Sneak Peek: Our New and Improved Deck Design Guide

One of the ways I get through winter every year is by looking forward to the weekend in March when we set our clocks ahead and “spring forward” into Daylight Savings time. Some people don’t like this change because of the lost hour of sleep, but to me it means the weather shouldn’t be cold for much longer.

The coming of spring means getting to walk to the car in daylight at the end of the workday. It also means getting the garden started for the year and spending more time outside in general.

Of course, I’m not alone in being happy to see winter go.

In the residential world, the phenomenon of “deck season” coincides with this time of year.  Homeowners with decks are getting ready for summer by giving their decks a cleaning and looking them over for any needed maintenance. Now’s the time that new or replacement decks are being planned and built to be enjoyed for the rest of the year.

deck-season

It’s no coincidence, then, that our deck-code guide has been updated again in time for warmer weather. The Deck Connection and Fastening Guide goes detail by detail (ledger connection, joist-to-beam connection, beam-to-post connection, etc.) through a typical deck and identifies the relevant building-code requirements (2012 and 2015 IRC/IBC) and connection options.

Our deck-code guide can be a helpful reference to an engineer who is just getting acquainted with decks, and can also bring you up to speed on revisions to the IRC that can necessitate engineering changes to even a relatively simple residential deck. Multilevel decks, guardrail details, ledger details and foundation challenges are all examples of things a deck builder could call you for assistance with.

For more information on resources available to engineers on deck design, feel free to consult my previous blog article, Wood-framed Deck Design Resources for Engineers.

The Deck Connection and Fastening Guide

F-DECKCODE17

This guide provides instructions on how to recognize defects and deficiencies in existing decks, and guidance for building a strong, safe, long-lasting new or renovated deck structures.


For more deck-related blog posts, check out the links below:

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

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.

Wood-framed Deck Guard Post Resources and Residential Details

A deck and porch study reported that 33% of deck failure-related injuries over the 5-year study period were attributed to guard or railing failures. While the importance of a deck guard is widely known, there was a significant omission from my May 2014 post on Wood-framed Deck Design Resources for Engineers regarding the design of deck guards.

A good starting point for information about wood-framed guard posts is a two-part article published in the October 2014 and January 2015 issues of Civil + Structural Engineer magazine. “Building Strong Guards, Part 1” provides an overview of typical wood-framed decks, the related code requirements and several examples that aim to demonstrate code-compliance through an analysis approach. The article discusses the difficulties in making an adequate connection at the bottom of a guard post, which involve countering the moment generated by the live load being applied at the top of the post. Other connections in a typical guard are not as difficult to design through analysis. This is due to common component geometries resulting in the rails and balusters/in-fill being simple-supported rather than cantilevered. “Building Strong Guards, Part 2” provides information on the testing approach to demonstrate code-compliance. Information about code requirements and testing criteria are included in the article as well.

Research and commentary from Virginia Tech on the performance of several tested guard post details for residential applications (36” guard height above decking) is featured in an article titled “Tested Guardrail Post Connections for Residential Decks” in the July 2007 issue of Structure magazine. Research showed that the common construction practice of attaching a 4×4 guard post to a 2x band joist with either ½” diameter lag screws or bolts, fell significantly below the 500 pound horizontal load target due to inadequate load transfer from the band joist into the surrounding deck floor framing. Ultimately, the research found that anchoring the post with a holdown installed horizontally provided enough leverage to meet the target load. The article also discussed the importance of testing to 500 pounds (which provides a safety factor of 2.5 over the 200-pound code live load), and the testing with a horizontal outward load to represent the worst-case safety scenario of a person falling away from the deck surface.

Simpson Strong-Tie has tested several connection options for a guard post at the typical 36” height, subjected to a horizontal outward load. Holdown solutions are included in our T-GRDRLPST10 technical bulletin. In response to recent industry interest, guard post details utilizing blocking and Strong-Drive® SDWS TIMBER screws have been developed (see picture below for a test view) and recently released in the engineering letter L-F-SDWSGRD15. The number of screws and the blocking shown are a reflection of the issue previously identified by the Virginia Tech researchers – an adequate load path must be provided to have sufficient support.

SDWS Detail C: Interior Post on Rim Joist between Joists, at 500-Pound Horizontal Test Target Load

SDWS Detail C: Interior Post on Rim Joist between Joists, at 500-Pound Horizontal Test Target Load

Have you found any other resources that have been helpful in your guard post designs? Let us know by posting a comment.

2015 IRC Adds New Options for Deck Construction

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.

2015 International Residential Code

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

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.

Minding the Gap in Hangers

Mind The Gap sign

Mind The Gap sign

Have you ever seen this famous sign? You may have seen it while riding the London Underground, to draw attention to the gap between the rail station platform and the train door. The warning phrase is so popular that you may also recognize it from souvenir T-shirts or coffee mugs.

In the connector world, the phrase comes to mind when thinking of the space, or “gap” between the end of the carried member and the face of the carrying member. Industry standards for testing require that a 1/8” gap be present when constructing the test setup (in order to prohibit testing with no gap, where friction between members could contribute significantly), so this is the gap size that is typically permitted for the joist hangers listed in our catalog.

Gaps exceeding 1/8” can affect hanger performance in several ways. A larger gap creates more rotation for the connector to resist by moving the downward force further from the header. Fasteners may also have reduced or no penetration into the carried member due to the gap. Testing confirms that these factors decrease hanger allowable loads for larger gaps.

Hanger installation with gap

Example of field installation with a 1” gap (approx.)

What are my options then if the field conditions create a gap larger than 1/8”? We have performed testing to establish allowable loads for many common joist and truss hangers with gaps up to 3/8” (up to ½” for HTU hangers), as well as testing for possible field remedies and repair scenarios. Our technical bulletin T-HANGERGAPS10 provides this information, along with a design example, and general recommendations and guidelines for preventing gaps. Notes on shim details are also included – shim size, material, and attachment (independent of the hanger fasteners) are key design considerations that must be covered by the engineer or truss designer.

What is your experience dealing with hangers that exceed 1/8” gaps? Let us know in the comments below.

Wood-framed Deck Design Resources for Engineers

This week’s blog was written by David Finkenbinder, P.E., who is a regional engineer working out of the Simpson Strong-Tie Ohio branch which services 24 states through the Northeast, Midwest, and Mid-Atlantic. He graduated from Penn State with a B.S. in Agricultural and Biological Engineering in 2004 and earned his M.S. in Civil Engineering with a focus on Structural Engineering from Virginia Tech in 2007. His master’s thesis investigated the splitting strength of bolted connections in solid-sawn lumber and structural composite lumber. Since joining Simpson Strong-Tie in 2007, David has shown a passion for deck safety and has served on committees developing prescriptive information and building code provisions for decks. Here is David’s post.

“Decks cause more injuries and loss of life than any other part of the home structure. Except for hurricanes and tornadoes, more injuries may be connected to deck failures than all other wood building components and loading cases combined.”

This quote, taken from Washington State University’s magazine article Making Decks Safer, underscores the critical importance of proper deck design, construction, and maintenance. An engineer who is encountering their first deck may be surprised that the deck design resources available are not as plentiful as he/she might have expected. The following resources can be helpful start:

For decks built to the IRC, the book Deck Construction Based on the 2009 International Residential Code provides a review of applicable code provisions and related commentary. The book gives background on important durability considerations such as flashing at points where the deck connects to an adjacent structure. The book also briefly discusses variations with IBC provisions, which can be significant for examples such as minimum guard height and live loads.

The American Wood Council (AWC) has several tools available in addition to using the NDS for wood member and connection design. Calculators for evaluating simple span joists and single fastener connections are available in both web-based and mobile app format. Technical Report 12, which was the topic of our May blog post, provides the ability to design connections with a gap between members, or with members having a hollow cross section. AWC’s DCA6 – Prescriptive Residential Wood Deck Construction Guide presents information for common deck details and a commentary covering important considerations for alternate designs. While the guide is helpful, please note that it is limited in scope to single level residential decks and does not address wind or seismic design.

Researchers at Virginia Tech and Washington State University conducted laboratory testing and published information to help in several common topics needing attention. An article in the May 2008 issue of Structure Magazine featured test performance of ledger-to-band joist connections using bolts or lag screws – this information has since been adopted into the IRC.

For lateral design there has been some uncertainty regarding lateral loads that can be generated by occupants, and if the magnitude of such is significant in comparison with wind and seismic forces calculated from ASCE 7. Tests were conducted of occupants performing several types of movement on a deck floor configuration. Separate articles summarizing results for each load type were published in the Summer 2013 issue of Wood Design Focus, along with a fourth article on the lateral performance of IRC ledger attachments (online copies of the articles courtesy of Professional Deck Builder magazine: Wind Loads; Seismic Loads; Occupant Loads).

Our January 2013 post, Corrosion: The Issues, Code Requirements, Research, and Solutions, touches on the corrosion considerations that are significant for most projects as well.

Have you found any other resources that have been helpful in your designs? Let us know by posting a comment.