Thin walled structures so extensively used nowadays in industry and
civil engineering are usually loaded by very complex systems of forces
acting on their edges or over their surfaces. In calculating the
strength of a structure we replace real loads by certain idealized loads
distinguishing between typical surface loads distributed over a great
area of the structure and loads acting over a small area. The latter are
called concentrated loads. When the area under the load is very small in
comparison with the dimensions of the surface of the structure, for
example, when the diameter of the loaded area is smaller than the wall
thickness, the load can be considered as a single force or a mo- ment
acting on the structure at one point only. The real loads which are met
in practice can always be replaced by a combination of components such
as forces normal and tangential to the wall as well as bending and
twisting moments. Knowing the distribution of the stresses in the
structure produced by each component, we can find it under any arbitrary
load using the principle of superposition. There are two main reasons
for the appearance of the concentration of stresses in the structure. It
can be produced by notches, rapid changes of the cross-section, holes,
cutouts, etc. on one hand and by concentrated loads resulting from the
interaction of the elements of the structure on the other.