17.57
17.58
17.59
allows the use of a smaller, less expensive beam than would have to be used if only simply supported spans were employed. Beams are used in sloping planes as roof rafters. If a rafter is supported vertically at both its ends, it does not exert any horizontal thrust (17.57). However, if two rafters are set against each other from opposite walls of the building without a center support, they will act as a simple arch. Each thrusts the other horizontally, and so a tie or buttresses must be provided. In reinforced concrete construction, most or all of the compressive forces are resisted by the concrete, with round steel bars located so that they resist all the tensile forces. Reinforcement against the strong diagonal forces near the ends of the beam is provided by vertical stirrups of steel bars (17.58). For maximum economy, continuous spans are almost always used in concrete building frames. The heaviest concentrations of steel bars alternate between the bottoms and tops of the beams as the direction of bending changes. Wind and earthquake forces can cause the directions of bending in the beams of a building to reverse, so in most cases at least some of the steel reinforcing bars are installed over the full length of the beam at both the top and the bottom (17.59).
194