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Fillet vs. Groove Welds for Plate Girder Web-to-Flange Connections
Most plate girders are designed using fillet welds at the web-to-flange connections, but some engineers choose groove welds. Designers should be aware that while either type of weld will work — both offer the same quality — in the author’s experience, groove welds can require more than 20 times as much effort as fillet welds. This effort translates into significant time and cost. Hence, in design situations where fillet welds will work, groove welds are wasteful, and fillet welds are the prudent choice.
Preparation for Welding
Groove welds require many more steps to make than fillet welds. Unlike fillet welds, groove welds require bevels, which take considerable effort to make, particularly over the length of an 80- or 100-ft girder. Bevels require cutting, grinding, and further finishing to produce a surface suitable for welding.
Complete joint penetration (CJP) groove welds require back gouging to ensure complete penetration at the root of the weld. Back gouging first involves air carbon-arc gouging, a slow, tedious, and loud process in which a carbon-graphite electrode melts the metal at the weld root while compressed air blasts the molten metal away. Gouging is performed on the back side of the joint to reach sound metal from the front side and to produce the proper root opening for the back-side passes. Afterward, the joint is ground and further finished to produce a surface suitable for welding and to ensure that the groove created by back gouging is at the proper angle to facilitate access by the welding equipment.
Following back gouging, fabricators may choose to perform surface nondestructive examination (NDE), such as liquid penetrant testing (PT), to ensure the back-gouged surface is sound before back-side welding begins, especially if the finished weld is to be tested, as is common in the bridge world. This surface NDE is optional, but experience shows that not performing a very close inspection of a back-gouged joint can result in failure to reach the front-side weld, which can result in incomplete fusion at the root of the finished weld.
Steps associated with PT are as follows:
- Clean the steel with PT cleaner.
- Spray the penetrant and wait to ensure it
penetrates any discontinuities. - Wipe away any excess dye.
- Spray the developer.
- Visually inspect the developer.
- Clean the developer in preparation for welding.
If anything is discovered with the PT, first remediate the condition and recheck the remediated surface with PT. This process requires three different products applied over an 80- or 100-ft joint.
By contrast, preparation for fillet welds does not require beveling, back gouging, or back-side surface NDE.
Welding Processes
Beyond the considerable effort required to prepare and check long web-to-flange joints for groove welding, there are the number of weld passes to consider. For long girder joints such as web-to-flange welds, including both groove and fillet welds, most plate girder fabricators choose submerged arc welding (SAW). Submerged arc welding is advantageous because it uses larger wires and higher heat inputs than other processes, such as flux cored arc or gas metal arc welding. This means that long welds, such as girder web-to-flange welds, can be accomplished more efficiently than with processes that use smaller wires.
Even with SAW, filling a web-to-flange groove can require one to two dozen weld passes, or more, depending on the web thickness. As webs get thicker, grooves get larger. Thus, for groove welds, the number of weld passes increases significantly with web thickness. These passes must be carefully produced to ensure weld soundness and to avoid defects such as incomplete fusion, porosity, or trapped slag. Fillet welds require far fewer passes. Most plate girders have 5/16-in. fillet welds, which can be made in one pass. Replacing these fillet welds with CJP groove welds can increase the number of weld passes by ten or 20 times, depending on the web thickness. Further, given the high volume of fillet welds used on plate girders, most fabricators have equipment that produces fillet welds more efficiently than groove welds.
Weld Quality
Although some engineers may perceive otherwise, the quality of fillet and groove welds used in web-to-flange girder welding is the same. There are several reasons that this is the case.
First, regardless of weld type, it is the fabricator’s responsibility to produce the weld in conformance with the contract requirements. In this sense, regardless of weld type, the welds are made to the contract requirements and thus, equally, satisfy quality requirements and expectations.
Second, AWS D1.1, Structural Welding Code — Steel, and AASHTO/AWS D1.5, Bridge Welding Code, define weld quality requirements for plate girder performance, including requirements for welding practices and procedures, weld design, visual weld acceptance, inspection, and, as applicable, NDE and associated acceptance criteria. The codes have different requirements for fillet welds and groove welds, as appropriate, given the inherent differences between these two weld types. In both codes, these requirements are set at the level required for performance, as has been established over time in the structural welding community. Therefore, conformance with code requirements for both fillet and groove welds ensures that web-to-flange welds meet the required quality.
Third, the fact that groove welds can be tested with ultrasonic testing (UT) and fillet welds cannot does not mean the quality of groove welds is fundamentally better than that of fillet welds. The welding codes establish the inspection practices required for each weld type to ensure its performance. The codes are written holistically. Requirements for fillet welds are established with the knowledge that UT cannot be prescribed; for CJP groove welds, requirements are established with the knowledge that UT can be prescribed.
Groove welds and fillet welds are fundamentally different, and the codes address each differently, including considerations for the application of NDE. Thus, the suitability of CJP groove welds for UT does not improve the quality of groove welds. Although UT cannot be performed on fillet welds, this has been addressed by the codes, which define what is fundamentally required for both weld types.
Conclusion
Welded plate girders can be made with either fillet welds or groove welds at the web-to-flange connections. Service history has demonstrated that both weld types perform excellently and are of equal quality. However, the effort required to produce each type of weld varies widely. Groove welds can take much longer and cost much more to produce than fillet welds. Therefore, if both fillet and groove welds satisfy the design requirements, use fillet welds to provide the best value for the plate girder. Fabricators can produce either type of weld, but if a fillet weld meets the design requirements, using a groove weld at the plate girder web-to-flange connection isn’t the best choice.
This article was written by Ronnie Medlock, vice president of technical services at High Steel Structures LLC and vice chair of the AWS D1 Committee on Structural Welding, for the American Welding Society.