Industry Testing at SBCRI Is Multi-ply-ing


Industry Testing at SBCRI Is Multi-ply-ing

Truss ply-to-ply connection design is
a high-priority for industry testing at SBCRI

What will happen if the plies of a multi-ply girder truss are not fastened together properly? Overloading and, potentially, chord fractures can occur.  

Currently, there are no load distribution provisions in ANSI/TPI 1-2014 (TPI 1) for the design of multi-ply girders. Section of TPI 1 states: “Connections shall be designed to transmit load from ply to ply in accordance with the ply-to-ply load distribution assumed in the design of the girder.” The goal of conducting industry testing on this issue is to determine the appropriate loads for use in designing the ply-to-ply connections. As a consequence, SBCA has identified truss ply-to-ply connection design as its next high-priority topic for industry testing.

The SBC Research Institute (SBCRI) has come a long way in its testing capabilities since it opened in 2007. One of the most recent endeavors was to develop a test setup for evaluating the load distribution between plies of a multi-ply girder truss. The setup needed to measure the reaction of each ply of the multi-ply truss.

Failure of a multi-ply girder at the SBC Research Center (SBCRI) in Madison, WI.

Last October, attendees of a full-day tour of SBCRI witnessed a demo test of a multi-ply girder truss. The truss plies were connected with two rows of 3-inch long by 0.131-inch diameter nails spaced 24 inches on center along the bottom chord and one row of 3-inch long by 0.131-inch diameter nails spaced 24 inches on center for the top chord and web members. This nailing pattern exceed the maximum spacing of 12 inches on center specified in TPI 1, but was selected to force the first ply of the truss to separate from the rest of the girder at failure.

The demonstration test was a success. SBCRI now has the test fixturing in place to better understand the load transfer mechanisms that take place in a girder truss or multi-ply LVL assembly. The future research conducted with this setup can help component manufacturers better understand and improve techniques for connecting girders, both in the truss plant and on the jobsite. Ease of application and labor efficiency, no matter where it occurs, will always benefit of the truss industry.

Further, this test setup can help those who manufacture fasteners develop connection systems based on an accurate understanding of efficient girder connections. Only time will tell where proprietary research and development will go from here.

Left: The multi-ply test setup. Loads were applied every 2 feet on center using truss hangers.
Right: Lateral restraints supported the test assembly and prevented out-of-plane movement.

Left: Two load cells were used for each ply of the multi-ply girder to measure the reaction for each truss.
Right: String potentiometers measured the deflection of each ply.

Load-deflection plot for each ply of the girder truss.
Many thanks to Bob Dayhoff, P.E. at Shelter Systems Limited for his input on this article.

About the Author: As technical manager of the SBC Research Institute, Daniel Lawless has been involved in the testing and analysis of a wide variety structural systems for the building component industry. Daniel graduated from the UW-Madison with an M.S. in Civil Engineering in 2013 and enjoys the opportunity to use applied research and testing to expand the engineering communities’ knowledge of structural systems.