Fire design of CLT elements. Glue line integrity maintained or not?

The check of CLT elements under fire conditions can be crucial in the design of a timber structure. Due to the layered structure of their cross-section, the calculation process is peculiar as pointed out in the examples below.

The charring depth of a CLT panel under fire conditions depends on the properties of the glue used in the panel assembly. The polyurethane glue (PUR), often used by manufacturers, is not resistant in case of high temperature meanwhile MUF glue (melamine-urea-formaldehyde) shows better performance.
This is why the new TimberTech Buildings additional module for the fire design of timber structures implements two calculation models to be applied according to the CLT properties certified by the manufacturer:

  • Glue line integrity maintained: the charring rate is assumed to be constant through the whole cross-section. This method can also be used when the manufacturer’s certificates suggest a constant charring rate (higher than that of the wood) to take into account the lower performance of the glue in a simplified manner.
  • Glue line integrity not maintained: the charring rate is not constant and depends on the panel stratigraphy. The CLT cross-section can be seen as a sequence of layers where each one acts as a protective layer for the following one. Therefore, the method for protected elements provided by EN 1995-1-2 can be applied to obtain a charring rate trend such as the one reported in the figure below where the k3 factor usually has a value of 2.

CLT Charring Rate

CLT design example: R60 with glue line integrity not maintained

CLT - Glue line integrity not maintained

  • Cross-section: 120 mm (20-30-20-30-20)
  • Glue: glue line integrity NOT maintained
  • Fire exposure: 60 minutes on one side
  • Unidimensional charring rate: beta0 = 0.65 mm/min
  • Charring depth: dchar,0 = 53 mm. Layer 1 (constant charring rate), layer 2 (double charring rate in the first 25 mm, then constant charring rate), strato 3 (double charring rate up to the required 60′)
  • Effective charring depth: def = dchar,0 + k0d0 = 60 mm
  • Effective cross-section: 60 mm (20-30-10)

 

Bending check 101% (MEd = 11.36 kNm)
Shear check 14% (VEd = 7.81 kNm)

 

Download the TimberTech Buildings calculation report about this example

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CLT design example: R60 with glue line integrity maintained

CLT - Glue line integrity maintained

  • Cross-section: 120 mm (20-30-20-30-20)
  • Glue: glue line integrity NOT maintained
  • Fire exposure: 60 minutes on one side
  • Unidimensional charring rate: beta0 = 0.65 mm/min
  • PCharring depth: dchar,0 = 39 mm. Constant charring rate through all layers
  • Effective charring depth: def = dchar,0 + k0d0 = 46 mm
  • Effective cross-section: 74 mm (20-30-20-4)

 

Bending check 55% (MEd = 11.36 kNm)
Shear check 12% (VEd = 7.81 kNm)

 

Download the TimberTech Buildings calculation report about this example

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Cross Laminated Timber (CLT) Beams Loaded in Plane: Testing Stiffness and Shear Strength

New paper by Francesco Boggian, Mauro Andreolli and Roberto Tomasi on the assessment of stiffness and shear strength of CLT beams loaded in-plane.

Set up

Abstract: Cross Laminated Timber (CLT) is a relatively new timber product used in construction that has gained popularity over the last decade. The product itself is constituted by multiple glued layers of juxtaposed boards, usually arranged in an orthogonal direction between one layer and the adjacent ones. This particular structure brings several benefits, such as the possibility to use the same product both for walls and slabs, since it can bear in-plane and out-of-plane loads. However, the mechanical behavior differs from usual timber products, and research is still ongoing to achieve common agreement on standard procedures for testing products and theories for evaluating stresses for safety verifications. This paper focuses on the in-plane shear behavior of CLT and analyzes the existing methods to evaluate shear stresses. An experimental part then presents a four-point bending test of CLT beams with a specific geometry to induce shear failure. Results are reported both for the elastic range test, measuring the Modulus of Elasticity, and for the failure test to investigate shear behavior with regard to different mechanisms. Previously exposed methods are used for the calculation of shear stresses and to analyze the correspondence between them, and the results are then compared with other existing tests and values in literature. A new test setup for future research is eventually proposed.

Failure modes

Continue…

Download the full paper

CLT multi-family residential building

Multi-family residential building with n. 8 lodgings, arranged on three levels above ground and a basement level used as a garage. The above-ground structure was build with CLT panels for walls and floors and with glulam timber beams and HEB steel beams.
Site of construction: Ischia di Pergine, Trento (TN)

Structural designer: Dr. Ing. Rossano Stefani
Architectural designer: Arch. Aldo Tomaselli
Construction company: Domus Immobiliare S.r.l.

Five floors residential buildings with framed walls

 

Residential building complex commissioned by ATER Trieste consisting of 5 buildings. Each building has 5 floors above ground plus the pitched attic floor. The walls are made with timer frames, while the floors are in CLT, including the ramps and the stairways, with the exception of the roof which is made of joists. The terraces are partially supported by steel structures.

Structural designers of the wooden structures: Ing. Sandro Rossi – Ing. Raffaele Cruciani, Ascoli Piceno, Italy

studioing-cru.ro@libero

Timber light-frame and CLT school

School complex made of three buildings separated by two seismic joints.
The strucures in elevation are made of both CLT (XLAM) panels and light frame walls. More in depth, the buildings hosting the classrooms and the locker rooms are built with structural light-frame walls while the gym building is made of CLT (XLAM) panels. Finally, the outdoor stairs are made of steel beams while the indoor stairs and the foundations are built in reinforced concrete.

Structural project: ReWis, Comano Terme (Trento)
www.rewis.it

Multi-storey residential CLT building

Multi-storey residential CLT building built in Sesto Fiorentino (Florence).
The building is made of four floors and has a maximum height of 13 m while the building base covers a 24.8 x 15.5 m rectangle.
The structure in elevation is entirely built with CLT (XLAM) panels while the basement and the foundations are made of reinforced concrete.

Structural project: ReWis, Comano Terme (Trento)
www.rewis.it

TimberTech Buildings: sold in 14 states

After long and hard work we are proud to share our important result regarding the software TimberTech Buildings. Thanks also to the translation in four languages (Italian, English, Spanish and Greek) we were able to sell it and distribute it in more than 14 countries worldwide.
In the map below you can view the places where some of the licenses have been activated (purchased by professionals and companies and educational versions given in use to students and professors).

Our resellers abroad.