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DeckLok in the PressBack to the main In The Press Page Alternative Hardware for Guardrail to Post Connection Test resultsBy Michael G. Morse and Daniel P. Walsh, P.E. Mar/Apr 2005 Professional Deck Builder Magazine Almost simultaneously to the research conducted at Virginia Tech, testing of the guardrail post to deck connection was performed at the University of Maryland. The purpose was to determine the effectiveness of the Decklok bracket, manufactured by Morse Technologies, in preventing a collapse of the guardrail to post connection when 500 lbs of lateral force was applied. During the research test units were evaluated that replicated five different configurations. These test units simulated current industry-standard methods for connecting timber deck guardrail rail posts to deck structures. For each of the five configurations, three samples were tested to eliminate anomalies and for comparison of results.
All of the test units were constructed using pressure-treated No. 2 Southern Pine 4x4s attached to No. 2 Southern Pine 2x8 band boards using ½” hex-bolts, simulating the common practice of attaching posts with carriage bolts and retaining nuts. The band boards were then connected to the deck joists using four 16-penny nails. 5/4x6 deck boards were screwed to the deck joists using two 2”, No. 8 galvanized decking screws at each joist. Due to the introduction of multiple decking products being used in place of solid-sawn lumber decking, we did not rely on the decking to provide structural support to the tested post assembly. Therefore, the decking installed on the test units did not contact the band board being tested and thus was not allowed to provide support for the band board and post rail connection. Each unit was then tested on a fabricated steel load frame. An increasing lateral load was applied to the 4x4 at a point 36” above the deck surface to measure resistance and to record the necessary data to demonstrate code compliance by the test approach. These tests did not take into account the effects of weathering. Deck components and their connections deteriorate over time. The load capacity of screws, nails, hardware, and wood products diminish with use and age. The test units were constructed with new lumber and fasteners. It is most probable that the load capacity of these, or any connections, would diminish with the effects of time and weathering. Summary and ConclusionCommon and industry accepted practices for deck railing construction and attachment can lead to unsafe and potentially life threatening installations. Many of these installations may quite possibly fail to meet the minimum IRC requirements for lateral loading. A metal bracket system, Decklok, has been developed by Morse Technologies that permits the deck railing system to meet and exceed the IRC lateral load requirements. The bracket has been tested in many of the typical deck railing construction configurations, and compared to the load resistance of the more common construction techniques. This system strengthened the connection of the rail post to the deck and allowed the lateral force to be transferred to other deck components. With every test that was run, a control test unit was tested first to establish a baseline performance level that represents the current construction methods used in the deck building industry. In all cases, the deck railing system represented by the control test unit failed at the connection points, and failed under lateral loads that were below the minimum IRC load requirements. The alternative deck railing construction method that utilizes the DeckLok bracket system resulted in significant improvement in the deck railing performance. All bracket tests resulted in deck railing load resistance that surpassed the minimum IRC lateral load requirements. Under continued loads far beyond the minimum requirements, the primary failure was ductile, rather than timber failure. That is, the rail post (4x4) was allowed to shift gradually, transferring load to the decks tructure and remaining substantially intact. This introduces a controlled, predictable mode of failure and moves catastrophic failure to a secondary failure position. The brackets exhibited excellent ductility and provided reasonable warning that the deck guardrail was in danger of being overloaded and pushed to failure. That same ductility also allowed for a realignment of the timber members of the deck to more effectively absorb energy. It should be noted that there was an inconsistency in the load capacity between the 4x4 posts used in the testing. Each timber was examined carefully, picked for those with the fewest knots, cracks, or checking. However, the difference in their load strength was surprising. The 4x4 post from test 2A experienced total failure at 525 lbs, where a similar 4x4, bought from the same lot, the same day, at the same store, withstood 1325 lbs and did not fail (the test was stopped to avoid damage to testing equipment and testing personnel). The table below summarizes the test results. With the DeckLok brackets installed there is substantially improved lateral load capacity. The load capacities shown in the column titled “Load capacity with DeckLok” reflect the average of the three test units in each configuration. Please note that, in each test of every configuration, the minimum load requirement was met or exceeded.
About the AuthorsMichael Morse is the president of Morse Technologies, which develops safety related devices in the construction and medical fields. Daniel Walsh, MS, P.E. is vice president of structures for Athavale, Lystad & Assoc. in McLean, Virginia. He has been designing structures since 1980. Mike and Dan are long time “Do It Yourselfers” who have been collaborating on projects for the last twenty years. And still, they remain friends! © Professional Deck Builder Magazine |
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For Lateral Loads, you need a Lateral Anchor!
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