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Review Of Pre Stressed Timber Bridge Decks: Durability, Lay Out And Structural Systems

2473 words - 10 pages

Summary
Stress-laminated timber bridges are more and more used nowadays but the knowledge about their long-term behavior are still not to satisfaction. The present review shows critical details of the timber bridges with regards to durability. Despite some of them are very common, there is still a need to improve their properties to limit the effects of high moisture content. Decks are often protected physically and chemically against external biological attacks but environmental issues have raised doubts about the use of toxic chemical treatments. Moreover, it is known that the mechanical pre-stress level decreases during the lifetime. All these issues have to be investigated to guarantee ...view middle of the document...

This kind of system for timber decks is used for new constructions or for improvements and restorations of existing bridges.

Mechanics of stress-laminated timber decks
The stress-laminated timber decks have to be considered as plates with orthotropic behaviour, according to [2].
Generally the elastic parameters EL (modulus of elasticity parallel to the grain) and GRL (shear modulus in longitudinal direction) are clearly specified in timber specification, whereas the parameters in the other directions are subject of test research and they are generally expressed as a percentage of EL.
Ritter in [1] suggests to consider the transversal modulus of elasticity ET and the in-plane shear modulus GLT respectively equal to the 1.3% and to the 3% of EL.
Eurocode in [2] suggests the use of the value shown in Table 1 for softwood laminations.

Table 1: Eurocode suggested values for elastic parameters
Type of wood ET/EL GLT/EL GLT/GRL
Sawn 0.015 0.06 0.08
Planed 0.020 0.06 0.10

The pre-stressing force Fps necessary to have this behaviour, avoiding gaps among the lamellas, can be easily calculated, considering that it has to support the transverse moment MT given by the loads (1).

(1)
Moreover, it has to be able to prevent vertical slips, thanks to the friction among the lamellas according to Eq.(2), wherein VT is the transverse shear acting on the deck, µ is the friction coefficient and the coefficient 3/2 is relative to a rectangular cross-section.
(2)
The respect of these two conditions is strictly necessary to allow the laminations to work as a plate.
The level of pre-stress is extremely important for stress-laminated deck because it generates the friction force between the laminations. Their dependency is shown in Eq.(3) .
(3)
The friction force Ffr is function of the compression N induced by the pre-stress and of the coefficient of static friction µ, which is a material property.
However, the pre-stressing force cannot be higher than the strength of the steel bar or than the compression strength perpendicular to grain of timber under the anchorage plate.

It has to be taken into account that the pre-stressing force is not constant during the lifetime of the bridge but it will suffer variations due to the changes of temperature and moisture content.
The above-mentioned variation ΔFps can be evaluated as [3]:
(4)
(5)
In the equations above ΔσS is the variation of stress in the steel bars, AS and AG are the areas of timber and of steel bar respectively, EG90 and ES are the modulus of elasticity of timber orthogonal to grain direction and of steel respectively, αG90 and αS are the thermal expansion coefficient of timber orthogonal to grain and of steel, βG90 is the moisture expansion coefficient of timber, ΔT and Δm are the variations of temperature and moisture content.
The influence of moisture content on the variation of pre-stressing force is higher than the influence of temperature, as shown in [4].

Durability issues
In...

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