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Field splices provide a means of connecting two truss sections together at the jobsite to create a single larger and/or deeper component. The goal of field splicing is to allow truss manufacturing, shipping and installation greater flexibility in serving customer needs. To successfully use field splices on a project, however, there are a number of issues to consider during the design and installation phase.
Why are field splices required in the first place?
There are several reasons why it may be necessary to design a field splice. For starters, a truss may simply be too large to manufacture as a single component. High pitch scissors trusses are a typical example, which can be difficult to fit on a truck and to handle. A retrofit may also require a field splice due to a change in the truss profile (such as adding a vault to a truss with a flat ceiling), a change in loading, or other modifications discovered after the truss is built and/or installed.
Common Materials Used for Field Splices
If connection forces and deflections are low, plywood or oriented strand board (OSB) gussets provide a flexible and cost-effective field splice option. One advantage of using plywood or OSB is that the material can easily be cut to the shape needed. For any field splice, make sure the splice material and its connection to the truss have adequate strength to resist the maximum forces that will transfer through the splice. (See photo: Example of field-spliced scissors trusses.)
Lumber scabs used as field splice material provide an easy option if it is possible to make the splice at a suitable location. Lumber scabs can often be used with high pitch scissors trusses if the splice location can be shifted away from the peak. In these instances, the truss is spliced by applying lumber scabs across the splice joint parallel to the top and bottom chord. The long available lengths and ease of installation make lumber scabs a very effective choice.
Metal plate connected wood scab trusses provide an excellent alternative as field splice material, especially when the forces to be transferred through the splice are very large. Scab trusses are designed to transfer the forces across the splice and should match the profile and configuration of the trusses being connected. Attaching the scab truss(es) to the spliced trusses through the aligning members provides a strong and stable splice connection.
Field splices using steel plates with bolted connections are another possibility, although they are far less common due to the relatively high cost of ordering custom plates. Benefits of using steel plates include the ability to withstand and transfer very high forces and relatively easy installation.
Common Errors with Field Splices: Design Department
A common error when designing trusses that will be spliced in the field is designing each part as an individual truss instead of as a single truss covering the entire span. Designing the trusses as individual parts typically results in truss members and connector plates that are undersized for the loads and forces that will be resisted by the completed field-spliced truss.
Some of the available truss design software allows the designer to assign different part numbers when designing a field-spliced truss. For example, setting the members on the left side of the splice location as “part 1” and the members on the right side as “part 2” will design each part as members of the full-length truss, but direct the program not to design a plated connection between the two sections.
Field splices for multi-ply trusses are more difficult to design and require more attention to detail. Since multi-ply trusses are wider than single-ply trusses, field splices are required on both sides of the truss to minimize eccentricity and transfer the forces on both sides of the truss. Field splices for trusses greater than two plys typically will require a greater depth and/or a higher grade of lumber than the material in the trusses themselves because the forces at the splice must be transferred through fewer pieces. The connection between the field splice and truss is also critical and must ensure that all of the plys are adequately connected to the field splice. This is a complicated splice condition and will require good communication between the truss designer and the customer installing the truss scab splice. (See figure: Example of a steeply pitched scissors truss requiring a specially designed field splice at the top and bottom peak joints.)
Common Errors with Field Splices: Jobsite
BCSI-B2 provides guidelines and industry best practices for installing field-spliced trusses:
Installers must ensure that everything is connected tightly. Failure to do so can result in increased deflection of the field-spliced truss and potential serviceability problems long after the building has been completed. When using OSB or plywood, make sure the sheets are not overcut. Use a rounded edge along the bottom chord at a scissors connection to help reduce the stress concentrations at this location and the risk of cracks developing. (See photo: Example of a field-spliced scissors truss being lifted into place.)
If using bolted steel plates for the field splice, make sure the connections are tight and all of the holes are properly sized in accordance with the requirements specified in the National Design Specification® (NDS®) for Wood Construction. Failure to do so can lead to an uneven distribution of the load between the bolts, resulting in a few of the bolts carrying a disproportionately large amount of load, which can potentially lead to a failure of the connection.