G450 steel to AS 1397 is a cold-reduced sheet steel with in-line galvanizing. Its grade is 450 MPa yield and 480 MPa tensile strength. It is widely used in Australia for purlins, and is being used to fabricate lightweight portal frames, often by welding. The effects of welding on G450 sheet steel in the heat affected zone with respect to the connection strength were unknown. In the present work, transverse and longitudinal fillet, and flare-bevel welded connections in 1.5 and 3.0 mm G450 steel were tested to failure. The test results are used to check the relevant design rules in the North American Specification for Cold-Formed Steel Structural Members and the Australian/New Zealand Standard for Cold-Formed Steel Structures AS/NZS 4600. It was found that the current design rules are not adequate for certain connection configurations in G450 steel to achieve the target safety index of 3.5. Reduced resistance (capacity) factors are proposed for some connections. It was also found that the current design rule for longitudinal flare-bevel welded connections is unnecessarily conservative. The quality of welded connections in thin sheet steels produced by industry fabricators was investigated. The need to complement the macrotest with the destructive prying test as part of the prequalification procedure for such connections is demonstrated. Part of the results of the present work has recently been incorporated into the North American Specification for Cold-Formed Steel Structural Members with a lowering of the thickness to 2.5 mm above which weld throat failure should be checked. © ASCE.