Civil Engineering Reference
In-Depth Information
For this design, the uniformly distributed load will be idealised as a series
of concentrated loads spaced at 600 mm centres along the length of the
beam. Similarly, the initial design will consider stirrups at a hypothetical
spacing of 600 mm coinciding with the concentrated loads. When the area of
stirrups required per 600 mm segment has been calculated, it will be
provided by stirrups appropriately spaced throughout the segment.
After two or three iterations, the truss shown in Figure 4.8 was
developed. Through the iterations, the slopes and widths of the struts, and
the size and location of nodal zones were adjusted to ensure that equilibrium
is maintained without overstressing the concrete.
Figure 4.8
Truss selected in design example
There is no unique 'correct' final design. Any one of several trusses will
prove satisfactory provided that the detailing of the structure allows the truss
to carry the loads in the manner assumed. For example, the left shear span
designs could have varied from having all of the shear carried by concrete
strut, through to having all of the shear carried by stirrups. The stirrup
reinforcement for the left shear span was selected on the basis of having 30-
35% of the shear carried by stirrups. This reduces the size of the direct
compression strut and improves ductility. The stirrups in the right shear span
were selected so that all of the shear across section 1-1 is carried by
stirrups. Stirrups loaded by struts steeper than 65° were ignored as these
stirrups are not likely to reach yield before beam failure.
Longitudinal flexural reinforcement requirements at mid span and at the
right support were determined from the moments at these locations assuming
that the steel yields at both locations.
Figure 4.9
illustrates the calculation of
force in the top chord. At support U, the bar force is 6565 kN. At joint T, the
vertical applied load is equilibrated by a steep inclined strut T-UV (Figure 4.8)
Horizontal equilibrium at the joint shows that the top chord force drops to
