Allowable Quality Tolerances
Question:
What tolerances are allowed for metal plate connectors in wood trusses? How do you take into account the possible presence of splits, waves and knots?
Answer:
Chapter 3 of the Truss Plate Institute's ANSI/TPI 1 addresses the tolerances for metal connector plate positioning: “The placement of the metal connector plate shall not interfere with other design aspects or the function of the truss (i.e., interfering with the duct space in the chase opening of a floor truss or exceeding the members at heel joints thus affecting the bearing area). Teeth placed in knots, bark, pitch pockets, holes, and joint gaps shall be considered ineffective.”
There are also stated tolerances for metal connector plate tooth embedment into the wood members. The tooth embedment gap is defined by ANSI/TPI 1 as the distance between the underside of the embedded metal connector plate and the surface of the wood member. If there is a tooth embedment gap equal to or less than 1/32 in., the teeth are 100% effective. If the tooth embedment gap is between 1/32 in. and 1/16 in., the teeth are only 60% effective. If the tooth embedment gap is between 1/16 in. and 3/32 in., the teeth are only 40% effective. The teeth are 0% effective if the gap is greater than 3/32 in.
Gaps between wood members cannot exceed the tolerances listed in ANSI/TPI 1. These maximum gaps range between 1/16 in. and 1/4 in. depending on where the joints are located (i.e., heel, splice, compression, tension).
According to ANSI/TPI 1, splits in any wood member caused by metal connector plate teeth or the manufacturing process shall not exceed those permitted in the grade and species of the lumber used and specified in the truss design drawing.