Civil Engineering Reference
In-Depth Information
If the slab has been cast into frogs in the brickwork,
horizontal displacement may show one course below the
bearing
Stress in members
When a potential change of size is
fully restrained, the induced stress
and force can be calculated using the
expressions:
Movement joints
Walls will need to be provided with
movement joints which also reflect
the positions of movement joints in
the remainder of the structure; these
joints will need to continue through
the wall finishes, and normally will
need also to provide continuity in
other aspects of the performance of
the wall such as its fire resistance
(Figure 1.18). If not provided, it is
not inevitable that cracking will
occur, but it is likely. Problems which
arise in this connection are dealt with
in later chapters appropriate to
various structures and finishes.
Stress =
E
×
strain
Force = stress
×
area
For thermal movements the
following expressions apply:
Strain =
α
t
Stress =
α
tE
Force =
α
tEA
Movements from the ground
The characteristics and performance
of foundations will be dealt with in
another topic in this series.
However, a brief note is included
here on the behaviour of
foundations as it affects the walls
above. It should be appreciated that
in most cases it is uneconomic and
unnecessary to design the
foundations to reduce the
transmitted movements to zero,
since most buildings can tolerate a
small amount of movement.
Although there are no definitive
criteria for the majority of buildings,
Further information will be found
in BRE Digest 228.
Vibrations
Ground-born vibration sometimes
affects buildings, though there is
little evidence that they produce
even cosmetic damage, such as
small cracks in plaster. Guidance on
the measurement of vibrations is
given in BRE Digest 403
(33)
and on
human exposure to vibrations in
BS 6472
(34)
.
Figure 1.17
Horizontal displacement of the top course
of a wall carrying a slab
generally small and can be borne by
the element. With large structures, if
one or more of these requirements
are not met, stresses may accumulate
over large areas and damage result.
In small structures such as detached
domestic houses, it is often possible
to omit explicit movement design
and depend on restraint from other
elements to accommodate
movement loads.
Progressive movement may
conceivably occur at the edges of
floors at the junction with external
walls, mainly as a result of ratcheting
where movement resulting from
expansion (usually thermal) is not
fully compensated by contraction.
Detritus then builds up, progressive
movement occurs, and the material
in a wall cracks. However, the crack
may also show at the head of the wall
just underneath the bearing of the
floor, depending on the detail
(Figure 1.17). Design data on
movements of the structure is given
in BRE Digests 227-229
(32)
.
Movement joints
If concrete block:
not more than 6 m
without movement
joints
If calcium silicate: not
more than 7.5 m without
movement joints
If fired clay: not
more than 12 m
without movement
joints
Figure 1.18
Recommendations for the provision of movement joints in walling
