Civil Engineering Reference
In-Depth Information
Dimensional stability
Chapter 1.2
This chapter covers the movements
of materials caused by changes in
their temperature and moisture
levels, and also to some extent with
inherent changes in dimension
caused by continuous loads - creep,
for example - or the absence of
support - movements from the
ground, for example. Tables of
movements for common materials
are given at the end of this chapter.
expansion or contraction or both.
So far as walls are concerned, it is
the larger components which need
most consideration, especially
where the building is only
intermittently heated. However, even
small dwellings made from large
components can suffer
(Figure 1.16).
Linear size changes due to thermal
movements are calculated using the
expression:
Moisture movement is mainly a
property of porous materials. Thus
aggregate concretes, autoclaved
aerated concrete (AAC) and calcium
silicate units have reversible
movements in the range 0.02-0.2%;
softwoods sawn tangentially across
the grain in the range 0.6-2.6%, and
sawn radially in the range 0.45-2.0%.
Concretes, including AAC, also
have an irreversible drying or
carbonation shrinkage in the range
0.02-0.1%, calcium silicate units in
the range 0.02-0.06%, and fired clay
has an irreversible volume expansion
in the range 0.02-0.1%. This
expansion is rapid at first and slows
down with time, but can continue for
30 years or more. In most real
situations there is some restraint,
though rarely complete restraint,
afforded to materials undergoing
these movements.
Linear size changes due to
moisture movements are calculated
using the expression:
Movements of the fabric
Expansion and contraction of any
part of the building fabric subjected
to variations of moisture content and
temperature will have the potential
to cause problems if not
accommodated in the design. As a
general rule, all common building
materials will be subject to thermal
R
=
α
Lt
where
R
= change of size,
α = coefficient of linear size change,
L
= length of dimension, and
t
= temperature difference.
Values of
are to be found in
Table 1.1 at the end of the chapter.
α
R
= factor
dimension
100
×
Where two differing materials are
joined, differential movement can
occur, which usually exacerbates the
problem. The best practice is to try
to accommodate movements at the
smallest and most elemental level,
provided this does not prejudice the
structural function. Thus, provided
wall components are simply shaped,
not too thick, not too large, have
movement-tolerant fixings and have
joints at their periphery which
accommodate movement, the
movement strains and the
corresponding loads (stresses) are
Figure 1.16
A REEMA large concrete panel dwelling showing signs of movements which are disrupting
the joints
