The doorway and windows are made possible by placing over the open spaces thick stone beams that support the weight from above.(中文：可在空地用石条搭建门窗,石条可以承受门窗上面的重量.）
1.这句话是被动语态.
2.句子后半部分placing over open spaces thick stone beams 从语法上讲可改写为placing thick stone beams over open spaces.比如：put+名词+over=put+over+名词,但若这里的名词是代词如：it 的话,语法规定则只能发在中间,即put it over.
3.而原文那样的语序,也是有道理的.因为该句后面有一个定语从句（that support the weight from above),因此为了避免先行词混淆,故把thick stone beams 放在over后面,以便紧跟从句,避免混淆.
仅供参考!