A Brief History of One Prudential Plaza - AISC HQ

AISC recently hosted a social event during the Council on Tall Buildings and Urban Habitat's 2019 World Congress in Chicago. At the event, Sherwin Asrow, who performed the lateral force analysis for One Prudential Plaza, shared his experience with AISC's own tall building. Read on to hear Asrow's take on One Prudential Plaza, which has been home to our headquarters since 2016.

By Sherwin P. Asrow, FASCE 

The Prudential Building at the north side of Randolph Street just east of Michigan Avenue was designed in 1947. John Roche, the chief structural engineer, had previous structural design experience in the design of the Chicago Civic Opera Building. The Prudential Building architect, Sigurd Naess, was also responsible for the architectural design of the Old Main Post Office Building over Ida B. Wells Drive when he was with Graham, Anderson, Probst and White. 

The building was built over the ICC railroad tracks. Each column and supporting caisson had to be placed so as to clear the tracks. Each caisson plot was separately purchased from the railroad. Lorry Johanson prepared a site plan for this so that about 670 separate lots could be purchased as well as a narrow basement extending north-south from Randolph Street to Lake Street for the  mechanical equipment and elevator shafts. Johanson went on to become the State of Illinois Architect. Roche drew on his experience with air rights issues at both the Civic Opera Building and the Daily News Building west of the Chicago River on Jackson Boulevard.

The façade of the tower was designed by an architectural designer that had also designed the façades for the RCA Building (now known as 30 Rockefeller Plaza) in New York City and is quite similar. The façade uses limestone piers with vertically pivoted windows and metal panels between the piers. The windows were opened to be washed and were not used for ventilation. The windows have since been replaced due to leakage of the gaskets and are now washed using scaffolding.

Sherwin Asrow performed the lateral force analysis for One Prudential Plaza that took almost three months. The analysis followed the portal method with a slide rule (pictured) and mechanical calculator. The steel framing system above the Plaza Level was conventional beam and column framing. The floors used metal deck and concrete topping. That was unusual for tall buildings at the time. The HH Robertson deck used had hexagonal cells and the cells were used for electric, telephone, and communication lines and were often referred to as "electrified". 

The east wall of the tower needed to be out of square about 10 degrees in order for the columns to land on a transfer girder at the Plaza Level. The transfer girder was turned about 10 degrees in order to be supported on the columns between the tracks. All of the columns to grade below the Plaza Level were located between the railroad tracks. At the Plaza Level, transfer girders were used to transfer the load of columns above to the columns between the railroad tracks. Unique to the design of the building is that the lateral loads and vertical loads of the tower were transferred at the Plaza Level framing elevation. The lateral loads were then distributed to the columns below Plaza. The amount of the lateral load on the column below was based on its stiffness and proximity to the lateral center of gravity of the lateral load on the tower. There was a north-south area without train tracks that was purchased and used for a basement. The two-level basement was used for air conditioning systems and other equipment. The basement also provided a lateral bracing system for the building. The excavation extended into the hard clay that was below the Lake Michigan water level. In order to provide cooling water for the air conditioning system, easements were obtained to extend water lines to and from the Chicago River about two blocks to the north.

During the installation of the bracing system for the basement, the contractor didn't install the horizontal members properly and the east side of the vertical sheet pile bracing moved inward on the east side of the excavation. The hand-dug caissons had been installed down to rock at about 90 ft on the east side of the excavation. The upper part of about 10 caissons cracked at about the bottom of the basement excavation as a result of the movement. Sherwin worked with Professor Ralph Peck of the University of Illinois (an expert in soil mechanics involved with the installation of the Chicago subway tunnels), to inspect the bracing system and evaluate the means of jacking back the bracing that had moved. 

After the sheeting had been restored to vertical, the upper part of the caissons were removed and replaced after reinforcing bars extending to the top of the caisson were embedded into the caisson below. This experience resulted in a Chicago Building Code requirement that all caissons thereafter would have full-height vertical steel reinforcing bars to prevent a crack from occurring.

During the construction of the building, Sherwin Asrow would visit the building to observe the work. The building was the first high-rise building built after World War II. In one instance, he rode the outside construction elevator up to the top of the 40-story building and wasn't aware that the workmen had left and had to walk down the unfinished stairs to grade.

After construction was initiated, an observation deck at the top of the building was added and Sherwin designed the framing for this addition. In addition, WGN negotiated to install a TV broadcast pylon atop the building. The building was under construction and he was given the responsibility to design and analyze the pylon for its natural frequency. This was important in that wind at low velocities could cause movements that could cause the pylon to fail. He was able to do this using the numerical method that was taught in graduate school by Professor Nathan Newmark at the University of Illinois. When the steel shop drawings were submitted, a review of the shop drawings indicated that the steel for the pylon anchor bolts that had already been sent to the site had a lower strength than that specified. Erection of the pylon was halted until such time as the correct strength steel for the anchor bolts had been delivered to the jobsite.

While Sherwin was on another project site in Peru, Indiana in 1952, he received a call from John Roche regarding the WGN pylon atop the Prudential Building. The American Bridge Co. that was fabricating and erecting the steel said their engineers had checked the pylon. They determined that the natural frequency was such that catastrophic movements would occur at fairly low wind velocity. John Roche sent Steve Tang, who had been employed in the Chicago office, down to Peru, Indiana to recheck Sherwin’s calculations. It was determined that Sherwin's calculations were correct and, in reviewing the American Bridge calculations, it was determined that they had forgotten to divide their result by 2π. The American Bridge result was therefore off by a factor of 6.28.


The American Institute of Steel Construction would like to extend a heartfelt "thank you" to Sherwin Asrow for sharing his stories with us both here and in person at the AISC social event during the Council on Tall Buildings and Urban Habitat's 2019 World Congress in Chicago.