Whether you’re starting a new project that includes trusses or retrofitting an existing one, you’re going to need Truss Design Drawings (TDDs) that are certified by a Professional Engineer. A TDD includes the essential characteristics of the truss itself, the inputs that went into loading and designing it, and the key outputs of the analysis. A TDD must include certain information in order to be valid; let’s walk through what information that is together.
Required Information
According to ANSI/TPI 1-2014 section 2.3.5.5, the following information must appear on a valid TDD:
- Building Code used for design: This is the building code used to develop the loading conditions and other constraints for the truss design, e.g. IBC 2018. Which code to use is determined by the jurisdiction in which the truss will be erected.
- Slope or depth, span and spacing: This includes how much space is occupied by the truss profile as well as the distance between this truss and the next one
- Location of all joints and support locations: This means dimensional information about where members of the truss intersect and where the bearings on which the truss rests are located relative to the truss
- Number of plies: A multi-ply truss is one where the same design is duplicated a certain number of times and then all the duplicates are fastened together into a single, much stronger component. The TDD must specify the number of these duplicates if one or more exist
- Required bearing widths: The bearing area required to support the truss is a function of the material of the member that rests on each bearing and the forces at that location (see TPI 7.4.1)
- Design loads as applicable: this includes all of the loading conditions the truss has been designed for, including the live and dead loads that span the entire top and bottom chords, environmental loads due to rain, snow, seismic, and wind, additional loads and where they’ll be applied, and lateral loads like drag strut loads. Any factors used in loading calculations, such as duration factors, need to be listed as well
- Truss Pal TDDs include a list of every load case that was considered. Each load case is a combination of load types along with their duration factors. See the Notes section for any additional loads that were applied
- Adjustments to Wood Member and Metal Connector Plate design values for conditions of use: These are factors that adjust the typical material strength properties of the members or plates. Examples of this include the Wet service factor, Quality control factor, Temperature factor, and Incising factor
- Maximum reaction force and direction: This refers to the reactions at each bearing in the down, up, and horizontal directions
- Metal Connector Plate types and joint offsets: The types of metal plates must be specified as well as the offset distances for any plates that are not centered on a joint
- Size, species, and grade for each Wood Member: The basic lumber properties of each wood member must be specified
- Truss-to-Truss connection and field assembly requirements: For trusses that carry other trusses, the TDD must describe how they should be connected. Any other detail that directly impacts the truss’s performance, like material that will be attached to it in the field, should be specified as well
- Deflection ratio and/or max vertical and horizontal deflections: This refers to the “L/d” ratio and total truss deflection under particular load combinations (see TPI 7.6)
- Max axial tension and compression forces in Truss members: The largest forces experienced by each wood member and whether they are tensile or compressive must be listed
- Truss Pal TDDs include all of the forces that occur between each truss joint that are greater than or equal to 250 lbs
- Fabrication tolerance: This is captured in the Quality Control Factor (Cq) and is used to account for imprecision in the placement of connector plates on the truss (see TPI 6.4.10)
- Required permanent member restraints: This could be sheathing, a rigid ceiling, or evenly spaced purlins over the chords or lateral bracing for webs. The locations of the restraints must be specified as well
- Truss Designer: The individual or organization who produced the truss design
Additional Information
Besides the additional information about forces and load cases already mentioned, Truss Pal TDDs also provide the following:
- Maximum CSI: A summary of key member check results that indicates how stressed the members are
- Maximum JSI: A summary of key plate check results that indicates how stressed the connector plates are
- Exposure Criteria: This table reflects information provided about whether or not portions of the truss, such as an end vertical or cantilever, are exposed or covered up, which primarily affects whether wind loads are considered in those areas
- Camber: Camber is the curvature of the bottom chord that would be required to compensate for the deflection of the bottom chord due to dead load
- Weight: This is the self-weight of the truss including all lumber and plates