Civil Engineering Reference
In-Depth Information
design of the joints, and in the discontinuity in the bending-moment
diagram at a supporting internal column, caused by the flexural stiffness
of the column.
For a given floor slab and design load per unit length of beam, the
advantages of continuous beams over simple spans are:
•
higher span/depth ratios can be used, for given limits to deflections;
•
cracking of the top surface of a floor slab near internal columns can be
controlled, so that the use of brittle finishes (e.g., terrazzo) is feasible;
•
the floor structure has a higher fundamental frequency of vibration,
and so is less susceptible to vibration caused by movements of people;
•
the structure is more robust (e.g., in resisting the effects of fire or
explosion).
The principal disadvantage is that design is more complex. Actions
on one span cause action effects in adjacent spans. Even where the steel
section is uniform, the stiffness and bending resistance of a composite
beam vary along its length, because of cracking of concrete, changes in
effective width, and variation in longitudinal reinforcement in the con-
crete flange.
It is not possible to predict accurately the stresses or deflections in a
continuous beam, for a given set of actions. Apart from the variation over
time caused by the shrinkage and creep of concrete, there are the effects
of cracking of concrete. In reinforced concrete beams, these occur at cross-
sections of both sagging and hogging bending, and so have little influence
on distributions of bending moment. In composite beams, significant
tension in concrete occurs only in hogging regions. It is influenced by
the sequence of construction of the slab, the method of propping used
(if any), and by effects of temperature, shrinkage and longitudinal slip.
The flexural stiffness (
EI
) of a fully cracked composite section can be
as low as a quarter of the 'uncracked' value, so a wide variation in flexural
stiffness can occur along a continuous beam of uniform section. This
leads to uncertainty in the distribution of longitudinal moments, and hence
in the amount of cracking to be expected. The response to a particular set
of actions also depends on whether it precedes or follows another set of
actions that causes cracking in a different part of the beam.
For these reasons, and also for economy, design is based as far as
possible on predictions of ultimate strength (which can be checked by
testing) rather than on analyses based on elastic theory. Methods have
been developed from simplified models of behaviour. The limits set to
the scope of some models may seem arbitrary, as they correspond to the
range of available research data, rather than to known limitations of the
model.
