Engineering and Construction

Where heavy timber beams join masonry or concrete walls, three problems must be solved: First, the beam  must be protected from decay caused by moisture seeping through the wall.  This is done by leaving a ventilating  airspace of at least ½ inch (13 mm) between the masonry and all sides of the beam except the bottom, unless the beam is chemically treated  to resist decay. The second and third  problems have to be solved together:  The beam must be securely anchored  to the wall so that it cannot pull away under normal service, yet it must be able to rotate freely so that it does not pry the wall apart if it burns through and collapses during a severe fire (Figure 4.12). Traditional and more contemporary methods of accomplishing these dual needs are shown  in Figures 4.11 and 4.13, respectively.

Figure 4.11 Two alternative details for the bearing of a beam on masonry in traditional Mill construction. In each case, the beam end is fi recut to allow it to rotate out of the wall if it burns through (Figure 4.12), but it is also anchored against pulling away from the wall by means of either lag screws or a lug on the iron bearing plate.

Figure 4.12 A timber beam that burns through in a  prolonged fi re is likely to topple its sup- porting masonry wall (left) unless its end  is fi recut (right). In this illustration, the beam is anchored to the wall in each case  by a steel strap anchor.

Figure 4.13 An example of contemporary heavy timber construction, based on steel plate connectors and an insulated cavity wall.