11 Aug 2022

Fibre-reinforced polymers in timber structures: description and evaluation of possible applications and overview research and development work carried out until 2020

WHFF Project 2017.20

Pedro Palma, René Steiger, Empa

The most important facts in brief

  • Fibre-reinforced plastics (FRP) offer interesting properties that can be advantageous in combination with wood.
  • The thermal conductivity of FRP is significantly lower than that of steel, which is why FRP-wood joints can be advantageous when exposed to fire.
  • FRP composites in timber connections can be successfully used to reinforce the timber in the connection area or to replace some of the connection parts that are usually made of steel.
  • The most studied methods are for improving the bending behaviour of glulam beams.
  • FRP reinforcements are often tested under deformation-controlled tests, which shows brittle fracture behaviour. This gives a false impression of deformation capacity or ductility. In reality, however, most loading scenarios are force-controlled.
  • Except when fire is involved and exposed steel parts are to be avoided, internal reinforcements placed inside the component with FRP composites in shear and tension perpendicular to the direction of the fibres do not offer any significant advantages over the reinforcement with self-drilling screws that is common today.
  • The long-term behaviour of FRP-wood joints is currently only sparsely studied in literature.
  • While FRP performs well in many applications, other materials offer a better performance/cost ratio.

 

Project description

Fibre-reinforced plastics (FRP) offer several advantages over other types of reinforcement: for example, they have a high load-bearing capacity, resistance to harsh environmental conditions and a high strength-to-weight ratio. Wooden structures can be reinforced directly during manufacture or later on site, both passively and actively.

FRP can also be used instead of steel parts (e.g. instead of pin-shaped fasteners).

Due to the numerous advantages but also disadvantages of FRP in timber structures, it is useful to fully understand the properties of both materials as well as their behaviour in order to fully exploit the potential of the connections. For this reason, researchers Pedro Palma and René Steiger have summarised the research and development work carried out up to 2020 in a comprehensive publication.

The literature review includes:

  • The collection and description of the material properties of wood and FRP (advantages and disadvantages of each material) and a comparison of the properties of FRP with alternative reinforcement materials that are not made of FRP.
  • An overview of the main research and development studies that have been carried out and a brief summary of their results.
  • Examples of practical applications.

In this report, the researchers also focused on the effectiveness of different reinforcement methods and analysed various aspects and methods of FRP applications.

In particular, they analysed the reinforcement of timber elements with FRP, the behaviour of these materials in fire, shear and tension perpendicular to the grain, the comparison with steel reinforcement elements, the timber-FRP composite behaviour and finally the environmental aspects related to the reuse, recycling and disposal of FRP.

Conclusions

Most studies from the past two decades deal with glulam beams and the improvement of their flexural strength by FRP. Improvements in the load-bearing capacity and stiffness of glulam beams have been observed by bonding FRP composites directly to the tension side of the beams and also by embedding only the fibres in the wood adhesive.

In terms of shear and tension perpendicular to the grain, research is much more limited, but strengthening by placing FRP composites on the side faces of timber members is an effective but aesthetically unappealing strategy. Internal shear and tensile reinforcements with bonded FRP composites do not appear to offer significant advantages over the self-tapping screw reinforcement commonly used today.

FRP-based reinforcements face numerous established competing materials. For longitudinal stiffening (i.e. bending, compression), the main alternative is the use of high-performance wood-based materials (e.g. laminated spruce or beech veneer lumber), which can be easily integrated into existing production processes. For transverse bracing (i.e. shear and tension perpendicular to the grain), the main alternatives are self-tapping screws and glued-in steel bars. Steel reinforcements have the great advantage that they can be easily machined and connected to other structural elements and that they easily allow ductile failure. Often these materials are either better performing than FRP or less expensive, and their performance is more extensively described in the literature.

A challenge for the establishment of FRP-wood joints is the still far from solved – but highly considerable and relevant – question of the ecological aspects related to the reuse, recycling and disposal of FRP. As long as this question remains unanswered, the ecological advantage of using wood seems to be eclipsed.

The project was supported by the Forest and Wood Research Promotion Switzerland WHFF-CH of the Federal Office for the Environment FOEN.

You can download the entire publication here:

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