Load combinations and concrete cover to the Eurocodes: 6.10, 6.10a, 6.10b and nominal cover

Two things come before any section design: the design loads, from EN 1990, and the cover to the steel, from section 4 of EN 1992-1-1. Both are easy to get slightly wrong, and both change every number that follows.

Characteristic loads

A design starts from characteristic values: the permanent load (self-weight, finishes, fixed partitions, services) and the variable imposed load from EN 1991-1-1, chosen by the use of the floor. The use category (A domestic, B offices, C places where people gather, D shops, E storage) sets both the recommended imposed load and the combination factors .

In the slab example, the slab's own weight is worked out from its thickness and added to the superimposed dead load:

Ultimate limit state: Expression 6.10

For the strength of the structure (the STR limit state), EN 1990 gives Expression 6.10, with the recommended partial factors of Table A1.2(B):

The alternative: 6.10a and 6.10b

EN 1990 also allows the less favourable of two expressions to be used instead, which usually gives a smaller load:

with the reduction factor recommended and from Table A1.1 (0.7 for categories A to D). The National Annex decides which expressions may be used; the UK, for example, sets . For the same slab:

The larger of the two governs: here 10.031 kN/m², about 8 % less than the 10.931 kN/m² of Expression 6.10. 6.10a tends to govern where the permanent load dominates, as in heavy concrete floors with light imposed loads.

Which one governs depends only on how large the imposed load is against the permanent load. The chart works all three expressions with the calculator's own load combination for a range of ratios, as multiples of :

When a second variable action is present, such as wind, each one takes a turn as the leading action at its full value, with the others reduced by . The calculator forms every combination and uses the worst; without wind, only the imposed-load combinations appear.

Serviceability combinations

Deflection and cracking are checked under smaller, longer-term loads (6.5.3):

Concrete cover

The cover is the distance from the surface of the concrete to the nearest bar, which for a beam or column is usually a link. Clause 4.4.1 sets a nominal cover, the value shown on the drawings:

• is for bond: at least the bar diameter (Table 4.2), more for aggregate over 32 mm.
• is for durability, from Table 4.4N by the exposure class and the structural class.
• allows for workmanship, 10 mm recommended.

The structural class starts at S4 for a 50 year working life and is moved up or down by Table 4.3N: up two classes for 100 years, down one for a strong enough concrete for the exposure class, down one for slab geometry, and down one for special quality control. For the slab example, in exposure class XC1:

Across the exposure classes, for the usual 50-year building and H12 bars, the calculator's cover rules give:

Footings and members cast against the ground

Concrete cast on blinding or directly against the soil needs more cover, whatever the calculation above gives (4.4.1.3(4)): at least 40 mm on blinding and 75 mm against the soil, in the recommended values. The pad footing example in exposure class XC2:

Points to remember

• Check which expressions your National Annex allows before using 6.10a and 6.10b.
• Cover is to the outermost bar (usually the link), and the effective depth then follows from it.
• Fire resistance may need more cover than durability; the calculator lets you enter a larger cover for fire, though it does not check fire resistance itself.

Open the slab calculator and switch the combination to the less favourable of 6.10a and 6.10b to compare.