【正文】 sis of Meinheit and J irsa , et [5 ] . The second factor is horizontal stirrup . Horizontal stirrup has no effect on t he initial cracking shear of abnormal joint , while greatly improves t he t horough cracking shear . Af ter crack appeared , t he stirrup begins to resist t he shear and confines t he expansion of concrete[ 6 ] . This experiment shows t hat t he st ress of stirrup s in each layer is not equal . When the joint fail s , t he stirrup s don’ t yield simultaneous. Fig. 10 shows t he change of st ress dist ribution of stirrup s along core height wit h t he load increasing. Through analyzing test result s , it can be known t hat 80 percent of the height at the joint core can yield. The last factor is the change of sec2 tion size of t he beam and column. The section change decreases t he initial crack2 ing load about 30 p resent of abnormal joint and makes t he initial crack appear at t he position of joint mi nor core. The rea2 son for t his p henomenon is t hat small up2 per column section makes t he confinement of mi nor core concrete decrease and t he edge shear increase. But t he section change has lit tle effect on thorough cracking load. Af ter t horough cracking , the joint enter s ultimate state while the external load can’ t increase too much , which is dif2 ferent f rom t he behavior of abnormal joint t hat can carry much shear af ter thorough cracking. 3. 5 Shear force formula of abnormal joint As a part of f rame , t he design of joint shall meet t he requirement s of the f rame st ruct ure design , namely , t he joint design should not damage t he basic performance of t he st ruct ure. According to the principle of st ronger j oi nt , it is necessary for joint to have some safety reserva2 tion. The raised cost for conservational estimation of t he joint bearing capacity is small . But t he con2 servational estimation is very important to t he safety of the f rame st ruct ure. At t horough cracking stage , t he widt h of most cracks is more t han 0. 2 mm , which is bigger than t he suggested limit value in t he concrete design code. Big cracks will influence t he durability of st ruct ure. Hence , the bearing capacity at t horough cracking stage is applied to calculating t he bearing capacity of joint . According to t he analysis of t he working mechanisms of abnormal joint , it could be concluded t hat t he bearing capacity of joint core mainly depends on mi nor core when t he force t ransferred f rom maj or core to mi nor core. All kinds of working mechanisms are suitable to mi nor core element . Thus , a formula for calculating t he shear capacity of abnormal joint can be obtained based on Eq. 1. According to the above analysis of influential factor s of shear capacity of abnormal joint , and ref2 erence to Eq. 1 , a formula for calculating t he shear capacity of reinforced concrete f rame abnormal joint is suggested as follows V j = 0. 1η jξ 1 f c bj hj + 0. 1η j nξ 2 f c bj hj +ξ 3 f yv Asvjh0 a′ ss (2) Where h0 = effective dept h of small beam section in abnormal joint 。 N = design value of axial pression at t he bot tom of upper column wit h considering the bi2 nation of the eart hquake action , When N 015 f c bc hc , let N = 0. 5 f c bc hc 。 η j = influential coefficient of t he orthogonal beam to the column 。 minor core 。 seismic behavior 。 f t = design value of concrete tensile st rength 。 bc = widt h of t he column section 。ξ 1 = influential coefficient consider2 ing mi nor core on working as cont rol element for calculating 。在分析鋼筋混凝土框架異型節(jié)點(diǎn)受力的基礎(chǔ)上，提出了由小梁小柱決定的“小核芯”概念，得出框架異型節(jié)點(diǎn)的承載能力取決于異型節(jié)點(diǎn)“小核芯”的結(jié)論。本文通過對(duì) 8個(gè)鋼筋混凝土異型節(jié)點(diǎn)的試驗(yàn)研究，分析了這類異型節(jié) 點(diǎn)不同于常規(guī)節(jié)點(diǎn)的受力特點(diǎn)，建立了這類異型節(jié)點(diǎn)的抗剪承載力計(jì)算公式，以期對(duì)實(shí) 際工程設(shè)計(jì)有所幫助。試件屈服前，采用荷載控制分級(jí)加載，屈服后采用位移控制加載直至試件破壞。 試件破壞歷程 本次試驗(yàn)的 8個(gè)節(jié)點(diǎn)雖然在尺寸、配筋、軸力、各階段施加的外荷載上有所不同，但其破壞的歷程和形態(tài)卻都經(jīng)歷了初裂、通裂、極限和破壞四個(gè)階段。初裂一旦發(fā)生，裂縫處箍筋應(yīng)力會(huì)有較大增加，箍筋應(yīng)變大約為 200*10)。裂縫寬度明顯加寬， 較寬者一般在 左右，此時(shí)稱為核芯區(qū)的通裂階段。 （ 3）極限階段 通裂后，荷載繼續(xù)增加，核芯區(qū)裂縫在原有裂縫的基礎(chǔ)上明顯加寬，可達(dá)到 1mm甚至更寬，但也有少量新裂縫出現(xiàn)，核芯區(qū)剪切變形要比初裂時(shí)增大很多，混凝土開始剝落，經(jīng)過幾次加載環(huán)后，試件承載能力達(dá)到最大值，稱為極限 狀態(tài)。試驗(yàn)中發(fā)現(xiàn)梁筋滑移現(xiàn)象十分嚴(yán)重，隨荷載增加和循環(huán)次數(shù)增加，粘結(jié)力的喪失區(qū)段逐漸向節(jié)點(diǎn)核芯區(qū)內(nèi)部滲透，造成斜壓桿機(jī)構(gòu)的負(fù)擔(dān)加重，進(jìn)一步加快了混凝土的被壓壞。所以對(duì)異型節(jié)點(diǎn)而言，節(jié)點(diǎn)核芯區(qū)的受剪承載能力大小取決于“小核芯”區(qū)，即取決于“小核芯”的混凝土強(qiáng)度等級(jí)、尺寸和配筋狀況，但是大核芯的大小對(duì)其有影響。 （ 3）從圖 6 可以看出，將大核芯所受的作用力向小核芯傳遞，就可以得出“小核芯”的受力狀況，因?yàn)樽饔昧π再|(zhì)與常規(guī)節(jié)點(diǎn)是相同的，所以各種機(jī)理（桁架機(jī)理、斜壓桿機(jī)理、箍筋的約束機(jī)理）對(duì)“小核芯”顯然仍然適用。顯然這個(gè)比值越大，約束作用就越強(qiáng)，其承載能力就越強(qiáng)。 產(chǎn)生這種情況的原因是軸壓力的存在能增加柱的受壓區(qū)面積 C 9 D，進(jìn)而提高斜壓桿的寬度，使節(jié)點(diǎn)核芯區(qū)混凝土受壓的范圍加大，即參與斜壓桿機(jī)構(gòu)的混凝土面積增大，同時(shí)梁筋傳遞給節(jié)點(diǎn)核芯混凝土的邊緣剪力中會(huì)有更多的部分匯入斜壓桿機(jī)構(gòu)，那么造成 節(jié)點(diǎn)核芯混凝土開裂的那部分邊緣剪力就會(huì)減小，所以軸力對(duì)提高節(jié)點(diǎn)核芯區(qū)的初裂是有利的。所以，在目前異型節(jié)點(diǎn)的試驗(yàn)數(shù)據(jù)較少的情況下，考慮到規(guī)范公式的連續(xù)性，依 據(jù)本次試驗(yàn)結(jié)果，并參考國(guó)內(nèi)外學(xué)者 MEINHEIT和 JIRSA 等人的試驗(yàn)研究和統(tǒng)計(jì)分析，在建立異型節(jié)點(diǎn)承載能力公式時(shí)，對(duì)與軸壓力有關(guān)的項(xiàng)折減一半，即取為。 箍筋沿節(jié)點(diǎn)核芯高度方向的應(yīng)變隨加載過程分布 如圖 8 所示（節(jié)點(diǎn) 35）。 我國(guó)規(guī)范在節(jié)點(diǎn)抗剪承載能力公式中箍筋所提供的抗剪能力項(xiàng)的系數(shù)為 1，未考慮箍筋應(yīng)力的非均勻性，即認(rèn)為各層箍筋都能全部屈服并發(fā)揮作用。上下柱截面不一樣對(duì)節(jié)點(diǎn)核芯的通裂影響不大，但是對(duì)節(jié)點(diǎn) 1節(jié)點(diǎn) 1節(jié)點(diǎn) 13 而言，通裂后外荷載增加的幅度不大就達(dá)到了極限狀態(tài)。從節(jié)點(diǎn)核芯產(chǎn)生破壞的試件節(jié)點(diǎn) 3節(jié)點(diǎn) 37 來看，同樣在節(jié)點(diǎn)核芯通裂后，節(jié)點(diǎn)承載能力即刻就達(dá)到了極限，外荷載的增長(zhǎng)幅度不大。而當(dāng)梁端和節(jié)點(diǎn)核芯裂縫出現(xiàn)過多后，曲線則有一明顯拐點(diǎn)，可以認(rèn)為試件達(dá)到了屈服。但應(yīng)注意這次試件設(shè)計(jì)是以節(jié)點(diǎn)核芯破壞 為原則進(jìn)行的，梁的配筋、柱的配筋與實(shí)際工程相比偏 小，若梁柱配筋比較大，則節(jié)點(diǎn)破壞要先于梁、柱破壞，那么滯回曲線反映的更多的是節(jié)點(diǎn)核芯的延性性能，因?yàn)楣?jié)點(diǎn)核芯的剛度是比較大的。節(jié)點(diǎn)在達(dá)到極限階段之前，節(jié)點(diǎn)核芯在多次荷載循環(huán)后剛度僅有微小的下降和少量不可恢復(fù)的殘余變形。經(jīng)過本次試驗(yàn)研究 9 本文采用與《建筑抗震設(shè)計(jì)規(guī)范》 amp。節(jié)點(diǎn)試驗(yàn)中的荷載 : 位移滯回曲線是綜合反映“子結(jié)構(gòu)”性能 的曲線。鋼筋混凝土框架節(jié)點(diǎn)抗震 [ M ]。 [ 3]框架節(jié)點(diǎn)專題研究 組?？紤]軸壓比影響的鋼筋混凝土框架內(nèi)節(jié)點(diǎn)抗震性能試驗(yàn)研究 [ J]。 。 [ 6]MEINHEIT D F,JIRSA J strength of R\C beamcolumn connections[ J].ACI Structural Journal, ,1993:6171. 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