Creating 9 miles of seating risers in the first level of the Hubert Humphrey Metrodome in Minneapolis presented some unusual forming problems. No suitable ready-made forming systems were available for rent, and those available for sale were too costly and could not cast the stadium's corner bays. Though they could cast several feet of riser at a time, standard riser forming systems ran on tracks and could only form straight risers, not corners. Setting forms by hand would have required casting one riser at a time all the way around the stadium. This would have been not only time-consuming but difficult.
The solution was a form assembly that could cast five risers at one time. The form for each riser was made of two 6-inch aluminum beams butted together side by side and faced with 3/4-inch-thick birch plywood. Five such forms were connected together in stair-step fashion by two strongbacks. Each strongback was made from two 8-inch aluminum channels. The strongbacks were attached to each form by steel angles bolted to the forms. To accommodate the height variation in risers, from 11 inches at the bottom of the grandstand to 14 inches at the top, each steel angle had a series of holes that enabled the forms to be raised or lowered.
The first riser and walkway were formed with hand-set wooden forms. High early strength concrete was used so the single riser could withstand the load of the five risers cast above it the following day. The remaining risers were cast with regular, 4000-psi concrete made using a water-reducing admixture. All concrete was pumped. Workers placed 7 lineal feet at a time, working from the bottom riser to the top riser. Any uplift on the forms created by the concrete pour was resisted by the weight of the forms.