huangyhg

welding finished steel
hi all,
i'm currently working on a project that will have a trellis on the exterior. this trellis will be exposed steel hss tubes. the trellis itself will be delivered in components and assembled in the field. the architect wishes to have all connections welded rather than bolted. now, assuming that the steel will be finished in the shop, do i need to be concerned with the effects of the finishes (galvanizing, painting, etc.) on the field welding? additionally, what is the best way to finish the steel to protect it against heat, moisture, etc. that it will always be exposed to?
thanks
if you are going to use galvanized steel, i would not recommend field welding for exposed applications.
you might consider weathering steel or stainless steel. either can be welded and will perform well for exposed conditions. no finish is required on either.
if you use plain steel, prime the steel at the fab shop but tape off the weld areas so that you're not burning paint when welding. then clean the welds, prime and paint the whole thing in the field.
the architect should be aware that field applied paint application is required and will never look as good as shop appield paint and that it will require more frequent maintenance. i would assume he would spec the field applied finish on the welded areas.
welding galvanized metal is never a "good" idea: at best you destroy the galvanized protection under and around the weld, you produce burning zinc fumes in the welder's face, and you are faced with a (usually unsatisfying!) field "repainting" with a cold galvanizing spray paint that won't match the rest of the finsih.
same with field welding the most common shop finish for outside iron and steel - the powder coating is destroyed by the grinding to clean the surface(s) and the immediate heat of the welding melts and burns the finish around the joint. field "re-powder coating is impossible, and can (at best) merely be approximated by repainting the surface.
what does work for this kind of iron?
i've used a good primer painted very, very carefully into all parts of the joints (under, around, above, and in the cracks and crevices of the joints) and then followed by two coats of spray paint: each coat coming from a 45 degree angle from the horizontal from the four quarters of the small iron: left front down, left rear down, right rear down, right front down, (turn the piece over or spray up from below) and repeat: left front up, left rear up, right rear up, right front up.
figure outside steel painted with a good primer (a galvanizing one is ok - just not as good a hot-dipped galvanizing process) wil last ten years. the cover paint to protect the primer from the sun and from the weather - and the primer is to protect the steel surface.
a bad paint job? 2-4 months.
and, of course, you can build the trellis (which has relatively little "mass" of the expensive stainless to buy) out of stainless steel. you will find little "wrought iron" or cast iron fittings available from the standard catalogs - which are al and cast iron and steel (for figures, points, ends, caps, etc.) but a trellis might not need them.
i disagree that galvanized steel should not be field welded. sure, there are issues which make it a bit different from welding black steel, but it is common practice, with satisfactory results, at least structurally. with closed sections, galvanized protection has the advantage of coating the inside of the section.

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