Civil Engineering Reference
In-Depth Information
Chapter
3
/
Normal Force, Shear
Force, Bending Moment
and Torsion
The purpose of a structure is to support the loads for which it has been designed. To
accomplish this it must be able to transmit a load from one point to another, i.e. from
the loading point to the supports. In Fig. 2.21, for example, the beam transmits the
effects of the loads at B and C to the built-in end A. It achieves this by developing an
internal force
system and it is the distribution of these internal forces which must be
determined before corresponding stress distributions and displacements can be found.
Aknowledge of stress is essential in structural designwhere the cross-sectional area of a
member must be such that stresses do not exceed values that would cause breakdown in
the crystalline structure of thematerial of themember; in other words, a structural fail-
ure. In addition to stresses, strains, and thereby displacements, must be calculated to
ensure that as well as strength a structural member possesses sufficient stiffness to pre-
vent excessive distortions damaging surrounding portions of the complete structure.
In this chapter we shall examine the different types of load to which a struc-
tural member may be subjected and then determine corresponding internal force
distributions.
3.1 T
YPES OF
L
OAD
Structural members may be subjected to complex loading systems apparently com-
prised of several different types of load. However, nomatter how complex such systems
appear to be, they consist of a maximum of four basic load types: axial loads, shear
loads, bending moments and torsion.
AXIAL LOAD
Axial loads are applied along the longitudinal or centroidal axis of a structural mem-
ber. If the action of the load is to increase the length of the member, the member is
said to be in
tension
(Fig. 3.1(a)) and the applied load is
tensile
. A load that tends to
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