Civil Engineering Reference
In-Depth Information
F
IGURE
1.22
Examples of
cold-formed sections
steel
is stronger than mild steel but has a similar stiffness. High yield steel, as well as
mild steel, is used for reinforcing bars in concrete construction and very high strength
steel is used for the wires in prestressed concrete beams.
Low carbon steels possessing sufficient ductility to be bent cold are used in the manu-
facture of
cold-formed
sections. In this process unheated thin steel strip passes through
a series of rolls which gradually bend it into the required section contour. Simple pro-
files, such as a channel section, may be produced in as few as six stages whereas more
complex sectionsmay require 15 ormore. Cold-formed sections are used as lightweight
roof purlins, stiffeners for the covers and sides of box beams and so on. Some typical
sections are shown in Fig. 1.22.
Other special purpose steels are produced by adding different elements. For example,
chromium is added to produce stainless steel although this is too expensive for general
structural use.
CONCRETE
Concrete is produced by mixing cement, the commonest type being
ordinary Portland
cement
, fine aggregate (sand), coarse aggregate (gravel, chippings) with water. A
typical mix would have the ratio of cement/sand/coarse aggregate to be 1:2:4butthis
can be varied depending on the required strength.
The tensile strength of concrete is roughly only 10% of its compressive strength and
therefore, as we have already noted, requires reinforcing in its weak tension zones and
sometimes in its compression zones.
TIMBER
Timber falls into two categories,
hardwoods
and
softwoods
. Included in hardwoods are
oak, beech, ash, mahogany, teak, etc. while softwoods come from coniferous trees,
such as spruce, pine and Douglas fir. Hardwoods generally possess a short grain and
are not necessarily hard. For example, balsa is classed as a hardwood because of its
short grain but is very soft. On the other hand some of the long-grained softwoods,
such as pitch pine, are relatively hard.
Timber is a
naturally
produced material and its properties can vary widely due to vary-
ing quality and significant defects. It has, though, been in use as a structural material
