Too much pressure is not only a problem in our lives, it can also be a problem in our plants. One of the most important issues when manufacturing quality trusses is plate embedment. Without proper embedment, the connector plate cannot adequately transfer load forces through the members down to the bearing or supports. Enough pressure is required to achieve proper embedment, but too much pressure creates additional problems.
Proper embedment is when the plate is pressed into the member with no more than 1/32” gap remaining between the plate and lumber face. In the early years of the industry, the truss building process depended on hydraulic presses to make this happen. These presses were very effective, and many are still used today.
Through the years, additional methods were developed, including huge weights of concrete and other mechanical methods. The most common method in the US today is the use of rollers, sometimes referred to as finish rollers. On roof lines, the final truss is pressed home using a set of pinch rollers. The rollers are set a specific dimension apart. Anything fed through the roller comes out 1 ½” plus a little bit. On most modern floor machines, a finish roller is not necessary because the floor machine is designed to plate the truss home within the jigging of the machine. These machines work similar to the pinch roller. Usually two rollers pinch the truss along the table and force the connector home, or a single roller pushes against a firm table and presses the connectors home.
Unfortunately, sometimes these finishing methods fail to embed connectors. This can occur if the material is not the correct width or thickness, the lumber thickness or widths vary, the density of the lumber is such that embedment is difficult on one pass, or the machine is not adjusted correctly. A standard in-process inspection should be made by the builders and stackers to look for embedment issues. This should be done on every truss.
To inspect the trusses, a simple gauge can be used to determine if the plates are not embedded. A credit card or plastic hotel key works great. If the card can be pushed under the edge of the plate and is tight, then the embedment is good. If it is not tight, then the gap exceeds requirements and should be repressed. On occasion, in an effort to make sure plates are embedded, a plant might tighten up the finish roller or gantry to guarantee embedment. If the adjustment is made too tight, the lumber can be damaged by crushing. This crushing seriously degrades the lumber’s ability to carry loads and should not be allowed. Look for cracks in the lumber or crushing sounds as the lumber is fed through the roller. If lumber is crushed, a rim will be created on the edge of the lumber. This rim occurs when large forces cause the lumber to squeeze thinner and the result is a muffin top. When this happens, reduce the pressure on the roller until the crushing is eliminated. Let’s only see muffin tops at breakfast where they belong!