Design detailing
When designing a structure which is to be hot dip galvanized, it must be borne in mind that articles are immersed into and withdrawn from a bath containing molten zinc heated to a temperature of 450ºC.
Design and fabrication is required to conform to acceptable standards which apply, regardless of whether a galvanized or a painted coating is to be applied. In the case of galvanizing, some additional requirements which aid access and drainage of molten zinc, will improve the quality of the coating and also reduce costs.
With certain fabrications, holes which are present for other purposes may fulfil the requirements of venting of air and draining of zinc; in other cases it may be necessary to provide extra holes for this purpose.
For complete protection, molten zinc must be able to flow freely over all surfaces of a fabrication. With hollow sections or where there are internal compartments, the galvanizing of the internal surfaces eliminates any danger of hidden corrosion occurring in service.
Design detailing
If the design of an article includes sealed hollow sections guidance on correct venting and drainage must be incorporated
- a examples of holes in end plates
- b examples of crops and cut-outs in stiffeners
- c cut-outs in fabricated beams
- d holes in web of beams
External stiffeners, welded gussets and webs on columns and beams and gussets in channel sections should have their corners cropped. The gaps created should be as large as possible without compromising structural strength. If welding is required around the edge created, a radiused corner is desirable, to facilitate continuity of the weld around the cut end to the other side. Circular holes are less effective; if used, they should be as close to corners and edges as practicable. Consultation with the galvanizer regarding the appropriate vent and drainage hole sizes is recommended.
Angle bracings should, if possible, be stopped short of the main boom flange. This will allow the free flow of molten zinc across the surface of the flange, enhancing drainage from the structure. This will assist the development of a smoother galvanized coating, reduce the potential for retention of ash on the surface of the flange and help to avoid air traps within the structure, which could lead to uncoated areas.
Internal diaphragms in large box sections should have cropped corners and a ‘manhole’. Internal diaphragms on small box sections should have cropped corners.
Large open top tanks should be stayed to minimise distortion. Where angles are used to rim the tank, appertures must be provided in the corners. Angles or flats used as stays should be as close as possible to the tank wall thickness.
Additional guidance for design of structural steelwork for hot dip galvanizing
In circumstances where, due to design restrictions, general design guidance (as set out above) cannot be followed and the introduction of holes or other fabrication features into the ‘K’ areas of a section (where the web and flange meet) is unavoidable, please consult GA to discuss how best to finalise the design of the fabrication.
Cope cutting of beams is a common feature in modern steel construction. For optimum results during galvanizing, where flame-cut copes have been introduced into a fabrication, the following steps are recommended:
- Use a large radius for the cope – 20 mm minimum if possible.
- After cope cutting, grind off any hardened steel surface layer.
- Provide a smoothly ground cope cut surface avoiding notches, grooves and other surface irregularities.
- Chamfer the edges to the cope cut.
It is very important when considering design detailing of steelwork for hot dip galvanizing that considerations for correct venting and drainage of sealed hollow sections are taken into consideration.
Size and shape
Bath Dimensions
In recent years, the size and capacity of galvanizing plants has increased significantly. Reference should be made to the galvanizing plants section for an indication of the bath sizes available in the UK and Republic of Ireland. When the length or depth of the item exceeds the size of the bath, special techniques may be employed to facilitate dipping, in this case advice must be sought from the galvanizer.
The design of the component may be important for successful galvanizing and early consultation between galvanizer, fabricator and designer is the key to obtaining the best result. Design features which aid the access and drainage of molten zinc will improve the quality of the coating (refer to Designing Articles for Galvanizing).
The bath dimensions noted within the following pages indicate the length, depth and width of the galvanizing bath(s) at each works. These bath dimensions give an indication of maximum size of fabrication which can be processed at that plant. It is important to recognise that these dimensions do not, in themselves, represent the maximum size of fabrication which can be dipped. The precise maximum single dip size can be agreed with the galvanizer and will usually depend upon the angle of immersion and overall dimensions of the fabrication.
Articles which are larger than the bath dimensions can sometimes be galvanized by “double-dipping”. Here, the article is partially dipped in the zinc, removed, reversed for length or depth to allow dipping of previously uncoated section. Although generally it is preferable to process work in single dip, the corrosion protection offered through double dipping is no different to that provided in a single dip. Sizes of articles which can be double-dipped should always be agreed with the galvanizer.
Connections, handling and masking
Connections
Handling of articles
Overlapping surfaces
Castings
Moveable parts
Adequate clearance on mating surfaces, such as hinges, should be allowed if they are to move freely after galvanizing. An extra clearance of at least 1mm is usually sufficient.
Masking
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