Experimental Study of a New Type Buckling-Restrained Brace. Abstract

Transcription

1 Experimental Study of a New Type Bukling-Restrained Brae S. G. CHENG, X.X. KONG AND K.H. LUO Institute of Earthquake Engineering, China Aademy of Building Researh Abstrat The steel ore and unbonding agent material of the existing bukling-restrained braes (BRB) are usually made of speial material, whih adds to the manufaturing ost and makes it hard to promote the use of BRB. In order to avoid the disadvantages of the existing BRB, a new type of BRB with gap and protuberane was developed. Six new type BRBs were tested with low-reversed yli loading history inluding standard loading and low-yle fatigue loading. The fatigue modes of BRB during the test were studied. Some mehanis properties indexes inluding elasti stiffness, seondary stiffness were identified based on the hysteresis urves of the BRB. And energy dissipation indexes from low-yle fatigue loading test were alulated aording to Chinese ode and FEMA356. In order to study seismi behavior of frames with BRB, two more bukling-restrained braed frames (BRBF) were tested inluding a plane BRBF with yli loading test and a BRBF with shaking table test. The test results of BRB indiate that the measured mehanial harateristis of BRB agree well with theoretial results and the energy dissipation behavior of BRB is stable. This new type of BRB an meet the requirement of existing norms and reah the standard of appliation. The test results of BRBF indiate that the seismi behavior of BRBF an be improved by the energy dissipation of BRB. Keywords:bukling-restrained braes (BRB), bukling-restrained braed frame (BRBF), yli loading test, hystereti harateristis, seismi behavior; shaking table test. Introdution Bukling-restrained braes (BRB) an yield both under tension and ompression without bukling, whih an avoid the disadvantages of onventional brae by ahieving better energy dissipation []-[5]. Aording to the researh results, the steel ore of BRB is usually made of speial steel with high dutility, whih raises the ost of BRB produt. Apart from this, unbonding agent material has not only the funtion of dereasing the frition fore between steel ore and onrete, but also the funtion of providing enough spae to aommodate lateral expansion of steel ore under ompression. Due to the above reasons, BRB depend on unbonding agent material exessively, whih makes BRB diffiult to develop. 2. Design of New Type BRB Based on the prophase researhes [6] [7], a new type of BRB with own intelletual property right [8] is invented. The sizes of BRB are shown in Fig. and Table. The features of this

2 new type BRB are as following: ) The ore plate is made of normal steel material Q235B, whih an redue the ost of produt. 2) Two protuberanes are set in the middle of steel ore at both sides along the thikness diretion to avoid the slippage between onrete and steel ore when BRB is under transport or installation. 3) A gap is set along the unbonding agent material interfae filled with ompressive material to meet the need of lateral expansion when steel ore is ompressed, while the unbonding agent material is used to redue the shear transfer between steel ore and onrete. That resolves the problem that BRB depend on unbonding agent material exessively. Fig.. Shemati of BRB Table. Sizes of BRB Steel ore Outer tube Steel type b ore (mm) t ore (mm) A ore (mm 2 ) Steel type Sizes(mm) Q Q Experimental Program 3. Test Setup Six BRB of the same sizes (as shown in Table ) were tested in the Laboratory of Strutural Engineering, at China Aademy of Building Researh. Fig. 2 shows test equipment and speimen setup. The test equipment is an atuator with eletro-hydrauli servo system and BRB speimen is fixed in the middle of the equipment. A displaement sensor is set along the length diretion of BRB and is fixed at both side of BRB. 3.2 Loading History Cyli loading test inluded standard loading and low-yle fatigue loading. Aording to the Fig. 2. Shemati of the test equipment setions of energy dissipation devie from China Code foe Seismi Design of Buildings (GB ) [9] in China mainland and Reommended provisions for bukling-restrained braed frame from SEAOC and AISC [0]

3 in USA, test loading histories of standard loading and low-yle fatigue loading are shown in Fig. 3. The maximum deformation of BRB and interstory drifts whih math along with maximum deformation are shown in Table 2. Speimen 0 ~ speimen 5 were tested with standard loading history and speimen and speimen 4 were tested with low-yle fatigue loading history after standard loading. (a)standard loading history (b)low-yle fatigue loading history Fig. 3. Loading history Table 2. Standard loading history Test Number No. of yles BRB deformation (mm) Interstory drift Comments 2.4 /75 elasti 0.5Dy /360 elasti Dy /45 elasti-plastiity 0.5Dm /70 elasti-plastiity Dm /50 elasti-plastiity.5dm. The alulation of interstory drift is based on the angle between BRB and frame olumn is 45 degree. 2. Dy is the yield deformation of BRB. 3. Dm is the design interstory drift of frame. 4. Experimental Results 4. General Behavior and Failure Mode 4.. Standard Loading Test There are six BRB speimens in standard loading test as shown in Table 3. Speimen 0 and speimen were tested in the parallel way in whih the steel ore and lamp of atuator were in the same plane. Speimen 2 ~ speimen 5 were tested in the perpendiular way in whih the steel ore plane and lamp plane of atuator were orthometri. The out-of-plane stiffness of Speimens whih were loaded in parallel way was not enough to resist larger deformation when BRB was ompressed. Speimen 0 was damaged with out-of-plane bukling failure of steel ore when the axial fore of atuator reahed about -2kN, as shown in Fig. 4 (a). Damage loation of speimen 0 was between yielding portion and transition portion of steel ore, where the ross-setion of steel ore hanges as shown in Fig. 4 (b). Speimen didn t have obvious failure in standard loading test but was damaged

4 with out-of-plane bukling failure in low-yle fatigue loading test after 5 yles, and the failure mode was the same as speimen 0 s whih is shown in Fig. 4 (b). Speimen 2 ~ speimen 5 whih were loaded in perpendiular way didn t have obvious failure in standard loading test. The test results indiate that speimens loaded in perpendiular way have better out-of-plane stability. Speimen number Table 3. Summary of the speimens in standard loading test Loading speed (mm/min) Relationship of steel ore and lamp Failure desription speimen 0 5~0 parallel out-of-plane bukling failure, no data left speimen 5~0 parallel finish the standard loading, no failure speimen 2 5~0 perpendiular finish the standard loading, no failure speimen 3 20~30 perpendiular.5dm loading haven t done, no failure speimen 4 20~30 perpendiular.5dm loading haven t done, no failure speimen 5 20~30 perpendiular.5dm loading haven t done, no failure (a) Out-of-plane buking failure of speimen 0 (b) Damage loation of speimen 0 ()Tension deformation of the steel ore of speimen Fig. 4. Phenomenon of standard loading test 4..2 Low-yle Fatigue Loading Test Aording to Setion in China Code for Seismi Design of Buildings (GB ), with max permitted deformation under low-yle fatigue loading test, the main energy dissipation indexes of energy dissipation devie should not attenuate more than 0% after 60 yles [9]. After the standard loading test, speimen and speimen 4 were tested in low-yle fatigue loading test as shown in Table 4. The maximum deformation of low-yle fatigue loading test was Dm (4mm) as shown in Fig. 3 (b). Both speimen and speimen 4 were damaged less than 60 yles. Speimen was damaged with out-of-plane bukling failure of steel ore after 5 yles as shown in Fig. 5 (a) while speimen 4 was damaged with tension failure of steel ore after 40 yles as shown in Fig. 5 (b). Table 4 and Fig. 5 indiate that speimens whih were loaded in parallel way had better stability. They also indiate that loading history and loading speed are important fators whih an influene the results. Before the low-yle fatigue loading test, speimen was loaded 2 more yles at.5dm than speimen 4 was. That s another reason why speimen was damaged earlier than speimen 0.

5 For the BRB speimen whih dissipates energy by elasti-plastiity deformation, 60 yles of low-yle fatigue loading test whih is quoted from China Code for Seismi Design of Buildings (GB ) is still open to disussion based on the test phenomenon. Speimen number speimen speimen 4 Table 4. Summary of the speimens in low-yle fatigue loading test Loading Relationship Loading history speed of steel ore Failure desription (mm/min) and lamp standard loading is already done, out-of-plane bukling 5~0 parallel the max deformation is..5dm failure after 5 yles standard loading is already done, tension failure after perpendiular the max deformation is Dm yles (a) Out-of-plane bukling failure after 5 yles (b)tension failure after 40 yles Fig. 5. Damage phenomenon of low-yle loading test 4.2 Hysteresis Curves There were six BRB speimens that were tested with standard loading history and two with low-yle fatigue loading history. Beause of the limited length of this paper, not all test hysteresis urves are shown here. Hysteresis urve of speimen 2 with standard loading test and speimen 4 with low-yle fatigue loading test are shown in Fig. 6 and Fig. 7. The hysteresis urves indiate that BRB has stable apability of energy dissipation as shown in Fig. 6 and the failure mode of speimen 4 as shown in Fig. 7. Fig. 6. Hysteresis urves of speimen 2 at standard loading test Fig. 7. Hysteresis urves of speimen 4 at low-yle loading test

6 5. Evaluation and Comparison Aording to experimental data as shown in setion 4.2, some mehanis properties indexes inluding elasti stiffness, seondary stiffness of BRB an be identified and ompared with the theoretial results. Some energy dissipation indexes of BRB also an be alulated. 5. Elasti Stiffness The elasti stiffness of the BRB is the sum of the individual stiffnesses of the different brae segments as the following equation K = K K K yi tr on () where K = EA / L is the elasti stiffness of the yielding portion, K = EA / L is yi yi yi on on on the stiffness of the onnetion portion and K = EA / L is the stiffness of the transition tr tr tr portion. Supersript of K indiates that the values are obtained from omputed results. Table 5 shows the measured stiffness, omputed stiffness and the differene between these e values for eah BRB speimen. The measured stiffness K was taken from hysteresis urves data by fitting a straight line. Table 5. Comparison of measured and theoretial elasti stiffness values Measured Stiffness Computed Stiffness Differene % Speimen number e e K K K (kn/mm) K (kn/mm) K speimen speimen speimen speimen speimen mean value * *the result of speimen 0 is eliminated when alulate the mean value. 5.2 Seondary Stiffness The seondary stiffness, also known as plasti stiffness, an be omputed as K = α K (2) 2 where K is the omputed elasti stiffness and α is defined as the ratio of post-yield to elasti stiffness. Aording to the material property of BRB steel ore, here α is equal to 0.02.

7 Table 6 shows the measured seondary stiffness, omputed seondary stiffness and the differene between these two values for eah BRB speimen. The measured seondary stiffness K 2 was taken from hysteresis urves by fitting a straight line. Displaement data between 3mm and 3mm from hysteresis urves were olleted to alulate the seondary stiffness as shown in Fig. 8. Fig. 8. Shemati of seondary stiffness values Table 6. Comparison of measured and theoretial seondary stiffness values Measured Stiffness Computed Stiffness Differene % Speimen number e e K 2 (kn/mm) K 2 (kn/mm) ( K ) K K 2 2 / 2 speimen speimen speimen speimen speimen mean value 5.3 Energy Dissipation Analysis Aording to setion from FEMA356, Some energy dissipation inluding the effetive stiffness ( K eff ), the maximum fore at zero displaement ( F 0max ) and minimum fore ( F 0min ) at zero displaement, the area of the hysteresis loop (W D ) of a prototype energy dissipation devie for any one yle does not differ by more than plus or minus 5% from the average values as alulated from all yles in that test []. For speimen and speimen 4 whih were tested in low-yle fatigue loading history, some energy dissipation indexes an be alulated based on the above FEMA setion. Table shows the results of energy dissipation indexes of speimen 4. Beause of the limited length of this paper, only the first loop results and the last loop results are shown in Table and results for speimen 4 are not shown here. No. of yles Table 7. Energy dissipation indexes results of speimen 4 Area of the hysteresis Maximum fore at Minimum fore at loop zero displaement zero displaement Effetive stiffness W D (kn mm) F0max F K ( kn) 0min eff ( kn) ( kn/mm) mean value from all yles Differene %

8 6. Further Experimental Study on Frame with BRB In order to study the seismi behavior of struture with new type of BRB, some further tests on bukling-restrained braed frame (BRBF) were studied. Beause of the limited length of this paper, only brief summaries are listed in this setion. And more researhes of those tests will be shown in other papers in the future. 6. Low-yle loading test of BRBF A plane bukling-restrained braed frame (BRBF) and a onventional braed frame (CBF) as omparison were tested with low-reversed yli loading history. The view of BRBF and CBF are shown in Fig. 0. The sizes of onventional brae are the same as the BRB steel ore without outer tube and onrete. The loading history of BRBF is shown in Fig.. Failure mode of BRBF and Fig. 9. Loading history of BRBF CBF are shown in Fig. and Fig. 2. The hysteresis urve of BRBF and CBF are shown in Fig. 3 and Fig. 4, Fig. 3 also shows the theoretial urve as omparison. (a)brbf Fig. 0. View of the frames (b)cbf Fig.. Tension deformation of the steel ore of BRB Fig. 2. Bukling failure of onventional brae

9 Fig. 3. Comparison of measured and theoretial hysteresis urve for BRBF Fig. 4. Hysteresis urve of CBF 6.2 Shaking Table test on frame with BRB A bukling-restrained braed frame was tested on the shaking table as shown in Fig. 5. Three natural earthquake waves and one test wave were seleted to load the model. The PGA of the waves inreases from 0.g to.0g. Fig. 6 shows the measured and theoretial hysteresis urve of No. BRB whih is in story one under the test wave loading when the PGA is 0.6g. Fig. 7 shows the measured and theoretial Fig. 5. View of model hysteresis urve of the same BRB under the USA223 wave loading when the PGA is.0g. Fig. 8 shows the energy dissipation of BRBF under test wave loading when the PGA is 0.6g and when the PGA is.0g. The energy dissipation urves indiate that the rate of BRB energy inrease from 5% of the whole input energy to 23%. (a) Theoretial urve (b) Measured urve Fig. 6. Measured and theoretial hysteresis urve of BRB in test wave when PGA is 0.6g

10 (a) Theoretial urve (b) Measured urve Fig. 7. Measured and theoretial hysteresis urve of BRB in USA223 wave when PGA is.0g 7. Conlusion (a)pga is 0.6g (b)pga is.0g Fig. 8. Energy dissipation of BRBF under test wave loading From the studies of presented in this paper, we an draw the following onlusions: ) A new type of BRB is developed and it an overome the disadvantages of the existing BRB. It also redues the ost of BRB produt. 2) The measured mehanial harateristis of BRB agree well with theoretial results and the energy dissipation behavior of BRB is stable. This new type of BRB an meet the requirements of existing norms and reah the standard of appliation 3) The failure modes of BRB indiate that speimens loaded in perpendiular way have better stability. Some improvement of installation will be disussed in the future study. 4) Seismi behavior of BRBF an be improved by the energy dissipation of BRB. BRBF have better energy dissipation harateristis than the CBF. Referenes []. UANG Chia-ming, NAKASHIMA Masayoshi, LU ye. The Pratie and Researh Development of Bukling-Restrained Braed Frames (I), Progress in Steel Building Strutures, 2005,7(): -2 (in Chinese) [2]. UANG Chia-ming, NAKASHIMA Masayoshi, LU ye. The Pratie and Researh Development of

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