Omer W. Blodgett, Sc.D., P.E.

The welded detail looks good on paper, but the engineer isn t quite sure: will the welder have enough access to make the weld? A skilled welder, let s call him Joe, is brought in, and declares, Sure, I can do it. With that assurance,, the engineer finalizes the design, confident that the required weld quality will be achieved. The problem is that Joe is an exceptional welder. He can weld anything but the crack of dawn. What will happen when an average welder, or the below average welder, tackles the same job?

Too often, designers specify a joint detail where the skill of the welder determines the strength and performance of the weldment. This should be avoided. It is true some designs demand more of the welder than others, and legitimately so. But there should be a consistent effort to make weld designs as foolproof as possible, so average quality welds are sufficient to provide consistently good performance. When this rule is violated, the result may be a level of quality that varies from shift to shift, day to day, and one welder to another. Obviously, this is unacceptable.

Cover Plate
Figure 1a illustrates a situation in which a cover plate is wider than the beam flange. Let s suppose the weld shown is placed during a day shift. When the whole assembly is turned over for finishing during the night shift, will the welder placing the longitudinal fillet weld tie it in properly to the transverse weld? Or will a crater, concave end, or notch be created? In this case, the designer can (and should) avoid questions of welder performance by using a cover plate narrower than the beam flange as shown in Figure 1b. If the cover plate width cannot be changed, the welds should be interrupted at the corners to avoid these potential problems.

Bearing Support
Figure 2 shows another instance in which trouble can arise when the designer fails to create a foolproof design. Figure 2a depicts the detail used to weld a bearing support for the crank of a press. Here, the outer fiber of the member in bending is made up of a member with a rough flame-cut edge, two fillet welds not tied together, and a weld crater. Talk about a no-win situation. This detail may work

acceptably when the welds start or terminate perfectly, and when the flame cutting is smooth. However, the sensitivity to the skill or the worker goes away when the design is changed to the detail shown in Figure 2b. Now, the outer fiber in bending is a wide, smooth plate with a long fillet weld.

Gear Box
Figure 3 presents sketches of a gear box used in mining machinery. Figure 3a shows numerous locations where welds on opposite sides of a plane were tied together, forming points of weakness. The unit began to break up after only a short time in service. The weld detail illustrated in Figure 3b avoided these problems.

Bridge Girder
A small, existing county bridge needing additional load bearing capacity was being retrofitted. The bridge would not be receiving a new concrete deck at the time of rehab, so access from the top of the bridge was restricted. The County Engineer devised the scheme shown in Figure 4a. Both flanges would be strengthened by the use of cover plates. To the bottom flange, a wider plate was to be attached with a pair of fillet welds made in the horizontal position. To the top flange, embedded into the concrete deck, a split cover plate would be used joined by four fillet welds. Because the top flange was only 1 ¹ 2-in. thick, making the fillet weld at position A would be difficult. Gaining access for making the fillet weld at B was impossible.

An alternative was devised; the split cover plate would be beveled on one edge for clearance around the radius of the rolled shape from which the beam girder was made. The width of the plates was reduced and the thickness increased (in order to achieve the same area as the proposal shown in 4a.) While the alternative required overhead welding, the welder had good access and a quality weld was achieved.

Conclusion
In my lifetime, I have seen scores of situations in which the ultimate performance of a weldment rested inappropriately on the welder s shoulders, rather than on those of the designer. This column has described just a few of the examples I can offer to illustrate my point that welder skill is not a substitute for good design.

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Company, struck his first arc on his grandfather s welder at the age of 10. He is the author of Design of Welded Structures and Design Weldments, and an internationally recognized expert in the field of weld design. In 1999, he was named on of the Top 125 People of the Past 125 Years by Engineering News Record.. He can be reached at (216) 383-2225.