Civil Engineering Reference
In-Depth Information
Chapter
12
/
Composite Beams
Frequently in civil engineering construction beams are fabricated from comparatively
inexpensive materials of low strength which are reinforced by small amounts of high-
strength material, such as steel. In this way a timber beam of rectangular section
may have steel plates bolted to its sides or to its top and bottom surfaces. Again,
concrete beams are reinforced in their weak tension zones and also, if necessary, in
their compression zones, by steel-reinforcing bars. Other instances arise where steel
beams support concrete floor slabs inwhich the strength of the concretemay be allowed
for in the design of the beams. The design of reinforced concrete beams, and concrete
and steel beams is covered by Codes of Practice and relies, as in the case of steel
beams, on ultimate load analysis. The design of steel-reinforced timber beams is not
covered by a code, and we shall therefore limit the analysis of this type of beam to an
elastic approach.
12.1 S
TEEL-REINFORCED
T
IMBER
B
EAMS
The timber joist of breadth
b
and depth
d
shown in Fig. 12.1 is reinforced by two steel
plates bolted to its sides, each plate being of thickness
t
and depth
d
. Let us suppose
that the beam is bent to a radius
R
at this section by a positive bending moment,
M
.
Clearly, since the steel plates are firmly attached to the sides of the timber joist, both
are bent to the same radius,
R
. Then, fromEq. (9.7), the bending moment,
M
t
, carried
y
G
z
d
t
t
b
F
IGURE
12.1
Steel-reinforced timber beam
300
