Civil Engineering Reference
In-Depth Information
comparison, typical imposed loads for office areas are given by Equations
1.12 and 1.13. An allowance of 1.2 kN/m
2
for non-structural partition
walls is treated as an additional imposed load, because their positions are
unknown.
3.3
Composite floor slabs
Composite slabs have for many decades been the most widely used method
of suspended floor construction for steel-framed buildings in North
America. Within the last thirty years there have been many advances in
design procedures, and a wide range of profiled sheetings has become
available in Europe. The British Standard for the design of composite
floors [19] first appeared in 1982. There are Eurocodes for design of both
the sheeting alone [15] and the composite slab [3].
The steel sheeting has to support not only the wet concrete for the floor
slab, but other loads that are imposed during concreting. These may include
the heaping of concrete and pipeline or pumping loads. For construction
loading, EN 1991-1-6 recommends a distributed loading between 0.75
and 1.5 kN/m
2
. The loading used here is:
q
k
=
1.0 kN/m
2
Profiled steel sheeting
The sheeting is very thin for economic reasons, usually between 0.8 mm
and 1.2 mm. It has to be galvanised to resist corrosion, and this adds
about 0.04 mm to the overall thickness. It is specified in EN 1993-1-3 that
where design is based on the nominal thickness of the steel, the sheet
must have at least 95% of that thickness - but it is not a simple matter for
the user to check this. The sheets are pressed or cold rolled and are
typically about 1-m wide and up to 6-m long. They are designed to span
in the longitudinal direction only. For many years, sheets were typically
50-mm deep and the limiting span was about 3 m. The cost of propping
the sheets during concreting, to reduce deflections, led to the development
of deeper profiles; but design of composite slabs is still often governed by
a limit on deflection.
The local buckling stress of a flat panel within sheeting should ideally
exceed its yield strength; but this requires breadth/thickness ratios of less
than about 35. Modern profiles have local stiffening ribs, but it is difficult
to achieve slendernesses less than about 50, so that for flexure, the sections
are in Class 4 (i.e., the buckling stress is below the yield stress). Calcula-
tion of the resistance to bending then becomes complex and involves
iteration.
