Civil Engineering Reference
In-Depth Information
Low-cycle fatigue
Low-cycle fatigue occurs when the
structure is subjected, by the
repeated action of severe gust
loading, to relatively few load cycles
close to the ultimate strength of the
structure; that is, for a few tens to a
few thousands cycles. Such high
stresses tend to occur in thin metal
claddings around the fixing points.
There could also be loss of bond
between insulation and its substrate
in composite sheets. Tests can be
specified to check for this.
Owing to the intermittent nature
of storms, low-cycle fatigue occurs in
a few short periods during the
strongest storms, and a satisfactory
inspection of a structure after one
severe storm may not guarantee
survival during the next storm.
Prototypes can be subjected to a
simulation test using a suitable
suction device. From continuous
wind records, BRE has proposed a
test regime which simulates the 50-
year design life of a component
exposed to the wind in the UK. The
pressures applied are given as a
percentage of the full design pressure
at the site in question.
Fluctuations in external surface
temperatures can lead to fatigue of
material at the interface in metal-
skinned sandwich or laminated
panels used for cladding external
walls. This has been known to cause
local delamination which has
adversely affected both appearance
and durability, and BRE tests have
shown that it is possible to predict
the risks of delamination.
Fixing devices and their structural
performance
The most common devices and their
structural performance requirements
are as follows:
●
Lateral support and restraint
Lateral support is often required to
be provided by floors to walls, and
such requirements are set out for
new buildings, for example, in
Approved Document A of the
Building Regulations for England
and Wales, Table 11. All external,
compartment or separating walls
greater than 3 m long will require
lateral support by every floor
forming a junction with the
supported wall, and all internal
loadbearing walls of whatever length
will require support from the floor at
the top of each storey.
Straps may not be needed if the
floor has an adequate bearing on the
wall - for example 90 mm in the
cases both of the bearing of timber
joists and of the bearing of
concrete joists.
BRE site studies have shown that
these requirements have not always
been complied with in practice, and
surveyors should be aware that a
proportion of relatively new
construction will be deficient in this
respect. Information from Housing
Association Property Mutual, for
example, indicates that lateral
restraint strapping was not provided
in 1 in 5 cases where such strapping
should have been provided in
accordance with BS 8103-1. For
wall ties for connecting one leaf of
a cavity wall across the cavity to
another leaf or to a frame
structure, and cladding restraint
ties for connecting cladding
elements to frames. They are
required to resist tension and
compression forces while allowing
limited differential lateral
movement
shear ties for connecting two
adjacent masonry leaves together,
for connecting masonry walls
which need to interact to produce
composite action and for
connecting masonry walls to
frame structures. They are
required to resist shear, tension
and compression forces
●
slip ties for connecting two
adjacent walls or masonry
cladding to frame structures at
movement joints and allowing in-
plane movements. They are
required to resist shear but not
tension and compression forces
●
straps for connecting masonry
walls to other adjacent
components such as floors and
roofs. They are required to resist
tension forces and need to be
blocked to resist compression
forces
●
joist hangers for supporting joists,
beams or rafters on masonry walls
by direct loading via a flange
which is embedded in a mortar
joint. They are required to resist
vertical loads, but occasionally
horizontal loads where lateral
restraint is needed (Figure 1.10)
●
support (shelf) angles for
supporting masonry walls on
other structural elements. They
are required to resist vertical
loads and on some occasions
horizontal loads
●
brackets comprising an individual
support for two adjacent
masonry units, which form part
of a masonry wall, on other
structural elements. These have the
same requirements as
support angles
●
Lateral restraint
straps
Compressive strengths of masonry
materials
It may be important to be able to
obtain some indication of the
compressive strengths of masonry
materials, particularly in older
buildings when change of use is
contemplated. A test has been
developed by BRE for use on mortars
and some masonry units with
strengths up to 7 N/mm
2 (29)
.
Occasionally,
lateral restraint
shoes may be found
Lateral restraint straps
Figure 1.10
Typical provision of lateral restraint for
external walls from floors. The ground floor
of a two storey building is shown
