Experimental Study On Creep Behavior Of Poplar LVL Orthogonal Rib Beam Box Floor

Poplar Laminated Veneer Lumber（LVL）orthogonal rib box floor is a new type of floor composed of orthogonal LVL rib beams and OSB（Oriented Strand Board）roof and floor.In order to study the creep behavior of the floor under long-term load,the creep tests of box floor under local damage loading,repeated loading and different stress ratio loading were carried out.Combined with the creep mechanism of wood materials,the creep theoretical calculation formula and finite element analysis model of the box floor with LVL orthogonal ribs were established.The main research work and results are as follows:（1）The physical and mechanical properties of Poplar LVL were tested,and the physical and mechanical properties parameters such as density,moisture content,flexural strength,flexural elastic modulus and fracture toughness of Italian Poplar LVL were measured.（2）The effects of loading after local damage,repeated loading,and loading with different stress ratios on the creep of Poplar LVL orthogonal rib box-girder floor were studied.Due to the local damage of the floor,the strength of the nail joint of the floor slab decreases,the ability to resist deformation decreases,and the creep increases.Repeated loading will make the floor residual deformation accumulation,creep rate decreases,but the floor creep deformation increases;increasing the load will accelerate the creep of the floor.（3）According to the creep mechanism of wood materials,the creep constitutive model of LVL orthogonal rib box floor was established.By comparing the creep model with the test results,it is found that the modified Burger model is more in line with the creep development characteristics of LVL box floor under natural environment,and the fitting effect is the best.The deformation of each specimen is predicted by the modified Burger model,and it is found that the floor deflection tends to a certain limit value when the creep time is about 700 days.Due to the existence of local damage,the strength of the nail joint decreases,and the creep of specimen L2-1 still increases slightly.Under the long-term load of 4.5 kN/m2,the maximum deformation of the floor in about three years is 14.27 mm.Under the long-term load of 7.5 kN/m2,the maximum deformation of the floor in about three years is 24.34 mm.（4）The finite element software was used to establish the model of Poplar LVL orthogonal rib box floor,and the influence of rib height and stress ratio on creep performance was analyzed.Through the finite element simulation,it is found that the stiffness of the floor increases and the creep is small when the height of the rib beam increases.By analyzing the creep deflection under the same load of 2.5 kN/m2,it can be found that the deflections of the specimens with two rib heights are far less than the allowable values of the specification under the standard live load of 2.5 kN/m2 for 55 days,which meets the specification requirements.The finite element software was used to establish the model of Poplar LVL orthogonal rib box floor,and the influence of rib height and stress ratio on the creep performance of the floor was analyzed.Through the finite element simulation,it is found that the floor stiffness increases and the creep decreases with the increase of rib beam height.Under the load of 2.5 kN/m2,it can be found that the floor creep at the height of 235 mm and 285 mm rib beams is far less than the allowable deflection value under the live load of 2.5 kN/m2 in the《Design Standard for Wood Structures》（GB50005-2017）for 55 days,which meets the specification requirements.Through the experimental study on the creep performance of Poplar LVL orthogonal rib box floor,it is found that the loading after local damage,repeated loading and stress ratio will affect the creep performance of the floor,and avoid excessive deformation of the floor and affect normal use.According to the creep mechanism,the floor creep model is established and the floor creep is predicted,and it is found that the creep tends to a certain limit value.Through the finite element software analysis,it is found that the creep of the floor is small and has sufficient safety reserve after increasing the height of the rib beam.The research results of this paper provide theoretical support for the calculation of long-term deformation of LVL box floor.