WHAT IS the purpose of a footing system in structure engineering?

■ the prevention of excessive movement
of building components relative to each
other; and
■ the prevention of unsightly or structurally
damaging cracks in masonry walls.
To some extent, these two criteria place
different requirements on the footing
system. While both will be satisfied by
strong stiff footings, this is not always
practical. The footings alone often do not
have sufficient stiffness and the designer
must either find some means of enhancing
their stiffness, or arrange the walls in such
a way that any movement does not lead to
cracks or excessive differential movement. A
crack differs from a movement joint in that
it is unintentional and its exact location is
often unpredictable. However, not all cracks
significantly diminish the structural integrity
or aesthetics of a building as demonstrated
by the following examples:
■ Reinforced concrete slabs and reinforced
concrete masonry walls crack under load,
but the steel reinforcing bars provide
tensile strength to the cracked sections
and control the width of the cracks once
they have formed.
■ A relatively flexible paint may bridge
small discontinuous cracks in mortar or
masonry units, thus ensuring that these
cracks do not detract aesthetically.
The first task is to define permissible crack
widths in various combinations of masonry
wall and coating type. The second is to
predict what foundation movement can
be tolerated before cracks exceeding those
permissible limits will form.
AS 2870 Table C1 Classification of Damage
with Reference to Walls assesses the degree
of damage associated with cracks of certain
widths. AS 2870 Clause 1.3 Performance
Requirements states:
Buildings supported by footing systems
designed and constructed in accordance with
this Standard on a normal site that is not
subject to abnormal moisture conditions
and maintained such that the original
classification remains valid and abnormal
moisture conditions do not develop are
expected to experience usually no damage, a
low incidence of damage Category 1 and a
occasional incidence of damage Category 2.
The fundamental design questions are:
■ Should the building be designed as a
series of discrete unreinforced masonry
panels which move independently in
sympathy with the sagging or hogging
footings? ie Unreinforced Brickwork; or
■ Should the building be designed as a
rigid reinforced masonry box (or series of
rigid elements) which spans over dishing
or doming foundations? ie Integrated
Reinforced Masonry/Footing Systems.