The Study On The Mechanical Performance Of CFRP-bolted Timber Connection Joints With Slotted-in Corrugated Steel Plates

With the popularization and promotion of green recycling building,timber structure had been widely applied to all kinds of buildings again with its advantages of light weight,low energy consumption as well as environmental protection.Currently,timber joints are mostly in the form of bolted with slotted-in steel plates to accommodate the efficient construction characteristics of modern building structures as well as span and appearance requirements.It should be noted that,for this connection form,in order to meet the appearance design and assembly requirement,the connection joints usually exists a certain shaped section,i.e.the cross-section of slotted-in steel plates and timber groove are not purely flat cross-section form.The existence of the shaped cross-section will have a certain impact on the mechanical properties,especially in the strength and ductility,and may even lead to the generation of visible fracture cracks in the region.However,the current design and research of this type of connection joints had neglected this impact.For this,in order to intuitively reflect the influence of shaped section on the bolted timber connection joints with slotted-in steel plates,and provide certain experience for the safety design and ductility improvement of such joints in future engineering applications,this dissertation proposed a CFRP(Carbon Fiber Reinforced Plastics)-bolted timber joints with slotted-in corrugated steel plates and conducted innovative research on the tensile properties in terms of force mechanism,failure mode and main mechanical properties in terms of experiment,theory and numerical simulation,as follows:(1)Based on the test,the base material properties of the wood were determined and the effects of each parameter on the shear resistance,fracture toughness and pin groove compression were clarified by orthogonal test.It was found that the density of the substrate was uniform and stable,the mechanical properties basically satisfied the conventional law,and the six factors of shear length,shear height,notch angle,shear width,shear thickness and the number of CFRP layers had a significant effect on the shear performance;among them,the smooth shear strength and shear modulus tended to decrease with the increase of shear length and tended to be relatively stable,and increased with the increase of shear height,shear width,shear thickness,shear angle and the number of CFRP layers and tended to be relatively stable.Stabilization.The crack length has no significant effect on the toughness of TL fracture(tensile fracture toughness,where"T"indicates chordal direction,which is the direction normal to the crack extension surface,and"L"indicates linear direction,which is the direction of crack extension),whereas the specimen height and the specimen thickness have a more significant effect;among them,the fracture toughness increases with increasing height when the height of the specimen ranges from 40 mm to 50 mm.Among them,the fracture toughness increases with the increase of height when the height of the specimen is between 40 mm and 50 mm,but it is basically unchanged when the height of the specimen is less than 40 mm or greater than 50 mm;the fracture toughness increases with the increase of thickness when the thickness of the specimen is between 40 and 60 mm,and it is basically unchanged when the thickness of the specimen is less than 40 mm or greater than 60 mm.The bond length,specimen height,specimen thickness and the number of CFRP layers have significant effects on the toughness of TL fracture(open fracture specimen,where"R"denotes the radial direction of transverse stripe,which is the direction normal to the crack extension surface,and L denotes the linear direction,which is the direction of crack extension);in which the fracture toughness is basically equal when the lateral bond lengths are 10 mm and 30 mm,while the fracture toughness increases with the increase in thickness between 40 and 60 mm.The fracture toughness is basically equal when the lateral bond length is 10 mm and 30 mm,and when the specimen is larger than 30 mm,the fracture toughness increases with the increase of the lateral bond length;in addition,the fracture toughness increases with the increase of the specimen height,thickness,and number of layers of CFRP,but the trend of its increase gradually decreases and tends to be stabilized.Specimen height,specimen width,specimen thickness and the number of CFRP layers have a significant effect on the fracture toughness of CS fracture specimens(slip-type shear fracture specimens)with U-type CFRP at the end,and the fracture toughness increases with the increase of these four factors,but the trend of its increase gradually decreases and tends to be stabilized.The lateral bond length of CFRP and the number of layers of CFRP have a more significant effect on the pin groove compression strength.The pin channel compressive stiffness and strength are basically equal when the lateral bond length is 20 mm and 40 mm,and when the specimen is larger than 40 mm,the pin channel compressive stiffness increases with the increase of the thickness;in addition,the pin channel compressive stiffness and strength increase with the increase of the number of CFRP layers,but the trend of its increase gradually decreases and tends to be relatively stable.(2)The force mechanisms of CFRP-single bolted timber connection joint with slotted-in corrugated steel plates and CFRP-single bolted timber connection joint with slotted-in flat steel plates were revealed,and the distinctions and merits were clarified and the influence of each parameter on the main mechanical properties of CFRP-single bolted timber connection joint with slotted-in corrugated steel plates.It was found that the two type joints had obvious distinctions in force characteristics,failure modes and bolt yielding modes,which are mainly manifested in the obvious force characteristics such as TL fracture damage,end shear damage,pressure damage,and CFRP spalling and slipping or rupture of the timber in the CFRP-single bolted timber connection joint with slotted-in corrugated steel plates.In additional,the CFRP-single bolted timber connection joint with slotted-in corrugated steel plates had a better stiffness and strength,especially strength,but the ductility was relatively poor,but still had a large failure displacement under the appropriate size conditions,thus had relatively better ductility;addition,wavelength,wave height,wave angle,timber thickness,and the number of CFRP layers have significant effects on the mechanical properties of CFRP-single bolted timber connection joint with slotted-in corrugated steel plates,but the stiffness was significantly less affected compared to the strength and ductility.Among them,the elastic stiffness decreased with increasing wavelength,fluctuated with the increase of wave height,and was positively correlated with the wave angle,timber thickness,and the number of CFRP layers;the strength fluctuated with the increasing wavelength and wave height,and was negatively correlated with the wave angle,but positively correlated with the timber thickness and the number of CFRP layers;and the ductility fluctuated with increasing wavelength and timber thickness,and was negatively correlated with the wave height and wave angle,but positively correlated with the number of CFRP layers.Ductility fluctuated with increasing wavelength and timber thickness,and was negatively correlated with wave height and wave angle,but positively correlated with number of CFRP layers.(3)The effect of each parameter on the group bolting effect of CFRP-group bolted timber connection joints with slotted-in corrugated steel plates were clarified based on orthogonal and single factor tests.It was found that wavelength,wave height,wave angle,timber thickness,and number of CFRP layers had a significant effect on the group effect(number of effective bolts).Among them,the number of effective bolts decreased with increasing wavelength,timber thickness and the number of CFRP layers but the trend was gradually decreasing;the number of effective bolts was the same as the number of wave height in the wave height of 10 mm and 15 mm,and the number of effective bolts in the wave height of 10 mm and 15 mm.The effective number of bolts was inversely proportional to the wave height under the conditions of wave height of 10mm and 15mm,and positively proportional to the wave height under the conditions of 15 mm,20 mm and 25 mm;the effective number of bolts fluctuated increasing wave angle;in addition,the number of columns of bolts had a more significant effect on the group effect.(4)The theoretical calculation formula of strength of single bolted timber connection joint with slotted-in corrugated steel plates was established based on the European yielding theory and fracture deformation calculation method,the formula for calculating the effective number of bolts of group bolted connection joints considering the wave size and the number of bolt columns was constructed based on the experimental data,then the formula for calculating the strength of group bolted connection joints was constructed;additionally,the theoretical calculation formula for stiffness of single and group bolted connection joints were established based on the spring force model.(5)The numerical model of CFRP-bolted timber connection joints with slotted-in corrugated steel plates was constructed by combining the timber foundation model and CZM model,and fracture simulation was carried out.It was shown that the range coefficients of timber foundation can be taken at[min(bolt spacing/d),min(bolt spacing/d)-1]or[min(bolt spacing/d)+min(bolt spacing/d)-1]/2 to better respond to the stiffness and strength characteristics,and the CZM model can effectively respond to the brittleness or ductile behavior under the consideration of the fracture characteristics,which reflects the applicability of the model.(6)Combined with experiments and numerical simulation analysis,the simple judgment basis of the damage model for the end of timber in the bolted timber connection joints with slotted-in corrugated steel plates was clarified,refined the strength calculation criterion,and proposed the appropriate design suggestions combined with the results of experimental and numerical analyses.It was found that the wave height and wave angle are the key parameters affecting the damage mode of the end of the timber.In which,when the wave angle is between 0 and 90°,the end of the timber appears to be pressurized damage when the h×sin~2θis less than 11.00,and the end of the timber appears to be shear damage when the h×sin~2θis greater than 11.00,while the end of the timber appears to be obvious shear damage when the wave angle is equal to 90°when the distance between the ring-oriented CFRP and the end of the timber groove is larger;moreover,the double slotted-in Z-type steel plates can effectively delay the RL fracture effect on both sides of the bolted timber joints with slotted-in steel plates,thus improving the ductility,and also improving the stiffness and strength to a certain extent.