| In the course of using glued wood beams,due to the degradation of material strength,change of service conditions and other reasons,the bearing capacity and stiffness are often insufficient.For this reason,scholars at home and abroad have proposed some strengthening methods for glued wood beams,such as pasting fiber composite materials,pasting steel plates and external prestressing,but most of these methods can only provide the bearing capacity of the beam,and the stiffness of the beam is limited.In this paper,the glued wood beam is strengthened by increasing the section,and the new section is connected with the glued wood beam by glue nails.At present,scholars at home and abroad have not carried out too much research on the flexural performance of glued wood beams strengthened by enlarging the section,but for improving the flexural performance of glued wood beams,this simple and effective strengthening method has important research significance.To this end,the main research work of this paper is as follows:(1)In this paper,48 pieces of 2850mm×120mm×50mm plywood beam made of pinus sylvestris var.mongolica.Pinus sylvestris board,reconstituted bamboo board,douglas fir board and new self tapping screws are selected as reinforcement materials to form a "reinforced beam with enlarged section" through the joint action of glue and glue nails.Through the flexural performance test of 6 groups of 12 specimens strengthened with adhesive and 6 groups of 36 specimens strengthened with adhesive and nails,and studies were in order to checkout the impact of flexural performance under various conditions,such as section height,new section materials,adhesive technique and the layout of self-tapping screw.And the final results indicate that the limit load of consolidated beam is in direct proportion to the depth of the secton.Under the same material,the ultimate load of the specimens strengthened by double layer plates is increased by 22.44%～43.7% compared with the specimens strengthened by single layer plates;Among the three kinds of reinforcement materials,the mechanical properties of reconstituted bamboo are superior.At the same section height,the ultimate load of the specimen strengthened with reconstituted bamboo plate is the highest,8.52%～30.68%higher than that of the specimen strengthened with other two materials;In the same group,the ultimate load of the specimens strengthened with standard compressive stress adhesive is slightly higher than that of the specimens strengthened with half standard compressive stress adhesive which also indicate that there is some effect that the adhesive technique process may exert on the mechanical behavior of the test piece in the course of processing;The ultimate load of the same group of specimens is basically proportional to the deflection.It shows that under the same section form,the specimen with larger deflection under bending is more fully stressed;The bending stiffness of the specimen strengthened with the same material is proportional to the section height of the specimen.Compared with the single-layer plate strengthened specimens,the bending stiffness of the double-layer plate strengthened specimens is increased by 20.9%～93.1%.It can be seen that the stiffness of the specimens can be effectively improved by increasing the section method.(2)Through the comparison between the beam strengthened by adhesive and the beam strengthened by adhesive nailing,it can be seen that the ultimate load and deflection of the specimen strengthened by adhesive nailing are higher than that of the specimen strengthened by adhesive under the same section form.After the self tapping screw is screwed in,the ultimate load of the strengthened beam is increased by 9.2%～12.5%,and the deflection of the strengthened beam is increased by 4.58%～33.08%,indicating that the self tapping screw makes the connection between new and old laminates better.At the same time,it also plays a role in limiting the shear failure of the adhesive layer and restraining the development of cracks to reduce the brittle failure;Thearrangement of self tapping screws also has a certain influence on the mechanicalperformance of strengthened beams.In the same group of specimens with the same screw angle,the beams with larger screw length and smaller spacing have lower ultimate bearing capacity.This also shows that if theself tapping screws are too long and the spacing is small,the local wood fiberdamage in the composite beam will be too concentrated,which will reduce thetensile strength of the strengthened beam.Based on this,this paper gives suggestions on the reasonable length and spacing of self tapping screws.It is recommended to screw in120 mm tapping screws at a 90 ° angle at 25 mm intervals.(3)Through the theoretical analysis of the strengthened beam,the equivalent section method is used to calculate its bending bearing capacity.It can beconcluded after economic evaluation with the results that improving the mechanical properties of the materials in the tensile zone of the specimen can effectively enhance the flexural bearing capacity ofthe specimen;The flexural capacity of the text piece is in direct proportion to the depth of valid section;Under thecondition of the same depth of section,text pieces which consolidated byreconbinational bamboo clappers have the biggest flexural bearing capacity;Withthe screw of the tapping screw,mechanical property and flexrual bearing capacityof the consolidated beam is of great improvement;When contrasting calculated value with experimental value of test pieces’ flexural bearing capacity,the error rate is 0.6%～16.6%,not more than 20%,thus it can prove that this design formula is workable,and is available for all types of text pieces’ sections in this thesis to calculate the flexural bearing capacity;When the section height is the same,the cost of the specimens strengthened by reconstituted bamboo plates and self tapping screws is the largest.Hence,we can draw a conclusion that the method of combining the recombination bamboo clappers and tapping screw to consolidate the glued beam is most cost efficient and rational.(4)The experiment has been done to proceed Finite Element Modeling of test stiffening beam by making use of ABAQUS finite element software,and have also stimulated flexural experiment of stiffening beam.When analyzed and compared the damage mode between test beam and model beam,the curve chart and variation between limit load and deflection,and the rate of deviation of limit load,it can be seen that the model beam and the test beam can be well matched.Three stimulation conditions have been developed correspondingly.These conditions are aimed at the finite element analysis of the impact on the flexural capacity of model beam,which include the depth of section,the length and screwing angle of tapping screw.The analysis results show that: the bending stiffness and ultimate load of the model beam are proportional to the section height of the model beam;Under the same material reinforcement,when the length and spacing of self tapping screws are the same,the ultimate load of the model beam is proportional to the screw in angle of the self tapping screws.The ultimate load of the model beam is proportional to the length of the self tapping screws when the screw in angle and the arrangement spacing of the self tapping screws are the same.To sum up,the beam strengthened by adhesive nailing with enlarged section proposed in this paper is simple in structure,reliable in stress,and can be used in practical projects.By comparing the mechanical performance and economic performance of the test pieces,the actual reinforcement effect of the "composite beam with expanded section and glued and nailed with recombined bamboo boards" composed of recombined bamboo boards pasted at the bottom of the plywood beam and 120 mm long self tapping screws screwed 90 ° into the beam at an interval of 25 mm is the best. |
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