Welcome to our Structural Engineering Blog! I’m Paul McEntee, Engineering R&D Manager at Simpson Strong-Tie. We’ll cover a variety of structural engineering topics here that I hope interest you and help with your projects and work. Social media is “uncharted territory” for a lot of us (me included!), but we here at Simpson Strong-Tie think this is a good way to connect and even start useful discussions among our peers in a way that’s easy to use and doesn’t take up too much of your time. Continue reading
Over the weekend, I had the pleasure of watching my daughter in her cheer competition. I was amazed at all the intricate detail they had to remember and practice. The entire team had to move in sync to create a routine filed with jumps, tumbles, flyers and kicks. This attention to detail reminded me of the new ratcheting take-up device (RTUD) that Simpson Strong-Tie has just developed to accommodate 5/8″ and ¾” diameter rods. The synchronized movement of the internal inserts allows the rod to move smoothly through the device as it ratchets. The new RTUDs are cost effective and allow unlimited movement to mitigate wood shrinkage in a multi-story wood- framed building. When designing such a building, the Designer needs to consider the effect of shrinkage and how to properly mitigate it.
Our SE blog post on Continuous Rod Restraint Systems for Multi-Story Wood Structures explained the importance of load path and the effects of wood shrinkage. This week’s blog post will focus on the importance of mitigating the shrinkage that typically occurs in multi-story light-frame buildings.
Shrinkage is natural in a wood member. As moisture reaches its equilibrium in a built environment, the volume of a wood member decreases. The decrease in moisture causes a wood-framed building to shrink.
The IBC allows construction of light-framed buildings up to 5 and 6 stories in the United States and Canada respectively. Based on the type of floor framing system, the incremental shrinkage can be up to ¼” or more per floor. In a 5-story building, that can add up to 1-¼” or more and possibly double that when construction settlement is included.
The Simpson Strong-Tie Wood Shrinkage Calculator is a perfect tool to determine the total shrinkage your building can experience.
In order to accommodate the shrinkage that occurs in a multi-story wood-framed building, Simpson Strong-Tie offers several shrinkage compensating devices. These devices have been tested per ICC-ES Acceptance Criteria 316 (AC316) and are listed under ICC-ES ESR-2320 (currently being updated for the new RTUD5, RTUD6, and ATUD9-3).
AC316 limits the rod elongation and device displacement to 0.2 inches between restraints in shearwalls. This deflection limit is to be used in calculating the total lateral drift of a light-framed wood shearwall.
The 0.2-inch allowable limit prescribed in AC316 is important to a shearwall’s structural ability to transfer the necessary lateral loads through the structure below to the foundation level. This limit assures that the structural integrity of the nails and sill plates used to transfer the lateral loads through the shearwalls is not compromised during a seismic or wind event. Testing has shown that sill plates can crack when excessive deformation is observed in a shearwalls. Nails have also been observed to pull out during testing. Additional information on this can be found here.
In AC316, 3 types of devices are listed.
- Compression-Controlled Shrinkage Compensating Device (CCSCD): This type of device is controlled by compression loading, where the rod passes uninterrupted through the device. Simpson Strong-Tie has several screw-type take-up devices, such as the Aluminum Take-Up Device (ATUD) and the Steel Take-Up Device (TUD), of this type.
Tension-Controlled Shrinkage Compensating Device (TCSCD): This type of device is controlled by tension loading, where the rod is attached or engaged by the device and allows the rod to ratchet through as the wood shrinks. The Simpson Strong-Tie Ratcheting Take-Up Device (RTUD) is of this type.
- Tension-controlled Shrinkage Compensating Coupling Device (TCSCCD): This type of device is controlled by tension loading that connects rods or anchors together. The Simpson Strong-Tie Coupling Take-Up Device (CTUD) is of this type.
Each device type has unique features that are important in achieving the best performance for different conditions and loads. The following table is a summary of each device.
The most cost-effective Simpson Strong-Tie shrinkage compensation device is the RTUD. This device has the smallest number of components and allows the rod unlimited travel through the device. It is ideal at the top level of a rod system run or where small rod diameters are used. Simpson Strong-Tie RTUDs can now accommodate 5/8″ (RTUD5) and ¾” (RTUD6) diameter rods.
How do you choose the best device for your projects? A Designer will have to consider the following during their design.
Rod Tension (Overturning) Check:
- Rods at each level designed to meet the cumulative overturning tension force per level
- Standard and high-strength steel rods designed not to exceed tensile capacity as defined in AISC specification
- Standard threaded rod based on 36 / 58 ksi (Fy/Fu)
- High-strength Strong-Rod based on 92 / 120 ksi (Fy/Fu
- H150 Strong-Rod based on 130 / 150 ksi (Fy/Fu)
- Rod elongation (see below)
Bearing Plate Check
- Bearing plates designed to transfer incremental overturning force per level into the rod
- Bearing stress on wood member limited in accordance with the NDS to provide proper bearing capacity and limit wood crushing
- Bearing plate thickness has been sized to limit plate bending in order to provide full bearing on wood member
Shrinkage Take-Up Device Check
- Shrinkage take-up device is selected to accommodate estimated wood shrinkage to eliminate gaps in the system load path
- Load capacity of the take-up device compared with incremental overturning force to ensure that load is transferred into rod
- Shrinkage compensation device deflection is included in system displacement
- System deformation is an integral design component impacting the selection of rods, bearing plates and shrinkage take-up devices
- Rod elongation plus take-up device displacement is limited to a maximum of 0.2″ per level or as further limited by the requirements of the engineer or jurisdiction
- Total system deformation reported for use in Δa term (total vertical elongation of wall anchorage system per NDS equation) when calculating shearwall deflection
- Both seating increment (ΔR) and deflection at allowable load (ΔA) are included in the overall system movement. These are listed in the evaluation report ICC-ES ESR-2320 for take-up devices
Optional Compression Post Design
- Compression post design can be performed upon request along with the Strong-Rod System
- Compression post design limited to buckling or bearing perpendicular to grain on wood plate
- Anchorage design tools are available
- Anchorage design information conforms to AC 318 anchorage provisions and Simpson Strong-Tie testing
In order to properly design a continuous rod tie-down system for your shearwall overturning restraint, all of the factors listed above will need to be taken into consideration.
A Designer can also contact Simpson Strong-Tie by going to www.strongtie.com/srs and filling out the online “Contact Us” page to have Simpson Strong-Tie design the continuous rod tie-down system for you. This design service does not cost you a dime. A few items will be required from the Designer in order for Simpson Strong-Tie to create a cost-effective rod run (it is recommended that on the Designer specify these in the construction documents):
- There is a maximum system displacement of 0.2″ per level, which includes rod elongation and shrinkage compensation device deflection. Some jurisdictions may impose a smaller deflection limit.
- Bearing plates and shrinkage compensation devices are required at every level.
- Cumulative and incremental forces must be listed at each level in Allowable Stress Design (ASD) force levels.
- Construction documents must include drawings and calculations proving that design requirements have been met. These drawings and calculations should be submitted to the Designer for review and the Authority Having Jurisdiction for approval.
More information can be obtained from our website at www.strongtie.com/srs, where a new design guide for the U.S., F-L-SRS15, and a new catalog for Canada, C-L-SRSCAN16, are available for download.
Are you an engineer working with California clients whose homes were built before 1979 on a raised foundation?
If you are, these clients may be among the 1.2 million California homeowners eligible for a seismic home retrofit. The state of California has approved the continuation of an initiative known as Earthquake Bolt + Brace (EBB). In its second year, this program plans to make as many as 1,600 grants to selected homeowners, nearly three times the number given the previous year. The EBB grant program provides up to $3,000 to homeowners residing in more than 150 California zip codes. Check to see whether your clients live within one of these communities here.
Simpson Strong-Tie has several different resources to assist you in helping your clients understand how to mitigate seismic risks to houses with raised foundations. The Seismic Retrofit Details sheet provides various ways to retrofit the cripple wall system using prescriptive methodologies, which can be adapted for engineered solutions. The picture below highlights the use of the Simpson Strong-Tie universal foundation plate (UFP) to attach the boltless sill plate of the cripple wall to the concrete stemwall. This simple step can help prevent the house from sliding off its foundation. The picture also reveals plywood sheathing used to reinforce the weak cripple wall system. Additional resources for retrofit can be found here.
To help your clients better understand the impact these simple steps can have in preventing structural damage in an earthquake, click here to watch the story of a Napa business women who had purchased a structure with a raised foundation for her business and retrofitted it just prior to the 2014 M6.0 Napa earthquake, which caused considerable damage to many similar structures.
Let your clients know that the time to apply is very limited if they think they qualify for a retrofit grant. Registration for the 2016 EBB program ends on February 20. To register or learn more about the program, visit www.earthquakebracebolt.com.
When you finish a retrofit for one of your clients, we want to hear how it went. Let us know in the comments below.
The following FRP Design example walks the reader through the typical process for designing an FRP strengthening solution for a concrete T-beam per ACI 440.2R Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures.
One of the most important initial checks for an Engineer of Record is to confirm that the unstrengthened structure can support the load combination shown in Equation 5.5.1 in ACI 562 Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings:
Eq. 5.5.1: (φRn)existing ≥ (1.2SDL + 0.5SLL)new
This check is to prevent a structural failure in case that the strengthening is damaged in an extraordinary event. If the structural element cannot pass this check, then external reinforcement is not recommended.
We have a Design Questionnaire where we ask Engineers of Record for more specific information related to the element to be strengthened:
For this particular example, the following information was provided for the concrete T-beam.
1. Structure Type (e.g., building, bridge, pier, garage):
- 5-story commercial concrete building
2. Element(s) to be Strengthened/Repaired (e.g., beam, column, slab, wall):
- Reinforced concrete beams
3. Type of Deficiency (e.g., shear, flexural, axial):
4. Existing Factored Capacity of Section (e.g., kips, kip-ft):
- 265 kip-ft
5. Ultimate Demand to be Supported (e.g., kips, kip-ft):
- 320 kip-ft
6. Existing Concrete Compressive Strength:
- 4,000 psi
7. Existing Rebar Yield Strength:
- 60 ksi
8. Existing Reinforcement Layout:
- 3 #7s 2.6875 inches from bottom of web to centroid of steel
9. Existing Dimensions:
- 36 inches total beam height, 8 inches slab, 24 inches web width, 120 inches effective slab width
10. Relevant Existing Drawing Sheets and/or Pictures:
- See attached
11. Finish Coating Requirements/Preferences:
12. For Flexural Strengthening:
- Dead Load Moment Applied at Time of Installation
- 60 kip-ft
- Service Dead Load Moment After Installation
- 80 kip-ft
- Service Live Load Moment After Installation
- 140 kip-ft
We then plug this information into our design program to come up with an FRP solution that meets the needs of the member:
For a beam that was at 83% of the capacity required for the new loading, we specified a simple, low-impact FRP solution to maintain clearances under the beams. If a traditional fix of adding cross-section to the beam had been specified instead, then additional concrete and rebar would need to be added to the beam, which would impact clearances under the beam and also increase the seismic weight of the building. The additional weight could also translate all the way through the building and even impact footing designs.
FRP can be used to increase the flexural strength up to 40% per ACI 440.
For your next retrofit project, please contact Simpson Strong-Tie to see if FRP would be an economical choice for strengthening your concrete or masonry element.
In my past life as a Design Engineer, when specifying a screw the size of the screw was the key feature that I considered. In my mind, a #10 screw performed better than #8, and a #12 was better than #10 and all #10 screws were the same. But that is not always true. Just as a shoe size or a dress size may not be exactly the same for all brands, a screw of the same size from different manufacturers may perform differently. The head type, head design, thread design (fine, coarse, thread angle, pitch), thread type (like box threads, buttress threads, unified, square) and drill point type (like #1, #3, #5 drill point) can influence the performance of a screw. When innovatively designed, a #10 engineered screw can meet or exceed the performance of a #12 or #14 screw in loads and drill time and could result in cost savings. You can use fewer screws, which would mean labor savings. For example, our newly designed XU34B1016 screw, which is a #10 screw with 16 threads per inch, a hex washer head and a #1 drill point, that performs better than a #14 standard screw in lighter gauge steels.
What Are Self-Drilling Tapping Screws?
Self-drilling tapping screws, or self-drilling screws, as the name implies, drill their own hole, eliminating the need for predrilling, and form or cut internal mating threads. They are relatively fast to instal compared to bolts or welds. Unlike pins, they do not require a thick support material to be used. They can be used in very thin steel, such as 26 gauge, up to steel that is ½” thick. Self-drilling screws may be a perfect choice for most applications involving cold-formed steel (CFS). They are most commonly used for CFS connections: either attaching CFS to CFS, wood to CFS or CFS to wood. They are a logical choice when the other side of the connection or material is not accessible.
Most self-drilling screws are made of steel wire that meets the specification of ASTM A510 minimum grade 1018 material as specified in ASTM C1513 standard. Self-drilling screws are heat treated to case harden then so that they meet the hardness, ductility, torsional strength and drill drive requirements as specified in ASTM C1513 standard. ASTM C1513 refers to SAE J78 for the dimensional and performance requirements of self-drilling screws.
While selecting the screw, you need to figure out the head type that works for the application. For example, a flat-head screw would be a good choice for wood-to-steel applications, but for steel-to-steel applications, a hex head or a pan head may be a better choice. Similarly, the length of the screw should be sufficient to fasten the members of the connection together. According to Section D1.3 of AISI S200, the screw should be at least equal in length to the total thickness of the material including gaps with a minimum of three exposed threads. The length of the drill point is another important feature to consider. It should be long enough to drill through the entire thickness of the material before engaging the threads. This is because thread forming occurs with fewer revolutions than the drilling process. if the drill point length is not long enough, the screw threads can engage the connection material and the screw can bind and break.
Some drill points also have “wings” to drill a hole in the material that is larger in diameter than the threaded shank. Screws with this kind of point are mainly used for wood-to-steel applications. The blog post by Jeff Ellis titled “Wings or No Wings” provides some useful insights for screws with wings when used in shearwall applications.
The Test Standards and Evaluation Criteria for Standard and Engineered Screws
Per Section D1 of AISI S200, screws used for steel-to-steel connections or sheathing-to-steel connections shall be in compliance with ASTM C1513 or an approved design or design standard.
For ASTM C1513–compliant screws (per AISI S100), Section E4 provides equations to calculate shear, pullout and pullover of screws used in steel-to-steel connections. It also provides safety and resistance factors for calculating allowable strength or design strength. These equations are based on the results of tests done worldwide and the many different types of screws used in the tests. As a result, these equations seem to have a great degree of conservatism.
As discussed earlier, many factors, such as the head type and washer diameter, thread profile, drill point type and length, installation torque and the installation method affect or influence the performance of a screw. In order to qualify the screws as ASTM C1513–compliant or better performing, manufacturers need to have their screws evaluated per Acceptance criteria for Tapping Screw Fasteners AC118 developed by International Code Council – Evaluation Service. The criteria have different requirements depending on whether the intention is to qualify as standard screws or proprietary screws. For proprietary screws, connection shear, pullout and pullover tests are performed in accordance with the AISI S905 test method. The shear strength and tensile strength of the screw itself are evaluated per test standard AISI S904. The safety and resistance factors are calculated in accordance with Section F of AISI S100. The pictures below are some test set-ups per AISI S905 and AISI S904 test procedures.
Another important consideration is corrosion resistance. AC118 has a requirement for testing the fasteners for corrosion resistance in accordance with ASTM B117 for a minimum of 12 hours. The screws tested shall not show any white rust after 3 hours or any red rust after 12 hours of the test. At the same time, it is important to keep in mind that hardened screws are prone to hydrogen embrittlement and are not recommended for exterior or wet condition applications. Also, these screws are not recommended for use with dissimilar metals. If self-drilling screws are to be used in exterior environments, the screws need to be selectively heat treated to keep the core and surface hardness in a range that reduces the susceptibility to hydrogen embrittlement. Other fastener options for exterior environments are stainless-steel screws.
This table shows are some of our screw offerings for CFS applications. Our stainless-screw options can be found in Fastening Systems Catalog (C-F-14) or at www.strongtie.com.
What are the screws that you most commonly specify? Share your screw preferences and your ideas on self-drilling screws in your comments below.
This week’s post comes from Hien Nguyen, one of our R&D engineers at the Simpson Strong-Tie Home Office in Pleasanton, CA. Hien has worked in new product development for 17 years on a variety of products. While she still does a few connector projects for wood, her skills and passion for cold-formed steel construction have allowed her to become our expert in CFS product development. Before joining Simpson Strong-Tie, Hien worked as a consulting engineer doing building design. She has a bachelor of science in Civil Engineering from UC Davis, and is a California Licensed Civil Engineer. Here is Hien’s post:
A previous blog post described how Simpson Strong-Tie tests and loadrates connectors used with cold-formed steel structural members per acceptance criteria ICC-ES AC261.
This week, I would like to describe how we test and determine engineering design values for RCKW, Rigid Connector Kneewall, in a CFS wall assembly and how the data can help designers perform engineering calculations accurately and efficiently.
The RCKW was developed to provide optimal rotational resistance at the base of exterior kneewalls, parapets, handrail and guardrail systems as well as interior partial-height walls.
RCKW connectors were tested in CFS wall assemblies for 33 mil, 43 mil, and 54 mil steel thicknesses and in stud members with depths from 3½ to 8 inches. RCKW connectors with stiffeners, RCKWS, were also tested in CFS wall assemblies for 43 mil, 54 mil, and 68 mil stud thicknesses.
The wall assembly is built using CFS stud framing, bottom and top tracks simulating the kneewall application in the field. The RCKW connectors are fastened to a stud using self-drilling screws and an anchor to the test bed foundation. The horizontal load (P) is applied to the CFS wall assembly at a height (hwall) of 38 inches. The instruments are also placed at the same height as the applied load to measure wall deflection. The load and deflection data are recorded concurrently until the wall assembly fails.
The allowable moment, MASD, is determined by multiplying PASD, the allowable horizontal load, by hwall, wall height (MASD = PASD * hwall).
PASD is calculated from peak load or nominal load, PNominal, divided by Ω, a safety factor per AISI 100 Chapter F. The blog post on Cold-Formed Steel Connectors discusses safety factors for CFS testing.
Similarly, the allowable rotational angle, θASD, is also determined by wall deflection at allowable load, ∆ASD, divided by hwall (θASD = ∆ASD / hwall).
So the assembly rotational stiffness, β, is calculated by MASD, divided by θASD (β = MASD / θASD).
The typical test performance curve for moment versus rotational angle is concave down and increasing as shown in the blue color curve. As a result, the rotational stiffness for RCKW is established by the secant stiffness, which is a red color straight line from zero to the allowable moment as shown below.
The rotational stiffness captures connector deflection, stud deflection and fastener slip in various stud thicknesses. Whereas when the connectors are tested in a steel jig fixture, the rotational stiffness includes connector deflection only and not the fastener and stud deflection behaviors. The photos below are examples of member failures which include stud buckling, bottom track tearing, and screws tilting and bearing. These failure modes are reflected in our tabulated loads because of our assembly testing.
Designers might wonder why the rotational stiffness is so important and how significant it is in Engineering Design. The IBC 2012 Building Code, Section 1604.3 indicates that structural systems and members shall be designed to have adequate stiffness to limit deflections and lateral drift. Table 1604.3 also provides deflection limits for various construction applications to which the Engineer must adhere.
For example, one of many common applications in CFS construction is the exterior kneewall system below a large window opening subject to the lateral pressure load. This kneewall system must not only be designed to provide moment strength to avoid the hinging failure at the base, but it must also be designed for deflection limits to prevent excess lateral drift that could result in cracking from various types of finish materials.
Since we performed comprehensive testing of full assemblies, engineers do not need to add stud deflection and fastener slip to the calculation. This saves time and eliminates guesswork with their specifications in a common 38 inch kneewall height.
Furthermore, we analyzed the test data to determine connector rotational stiffness, βc, which includes connector deflection, fastener slips, but not the stud deflection. Connector rotational stiffness allows engineers to perform deflection calculations for assemblies of any height. Design examples are available in the RCKW Kneewall Connectors flier.
Simpson Strong-Tie recognizes the complexity of performing hand calculations to accurately determine the anchorage reactions for the RCKW connectors. This post on Statics and Testing described how we established loads for our CFS SJC products through testing. We have also provided anchor reaction loads for connectors at allowable moments so engineers could skip this step in the calculations. We measure the anchor reactions by connecting the calibrated blue load cells with the threaded rod that anchors the RCKW connector. The load cell measures the tension forces in the rod directly.
Connector strength and stiffness are critical for RCKW products where calculation or interpolation cannot capture the true performance accuracy the same way that testing would. For this reason, we have tabulated values for various stud member depths and thicknesses. Like Paul, I am amazed at the number of tests that go into this product. Ultimately, we can provide complete Engineer Design values that our specifiers can trust in determining adequate strength and stiffness to meet the code requirement.
This blog has described how we load rate different products based on test standards, which are covered under various ICC-ES Acceptance Criteria, or ACs. The first was a post on wood connectors (AC13), then holdowns (AC155), threaded fasteners (AC233) and cast-in-place anchors for light-frame construction (AC398 and AC399). I realized today that I have never talked about how we test and load rate connectors for cold-formed steel.
But first, a confession – it has taken me many years to stop calling it “light-gauge steel.” When I started designing with cold-formed steel, I called it “light-gauge” because I had a binder of design information put together by the Light Gauge Steel Engineers Association. Advocates for CFS felt that “light-gauge” may make people think “weak” or “non-structural,” and that perception would limit the use of cold-formed steel in construction. So there was a deliberate effort to banish the word light-gauge and replace it with cold-formed steel, or CFS. I still slip every once in a while.
Connectors for light-gauge, ahem, I mean cold-formed steel members are covered under ICC-ES AC261 – Acceptance Criteria for Connectors Used with Cold-formed Steel Structural Members. The physical testing for cold-formed steel is similar to wood connectors. Build a setup representative of field conditions, apply load till failure and measure the load and deflection data. Both wood-to-wood and CFS connectors have a service limit state of 1/8” deflection.
Strength data for CFS connectors is analyzed much differently, however. Wood connectors generally use a safety factor of 3 on the lowest ultimate load (or average ultimate if six tests are run). We are often asked what the safety factor for CFS connectors is.
AISI S100 Chapter F details how to determine design strengths for tested CFS products. The design strength is the average test value, Rn, multiplied by an LRFD resistance factor, Φ, or divided by an ASD safety factor, Ω. Determining the resistance factor or corresponding safety factor is based on a statistical analysis dependent on several variables. This is similar in concept to how embedded concrete connectors tested to AC398 or AC399 are evaluated, which I discussed in this post.
I don’t want to get too deep into the Greek letters involved in the calculation. The factors that affect the allowable load calculation are type of member tested, variation in the test values, type of manufacturing, and number of samples tested. One factor that has a large impact on the calculation is the target reliability index, βo. In connector testing, this factor is 2.5 if the structural member (joist, stud, track, etc) fails and 3.5 if the connection fails. The net result is a higher safety factor for test values limited by the connection, and lower safety factors if the structural members governed the test load. Typical safety factors for CFS connectors are 1.8 to 2.0 where the failure mode is in the structural members and 2.2 to 2.9 for tests where the connection failed.
AC261 has a reduction factor, RS, which is used to adjust test values if your steel strength and/or steel thickness are over the specified minimum. CFS test setups often use different steel in the joist, header and the connector. Reductions are calculated based on the tested and specified strength and thickness for each member. The lowest reduction is used to adjust the test values.
One additional complexity in CFS testing is the multiple gauges of steel which must be evaluated. This requires more CFS test setups than a comparable wood connector would require. In the end, we have what we are really after. Design loads that specifiers can be confident in.
Bruce Lindsey is the South Atlantic Regional Director for WoodWorks – The Wood Products Council, which provides free project assistance as well as education and resources related to the design of nonresidential and multi-family wood buildings. Based in Charlotte, NC, Bruce’s multi-faceted career with the industry spans 20 years and includes architectural design, structural design and roles within the engineered wood products industry related to marketing, product management, distribution, consulting and sales.
Last week’s post reviewed some of the common questions WoodWorks receives from engineers designing five-story, Type III wood-frame buildings—including those related to fire retardant-treated building elements, and fire-rated floor and wall assemblies. This week, we extend that conversation to another common issue—details and fire rating of floor-to-wall intersections.
The fire rating of an exterior wall assembly in Type III construction causes a detailing issue where the floor intersects the exterior wall assembly. There are no testing criteria established by the code for system intersections of any material, so detailing must rely on code interpretation. The two points of interpretation focus on continuity of the two-hour wall fire rating and the FRT requirement.
Section 705.6 of the 2012 IBC 1 requires that an exterior wall have “sufficient structural stability such that it will remain in place for the duration of time indicated by the required fire resistance rating.” The ‘interruption’ of the floor in the plane of the exterior wall may be seen by authorities as affecting the structural stability. It is not clear how designers are to comply with this language; for that reason, the language has been removed in the 2015 IBC.
The implication of FRT continuity is derived from the primary requirement that Type III buildings have noncombustible exterior walls. FRT wood is permitted in these walls per IBC Sections 602.3 and 602.4. Since the noncombustibility or acceptable FRT alternative is intended to reduce fire exposure to other buildings, some code officials require FRT material in the plane of exterior walls through the floor intersection. The degree to which a building official believes that the rim joist, floor joist and/or sheathing present a risk of fire spread will determine the degree of FRT material required through the floor-wall intersection.
The manner in which this floor-to-wall connection can be detailed first depends on the type of framing being used—traditional platform framing or semi/modified balloon framing. Platform framing relies on the fact that the floor system bears directly onto the wall below. Semi-balloon framing relies on hangers to support the floor framing.
Typical platform-framed floor-to-wall intersections have been accepted by many jurisdictions without any special detailing according to the rationale that the area of intersection represents “floor framing” and not “wall framing.” In these intersections, the “floor” is not required to be FRT and its fire resistance is limited to one hour. This is similar to the floor conditions found in Type V construction; where such conditions obtain, it’s also logical to extend the same detailing allowances at this intersection to Type III buildings.
While local code interpretation varies widely, a variety of detailing concepts have arisen across the country as possible solutions to this issue.
In one solution, a solid sawn, glulam or engineered rim board is used to create continuity of the two-hour rating through the plane of the wall by using the charring capability of the rim board calculated using Chapter 16 of the NDS. Variations of this detail include a built-up rim board. In some solutions, the member closest on the outside of the wall may also be FRT to provide some degree of FRT continuity. If continuity of FRT through the floor for the entire width of the wall is also required, the entire thickened rim board and possibly the first sheet of floor sheathing may need to be FRT. In some scenarios without heavy FRT requirements, a hanger is not needed if the rim board width that can accommodate the charring is narrower than the width of the wall and the joist can bear on the top plate itself.
Another option is to use a continuous 2x block to achieve one hour of fire resistance, again calculated using Chapter 16 of the NDS. The second hour of resistance is provided by the horizontally applied drywall on the underside of the floor. While the two layers of drywall may not be in the plane of the wall, they still provide two hours of fire endurance. This detail may or may not require that the block and the floor sheathing be FRT, depending on the FRT continuity interpretation. Variations of this detail include an option where the blocking is moved inside the plane of the wall between the joists. Some jurisdictions object, citing concerns about fires starting in the floor cavity. There are other measures, such as fire blocking or cavity sprinklers, provided to minimize spread of fire in these situations. The same question could be asked about fires starting within a wall cavity.
A third option is a slight variation of the second. Instead of using blocking to achieve the one hour of fire resistance, one layer of drywall can extend up behind certain proprietary top flange joist hangers (for SST example, click here). This provides one hour of fire resistance in the plane of the wall, and the second hour is provided by the drywall on the underside of the floor. Some contractors find this detail difficult to accommodate because of construction sequencing — the drywall crew typically does not arrive on site until after rough framing is complete. A variation seen in some areas is using a top-chord-bearing truss, which eliminates the hanger hardware and minimizes the non-treated penetration in the plane of the exterior wall. Addressing full FRT continuity may be more difficult with this variation depending on the truss manufacturer.
A fourth option requires relatively new concepts using connector solutions that allow two layers of gypsum to be applied behind the floor joist connection to the wall (for SST example, click here). Hardware solutions can be a useful option to have available when an Authority Having Jurisdiction is particularly wary of maintaining the integrity of a rated wall assembly, but Designers should consider both the labor and the cost of these details to determine the best fit for the project.
In addition to regional nuances and differing (and evolving) code interpretations, there isn’t one solution that fits all applications. Designers should determine the local availability of FRT products, review manufacturer product specifications and discuss the proposed solution with their jurisdiction.
If you’re designing a mid-rise wood building and have questions—e.g., about fire and life safety, lateral and vertical loads, how to address shrinkage, etc.—I encourage you to contact your local WoodWorks regional director. The WoodWorks website (woodworks.org) also offers a wide range of technical information on mid-rise structures and we welcome inquiries to the project assistance help desk (firstname.lastname@example.org).
1Information is based on the 2012 International Building Code unless otherwise indicated.
Bruce Lindsey is the South Atlantic Regional Director for WoodWorks – The Wood Products Council, which provides free project assistance as well as education and resources related to the design of nonresidential and multi-family wood buildings. Based in Charlotte, NC, Bruce’s multi-faceted career with the industry spans 20 years and includes architectural design, structural design and roles within the wood products industry related to marketing, product management, distribution, consulting and sales.
As a regional director for WoodWorks, my job is to provide technical assistance related to the design of nonresidential and multi-family wood buildings. I’ve been with the program since it launched in 2007 and, although we support a full range of building types, I’ve seen a steady increase in the number of design professionals looking for information and support related to mid-rise wood structures in particular.
Reasons for this are summed up in a recent Wood Solution Paper by my colleague, Lisa Podesto, PE, Maximizing Value with Mid-Rise Construction, in which she points out that wood-frame construction is a cost-effective choice because it allows high-density use (five stories for many residential occupancy groups, six for office) at relatively low cost, while providing other benefits such as construction speed, structural performance, design versatility, sustainability, and a light carbon footprint.1
In particular, WoodWorks gets a lot of calls from engineers designing five-story, Type III wood-frame buildings, since the structural challenges are considerably different than they are for buildings up to four stories. We provide technical support (at no cost), from conceptual design through construction of a project, helping to work through issues such as the following:
Fire Retardant-Treated Building Elements
Type III buildings are required to have fire retardant-treated (FRT) exterior walls, and designers often struggle with how to specify FRT. While preservative-treated products are typically applied under a set of prescriptive requirements according to the American Wood Protection Association (AWPA) U1 standard, FRT wood is defined in IBC Section 2303.22 and differs from preservative-treated specification because treatments include proprietary formulations and application processes that instead meet a performance standard. Each of the treatment formulations has it’s own recommendations with regard to corrosion resistance of fasteners and strength reduction factors for wood members and connections. Full recommendations can be found in individual evaluation reports from FRT suppliers. Engineers might consider using the worst-case reduction factors for design to allow contractors the flexibility to source FRT from different suppliers.
Fire-Rated Wall Assemblies
While all Type III construction requires two-hour fire-rated exterior walls, it can be challenging to find tested assemblies that meet this criterion. When looking for these assemblies—and indeed all assemblies—it is helpful to keep a few things in mind:
- Structural panels may add to fire resistance – Many assemblies may not show wood structural panels in the approved assembly, but exterior walls usually require wood sheathing for lateral resistance of the building, sometimes on both sides of the wall. The addition of wood structural panels to assemblies should not diminish the fire rating, as acknowledged in the General Notes section of the Gypsum Association Fire Resistance Design Manual, which allows their addition. The second rule in Ten Rules of Fire Endurance Rating by Tibor Harmathy, presented in the American Wood Council publication, CAM for Calculating and Demonstrating Assembly Fire Endurance, says, “The fire endurance does not decrease with the addition of further layers.” Another resource that may assist designers is the ICC-ES Evaluation Report ESR-2586, Performance Standards and Qualification Policy for Structural-use Panels, which states, “Structural-use panels may be installed between the fire protection and the wood studs on either the interior or exterior side of fire-resistance-rated wood frame wall and partition assemblies described in the applicable code, provided the length of fasteners is adjusted for the added thickness of the panel.”
- FRT studs may be used – For Type III construction, FRT wood is also a requirement in exterior wood wall assemblies, in addition to the two-hour rating. Some two-hour-rated assemblies may not specifically state that FRT studs may be used, but the UL Guide Information clarifies that FRT may be used in place of non-treated wood in any assembly.
Fire-Rated Floor Assemblies
Both Type IIIA and Type VA construction require one-hour-rated floor assemblies. Even when using Type B, generally considered unprotected construction, with a residential occupancy, floors between dwelling units still need protection per IBC Section 711.3.
- Floors less than 10 inches deep – As with wall assemblies, finding fire-rated floor assemblies that meet the design parameters can be challenging. In mid-rise applications, it is common for designers to go to great lengths to minimize the floor depth in order to maximize the plate height at every level and still stay beneath the overall height limit of the structure. However, there are few available UL assemblies with a minimum joist depth of less than 10-inch nominal. Designers can use either IBC Section 721 with the Deemed to Comply tables, or Section 722 on calculated fire resistance to address this issue.
- Using structural composite lumber in floors – While a similar lack of published options is true of assemblies with structural composite lumber (such as laminated veneer lumber, laminated strand lumber or parallel strand lumber), the argument for using these products in fire-rated assemblies lies in their ICC-ES reports. The section under Calculated Fire Resistance states that the fire resistance of an exposed wood member—solid sawn, structural glued laminated timber (glulam) and structural composite lumber—can be calculated using Chapter 16 of the National Design Specification® (NDS®) for Wood Construction, which implies that the fire resistance is equal to that of solid sawn members. The structural adhesives used can withstand temperatures beyond that of wood.
- Heavy timber corridor decking – Some designers use a heavy timber decking over corridors allowing taller plate heights and/or unencumbered area for utilities to run above a drop ceiling. This accomplishes a one-hour resistance by using char calculations for exposed wood elements as outlined in Chapter 16 of the NDS stipulated as an alternate method in IBC 722.1.
If you’re designing a mid-rise wood building and have questions—e.g., about fire and life safety, lateral and vertical loads, how to address shrinkage, etc.—I encourage you to contact your local WoodWorks regional director. The WoodWorks website (woodworks.org) also offers a wide range of technical information on mid-rise structures, and we welcome inquiries to the project assistance help desk (email@example.com).
1For more information, visit www.woodworks.org/why-wood
2Information is based on the 2012 International Building Code unless otherwise indicated.