Experimental Study On Shear Bearing Capacity Of Underpinning Beam For Underground Increase Of Existing Buildings

With the continuous advance of China's urbanization process,many of the existing buildings in the use of functional defects continue to show due to the lack of underground parking or other facilities.More and more people pay attention to the development and utilization of underground space.The foundation underpinning is the key technology to the underground increase of the existing building,the setting method of the underpinning pile is divided into setting on both sides of the upper vertical member and in the middle of the vertical member.As the supporting pile is set in the middle position,because the load of the superstructure is applied on the supporting beam through the vertical members,the shear capacity of the beam is put forward very high requirements.Due to the existence of frame columns in the upper part of the existing building,the beam-column joint connection between the frame column and the underpinning beam adopts the double-beam underpinning construction that wraps the frame column in four directions,in order to ensure the smooth completion of the replacement process.Taking the underground increase of a building in Huaiyin as the project background,The supporting method of setting the supporting pile in the middle of the original vertical member is adopted,Midas gen is used to compare and analyze the three schemes of ordinary reinforced concrete underpinning beam,shaped steel reinforced concrete underpinning beam and shaped steel reinforced concrete underpinning beam as horizontal support for support structure,to study the mechanical mechanism,displacement and deformation of the underpinning beam and to optimize the design.The results show that:(1)The middle span of the supporting beam bears a large upper load,and the main shear beam is a double beam structure with approximately deep flexural members.(2)Beam height reduced from 1.4 meters to 1.0 meters and the headroom can be obtained by usingsteel reinforced concrete underpinning beam,the self weight and internal force are also reduced accordingly.(3)The shaped steel reinforced concrete underpinning beam as horizontal support for support structure,which further reduces the stirrup of the underpinning beam.In order to further study the shear mechanism of underpinning beam,a 1:2 large scale underpinning beam model was developed for comparative experimental study.A total of 6specimens of joist beam were designed for different working conditions(section form,reinforcement,single beam or double beam,built-in h-beam).The following conclusions are obtained by analyzing the failure process,load-deflection curve and load-strain curve:(1)When only the section form is changed,the ultimate load of the single beam section is roughly the same as that of the double beam section,but the double beam section is slightly higher than that of the single beam section.(2)Compared with reinforced concrete jobbing beams with internal h-beams,the internal h-beams can improve the shear capacity of the inclined section,improve the ductility of the members and delay the brittle failure.(3)The shear force of the supporting beam is mainly borne by the stirrup and section steel at the midpoint of the connection between the bearing and the loading point.Based on the test results and relevant specifications,the truss-arch calculation model was adopted to the shear capacity formula of the inclined section of reinforced concrete underpinning beam with h-shaped steel embedded in the method is established.