Deck Guardrail Update

This post is an update to David Finkenbinder’s post on Guard Post Resources from August 13.

As David explained, the requirements in the IRC and IBC for guards are intended to prevent people from falling off of raised surfaces. The failure of this guard is a common source of injuries caused by failures of deck components.

Section R312.1.1 of the 2012 International Residential Code (IRC) states that “Guards shall be located along open-sided walking surfaces, including stairs, ramps and landings, that are located more than 30 inches measured vertically to the floor or grade below at any point within 36 inches horizontally to the edge of the open side.”

Table 301.5 of the 2012 IRC requires that guards and handrails be designed for “[a] single concentrated load” of 200 pounds “applied in any direction at any point along the top.”

David mentioned the article Tested Guardrail Post Connections for Residential Decks, which described a testing program at Virginia Tech that examined the ability of various assemblies to resist this concentrated load at the top of the guard post. But rather than test in any direction, the researchers decided to test in what they considered the most critical direction: outward away from the deck.

Deck guardrail deflection

Simpson Strong-Tie subsequently developed a new tension tie, the DTT2Z, to make an economical connection from the top bolt in a deck post back into the framing of the deck to resist the high tension forces that develop in the top bolt when the top of the post is pushed outward. Several details were developed to try to address the various orientations of the post and deck framing.

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To allow evaluation of assemblies used to resist this deck guardrail force, ICC-ES developed AC273, Acceptance Criteria for Handrails and Guards. AC273 is available for purchase through the ICC bookstore.

Even with the connectors being readily available, deck builders have asked for guard post connection details that do not involve the use of connection hardware. So Simpson Strong-Tie again tested several framing configurations according to the AC273 criteria, using our Strong-Drive® SDWS TIMBER screws and additional blocking to try to prevent the post from rotating. These details are shown in the engineering letter L-F-SDWSGRD15.

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That brings us to the update part.

A committee made up of building officials, manufacturers, deck builders, designers and other interested parties is currently developing a set of code proposals on deck construction for inclusion in the 2018 International Residential Code (IRC). Even though more and more deck information has been incorporated into the last few editions of the IRC, there is still insufficient information in the code to be able to completely build a deck prescriptively. One area of interest is this guard connection. There is a desire to develop prescriptive details for both connection of a 4×4 post to deck framing with blocking and fasteners and for connecting the deck band joist back to the deck framing so that pre-manufactured guard rails can simply be fastened to the deck band with the knowledge that the connection is secure.

The problem is that, with the current requirement, the guard must resist the 200-pound load in ANY direction. All current testing, including AC273, only uses testing in the outward direction away from the floor of the deck. If the post were really required to resist a 200 pound load in the inward direction as well, then two hardware connectors would be required, one on each bolt. However, the belief of the committee is that resistance of 200 pounds in the outward and downward direction is primarily what is needed to ensure the safety of the occupants of the deck.

So they are working on a code proposal to change Table R301.5 of the IRC to require that the guard only resist the 200 pounds in the outward and downward direction and reduce the load to 50 pounds in the inward and upward direction.

The committee recognizes that while this is not necessarily a departure from current practice, it is a departure from current loading requirements in the IRC, IBC, and ASCE 7. So representatives of Simpson Strong-Tie met on September 30 with the NCSEA Code Advisory Committee – General Requirements Subcommittee to get the opinions of this group of active structural engineers. They provided valuable input, including the consideration that at some locations near landings and other changes in elevation, resistance to 200 pounds in the inward direction could be important.

Prior to incorporation of NCSEA’s input, the committee thought the code change might look as shown below.

We are interested in getting additional comments on this code proposal. What do you think? Let us know in the comments below.

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d) A single concentrated load applied in any direction at any point along the top, in pounds.

f) Guard in-fill components (all those except the handrail), balusters and panel fillers shall be designed to withstand a horizontally applied normal load of 50 pounds on an area equal to 1 square foot. This load need not be assumed to act concurrently with any other live load requirement.

h) Glazing used in handrail assemblies and guards shall be designed with a safety factor of 4. The safety factor shall be applied to each of the concentrated loads applied to the top of the rail, and to the load on the in-fill components. These loads shall be determined independent of one another, and loads are assumed not to occur with any other live load.

j) A single concentrated load applied at any point along the top, in pounds. The 200-pound load is required to be applied in either the outward or downward direction, and it is permitted to be reduced to 50 pounds in either the inward or upward direction. The guard is not required to resist these loads applied concurrently with each other.

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.