Stress-laminated timber decks in bridges: pre-stressing system, long-term effects and modelling

Abstract

Stress-laminated timber (SLT) decks are a popular solution for the superstructure

of timber bridges. The aim of the research presented in this thesis was to increase

the knowledge on the behaviour of SLT decks. The timber beams that form a SLT

deck are compressed in the perpendicular to grain direction in order to work as a

plate. The stress is transferred by friction between the laminations. However, the

pre-stress level in the decks decreases during the lifetime of the bridges.

Therefore, the long-term behaviour of glulam loaded perpendicular to grain was

studied. Several compression perpendicular to grain experiments were performed

on glulam blocks in a climate controlled environment, keeping either the load or the

displacement constant, thereby resulting in creep and relaxation tests, respectively.

Hence, parameters describing the viscoelastic creep behaviour were determined, as

well as hygroscopic and mechanosorptive coefficients. Thus, the results of the longterm

tests helped the development and the validation of a material model for timber

under compression perpendicular to grain. The material model considers both time

and moisture content variations, describing thereby elastic, hygroscopic,

viscoelastic and mechanosorptive behaviour of timber. Moreover, the pith location

of each lamella is taken into account either by the use of effective properties, or by

defining the timber properties for each lamella in a cylindrical coordinate system

with origin in the pith.

Furthermore, the frictional behaviour between glulam elements was investigated in

an experimental campaign. The friction coefficient was evaluated in both of the

sliding directions (longitudinal and perpendicular to the grain) and with different

levels of compression stress. The results of the friction tests were thereafter

numerically modelled and employed in the subsequent study about the design of

SLT decks.

Finally, numerical simulations of SLT decks were performed in order to study the

influence of the butt joints distribution and the pre-stressing scheme on the

structural behaviour of SLT decks. Hence, the reduction of flexural stiffness for different butt joints configurations and the development of the stresses in the

perpendicular direction to the grain for three different pre-stressing patterns were

assessed. Furthermore, the influence of the geometrical layout of the anchor plates

on the perpendicular to grain compression of the outer deck beams in SLT decks was evaluated.