Civil Engineering Reference
In-Depth Information
British code CP 110, Beeby [34] identified nine reasons why deflections
of reinforced concrete beams in service were usually less than those cal-
culated by the designers, and increased his theoretical span/depth ratios
by 36% to allow for them. Many of the reasons apply equally to compos-
ite beams, the most significant of them being the variations in the elastic-
ity, shrinkage and creep properties of the concrete, the stiffening effect of
finishes, and restraint and partial fixity at the supports.
The other problem is the difficulty of defining when a deflection becomes
'excessive'. In practice, complaints often arise from the cracking of plaster
on partition walls, which can occur when the deflection of the supporting
beam is as low as span/800 [34]. For partitions and in-fill panels generally,
the relevant deflection is that which takes place after their construction.
This can exceed that due to the finishes and imposed load, for dead-load
creep deflections continue to increase for several years after construction.
It is good practice to provide partitions with appropriate joints and
clearances. When this is not done the deflection under the characteristic
load combination should not exceed span/300, or span/500 if the parti-
tions are of brittle construction. Where appearance is the only criterion, a
greater deflection may be acceptable where there is a suspended ceiling,
and for roof beams constructed to a fall. The difficulty of assessing the
accuracy and significance of a calculated deflection is such that simplified
methods of calculation are justified.
3.8
Effects of shrinkage of concrete and of temperature
In the fairly dry environment of a building, an unrestrained concrete slab
can be expected to shrink by 0.03% of its length (3 mm in 10 m) or more.
In a composite beam, the slab is restrained by the steel member, which
exerts a tensile force on it, through the shear connectors near the free ends
of the beam, so its apparent shrinkage is less than the 'free' shrinkage.
The forces on the shear connectors act in the opposite direction to those
due to the loads, and so can be neglected in design.
The stresses due to shrinkage develop slowly, and so are reduced by
creep of the concrete, but the increase they cause in the deflection of a
composite beam may be significant. An approximate and usually con-
servative rule of thumb for estimating this deflection in a simply-supported
beam is to take it as equal to the long-term deflection due to the weight of
the concrete slab, excluding finishes, acting on the
composite
member.
In the beam studied in Section 3.11, this rule gives an additional deflec-
tion of 5.4 mm, whereas the calculated long-term deflection due to a
shrinkage of 0.03% (with a modular ratio
n
=
20.2) is 5.9 mm.
