Civil Engineering Reference
In-Depth Information
If, however, control of crack widths to 0.4 mm were required, reference to
Fig. 4.7 shows that 8-mm bars would be required. To provide 679 mm
2
,
their spacing would have to be 90 mm, or bars in pairs at 180 mm.
The alternative of increasing the top reinforcement to, say, 10-mm bars
at 100 mm (883 mm
2
) would require some re-calculation. It increases the
ultimate hogging bending moment
M
Ed,B
and so makes susceptibility to
lateral buckling more likely. It is not obvious whether it would reduce the
value of the tensile stress
σ
s
to below 360 N/mm
2
, the limit given in both
Figs 4.7 and 4.8 for crack control to 0.4 mm. This illustrates interactions
that occur in design of a hogging moment region.
4.7
Continuous composite slabs
The concrete of a composite slab floor is almost always continuous over
the supporting beams, but the individual spans are often designed as
simply-supported (Sections 3.3 and 3.4), for simplicity. Where deflections
are found to be excessive, continuous design may be used, as follows.
Elastic theory is used for the global analysis of continuous sheets acting
as shuttering. Variations of stiffness due to local buckling of compressed
parts can be neglected. Resistance moments of cross-sections are based on
tests (Section 3.3).
Completed composite slabs are generally analysed for ultimate bending
moments in the same way as continuous beams with Class 2 sections.
'Uncracked' elastic analysis is used, with up to 30% redistribution of
hogging moments, assuming that the whole load acts on the composite
member. Rigid-plastic global analysis is allowed by EN 1994-1-1 where
all cross-sections at plastic hinges have been shown to have sufficient
rotation capacity. This has been established for spans less than 3.0 m with
reinforcement of 'high ductility' as defined in EN 1992-1-1 for reinforced
concrete. No check on rotation capacity is then required.
At internal supports where the sheeting is continuous, resistance to
hogging bending is calculated by rectangular-stress-block theory, as for
composite beams, except that local buckling is allowed for by using an
effective width for each flat panel of sheeting in compression. This width
is given in EN 1994-1-1 as twice the value specified for a Class 1 steel
web, thus allowing for the partial restraint from the concrete on one side
of the sheet. Where the sheeting is not continuous at a support, the section
is treated as reinforced concrete.
For the control of cracking at internal supports, EN 1994-1-1 refers to
EN 1992-1-1. In practice, the reinforcement to be provided may be gov-
erned by design for resistance to fire, as in Section 3.3.7, or by the trans-
verse reinforcement required for the composite beam that supports the slab.
