Theoretical And Experimental Research On The Structural Behaviour Of Cross-laminated Timber Structures Exposed To Compartment Fire

With the rapid development of society and improvement of people's live standards,modern timber buildings are gradually favored because they are green,environmentally friendly and can be industrialized.The Crosslaminated timber(CLT)structure is the mainstream system of multi-storey and high-rise timber buildings.However,due to the combustibility of wood,the fire safety of timber buildings has always been one of the highlight issues needs to be focused on.The fire behaviour and development of CLT compartments with exposed timber boundaries is very much different with ordinary compartment fires.Traditional fire design methods based on standard fire curves are therefore no longer fully applicable.Recently,some studies have been carried out to investigate the fire design of timber buildings.However,current research is still inadequate,and it is necessary to conduct further research on the fire design methods of CLT structures and elements.In this thesis,theoretical analysis,numerical simulation and experimental tests were conducted to investigate the fire bahaviour of the CLT compartments and panels.The main contents and results are summarized as follows:(1)The fire dynamic and development of the CLT compartment fires were studied and discussed by reviewing the relevent literatures.Key parameters to be considered in the analysis of the fire dynamics of the CLT compartments with combustible interior surface were introduced.Fire dynamic model for the CLT compartment considering exposed timber surfaces and fall off of the charred layers were proposed based on the two-zone fire model.The model was validated by comparing the temperature distribution and heat release rates with the reference fire tests.The applicability and limitations of the fire model were discussed accordingly.(2)CLT compartments with interior dimensions of 1.8m deep,1.2m wide and 1.5m height were designed and built,one of which was acted as a reference test with fully encapsulated interior surfaces.Four compartment fire tests were carried out using wood cribs as fuel to investigate the contribution of exposed timber surfaces and fall off of the charred layers on the fire dynamics,temperature distriburion and charring rates within the compartment.Randomness was found in terms of both time and area of the fall off of the charred layers,which led to sigifinicant differences of the fire development of the compartment fires.Self-extinguishment,secondary flashover and fire with no dacay phase were both observed in the tests.The mass loss rates of the CLT compartmentes with exposed timber surfaces were increased by 7.54-44.53% compared to the reference test,but no significant difference was found regarding the maximum temperature between the four fire tests.(3)Full-scale fire resistance tests on the CLT panels were carried out using a large horizontal furnace under standard fire and compartment fire curve,respectively.Effects of different fire curves,CLT lay-up and load ratios on failure mode,fire resistance,temperature distribution and charring were analysed.The heating rate of the fire was found to have a significant influence on the charring rates of the CLT panels.The charring rates of CLT in the heating phase of a compartment fire is much higher than that of a standard fire.But in the cooling phase,the charring rates is a lot lower than both in the heating stage and the standard fire.The fire resistance of the CLT panels depends heavily on the exposed fire curves,load ratio and CLT lay-up.In most cases,the fire resistance of 5-layer panel was shown to be longer than the 3-layer ones.The reason is that the 5-layer CLT bending member has a layer parallel to grain in the middle.The remaining cross sections can provide more than 40% of the flexural capacity when the first layer is fully consumed in fire.Futhermore,the second layer perpendicular to grain can act as an encapsulation to prevent the inner layers from heat penetration.(4)Three-dimensional finite element model was developed using Abaqus software to simulate the orthogonal anisotropy behaviour of timber and cohesive behaviour of adhesives,respectively.The constitutive relation of wood was implemented by using a VUMAT user-defined subroutine,which considers the Hashin failure criterion,damage variable based continuum damage mechanics(CDM)theory and reduction of the material properties at elevated temperatures.Quasi-brittle fracture under tension and shear,ductile failure under compression can thus be introduced for wood.Cohesive elements were hired for the adhesives to simulate a bilinear traction-separation failure mode.The FE model at ambient temperature was validated by comparing with the results of the material tests.Thermo-mechanical model was implemented through sequentially heat-structure coupling model and was validated by comparing the results from fire resistance tests of CLT panels.The model was validated by comparing with the test results in terms of mid span deflection versus time and fire resistance time.Good agreement between numerical simulation and experimental results was found,existing error was due to the randomness in the fall off of the charred layers.(5)Theoretical model based on the effective residual cross section theory was developed to calculate the bending capacity of CLT panels under different fire exposures,including the calculation methods of the bending capacity at ambient temperature,charring rate and the thickness of zero-strength layer.The charring model was implemented using FE heat transfer model that can consider the effectes of fall off of the charred layer.The time dependent zero-strength layer thickness was derived based on this,which took into account the shifting process of neutral axis of CLT panel exposed to fire and different reduction factor of tensile and compressive modulus.The theoretical model was validated by comparing with reference fire resistance tests,results from the method by EN 1995-1-2 and NDS 2018 were also provided for comparison.The results from theoretical model showed that due to the inherent characteristics of CLT panels,the thickness of zero-strength layer suddenly increased after the first layer is out of service.It is impossible to directly use the constant value of zero-strength layer from current codes,which was originally designed for solid and gluedlaminated timber products.The thickness of zero-strength layer was also found to have a lower value in the heating phase of a natrual fire than in the standard fire.After the charring depth and residual cross section stop increasing in the cooling phase,however,the effective residual cross section continue to decrease by the heat transfer process inside the char,lead to the final failure and fire resistance of CLT panel in the cooling phase.