My reasoning went like this: In the old days, when buildings were made of wood, the beam was the weakest part of the house, so the altar was placed away from it for protection.
我的解釋是：過去的房子是木結構的，樑是最弱的部分，而中國人的觀念是：即使房子榻了，祖先牌位可不能砸，所以牌位忌放屋樑之下。