SEISMIC PERFORMANCE OF A CLT STRUCTURE

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Seismic loads on the structure are due to forces which are created by ground accelerations (earthquakes). The magnitude of these loads is influenced by the following factors weight of the building, the intensity of the seismic loads, duration of the earthquake/ground acceleration, frequency of the seismic waves, soil-structure interaction. With regards to this, all tall structures are and must always be designed against heavy loads imposed by seismic loads while minimizing the environmental costs associated with their construction and maintenance.

Timber in this case, has proved to be environmentally friendly, easy to construct and maintain. However, major concern is on the ability of the CLT structure to structurally stand and function when subjected to intense seismic loads.

 

Cross-Laminated Timber (CLT) panels have low embodied carbon content, high strength-to-weight ratio and high degree of prefabrication which are considered of great advantage in a structure. But with regards to seismic forces, the ability of CLT walls to withstand forces exerted by seismic loads heavily depends on its joinery sections, therefore the amount of vertical load has a high influence on the wall stiffness/ lateral resistance. Therefore, the following influential factors must be considered:

  • Boundary conditions of the CLT
  • Shear wall imposed by CLT diaphragm
  • Presence of gravity loading. Connector type.
  • Connector plate thickness, CLT grade, CLT panel aspect ratio,
  • Panel thickness.
  • Presence of inter-panel connector (vertical joint).

 

Minimizing failure in CLT structure due to seismic loads

According to the underlying capacity-based design principle, all non-dissipative connections are expected to remain elastic under the force and displacement demands that are induced in them when the energy-dissipative connections reach the 95% of their ultimate resistance or target displacement. Energy dissipative connections of CLT structures need to be designed such that:

  • The yielding mode governs the connection resistance
  • The connections also need to be moderately ductile in the directions of the CLT panel’s assumed rigid body motions
  • The connections should have sufficient deformation capacity to allow for the CLT panels to develop their assumed deformation behavior.

 

When joining the timber beams and column joints, steel brackets and their respective fasteners are used. Connectors define the behavior of the CLT wall in terms of strength, stiffness, deformation capacity, and energy dissipation.

CLT wall panels behave as rigid bodies when subjected to seismic loads. CLT structures have an adequate seismic performance when nails or screws are used with the steel brackets. Therefore, the use of hold-downs with nails on each end of the wall improves its seismic performance.

Use of step joints in longer walls can be an effective solution not only to reduce the wall stiffness and thus reduce the seismic input load, but also to improve the wall deformation capabilities.

 

 

However, the use of diagonally placed long screws to connect the CLT walls to the floor below is not recommended in seismic prone areas due to less ductile wall behavior.

If panels are poorly joined, slight shear deformations (panel deflections) are likely to occur as a result of the deformation in the joints. In case of multi-panel walls, deformations in the step joints might have significant contribution to the overall wall deflection.

 

 

 

 

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Posted on: December 24th, 2021 by London Building Contractors No Comments

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