The longitudinal shear capacity of the lumber is one of several ultimate limit states assessments that need to be undertaken when designing a metal plate-connected truss. In most cases, the shear of the lumber is not the controlling parameter when selecting a size, grade, or species of lumber for use in a truss. When it does control, the size of the affected member size is either increased and/or the member is replaced with a member from a species grouping that is assigned higher specified shear strength. Except for some grades of machine-graded lumber, higher shear capacity cannot be obtained by selecting a higher grade of lumber.
In 1996, the Truss Plate Institute of Canada (TPIC) adopted the stiffness method as the basis of truss design in Canada. Along with this new analytical approach were new truss analogues, the most significant being the heel analogue of pitched chord trusses. One of the ramifications of adopting the new TPIC design procedures noted by users of the new procedures was the increased incidence of designs where shear strength dictated the selection of the chord member, but only if the truss were analyzed using the new TPIC procedures.
To better understand this situation and develop a possible solution, a 2-ply pitched chord truss with a girder heel detail was selected for analysis using the girder heel analogues from the 1988 and the 1996 editions of the TPIC “Truss Design Procedures and Specifications for Light Metal Plate Connected Wood Trusses.” These results were then compared to results from the Forintek “Structural Analysis of Trusses” (NSAT) program. Based on the member forces estimated by the NSAT program, a simplified 2-dimensional finite element analysis was carried out on the bottom chord at the girder heel.
The following were observed:
The discrepancy in the computed shear forces between the TPIC-88 and TPIC-96 is confirmed. At the traditional design points (location displaying the highest shear force in the bottom chord), both the TPIC-88 and NSAT analogues gave similar results. Results from the TPIC-96 analogues were typically 18 to 22% higher than the TPIC-88 and NSAT analogue results.
Both the TPIC-88 and TPIC-96 did not report the presence of a high shear force in the bottom chord of the girder heel joint (i.e. a “blind spot”). Only the NSAT model was able to highlight this shear force. Although a more detailed finite element analysis of the bottom chord in the girder heel joint suggests significantly lower shear stresses, the stresses were still found to be higher than that permitted.
The following are recommendations for consideration by the TPIC and the provincial wood truss associations:
The TPIC-96 analogue may be adjusted to give shear force values in the bottom chord that are comparable to that in the TPIC-88 and NSAT models. This is achieved by reducing the modulus of elasticity of the web members by approximately 10%.
Given the general uncertainty in estimating member shear force using truss analogues, the industry should consider whether or not the adjusted analogue is necessary given that significant increases to the specified shear strength of lumber is currently under consideration by the CSA Technical Committee on Engineering. The proposed increases would raise the shear strength of lumber by about 50%.
The high shear stress in the girder heel joint identified by the NSAT analogue, but missed by the TPIC-88 and TPIC-96 analogues should be assessed more closely before considering any remedial action. Alternate combinations of truss pitch and chord sizes may offer a more severe stress condition and thus a more appropriate configuration for developing empirical data is needed for supporting the girder heel joint detail.
The permitted practice of ignoring the “shear due to loads applied near the supports” as described in the CSA O86 and the NDS should be considered carefully by the TPIC to determine whether the intent of the provisions are properly applied in truss applications.