The following example is intended to demonstrate, for the inexperienced specifier, the step-by-step approach to selection of a coating system for a particular fabrication, taking all appropriate factors into account.
The protective system to be selected and specified is required for a girder linking a sea-water pontoon to a fixed point on land at a site in the United
Kingdom.
It is assumed that the girder has been conceived as a welded box-girder with lightening holes and pin joints at the ends. The pin joints, which are
considered to be purpose-made units to be attached to the ends of the girder by site bolts, are not included in the example.
Figure 11 — Assessment of steel cleaning
The selection and specification of a coating system or location in the codeSource of information
Step 1 Acquire an appreciation of what is included in the code and where
it is located Table of contents and index
Step 2 Consider if any modification of the design is desirable in the light
of the advice in the code 45.3 and A.3
In the example it is seen that a box section with lightening holes will not only trap water but will be difficult to maintain internally, so it is decided to redesign the girder as a welded plate girder with lightening holes in the web. This reduces water trapping and makes all surfaces readily accessible. Special attention is paid to the ends to ensure that any water resting on the bottom flange can run off as the pontoon rises and falls.
Step 3 Ascertain and list the basic facts concerning the structure by answering the questions in 4.2
In the example, the facts (and assumptions) are tabulated as follows.
4.2.1 Function.
a) Structure is required to link pontoon to shore. Design remit b) Nil. 4.2.2 Life. a) 50 years. Design remit b) Reconsider later. 4.2.3 Environment. Clause 5 a) Coastal.
b) Structure is a few feet above salt water and is likely to be covered with spray from time to time.
c) At high tide, in storm conditions the under side may be hit by water-borne flotsam, but there is unlikely to be mechanical damage arising from usage or contamination by de-icing salts.
Design remit
4.2.4 Appearance.
a) The owning authority requires the colour to be green overall. Design remit b) Yes.
4.2.5 Special properties.
a) No requirement.
4.2.6 Maintenance
a) Because of pontoon use, access for maintenance will be possible only in
November and December. Design remit b) It is clear that maintenance will be difficult. Practical
experience
4.2.7 Health and safety.
a) No special points requiring attention are apparent at this stage. b) The existence of offices adjacent to the shore connection suggests that limits may be placed on permissible noise and/or dust created during maintenance work.
Section 6 and practical experience
4.2.8 Tolerance.
a) A high standard is essential at this site, requiring works application of
initial coatings. Clause 9
118 © BSI 11-1998
The selection and specification of a coating system Source of information or location in the code
Step 4 Examine Table 1. Select the description nearest to 4.2.3 above.
Turn to the corresponding part of Table 3 Table 1
In the example, the nearest description is “sea water: splash zone ...
frequent salt spray”. Table 3, part 9
Step 5 Decide on a life requirement, taking into account the answers to
questions 4.2.2, 4.2.4, 4.2.6 Clause 6
It is clear that a durable protective system is required. However, the decorative coating may be expected to require attention at shorter
intervals than would normally apply with a durable protective system. It is decided, therefore, to select a “long life” system to reduce the frequency of maintenance and (incidentally) reduce the likelihood of potentially noisy surface preparation being needed for corroded areas. At the same time a greater frequency of maintenance for the decorative coating is considered acceptable.
6.2.1 6.2.2
6.3b)
Step 6 Consider the “long life” systems listed in Table 3, part 9; eliminate any that are thought to be unsuitable. Make preliminary estimates of the approximate cost per square metre and inquire into the suitability (for the actual conditions) of the remaining systems, so far as time permits
Table 3, part 9. Appendix E
It is clear that some of the systems listed (SB1 + SK5; SB3; SB9; SC3Z; SC6Z; SC10A; SC10Z; SE7; SK8; SL5) will require the decorative finish as an addition; yet there are no immediately obvious reasons for eliminating any of those systems at this stage. Therefore:
a) if time permits, make inquiries (preferably of the manufacturers) about all of these systems; or
b) if there is not sufficient time, confine the inquiries to those systems that include a finish that gives or seems to give the required
appearance.
Step 7 On the basis of the conclusions reached in step 6 and any experience or justified preference, select a limited number of systems for detailed examination
In fact, time is short (step 6 b) applies) so the selection of systems for
detailed study is limited to SB9; SC10A; SC10Z; SE7; SL5. Table 3, part 9
Step 8 Make a detailed study of the characteristics of the selected systems, asking the questions listed in 4.3
In this example, this study could have been made as part of step 6 because the time limitation discussed in step 7 was then already known. 4.3 A check list in the following form may be found useful. It tabulates the
answer for each of the five systems against each question.
4.3.1 Coating systems.
a) SB9 SC10A SC10Z SE7 SL5 Table 3, part 9 b) Yes Yes Yes Yes Yes Supplier/experience c) Yes Yes Yes Yes Yes Supplier/experience d) Application method to suit material. No apparent limitation.
The selection and specification of a coating system Source of information or location in the code
4.3.2 Coating facilities
SB9 SC10A SC10Z SE7 SL5 a) 1) Depends on
size and shape of structural member
Yes Yes Yes Yes Supplier/experience
2) Yes Yes Yes Yes Yes Supplier/experience b) Very large
members may be a problem
Yes Yes Yes Yes
c) Yes Yes Yes Yes Yes
d) Yes Yes Yes Yes Yes
4.3.3 Compatibility
SB9 SC10A SC10Z SE7 SL5 Clauses 7 and 8
a) Yes Yes Yes Yes Yes
b) Not with normal structural steels; check if special steels may be used in the fabrication
No No No No
c) N.A. (not applicable)
4.3.4 Delays.
SB9 SC10A SC10Z SE7 SL5 a) N.A. N.A. N.A. N.A. N.A. b) N.A. N.A. N.A. N.A. N.A. c) N.A. N.A. N.A. There may be problems
of intercoat adhesion. d) N.A. N.A. N.A. There may be problems
of intercoat adhesion. e) The final coat should be applied as soon as possible after erection in view of possible salt contamination. Ensure adequate pretreatment before applying paint coat.
4.3.5 Transport, storage, handling (works-applied coats only).
SB9 SC10A SC10Z SE7 SL5
Experience applied to manufacturer’s data
a) Quite good Good Good Fair Fair b) Quite good Good Good Fair Fair c) Good Good Good Doubtful Doubtful d) N.A. N.A. N.A. N.A. N.A. NOTE These are qualitative, not absolute, terms.
4.3.6 Experience.
Assumed reported equal for all systems.
120 © BSI 11-1998
The selection and specification of a coating system information or Source of
location in the code
4.3.8 Maintenance.
Yes, if steel is exposed. Deterioration of finishing coat only is not serious.
4.3.9 Costs.
a) 1)
It is not practical to quote costs in a hypothetical exercise. 2)
3)
So far as can be seen these will be the same for all the systems considered.
4) b)
Step 9 Make final selection
In the example, all five systems examined are of about equal merit. Assuming that there is no large disparity in the Net Present Value of the corrosion protection, it is decided to reduce the choice to the two
sprayed-metal systems, since these seem to have some slight advantage.
Step 10 Review the design in the light of the information now assembled
No further basic modification of the design seems necessary. However, detailing should ensure adequate access for surface preparation and application of coatings, with provision for slinging.
Step 11 Draft the Specification for inclusion in the tender documents
It may be advantageous to permit the contractor to include alternative prices in his tender for different systems where the performance is equal. The specifications for these items are based on the recommendations included in this code. The clauses for fabrication, handling, transport, storage and erection of steelwork cover all necessary precautions.
Section 3
Full details of the protective system are given, including references to British Standards; and clauses covering quality of other materials, standards of workmanship and limitations on application environments are included.
Section 2 Section 3
Step 12 Draft the inspection requirements Section 4
Since undetected or unrepaired damage to the coating system can have serious consequences it is considered necessary to specify “full” level inspection for both works- and site-applied coats.
4.3.8
Clause 37 Furthermore, it is required that all inspection reports and other relevant
information be kept available for future reference in connection with maintenance.
Clause 39
Step 13 Final-check Clause 34