Experimental Study On Mechanical Properties Of Friction Steel Coupling Beams

As one of the important lateral force resisting components in the shear wall system,coupling beams play the role of the first defense to earthquakes.Multiple earthquake disasters show that coupling beam with small span-to-depth ratio is prone to fail in a shear mode,which is extremely unfavorable to the strength and ductility of coupling beams,and difficult to be repaired after an earthquake.The energy-dissipating coupling beam,if designed rationally,can concentrate most of the deformation and energy into the associated dampers,which can be quickly replaced after a strong earthquake,thus making the system more resilient.To this end,a friction damper is selected as the energy dissipating component connected through steel members to the concrete walls.This study covered the experiments of the connection to RC walls,the friction damper,and the friction coupling beam.Major contents and findings include:Firstly,three kinds of connection joints with different structural patterns are designed and tested.Among them,the specimen SJ1 is the rebar anchorage where the steel rebars bear the bending moment and shear force simultaneously.Specimen SJ2 is the angle steel-shear plate anchorage,and the specimen SJ3 is the steel rebars-shear plate anchorage.Both of them are designed to resist bending and shear separately by different components,i.e.,the bending moment sustained by either angle steels or steel rebars,while the shear force by the shear plate.The test results show that SJ1 has a large initial stiffness,uniformly distributed damages,full hysteresis,and moderate rotational stiffness.However,with the loading force increased,the concrete under the embedded plate tends to be partially crushed,resulting in a serious vertical slip of the embedded plate.The vertical stiffness is significantly degraded.The hysteresis loop of SJ2 is narrow,and the damages of wall is concentrated and difficult to be repaired.SJ3 has uniformly distributed damages.The rotational and vertical stiffness are both large so that the internal forces can be transferred sufficiently.The calculated value from the design equations is close to the experimental value for SJ1,but quite conservative for SJ2 and SJ3.Secondly,four types of friction materials are tested cyclically in order to develop a friction damper satisfying the design and application standards.The friction coefficients of carbon-titanium alloy,copper-iron alloy and ceramic material are similar,and the friction coefficient of semi-metal(brake pad)material is relatively larger.Different from the carbon titanium alloy,the behavior of the rest three types of material is temperature dependent.With the temperature increase,the friction coefficient decreases.In addition,the configuration of damper is optimized by using disc springs to control the contact pressure and preload force.The performance test of the improved damper is carried out,and stable mechanical behavior is realized for both configurations B and C,particularly under low-frequency loading.The change of friction force is less than 10%,and the change of bolt preload force is also less than 10%.Their hysteretic curves are close to rectangle with full energy consumption.Finally,four coupling beam specimens are designed and tested quasi-statically,including one RC coupling beam and three friction steel coupling beams.One friction damper is inserted in the mid-span of each friction steel coupling beam,which is connected to RC walls through steel members with I-shaped cross-sections.The traditional RC coupling beam is dominated by bending deformation in the early stage of loading,and horizontal cracks appear in the beam.With the increase of loading,the plastic hinges form at the both ends.After that,the plastic regions are expanded around the plastic hinge,and the wall limbs and floor slabs are seriously damaged.Contrast to the RC coupling beam,the main deformation and energy dissipation of friction steel beam are concentrated on the friction damper.The hysteretic curve is close to square,implying a much larger energy dissipation capacity than the RC coupling beam.Friction steel coupling beam can effectively control damages of RC components.The maximum crack width of wall limb is less than 1mm.Those cracks on the floor slab are all less than 1.2mm,and are no concentrated damages.It is also observed the bearing force contributed by the floor slab is very limited.For the prefabricated specimen,it takes two workers 80 min to complete the assembly of the components.This paper systematically studies the configuration and design method of friction steel coupling beam.Experimental results demonstrate that the proposed coupling beam is an effective seismic structural member,which can dissipate large amount of seismic energy,control structural damages,and improve the reparability after strong earthquakes.Combined with the assembly technology,the performance targets of high ductility,low damage,and replaceability can be effectively achieved for key structural members.