Truss design software simply uses the lumber inventory to determine the least expensive solution when designing a truss simply based on price. Although this is a convenient function, there are several issues the software does not consider – and for these issues a designer should be making the determination. To illustrate this, here are two similar examples.
In the first example, shown in Figures 1 and 2, a top chord bearing 2 x 4 truss calls out for a bearing block to be plated in the bearing. The bearing block selected is a 2 x 8. Unfortunately, due to the short length of the member, the material must be carefully selected to prevent the splitting of the material when pressing. Even careful selection does not guarantee the member will survive pressing, much less installation, without breaking into multiple pieces. [For all photos, See PDF or View in Full Issue.]
In the second example, it is apparent that 80% of the trusses suffered damage when pressing. Each time the truss was pressed, disaster resulted. So, what would be a better solution?
One solution would be to carefully select lumber with significant figuring (grain pattern) resulting in a stable web member.
A better solution would be to utilize laminated veneer lumber suitable for pressing. This lumber needs to be 1-½” thick like the truss members and be approved for truss use. The concern might be that some products are too dense to be pressed. This needs to be approved by your engineer.
A third solution might be to use a hanger or device to achieve the design connection.
A fourth solution might be to require field-installed bearing plates to raise the truss to the required elevation.
The Bottom Line
The worst solution is to depend on lumber to achieve the impossible. Small blocks do not have the strength parallel to the grain to handle this sort of application and should not be used.
It is important to remember that design software needs to be used by qualified individuals in order to perform effectively. The software is not autonomous, nor should it be expected to provide the best answers without any human input. Especially when it comes to quality, brain power can be more important than computing power.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.