When researchers at Virginia Tech applied measured loads to deck posts to see which connections could meet code, they discovered that two of the most common post connections (4x4 pressure-treated post notched and lag-screwed to the band joist, and post through-bolted to the deck band joist) cannot stand up to code-protocol test load.
Test Specs Height: We assumed that the top of the railing was 36 inches above the deck surface (the minimum height allowed by the IRC).
Deck boards: at most 1.5 inches thick.
Lumber: pressure-treated (ACQ or CA-B) 2x8 Southern pine to simulate the joists and No. 2 Southern pine 4x4 posts. The lumber was bought in “wet” condition (moisture content greater than 19%) to eliminate the need for applying an adjustment factor for “wet use” to the test data.
Test Results Five samples each of the bolted and lag-screwed post connections (see “Rail-Post Connections for Wooden Decks, Part 1,” May 2006) were tested by applying a horizontal force at the top of the post until the post detached from the joist structure or the band joist itself came off. Although the bolted and lagged connections appeared sturdy, neither came close to meeting the target 500-pound test load. A piece of 5/4 decking nailed to the simulated joists and band joist did little to prevent failure.
The lag-screwed connection failed at less than 200 pounds when the lags pulled out of the band joist. The bolted connections failed at an average load of 237 pounds — barely surpassing the code design load but with almost no safety factor for the service life of the deck.
After testing the bolted and lagged connections, the authors tried reinforcing the post with blocking in a variety of configurations, each of which failed when the block split along the grain.
Post Connections That Passed As testing progressed, we realized that the high forces at the base of the post were not going to be resisted by fasteners loaded in withdrawal or by blocking loaded perpendicular to the grain.
We turned to a commercial steel connector — a Simpson Strong-Tie HD2A — designed to resist wind and earthquake loads in shear walls. By orienting the connector sideways along the joist, we were able to use it to secure the post. We installed the HD2A with three ½-inch-diameter bolts: The two bolts in the joist are loaded in shear, while the third bolt, passing through the post, the band, and the connector itself, is loaded in tension. As part of the tested design, we also installed another ½-inch bolt in the lower part of the post and the band joist. We applied at least 650 pounds to the top of the post; every specimen successfully resisted the load.
We tested the connection two ways: with the post located inside the band and on the outside. We observed different types of failure for the two cases as the load increased.
When the post was mounted inside the band, the washers under the bolt head embedded into the wide face of the the 2x8 band joist. When the post was located outside the band, the bolt head and washer pulled well into the 4x4 post, crushing the wood fibers beneath the washer.
Limitations of Test Results We didn't test the HD2A connector post-to-deck assembly in the inward loading mode — say, the load of a tree falling against the railing. In our judgment, the assembly as tested would not carry a 500-pound inward force. However, we believe that the assembly would carry 500 pounds in either direction if you were to install two HD2A connectors per post, one 2 inches from the bottom of the 2x8 band joist and one 2 inches from the top.
Our test also applies only to the grade and species of lumber that we used. Keep in mind that Southern pine is denser than most other common framing species (specific gravity [SG]=0.55), which affects its ability to hold fasteners. Pressure-treated hem-fir is commonly available in the Western states, but because hem-fir is less dense (SG=0.43) than Southern pine, the same connections made with hem-fir lumber would probably fail at a lower load. —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.
We used only one Simpson Strong-Tie HD2A per post, placing the centerline of the connector 2 inches below the top edge of the 2x8 joist. If you use this detail in the field, be sure to maintain this centerline distance, because it helps to limit the forces involved in the connection. If you place the HD2A lower, you're reducing the resisting lever arm, which extends from the bottom of the band joist to the centerline of the connector. Losing even an inch of this resisting lever arm would greatly increase the forces in the connector.
We used a hot-dipped galvanized (HDG) HD2A connector for our tests. Because of the corrosive nature of the new lumber treatments, this is the version that should be used in practice. The ½-inch bolts, washers, and nuts should also be hot-dipped galvanized.