Reinforced multiple bolt timber connections

Bolted connections have been used in heavy timber construction for centuries. Yet design rules are very inconsistent and failure modes are often of a brittle nature, contrary to design rules that are based on a yield model approach. To improve the ductility of bolted connections especially in seismic design applications, reinforcing measures can be applied in the connection region. The reinforcement restrains the expansion of timber in the perpendicular-to-grain direction and is meant to prevent a sudden release of energy and the associated brittle failure of the connection. This thesis reports on an experimental investigation into the effect of various reinforcing techniques on multiple bolt connections in glue-laminated timber (Glulam) and parallel strand lumber (PSL). Several reinforcing techniques were investigated; (i) threaded rods and glued-in rods were applied internally; (ii) truss plates, nailed plates and glued-on plates were used as a surface reinforcement. Different configurations and materials within each reinforcement group were compared. A total of 79 specimens were tested, 58 under monotonic tension and 21 under reverse cyclic loads. All connections consisted of 2 rows of 5 bolts each, attached to 19mm thick steel side plates. As a pilot study, three replicas of each ten-bolt connection were tested in monotonic tension. From these results, the joints considered to display good strength, ductility and energy dissipation characteristics were later tested in cyclic static loading. The parallel strand lumber specimens were 89x140mm and glue-laminated specimens 89x130mm in cross section with the bolts penetrating the short distance (89mm). The connection geometry was in all cases based on the bolt diameter and followed the Canadian code rules in CAN/CSA 086.1-94. The end distance, and the bolt and row spacing were respectively lOd, 4d and 4d in all the connections. The slenderness of the bolts (1/d ratio) was varied by using different bolt diameters in the tests - 9.5mm(3/8"), 12.7mm(l/2") and 15.9mm(5/8"). The bolts (Grade 5) were tested in bending to obtain their yield stress. The larger diameter (1/2", 5/8") bolted connections with lag screw and truss plate reinforcement exhibited brittle fractures (splitting and shear plug). The reinforcement helped to maintain the integrity of the connection and considerable nonlinear deformations occurred with a relatively small reduction in load. The most promising reinforcement method consisted of coarse threaded lag screws (4mm thread) inserted perpendicular to the grain halfway between each of the connection bolts. These specimens reached the highest displacement ductility ratio (15 on average) in the 3/8" bolt connections. Further achievement was achieved when the reinforcing rod was offset from the bolt, and higher ductility was reached when compared to the reinforcing rod in the mid-position. Fine threaded ready rod (1.8mm thread) did not have adequate bond to prevent perpendicular-to-grain tension splitting. In these cases the connections failed suddenly and in brittle failure modes with a ductility ratio of 4.7 in 5/8" PSL and 6.1 in 1/2" Glulam connections. The tests with the truss plates showed that the teeth of the truss plates were not long enough. Especially in the cyclic tests, the huge cumulative displacement caused the truss plates to prematurely pull out from the timber. Thus a more sudden drop in the strength and stiffness followed after the peak load, compared to the lag screw reinforced cases. The use of epoxy glue with different forms of reinforcement (glued-on plates, glued-in rods and rebars) mostly increased the strength (-1.7%, 32.1% and 14% on average) but did not prevent sudden failures. Ductility ratios of 2.9, 4.1 and 4.2 (on average) were reached for the three bolt sizes. Little plate crushing in the bolt location and nail bending were observed when finishing nails and thin plates (0.6mm) were used as reinforcement. When 2"spiral nails with 1.2 mm thick plate were used, the stiff nails caused cracking of the specimen at peak load and no plate crushing was observed. Both configurations failed in a brittle manner. One of the specimens was reinforced with a stiffened truss plate. Whereas the truss plates in general acted as passive reinforcement only, in this case most of the force was directly transferred through truss plate teeth to the wood. The plate prematurely pulled out off the wood, which caused a brittle failure (ductility of 3.3). The Glulam connections were stronger than their PSL equivalents, but less ductile. The damage on the Glulam specimens was less predictable and cracks typically propagated along the entire specimen length. Cracks in PSL joints stopped at 10-15cm from the last bolt due to the denser and random wood strand orientation in the PSL cross section.

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