Experimental Study Of The Base-isolated Brick Wall With Assembled Steel Underpinning Beam

In the existing buildings in our country,the masonry structure which does not meet the current requirements for fortification still accounts for a large proportion.Some old primary and secondary school buildings and hospitals are mostly multilayer masonry structure.Although the requirements for seismic resistance were considered in the design,with the continuous improvement of the requirements for seismic resistance,some buildings have not meet the current requirements,and it is necessary to adopt seismic reinforcement or isolation renovation measures for such buildings.Due to its special use,such buildings require a short construction period.The traditional seismic reinforcement method,on the one hand,the size of structural components in the high intensity areas is usually designed very large,which affects the headroom of the structure and destroys the architectural form of the structure during the building reinforcement,so that the building can’t keep the original state.On the other hand,arbitrarily increasing the size of the structure will increase the stiffness of the structure,which will increase the seismic force,forming a vicious circle.Concrete underpinning structure is generally used in solation renovation design of existing buildings,which has long construction period and high labor intensity.Assembled steel underpinning beam have the characteristics of safety and reliability,block assembly,quick installation,fast construction speed,and can be used for factory processing,which greatly shorten the construction period.The transfer mechanism of the two underpinning structures is also different.For concrete underpinning structures,the load is transferred through the bond between the underpinning beam and the underpinning wall.Assembled steel underpinning structure uses friction principle to realize effective underpinning.It converts the bolt pretightening force into the friction force between the column or wall and the steel beam to bear the vertical and horizontal load,and transfers this force to the lower column or foundation.To sum up,it is of great significance to study the application of assembled steel underpinning beams for base-isolated brick wall.In this thesis,quasi-static tests and theoretical analysis are carried out on 5 brick wall specimens of assembled steel underpinning beams（3 non-isolated specimens and 2 isolated specimens）.The main conclusions are as follows:（1）The failure mode of the specimen is that both sides rotate up and down with the axis of the middle span of underpinning steel beam.The slippage occurs between the combining face and the assembled steel underpinning beam,while there is no damage between the brick wall and the combining face.（2）In the non-isolated structure,the skeleton curve has no descent part and the bearing capacity keeps rising,which indicates that there is enough safety reserve for using assembled steel underpinning beams in brick wall underpinning.The bearing capacity,deformation capacity and energy dissipation capacity of the structure can be greatly improved by increasing the bolt pretightening force.（3）In the isolated structure,the energy dissipation capacity of the structure can be greatly improved and the relative slip between the brick wall and the assembled steel underpinning beam can be reduced by increasing the bolt pretightening force.Compared with specimen LW-25,the total bolt pretightening force of the underpinning steel beam of specimen LW-35 increased by 39.76 %,and the overall energy dissipation capacity increased by 8.29 %.Compared with the superstructure of specimen LW-25,the energy dissipation capacity of specimen LW-35 is increased by 26.24 %,and the relative slip decreases by 54.90 % when the isolation bearing reached 250 % shear strain（rare earthquake）.When selecting lead rubber bearings,the effects of bearing pressure and bolt pretightening force on the horizontal equivalent stiffness and equivalent damping ratio of the isolation layer should be considered.The isolation bearing of specimen is mainly translational,so the torsion effect need not be considered in engineering design.（4）Compared with non-isolated specimens,the deformation capacity and energy dissipation capacity of isolated specimens are greatly improved.Compared with specimen W-25,the ultimate displacement Angle and energy dissipation capacity of specimen LW-25 are increased by 4712.78 % and 10907.07 % respectively.Compared with specimen W-35,the ultimate displacement Angle and energy dissipation capacity of specimen LW-35 are increased by 2930.03 % and 3228.67 % respectively.It shows that the lead rubber bearings can reduce the input of ground motion energy and the seismic response of the superstructure for assembled steel underpinning beam in brick wall structure,and meet the basic fortification target of "no damage under moderate earthquake,repairable under major earthquake,no collapse under huge earthquake".（5）According to the bearing capacity of non-isolated specimens,the theoretical formula is derived.The research results have a good reference significance for the base isolation and transformation of the existing brick wall structure with assembled steel underpinning beams.