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Durability - Structural steel corrosion protection

European structural design codes will introduce

the concept of design working life to British

engineering.Peter MayerofBuilding LifePlans

explains exactly what that means .

The Eurocodes are a series of standards that will provide a common approach to structural design

across the EU. As conditions a ffecting structures vary across the EU, each country can vary certain

parameters. These will be introduced as annexes to the standards in mid 2008 with conflicting

standards withdrawn by 2010.

When the Eurocodes for designing steel structures are fully adopted, one of the biggest changes for UK

engineers will be the concept of design working life (see table).

While British Standards consider durability for a range of environments, Eurocodes take a more detailedapproach, focusing on:

The material deterioration processes

Design options to resist degradation and actions on the structure

Maintenance strategies to ensure functional performance.

These provide the basis for life-cycle costing analysis.

Design working life

Design working life is the period of intended use, with maintenance but no major repair anticipated, for a

structure. Most buildings fall into one of two categories. Category 4 has an indicative working life of 50

years (building structures), while category 5 has a life of 120 years (monumental building structures and

infrastructure construction).

The Eurocodes set out how materials, design, details, protective meas ures, workmanship and quality

controls can be managed to achieve a required life.

Achieving the design working life

Putting these concepts into practice may involve standards needing to be cross-referenced to arrive at an

answer. In certain cases, advisory organisations have published guidance as a shortcut.

With steel, for example, to determine a solution for the design working life, the EN 1993-1.1 refers to the

EN 1090, which outlines the process for achieving the design working life for steel structures.

Once the solution has been determined, the risk of corrosion needs to be managed by some form of

surface protective treatment. Issues to take into account include:

The durability required

The atmospheric-corrosivity category or exposure classification

Metal spraying, galvanising, painting, subsequent decorative coatings, choices of colour and bimetallicinterfaces.

Relevant standards

A suitable protective system can be specified by cross-referencing relevant standards for a given design

working l ife, corrosivity category and main tenance strategy.

Classification of corrosivity atmospheres: ISO 9223

Preparation of surfaces : BS EN ISO 8501

Protection of steel structures, zinc and aluminium coatings: BS EN ISO 14713: 1999

Sprayed zinc or aluminium coatings: BS EN ISO 2063

Paints and varnishes: BS EN ISO 12944

Hot dip galvanized coatings: BS EN ISO 1461:1999

Post application testing: National structural steelwork specification

Calculating the steel design working life

For atmospheric corrosion, refer to the Galvanizers Ass ociation Millennium Map of average zinc

corrosion rates. About 50% of England and Wales has a rate of under 1m/year. Steel with a 85?m

galvanised zinc coating, in an environment with a corros ion rate of 1m/year would have an expected life

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Retail lighting

Sealants

of 85 years. This falls within the 50 year category. For 120 years, thicker coatings of zinc galvanising is an

option.

Internal steel structures where the internal relative humidity does not exceed 80% do not require

corrosion protection. Information on steel corrosion is provided by Corus, the Galvanizers Association,

National Physical Laboratory and Steel Construction Institute.

Specification options

Steelwork exposed to atmosphere Capital cost

/m2

Life Cycle Cost

for 50 or 120

years

/m2

Period to

first major

maintenanceYears

Category 4: 50 years

Hot dip galvanized to BS EN ISO 1461 mean coating

thickness 85m35 42 80

Epoxy micaceous i ron oxide (MIO) finish: 100?m ; Zinc

phosphate epoxy primer: 80m. Steel: blast cleaned

to Sa 2.5

22 61 20

High s olids polyurethane finish:100m. Zinc

phosphate epoxy primer: 100m. Steel: blast cleaned

to Sa 2.5

17 48 20

Category 5: 120 years

Hot dip galvanized to BS EN ISO 1461 mean coating

thickness 85m35 43 80

Hot dip galvanized mean coating thickness 140m

includes s hotblast preparation50 58 120

Epoxy micaceous i ron oxide (MIO) finish: 100m ; Zinc

phosphate epoxy primer: 80m. Steel: blast cleaned

to Sa 2.5

22 73 20

High s olids polyurethane finish:100m. Zinc

phosphate epoxy primer: 100m. Steel: blast cleaned

to Sa 2.5

17 57 20

Table notes

External environment category C3 to ISO 9223, Average annual zinc corrosion rate: 1?m/yr. First

protective system factory applied. Subsequent coating assumed at 8 10 year intervals including

allowance for treating corrosion and quinquennial inspections. Dimensions refer to dry film thicknesses.

Discount rates of 3.0% (50 years) and 2.5% (120 years) are used to calculate net present values.

There are a huge range of alternative coating systems; costs and options are indicative. Use specialist

advice for suitable options and a cost analysis based on project specific information for a realistic best

value appraisal.

First published in Building2007

Further information

BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450

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