Experimental Study On The Mechanical Performance Of Assembled Steel Underpinning Beams In The Underpinning Of Brick Walls

As urban construction is in full swing,building displacement is becoming more and more common in urban reconstruction.Underlaying technology is one of the important guarantees for the success of displacement engineering and relates to the safety of buildings in the process of displacement.In the existing displacement technology,reinforced concrete double beam or single beam is mainly used for underpinning of masonry structure wall.In recent years,the research group has carried out the underpinning test research on the assembled steel underpinning beams for the axial and eccentric compression members such as concrete columns,wooden columns and brick columns.The results show that all the failures are the sliding failure of the interface between the underpinning beam and the underpinning member,and the bearing capacity does not decline rapidly after the sliding of the interface.But the research on mechanical behavior of assembled steel underpinning beams in brick wall underpinning is relatively few.In this thesis,in view of the brick masonry packaged steel underpinning method,designed the eight brick masonry packaged steel underpinning components,consider and tighten the screw,the screw quantity and in combination with surface plumpness shear bearing capacity for the underpinning structure,the influence of different working conditions is underpinning the mechanical characteristics of components,the relative displacement change rule and the ultimate bearing capacity.1.In the case of the same number of screws,the influence of different screw preload on the underpinning capacity is studied.The number of screws in JD1,JD6 and JD8 specimens is equal,the total preload of screws is 450 k N,540 k N and 630 k N,respectively,and the ultimate undercarriage capacity is 460 k N,750 k N and 920 k N,respectively.The load-relative displacement curves of wall beams of JD1,JD6 and JD8 specimens are basically the same,and the undercarriage capacity increases obviously with the increase of the total preload of screws.In terms of the relative displacement between the wall and the underpinning beam,the smaller the screw preload is,the faster the relative displacement between the wall beam changes,and the larger the final slip of the specimen is and the earlier the slip failure occurs.2.In the case of the same screw preload,the influence of screw number on underpinning capacity is studied.The total pretightening force of bolts in JD1 and JD2 specimens is 450 k N,the number of bolts in JD1 is 12,and the number of bolts in JD2 is 18.The ultimate bearing capacity of the two specimens is about 465 k N,and the maximum relative displacement of the corresponding wall beam is not much different,less than 1.5mm.3.The influence of joint plumpness on underpinning capacity is studied.The test results show that the plumpness of the joint surface of the joist beam and the wall has a certain effect on the bearing capacity of the joist.The total preload of the screw of specimen JD5 is equal to that of specimen JD6,and the ultimate bearing capacity of JD6 is significantly greater than that of JD5.After the test,it is found that the plumpness of the joint surface of JD6 is about70%,while the plumpness of JD5 is only about 35%,indicating that the plumpness of the screed mortar on the junction surface has a significant influence on the underburden bearing capacity.Therefore,increasing the plumpness of the joint surface can effectively improve the bearing capacity of the undercarriage and reduce the relative displacement between the undercarriage beam and the wall.4.The influence of bonding force on underpinning capacity is studied.The experimental results show that the bonding force between the underpinning beam and the wall surface has a certain effect on the underpinning capacity.Specimen JD7 and specimen JD8 in equal screw total pre-tightening force,screw number under the condition of no effect,the result of the test wall beam load-the relative displacement curves in basic same,underpinning components of jack-up underpinning beam and wall body between the combination surface no more cohesive force,is not the ultimate bearing capacity of jack-up specimen JD8 JD7 significantly greater than jack.In terms of relative displacement of wall beam,the maximum slip of JD7 after jacking is 3.23 mm,and the maximum slip of JD8 without jacking is 1.21 mm.The maximum slip of specimens after jacking is significantly increased and the slip occurs earlier.Finally,based on the test results,the calculation formula of underpinning capacity of brick masonry composite steel underpinning method is put forward,which can provide reference for brick wall underpinning design in brick masonry building displacement engineering.