Many in the construction industry have run into this problem—not enough bearing. When the forces transferred through the interconnecting triangles of a truss are more than the beam or the top plate of the wall it bears on can handle, this can lead to crushing of the bearing over time. To avoid crushing, the capacity of the bearing must be higher than the compressive force.
Let’s look at an example where the top plate of the wall is 2x6 SPF No. 2. Assuming a single-ply truss, the total bearing area of the truss on the top plate of the wall is 8.25 in2 (1.5 in x 5.5 in). The allowable compression perpendicular to grain of SPF is 425 psi (pounds per in2). Therefore, the allowable reaction onto this bearing example would be 3,506 lbs (425 psi x 8.25 in2). If the truss has a higher reaction than 3,506 lbs, then the bearing area is insufficient.
What’s the best way to deal with insufficient bearing area?
When calculating the required bearing length, the Truss Designer should base the calculation on the species of wood for the material that will support the truss. Most truss design software allows the designer to select the lumber grade of the bearing the truss is set on. The Truss Designer calculates the required bearing length and compares this to the length of the bearing indicated in the Construction Documents provided by the Building Designer. It is essential for the Building Designer to provide accurate wall or beam/header information in the Construction Documents in order for the Truss Designer to arrive at the correct bearing length. When the bearing area is insufficient, there are a few options to resolve the issue.
Upgrade Lumber: The species and grade of lumber used in the truss, as well as the lumber used for truss bearing, affects the bearing area requirements. When the bearing width is insufficient, one option is to upgrade the lumber used in the truss or truss bearing. In some instances, the bearing area of the wall or beam may need to be increased. If the bearing material, wall or beam is altered, the Building Designer would need to be involved in making this change.
Add a Bearing Block: Bearing blocks are another way to deal with insufficient bearing area. Adding bearing blocks widens the surface contact, providing more bearing area to distribute forces from the truss. The truss design drawing will call out if a truss requires bearing blocks, along with the proper size, position and nailing pattern. Similarly, adding plies to the truss can increase the contact area (by increasing the width).
Raised Heel Trusses and Compression Parallel vs. Perpendicular to Grain: There is also the possibility of using a raised heel. With the end vertical running up and down through the bottom chord, this option can significantly increase the allowable compressive stress. While this provides a higher crushing value of the truss, it does not increase the crushing value for the bearing material.
See Figure 1, which builds off of the example provided in the introduction to this article. The graphics illustrate the differences between compression parallel to grain and compression perpendicular to grain. In addition, Table 1 lists wall top plate compression perpendicular to gain values for some common construction lumber species. Obviously, these numbers would be different for other materials and sizes.
|Common Construction Lumber Species||Wall Top Plate Compression Perpendicular to Grain Stress||2x4 Wall Truss Reaction Load Capacity (lbs)||2x6 Wall Truss Reaction Load Capacity (lbs)|
|Southern Pine||565 psi||2,966||4,661|
|Douglas Fir||625 psi||3,281||5,156|
Use a Bearing Enhancer: Truss bearing enhancers are metal plates that take on load from the truss. These products are available through a number of suppliers in different sizes and configurations. While it’s outside the typical scope of work for component manufacturers, some offer bearing enhancers as an add-on, much like supplying hangers to their customers. When using a truss bearing enhancer, it’s important to follow the manufacturer’s installation instructions and use the correct fasteners and top plate size.
Run Plates Over the Bearing Flush to the Bearing: When designing with TPI 1 2007, Section 18.104.22.168 allows the bearing capacity of the truss lumber to be increased by 18 percent. To take advantage of this increase, the truss must bear on the 1.5 in face of the lumber, and the plates must be no more than ¼ in from the bearing. Again, while this will improve the bearing capacity of the truss, the capacity of the truss bearing must also be considered.
Which Option Works Best? Choosing the best method to manage insufficient bearing width will depend on the project. Bearing blocks may work well on one project, while bearing enhancers may be the best way to go in another situation. Sometimes it requires a combination of both methods. All of the options above should be considered on a case-by-case basis.