- Page 26 of 50 - News, Notes and Discussion from the Simpson Strong-Tie Engineering Department

Cold-Formed Steel Curtain-Wall Systems

In August 2012, Simpson Strong-Tie launched a comprehensive, innovative solution for curtain-wall framing. Our lead engineer for developing our line of connectors for curtain-wall construction explains the purpose of the curtain wall with the illustrations below.

First, curtain walls are not what you put up if you shared a room with your brother and sister when you were growing up. When I first learned about the use of cold-formed steel curtain walls, I laughed and thought: “Gosh, how useful this would be for someone growing up with 5 siblings in one bedroom!” I have always enjoyed the sense of humor that our engineers use to help explain technical topics.

Curtain walls can be described as exterior building walls with the primary purpose of protecting the interior building against the exterior weather and natural phenomena such as sun exposure, temperature changes, earthquakes, rain and wind.

To put it in structural terms, a curtain-wall system consists of non-load-bearing exterior walls that must still carry their own weight. Curtain walls are not part of the primary structural framing for the building, but they typically rely on the primary structural framing for support. Additionally, curtain walls receive wind and seismic loads and transfer these forces to the primary building structure.

Types of Curtain Walls

Glass and cladding curtain walls make up two basic types of curtain-wall systems. Glass curtain-wall systems are usually designed using aluminum-framed walls with in-fills of glass. The cladding curtain wall is a system with back-up framing that is covered in some type of cladding material. The cladding curtain-wall system is the type in which Simpson Strong-Tie products can be used.

The back-up framing is the structural element of the curtain-wall system. It is typically constructed with cold-formed steel studs ranging from 31/2″ to 8″ deep, in 33 mil (20 ga.) to 97 mil (12 ga.) steel thicknesses. The framing studs are typically spaced at 16″or 24″ on center. There are many different types of cladding materials. They include, but are not limited to, exterior insulation finish systems (EIFS), glass-fiber-reinforced concrete (GFRC), bricks, metal panels and stone panels.

Deflection

One essential function of the curtain wall is to allow for relative movement between the curtain-wall system and the main building structure. At first, it was not obvious to me why making this allowance was necessary, but our product development team creatively explained some of the reasons why this is an important must-have feature for curtain walls.

First, the primary building will move up and down as it is loaded and unloaded by the live-load occupancy, similar to beam live-load deflections.

Second, the structure sways and has torsional displacement due to movement from lateral wind or seismic loads.

Third, concrete structures typically encounter creep and shrinkage, and there may be foundation differential settlement or soil compression from high-gravity loads.

Lastly, the temperature differential may cause the building elements to expand and contract, which, again, can result in relative movement between structural elements. This is similar to a bridge’s steel plate expansion joint system.

And if you are a curious designer like me, you probably wonder why the relative vertical moment is so significant in engineering design.

One key reason is to ensure that the curtain walls do not collect gravity loads from the building, so as to prevent overloading and possible failure of the stud framing. In addition, a well-designed curtain-wall system needs to retain the primary structural load path as intended by the building designer.

The other reason is to protect the cladding of the building. If you remember earlier, the cladding material may be marble, granite or natural stones that are often very expensive and heavy. In some cases, the cladding can be one of the most expensive systems in a building. And there are times when it’s much more cost-effective to design for relative movement than it is to over-design structural framing to address the stringent deflection requirements.

Construction Type

Bypass framing is a term that is often used in curtain-wall construction. In this system, the metal studs bypass the floor and hang off the outside edges of the building. You can see from the illustration how the studs run past, or bypass, the edge of the slab. In this case, the studs are supported vertically on the foundation at the bottom, and then run continuously past multiple floor levels.

Picture by Don Allen of Super Stud Building Products.

In steel construction, concrete fill over metal deck is typically constructed with a heavy-gauge bent plate or structural angle. Connectors can attach directly to the steel angle or the web of an edge beam.

Simpson Strong-Tie SCB Bypass slide clip connections.

SSB Bypass Framing Slide-Clip Strut connections.

It may seem that this type of construction is too complex and requires great efforts to detail the many connections needed to hang the curtain wall off the outside of the building. So what are the compelling reasons to choose bypass framing construction?

Bypass framing can accommodate flexibility for the architect. In another words, the bypass configuration easily allows architects to create reveals, set-backs and other architectural features. Plus, there are fewer joints to detail for movement when stud length can run continuously for several floors. Another benefit is that the exterior finish can also be installed on a curtain-wall system with a tighter tolerance than the edge of the structure.

One other special bypass framing type is known as ribbon window or spandrel framing. Ribbon windows are a series of windows set side by side to form a continuous band horizontally across a façade. The vertical deflection for this type of bypass framing is typically accommodated at the window head. This type of bypass usually works well for panelized construction.

Another common curtain-wall system is infill framing, where the studs run from the top of one floor to the underside of the floor above. Sometimes it’s a challenge to attach bypass framing to the edge of thin concrete slabs. In the following illustration, deflection is designed at the top track of wall panels.

In Part 2 of this blog post series, I will provide more details about how we have innovated products to be used for this application, plus a more comprehensive post about the products we offer and how they are typically used.

In the meantime, you can check out our product offering. Our recent SC slide-clip and FC fixed-clip connectors are designed for high-seismic areas.

I would like to invite you to comment and provide feedback on this topic and tell us whether you’ve had any experience working with a Designer on a CFS curtain-wall project. If you are a Designer who specializes in this discipline, how are you designing curtain-wall systems for seismic forces?

How to Pick a Connector Series – Truss Hangers

In our second blog in the “How to Pick a Connector Series,” Randy Shackelford discussed the various considerations involved in selecting a joist hanger. So why is this blog post about truss hangers? A hanger is a hanger, right? Before I moved into the Engineering Department at Simpson Strong-Tie, I was the product manager for our Plated Truss product line. I can assure you that there is a bit more that goes into the selection (and design) of a truss hanger than does into selecting a joist hanger!

Of course, all of the considerations that were covered in the joist hanger blog apply to truss hangers as well. This blog post is going to discuss some additional considerations that come into play in selecting a hanger for a truss rather than a joist, and how some hangers have features designed especially for trusses.

The first (and most obvious) truss-specific consideration is the presence of webs. Because of truss webs, top-flange hangers are not as conducive to truss applications as they are to joist applications. A better alternative for trusses is an adjustable-strap hanger that can be installed as a top-flange hanger or face-mount hanger. Take the THA29, for example, Simpson’s first hanger developed specifically for the truss industry (circa 1984). It can accommodate different girder bottom chord depths, which eliminates the need for multiple SKUs, and the straps can be field-formed over the top of the girder bottom chord to reduce the number of fasteners (just like top-flange hangers). When a web member is in the way of the top-flange installation method, the straps can be attached vertically to the web in a face-mount installation instead.

What if the web at that location isn’t vertical? You can still install the strap onto the web, but if any nails land in the joint lines formed by the intersection of the wood members, they cannot be considered effective. Therefore, the hanger allowable load may need to be reduced to account for ineffective header nails. This alternative installation is acceptable for any face-mount hanger located at a panel point as shown in our catalog (see detail below).

Although very versatile, not all adjustable-strap hangers can be installed on all sizes of bottom chords. Our catalog specifies a C-dimension for these hangers, which corresponds to the height of the side-nailing flanges. If that dimension exceeds the height of the bottom chord, then the straps cannot be field-formed as needed for the top-flange installation. And if the hanger isn’t located at a panel point, nailing the straps to any diagonal web that the straps can reach (see photo below) is not an acceptable option!

The wrong hanger selection for the application

Another unique consideration that goes into the selection of a truss hanger is the heel height of the carried truss. A truss with a short heel height installed into a tall hanger will likely leave air (or “daylight,” as I call it) behind a lot of the nail holes running up the side flanges. When nail holes in a hanger have air behind them instead of wood, this equates to a reduction in hanger capacity. So when the carried truss has a heel height that is much less than the depth of the carrying member (and the hanger), it is important to use the appropriate hanger capacity for that condition and not overestimate the hanger’s capacity. Refer to our technical bulletin T-REDHEEL for allowable loads for reduced heel height conditions.

Example of a short heel installed in a tall hanger.

Because trusses are capable of carrying a lot of load – and producing large reactions – hangers for truss applications often require larger capacities than joist hangers. Unfortunately, there is only so much capacity that can be achieved from a hanger that fits entirely onto a girder truss bottom chord. Therefore, in order to use our highest load-rated truss hangers, a properly located vertical web is required, and the web must be wide enough for the hanger’s required face fasteners and minimum edge distances. The more capacity that is required, the more fasteners it takes, and the wider the vertical web must be. Our highest-load-rated truss hanger that installs with screws is the HTHGQ. It has a maximum download capacity of 20,735 lb., but it requires a minimum 2×10 vertical web. The THGQ/THGQH series can be installed onto as small as a 2×6 web, but the maximum possible capacity on a 2×6 web is 9,140 lb.

In addition to high-capacity hangers, truss applications often require high-capacity skewed hangers. When selecting skewed hangers, it’s important to realize that hangers with custom skew options usually have a reduction that must be applied to the hanger’s 90-degree capacity. Another important factor that is sometimes overlooked in the selection of skewed hangers is whether the carried member is square-cut or bevel-cut. When the member is square cut – as in the case of trusses – not only does this typically result in a greater reduction in capacity, but some skewed hangers cannot be used at all with square-cut members. For example, the fastener holes on the side flange may not be located far enough away from the header to accommodate square-cut members. See the photo below for an example of what can happen if a skewed hanger that is intended for a bevel-cut member is used for a truss.

Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.

Not all skewed hangers can be used with square-cut members (trusses).

As discussed in the previous hanger blog, face-mount hangers offer the advantage of being installed after the joist (or truss) is installed. What if the truss is installed prior to the hanger and a gap exists between the truss and the carrying member? In that case, the best option may be to select a truss hanger that was designed with this type of installation tolerance in mind, the HTU hanger. Other face-mount truss hangers that use double-shear nailing are great when gaps are limited to ⅛” or less, but their capacities take a pretty large hit when the gap exceeds ⅛” (see our previous blog Minding the Gap in Hangers for more information). The HTU was designed to give an allowable load for up to a ½” gap between the end of the truss and the carrying member. In addition, it has built-in nailing options to accommodate short heel heights even in the taller models – definitely a truss hanger!

Finally, there is one more thing to consider when selecting a face-mount hanger for a truss application, which relates to how tall the carrying member is compared to the hanger. Assuming the bottom of the hanger will be installed flush with the bottom of the girder bottom chord, a hanger that is much shorter than the bottom chord will induce tension perpendicular to the grain in the chord. Due to wood’s inherent weakness in perpendicular-to-grain tension, a hanger that is too short may limit the amount of load that can be transferred– to something less than the hanger’s published allowable load. Therefore, it isn’t enough to check whether the hanger fits on the bottom chord; the hanger must also cover enough depth of the chord to effectively transfer the load (or else the allowable hanger load may need to be reduced to the member’s allowable cross-grain tension limit).

Cross-grain tension is not a truss-specific issue, but because it is an explicit design provision in the truss design standard (TPI 1), it is a necessary consideration to mention in a discussion about truss hanger selection. In fact, proper detailing for cross-grain tension in different wood applications could be a future topic in and of itself.

Add to all this the specialty truss hangers that can carry two, three, four, and even five trusses framing into one location, and it is no wonder that there is an entire section in our catalog that is dedicated to truss hangers. Are there any other truss hanger needs that you would like to discuss? Please let us know in the comments below!

Great ShakeOut Earthquake Drill 2016

The Great ShakeOut Earthquake Drill is an annual opportunity for people in homes, schools and organizations to practice what to do during earthquakes and improve their preparedness. In a post I wrote last October about the Great ShakeOut, I reminisced about the first earthquake I had to stop, drop and cover for – the Livermore earthquake in January, 1980. This year got me thinking about how our evacuation drills work.

At Simpson Strong-Tie, we use the annual Great ShakeOut drill to practice our building evacuation procedures. Evacuation drills are simple in concept – alarms go off and you exit the building. We have volunteer safety wardens in different departments who confirm that everyone actually leaves their offices. There are always a few people who want to stay inside and finish up a blog post. Once the building is empty and we have all met up in the designated meeting area, we do a roll call and wait for the all-clear to get back to work.

Several years ago the alarms went off. While waiting for the drill to end, we were concerned to see fire fighters arrive and rush into the building. Realizing this was not a drill, there were some tense moments of waiting. The fire chief and our president eventually walked out of the building and our president was yelling for one of our engineers. Turns out the engineer (who shall remain nameless) was cooking a chicken for lunch. Yes, a whole chicken. The chicken didn’t make it – I’m not sure what the guilty engineer had for lunch afterwards. At least we received extra evacuation practice that year. We aren’t allowed to cook whole chickens in the kitchen anymore.

Simpson Strong-Tie is helping increase awareness about earthquake safety and encouraging our customers to participate in the Great ShakeOut, which takes place next Thursday on October 20. It’s the largest earthquake drill in the world. More than 43 million people around the world have already registered on the site.

On October 20, from noon to 2:00 p.m. (PST), earthquake preparedness experts from the Washington Emergency Management Division and FEMA will join scientists with the Washington Department of Natural Resources and the Pacific Northwest Seismic Network for a Reddit Ask Me Anything – an online Q&A. Our very own Emory Montague will be answering questions. The public is invited to ask questions here. (Just remember that this thread opens the day before the event and not sooner.)

Emory Montague from Simpson Strong-Tie — Emory, ready to answer some seismic-related questions.

We’re also providing resources on how to retrofit homes and buildings, and have information for engineers here and for homeowners here.

Earthquake risk is not just a California issue. According to the USGS, structures in 42 of 50 states are at risk for seismic damage. As many of you know, we have done a considerable amount of earthquake research, and are committed to helping our customers build safer, stronger homes and buildings. We continue to conduct extensive testing at our state-of-the-art Tye Gilb lab in Stockton, California. We have also worked with the City of San Francisco to offer education and retrofit solutions to address their mandatory soft-story building retrofit ordinance and have created a section on our website to give building owners and engineers information to help them meet the requirements of the ordinance.

Last year, Tim Kaucher, our Southwestern regional Engineering Manager, wrote about the City of Los Angeles’s Seismic Safety Plan in this post. Since that time, the City of Los Angeles has put that plan into action by adopting mandatory retrofit ordinances for both soft-story buildings and non-ductile concrete buildings. Fortunately, California has not had a damaging earthquake for some time now. As a structural engineer, I find it encouraging to see government policy makers resist complacency and enact laws to promote public safety.

Participating in the Great ShakeOut Earthquake Drill is a small thing we can all do to make ourselves more prepared for an earthquake. If your office hasn’t signed up for the Great ShakeOut Earthquake Drill, we encourage you to visit shakeout.org and do so now.

Can Decorative Hardware Add Structural Strength?

At Simpson Strong-Tie, we really try to listen to our customers. Our products are developed with your needs in mind.

Last year, at my daughter’s college orientation, I found myself in an interesting conversation with one of the other parents. It turned out that he owns a deck-building company. When he found out that I’m an engineer at Simpson Strong-Tie, his first question was “why don’t you guys make some nice-looking connections that I can use on my decks?”

I had to choke back a laugh because that’s exactly what I was working on at the time. What he didn’t mention (but we knew he also needed) were connectors that are fast to install, suitable for outdoor use and structurally rated for engineered designs. We also knew code approval was critical to help building departments approve the designs.

The Outdoor Accents® connectors we designed include some basic T’s, L’s, angles and post bases with a nice architectural feature of decorative edges from our Mission Collection®. The steel has our ZMAX® (G185) galvanizing (which is twice as heavy as our standard G90) to resist corrosion and a black powder-coat finish for aesthetics.

But the real innovation is in the fastener. Architectural connectors and big bolts go hand in hand, but big bolts are expensive, time consuming and often structurally unnecessary. To solve the installation issue, we designed a decorative washer that looks like a washer and nut and perfectly fits our SDWS22DBB Structural Wood screw.

We named it the shear tube nut (STN) because the extended tube increases the shear area in contact with the connector.

Together with the SDWS22DBB screw, this solution looks like a bolted connection but installs with the speed and ease of a self-tapping screw. Structurally as well, the hardware is comparable to a bolted connection with a shear capacity of 470 lb. per fastener when used with metal side plates, i.e., connectors.¹ The solution has also been tested and load rated for use directly on wood, so it can be used for a variety of other connections such as joining multi-ply beams, knee braces, etc.

In order to be code approved, the SDWS22DBB screws were tested with and without the STN in both wood-to-wood and metal-to-wood per AC233 Acceptance Criteria for Alternate Dowel-Type Threaded Fasteners. The connectors and fasteners, including STN, were tested as assemblies per ASTM D7147. Code agency reviewers quickly saw the benefits of the design and issued evaluation reports verifying the loads. The Outdoor Accents® connectors and SDWS22DBB screws are recognized under IAPMO UES ER-280 and ER-192, respectively. The smaller APA21 angle uses our new SD10112DBB screw, which is listed in ICC-ES ESR-3046.

My deck-builder friend will be pleased to see the new connectors are now available at select Home Depot stores.

I can’t wait to see what he thinks of them and to get his ideas for the next big project. How about you? What would you build with these new architectural products? Let us know in the comments below.

Ref. IAPMO UES ER-192 Table 6A steel side member DF = 470 lb.; 2015 NDS Table 12B 3 1/2″ main member, 1/2″ bolt, DF perpendicular-to-grain = 510.

Outdoor Accents®

Add Beauty and Strength to Your Custom Outdoor Living Structures.

Designing Gable End Overhangs

It seems that each major hurricane tends to teach those of us in the construction industry some lesson. With Hurricane Andrew, the lessons were the importance of protection from windborne debris, and the importance of proper construction of gable end overhangs.

There are two main areas where gable ends can fail.

Being an Engineering Intern at Simpson Strong-Tie

Editor’s Note: This week’s blog post is written by one our college interns in the Engineering Department. Ian Kennedy spent the summer of 2016 as an intern for the McKinney office of Simpson Strong-Tie. He will be starting his second year at Calpoly San Luis Obispo in Fall 2016 studying Mechanical Engineering. As an intern, he spent his time helping the branch engineering department with numerous projects, as well as exploring projects of his own. He enjoys metalworking, fitness, and the outdoors. Thank you to Ian Kennedy for this week’s post.

As I write this, I can’t help but laugh that of all the interns studying structural, civil or architectural engineering in school, the intern writing the post for our Structural Engineering Blog is studying mechanical engineering. I haven’t met too many mechanical engineers during my time here at Simpson Strong-Tie. I know there are a few, but while a lot of mechanical engineers are focused on making things move, most of the people here concentrate primarily on making things stay still. I’ve found what Simpson does to be more important than a lot of my peers at school may realize – it seems ME students are more preoccupied with cars and equipment than with what’s keeping the roof from coming down on top of them. Still, my exigence alone wasn’t enough to cancel the uneasiness of a first-time intern doing things he never knew he would be doing.Continue Reading

Bucket Lists for Structural Engineers and Some Resources for Helping Cross Post-Frame off Your List

Bucket lists are mentioned regularly today, which got me to thinking – what about a bucket list for structural engineers? ASCE and others have put together lists of engineering wonders of the modern world, so those seem like a good start for sights to see. But for a practitioner, I’d propose the next most obvious things to add would be working with each of the common structural building materials and system types. For engineers working with buildings, the “list” would include the various types of steel, concrete, wood and masonry materials, and then the different respective building systems.

Maybe this list can also offer a refreshing perspective when you’re wading into uncharted territory; a new material or system presents the chance to cross another item off your list! For most engineers, I would guess a post-frame building will be one of the final remaining items on their list. Post-frame is rightly known for its historical origins in agricultural buildings; however, today there is more developed design information, and post-frame buildings are being built for many different uses. If you do find yourself looking at post-frame for the first time, there are a few resources to be aware of that can help guide and inform your experience.

Post-frame buildings comprise a primary framing system of wood roof trusses or rafters that are supported by large solid-sawn or laminated lumber columns. The secondary roof purlins and wall girts support the roof and wall sheathing. The columns are either embedded into the ground or anchored to concrete piers, walls or slabs. The buildings offer efficiency in materials, construction time and costs, and energy. An engineer can design a post-frame building in compliance with the IBC, with allowances for high-wind and seismic conditions.

Two free resources that are good starting points for an engineer considering post-frame are the American Wood Council’s Design for Code Acceptance (DCA5) – Post Frame Buildings, and the Post-Frame Construction Guide by the National Frame Building Association (NFBA). The DCA5 gives a brief overview of the pertinent section of the IBC that relates to post-frame. The Post-Frame Construction Guide is a 20-page document that describes the components of a post-frame system, fire performance, examples of common details and different building uses, and a summary of resources for additional information.

A manual for purchase that is an excellent resource is the NFBA’s Post-Frame Building Design Manual – Second Edition. The manual presents a comprehensive scope of content including sections on code provisions, guidance for design, diaphragm design, post design and foundation design. Lesser-known IBC-referenced standards that are commonly utilized in post-frame, such as ASABE EP 484.2 for diaphragm design and ASABE EP 486.1 for shallow post foundation design, are covered by the manual.

What do you think of the idea of a bucket list for structural engineers? Would you already be able to cross off post-frame building from your list? Let us know by posting a comment.

How to Pick a Connector Series – Selecting Fasteners

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

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

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

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

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

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

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

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

Figure 3. Corrosion resistance recommendations.

NAILS

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

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

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

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

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

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

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

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

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

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

SD Screws

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.

SDS Screws

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

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

Table 5. SDS Heavy-Duty Connector Screws.

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

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

Concrete Anchorage for ASD Designs

One of the first things I learned in school about using load combinations was that you had to pick either Load and Resistance Factor Design (LRFD)/Strength Design (SD) or Allowable Stress Design (ASD) for a building and stick with it, no mixing allowed! This worked for the most part since many material design standards were available in a dual format. So even though I may prefer to use LRFD for steel and ASD for wood, when a steel beam was needed at the bottom of a wood-framed building that was designed using ASD load combinations, the steel beam could easily be designed using the ASD loads that were already calculated for the wood framing above since AISC 360 is a dual- format material standard. And when the wood-framed building had to anchor to concrete, ASD anchor values were available in the IBC for cast-in-place anchors and from manufacturers for post-installed anchors in easy-to-use tables, even though ACI 318 was not a dual-format material standard. (Those were good times!)

Then along came ACI 318-02 and its introduction of Appendix D – Anchoring to Concrete, which requires the use of Strength Design. The 2003 IBC referenced Appendix D for Strength Design anchorage, but it also provided a table of ASD values for some cast-in-place headed anchors that did not resist earthquake loads or effects. This option to use ASD anchors for limited cases remained in the 2006, 2009 and 2012 codes. In the 2015 IBC, all references to the ASD anchor values have been removed, closing the book on the old way of designing anchors.

ICC-ES-equation-tension So what do you do now? Well, there is some guidance provided by ICC-ES for manufacturers to convert calculated SD capacities to ASD allowable load values. Since there is no conversion procedure stated in the IBC or referenced standards, designers may want to use this generally accepted method for converting anchor capacities designed using ACI 318. ICC-ES acceptance criteria for post-installed mechanical and adhesive anchors (AC193 and AC308) and cast-in-place steel connectors and proprietary bolts (AC398 and AC399) outline a procedure to convert LRFD capacities to ASD using a weighted average for the governing LRFD/SD load combination. So if the governing load combination for this anchor was 1.2D + 1.6L and the dead load was 1,000 pounds and the live load was 4,000, then the conversion factor would be (1.2)(0.2) + (1.6)(0.8) = 1.52 (keep in mind that the LRFD/SD capacity is divided by the conversion factor in the ICC-ES equation shown here for tension).

Right away, there are a few things that you may be thinking:

What about load factors that may exist in ASD load combinations?
It may just be easier to just recalculate my design loads using LRFD/SD combinations!
The resulting allowable loads will vary based on the load type, or combination thereof.
If the ACI 318 design strength is limited by the steel anchor, then the conversion will result in an allowable load that is different from the allowable load listed for the steel element in AISC 360.

Let’s take a look at these objections one by one.

Item 1: Since unfactored earthquake loads are determined at the ultimate level in the IBC, they have an LRFD/SD load factor of 1.0 and an ASD load factor less than 1.0, which is also true for wind loads in the 2012 and 2015 IBC (see graphic below). Using the LRFD/SD load factor of 1.0 obviously does not convert the capacity from LRFD to ASD so you must also account for ASD load factors when calculating the conversion factor. To do so, instead of just using the LRFD load factor, use the ratio of LRFD Factor over ASD Factor. So if the governing load combination for an anchor was 0.9D + 1.0E and the dead load was 1,000 pounds and the seismic load was 4,000, then the conversion factor would be (0.9)(0.2) + (1.0/0.7)(0.8) = 1.32.

Item 2: Even though the weighted average conversion requires you to go back and dissect the demand load into its various load types, often this can be simplified. ICC-ES acceptance criteria permit you to conservatively use the largest load factor. The most common application I run into is working with ASD-level tension loads for wood shearwall overturning that must be evaluated using SD-level capacities for the concrete anchorage. Since these loads almost always consist of wind or seismic loads, using the largest factor is not overly conservative. Depending on the direction in which you are converting the demand loads or resistance capacities, the adjustment factors are as shown in the figure below. Affected Simpson Strong-Tie products now have different allowable load tables for each load type. (For examples, see pp. 33-36 of our Wood Construction Connectors catalog for wind/seismic tables and pp. 28-30 of our Anchoring and Fastening Systems catalog for static/wind/seismic tables.)

Item 3: I am unsure whether there is any sound rationale for having allowable loads for an anchor resisting 10% dead load and 90% live load differ from those of an anchor that resists 20% dead load and 80% live load. Perhaps a reader could share some insight, but I just accept it as an expedience for constructing an ASD conversion method for a material design standard that was developed for SD methodology only.

Item 4: We have differing opinions within our engineering department on how to handle the steel strength component of the various SD failure modes listed in ACI 318. Some believe all SD failure modes in ACI 318 should be converted using the load factor conversion method. I side with others who believe that the ASD capacity of a steel element should be determined using AISC 360. So when converting SD anchor tension values for a headed anchor, I would apply the conversion factor to the concrete breakout and pullout failure modes from ACI 318, but use the ASD steel strength from AISC 360.

Finally, I wanted to point out that the seismic provisions in ACI 318, such as ductility and stretch length, must be considered when designing anchors and are not always apparent when simply converting to ASD. For this reason, I usually suggest converting ASD demand loads to SD levels so you can use our Anchor Designer™ software to check all of the ACI 318 provisions. But for some quick references, we now publish tabulated ASD values for our code-listed mechanical and adhesive anchors in our C-A-2016 catalog — just be sure to read all of the footnotes!

A Tale of Two Houses: Design Loads for Metal Plate Connected Wood Trusses

Take two trusses with identical profiles and environmental surroundings, and they should have the same design loads, right? Early in my career, I recall hearing a story about two identical buildings right next to each other that were designed for two different magnitudes of environmental loads. I remember wondering – how do the loads know which building is which?

There used to be a time when it was not uncommon for 5 substantially different wood truss designs to come from 5 different companies – all designing to the exact same spec. Whereas some differences are always to be expected (manufacturer-specific plate design values and proprietary analogues come to mind), the truss design disparities that used to exist from one company to the next were compounded by variations in something which really shouldn’t vary at all – the application of the specified loads to the truss. Differences in loading can occur whenever there is room for interpretation. In cases where the loading specs for fabricated wood trusses are not very detailed, there is a lot of room for interpretation. And when that happens, everyone knows how many different answers you get when you ask 5 different engineers!

Fortunately, the truss industry has come a long way in this area. In some cases, the codes and standards that govern the loading of structures have improved and helped the cause. But the truss industry also made a concerted effort to minimize these loading differences. Everyone agreed that a truss bid shouldn’t be won based on “less loading,” so they set out to change that. One of the best efforts in accomplishing this was the development of the SBCA Load Guide entitled “Guide to Good Practice for Specifying & Applying Loads to Structural Building Components.” Produced by the Structural Building Component Association (SBCA) in cooperation with the Truss Plate Institute (TPI), the Load Guide was developed with the stated goal of “helping everyone that uses it to more easily understand, define and specify all the loads that should be applied to the design of each structural building component” and “to help assure that all trusses will be designed using a consistent interpretation and application of the code.”

If you are an architect, engineer or a Building Code Official who deals with trusses and you don’t already have the current SBCA Load Guide, I strongly encourage you to check it out (free downloads are available from the SBCA website here.) When fielding questions about loading on trusses, I inevitably refer the inquiring party to the SBCA Load Guide not only for the answer to the question, but for future reference as well. The SBCA Load Guide isn’t just a handy reference to read, it also offers a spreadsheet tool that can be used to calculate loads as well as output the load calculation worksheets. The worksheets can be submitted with the construction documents for plan approval or submitted to the truss manufacturer to be used in the design process.

In addition to providing all of the code and standard loading provisions that apply to metal-plate-connected wood trusses, the SBCA Load Guide also presents the truss industry’s consensus positions and interpretations on provisions that are either unclear as to how they apply to trusses or that have resulted in loading inconsistencies in the past. With the many truss-specific examples and applications covered, it leaves very little room, if any, for further interpretation or question as to how the various code provisions should be applied to trusses.

Take wind loads, for example. Wind loading on trusses has been a heavily debated topic over the years, such as whether a truss should be designed for Components & Cladding (C&C), Main Wind Force Resisting System (MWFRS) or both. In fact, wind loading used to be one of the main sources of inconsistencies in truss designs from one company to another. The truss industry has since established a consensus position on this matter and the SBCA Load Guide presents it as follows:

The SBCA Load Guide also pulls information from a variety of resources to help provide more insight into some of the code provisions. For example, in the wind loading section a graphic is reproduced from a Structural Engineers Association of Washington’s handbook (SEAW RSM-03) to clarify the effect of wind directionality on C&C wind pressures for gable/hip roofs, since this consideration is not made clear in ASCE 7.

Graphic from SEAW RSM-03 As Reprinted in the SBCA Load Guide

This clarification is further illustrated in the example wind loading diagrams, which show how wind pressures are evaluated when taking the directionality of the wind into account, i.e., by evaluating the pressures separately with the wind from the left and from the right.

Example Wind Loading Diagrams in the SBCA Load Guide

Of course, the SBCA Load Guide is only a guide and is NOT intended to supersede a Building Designer’s design specification. As specified in ANSI/TPI 1, the Building Designer is responsible for providing all applicable design loads to be applied to the trusses:

If you are an architect or engineer who specifies detailed loading schedules for truss systems, great! Your specifications may not need the SBCA Load Guide to ensure that the trusses are accurately loaded as intended in the design of the building. But the SBCA Load Guide still provides a lot of insight as to how the truss industry – and anyone who uses the Load Guide – applies various code provisions to trusses. It might even be an interesting study to see how your specified loads compare to the loading examples in the SBCA Load Guide.

For everyone else who isn’t well-versed in the application of code provisions to wood trusses, the SBCA Load Guide is an invaluable tool. Building Designers, building code officials, truss technicians and truss Designers can all benefit from the Load Guide. As stated in the SBCA Load Guide, one of the industry’s goals is to achieve a greater level of consensus among the largest audience possible on how to load trusses and other structural building components. The more people who read and use the SBCA Load Guide, the more consistency there will be in the interpretation and application of code provisions pertaining to wood trusses, which will help make projects run smoother and most importantly, improve building safety. At Simpson Strong-Tie, we are big fans of tools that work to do that.

If you’ve had experience using the SBCA Load Guide, we’d love to hear about it – please let us know in the comments below!