Test Research Of Bending Performance Of Prestressed High Strength Glue-lumber Beam

With the characteristics of green environmental protection,excellent properties and being capable of sustainable development,the timber structure building is highly anastomosed with country's policies of energy saving and development of green economy.The glue-lumber laminated and glued along the parallel to grain of timber is widely used in modern timber structure for the advantages of materials defects dispersion and high strength.For common glue-lumber beam under bending,the brittleness failure happened due to the lack of tensile strength of glue lumber beam bottom,at the moment,the compressive strength of glue-lumber beam upper timber cannot be exerted fully.Besides,because of glue-lumber the lesser elastic modulus and the characteristics of creep,the deformation plays the control part in the design normally,which was bad for the full play of material strength,and the economy was difficult to guarantee.Therefore,the following researches were made in this dissertation.Aimed to the above problems existed in common glue-lumber beam,a new type of steel and wood composite component,prestressed high strength glue-lumber beam was proposed,which can make full use of compression strength of glue-lumber,decrease the deformation and improve the failure mode of beam.A kind of prestressed applied device suitable for the new type of glue-lumber beam——screw jack stretching device was developed,which was easy to operate and install,without being limited by space and being able to adjust at any time in the using process.To reflect the laminates deflect and bonding quality overall,and to avoid the bad influence of end local pressure and testing machine loading plate transverse constraint,a kind.of prism test block that applied to measure glue-lumber parallel to grain compression strength was designed.To measure the elastic deformation of glue-lumber prism test block more accurately,double side clip long edge extensometer was developed.On the basis of making as much use as possible of China's fast-growing forest resources,suitable timber and adhesive were chosen to manufacture glue-lumber prism compression test block,through the parallel to grain compression test of four batches 17 groups(in all:102)test blocks,the influence of material type,composition pattern,layer plate thickness,lamina lay-ups type to the glue-lumber parallel to grain compression performance were researched systematically,which can obtain the compression strength parallel to grain?elastic modulus?load-deformation relation curve and compressive failure modes of glue-lumber prism test block.The results showed that:the compression performance of glue-lumber made up of commonly used domestic timber—northeast larch and poplar was better than the other timber;different composition pattern had certain possibility;the influence of layer plate thickness and lamina lay-ups type to compression performance weren't big.The load-deformation relation curve of glue-lumber prism test block parallel to grain compression test showed a trend of slow decrease and behaved good plastic characteristics;the failure modes were mainly end local pressure failure?oblique shear failure?scuffing surface cracking failure?splitting failure and internal fiber extrusion failure etc.To research the influence of material type,composition pattern,layer plate thickness,lamina lay-ups type to the prestressed high strength glue-lumber beam bending performance,by using the self-developed prestressed applied device,3 groups(in all:9)prestressed high strength glue-lumber beams were manufactured and the bending test were conducted,which can obtain the parameters of ultimate load and mid-span deflection.The research showed that:compared with ? c??c level SPF timber and poplar,the prestressed high strength glue-lumber beam made of northeast larch had the higher flexural bearing capacity;the influence of layer plate thickness and lamina lay-ups type to beam weren't big.Based on prism test block parallel to grain compression test and bending test results of beam,the northeast larch with the layer plate thickness of 20mm was selected as the material of prestressed high strength glue-lumber beam.Through the bending tests of 4 batches(in all:36)beams,the influence of prestressed steel wires number and pre-tension value to prestressed high strength glue-lumber beam short-term bending performance were researched,which can obtain the ultimate load?load-displacement relation curve?load-strain relation curve?section strain and failure mode of beam.The results showed that:when pre-tension value was the same,the beam bearing capacity and stiffness increased along with the increase of prestressed steel wires number;when the number of prestressed steel wires was the same,the beam bearing capacity was increased with the increase of pre-tension value,while the stiffness changed a little.With the increase of prestressed steel wires number and pre-tension value,the failure mode of beam changed from brittle failure of tackless in the bottom of beam or the direct abruption gradually to ductility failure of fold compression in the top of most beam;with the increase of prestressed steel wires number and pre-tension value,the neutral axis of high strength glue-lumber beam shifted down,the compression zone of section was largen before failure,and the maximum compressive strain became smaller,and the beam basically reached to the state of whole section unber compression,which can make full use of compressive strength of timber,from the compared tests of three batches(in all:9)common glue-lumber beams,it had been discovered that the failure mode of common glue-lumber beams was brittle and the discreteness was bigger;when the ultimate bearing capacity was the same,the prestressed high strength glue-lumber beam saved wood by 35.1-44.5%and saved cost by 20-31%.The experimental data verified the plane section assumption of prestressed high strength glue-lumber beam,and the proposed method of bearing capacity of such components was proposed according to the test results.The 45 days long-term loading bending test of two batches(in all:10)beams were conducted under the normal indoor environments to research the the influence of prestressed steel wires number and pre-tension value to prestressed high strength glue-lumber beam long-term bending performance,which can obtain the beam deflection change law and prestressed steel wire stress loss law.The results showed that:When the pre-tension value was the same and with the increase of prestressed steel wire,the stress loss value of prestressed steel wire was bigger,and the change value of the beam mid-span deflection increased over time was smaller;when the prestressed steel wires number was the same and with the increase of pre-tension value,the stress loss value of prestressed steel wire was bigger,and the change value of the beam mid-span deflection increased over time was not big.According to test results,the enhancement coefficient of beam mid-span long-term deflection was obtained,and proposed method of long term deflection calculation of prestressed high strength glue-lumber beam with the influence of timber creep was proposed.