Civil Engineering Reference
In-Depth Information
durability. Other things being equal,
the lighter the colour the longer the
life. This should not be taken to
extremes, however, since the glare
from a light coloured surface,
especially on a tall building, on a
sunny day can be disabling and a
nuisance; lighter colours may also
suffer more from soiling. A suitable
compromise can often be found in
reflectances in the range of 40-65%.
Colour fastness may be identified as
a problem. The degree to which this
will occur will depend on the material
used, and its susceptibility to, for
example, ultra-violet radiation. This
phenomenon may prove impossible to
control in advance by performance
specification, and needs to be assessed
in conjunction with suppliers. Colour
changes have been especially marked
in the case of some glass-reinforced
polyester (GRP) examples. Specialist
advice should be sought.
opposite. This may be important in a
courtyard or atrium.
Glare or dazzle can occur when
sunlight is reflected from vertical
glazing or shiny cladding (Figure
1.47). For most walls and windows
this problem usually occurs only when
the sun is low in the sky; but some
types of modern design incorporate
sloping glazed or metallic wall
surfaces which can, under certain
circumstances, reflect unwanted high
altitude sunlight into the eyes of
motorists, pedestrians and people in
nearby buildings
(48)
(Figure 1.48).
It is important that the possible
effects of unwanted external solar
reflection are considered at the design
stage; failing that, remedial measures
may be needed. From simple input
data, the times of the year and of each
day at which reflected sunlight might
occur, and their duration, can be
calculated. It is necessary to obtain the
relationship between the angles of
incidence and reflection to derive the
sun positions at which solar dazzle
may be a problem in each particular
case.
For a wall design where solar dazzle
has been identified as a potential
problem, the geometry of the wall may
have to be altered. Initial experience
suggests that, in Europe and the USA
at least, the greatest problems occur
with surfaces facing within 90
Figure 1.47
Low level sun reflected off vertical glazing
Internal reflectivity
The reflectance of the wall surfaces
will affect the amount and distribution
of light in a space. A light coloured
floor will increase the illuminances on
walls and especially the ceiling,
making the room look less gloomy and
reducing glare from overhead light
fittings. In a daylit space the light
levels away from the windows, and
hence the overall balance of light in
the room, will improve if wall
reflectances are increased.
Reflectance values will therefore
always need to be considered when
choosing finishes.
The part played by reflectivity of
the surfaces can be demonstrated by
calculation of daylight factor. Average
daylight factor
DF
is used
as the main criterion of good
daylighting - the ratio of indoor to
outdoor daylight illuminance under
the standard overcast sky. Good
housekeeping also demands that
windows and especially laylights in
ceilings be kept clean. The
transmittance of single glazing will
normally be around 0.85 but can be as
low as 0.6 if the glazing is dirty.
Figure 1.48
Bright sunlight can reflect from polished
metal surfaces; vertical glazing can dazzle
people in adjacent buildings or motorists
of due
south, sloping back at angles between
5
°
External colour
BS 4904
(47)
specifies thirty eight
preferred colours for external
cladding. These are designed to:
●
bring the colours of building
materials and finishes into a
systematic relationship
to the vertical. Where the
surface slopes at more than 40° to the
vertical, solar reflections are likely to
be less of a problem, unless nearby
buildings are very high. It is very
unlikely that surfaces which slope
forward, so that the top of the building
forms an effective overhang, will
cause problems in this respect.
°
and 30
°
combine economy in the number of
colours used with sufficient
flexibility to meet design and
technical requirements. The
standard colours are applicable to
aluminium, fibre-cement, glass,
plastics and steel cladding; and the
colour co-ordination framework
may also be used for other materials
such as concrete, brick, stone and
clay tiles
●
Generally speaking the darker the
colour chosen for a wall, the higher the
temperature will be reached when
exposed to sunlight; this affects
External reflectivity
Light coloured external surfaces can
reflect some extra light to windows
