The Big Bang, at least the British version of it, took place on Oct. 27, 1986, when Prime Minister Margaret Thatcher deregulated the London stock market. Very swiftly after that, the City of London morphed from an insular old-boys club into a modern, global financial center. Foreign banks rushed in, and they needed somewhere to go.

Broadgate, which had broken ground a year earlier, became that somewhere. With 14 buildings planned for a 32-acre site east of the Barbican and St. Paul’s, it was the largest-ever office development in Britain at the time. Working with Rosehaugh Stanhope Developments and British Rail (BR), Arup designed the initial master plan and several buildings, while Skidmore, Owings & Merrill (SOM) designed the rest.

Broadgate in 2010
Dennis GIlbert/View Pictures Broadgate in 2010

The main challenge facing SOM was that the land, then owned by BR, was covered by tracks going in and out of Liverpool Street Station, a major commuter hub. The trickiest part of the site was the “throat” of jumbled tracks on the north side of the station. The brilliant way that SOM solved this problem in building the Broadgate Exchange House—by spanning the tracks with parabolic steel arches, which it happily exposed—has earned the firm this year’s Twenty-Five Year Award. The award is given each year to a building that has stood the test of time, and the Exchange House—completed in 1991—is the first building outside of the United States to receive it.

The issue of Architecture (the precursor to ARCHITECT magazine) that featured the Broadgate project. (See more Architecture images below.)
April Greer The issue of Architecture (the precursor to ARCHITECT magazine) that featured the Broadgate project. (See more Architecture images below.)

Here the key players recall how they created this unlikely icon:

Bill Baker, structural engineering partner, SOM:

 The initial phases of Broadgate were built where the tracks are kind of on a grid. Then the leftover piece was where, when the trains come out of the station, they start to commingle and merge. You’ve got a very complicated set of tracks coming out of there. That’s where the Exchange House is. You couldn’t bring down regular columns and foundations on a regular grid, except on two strips that were 78 meters [256 feet] apart.

Richard Tomlinson, FAIA, retired partner, SOM: I don’t want to oversimplify it, but it was one of those things where the client felt—and we felt, even from an urban-design perspective—that it would be crazy to leave a hole in the site in that location. The client said, “Isn’t there any way we can put a building there?” and we said, “Sure.” The design, that’s all Bruce Graham [who died in 2010]. It was his vision to span the tracks and to make a bridge a building, and a building a bridge.

Baker: If you took this building and hung it as a series of weights, it would drape to exactly this curve. We built, essentially, the natural shape of the structure. And because of that it was very economical. It wasn’t a bunch of hairy transfers and trusses and girders, but these very simple arches. When I first started to do the details, I tended to use a machine aesthetic. Bruce said no, no—it should be a structural aesthetic, a bridge aesthetic. There are no fancy, machine-like connections.

Sir Stuart Lipton, developer, former chief executive of Stanhope Properties: SOM did buildings on Bishopsgate [the road that forms the eastern edge of the site] in a sort of Beaux-Arts style. Local officials were keen to have stone façades. But when it came to Exchange House, in the middle of the site, there was no real vision from the city. So the architect was much freer.

Tomlinson: A building as a bridge over an operating railway station: Something of that magnitude had never been done before. When you innovate, every day is a surprise. The building actually spanned two boroughs. Part of it was in Hackney, the other in the City of London. Each had different building codes; each had its own fire department with its own fire regulations. One of the challenges was getting all these people together and then going through all the testing to prove to everybody, including ourselves 

and the client, that this was going to work the way it was imagined.

Baker: We were working around the clock because of the time difference. At the end of the day, the architects and engineers in Chicago would take a drawing, cut it up into rectangles, fax it to London, and then they’d tape it back together.

Tomlinson: I was leading the project in the London office. Getting FedEx packages every morning, it was sort of like Christmas.

Baker: The construction sequence was a big deal. We had the steel on temporary supports until we could close the arch. The engineers were worried about loads jumping around. We jacked up the entire building 50 millimeters, to be able to control the unloading of the temporary shores. Then we put the building back down again.

Tomlinson: The public realm at Broadgate is extremely important. The plaza covers all the tracks; the building only spans them. It’s a great plaza. Hundreds of people gather there in decent weather. I visited a week and a half ago; it doesn’t look 25 years old.

Lipton: It looks almost new today. The finishes of the building have lasted well. The façade looks terrific. It looks good because they knew how to design it.

These interviews have been edited and condensed.