire or collapse includes many unknowns. Most of the truss Designers I have spoken to about truss damage due to fire or truss collapse often recommend replacement of the trusses rather than repair because it is usually too difficult to quantify the damage to the lumber and/or joints. In fires, there can be “hidden” damage due to the sustained high temperatures, while the truss appears to have no visible damage. Likewise, in a truss collapse, not only may there be too many breaks in the trusses involved in the collapse, but there may also be trusses that suffered severe stresses during the collapse and have damage that is not visible. To attempt a repair in either of these cases often requires an inspection at the jobsite, and the result may still end up being replacement of some or all of the trusses. Therefore, the cost of a full-blown inspection should be weighed against the cost of replacing the trusses.

The Structural Building Components Association website has a page with information pertaining to fire issues. It includes a couple of documents related to fire damage that are worth checking out.

Beyond the Blog: Where to Get More Truss Repair Information

The best bet for getting practical design information related to truss repairs is to keep an eye out for short courses, workshops or seminars. ASCE has hosted a Truss Repair Seminar (Evaluating Damage and Repairing Metal Plate Connected Wood Trusses) in the past and may very well offer something like it again. Virginia Tech recently hosted a short course on Advanced Design Topics in Wood Construction Engineering, which included a section on Wood Truss Repair Design Techniques.

What other references or resources for truss repair do you use? Are there any upcoming truss repair courses that you know of? Please let us know in the comments below!

Continuous Rod Restraint Systems for Multi-Story Wood Structures

This week was our new employee Sales and Product Orientation class. It reminded me of the post A Little Fun with Testing where we broke a bowling ball. Although breaking stuff is fun, my second favorite part of the class is teaching about the importance of a continuous load path. I think it is really the most important thing a Structural Engineer does. If we don’t pay attention to the loads, where they occur and create a path so they can get where they need to go, a building may not stand up. This week, we also released some new tools and information for our new Strong-Rod™ Systems, which are used to complete the load path for multi-story wood-framed shearwall overturning restraint and roof uplift restraint.

Two Load Paths

All wood-framed buildings need to be designed to resist shearwall overturning and roof-uplift forces. To transfer these tensile forces through the load path, connectors (hurricane ties, straps and holdowns) have been the traditional answer. Simpson Strong-Tie offers a few options there. With the growth in multi-story wood-framed structures, where the code requires shrinkage to be addressed and overturning and uplift forces are typically higher, rod systems have become an increasingly popular load restraint solution. Our Anchor Tiedown System (ATS) for shearwall overturning restraint has been around for many years. A new Strong-Rod Systems Design Guide and revamped web pages provide information on new design options, components and configurations.

Strong-Rod Systems Seismic and Wind Restraint Systems Guide

The guide and website focus more on the unique design considerations for rod systems, how you should specify the system and highlight the design services that we provide. They also provide more detail and design information for our relatively new Uplift Restraint System (URS) for roofs. Connectors are a common choice for transferring the net roof uplift forces from wind events down the structure. Although in some high-wind areas, rod systems are preferred.

ATS and URS Continuous Rod Tiedown Systems

I’ll touch on some of the design considerations for these types of systems below, but back to the load path. For shearwall overturning restraint using holdowns, the load path is fairly simple. Once the lateral load is in the shearwall, the sheathing and nailing lifts up on the post. The holdown connects to the post, holding it down and transferring the forces to the foundation or level below. A continuous rod tiedown system follows a little different path. The sheathing and nailing lifts up on the boundary posts and the posts push up on the framing above until the load is resisted in bearing by a bearing plate. The load is then transferred into the rod and down to the foundation. There has been a lot of testing and research on the effects of skipping restraint locations where a bearing plate restraint is installed at every other floor or only at the top level. Doing that will change the load path because the load has to continue to travel up until a restraint holds it down. It also negatively impacts the stiffness and drift of the shearwall stack, not to mention increases project cost because the boundary posts, rod and bearing must be sized to transfer the cumulative overturning forces from each level.

Wood Shrinkage, Take-up Devices and Displacement Limits

Shrinkage is not just a Seinfeld episode cult classic. It is also something that designers need to consider when designing wood structures. IBC Section 2304.3.3 requires that designers evaluate the impact of wood shrinkage on the building structure when bearing walls support more than two floors and a roof. The effects of wood shrinkage can impact many things in the structure from finishes to MEP systems to the continuous rod system. As the wood members lose moisture, the wood shrinks and the building settles. This can cause gaps at the bearing plate locations of continuous rod systems because the continuous steel rod doesn’t shrink. That is where the magic of take-up devices comes in. They allow the building to shrink but keep gaps from forming by filling the gap (expanding devices – can be screw style or ratcheting), ratcheting down the rod (ratcheting devices), or making the rod shrink as much as the wood (contracting coupling device).

In addition to keeping the rod system tight to insure the intended performance, it is important to consider the movement associated with the rod system when under wind or earthquake loading. The IBC requires shearwall displacements to be within story drift limits in moderate to high seismic regions. We highlighted some of the changes coming for the evaluation of shearwall deflection in the previous post discussing the New Treatment of Shear Wall Aspect Ratios in the 2015 SDPWS. For continuous rod systems, there are some additional limits. ICC-ES AC316 Acceptance Criteria for Shrinkage Compensating Devices requires designs to limit displacement between restraints to 0.20 inches (including rod elongation and device displacement) for shearwall restraint. The movement of the take-up device plays a big part in meeting this requirement and the rod diameter required. Screw-style devices have the lowest total movement. Ratcheting devices are appropriate in many cases as well such as the upper levels where loads are lower, but may require larger rod diameters to meet the displacement limit.

ICC-ES AC391 Acceptance Criteria for Continuous Rod Tie-down Runs and Continuous Rod Tie-down Systems Used to Resist Wind Uplift covers continuous rod systems for roof uplift restraint. The displacement limit for the Continuous Rod Tie-down Run (just the rod system components) is limited to 0.18 inches of rod elongation for the total length of rod. The Strong-Rod URS evaluates the Continuous Rod Tie-down System (the whole load path). Displacement limits for the system are L/240 for the top plate bending and 0.25 inches total deflection at the top plate between tie-down runs (including top plate bending, rod elongation, wood bearing deformation and take-up device displacement). The differences between the rod run and rod system analysis as well as other design considerations are explained in more detail in the design guide and on our website.

I always end my continuous load path presentation during orientation class with the same questions and if they were paying attention I get the response I want.

“What is the most important thing a Structural Engineer does?”

“Designs a continuous load path for the building!”

“What does Simpson Strong-Tie do?”

“Provides product and system solutions to help engineers do their job!”

Take a look at the new Strong-Rod Systems tools and information and let us know how we can help you with your next multi-story wood-framed project.

What related blog topics would you like to discuss? Let us know in the comments below.

Holdown Anchorage Solutions

A common question we get from specifiers is “What anchor do I use with each holdown?” Prior to the adoption of ACI 318 Appendix D, this was somewhat simple to do. We had a very small table near the holdown section of our catalog that listed which SSTB anchor worked with each holdown.

The good old days! (Don’t use this today.)

During the good old days, anchor bolts had one capacity and concrete wasn’t cracked. ACI 318 Appendix D gives us reduced capacities in many situations, different design loads for seismic or wind and reductions for cracked concrete. These changes have combined to make anchor bolt design more challenging than it was under the 1997 Uniform Building Code.

This blog has had several posts related to holdowns. So, What’s Behind a Structural Connector’s Allowable Load? (Holdown Edition) explained how holdowns are tested and load rated in accordance with ICC-ES Acceptance Criteria. Damon Ho did a post, Use of Holdowns During Shearwall Assembly, which discussed the performance differences of shearwalls with and without holdowns, and Shane Vilasineekul did a Wood Shearwall Design Example. So I won’t get in to how to pick a holdown.

Once you have determined your uplift requirements and selected a post size and holdown, it is necessary to provide an anchor to the foundation. To help Designers select an anchor that works for a given holdown, we have created different tables that provide anchorage solutions for Simpson Strong-Tie holdowns.

There is one Engineering letter that addresses slab-on-grade foundations and another version that covers stemwall foundations. The tables are separated by wood species (DF/SP and SPF/HF) to give the most economical anchor design for each post material. The preferred anchor solutions are SSTB or SB anchors, as these proprietary anchor bolts are tested and will require the least amount of concrete. When SSTB or SB anchors do not have adequate capacity, we have tabulated solutions for the PAB anchors, which are pre-assembled anchors that are calculated in accordance with ACI 318 Appendix D.

The solutions in the letters are designed to match the capacity of the holdowns, which allows the contractor to select an anchor bolt if the engineer doesn’t specify one. They are primarily used by engineers who don’t want to design an anchor or select one from our catalog tables. We received some feedback from customers who were frustrated that some of our heavier holdowns required such a large footing for the PAB anchors, whereas a slightly smaller holdown worked with an SB or SSTB anchor in a standard 12″ footing with a 1½” pop out.

To achieve smaller footings using our SB1x30 anchor bolts, we reviewed our original testing and created finite element (FEA) models to determine what modifications to the slab-on-grade foundation details would meet our target loads. Of course, we ran physical tests to confirm the FEA models. With a 6″ pop out, we were able to achieve design loads for HD12, HDU14 and HHDQ14.

HD12, HDU14 & HHDQ14 Solutions — HD12, HDU14 and HHDQ14 Solutions

The revised footing solutions for the heavier holdowns require less excavation and less concrete than the previous Appendix D calculated solutions, reducing costs on the installation.

What has been your experience with holdown anchorage? Tell us in the comments below.

An Introduction to the Helical Wall Tie

What do you do when brickwork is in bad condition? Depending on what state the brickwork is in, a tear-down may be called for. However, often brickwork can be restored and strengthened using helical ties such as Simpson Strong-Tie® Heli-Tie™ wall ties and stitching ties. This post introduces these two types of helical ties, which might be just what you need for your next brick restoration project.

Which Tornado Saferoom is Right for You?

There certainly seems to be increased awareness of the potential for damage and injury from tornadoes these days. Recent information published by the Federal Emergency Management Agency (FEMA) and the Federal Alliance for Safe Homes (FLASH) help explain that. This increased awareness has led to a growing interest in tornado shelters for protection of life and property.

This FEMA graphic shows that most areas of the United States have been affected by a tornado at some point since 1996, and many have been affected by one or more strong tornadoes (EF3 or greater).

Figure 1 - Tornado activity by county: 1996-2013 — Figure 1 – Tornado activity by county: 1996-2013

Living in North Texas near the Simpson Strong-Tie manufacturing plant in McKinney, Texas, I know all too well the sinking feeling of hearing the tornado sirens and turning on the TV to find you are under a tornado watch. FLASH recently published a graphic developed by the National Weather Service that shows the large number of U.S. counties that have been under a tornado watch between 2003-2014, and the high number of warnings that some counties experienced.

Figure 2 - Annual average number of hours under NWS/SPC tornado watches (2003-2014) — Figure 2 – Annual average number of hours under NWS/SPC tornado watches (2003-2014)

Other than moving to an area that has fewer tornadoes, one of the best ways to protect your family and at least have more peace of mind during tornado season is to have a tornado shelter or safe room. These structures are designed and tested to resist the highest winds that meteorologists and engineers believe occur at ground level during a tornado and the debris that is contained in tornado winds.

Tornado shelters can be either pre-fabricated and installed by a specialty shelter manufacturer, or can be site-built from a designed plan or pre-engineered plan. A good source for information on pre-fabricated shelters is the National Storm Shelter Association, a self-policing organization that has strict requirements for the design, testing and installation of its members’ shelters.

FEMA publishes a document, P-320, Taking Shelter from the Storm, that provides good information on safe rooms in general, as well as several pre-engineered plans for tornado safe rooms.

To highlight the different types of safe rooms covered by FEMA P-320, FEMA, FLASH and the Portland Cement Association (PCA) sponsored an exhibit at January’s International Builder’s Show. The exhibit was called the “Home Safe Home Tornado Saferoom Showcase.” It featured six different types of saferooms that builders could incorporate into the homes they build. Simpson Strong-Tie and the American Wood Council collaborated to build a wood frame with steel sheathing safe room meeting the FEMA P-320 plans. Other safe rooms shown at the exhibit included pre-cast concrete and pre-manufactured steel shelters manufactured by NSSA members, and reinforced CMU, ICF cast-in-place concrete and aluminum formed cast-in-place concrete built to FEMA P-320 plans.

Figure 4 - Home Safe Home Tornado Saferoom Showcase — Figure 4 – Home Safe Home Tornado Saferoom Showcase

Simpson Strong-Tie staff in McKinney, Texas, constructed the wood frame/steel sheathing safe room in panels and shipped it to the show. It was built from locally sourced lumber, readily available fasteners and connectors and sheets of 16 ga. steel (which we happen to keep here at the factory). It had cut-away sheathing at the corners to show the three layers of sheathing needed. Our message to builders was that this type of shelter would be the easiest for their framers to build on their sites.

tornado5 — Figure 5: Holdowns and plate anchorage

tornado6 — Figure 6: Roof-to-wall connections

tornado7 — Figure 7: A visitor examines our tested door, a vital component of any shelter. This one was furnished by CECO Doors.

The sponsors of the exhibit took advantage of the variety of safe rooms in one place to film a video series, “Which Tornado Safe Room is Right for You?” The videos are posted at the FLASH StrongHomes channel on YouTube. The series provides comparative information on cast-in-place, concrete block masonry, insulated concrete forms, precast concrete and wood-frame safe rooms, with the goal of helping consumers to better understand their tornado safe room options.

“Today’s marketplace offers an unprecedented range of high-performing, affordable options to save lives and preserve peace of mind for the millions of families in the path of severe weather,” said FLASH President and CEO Leslie Chapman-Henderson. “These videos will help families understand their options for a properly built safe room that will deliver life safety when it counts.”

FLASH released the videos earlier this month as part America’s PrepareAthon!, a grassroots campaign to increase community emergency preparedness and resilience through hazard-specific drills, group discussions and exercises. The overall goal of the program is to get individuals to understand which disasters could happen in their community, know what to do to be safe and mitigate damage from those disasters, take action to increase their preparedness, and go one step farther by participating in resilience planning for their community. Currently, the program focuses on preparing for the disasters of tornadoes, hurricanes, floods, wildfires, earthquakes and winter storms.

Do you know what the risk of disasters is in your community? If you are subject to tornado risk, would you like to build your own safe room, have one built to pre-engineered plans or buy one from a reputable manufacturer? Let us know in the comments below.

Pier Decking Fasteners

This week’s post is a case study featuring a recent restoration job on the central coast of California and how Simpson Strong-Tie® hot-dipped galvanized screws proved to be a better option than traditional spikes.

A pier originally constructed in the 1800s was closed a few years ago as general deterioration caused the structure to become unsafe. As preparation for rebuilding the pier began, one of the major concerns was the attachment of the deck boards to the framing.

Traditionally, the deck boards have been attached with hot-dip galvanized 60d (0.283″ x 6″) spikes. However, spikes have a low withdrawal resistance, are typically predrilled and have a multi-step installation process. In addition, spikes, over time, can begin to back out so that the heads protrude above the top of the deck boards. This creates an unsafe condition for pedestrians and also results in ongoing maintenance work. Here you can see one of the old spikes.

Corroded spike for deck board fastening.

Simpson Strong-Tie provided two options for replacing these spikes: the Strong-Drive® Timber-Hex HDG screw, SDWH27800G, and its stainless-steel counterpart, the Strong-Drive Timber-Hex SS screw, SDWH27800SS. The SDWH27800G screw measures 0.276″ x 8″ and has a hot-dip galvanized coating, conforming to ASTM A153 Class-C. The SDWH27800SS screw measures 0.276″ x 8″ and is made from Type 316 stainless steel. Both of these screws have integral washer, hex-drive heads and are self-drilling. They are not intended to be self-countersinking though, and as a result, installation with the heads below the deck surface requires a shallow dapped hole.

A comparison of the load values was provided to Shoreline Engineering, Inc. engineers Bruce S. Elster, P.E., and Jonathan T. Boynton, P.E., for their review and approval. In addition, Simpson Strong-Tie Fastening Systems/Dealer Sales Representative Darwin Waite expertly conducted on-site demonstrations for numerous decision makers including the contractor and city officials. These demonstrations allowed the contractor and owners to compare the labor costs and finished appearance of the different fastening methods.

Simpson Strong-Tie Fastening Systems Dealer Sales Representative Darwin Waite takes selfie on the completed dock.

Below is a comparison of the allowable load values* of the potential fasteners. We can see how each of the Simpson Strong-Tie screw options exceeds the spike load values in all load conditions.

Table 1. Comparative allowable properties for hot-dip galvanized spikes (60d), hot-dip galvanized screws (SDWH27600G, SDWH27800G) and stainless steel screws (SDWH27600SS and SDWH27800SS).

*Not to be used for design purposes as footnotes have been left out of this blog post. Table values include wet service factor adjustments.

In the end, the SDWH27800SS stainless-steel screw was specified for the project.

Some might consider a 316 stainless steel screw to be cost prohibitive, but when you factor in the lower cost of installation, the lower maintenance requirements and the actual cost of the fastener, this screw turned out to be the lowest cost alternate. In addition, it provided better withdrawal and lateral load values than the spikes.

This picture shows the deck fastening in progress. The screws are set and ready for driving with screw driving tools. — This picture shows the deck fastening in progress. The screws are set and ready for driving with screw-driving tools.

The Strong-Drive Timber-Hex SS screws made it possible to complete the deck restoration on time and on budget. Perhaps just as importantly, the pier looks beautiful and should last for many years to come.

Let us know if the comments below if you have any questions about specifying these fasteners for securing decks, docks, pilings and other heavy-duty, coastal applications.

As always, call our Engineering Department if you have any questions.

Have you used the SDWH27800SS screw for a project? Tell us about in the comments below.

Connectors and Fasteners in Fire-Retardant-Treated Wood

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

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

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

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

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

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

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

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

Figure 3: Simpson Strong-Tie Corrosion Resistance Classifications Chart

Figure 4: Simpson Strong-Tie Corrosion Resistance Recommendations Chart

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

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

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

From Structural Plans to Truss Designs – Collaborative Effort or Review Nightmare?

In an ideal world, a building is envisioned and a structural engineer begins the structural design. When the decision to use roof trusses is made, a component manufacturer is promptly involved in the design process. Using the loads and design parameters from the structural engineer, the trusses are designed and those designs are provided back to the engineer. The structural engineer then incorporates the truss designs into the building, transfers the loads through the structure and designs all the structural elements, bracing and connections necessary to provide a continuous load path down to the foundation. Finally, working from the complete and detailed plans, the contractor constructs the building flawlessly. Perfection!

But let’s get back to reality, where buildings are often designed and ground broken by the time the trusses are designed. The building Designer creates a roof framing plan to analyze and transfer the loads between the roof system and the rest of the building, and then designs the building (from the top plate down) accordingly. The component manufacturer (CM) eventually gets the plans and uses them to create a truss placement diagram and gather the truss design criteria. The CM then communicates the truss design parameters to the truss design engineer, who designs each of the individual trusses. The CM provides those truss designs and the truss placement diagram back to the building Designer. So far so good, right?

If every building were rectangular with a gable-style roof, this process would be fairly simple and hassle-free. But buildings are often more complicated than that. Roof systems can get very complex, so things don’t always go according to plan…such as when a wall that was intended to be non-load bearing gets used as a bearing location for the trusses; or when the CM moves a girder to a different location that works better for the trusses. Or maybe a truss can’t be designed for the entire span and an additional support is needed. In some cases, the CM may even flip the direction of the trusses completely because it results in a more efficient (lower-cost) truss package – that’s better for the building, right?

Changes during the design process can result in additional labor, material and cost. In today’s fast-tracked world, how can these expenses be avoided? Below are some suggestions from a truss Designer’s perspective.

Know each party’s design responsibilities

This may not seem like it has much to do with minimizing changes during the design process, but some problems arise from the fact that there is not always a common understanding of each party’s responsibilities. One misconception is that truss design engineers design an entire roof system, and they design the roof truss system to work within the framework of the building as set forth in the building structural plans. This is not true! Truss design engineers design single components, not systems. They do not even see the building Designer’s plans. It is the CM that receives the construction documents to obtain the truss design criteria and requirements. They in turn communicate the truss design parameters to the truss design engineer.

It is also important to understand the role the truss placement diagram plays in the overall process. This is a commonly misunderstood document, because many people think this is an engineered document prepared by the truss design engineer. Not only do truss design engineers not prepare these documents, they do not even review them. Instead, the CM creates the truss placement diagram after reviewing the construction documents. A truss placement diagram is only intended to identify the assumed location for each truss and serve as a truss installation aid. In fact, the truss industry intentionally changed the description of this document at one point from “truss placement plan” to “truss placement diagram” because the word “plan” has a connotation associated with engineering and design, and a truss placement diagram does not involve either of those.

Understanding how the truss industry operates and the responsibilities each party has in the design process will prevent incorrect assumptions from being made. If you find yourself saying, “I’m sure the truss design engineer will check this and take care of it…,” you probably need to think again.

The clearer the specifications, the better

This may seem obvious, but do the construction documents contain all of the information necessary for the preparation of the truss design drawings? ANSI/TPI 1 specifies the required information in the construction documents (see text box below). Anything less than this information will mean assumptions need to be made during the creation of the truss placement diagram and the design of the trusses. The building Designer can also specify additional requirements to help ensure that the trusses they get are the trusses they want. In short, the more complete, accurate and detailed the construction documents are, the better the design process will be.

Required information in the construction documents (excerpted from ANSI/TPI 1-2014)

Review, review, review

It is the building Designer’s responsibility to review (and approve or reject) both the truss placement diagram and the individual truss design drawings for conformance with the overall building design. As mentioned earlier, the truss design engineer is solely responsible for the design of the individual trusses; they do not review the entire truss system and they do not check to see that the truss placement diagram meets the intent of the framing plan and specifications. Further, the component manufacturer relies on the completeness and accuracy of the information in the construction documents to create the truss placement diagram and to communicate the truss design requirements to the truss design engineer. Therefore, the success of the building’s construction depends on the building Designer’s thorough review of the truss submittal package.

Collaboration is key

Building Designers know what is best for the overall building; component manufacturers know what is best for the trusses. The best-case scenario is when the building gets the best of both. This requires collaboration during the planning stages, so that the strengths of both parties can be utilized to their maximum potential. A building Designer who finds out from the CM what the best setback distance is for a particular hip roof will not only minimize changes to the design downstream, but will also likely end up with a lower-cost building. On the other hand, a building Designer who provides some input into the actual truss designs, such as strategically placed webs in a string of trusses, will end up with a more efficient, permanent bracing plan that can even be included in the original construction documents.

Some may think the answer to this design process challenge is to let the building Designers design the trusses themselves; that all they need is the truss design software and they can design the trusses as part of their building design. But the truss industry knows that truss design software cannot replace the ingenuity and creativity of talented CMs who have a passion for what they do; just the same way that engineering software cannot ever replace an experienced engineer. That is why the truss industry continues to work hard to make it easier to collaborate, in lieu of providing truss design software to people outside of the component manufacturing industry. In this way, everyone’s skill sets can be fully utilized.

What do you think about this challenge? Let us know your thoughts in the comments below!

Strong-Wall Bracing Selector: Bridging the Gap between Engineered Design and Prescriptive Construction

Asking a structural engineer to design wall bracing under the IRC^® can be like asking a French pastry chef to bake a cake using Betty Crocker’s Cookbook. The temptation is to toss out the prescriptive IRC recipe and design the house using ASCE 7 loads and the AWC SDPWS shear wall provisions per the IBC^®. But if only a portion of the house needs to be engineered, there may be an easier option.

The prescriptive IRC states an IBC engineered design “is permitted for all buildings and structures and parts thereof” but the design must be “compatible with the performance of the conventional framed system.” But how exactly does an IRC braced wall panel perform? The code doesn’t come right out and tell us, but there are two bracing methods that are essentially shear walls masquerading as braced wall panels: Method ABW and Method BV-WSP. Backing into their allowable loads gives us the key to determining equivalence and eliminates the need to develop lateral forces.

But before you can bust out the slide rule and start crunching numbers, you need to figure out how much bracing the prescriptive code requires. We developed our Wall-Bracing-Length Calculator in 2010 to help designers do just that. And last month, we launched our Strong-Wall® Bracing Selector tool to make it easier to specify equivalent solutions for tricky situations.

You can export the required lengths (and project information) from the Wall-Bracing-Length Calculator directly into the Strong-Wall Bracing Selector or you can manually enter in the required lengths. The selector app will provide a list of Strong-Wall panels that have an equivalent length, evaluate their anchorage loads and return a list of pre-engineered anchor solutions for a variety of foundation types.

If you’re familiar with our Strong-Wall Prescriptive Design Guide (T-SWPDG10), the selector automates this 84-page document in just a few steps. One big upgrade is the ability to select a solution to meet the exact amount of bracing that is required. If you needed 2.8-ft. of wall bracing, you have to round up to the tabulated 4-ft. solutions if you are using the guide, but now you can select a wall solution that is equivalent to 2.8-ft., which might mean a smaller wall width or better anchor options. You also have the ability to save the selector file for later modifications, create a PDF of the job-specific output, or email the PDF directly from the program.

So next time you get asked to “design” some wall bracing, see if our Wall-Bracing-Length Calculator and Strong-Wall Bracing Selector might save you some time. There is a tutorial and a design example on the Bracing Selector web page, but it’s very easy to use so you may just want to dive right in. I should also point out that the Strong-Wall SB panels have not yet been implemented into the program, but bracing information for them is available on strongtie.com in posted letters for wind (L-L-SWSBWBRCE14), seismic (L-L-SWSBSBRCE14), and seismic with masonry veneer (L-L-SWSBVBRCE14).

Let us know what you think of this new tool in the comments below.

Deck Fasteners – Deck Board to Framing Attachments

When you’re building a deck, it’s important to know the types of fasteners you need to use with the various materials that are available. On this week’s post we explore some deck fastener applications as well as offer suggestions on how to avoid a few common problems. We will address two generic types of deck boards fastened to wood framing: preservative-treated wood and composite decking.

Preservative-Treated Wood Decking

With preservative treated wood, it pays to know the board treatment. Wood is treated with a water-borne treatment chemical (typically micronized copper azole these days) and then it is either sent out wet or it is kiln dried. Wet-treated wood can have a moisture content (MC) greater than 30%. Wood that is subsequently kiln dried to remove excess moisture after the treatment process is labeled Kiln-Dried After Treatment (KDAT) and has a MC of about 15%. Wood deck boards with preservative treatments will be labeled as such regardless of their moisture condition.

The moisture condition of the deck boards determines how best to fasten and space your deck boards. Wet wood will shrink in width and thickness after installation. As a result, you should install these boards butted tight so that gaps will emerge after they dry in place. On the other hand, KDAT wood or wood that is dry should be installed with 1/8” gaps between boards so there is a slight gap after the boards get wet and swell due to rain, ice and snow. Some manufacturers suggest using an 8d common nail for spacing when installing KDAT decking, as seen in the figure below.

Shrinking and swelling of any installed deck board can cause the deck fastener to bend back and forth with the MC cycling. This causes many deck fasteners to break because of the fatigue loading, which can be exacerbated by the brittle steel used in most deck screws.

To combat this problem, Simpson Strong-Tie developed the DSV Wood screw. This screw is specifically designed with increased ductility to handle the bending induced by deck board movement. It is available in a variety of lengths with threads optimized to prevent jacking between the deck board and the framing, ensuring a snug long-lasting connection.

Composite deck boards are made from a mixture of wood fiber and plastic or are entirely “plastic.” Wood plastic composites and plastic materials exhibit thermal expansion, so they expand and contract in thickness, width and length as a function of temperature and solar heating. Consequently, they typically require a special screw designed for composite decking. Screws for this application will often utilize a two-thread design. The lower thread drives the screw into the framing while the upper thread pulls the loosened composite material back into the hole and holds the deck board tight to the joist. Composite screws also have a cap-style head that covers any residual material left around the screw body and leaves a clean finish. Ductility is important to these screws too.

Given the wide variety of composite deck producers, we designed a screw that works well with all of them. The DCU screw works in all types of composite decking fastened to wood framing.

A note about cellular PVC deck boards: the manufacturers’ recommendation of stainless-steel screws restricts the use of many deck board fasteners. Be sure to read and follow the decking material fastener requirements. Simpson Strong-Tie has a broad offering of painted stainless-steel deck screws available to match PVC deck boards. Find the proper match for your board here.

For other deck fastener applications, including decking fastened to steel framing, and information about other deck fasteners available, see our website product page.

Are there other applications that you want to know about that we didn’t share here? Let us know in the comments below. As always, call us in the Engineering Department if you have questions.