Civil Engineering Reference
In-Depth Information
4.4
Stresses and deflections in continuous beams
Values of bending stresses at serviceability limit states may be needed
in calculations for control of load-induced cracking of concrete (Section
4.2.5.3), and for prediction of deflections where unpropped construction
is used. Bending moments are determined by elastic global analysis
(Section 4.3.2). Those at internal supports are then modified to allow for
cracking and yielding (Section 4.3.2.3). Stresses are found as in Section
3.5.3 for sagging moments, or Section 4.2.1 for hogging moments.
Deflections are much less likely to be excessive in continuous beams than
in simply-supported spans, but they should always be checked where design
for ultimate limit states is based on rigid-plastic global analysis. For simply-
supported beams, the increase in deflection due to the use of partial shear
connection can be neglected in certain circumstances (Section 3.7.1) and
can be estimated from Equation 3.94. These same rules can be used for
continuous beams, where they are a little conservative because partial
shear connection is used only in regions of sagging bending moment.
The influence of shrinkage of concrete on deflections is treated in
Section 3.8. For continuous beams, the method of calculation is rather
complex, because shrinkage causes bending moments as well as sagging
curvature; but its influence on deflection is much reduced by continuity.
4.5
Design strategies for continuous beams
Until experience has been gained, the design of a continuous beam may
involve much trial and error. There is no ideal sequence in which decisions
should be made, but the following comments on this subject may be useful.
It is assumed that the span and spacing of the beams is known, that the
floor or roof slab spanning between them has been designed, and that
most or all of the loading on the beams is uniformly-distributed, being
either permanent (
g
) or variable (
q
). The beams add little to the total load,
so
g
and
q
are known.
One would not be designing a continuous beam if simply-supported
spans were satisfactory, so it can be assumed that simple spans of the
maximum available depth are too weak, or deflect or vibrate too much; or
that continuity is needed for seismic design, or to avoid wide cracks in the
slab, or for some other reason.
The provision for services must be considered early. Will the pipes and
ducts run under the beams, through the holes in the webs, or above the
slab? Heavily-serviced buildings needing special solutions (castella beams,
stub girders, haunched beams, etc.) are not considered here. The provision
