Building Rainier Square, Part 1: The Revolutionary Core

by Jordan Gevers, Marketing Manager

If you’ve ventured into downtown Seattle recently, you may have noticed some big changes happening at 5th and Union, home to the iconic tapered-base Rainier Tower and the rapidly-rising mixed-use tower known as Rainier Square. At 850 feet, Rainier Square will be the second tallest tower in Seattle. It will certainly change Seattle’s skyline, and it just might change the way tall towers are built around the world.

This project, one of the first in Seattle’s “Metropolitan Tract” in decades, represents a turning point in the surrounding neighborhood. In an area known more for its office towers than residences, the luxury apartments in the upper third of the building will provide homes for hundreds of new residents and the grocery store in the base of the building will be the first in the area.

Over the next several weeks, we’ll explore Rainier Square’s groundbreaking construction in more detail, how Lewis shepherded the idea from a theoretical concept to a tangible, buildable model, plus what this could mean for the built environment.

Rainier Square is quickly rising next to Seattle’s iconic Rainier Tower. Photo: NBBJ/Sean Airhart

So, what’s different?

When it comes to revolutionary construction concepts, Rainier Square keeps its secrets hidden behind a striking, multi-faceted curtainwall. When it’s complete, even the residents won’t notice a difference between this building and a typical one. So what, exactly, is Rainier Square’s secret, and why is it so important? The answer lies deep inside the tower: a revolutionary building core, the first of its kind ever built in a high-rise.

Most modern office high-rises in Seattle are built with a concrete core surrounded by steel framing. While this is a widely accepted design for office buildings across the country, the system has its drawbacks: perhaps the main one is that the concrete core must cure for a period before the steel structural system can be attached to it. The building cannot be built any faster than the amount of time it takes the concrete to cure, which is why it is typical to see a “leading core,” where the core is poured days in advance of the steel framing. Until now, this was  the “norm” when it came to constructing a high-rise.

West 8th, which Lewis completed in 2009, is an example of “leading core” construction, common among high-rise towers.

Rainier Square’s core, engineered by Magnusson Klemencic Associates, is a completely different, first-of-its-kind system, known as a “concrete-filled composite plate shear wall” (CF-CPSW). Instead of a giant tower of concrete, this core is made of steel and concrete sandwich modules assembled like building blocks. Each prefabricated module consists of two 1/2 -inch steel plates connected by dozens of rods. These plates are stacked vertically between a series of steel columns, welded together and filled with a special concrete mix to form the structural core of Rainier Square.

These modules are fabricated at Supreme Group’s facility in Portland, Oregon and trucked up to the site in the morning. Crews from Lewis and The Erection Company hoist them into place like building blocks, welding along the seams. Once the columns and plates form a complete ring, they’re filled with concrete.

Rainier Square’s innovative core allows the rest of the structure to be erected simultaneously, speeding up construction time significantly.

Rainier Square’s innovative core allows the rest of the structure to be erected simultaneously, speeding up construction time significantly.With typical concrete core construction, crews must wait for the concrete to cure and strengthen before attaching the surrounding steel structure. However, the CF-CPSW’s steel modules provide enough stability that crews can begin attaching the surrounding steel structure at the same rate the modules are placed and filled.

Since the waiting period for the concrete’s curing process is eliminated, Rainier Square will top out nine months faster than a traditional concrete-core building. This translates into serious financial savings during construction, not to mention a faster speed-to-market. And for all the people looking for a new home in Seattle’s tallest residential tower, their wait just got shorter.

In our next installment, we’ll explore how Lewis helped shepherd this revolutionary concept into reality, including solving some of the perplexing logistical issues this modular system presented.