Civil Engineering Reference
In-Depth Information
This clause from EN 1992-1-1 distinguishes between two types of crack-
ing. These are treated separately in EN 1994-1-1, which follows closely
the rules for crack-width control given in EN 1992. Even where no re-
inforcement is required to resist 'direct loading', it is necessary to limit the
widths of cracks that result from tensile strains imposed on the element
considered. The origin of these strains can be 'extrinsic' (external to the
member), such as differential settlement of the supports of a continuous
beam, or 'intrinsic' (inherent in the member), such as a temperature gradient
or shrinkage of the concrete.
In reinforced concrete, cracking has little influence on tensile forces
caused by direct loading, but it reduces stiffness, and so reduces the tensile
force caused by an imposed deformation. For example, the tensile force in
a restrained member caused by shrinkage of concrete is reduced when the
member cracks.
Calculations for load-induced cracking are therefore based on the tensile
force in the reinforcement after cracking (i.e., on the analysis of cracked
cross-sections), whereas calculations for restraint cracking are based on
the tensile force in the concrete just before it cracks.
These concepts are more difficult to apply to composite members,
where there is local restraint from the axial and flexural stiffnesses of the
structural steel component, applied through the shear connectors or by
bond. In a web-encased beam, for example, where the steel tension flange
is stressed by direct loading, the resulting strains and curvature impose a
deformation on the concrete that encases the web.
The presence of structural steel members, and the differences between
beam-to-column joints in composite and reinforced concrete frames, made
it impossible to cover cracking in Eurocode 4 simply by cross-reference
to Eurocode 2 for reinforced concrete. This led to a 'stand-alone' treat-
ment of cracking in a slab that is part of the tension flange of a composite
beam, and in the concrete encasement of a steel web.
'Cracking shall be limited to an extent that will not impair the proper
functioning or durability of the structure or cause its appearance to be
unacceptable.'
This quotation, also from EN 1992-1-1, refers to function and
appearance. For the concrete of composite members for most types of
building, the most likely cause of impairment to 'proper functioning' is
corrosion of reinforcement, following breakdown of its protection by the
surrounding concrete. Design is based on the exposure classes given in
EN 1992-1-1. The relevant class is likely to be X0 (very dry environment)
or XC1 (dry environment). For these classes, the limiting surface crack
