High-Rise Adapts To Lesser Foundation

Structural engineer prescribes five-story A-frames to carry tower without reinforcing foundations and disrupting rail traffic

Peter Reina/ENR Johnson had to rethink design when building height grew.

Now virtually complete externally, London’s Broadgate Tower owes its existence substantially to the engineering of its lead structural designer Ron Johnson, an associate partner with the project’s architect/engineer Skidmore, Owings & Merrill LLP, Chicago

Six boldly expressed, five-floor-tall A-frames allow the 35-floor tower to rise above an 8,000-sq-meter structural raft that was designed for a much lower building without disrupting railroad traffic beneath as it enters Liverpool Street station.

Told by the owner in the 1990s to plan for low-rise construction, Johnson initially designed the raft for various possible configurations. Later, with the raft built, the requirement changed to a 165-m-tall tower. Johnson’s challenge was to make that possible without having to reinforce existing foundations within the rail tracks, a task that would be hampered by access constraints

Taking increased loads off the raft with A-frames “was basically the first thought I had on how to solve the problem,” says Johnson. And the ability to use the A-frames’ external legs to roof the space between the tower and its new low-rise neighbor “caught people’s imagination,” adds Richard Elliott, construction director of owner British Land Co. Plc.

Designing buildings over railroads “is highly influenced by the sequence of construction,” says Johnson. “Some structures may become impossible because of the sequence they have to be built in,” he adds.

Designing over railroads has threaded through Johnson’s career since he worked in SOM’s team on Chicago’s McCormick Place convention complex in 1985. He recently had moved to Chicago after six years at SOM’s Denver office, where he acquired his engineering skills.

Multimedia  View Award of Excellence Luncheon and Acceptance Speech of Ron Johnson April 3, 2008

Johnson turned to structural design a year after gaining an architectural master’s degree at University of Illinois at Urbana-Champaign in 1977. “It became clear I was a better engineer than architect,” he says.

The railroad theme continued when Johnson began work on a series of major buildings at London’s Broadgate development over Liverpool Street station. Broadgate has consumed a large part of Johnson’s professional time over the last two decades.

Due for construction completion next July, the Broadgate tower ranks among Johnson’s structurally most difficult and favorite jobs, he says. Adds Elliott: “Many projects don’t give the opportunity for structural engineering skills to be so stretched.”

Related:

Slideshow: Building Broadgate's Five-Story A-Frames