The Importance of Adequate Venting and Draining
The purpose of venting and draining is to ensure the part can be immersed and withdrawn from each stage in the process in a safe, efficient and effective manner. The pre-treatment solutions from the galvanizing process and the molten zinc must be able to flow freely into and around the article so that contact is made with all surfaces and all air is displaced. In turn, the pre-treatment liquids must then be able to flow out of and run off the part.
Galvanizing open sections can usually be achieved without too much difficulty provided the general Welding Guidelines and the few simple rules for allowing the run-off of liquids are observed. However, hollow sections and prefabricated assemblies will always require additional attention by the detailer due to their disposition to create pockets of trapped air which will inhibit the flow of molten zinc contacting the inside surfaces preventing the galvanizing coating to form. In addition, trapped air and gasses will turn to superheated steam running the risk of a messy and dangerous explosion. So, it’s vitally important that all enclosed areas are adequately vented allowing the flow of liquids and the expulsion of gasses.
Galvanizing Open Sections
Provided the necessary procedures with regard to fabrication and welding are observed, galvanizing open sections such as hot-rolled I and H profiles will present few problems as the profiles are reasonably symmetrical with the ratio between the flange and web thicknesses being within the range of 3:1.
Channels and Angle profiles however are not symmetrical and may tend to bow in the direction of the toes, however, provided the bow is within manageable limits, it can usually be pulled straight during the erection process – if not, it may be necessary to straighten the member in the workshop before delivery to site.
Fabricated beams and girders may present their own difficulties. The unique web and flange steel thickness in modern girder design, coupled with weld lengths which can exceed 20 m. must be considered by the design team and detailer. Here are a few guidelines for effective girder design:
- The flange-to-web thickness ratio should be no more than 3-to-1 to avoid distortion of the web.
- Continuous welding should be used to prevent weld fracture from large forces generated when stress relieving the steel at the galvanizing temperature, or from trapped pre-treatment liquids expanding at the galvanizing temperature.
- Hot-dip galvanizing may accentuate or remove a camber. This is not usually problematic as the diaphragm members attached during construction can be used to draw the girder in to place. This is done by welding or fastening (with hot-dip galvanized connections) the diaphragm steel members to the girder, starting at one end of the girder and working to the other end.
- Stiffeners should be used and they should be liberally cropped to allow for the free flow of cleaning solutions and molten zinc within the web space.
- Any lifting (material handling) of the girder should be at the quarter points to avoid permanent deflection caused by the self-weight of the product.
- To completely support a positive or negative camber and avoid flattening, lay the newly galvanized girder on the strong axis and support with as many blocks as possible.
- The finished galvanized girder should be air cooled and not water quenched to minimize the induced stress from the cooling cycle.
For more information on detailing open sections for galvanizing with suggested details go to ……..
Galvanizing Hollow Sections
Hollow sections including Square, rectangular and circular, including pipes and fabricated box sections can all be successfully galvanized provided the basic rules for fabrication and welding are observed. Successful galvanizing means that all internal and external surfaces are properly coated. If the ends of the sections are left open, this will present few problems as the free flow of the cleaning fluids and molten zinc can be pretty much guaranteed. However, if the ends are closed off due to the placement of end-plates, or if the hollow sections are arranged to form an assembly such as a latticed girder, truss or frame, the part or assembly must be properly vented to allow for the flow of the molten zinc and the displacement of air. Failure to do this will mean the molten zinc will not be able to flow freely into and around the article coating all surfaces and the entrapped air will be expanded due to the superheating creating a pressure that could rupture, or explode the fabrication.
There are guidelines set out by most national galvanizing associations endorsed by ISO, AISC, AWS etc … to assist the detailer and design team providing effective and safe details to mitigate these affects.
For more information on detailing hollow sections for galvanizing with suggested details go to ……..