Continuous beams and slabs, and beams in frames - Composite Structures of Steel and Concrete - page 156

of holes in webs of continuous beams is easiest where the ratio q / g is low
[32], and a low q / g is also the situation where the advantages of continu-
ity over simple spans are greatest.
Continuity is more advantageous in beams with three or more spans
than where there are only two; and end spans should ideally be shorter
than interior spans. The least benefit is probably obtained where there are
two equal spans. The example in Section 4.6 illustrates this. Using a steel
section that could span 9.0 m simply-supported, it is quite difficult to use
the same cross-section for two continuous spans of 9.5 m.
A decision with many consequences is the Class of the composite
section at internal supports. Two distinct strategies are now compared.

(1) Minimal top longitudinal reinforcement is provided in the slab. If the

composite beam is in Class 1, rigid-plastic global analysis can be used,
unless l LT 0.4 (Section 4.2.4). If the beam is in Class 2, the hogging

bending moment will be redistributed as much as permitted, to enable
good use to be made of the available bending resistance at mid-span.

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(2) The reinforcement in the slab at internal supports is heavier, with an

effective area at least 1% of that of the slab. The composite section
will certainly be in Class 2, perhaps Class 3. Restrictions on redistribu-
tion of moments will probably cause the design hogging moments,
M ′ Ed , to increase (cf. case (1)) faster than the increase in resistance,

M ′ Rd , provided by the reinforcement, and further increase in the latter

may put the section into Class 4. So the steel section may have to be
heavier than for case (1), and there will be more unused bending
resistance at mid-span. However, that will allow a lower degree of
shear connection to be used. With higher M Ed the bending-moment

diagram for lateral-torsional buckling is more adverse. Deflections
are less likely to be troublesome, but the increase in the diameter of
the reinforcing bars makes crack-width control more difficult.

The method of fire protection to be used may have consequences for
the structural design. For example, web encasement improves the class of
a steel web that is otherwise in Class 3, but not if it is in Class 2; and it
improves resistance to lateral buckling.
Finally, it has to be decided whether construction will be propped or
unpropped. Propped construction allows a shallower steel beam to be used
- but it will be less stiff, so the dynamic behaviour may be less satisfactory.
Propped construction costs more, and crack-width control is more difficult;
but design is much less likely to be governed by excessive deflection.
The design presented next is based on strategy (1) above, using a
lightweight-concrete slab and an encased web. This is done to illustrate
methods. It may not be the best solution.