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Steel Designers' Manual - 6th Edition (2003)
1062 The Eurocodes
but the overall concepts are now well established. For certain parameters recommended
values will be given, and it will be left to national bodies to decide whether
to accept these or choose something different. Where necessary, recommended
values have been used in this section. The plan is for Parts 1.1 and 1.2 to be made
available by CEN in the first half of 2003, with Part 2 following a year later.
Eurocode 4 covers more than BS 5950 Part 3.1. It includes rules for partially
encased sections and, more importantly, includes guidance on composite columns.
As well as individual elements, the EC3 rules for moment connections are extended
to composite joints. The basis for the work was the ENV version published in 1994,
but there have been significant changes since then. Guidance on composite slabs
equivalent to that in BS 5950 Part 4 is included in EC4 Part 1.1.
Taking an overall view EC4 will give very similar results to BS 5950 for
composite beams and slabs. For composite columns the resistance will be larger than
that determined by current UK guidance. One area where EC4 is more conservative
than BS 5950 is in the capacity of stud shear connections.
36.4.2 Design rules
Materials
The Eurocode covers a wider range of properties than BS 5950. Steel grades S420
and S460 are included, as are concrete grades up to a cube strength of 75 MPa.
For concrete, the limiting stress used in EC4 is 85% of the design strength.
According to EC2 the concrete design strength (fcd) is afck/gc, where fck is the
cylinder strength. The recommended value for the material safety factor gc is 1.5.
To compare EC4 and BS 5950 you also need to account for the relationship
between cube and cylinder strength. Typical values, given in Table 36.5, show that
the limiting stresses are very similar.
For steel, both for sections and reinforcement, the limiting stress in EC4 is the
design strength. For structural steel, EC3 recommends that this is equal to the
nominal yield stress, but for reinforcement EC2 recommends a partial factor of 1.15
Table 36.5 Comparison of concrete strength in BS 5950 and EC4
Cube BS EC4
strength 5950
(MPa) limiting
stress
Normal weight concrete Lightweight concrete
Cylinder Limiting EC4/BS 5950 Cylinder Limiting EC4/BS 5950
strength stress strength stress
25 11.25 20.0 11.33 1.01 22.5 10.84 0.96
30 13.50 25.0 14.17 1.05 27.0 13.00 0.96
35 15.75 28.6 16.21 1.03 32.0 15.41 0.98
40 18.00 31.9 18.10 1.00 36.7 17.68 0.98