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

Lifting device A steel casting was one of two parts that made up a material-handling system used to lift coils of steel. The force F was applied to the lip and was ultimately transferred through the hole on the other end.	The designer charged with converting the casting to a weldment first proposed what was essentially a carbon copy of the casting design, consisting of two lips (c), an intermediate member (b), and two uprights (a), with holes to connect to the hoist. The force F was applied to the lip, transferred through the complete joint-penetration (CJP) groove welds between components (c) and (b), through the thickness of (b), through CJP groove welds between (b) and (a), and ultimately out of the hole into the supporting hoist. Since the weldment was to be made of high strength, quenched and tempered steel, matching strength E110 filler metal was required. In this design, each of the four CJP groove welds is critical, each transferring half of the applied load F; also, member (b) is loaded in the through-thickness direction.
A far superior design uses a continuous piece (a) for the lip and the upright, eliminating the through-plate. While the rest of the geometry remains essentially the same as that of the casting, the load is not transferred through welds, nor through the thickness of the steel. Retaining the geometry could raise questions about the protrusions (b) on the sides of the assembly and what loads, if any, they transfer to the continuous piece. In this design, the protrusions function simply as "bumpers" that protect the steel coils from damage. No significant loads are transferred through the welds that join the bumpers to the support (a). Similarly, the item (c) between the two supports functions as a spacer and is only lightly loaded.	The first design called for large CJP groove welds on either side of the column member. This concept might have been sensitive to through-thickness tearing.
	In the detail ultimately employed, the plate used for the cantilever runs through the column, eliminating through-thickness loading. The column, which is in compression, connects to the cantilever flanges with a simple partial joint-penetration (PJP) groove weld. Since the cantilever is tapered, a bent plate accommodates the desired shape.

Steel is not purely homogeneous, and as it is rolled, it develops "directionality." The material typically exhibits its best ductility in the longitudinal direction (that is, parallel to rolling). In the through-thickness direction, it usually has the lowest ductility. Designers should be aware of this, and wherever practical, avoid transferring major loads through the thickness of the steel. But when through-thickness loading can't be avoided, the application may require special material-property and fabrication techniques. Often, anticipating the problem eliminates the need for such special techniques simply by avoiding the transfer of force through the thickness of the material.

The final design eliminates loading in the through-thickness direction. In addition, it replaces large, matching strength CJP groove welds with small fillets using under-matching filler metal. The resulting use of thinner materials and smaller welds reduces overall welding costs.

Support for stadium seating
The challenge of supporting seating in a stadium provides a structural example of the same principle. The design of a large sports stadium featured a cantilevered beam to support spectator seats. Obviously, the connection of the cantilever to the column was critical, and the designer considered the possible consequences of transferring the force through the thickness of the steel.

Of course, there are situations where through-thickness loading cannot be avoided. With proper care, it can be safely accommodated. But often, the best way to minimize a potential problem is to eliminate it, in this case, by avoiding the transfer of force through the thickness of steel.

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