Civil Engineering Reference
In-Depth Information
Figure 3.27
Arching action in beams (Note: compression members are shaded)
3.12 Continuity between the concepts of bending and arching action
3.12.1 General
When a simply supported beam carries a series of loads it is subjected to bending
moments; the bottom fi bre extends as it is stressed in tension, and the compressed top
fi bre shortens, causing the beam to defl ect and the beam ends to rotate, Figure 3.27 (a).
The loads are carried to the supports by shear forces and bending moments, which
may be described by the truss analogy, Figure 3.27 (b).
If the extension of the bottom fi bre is impeded by external restraints, the loads are
carried to the supports in a different manner. Figure 3.27 (c) shows how a segmental
arch may be drawn within the depth of a stocky beam. As long as the restraints to the
thrust of the arch are suffi ciently rigid, it will carry all of the loads applied to the beam
and no bottom fi bre tensile reinforcement will be required, other than nominal anti-
crack steel. For more slender beams and for less than perfect restraint, the load will be
shared between arching and bending actions. In general, if it is possible to draw within
