Do You Know What It Takes to Pick the Perfect Plate?

Truss designers rarely think about what their software is doing in terms of plate selection, but they often need to make decisions about web connections and plating. Understanding the basics of metal connector plate testing and design can make it easier to spot potential problems and pick the best plate for a joint.

At a truss joint, the teeth of a connector plate transfer forces from one wood member into the plate itself. Shear and axial forces in the truss plate are then transferred across the joint line, where teeth on that side of the joint transfer the forces into the adjoining wood member. In essence, the truss plate acts like a bridge.

Several strength limits exist in this load path and each must be checked during the joint design. According to ANSI/TPI 1-2014 (TPI 1), truss designers are required to check the lateral resistance of the truss plate teeth as well as metal tension and shear in the truss plate. Other checks include combinations of shear and tension or flexure and axial load in the truss plate.

The key to being able to perform these checks is an understanding of the strength of the truss plate. The mechanical properties of sheet steel are well defined. However, once that material is stamped into a truss plate, its strength properties become much harder to measure.

First, there is significant variation among manufacturers in the size and placement of tooth slots, which means there can be significant variation in the shear and tensile strength of similarly-sized plates.

Second, the combination of unique tooth shapes for different vendors and multiple wood species means that the lateral resistance—or tooth holding power—is different for each plate and species combination.

Finally, the properties of plate-to-wood connection depend on the
orientation of the wood grain, and a myriad of angles can be formed between wood members in truss joints. So while the strength of steel is easy to measure, connector plates are a challenge to assess.

Due to the complexity of the issue, TPI 1 requires testing, rather than a theoretical analysis, to determine truss plate design values. Testing aims to determine three properties of metal connector plates: lateral resistance, shear strength and tensile strength. The metal connector plate design values determined from testing are then used to size the plates for each joint in a truss.

In addition to the checks for sizing the plate, TPI 1 requires calculations of the required number of teeth for each plate contact area and the plate positioning tolerance, or how much each plate can be rotated or shifted without compromising its strength properties.

Because these calculations are built into the checks truss design software performs, how a plate is designed has a direct impact on the QC process. With a good general understanding of the factors involved in plate design, truss designers are better prepared to spot potential issues and have a positive impact on QC.

“Our QC program helps ensure we send out trusses with the right size plates in exactly the right spot,” says Scott Ward, principal of Southern Components, Inc. in Shreveport, Louisiana and a user of SBCA’s In-Plant Wood Truss QC Program. Ward currently heads SBCA’s QC Committee, a group focused on helping component manufacturers ensure that every truss they ship out is of sufficient quality to resist the design loads despite any minor defects in lumber or plating. In this ongoing article series, we’ll continue exploring how plate design impacts placement tolerance and why QC is such a critical step on every production floor.