Abstract:

Objective: This study was conducted to evaluate the mechanical properties of the tensile joints of a paralleled string truss connected by self-tapping screws which were screwed from different angles, in order to provide basic scientific data for its practical engineering applications and improve the code for design timber structures. Method: The test pieces of the truss tensile joint were made by the imported SPF dimension lumber and self-tapping screws. By changing the angle between the axial direction of the screw and the grain direction of the web of the truss, the pull-out test was conducted on the 7 groups of tensile joints with angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90°. The test was conducted according to the American specification ASTM D1761-12. Each specimen was pulled out at a constant rate of 2.54 mm·min^-1. The destructive form record, the failure mode analysis and, the comparison of the load-displacement curves, ultimate load-carrying capacity, yield load and stiffness of each group of specimens were implemented. Result: There were 2 types of failure modes:head pulled through the member and the screw pulled out from the tensile joint. When the angle were 0° and 15°, the failure of the test piece was mainly due to the fact that the compressive strength radical to the grain of the wood was less than the characteristic withdrawal strength of the screw, and the screw finally pulled through the chord of the specimen; when the angle was 30°-90°, the cause of failure of specimens was that the shear force formed by the embedment strength reached the yield load of the screw. When the angle were 0° and 15°, the load-displacement curves of specimens kept very well near the maximum load; when the angle ranged from 30° to 60°, the load-displacement curves decreased slowly; when the angle were 75° and 90°, the load-displacement curves all had a significant yield stage. The bearing capacity of the joint was affected by the yield strength of the screw and the compression strength perpendicular to the grain of the wood. When the angle was 60°, the average load-carrying capacity of the specimens was the highest, reaching 7.423 kN, and the yield load was the highest when the angle was 0°, which was 4.859 kN. The stiffness of the joints decreased when the angle between screw axis and grain direction increased. Conclusion: The mechanic behavior of the tensile joint might be the best when the inclined angle is between 45° and 60°. Specimens with an angle of inclination of 0° could be suitable for use in buildings where stiffness is critical.

Key words: parallel chords wood truss, connected with inclined screws, tensile joints, uplift resistant performance