Failure Mechanism And Reinforcement Methods Of Laminated Veneer Lumber Joists With Round Holes

Laminated Veneer Lumber(LVL)is a kind of engineered wood product,made by peeling logs veneers and gluing these veneers together.Laminating decreases the detrimental effects of knots and imperfections by distributing them throughout the LVL members.Therefore,it can be realized that the use of short wood,low class and small size wood,and it is a good way to improve the utilization value of fast growing poplar.Structural LVL is characteristic of dimensional stability,load-bearing capacity and weather resistance,which is commonly used as bearing members such as beams and joists in a wooden building.Holes in beams or joists are many times necessary to allow piping system to pass through,such as water plumbing,thus to avoid additional space requirement.Introducing a hole in the shear dominant area of a beam or joist significantly changes the distribution of stresses in the vicinity of the hole.Tensile stresses perpendicular to grain appear and the bearing capacity of the member can accordingly be decreased.The hole likes a stress concentrator which enhances the local tension stress perpendicular to grain,crack changes the failure mechanism of the member,and fracture can occur at a very low load value because of low tension strength perpendicular to grain of timber.The poplar LVL and Douglas fir LVL were chose as the main materials in this project.The bearing capacity and stress distribution of LVL members with holes which subjected to bending were studied,joists as example.The bearing performance of joists made by the two kinds of materials and the bearing capacity of joists with holes were compared.Different reinforcement methods were applied to reinforce the joists with holes,to restore the bending failure mode and the ultimate bearing capacity of joists with holes.The failure mechanism of joists with holes was studied,and the stress distribution around the hole in the loading state was analyzed.Based on the stress condition of the joists with holes,the reinforcing mechanism of different methods was analyzed,and the design methods of joists with reinforcement were proposed.The results showed that shear strength was more important than the bending strength for joists with holes.The horizontal shear performance of poplar LVL was similar to that of Douglas fir LVL(both were 6.3MPa).Therefore poplar LVL could be used as joists with holes.The design methods in DIN 1052 and EC5 could be used in joists with holes made by poplar LVL and Douglas LVL.The ratio of theoretical value and experimental value in the range of 0.6-0.9,and with the ratio of hole diameter and beam height increased,the deviation decreased.In this paper,a theoretical analysis model and design formulations,which based on the classical material strength criterion,for calculating the ultimate bearing capacity of joists reinforced by nailed-on plywood were proposed.Through the verification of the test data,the ratio between the theoretical value and the experimental value of the test specimens was in the range of 0.85-0.95,which showed good evaluation.