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Sections with a spacing between the overlapped interface of less than 2.5 mm will restrict the flow of pre-treatment fluids and prevent the effective flow of zinc into and out of the crevice, resulting in uncoated areas. For this reason, spacings between overlapped surfaces less than 2.5 mm should be designed with fully sealed welds when the overlap is small, or with vented overlaps.
The size and amount of vent holes in overlapping surfaces are dependent on the size of the overlap – details are in Design Note 7. A minimum of 2 vent holes are required and the holes are only required to penetrate 1 surface. For tubular sections covered by plate, the hole can be drilled in the tube allowing the venting to occur into the tube.
The hole size for venting and draining must be at least the thickness of the steel. In other words, if the overlapping plate is 15 mm thick, then the minimum holes size is Ø15 mm.
In all cases, the location of the vent holes must conform to the requirements of AS/NZS 5131 and AS 4100.