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Steel Designers' Manual - 6th Edition (2003)
29.4 Durability
It is important that the long-term performance of the structure is considered both
in the choice of structural form and in the design of construction details. Failure to
do so may result in maintenance problems requiring costly repair.
29.4.1 Corrosion allowances
The means for countering the effect of corrosion of steel piles are well developed.
Guidance is given in the British Steel Piling Handbook. 20
BS 8002 8 considers that the end of the effective life of a steel sheet pile occurs
when the loss of section, due to corrosion, causes the stress to reach the specified
minimum yield strength. A pile section chosen for the in-service condition has to
be adequate at its end-of-design-life, at which time the effective pile section will
have been reduced by corrosion.
As the corrosion loss allowance varies along the pile according to the corrosion
environment, the designer needs to be aware that the maximum corrosion may
occur at a different level to that of the maximum forces and moments, and should
allow for this accordingly.
Also, since redistribution of earth pressures may occur as a result of increased
flexure of a corroded section, the end-of-design-life condition may be a critical
design load case in the selection of the sheet pile section.
29.4.2 Corrosion and protection of steel piles
Durability 901
The design life requirements for proposed buildings and individual components or
assemblies are defined in BS 7543 Guide to durability of buildings and building elements,
52 where a building design life can range from 10 years for a building with a
‘short’ life to 120 years for civic and other high quality buildings. A retaining wall
which is part of a building structure, i.e. a basement, must therefore comply with
these requirements and be designed with sacrificial thicknesses applied to each
surface, depending on the exposure conditions. The exposure conditions are based
on the advice given in BS 8002 clause 4.4.4.4.3 and are shown in Table 29.4. 8
The reduced (corroded) section properties can be obtained either by calculation
or from the British Steel Piling Handbook. 20
Another way of allowing for sacrificial thickness is to use a higher strength steel
than would be required if no corrosion were assumed (i.e. use steel grade S355GP,
to BS EN 10248, 25 in a wall designed for steel grade S270GP). This permits a greater
loss of metal before stresses become critical.