In the midst of negotiating a 50,000-square-foot office lease at 55 Hudson Yards, executives at The Related Cos. were hit with what should have been an impossible request. Prospective tenant Silver Lake, a $24 billion investment fund, wanted to add an outdoor terrace to the space it was looking to take in the under-construction building.

Making such a late-breaking modification would have been a nonstarter if the 51-story tower were being built from steel, the material of choice for most office towers. But 55 Hudson Yards is part of a new wave of properties being constructed with concrete. That allowed Related simply to adjust the mold for Silver Lake’s prospective floors and pour in the concrete to create the balcony.

“With steel, it takes months to engineer and fabricate, and once you have it, changes are costly,” said Related’s president, Bruce Beal, who could not comment on the Silver Lake deal, which has been reported but has yet to be signed. “Concrete is great. It can be less expensive [and] faster to build with, and if you need to, you can make changes right up to the moment before the pour.”

Pouring ahead

Several new city office buildings are following Related’s lead with their all- concrete designs, including boutique office projects at 61 Ninth Ave., 40 10th Ave. and 512 W. 22nd St.

The trend reflects the changing tastes of tenants, who have increasingly eschewed the office mainstays of yesteryear, such as drop ceilings, carpeted floors and corner offices, in favor of a stripped-down, industrial feel in which the building’s structural details are often left exposed.

That aesthetic is difficult to capture in a steel building, where structural girders need to be coated with layers of fireproofing and the ceilings are generally made out of corrugated metal.

“An unfinished ceiling in a steel building is pretty ugly,” said Matthew Abreu of Aurora Capital Associates, which is co-developing both 40 10th Ave. and 61 Ninth Ave. “Concrete has an appealing textural element, and there’s an authenticity to it.”

Of course, there’s another reason concrete has caught on: It’s generally cheaper to use, in large part because concrete pourers are less expensive to hire than steelworkers, especially on high-rise projects.

At the 12-story 61 Ninth Ave., however, concrete was actually slightly more expensive to use, Abreu said, but its benefits outweighed the additional costs. Concrete floors are thinner than steel ones, allowing for higher ceilings, which many tenants will pay a premium for.

Concrete has long been used for ultratall residential towers but not high-rise office buildings, because office floors have to bear the weight of a greater number of people and heavy equipment, such as commercial-grade heating systems. Steel is roughly three times stronger than concrete per pound and can allow for fewer structural columns on individual floors than in a commensurate-size concrete building.

But the gap is closing. In the past decade chemical additives have tripled the strength of some concrete mixes. At 55 Hudson Yards, Related is using a technique in which steel wires are inserted into the frame and tightened to high tensions, strengthening the structure and allowing for columns to be spaced farther apart.

“It used to be that you couldn’t get enough of a span between the columns with concrete,” Beal said. “You can solve for it now.”