Construction of Assembly Hall (Champaign)

The construction of Assembly Hall followed the process used for building any other building. The construction crews first built the foundation, then built the walls and supports for the roof, and, finally, built the roof. The Assembly Hall became known as an engineering marvel because of the unique method used to construct its roof. At its opening, and for the next few years, it was the largest dome structure in North America, a title that was retained until the Houston Astrodome opened in 1965.

According to the article "To Cover This Assembly Bowl: A 400 ft (120 m) Prestressed Saucer" by Engineering News-Record, the total weight of Assembly Hall is carried by 48 massive buttresses to a 16 feet (4.9 m) wide and 3.5 feet (1.1 m) thick concrete ring foundation, with a 109 feet (33 m) radius. The construction crews first dug down 15 feet (4.6 m) from ground level and poured the concrete ring on high-density, hard gray clay. The center floor of the stadium is 24 feet (7.3 m) below ground level, however the contractors decided not to excavate all of the dirt before the roof was constructed. This decision gave the construction crews more surface area to work on, and decreased the height of scaffolding used when forming the concrete roof. After the roof was completed and the scaffolding was removed, the rest of the soil was excavated through the building's entrances. By building the structure into the ground, the top row is only 36 feet (11 m) above the ground line. The construction of Assembly Hall required excavation of 60,000 cu yd of soil. Most of the foundation: ring footing, 48 buttresses, and columns and walls of the circular concourse, was completed in 1960.^[1]

Construction of the roof started with pouring the concrete. After the concrete finished curing, construction crews post-tensioned the concrete roof to its final strength.

After the buttresses were completed, the construction crews could then begin building the falsework on which the concrete would harden. While the concrete was drying, the roof was supported by a central steel tower, the buttresses around the periphery, a circular timber tower about 135 feet (41 m) from the center, and two movable timber scaffolds at the midpoints of 120 feet (37 m) and 60-foot (18 m)-span joists.^[1] These supports totaled at 800,000 square feet (74,000 m²) of wooden scaffolding.^[5] To aid in construction of the roof, a temporary steel tower was erected in the center of the stadium. As mentioned in the foundation section, the contractors decided not to fully excavate the interior of the building in order to reduce the required height of the steel tower and allow more surface area for construction crews. This decision decreased the tower height to only 100 feet (30 m) thus decreased cost.^[1]

According to the article "To Cover this Assembly Bowl: A 400-ft Prestressed Saucer, the roof was cast in 24 segments of 15° each. Each segment of the roof is 52 feet wide at the rim, and 6.5 feet wide at the center. The central ring beam, or very center of the dome, is a heavily reinforced 4 feet thick concrete slab that contains 170 cubic yards of lightweight concrete. The entire roof used 2,880 cubic yards of light weight concrete that weighed only 105 pounds per cubic feet."^[1]

Once the concrete had finished curing and dried, the top of the roof was painted with three applications of plastic material for water and weatherproofing.^[1]

The dome shape of the roof, by design, causes both vertical and horizontal reactions or forces on the buildings walls and monoliths. In order to make Max Abramovitz design a reality, a relatively new post-tensioning technique was required. The post-tensioning techniques required for the construction of the Assembly hall were first developed for Titan missile base construction.^[8] Felmley-Dickerson Co sub-contracted Preload Co. of New York City to post-tension a concrete ring girder around the perimeter of the dome.^[1] Preload Co. borrowed a special horizontal-wheeled tractor from the missile silo work to wind the steel wire around the dome.^[5] The wrapping machine required some slight modifications for this project.^[8]

In order to post-tension the concrete roof, the construction crews used the wrapping machine to wrap the edge of the roof in miles of steel wire and pull the wires extremely tight. The effect is similar to a rubber band. The rubber band (the steel wire) wants to constrict into a smaller circle, while the roof of Assembly Hall pushes radially outward. The two forces are designed to cancel each other out. In order to create the required stress, the wire was pulled through a steel die that reduced its diameter. The first 75% of steel wire was pulled through a 0.211-in.-dia die to create a 130,000-psi stress. The last 25% of steel wire was pulled through a 0.214-in.-dia die to create a 120,000-psi stress.^[8] The wrapping machine was propelled by a 200-hp motor, along an endless chain 1,260 feet (380 m) long around the circumference of the dome. Average speed of the machine was 8 mph.^[8]

As the wrapping machine traveled around the dome, 2,467 times, the wires were placed in 44 vertical rows and each row decreased from 77 wires on the innermost row to 37 wires for the outer row. Each row was separated by a 1+1⁄8 in. wide and 1⁄8 in. thick steel bar.^[1][5] These spacers were used in order allow each wire to be fully encase in grout, thus creating a permanent bond.^[8]

The construction crews torpedo-spliced spools of wire together to create 600 miles (970 km) of one continuous wire.^[1] Torpedo splicing consists of two half-sections screwed together at the center, with each section containing spring-loaded jaws that engaged the wire. The more tension put into the wires, the tighter they grip.^[8]

After all the post-tensioning was completed the 85 sq. in. of total cross-sectional area of wrapped steel was fully encased in 200 cu yd of grout, creating a permanent bond.^[8] The post-tensioning of the concrete worked so well that the 10 million pound roof was raised 2 in. off of the center tower by the wire structure when tensioning was finished.^[8] The post-tensioning of the roof was completed on March 16.^[8]

• Assembly Hall at Champaign, Illinois
• Assembly Hall Construction | University of Illinois Archives
• uitours.ncsa.uiuc.edu
• Historic Concrete Contractor Projects - Assembly Hall - University of Illinois
• "UIHistories: Buildings: Assembly Hall." UIHistories Project: A History of the University of Illinois by Kalev Leetaru

40°05′46″N 88°14′10″W / 40.096°N 88.236°W / 40.096; -88.236