DESIGN BASICS FOR GALVANIZING

Design Basic

The successful hot dip galvanizing of fabricated steelwork is heavily dependent on its design. To be satisfactorily galvanized, the fabrication must be immersed in a series of pre-treatment baths to prepare the surface for galvanizing, and then be submerged in a bath of molten zinc. If the pre-treatment chemicals and the molten zinc cannot get access to all surfaces, the item will not be effectively coated. Fabrication techniques can also either create or alleviate problems in the galvanizing process.

SGSI technical staff are available to advise on design, and will assist with design detailing prior to fabrication to ensure that a high quality hot-dip galvanized coating is applied to customers’ projects.

There are a number of fundamental considerations when designing a steel fabrication for hot-dip galvanizing. The key issues are:-

Items larger than the galvanizing bath in two dimensions cannot be galvanized.

Venting is Vital. If a hollow section traps 18% of its internal volume as air, it will not sink in the   molten zinc.

Steel sections of differing section thickness heat and cool at different rates. Unbalanced   sections causing differential heating and cooling will increase the risk of distortion (buckling).

Structural grades of steel lose 50% of their yield strength at galvanizing temperatures. The   design of the fabrications for galvanizing must take this into account to prevent sagging or   bending under self-loads.

Overlapping surfaces should be fully sealed. Large overlapping surfaces exceeding 400 cm2   should be provided with vent holes.

The size of the item that can be galvanized will be governed by the size of the galvanizing    bath. Long items can be galvanized by double-end dipping but items larger than the   galvanizing bath in two dimensions cannot be galvanized.

Items to be galvanized must be suspended or supported through the galvanizing process. This    may require holes in some work, or the welding of (removable) lifting lugs.

IMPORTANT FABRICATION FACTORS

For standard fabrications, a basic set of design rules should be applied. These design steps do not add to the fabrication cost but will ensure a good quality outcome. These are:
•	Gussets and stiffeners should be cropped.
•	End plates or base plates on columns:
•	Universal sections should have vent/drain holes in the corners of the web/flange connection.
•	Hollow sections should have vent/drain holes in the internal extremities
•	Outward facing channels and angles in fabricated frames eliminate the need for vent and drain holes, contrary to inward facing sections.
•	Terminating bracing short of adjacent flanges eliminates the need for cropping or vent holes.