Performance of Stress-Laminated-Timber Bridge Decks

Doctoral thesis, 2013

Stress-laminated-timber (SLT) bridge decks are a satisfactory alternative to conventional short-span bridges in terms of cost and performance. SLT decks are made from a number of timber or glulam beams positioned side by side and stressed together using high-strength steel bars. A concentrated load can therefore be distributed from the loaded beams onto adjacent beams due to the resisting friction caused by the stressing. A project has been conducted in three parts in order to fill some of the knowledge gaps relating to SLT decks constructed in Sweden. The first part of the project focused on determining the ultimate-load capacity of SLT decks, as well as studying their behaviour when subjected to non-destructive loads. A full-scale test of a 270 mm deep SLT deck showed that interlaminar slip already occurs at load levels equivalent to serviceability limit state (SLS) loads. The interlaminar slip resulted in non-linear load-deflection behaviour in the deck, making linear design models insufficient. The ultimate load capacity of the tested SLT deck was 4.5 times higher than the SLS load. The deck showed a great redundancy when reloaded after failure. The second part of the project aimed to study the cause and effects of interlaminar slip. Non-linear finite element (FE) models, which successfully simulated the interlaminar slip between the deck beams, were developed. Non-linear FE models can produce valuable knowledge about the stress redistribution that occurs in the deck and cannot be measured during tests. Even though the magnitude of the interlaminar slip is very small, it has a major influence on the load redistribution in SLT decks. Significant variations in the normal stress in the SLT deck were observed when the laboratory-tested full-scale test was simulated. The variations in normal stress have a significant influence on the amount of interlaminar slip in SLT decks, since the critical shear stress is dependent on the magnitude of normal stress. The final part of the project focused on studying timber bridges in service and summarising the existing literature on the durability of SLT decks. A reduction in prestressing has been a major durability concern for SLT decks constructed using wet timber beams which shrunk with time, causing a prestressing reduction. Field inspections have shown that the prestressing loss in Swedish SLT decks made from dry glulam is much smaller than that reported in the literature.