30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore. Article Online Id:

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1 EXPERIMENTAL RESEARCH ON JOINTS OF PRESTRESSED CONCRETE BEAM AND COMPOSITE CONCRETE COLUMN WITH CORE OF CONCRETE FILLED STEEL TUBE UNDER LOW-CYCLIC REVERSED LOADING Deng Zhi-heng*, Guangxi University, China Fan Ye-shu, Guangxi University, China Xiang Ping, Guangxi University, China Wan Yun-fang, Guangxi University, China 30th Conferene on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore Artile Online Id: The online version of this artile an be found at: This artile is brought to you with the support of Singapore Conrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the artile in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 30 th Conferene on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore EXPERIMENTAL RESEARCH ON JOINTS OF PRESTRESSED CONCRETE BEAM AND COMPOSITE CONCRETE COLUMN WITH CORE OF CONCRETE FILLED STEEL TUBE UNDER LOW-CYCLIC REVERSED LOADING Deng Zhi-heng*, Guangxi University, China Fan Ye-shu, Guangxi University, China Xiang Ping, Guangxi University, China Wan Yun-fang, Guangxi University, China Abstrat Based on experimental researh of four joints of prestressed onrete beam and omposite onrete olumn with ore of onrete filled steel tube and one joint of ordinary prestressed frame struture, the dutility, energy dissipation, hysteresis property and the mehanial behavior of joints have been analyzed. It also takes researhes on reliability of the design method and the influene of fator. Experimental results indiate that this kind of joints an raise the shear strength of ore area of joints and its seismi performane is also good, this design method an resolve the problem of the traditional onrete filled steel tubular struture effetively, and an also meet the requirement of the high-rise and large span struture. It an be useful referene to engineering design. Key words: prestressed onrete filled steel tube; joint; seismi performane 1. Introdution Conrete filled steel tubular struture is widely used in high-rise building. Generally, onrete filled steel tubular struture use the onrete inside and the steel tube wrapped around. This type of struture has some merits but its anti-rust, anti-orrosion property and fireproofing behavior are short. Beause of the omplexity of the struture of joints, it an hardly be used onsistent with prestressed onrete beam to form prestressed frame struture. This artile use omposite onrete olumn with ore of onrete filled steel tube, put forward the struture form whih is made up of prestressed onrete beam and omposite onrete olumn with ore of onrete filled steel tube. It an resolve the problem of the traditional onrete filled steel tubular struture effetively, and an also meet the requirement of the high-rise and large span struture. The joint design and the seismi behavior is the key tehnology of this struture. This artile suggests the method of design, and have the low-yli reversed loading test on four joints of prestressed onrete beam and omposite onrete olumn with ore of onrete filled steel tube and one joint of ordinary prestressed frame struture. The result indiates that the shear strength of this kind of joints an be largely improved and its seismi resistant behavior is also good.

3 2. General situation of experimental researh 2.1 Model design Speimens are middle storey joints on the edge of plan stress state frames. Diameter and setional area of the steel tube is the main experimental parameter. Aording to pratial dimension in engineering, speimens are designed in the ratio of 1:1 to 1:2. Column is the omposite onrete olumn with ore of onrete filled steel tube. Beam is the post-tensioning partially prestressed onrete beam with bond. Five speimens were prepared, JD-1 is the ommon joint without steel tube in it, it lies on the edge of partially prestressed frame and mainly used for omparison. Speimens in this artile are designed aording to size of tube. This artile also has researhes on the influene of protetive layer size to mehanial behavior of joints. Reinforement status of prestressed onrete filled steel tubular struture joints is designed as shown in Fig.1. In other joints, dimension of tube respetively are Φ ,Φ and Φ Material properties are shown in Table 1 and Table embeded steel-plate 1 3φ φj densifiation in olumn apitalφ8@50 densifiation dimension is 300mm 2 tendon longitude bar in upper region of beam embeded steel-plate φ8@100 3φ25 3φ vertial rib plate φ8@100 longitude bar at bottom of beam 2200 embeded steel-plate Φ20 φ φ8@ (JD-2) embeded steel-plate 1 Fig.1 Reinforement of prestressed onrete filled steel tubular joint 2.2 Experimental equipment and observational sites arrangement Test speimens are subjeted to vertial loads. Constant axial fore is given on olumn apital, and the value of fore is 1500kN. Low-yli reversed loading imposed on beam end is used to simulate seismi ation. Horizontal reating fore is balaned with bak strength wall. Almost about 60 observational sites were plaed in every speimen to proeed strain measurement of stirrups, tube and longitude bars. Fig.2 and Fig.3 present the arrangement of observational sties and experimental equipment.

4 Table 1 Material property of onrete Table 2 Material property of tube and steel Speimen Number f u f E Name Material Yielding Load Ultimate Strength Modulus of Elastiity JD JD JD JD JD Tube A Steel Φ Steel Φ Steel Φ Strand j (a) Strain gauge arrangement of hotizontal stirrup in joint ore (b) Strain gauge arrangement of stirrup in beam end () Strain gauge arrangement of steel tube Fig. 2 Observational sites arrangement gantry 70 irular steel reation buttress embeded steel plate 1 displaement sensor sheer deformation of joint ore speimen displaement sensor embeded plate 1 load sensor hoisting jak (300ton) loading at the end of beam embeded plate 2 embeded plate 3 steel plate Fig.3 Experimental equipment

5 2.3 Loading riteria Eah speimen will subjet to 1500kN fore on olumn apital. The loads given by 3000kN hoisting jak will be divided into four proedures. The low-yli loading on beam end is offered by servo-loading system. Aording to Experiment Code for Seismi Resistane of Joints, loading riteria shown in Fig.4 is adopted. It is ontrolled by value of fore before yielding and by displaement after yielding. Exeute the fore ontrol irulation twie before yielding, the fore value of the first irulation is 50% of yielding load, and the fore value of the seond irulation is 75% of yielding load. Displaement ontrol irulation should be exeuted step by step with the displaement... value of 1 times, 2 times, 3 times of yielding displaement and so on, repeat the irulation three times at eah step. With the ontrolling displaement Fig. 4 Loading riteria developing on, the fore will finally derease. When the value of fore is less than 80%~85% of P max, it is onsidered that the bearing apaity desend too low and the strength annot satisfy the requirement, so the speimen rushed. 3. Experiment result and analysis 3.1 Damage pattern of speimens First rak of speimen JD-1 ours in beam end. When reinforement bars at the bottom of beam end yield, raks spread longer and wider, and then more and more raks appear in juntion. Displaement ontrol is used after yielding, the raks at beam end extend and broaden under yli loads. Some new raks our in the joint area and finally form ross raks. At the third step of displaement ontrol, onrete at the bottom of beam end rashed, joint ore ould be divided into several diamond ross raks, seriously damaged. The raking status of other joints under the gradually loading steps is similar to JD-1. When longitude bars yield, the inreasing of raks in joint ore is slow, raks are very small and the development of raks is ompletely different with JD-1. After plasti hinge ourred in beam end, positive and negative yli loads an also be inreased to ertain extent in the displaement ontrol. Craks in beam end broaden, while raks in joint area progress little. At the third irulation of the triple yielding displaement ontrol, the onrete at the bottom of beam rashed, and then the longitude bars in upper beam yield and blow-up at the first irulation of quadruple yielding displaement ontrol. Craks in joint ore area are small and few when rush. It an be seen from the rak propagation and damage pattern of JD-2~JD-5 that the joint has good integrality and the design method is reliable. The steel tube and exterior onrete an work well together without stripping of onrete in ertain range of tube diameter, this kind of reinforement provides high shearing strength in joints. 3.2 Dutility and energy dissipation Beause most steel tube joints have approximately the same loading-displaement hysteresis urve, on aount of the limitation of artile spae, only a part of speimens loading-displaement hysteresis urve have been presented as shown in Fig.5. Generally it an be seen that with the developing of the loading step by step the hysteresis ring beome full and round, area ontained in the hysteresis ring is large, the skeleton frame urve appears smooth. It indiates that the energy dissipation ability is good. Hysteresis urve appears like full spindle-shaped, not shrinking. There is no prominent differenes of displaement hystereti urve between JD-1 and JD-2~JD-5 at beam end. It shows that the seismi behavior of joints of prestressed onrete beam and omposite onrete olumn with ore of onrete filled steel tube is good.

6 p(kn) f (mm) P(KN) JD f (mm) JD-3 P(KN) p(kn) f (mm) JD f (mm) JD-4 Fig. 5 Loading-displaement hysteresis urve at beam end The dutility ability of joints an be measured by dutility fator apaity an be measured by equivalent hystereti damping oeffiient, and the energy dissipation h e. Table 3 shows dutility fator and equivalent hystereti damping oeffiient. The experiment result shows that this kind of joints have good dutility apaity and seismi resistane behavior. Table 3 Dutility fator and equivalent hystereti damping oeffiient Speimen number Dutility fator Positive Reverse Equivalent hystereti damping oeffiient h e JD JD JD JD JD Shearing behavior Fig.6 shows the loading-shearing deformation hysteresis urve of JD-1 speimen and JD-2 speimen. Only some small rossing raks our in the joint ore of JD-2~JD-5, and shearing deformation is small. Wide raks our in JD-1 under servie load, and the joint ore area be divided into some small diamond piees under ultimate yli loading, then the joint ore area rushed. Aording to the experiment result of this artile, the joint of prestressed onrete beam and omposite onrete olumn with ore of onrete filled steel tube has strong restrain on onrete in the joint ore. Fore of longitude bars ould mostly be transferred to steel tube, so the anhorage is reliable. Therefore, raks develop slowly after first raking. The shear deformation of JD-2~JD-5 is only 1/8 of

7 this of JD-1. The shearing behavior is very exellent. P(KN) P( KN) γ 10e-3 ( rad ) γ 10e-3 ( rad ) Fig. 6 Load-shearing deformation hysteresis urve of joint ore 4. Mehanism analysis 4.1 Transmission of inner fore of joints The inner fore of joints is transferred through arrying bars of olumn and beam, ohesive fore between steel tube and onrete and ompressive fore of onrete. 1. Fore transferred by steel bars of beam Steel bars in beam are of two ategories, one is prestressing tendons and the other is non-prestressed reinforing bars. Aording to the diameter of inner tube, non-prestressed reinforing bars are ompletely or partially welding on the ring plate together. Tensile fore of the bar welding on the ring plate is transferred to the ore of tube diretly. Fore of other bars is transferred to ore area onrete that wrapped around the tube. It is transferred by the ohesive fore between bars and onrete and ompressive fore of hook at the end of bars. Bars inserted into joints transfer a large amount of fore to onrete in horizontal anhoring length. With the loading irulation ating on, ohesion between steel and onrete has partially been damaged. Shear fore transferred by horizontal anhoring derease, while ompression fore of hooks inrease at the same time, so the design of hooks at the end of bar is very important. Seen from fore transmission of non-prestressed reinforing bars, fore transmission is basially different from normal onrete strutures beause fore of bars is partially transferred diretly to the steel tube. This is a reason that performane of joints an be improved. The fore of prestressed tendons is transferred to the joint mostly by anhor plate and anhorage, only a little is transferred by ohesive fore between tendons and onrete. Beause of the funtion of prestressing fore, ohesive fore of prestressed tendons is smaller than ohesive fore of non-prestressed reinforing bars in joint area, and with the effet of anhor plate the glide of prestressed tendons is small. Prestressing fore strengthen the restrition of joints, and make the onrete in joints under two-way load bearing status. The funtion of prestressing fore ould enhane the raking load and shear strength of joints. In this artile, the raking load of joint JD-5 is small, beause there are only a few numbers of prestressed tendons in it, so it an prove this onlusion. Prestressing fore in prestressed beams an not only improve the mehanial performane of beam, but also improve the mehanial behavior of joints. 2. Compressive fore transferred by onrete Compressive fore exist around the joint ore, the ompressive fore C transferred from beam is deided by the ratio of steel in upper beam to steel at the bottom of beam, and it is also deided by strain of bars at the bottom of beam when loads yling. The ompressive fore C transferred form olumn ould be divided into three parts. The first part ats on the inner tube of joints diretly, the seond ats in joint ore between tube and longitude bars, and the last part of ompressive fore ats on protetive layer of onrete olumn. Beause stirrups are subjeted to transversal pulling fore and transversal expansion of onrete, the transversal tensile fore of protetive layer onrete will inrease, so the protetive layer might strip off when joints break up. At this time, the third part of ompressive fore C will diminish, and ompressive fore will be redistributed to the first part and the seond part, so an appropriate number of stirrups around longitude bars are neessary, it an provide effetive restrition on onrete between steel tube and longitude bars.

8 3. Fore transferred by steel tube and longitude bars In tensile area of longitude bars and tension parts of tube, fore an be transferred to ore area onrete, and ompressive area bars and steel tube at together with onrete on ompression area of joints. Strength of Joints an be largely improved beause of the restrition effet of steel tube. Steel tube ould funtion as longitude bars to transfer longitude fore, and also ould funtion as stirrups to resist shearing fore. 4.2 Mehanial analysis of joint ore Inlined raks our along the diagonal when shearing failure takes plae in the joint ore, inlined raks divide the ore area into two large parts. Analytial model is given in Fig.7. Frition exist T V T θ Fig. 7 Shearing model in these two parts, horizontal stirrups and steel tube that interset the raks will yield of tension. Shearing apaity of joints would be onsist of four major parts, first is the shearing fore of the yielding stirrups that interset the raks, seond is fritional fore between steel tube and longitude bars under the flexural-ompressing stress, this part of fore is related with axial ompressive fore and flexural-ompressing stress. The third part of fore is the shearing fore offered by ore onrete of joints. The last part is shearing fore offered by steel tube, shearing fore is the horizontal projetion of prinipal tensile stress of tube whih is perpendiular to the raks. The total shear strength of joints an be expressed as the following equation: V V V V V 1 2 s1 s2 Where V is horizontal projetion of frition fore between longitude bars and tube 1 V is horizontal projetion of shearing fore of onrete in tube 2 V is shear fore of the yielding stirrups in joint area s1 V is shear fore offered by yielding steel tube in joint area s2 From the equation above, it an be onluded that the shear strength of joints of steel tube reinfored onrete olumn ould be largely improved, beause of the self-resistane apaity of inner steel tube, and also beause the shearing apaity of onrete in the tube ould be largely enhaned. 5. Conlusions The following onlusions ould be made through the omparison of experiments and mehanism analysis of four joints of prestressed onrete beam and omposite onrete olumn with ore of onrete filled steel tube and one joint of ordinary prestressed frame strutures. (1) Composite onrete olumn with ore of onrete filled steel tube is a good reinforement style that has prominent benefiial ation on seismi behavior, and it an also improve the shearing apaity of joints. (2) Conrete ould be effetively restrited by steel tube in ore area. Development speed of the inlined raks is more slowly than normal onrete struture joints after the inlined raks ourred in joint area. (3) This kind of joints has good performane of dutility and energy dissipation apaity and the

9 equivalent hystereti damping fator is also larger than that of ordinary onrete frame struture joints. (4) Experiment results show that steel tube reinfored onrete olumn is a good reinforement form with exellent seismi performane, high bearing apaity and high shear strength. It an be available for referene in engineering design. Referenes [1] Zhong Shantong. The Conrete filled steel tubular strutures [M]. Tsinghua University Press, [2] Tang Jiuru. Seismi resistane of joints in reinfored onrete frames [M]. Southeast University Press, [3] Deng Zhiheng. Study on some problems of prestressed Mega-Frame-Tube with suspended damper ontrol strutural system[d]. Guangxi: Guangxi University, [4] Hsu,Thoma T.C.Nonlinear analysis of onrete membrane elements [J].ACI Materials. Journal, 1991,88(5): [5] Ottosen N S. Constitutive Model for Short-Time Loading of Conrete. ASCE,1979,105(EMI): [6] Cheng Xiaohui, Qian Jiaru, Fang Ehua. Analysis and shear alulation of RC mega-frame beam-olumn ore regions[j]. Journal of tsinghua University, 2000,40(6): [7] Park R, Thompson K J. Cyli load test on prestressed and partially prestressed beam-olumn joints[j]. PCI Joumal, 1997,22(5):66-83.