拱板設(she)備之(zhi)拱形(xing)承壓(ya)的原理
拱(gong)(gong)(gong)形(xing)所承(cheng)受(shou)的荷載(zai)不同，其壓力(li)曲(qu)(qu)線(xian)的線(xian)形(xing)也不相(xiang)(xiang)同，一般按(an)恒載(zai)下壓力(li)曲(qu)(qu)線(xian)確定;在活載(zai)作用下，拱(gong)(gong)(gong)內力(li)可能(neng)產(chan)生彎矩，這時鉸(jiao)的設置就會(hui)影(ying)響拱(gong)(gong)(gong)內彎矩的分布(bu)狀況(kuang)。與剛架相(xiang)(xiang)仿(fang)，只有地基良好或兩側拱(gong)(gong)(gong)肢(zhi)處有穩定邊(bian)跨結構(gou)時才采(cai)用無(wu)鉸(jiao)拱(gong)(gong)(gong)，這種拱(gong)(gong)(gong)很少用于房屋建筑。雙鉸(jiao)拱(gong)(gong)(gong)應用較多，為(wei)適應軟(ruan)弱地基上支座(zuo)沉降差及拱(gong)(gong)(gong)拉(la)桿變(bian)形(xing)，很好采(cai)用靜(jing)定結構(gou)的三(san)鉸(jiao)拱(gong)(gong)(gong)。
拱以曲(qu)桿抗(kang)衡并傳遞外(wai)力(li)(li)(li)(li)給支座，故鉸支座不僅承受豎(shu)向力(li)(li)(li)(li)，并有相當大(da)的水(shui)平(ping)向外(wai)的拱腳推(tui)(tui)力(li)(li)(li)(li)，其合力(li)(li)(li)(li)就(jiu)位(wei)于拱軸曲(qu)線在(zai)(zai)支座點(dian)的切線方向上。拱腳有推(tui)(tui)力(li)(li)(li)(li)是其主(zhu)要力(li)(li)(li)(li)學特征(zheng)之一，矢(shi)高f越小，推(tui)(tui)力(li)(li)(li)(li)越大(da)。一次(ci)超靜(jing)定的雙鉸拱，支座的垂直(zhi)或水(shui)平(ping)位(wei)移均(jun)會引起內力(li)(li)(li)(li)變(bian)化，對支座在(zai)(zai)推(tui)(tui)力(li)(li)(li)(li)作用下無(wu)變(bian)位(wei)的要求就(jiu)更(geng)嚴格。由此可見，為了使拱保持正常工(gong)作，務必(bi)確保其支座能承受住(zhu)推(tui)(tui)力(li)(li)(li)(li)而不位(wei)移，故拱腳推(tui)(tui)力(li)(li)(li)(li)的結構處理，是拱結構設計的中心(xin)問題。
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