The objective of designing a structure is to ensure that the structure is suitable for its function, has adequate stability, strength and durability, is constructed at an acceptable cost and is aesthetically pleasing.

The overall design shall be planned to facilitate surface preparation, painting, inspection and maintenance.

The shape of a structure can influence its susceptibility to corrosion. Therefore structures should be designed such that corrosion cannot easily establish a foothold (a corrosion trap) from which it can spread. It is therefore strongly recommended that the designer consults a corrosion protection expert at a very early stage in the design process. Ideally, the corrosion protection system should be selected at that time, with due consideration to the type of service of the structure, its service life and maintenance requirements.

The shapes of the structural elements and the methods used to join them should be such that fabrication, joining and any subsequent treatment will not promote corrosion. Similarly, consideration should be given to the shape of the structure and its elements with respect to the category of its environment (see IS0 12944-2) when specifying a protective paint system.

Designs should be simple and excessive complexity should be avoided. Where steel components are in contact, embedded or enclosed in other building materials, e.g. brickwork, they are no longer accessible, therefore, the corrosion protection measures shall be effective throughout the service life of the structure.

Structural design should incorporate features of the design that minimise the likelihood of corrosion. Poor design of the structure may initiate early breakdown of the coating and corrosion.

Design will affect

• Surface preparation
• Paint application
• Inspection
• Maintenance

Paint will give the best performance when applied to

• An even and continuous surface
• Good design should allow for sufficient space for surface preparation, paint application, inspection, maintenance
• Good design avoids water & contamination retention

This standard addresses  the basic criteria for the design of steel structures which are to be coated by protective paint systems, in order to avoid premature corrosion and degradation of the coating or the structure. Basic design criteria included in the standard:

• Accessibility
• Treatment of gaps
• Precautions to prevent retention of deposits and water
• Edges
• Welding surface imperfections
• Bolted connections
• Box chambers and hollow components
• Notches
• Stiffeners
• Prevention of galvanic corrosion
• Handling, transport and erection

Furthermore, ISO standard 8501-3 (Preparation grades of welds, edges and other areas of surface imperfections) describes preparation grades of imperfections. 

The steel structure should be accessible, visible and suitable for:

• Application
• Inspection
• Maintenance

Accessories, such as ladders, walkways and hooks of scaffolding need to be considered in the design. There must be sufficient opening for people to access box chambers and/or confined areas (tanks), narrow spaces should be avoided.

If a component cannot be properly maintained after fabrication, corrosion resistant materials or long lasting protective systems should be considered.

Notches / rat holes

Notches should have a radius of not less than 50 mm so they can both be prepared and coated to a good standard.

Spot welds / Stich welding / Intermittent welds

This type of welding may be quicker, cheaper and weigh less than full welding, but they may end up as corrosion traps. Paint is unable to fill the gaps between the pieces which are spot welded together. These areas will be at risk to corrosion. Spot welding is not recommended for areas exposed to severe / aggressive environments.

Other areas of a structure which may easily trap moisture, dirt, abrasives, etc.:

• Narrow gaps
• Blind crevices
• Lap joints
• Skip welding
• Mating surfaces

Future maintenance and repair may require use of scaffolding. By incorporating permanent scaffold supports in the design, damages to the main area may be avoided. Damage to the supports themselves is easily touched up. Material of retainer is commonly made by stainless steel (painted).

Sharp edges must be rounded or chamfered for the purpose of:

• Apply a uniform coating
• Provide adequate thickness
• Minimize damage of coating
• A minimum radius of 2mm is recommended

All types of stiffeners should be designed so as to avoid gaps and thereby removing the risk of retention of dirt deposits or water. There must be sufficient space for surface preparation and paint application.

Accumulation of water, be it rainwater or splashed seawater, as well as air-born pollutants, will provide an excellent electrolyte for corrosion and should be avoided.

• Good drainage will reduce corrosion
• Proper shape to allow easier water run off
• Drainage open in suitable location

• Avoid forming pockets and recesses

Many problems are caused by a poor welding job

• Sharp edges
• Narrow gaps
• Weld spatter
• Blow holes

Slip-resistant connections with high-tensile bolts

• Connection surface should be blasted to achieved agreed roughness
• A coating tested and approved for contact surfaces can be used

Preloaded Connections

• Paint system to be used should not decrease preloading force

Bolts, Nuts and Washers

• Bolts, nuts and washers shall be protected against corrosion to the same durability as the corrosion protection of the structure

Open box chambers and hollow components

• Require good drainage and air flow
• Effective protection-envelope barrier coatings

Sealed box chambers and hollow components

• Ensure air/moisture proof, check welds for pinholes
• Avoid water entrapment during fabrication

• The structural design for galvanising should meet relevant regulation and/or standards (e.g. ISO 1461 AND ISO 14713)
• Venting and drainage should be provided on all enclosed structural members that are to be hot dip galvanised to avoid air expansion and explosion in the zinc bath

A hole in a hollow component will allow moisture, oxygen and other corrosive ingredients to enter the item as a result of temperature changes (e.g. night to day). The ingredients will be trapped inside the hollow component and create corrosion.

Ways to avoid damage of the protective coatings at the different  stages of construction should be considered at the design stage:

• Handling
• Transportation (lifting)
• Erection
• On site operation (welding, cutting and grinding)