During the last several years, wood science researchers at Virginia Tech have scrutinized the structural connections commonly found in residential wood decks.
For this article we turned our attention to residential deck railings — the guardrails intended to prevent people from accidentally falling off the edge. When decks rise more than a couple of feet off the ground, such accidents can be serious or even deadly, as news reports have corroborated. With many decks standing 8 feet or higher above grade, this is not an issue a builder can afford to ignore.
For this article, we applied measured loads to deck posts to see which connections could meet code. Looking around our area of Virginia, we spotted railing connections that made it clear that some deck builders aren't aware of code requirements for deck railings (see Figure 1, right).
What Code Says The 2003 International Residential Code (IRC Table R301.5) specifies a minimum concentrated live load of 200 pounds for both guardrails and handrails. Footnote “d” defines the application of the 200-pound load as “a single concentrated load applied in any direction at any point along the top.” Judging by what we were observing in the field, it seemed obvious that many deck railings would not pass this loading requirement.
A guardrail is really a system of components, including posts, railings, and pickets (or balusters), connected together and fastened to the deck. Rather than look at the entire guardrail system, we narrowed our testing to post connections. There are many ways to attach deck posts; we decided to limit the possibilities to methods frequently used by carpenters in our geographic area.
What's Being Built By far the most common details we found locally were the cases shown in Figure 1, where the post attaches to a “band joist” at the outer edge of the deck structure. These posts are typically notched (see “Why Not Notch?,” below), but not always, so we decided to test the connections both ways.
Although many of the post connections we observed were obviously loose and allowed us to shake the railing, some of the posts seemed strong. But the question we wanted to answer was whether these connections would stand up to a code-protocol test load.
Lever-Arm Effect Making a connection to resist this force at the base of the post is harder than you might think, because of the lever-arm effect (force x distance): The magnification of this 200-pound horizontal force produces a couple of thousand pounds of load at the base of the post.
A guardrail post can behave like a lever: The force applied at the top gets multiplied by the length of the post — the lever arm — to produce a large moment, expressed in inch-pounds, at the base. The resisting force at the base, here represented by a single bolt, is also multiplied, but by a much shorter lever arm — 5¼ inches in this example. In the case shown here, representing a typical residential deck rail post 36 inches high, the bolt would have to provide nearly 2,000 pounds of resisting force. Although the steel might be up to the task, the wood fibers under the washers would not be strong enough, as the authors' tests indicated.
Next month we'll follow up with “Rail-Post Connections for Wooden Decks, Part 2.” —Joseph Loferski is a professor in the Department of Wood Science and Forest Products at Virginia Tech University, Blacksburg. Frank Woeste, P.E., is professor emeritus in the Department of Biological Systems Engineering at Virginia Tech. Dustin Albright is a graduate research assistant and Ricky Caudill is a lab technician. Adapted from “Strong Rail-Post Connections for Wooden Decks,” THE JOURNAL OF LIGHT CONSTRUCTION, February 2005.
Why Not Notch?
Several of the 4x4 posts we tested were notched around the band joist — a common detail in the field. Although you might expect the notch to be the weak point in the connection, in fact none of the test posts failed at the notch. Even so, notching should be avoided, because it does substantially reduce the strength of the post. Here's why: