Early this summer a package arrived at my office that I knew right away was either a copy of a new building code or design standard. Some codes or standards are more exciting than others to open up and see what’s new and different. As it turns out, this package was the just-published 2015 International Residential Code (IRC). With my interest in wood decks, I have to admit that this was new information that I was happy to see.
Why? Similar to my blog post in May mentioning the limited design resources currently available to engineers, the IRC itself is also a work in progress when it comes to the prescriptive details included for decks. Performance requirements for the framing and guards has always been included in Chapter 3, but it wasn’t until the 2009 and 2012 editions that prescriptive information for attaching a deck ledger to a wood band joist with lag screws or bolts, and a detail for transferring lateral loads to a support structure, were included. Key improvements for the 2015 IRC include provisions for composite materials, clarification of the prescriptive ledger information, and prescriptive information for decking, joist and beam allowable spans, post heights and foundations.
Lateral load connections at the support structure were a significant topic during the development of the 2015 IRC. The permitted method already in the code involves constructing the Figure 507.2.3(1) detail with 1,500 pound hold-downs, in two or more locations per deck. The detail transfers the lateral load by bypassing the joist hanger and ledger connections, and ultimately transfers it into the floor diaphragm of the support structure. The concentrated nailing on the floor joist and the need to have access from below to the install the hold-down can cause undesirable complications for builders with existing conditions. A number of common conditions also differ significantly from the detail, such as the floor joists running parallel to the deck ledger and alternate floor joist types, including i-joists or trusses. In response to frequently-asked-questions from the industry, our technical bulletin T-DECKLATLOAD provides commentary to consider for these situations. The technical bulletin also offers an alternate floor joist-to-sheathing connection that may save the builder from removing a finished floor in an existing condition or from adding additional sheathing nailing from above.
Figure: 2015 International Residential Code; International Code Council
In order to provide greater flexibility, a second option is now included in the 2015 IRC: constructing Figure R507.2.3(2) with 750 pound hold-downs in four locations per deck. This detail also transfers the lateral load in bypassing the joist hanger and ledger connections, but transfers the load to the wall plates, studs, or wall header by means of a screw anchoring the hold-down. In some cases, builders will hope this detail can save removing interior portions of an existing structure, but close attention will be required in terms of the deck joist elevation with respect to components of the wall and ensuring that hold-down anchor has proper penetration into the wall framing.
Figure: 2015 International Residential Code; International Code Council
There are still a number of scenarios where a residential deck builder may need or want to consider hiring a structural engineer. Prescriptive details for guards and stairs are still not included in the code, as well as lateral considerations such as the deck diaphragm or the stability of a freestanding deck. Alternate loading conditions, such as the future presence of a hot tub, are also outside the scope of the current code. The allowance for alternative means and methods permitted by Chapter 3 of the 2015 IRC, is also something to keep in mind when the prescriptive options do not fit well with the project conditions. For example, the IRC ledger fastening table applies for connections to a band joist only and not to wall studs or other members of the adjacent support structure.
Have you been involved with any residential deck projects? Let us know in the comments section below.
In my former life working as a consulting engineer, I reviewed many truss submittal packages. I remember during my reviews wondering how it was possible to get so much information on to an 8½ inch by 11 inch piece of paper. I also remember how a lot of what was being reported was difficult to understand without some help interpreting the information.
As many of you may know, Simpson Strong-Tie has ventured into the truss industry and we are now offering truss connector plates and software to component manufacturers around the country. So given my past experiences, I figure some of you might appreciate some insight into the engineering that goes on behind those truss submittal packages. So I have asked one of our great truss engineers, Kelly Sias, to put together some blog posts on the topic that we can share our knowledge with you. Kelly has worked in the truss industry for years and spent time as the Technical Director at the Truss Plate Institute. I am sure her blog posts are going to help all of us have a better appreciation for trusses.
Have you ever been involved in a discussion with someone on a project that ended with “but that’s the way we’ve always done it!”? I heard those words spoken by a contractor in my first engineering job when I tried explaining why his single stud would not work at a particular location. When he said something about his grandfather having always done it that way, I knew I could explain the calculations all day and it wouldn’t do much good.Continue Reading
When designing a shearwall according to the International Building Code (IBC), a holdown connector is used to resist the overturning moment due to lateral loading. From a structural statics point of view, a shearwall without dead load or holdowns would have zero lateral-resisting capacity without any restraint to resist the overturning moment. Since the wall assembly still has the sill plate anchorage providing resistance to overturning, testing can measure the capacity of a wall assembly without holdowns.
The IRC® contains several different narrow bracing methods that are made up of portal frames. One method that is useful if you are using intermittent wall bracing is the Method PFH Portal Frame with Holdowns. This method relies on low-deflection holdown anchorage at the bottom, and substantial nailing at the overlap of the sheathing and the header at the top to prevent overturning of the narrow panel. An identical method for use as wall bracing is in the Conventional Construction section in Chapter 23 of the IBC®. These portal frames were first included in the 2006 IBC and IRC.
Method PFH- Portal Fram With Holdowns
The method was originally tested with straps clamped to a steel test bed to simulate the embedded holdown straps. The straps were nailed to the wood with enough nails to mimic a 4,200 lb. strap anchor. The original test report is APA T2002-70. At that time, the Simpson Strong-Tie® STHD14 had a published allowable load in excess of 4,200 lbs. based on then-current Acceptance Criteria, so hardware was available to construct this frame throughout the country. However, in 2008, ICC Evaluation Service developed a new acceptance criteria for embedded connectors, AC398, Acceptance Criteria for Cast-in-place Cold-formed Steel Connectors in Concrete for Light-frame Construction. This was in response to the changes in ACI 318 for anchors in concrete. When re-tested and evaluated using the new Acceptance Criteria, the allowable load for STHD14 was reduced below 4,200 lbs. for use in buildings designed for Seismic Design Categories C through F. The same thing happened to other manufacturers’ embedded holdown allowable loads. That made it impossible to properly construct this bracing method in those areas. In response to this, Simpson Strong-Tie worked with APA, the Engineered Wood Association, to design a new test that would determine if a lower capacity holdown could be used with this portal frame method. APA performed the testing at their Tacoma, Washington testing lab. Since the initial testing of the portal frames with the 4,200 lb. holdown was performed using the outdated SEAOSC protocol with an older testing rig that used a stiff beam above the wall, both the old tests with a simulated 4,200 lb. holdown and new tests with a simulated 3,500 lb. holdown were rerun in accordance with the current ASTM E2126 test method using the CUREe protocol. The test was published as Test Report T2012L-24. The tests showed little to no effect of reducing the holdown from 4,200 lbs. to 3,500 lbs. allowable load. Here is one of the graphs of the backbone curves comparing the two assemblies for a 16-inch wide, 10-foot tall portal frame.
Comparison graph of two assemblies for a 16-inch wide, 10-foot tall portal frame.
With the testing complete, APA prepared and submitted code changes to both the 2012 International Building Code® and 2012 International Residential Code®. The IBC proposal is S291-12, and can be found on page 605 of the 2012 Proposed Changes to the International Building Code – Structural. The IRC proposal is RB311-13, and can be found on page 613 of the 2013 Proposed Changes to the International Residential Code-Building. With support from Simpson Strong-Tie, both of the proposals were approved. So in the 2015 IRC, bracing method PFH will require an embedded strap-type holdown with a minimum capacity of 3,500 lbs. instead of 4,200 lbs. The same will hold true for the Alternate Braced Wall Panel Adjacent to a Door or Window Opening bracing method in the 2015 IBC. APA also re-tested the portal frames with only two sill plates instead of three. This will allow the use of a 5/8” by 8” Titen HD® anchor as a retrofit anchor bolt. What are your thoughts? Let us know in the comments below.
The number of midrise structures constructed using light-frame cold-formed steel (CFS) certainly seems to be increasing each year. As with any material, there are benefits and challenges, especially in areas of moderate to high seismic risk. This post will discuss these as well as potential solutions.
Light-frame CFS midrise construction often uses ledger floor framing primarily to facilitate the load transfer detailing at the floor, tension anchorage (tie-downs or hold-downs) and compression chord studs or posts designed to resist the amplified seismic overturning loads. CFS framing is typically thin and singly symmetric.
Various CFS Construction Floor Framing Methods
Amplified Seismic Load
The AISI Lateral Design standard (AISI S213-07/S1-09) Section C5.1.2 requires that the nominal strength of uplift (tension) anchorage and the compression chord studs for shear walls resist the lesser of (1) the amplified seismic load or (2) the maximum load the system can deliver when the response modification coefficient, R, greater than 3. The amplified seismic load is defined as the load determined using the ASCE 7 seismic load combinations with the overstrength factor, Wo, which may be taken as 2.5 for CFS framed shear wall systems with flexible diaphragms.
Typically, the maximum the system can deliver to the uplift anchorage or chord studs is taken as the forces determined using the nominal shear strength of the shear wall assembly tabulated in the seismic shear wall table in S213 multiplied by 1.3. The S213 commentary accounts for the tabulated loads being based on Sequential Phased Displacement (SPD) rather than CUREE cyclic protocol and the degraded backbone curve. See theStructure magazine article that discusses the design of CFS framed lateral force-resisting systems.
Continuous Rod Tie-Down Systems
Light-framed CFS over three stories often use continuous rod tie-down systems rather than cold-formed steel hold-downs to resist shear wall overturning forces as they offer increased load capacity. Neglecting the dead load contribution, the amplified seismic load requirement for CFS shear walls using an R greater than 3 results in an 80% increase in the load used to size the continuous rod tie-down system compared to design level loads. For shear walls using an R greater than 3, it is important to note on the design drawings whether the uplift loads shown are ASD, LRFD, amplified ASD or amplified LRFD so the appropriate tie-down system may be designed.
Continuous Rod Tie-Down System Resisting Shear Wall Overturning Forces
Continuous rod tie-down systems are designed not only for strength, but also checked to ensure they do not deflect too much to cause the top of shear wall drift to exceed the code limit or to exceed the 0.20” vertical story deflection limit required by some jurisdictions and ICC-ES AC316. Take-up devices are used in CFS framed structures to take-up construction and settlement gaps that may occur. AISI S200 Section C3.4.4 states that a gap of up to 1/8” might occur between the end of wall framing and the track. The vertical elongation of the continuous rod tie-down system includes rod elongation (PL/AE) and the take-up device deflection due to the seating increment and the deflection under load.
In addition, coordination is important in using continuous rod tie-down systems in CFS structures because the walls are often prefabricated offsite. An example is the consideration of the appropriate detail for the steel bearing plate installed at the floor sheathing in the story above to resist the uplift (tension) force from the story below.
One possible detail is to install the bearing plate in the bottom CFS track under all the CFS chord studs, but it’s important to ensure the bottom track flanges are deep enough to screw them to the stud flanges as the bearing plate can have a thickness of 1 ½” or more and typical tracks use 1 ¼” flanges. It is also important to ensure that the bearing plate width fits in the track. Another possible detail is to install the bearing plate under the CFS track under all the CFS chord studs. However, then it must be cut into the floor sheathing and may cause the bottom track to be raised at the bearing plate. For this detail, the floor shear transfer must be detailed through the ledger into the CFS framing.
Continuous Rod Tie-Down System Steel Bearing Plate Coordination Issues
Concrete Tension Anchorage
The concrete tension anchorage is designed according to ACI 318 Appendix D using the continuous steel rod material and size in accordance with S213 to have the nominal strength to resist the lesser of the amplified seismic force or the maximum load the system can deliver. ACI 318-11 Section D.3.3.4.3 offers four force limits for design of concrete tension anchorage design in Seismic Design Category C through F:
(1) The concrete nominal tension anchorage strength shall be greater than 1.2 times the ductile steel rod nominal tension anchorage strength
(2) The anchorage design strength shall be greater than the maximum tension force that can be delivered by a yielding attachment;
(3) The anchorage design strength shall be greater than the maximum tension force that can be delivered by a non-yielding attachment; and
(4) The anchorage design strength shall be greater than the amplified seismic force.
Typically either option (1) or (4) is used where (1) would lead to less concrete required than (4) if the bolt is efficiently sized while (4) would be required for such conditions as a vertical irregularity. See the concrete anchorage and podium anchorage SE Blog posts for more details.
CFS studs are typically thin and singly symmetric and thus require bracing. AISI S211 (Wall Stud Design Standard) permits two types of bracing design that cannot be combined; sheathing based or steel based. There are limits on the stud axial strength when using sheathing braced design. It’s important to identify on the drawings that the sheathing braces the studs and another load combination must be used for the stud design.
2012 IBC Section 2211.4 requires stud bracing to be designed using either AISI S100 (North American Specification) or S211 (Wall Stud Design Standard). S100-07 Section D3.3 required nominal brace strength is to be 1% of the stud’s nominal compressive axial strength, but S100-12 Section D3.3 changes this to the required brace strength is to be 1% of the stud’s required compressive axial strength (demand load). In addition, D3.3 requires a certain stiffness for each brace. AISI S211 required brace strength is to be 2% of each stud’s required compressive axial strength for axially loaded studs and, for combined bending and axial loads, be designed for the combined brace force per S100 Section D3.2.2 and 2% of the stud’s required compressive axial strength.
There are two primary types of steel stud bracing systems: bridging and strap bracing. U-channel bridging extends through the stud punchouts and is attached to the stud with a clip, of which there are various solutions such as this post on Wall Stud Bridging. Bridging bracing requires coordination with the building elements in the stud bay. It installs on one side of the wall, and does not bump out the wall sheathing. It also requires periodic anchorage to distribute the cumulative bracing loads to the structure for axially loaded studs often using strongback studs and does not require periodic anchorage for laterally loaded studs since the system is in equilibrium as the torsion in the stud is resisted by the U-channel bending.
Flat strap bracing is installed on either side of the wall and at locations other than the stud punchout. It bumps out the sheathing and requires periodic anchorage to distribute the cumulative bracing loads to the structure for axially and laterally loaded studs.
Strap and Block Stud Bracing Anchored Periodically to Structure Using Strongbacks
Bridging and Clip Bracing Anchored Periodically to Structure Using StrongbacksBridging and Clip Bracing Anchored Periodically to Structure Using Diagonal Strap Bracing
It is hard to believe it has been almost two years since I posted The Anchorage to Concrete Challenge – How Do You Meet It? That post gave a summary of the challenges engineers face when designing anchorage to concrete. Challenges include just doing the calculations (software helps), developing a high enough load, satisfying ductility requirements or designing for overstrength. Over the past several years, Simpson Strong-Tie has worked closely with the Structural Engineers Association of Northern California (SEAONC) to help create more workable concrete anchorage solutions for light-frame construction.
Anchor FEAAnchor BreakoutAnchor Close Up
This month’s issue of Structure magazine has an article, Testing Tension-Only Steel Anchor Rods Embedded in Reinforced Concrete Slabs, which provides an update on the ongoing work of SEAONC and Simpson Strong-Tie. The goal of the testing program is to create a useful design methodology that will allow structural engineers to develop the full tensile capacity of high-strength anchor rods in relatively thin (10” to 14”) podium slabs.
Anchor capacity is limited by steel strength, concrete strength, embedment depth, and edge distances. One way to achieve higher anchor strengths is to design anchor reinforcement per ACI 318-11 Appendix D.
ACI318-11 Figure RD.5.2.9ACI318-11 D.5.2.9
Section D.5.2.9 requires anchor reinforcing to be developed on both sides of the breakout surface. Since this is not practical in thin podium slabs, alternate details using inclined reinforcing perpendicular to the breakout plane were developed and tested.
This month’s Structure magazine article summarizes the test results for anchors located at the interior of the slab, away from edges. Additional testing is needed for anchor solutions at the edge of slab. The anchor reinforcement concepts are similar, yet additional detailing is required to prevent side-face blowout failure modes. This testing is in progress at the Tyrell Gilb Research Laboratory and will be completed later this year.
Did you read the Structure article? What are your thoughts?
[Simpson Strong-Tie note: Shane Vilasineekul is the Simpson Strong-Tie Engineering Manager for the Northeast U.S. and one of our guest bloggers for the Structural Engineering Blog. For more on Shane, see his bio here.]
Are you finding it difficult to keep your fees competitive?
“Codes are becoming more complex.”
“Builders are demanding lower construction costs.”
“If I don’t allow this they will find an engineer who will.”
“Competition is stiffer.”
“New proprietary systems take too long to evaluate.”
“We have less time to do our job”
“Architects don’t give us enough to work with.”
“Other engineers are not doing it right.”
Working at Simpson Strong-Tie for 15 years, I have had the opportunity to speak with thousands of engineers and these are recurring themes. Some of these issues are way above my pay grade, but there may be something each of us can do to help keep our profession healthy.
A few years ago we had Susan Dowty from the California offices of S.K. Gosh speak at our SEA of Ohio conference. After her presentation, she stuck around to hear Steven Regoli from the Ohio Board of Building Standards. The gist of his presentation was that Ohio building officials don’t have the authority to reject sealed plans or even require calculations to be submitted unless there is clear evidence of a code violation. Midway through, a very lively discussion broke out between Susan and Steven about the responsibilities of plans examiners as they relate to structural design. On one side you have plans examiners who are licensed engineers and perform something akin to a peer review, and on the other side you have plans examiners with little engineering background that rely on the licensed engineer to ensure structural provisions are met. With some exceptions, the first view is held by many western states and the latter by many states in the South, Midwest and East Coast.
So how does this affect engineering fees? Well, when all it takes to collect a fee is a sealed set of structural plans, the temptation is there to cut corners in the design process and, in an increasingly competitive market, provide clients with a building that costs less to construct than one properly designed. I take pride in working in a profession that holds ethics in such high regard, but it only takes a few to give in and disrupt the market in a particular region. It seems like these “few” are gaining in numbers the last several years. Without proper checks and balances, this trend could continue.
So what can we do about it? I don’t think local government would be open to increasing the payroll for building departments to hire more engineers to review plans (building departments in Ohio saw some of the first and most severe cuts during the recent recession), but maybe we can help raise the bar for structural plan review. Steven Schaefer, the founder of Schaefer structural engineers in Cincinnati, decided years ago to take it upon himself to educate Ohio building departments on the fundamentals of structural engineering. He regularly presents at their meetings and has even created a guide to help plan reviewers look for proper load paths and lateral force-resisting systems. Next week he will be presenting four courses at their state conference and will be honored with an award for all his efforts over the years. We may not all be able to have the same impact, but most of us could spare a few hours each year to work with our local engineering association to reach out to building departments and offer training and support.
Leave a comment if you have some ideas on how to maintain our high standards, or better yet, share some successes you have seen in your area.
This week’s blog was written by Branch Engineer Randy Shackelford, P.E., who has been an engineer for the Simpson Strong-Tie Southeast Region since 1994. He is an active member of several influential committees, including the AISI Committee on Framing Standards, the American Wood Council Wood Design Standards Committee, and the Federal Alliance for Safe Homes Technical Advisory Committee. He is vice-president and member of the Board of Directors of the National Storm Shelter Association. Randy has been a guest speaker at numerous outside seminars and workshops as a connector and high wind expert. Here is Randy’s post:
In my last blog post, I gave an overview of FLASH, the Federal Alliance for Safe Homes, and how Simpson Strong-Tie partners with them. Last November, FLASH held their Annual Conference. The theme of this past meeting was “15 Years of Stronger Homes and Safer Families,” and it was one of their best conferences yet.Continue Reading
A few days ago, I was speaking to a customer about an application using nail substitutions for a joist hanger installation. Her questions come up often, so I thought I would dedicate a blog post to some of the resources available that cover the use of different nails in connectors.
Designers and builders often wish to use different fasteners than the catalog specifies. The application could require short nails that don’t penetrate through the back of a ledger or they want to use screws or sinker nails for easier installation. The Wood Connectors Catalog provides multiple options for alternate nailing for face mount hangers and straight straps on page 27.
Fastener Reduction Factors
The load adjustments for alternate fasteners cover substitutions from a common diameter of 16d to a 10d, or a 10d to an 8d. Multiple different replacement lengths are also covered, with reduction factors ranging from 0.64 to 1.0.
It is important to remember that double shear hangers require 3” minimum joist nails. Short nails installed at an angle in double shear hangers will not have adequate penetration into the header.
Fastener – Double Shear
Pneumatic nail guns used for connector installation are commonly referred to as positive placement nail guns. These tools either have a nose piece that locates connector hole, or the nail itself protrudes from the tool so that the installer can line the nail up with the hole. Most positive placement tools do not accept nails longer than 2½”, so framers using these tools will want to use 1½” or 2½” nails. To accommodate installers using pneumatic nails, we have a technical bulletin T-PNUEMATIC. This bulletin provides adjustment factors for many of our most common embedded holdowns, post caps and bases, hangers and twist straps.
The question of nail size also comes up when attaching hangers to rim board, which can range from 1” to 1¾”. The adjustment factors in C-2013 don’t necessarily apply with rim board, since the material may be thinner the length of the nails used. We also have a technical bulletin for that application – T-RIMBDHGR.
Rim Board Reduction Factors
Several of the reduction factors are the same as those in the catalog. Testing of hangers with 10dx1½ nails on 1” OSB or 1¼” LVL did not do as well, however. We observed that once the nails withdrew a little bit under load, they quickly lost capacity. For that reason, we recommend full length 10d or 16d nails on those materials.
Rim board failure
Understanding that alternate fasteners are available for many connectors can help you pick the right fastener for you application. When you specify a connector, it is important to also specify the fasteners you require to achieve your design load.
I attended a CFSEI and Steel Framing Alliance webinar last week entitled Specifying Cold-Formed Steel: Finding and Avoiding Pitfalls in Structural General Notes and Architectural Specifications. The presenter was Don Allen, P.E., from DSi Engineering, LLC, and he focused on issues specifically related to design and specification of cold-formed steel (CFS) in contract documents.
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