About the research
Buchanan County, Iowa, has been working with the National Center for Wood Transportation Structures and a timber fabricator to develop the next-generation timber bridge. The goal is to increase the structural efficiency of timber bridges and increase longevity by (1) creating a composite deck-girder system and (2) using an epoxy overlay. These design elements have the potential to increase viable bridge options for use not only on Iowa’s roadways, but nationally and internationally as well.
The bridge system developed for this research was a composite glue-laminated (glulam) girder-deck system utilizing epoxy for the connection and an epoxy overlay wearing surface on the deck. This design was investigated through small- and large-scale laboratory testing of the composite epoxy connection and a field demonstration bridge built utilizing this girder to deck connection detail and epoxy overlay.
The small-scale tests showed that the best overall joint connection is an epoxy and lag bolt connection. The joints with epoxy at least tripled the shear capacity of the lag bolt joint, and addition of mechanical fasteners to the epoxy connection marginally increased performance. The large-scale laboratory tests showed a small increase in the load capacity and movement of the neutral axis when the deck panels are affixed to the girders, both of which indicate potential composite action. Furthermore, the epoxied connection exhibited an improved composite connection over the lag bolt connection.
Three live load tests on the field demonstration bridge in 2015, 2016, and 2017 indicated that transverse load distribution for all load cases was adequate. The composite action observed was not likely substantial enough to be accounted for in design. The chip seal shows signs of cracking at the transverse deck panel joints, but because of the epoxy the joints remain sealed and show no signs of moisture intrusion on the underside of the deck. The epoxy wearing surface on the deck performed better as an impermeable joint filler than a wearing surface.