無梁拱,拱(gong)(gong)形(xing)屋面還具有承(cheng)壓(ya)(ya)效果，拱(gong)(gong)所承(cheng)受的荷(he)載不同，其壓(ya)(ya)力曲線的線形(xing)也不相(xiang)同，一般按恒載下壓(ya)(ya)力曲線確定;在(zai)活載作用(yong)下，拱(gong)(gong)內力可能產生彎(wan)矩，這時鉸(jiao)的設(she)置(zhi)就會影響(xiang)拱(gong)(gong)內彎(wan)矩的分(fen)布(bu)狀況。與剛架相(xiang)仿，只(zhi)有地基(ji)良(liang)好或兩側拱(gong)(gong)肢處有穩(wen)定邊跨(kua)結(jie)構時才采(cai)用(yong)無(wu)鉸(jiao)拱(gong)(gong)，這種拱(gong)(gong)很少(shao)用(yong)于房屋建筑。雙鉸(jiao)拱(gong)(gong)應用(yong)較(jiao)多(duo)，為適應軟(ruan)弱地基(ji)上支座(zuo)沉降差及拱(gong)(gong)拉桿變形(xing)，采(cai)用(yong)靜定結(jie)構的三鉸(jiao)拱(gong)(gong)。
拱以曲桿抗衡(heng)并傳遞外力(li)(li)給(gei)支座，故鉸支座不(bu)僅(jin)承受(shou)豎向力(li)(li)，并有(you)相當大的(de)水(shui)平向外的(de)拱腳推(tui)力(li)(li)，其(qi)(qi)合力(li)(li)就位(wei)于拱軸曲線在(zai)支座點的(de)切線方向上。拱腳有(you)推(tui)力(li)(li)是其(qi)(qi)主要(yao)力(li)(li)學特征(zheng)之(zhi)一(yi)，矢高(gao)f越小(xiao)，推(tui)力(li)(li)越大。一(yi)次(ci)超靜(jing)定(ding)的(de)雙鉸拱，金屬屋面支座的(de)垂直或水(shui)平位(wei)移均會引起內力(li)(li)變化，對支座在(zai)推(tui)力(li)(li)作用(yong)下無變位(wei)的(de)要(yao)求就更嚴格(ge)。由此可見，為(wei)了使拱保(bao)持(chi)正常工作，務必確保(bao)其(qi)(qi)支座能承受(shou)住推(tui)力(li)(li)而不(bu)位(wei)移，故拱腳推(tui)力(li)(li)的(de)結構處理，是拱結構設(she)計(ji)的(de)中(zhong)心(xin)問題。
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