1、Experimental research on seismic behavior of abnormal joint in reinforced concrete frame Abstract :Based on nine plane abnormal joint s , one space abnormal joint experiment and a p seudo dynamic test of a power plant model , the work mechanism and the hysteretic characteristic of abnormal joint are
2、 put to analysis in this paper. A conception of minor core determined by the small beam and small column , and a conclusion that the shear capacity of ab2 normal joint depends on minor core are put forward in this paper. This paper also analyzes the effect s of axial compres2 sion , horizontal stirr
3、up s and section variation of beam and column on the shear behavior of abnormal joint . Finally , the formula of shear capacity for abnormal joint in reinforced concrete f rame is provided. Key words : abnormal j oint ； minor core ； seismic behavior ； shear ca paci t y CLC number :TU375. 4 ； TU317.
4、1 Document code :A Article ID :100627930 (2006) 0220168210 1 Int roduction For reinforced concrete f rame st ructure , t he joint is a key component . It is subjected to axial comp ression , bending moment and shear force. The key is whet her the joint has enough shear capaci2 ty. The Chinese Code f
5、 or S eismic Desi gn of B ui l di ngs ( GB5001122001) adopt s the following formula to calculate t he shear capacity of the reinforced concrete f rame joint . V j = 1. 1 j f t bj h j + 0. 05 j N bj bc + f yv Asvj hb0 - a s s (1) Where V j = design value of t he seismic shear capacity of the joint co
6、re section ； j = influential coefficient of t he orthogonal beam to the column ； f t = design value of concrete tensile st rength ； bj = effective widt h of the joint core section ； hj = dept h of the joint core section , Which can be adopted as t he depth of the column section in t he verification
7、direction ； N = design value of axial compression at t he bot tom of upper column wit h considering the combi2 nation of the eart hquake action , When N 015 f c bc hc , let N = 0. 5 f c bc hc ； bc = widt h of t he column section ； f yv = design value of t he stirrup tensile st rengt h ； Asvj = total
8、 stirrup area in a set making up one layer ； hb0 = effective dept h of t he beam. If t he dept h of two beams at the side of t he joint is unequal , hb0 = t he average depth of two beams. a s = distance f rom the cent roid of the compression beam steel bar to the ext reme concrete fiber . s = distan
9、ce of t he stirrup . Eq. 1 is based on t he formula in t he previous seismic code1 and some modifications made eavlicr and it is suit2 able to the normal joint of reinforced concrete f rame , but not to t he abnormal one which has large different in t he section of t he upper column and lower one (3
10、 600 mm and 1 200 mm) , lef t beam and right beam (1 800 mm and 1 200 mm) . The shear capacity of abnormal joint s calculat2 ed by Eq. 1 may cause some unsafe result s. A type of ab2 normal joint which of ten exist s in t he power plant st ruc2 t ure is discussed ( see Fig. 1) , and it s behavior wa
11、s st ud2 ied based on t he experiment in t his paper 2 Experimental work According to the above problem , and t he experiment of plane abnormal joint s and space abnormal joint , a p seudo dynamic test of space model of power plant st ruct ure was carried out . The aim of t his st udy is to set up a
12、 shear force formula and to discuss seismic behavior s of t he joint s. According to the characteristic of t he power plant st ruct ure , nine abnormal joint s and one space abnormal joint were designed in t he experiment . The scale of the model s is one2fif t h. Tab. 1 and Tab. 2 show t he dimensi
13、ons and reinforcement detail s of t he specimens. Fig. 2 shows the typical const ruction drawing of t he specimen. Fig. 3 shows the loading set up . These specimens are subjected to low2cyclic loading , the loading process of which is cont rolled by force and displacement , t he preceding yield load
14、ing by force and subsequent yield by t he displacement . The shear deformation of the joint core , t he st rain of the longit udinal steel and t he stirrup are main measuring items. 3 Analysis of test result s 3. 1 Main results Tab. 3 shows t he main result s of t he experiment . 3. 2 Failure proces
15、s of specimen Based on t he experiment , t he process of t he specimens failure includes four stages , namely , t he initial cracking , t he t horough cracking , the ultimate stage and t he failure stage. (1) Initial cracking stage When t he first diagonal crack appears along t he diagonal direction
16、 in t he core af ter loading , it s widt h is about 0. 1mm , which is named initial cracking stage of joint core. Before t he initial cracking stage , t he joint remains elastic performance , and the variety of stiff ness is not very obvious on t he p2 curve. At t his stage concrete bear s most of t
17、he core shear force while stirrup bears few. At t he time when t he initial crack occur s , t he st ress of t he stirrup at t he crack increase sharply and t he st rain is a2 bout 200 10 - 6 300 10 - 6 . The shear deformation of t he core at t his stage is very small (less than 1 10 - 3 radian ,gene
18、rally between 0. 4 10 - 3 and 0. 8 10 - 3 radian) . (2) Thorough cracking stage Wit h the load increasing following t he initial cracking stage , the second and t hird crossing diago2 nal cracks will appear at t he core. The core is cut into some small rhombus pieces which will become at least one m
19、ain inclined crack across t he core diagonal . The widt h of cracks enlarges obviously , and t he wider ones are generally about 0. 5mm , which is named core t horough cracking stage. The st ress of stirrup increases obviously , and the stirrup in t he middle of t he core is near to yielding or has
20、yiel2 ded. The joint core shows nonlinear property on t he p2 curve , and it enter s elastic2plastic stage. The load at t horough cracking stage is about 80 % 90 % load. (3) Ultimate stage At t his stage , t he widt h of t he cracks is about 1mm or more and some new cracks continue to oc2 cur . The
21、shear deformation at t he core is much larger and concrete begins to collap se. Af ter several cyclic loading , the force reaches the maximum value , which is called ultimate stage. The load increase is due to t he enhancing of the concrete aggregate mechanical f riction between cracks. At t he same
22、 time t he st ress of stirrup increases gradually. On t he one hand stirrup resist s t he horizontal shear , and on t he ot her hand the confinement effect to t he expanding compression concrete st rengthens continuous2 ly , which can also improve t he shear capacity of diagonal compression bar mech
23、anism. (4) Failure stage As the load circulated , concrete in t he core began to collap se , and t he deformation increased sharply , while the capacity began to drop . It was found t hat t he slip of reinforcement in t he beam was very serious in t he experiment . Wit h t he load and it s circulati
24、on time increasing , t he zoon wit hout bond gradually permeated towards t he internal core , enhancing t he burden of t he diagonal compression bar mechanism and accelerates the compression failure of concrete. Fig. 4 shows t he p hotos of typical damaged joint s. A p seudo dynamic test of space mo
25、del of power plant st ruct ure was carried out to research t he working behavior of t he abnormal joint s in re2 al st ructure and the seismic behavior of st ructure. Fig. 5 shows the p hoto of model . The test includes two step s. The fir st is the p seudo dynamic test . At t his step , El2Cent ro
26、wave is inp ut and the peak acceleration varies f rom 50 gal to 1 200 gal . The seismic response is measured. The second is t he p seudo static test . The loading can t stop until t he model fail s. Fig. 7 Minor core The experiment shows t hat t he dist ribution and development of t he crack is infl
27、uenced by t he rest rictive effect of the ort hogonal beam , and t he crack of joint core mainly dist ributes under t he orthogonal beam ( see Fig. 6) , which is different f rom t he result of t he plane joint test , but similar to J 4210. 3. 3 Analysis of test results 3. 3. 1 Mechanical analysis In
28、 t he experiment , t he location of the initial crack of t he exterior joint and the crushed position of concrete both appear in the middle of t he joint core , and t he position is near t he centerline of t he upper col2 umn. The initial crack and crushed position of t he concrete of the interior j
29、oint both appear in t he mi2 nor core ( see Fig. 4 ,Fig. 7) . For interior abnormal joint t he crack doesn t appear or develop in t he ma2 j or core out side of the mi nor core until t horough cracking takes place , while t he crack seldom appears in t he shadow region ( see Fig. 7) as the joint fai
30、l s. Therefore , for abnormal joint , t he shear capacity of t he joint core depends on t he properties of t he mi nor core , namely , on t he st rengt h grades of concrete , t he size and the reinforcement of t he mi nor core , get t he effect of t he maj or core dimension can t be neglected. Mecha
31、nical effect s are t he same will that of t he normal joint , when t he forces t ransfer to t he mi2 nor core t hrough column and beam and reinforcement bar . Therefore , t he working mechanisms of nor2 mal joint , including t russ mechanism , diagonal compression bar mechanism and rest rictive mech
32、anism of stirrup , are also suitable for mi nor core of t he abnormal joint , but their working characteristic is not symmet rical when the load rever ses. Fig. 8 illust rates t he working mechanism of t he abnormal joint . When t he load t ransfer to mi nor core , t he diagonal compression bar area
33、 of mi nor core is bigger t han normal joint core2composed by small column and small beam of abnormal joint , which is due to t he compressive st ress diff usion of concrete compressive region of the beam and column , while at t he same time t he compression carried by the diagonal compression bar b
34、ecomes large. Because t he main part of bond force of column and beam is added to t he diagonal comp ression bar but cont rasting wit h t he increased area of diagonal compression bar , t he increased action is small . The region in the maj or core but out of the mi nor core has less st ress dist ri
35、bution and fewer cracks. The region can confine t he expansion of t he concrete of t he mi nor core diagonal compression bar concrete , which enhances t he concrete compressive st rengt h of mi nor core diagonal compression bar . Making t he mi nor core as st udy element , the area increment of concrete diagonal compression bar
