POINTS TO CONSIDER WHEN DESIGNING FABRICATIONS FOR HOT DIP GALVANIZING
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More extensive guidance on design for hot dip galvanizing can be found
in BS EN ISO 14713 and other Galvanizers Association publications.
 
Extenal stiffeners, welded gussets and webs on columns and beams, and
gussets in channel sections should have their corners cropped.
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Cropping the corners of these brackets will aid access and drainage of molten zinc and a cleaner coating will be obtained.
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.
On structural hollow sections, provision must be made for venting and
draining. With vertical members, drilled holes or V notches should be
provided diagonally opposite each other at top and bottom.
Every sealed section of a fabrication must be vented for reasons
of safety and to allow access and drainage of molten zinc. Holes
diagonally opposite each other should be as close as possible to the
sealed end.
Overlapping or contacting surfaces are potentially dangerous as
pretreatment solutions trapped between surfaces is converted to
superheated steam in the galvanizing bath and can lead to an explosion.
If contacting surfaces can not be avoided, as with these channels, then
the edges of the contacting areas should be continuously welded. One
hole should be drilled through both members for each 100 of
overlap, the minimum hole diameter being 10 mm, or the thickness of the
section, which ever is greater in order to eliminate the danger of an
explosion in the galvanizing bath.
If this is not possible, then welding should be intermittent: in
service there may be some weeping of trapped pretreatment solutions
from between the plates leading to brown staining but this will not be
detrimental to the protection given by the coating. It will not
generally be necessary to make any provision if the enclosed area is
less than about 100 (e.g.10 cm x 10 cm).
Internal diaphragms in large box sections should have cropped corners
and a 'manhole'. Internal diaphragms on small box sections should have
cropped corners. Where any hollow section is vented internally it is
essential, for safety reasons, that the galvanizer is able to view such
venting.
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Alternative designs for venting sections fixed to base plates.
External stiffeners for beams or channels should have the corners cropped.
Welded joints should be continuous if they are not enclosing an
otherwise unvented surface. Bolted joints are best made after
galvanizing.
To minimise the risk of distortion, flat panels should be braced, e.g.
dished or ribbed. Openings should be provided in the 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.
Vents should be diametrically opposite and at least 50mm in diameter.
Internal baffles should be cropped top and bottom. Lifting lugs are
required as indicated. It should be possible to view the baffles
through either the vent holes or an inspection hole - the placement of
the inspection hole should be discussed with the galvanizer.
DESIGNING USING HIGHT STRENGTH STEEL
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In rare circumstances, a fabrication sent for galvanizing can after
galvanizing, exhibit cracking around welds or other details of the
structure. Careful consideration of the design of the fabrication prior
to galvanizing can minimize the usually low potential for this to
happen.
Guidance on designing high strength steel fabrications when galvanizing for corrosion protection.
Introduction
Many hundreds of thousands of tonnes of steel are hot dip galvanized
each year, with galvanizing providing long-term, cost effective
corrosion protection in a wide range of environments and across a very
broad range of construction projects.
In common with other construction materials, it is a fact that steel
fabrications crack from time to time, for a variety of reasons. These
reasons may be related to the steel quality, the fabrication techniques
used and/or particular in-service conditions – or a combination of
these and other factors. In very rare circumstances this cracking might
exhibit itself immediately after withdrawal from the galvanizing bath.
A detailed guidance document, Publication No 40/05, 'Galvanizing
structural steelwork - An approach to the management of liquid metal
assisted cracking', has been developed by a British Constructional
Steelwork Association [BCSA] working group which included
representatives from Galvanizers Association, major fabricators and
major specifiers.
The guide sets out the steps to take in order to optimize the quality
of the galvanized steel fabrication when designs incorporate the use of
high strength steel. Following this guidance minimises the chances for
any single fabrication to combine those characteristics of general
design, steel supply, fabrication history and galvanizing conditions
which might give rise to a significant potential for such rare cracking
to occur.
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