Investigations of the Loadbearing Behaviour of Timber Bending Beams Reinforced Using Prestressed CFRP-Lamellas

The refurbishment of old buildings often goes hand in hand with an increase in both the dead
and live loads. The latter, combined with the higher safety factors, often make the reinforcement
of the old structures necessary. Most reinforcement methods involve transforming a
structural timber member into a composite beam. Composite sections have a long tradition in
timber construction. In the early days, multiple timber beams were connected with interlocking
tooth and wooden shear connecters, which resulted in an elastic connection. Although
historical timber structures are frequently upgraded, no method has yet been established and
fully accepted by all stakeholders such as owners, builders, architects, engineers and cultural
heritage organisations. Carbon fibre-reinforced polymers (CFRP) have already shown their
efficiency in structural reinforcement especially in concrete structures. Moreover, previous
studies have shown that CFRP has the potential to meet the expectations of all parties involved.
In order to reach the service-limit state, a high amount of carbon fibres has to be
used, or considering the cost of reinforcement, prestress has to be applied. However, prestressing
often goes hand-in-hand with delaminating issues. The camber method presented
here offers an efficient solution for prestressing timber bending members and overcoming the
known obstacles.
In the method proposed, the timber beam is cambered using an adjustable prop at midspan
during the bonding of the CRFP-lamella to the lower side of the bending member. After curing
the adhesive, the prop is removed and the prestressed composite beam is ready to be
used. The prestress introduced in the system is not constant, but has a triangular shape and
peaks at midspan, where it is used the most. The prestress force, which declines towards the
end of the beam, leads to a constant shear stress over the whole length of the reinforcement,
avoiding a concentrated anchorage zone.
An analytical calculation model has been developed to calculate and design prestressed timber-
bending members using the camber method. Numerical modelling, using a multi-surface
plasticity model for timber, confirmed the results from the analytical model, and clearly reduced
delaminating issues, comparing very favourably to traditional prestressing methods.
The experimental parametric study, including the determination of the short-term loadbearing
capacity of structural-sized beams, showed agreement with the analytical and numerical
calculation. The prestressed reinforcement showed a benefit of nearly 50% towards
the ultimate-limit state and up to 70% towards the service-limit state. Calculations revealed
that the use of high modulus CFRP allows even higher benefits, depending on the configurations
and requirements. The long-term design of the prestressed composite beam was investigated
by extending the analytical model. The creep of the timber leads to a load transfer
from the timber towards the CFRP, and therefore increases the benefit towards the ultimatelimit
design. Applying high modulus CFRP-lamellas allows for a complete utilisation of the
design capacity of timber and carbon fibre-reinforced polymer.
The thorough investigation conducted demonstrated that the camber method is an efficient
technique for prestressing and reinforcing timber-bending members. Furthermore, the calculation
model presented allows for a safe design and estimation of long-term behaviour.