Building strong Guards: Part 1

    397

    Building codes require a guard along most elevated, open-sided walking surfaces. The 2012 International Building Code (IBC) defines a guard as “a building component or a system of building components located at or near the open sides of elevated walking surfaces that minimizes the possibly of a fall from the walking surface to a lower level.” Even when perceived to be strong and safe, guards often do not meet the strength requirements of the building code because of an incomplete load path from the guards to the structure. Many types of buildings have non-code compliant guards, but this article focuses on wood-framed exterior decks typically found on residential buildings (single family homes, townhouses, condominiums, etc.), where creating an adequate connection between the guard post and the wood deck framing can be difficult.

    This two-part article provides a review of the design, construction, and testing of guard systems connected to wood-framed decks, with a focus on the wood guard post connection to the deck structure. Part 1 reviews the building code requirements related to guards, typical non-code compliant guard post details, corresponding code-compliant guard post details proven through analysis, as well as other guard design considerations.

    Typical wood-framed deck construction

    A commonly observed wood-framed exterior deck assembly consists of the following (variations are many but the principles of the article remain applicable):

    • Deck framing — 2×8 or 2×10 preservative-treated joists, ledger, and rim board. The joists transfer loads to the ledger through hangers and the ledger is fastened to the building structure. The outer edge of the deck is supported either by columns attached directly to the deck framing or by the joists bearing on a beam supported by columns.
    • Decking — Wood or composite.
    • Guard posts — 4×4 wood posts that are either a vertical extension of a post supporting the deck or shorter posts that are fastened to the deck framing and extend to the guard height. The posts are sometimes notched at the rim or interfering joists.
    • Guards — A guard generally consists of a top rail, bottom rail, and balusters, and is installed between guard posts or building walls. Guards are typically constructed of wood, PVC reinforced with metal (or other composite systems), or metal. The top rail of field-constructed wood guards is installed continuous over the top of the wood posts or framed to the posts.

    Code requirements for guards

    The code requirements reviewed in this article relate to guards at the perimeter of wood-framed exterior decks based on the 2012 IBC; the information is mostly applicable to the International Residential Code as well. A summary of the code requirements follows:

    • IBC Section “1607.8.1 Handrails and guards… shall be designed to resist a linear load of 50 pounds per linear foot (plf) (0.73 kN/m) in accordance with Section 4.5.1 of ASCE 7… Exceptions: 1. For one- and two-family dwellings, only the single concentrated load required by Section 1607.8.1.1. shall be applied…”
    • IBC Section “1607.8.1.1 Concentrated load. Handrails and guards shall also be designed to resist a concentrated load of 200 pounds (0.89 kN) in accordance with Section 4.5.1 of ASCE 7.”
    • ASCE 7-10 (2012 IBC refers to 2010 ASCE-7) Section 4.5.1 – Loads on Handrail and Guardrail Systems: “…shall be designed to resist a single concentrated load of 200 lb (0.89 kN) applied in any direction at any point on the handrail or top rail …” (authors’ emphasis). The 50 pound/foot must also be applied in any direction, when applicable, and is not assumed to act concurrently with the 200-pound load.

    The IBC does not define explicit serviceability (deflection) requirements, but designers should consider deflection in order to provide a design that feels strong and safe to occupants. Test standards define deflection limits that will be reviewed in Part 2 of this article.

    Figure 1: Non-code compliant common post detail at rim board.

    Guard post design

    The guard system must resist the code-defined loads in any direction. Some details provided by manufacturers, and others, that can resist Load Case 1 defined below, but cannot resist loads in any direction, include a footnote indicating the limitation.

    A guard post designer must consider at least the following four load cases:

    • Load Case 1 — Load applied to the top rail in an outward direction. This is the most obvious direction and appears to be considered on some decks based on the installation of additional resisting elements.
    • Load Case 2 — Load applied to the top rail in an inward direction. This direction appears to be ignored on many decks based on the lack of resisting elements.
    • Load Case 3 — Load applied to the top rail parallel to the rail (each direction). This direction also appears to be ignored.
    • Load Case 4 — Load applied to the top rail vertically (each direction). This direction is easily resisted on most decks by a detail capable of resisting Load Case 1.

    The load applied to a post at the top rail height (typically 36 inches or 42 inches) typically controls design and must be transferred to the deck framing and then to the building or ground. There are many framing configurations to consider. This article reviews one common post-to-rim connection and one common post-to-end joist connection, and shows that both are inadequate to resist code-required loads. Analogous code-compliant post connections at the rim and end joist also are reviewed.

    The analysis is based on the 2012 National Design Specification (NDS) and the following assumptions: 200-pound load applied to a 36-inch-high guard, 4×4 posts with 1/2-inch-diameter carriage bolts to the rim or end joist, 5/4 by 6-inch deck boards, 2×8 joists, all lumber is pressure-treated southern pine (specific gravity G=0.55), load duration factor Cd=1.6, and wet service factor Cm=0.7. This analysis doesn’t include a 42-inch-high guard (increased loads at the post-to-deck connection) or the 50-pound/foot load required at some guards (potentially increased loads at the post-to-deck connection), but the principles are similar.

    Figure 2: Code compliant post connection at rim board (by analysis).



    Non-code compliant common post detail at rim board

    A typical 4×4 post connection at a rim includes the post fastened to the rim with two 1/2-inch-diameter carriage bolts with no additional resisting elements (Figure 1). As described below, analysis of this connection shows that it is inadequate to resist the loads; testing by others confirms these results (Loferski, et al., 2007). Notching the post further reduces the capacity and is not discussed herein.

    Figure 3: Code compliant post connection at end joist (by analysis).

    Load Case 1 (Load Case 2 is similar) — Application of a horizontal 200-pound load at the top of the post creates a 8,125 pound-inch moment at the post base; the moment resolves into a force couple at the carriage bolts with a tension force of 2,221 pounds. The carriage bolts easily transfer this force to the rim, but require 2-1/2-inch-diameter washers (4-1/2 square-inch bearing area) to prevent the carriage bolt head/nut from crushing the wood. The moment from the post that is imparted on the rim must transfer to the adjacent joists through an adequate load path. The typical nail or screw fastening of the rim into the end grain of the joists is inadequate for the post load. The rim-to-joist connections can be strengthened by fastening the rim to the adjacent joists using two hold down anchors at each adjacent joist.

    Load Case 3 — Application of a horizontal 200-pound load creates the same forces as Load Case 1, but as horizontal shear forces applied to the bolts. The carriage bolts, with a shear capacity of approximately 450 pounds in wood, are inadequate to resist the 2,421 pounds shear. Designers may wish to rely on the continuity of the guards to transfer this load to the building structure or other posts along the length of the rail designed to resist the force, including connections.

    Load Case 4 — The carriage bolts easily resist the 200-pound vertical load in shear.

    Non-code compliant common post detail at end joist

    A typical 4×4 post connection at an end joist is similar to the post at the rim, but the post is fastened to the end joist. Analysis of this connection shows that it is also inadequate to resist the loads.

    Load Case 1 (Load Case 2 is similar) — Application of a horizontal 200-pound load creates the same forces as Load Case 1 (rim), except there are no adjacent joists to help resist the torsion from the post. As described above, the carriage bolts require 2-1/2-inch-diameter washers. The torsion in the end joist must be supported at the building/ledger and rim. These connections, whether with a joist hanger or fasteners, are typically inadequate to resist the torsion. In the non-code compliant installation, the post will experience large deflections and the end joist will likely fail in torsion.

    Load Cases 3 and 4 — Comments are the same as for the rim detail.

    Figure 4: Apparent shrinkage crack occurred after wet pressure treated lumber was installed wet and dried.

    Code compliant post connection at rim board

    An example of a code compliant 4×4 post connection detail that can be proven by analysis utilizes blocking and two hold-down type anchors (Figure 2). If the post must resist Load Case 3, blocking must be fastened to the rim on each side of the post and be in tight contact with the post to resist the forces. The blocking must be fastened to the rim with sufficient nails in the appropriate position to resist the force couple.

    Code compliant post connection at end joist

    An example of a code compliant 4×4 post connection detail that can be proven by analysis utilizes blocking and high-strength structural wood screws (Figure 3). The horizontal 200-pound load creates the same 8,125 pounds-inch moment. Using proper screw spacing, the moment generated by Load Cases 1, 2, and 3 is transferred into the blocking, and subsequently into the joists as vertical shear forces (blocking shear). The number of fasteners required to implement this detail can result in splitting. Contractors must use care, pre-drill holes where possible, and use lumber with less than 19 percent moisture content when installed to limit the risk of splitting during drying cycles.

    Additional design considerations

    Designers should also consider the following:

    • Consider the complete load path from the guard to the building structure or ground. There are often discontinuities that must be addressed.
    • Specify kiln dried after treatment and apply wet service factor Cm=0.7 to nominal fastener capacities. The wet service factor for lumber installed with greater than 19 percent moisture content that will dry to less than 19 percent moisture content is Cm=0.4, and that lumber has a risk of checks and splits forming after installation where fasteners restrain shrinkage (Figure 4).
    • Coordinate posts and deck framing to avoid interferences, eliminating the need for notched posts or joists. Notching posts reduces the post strength and increases the risk of splitting along the grain (Loferski, et al., 2005).
    • Consider extending the posts to the ground to support the deck where practical. All four guard load cases are easily resolved with this configuration.
    • Consider using guards with top rails that are continuous over posts, or provide continuity along the guard system, to distribute perpendicular and/or parallel loads (Load cases 1 through 3) to adjacent posts for load sharing.
    • Use 2×8 joists, or deeper, even if not required by the deck spans. The deeper joists allow the post base connection fasteners to be spaced further apart, which reduces the fastener forces.

    Guard system components are often code-compliant, but there seems to be a lack of consideration for the critical post-to-structure connection in the guard system. While it is not simple to design or construct a post base connection that meets the code, there are practical solutions that can be implemented.

    Analysis is not the only method for providing code-compliant guards. Designers may consider testing as a means for developing post details that meet the building code. In retrofit conditions, some details that do not work by analysis can be proven compliant through testing.

    Part 2 of this article will explore guard and post testing to demonstrate code compliance.


    References

    • Loferski, Joseph, Dustin Albright, and Frank Woeste, Ph.D., P.E., 2007, Tested Guardrail Post Connections for Residential Decks, Structure Magazine, July, pages 55-59.
    • Loferski, Joseph, et al., 2005, Strong Rail-Post Connections for Wooden Decks, The Journal of Light Construction, February.

    Scott A. Tomlinson, P.E., and Erik W. Farrington, P.E., are senior project managers at Simpson Gumpertz & Heger Inc. Reach them at satomlinson@sgh.com and ewfarrington@sgh.com. Makoto S. Weinstein, EIT, is a staff I at SGH. Reach him at msweinstein@sgh.com.