Engineering the Independent, Austin’s Next Tallest Tower

A rendering of the finished Independent tower superimposed on the Austin skyline. Image: DCI Engineers / Rhode Partners

Since it’s visible from almost everywhere in and around downtown, you probably won’t be surprised to learn that Austin’s next tallest tower, The Independent, is topped out and in a late stage of construction at 301 West Avenue in the emerging Seaholm District. Roughly 80% of the 58-story residential building’s 370 units are spoken for, and the project should deliver in 2019 at an estimated total cost of $300 million.

As you might imagine, the appearance of the so-called “Jenga Tower” is the first thing on everyone’s mind — the architects at Rhode Partners have certainly designed a one-of-a-kind building. But after spending some time with the folks at DCI Engineers, one half of the joint venture with Frank Lam & Associates collectively overseeing the Independent’s structural engineering, I’ve come away with a deeper appreciation of what it takes to keep this tower — which is obviously designed to look more than a little precarious — standing tall.

Today, we’re taking a closer look at two of the building’s most fascinating engineering features, which I’ll attempt to explain as simply as possible due to the inherent constraints of my liberal arts brain.

The Big Boat in the Sky

According to Kris Swanson, principal at DCI, it’s kind of helpful to imagine the Independent as a very tall, square trimaran — that’s one of those boats with two outriggers on either side of the main hull, which stabilize the craft and prevent it from tipping over when it turns. (I’m not gonna come out and encourage you to watch Waterworld, but that’s what Kevin Costner’s boat is.)

A diagram pointing out various design highlights of the Independent. We’ll be discussing several of these. Image: DCI Engineers

The tower is constructed around a central core, which is like the main hull of the trimaran, with four corner columns and eight “mega columns” running up the exterior of the building. The mega columns are what you’d consider the outriggers in this increasingly convoluted boat metaphor — they’re connected to the central core of the building via “outrigger frames” at the tower’s crown.

A rendering of the crown of the building, with the outrigger frames highlighted in red. Image: Rhode Partners

According to Swanson, the use of the outrigger frames primarily helps reduce the lateral deflection of the tower:

“With the main building core being proportionally tall and slender, the outrigger links the core to the outer mega columns, creating a wider base a which helps reduce lateral deflection and enable thinner concrete walls within the core.”
— Kris Swanson, DCI Engineers

When the structure experiences a strong enough gust of wind, it’s inevitably going to bend a little — every tower does — but without this feature, the building would sway more and require a thicker and stiffer central core:

A diagram showing how the outrigger trusses reduce lateral forces on the building from wind pressure. Image: DCI Engineers

But there’s an extra wrinkle here compared to other towers, since the Independent is designed with multiple cantilevered floors that shift 19 feet off-center from the rest of the tower — that’s what gives it the jaunty look responsible for the Jenga nickname. The cantilevered tiers of floors are connected to the outer “mega columns” via a system of steel tension rods and compression struts by steel fabricators Macalloy, which are respectively oriented diagonally and vertically, as you can see in the diagram:

Image: DCI Engineers

The weight of the extended cantilevered floors is supported by two-inch diameter tension rods, which are each tied back to the main columns — this enables the long sections of cantilevered floors which switch sides up the building. Each cantilevered tier is connected by six-inch diameter struts oriented vertically up the edges of these sections, such that the floors will move in unison as well as provide what DCI calls a “redundant load path” within the connected elements.

In addition to the off-center residential floors, this design enables the building’s 34th level, known by DCI as the tower’s “amenity truss,” to extend out even further than the other cantilevered tiers, with full story trusses anchored to the side of the main columns supporting the floor — which allows it to be cantilevered out a full 30 feet. If you’re looking for the amenity truss on renderings of the tower, it’s pretty easy to find — it’s the floor roughly halfway up the structure that sticks out further than the rest.

A rendering of the Independent, with the 34th-floor amenity truss highlighted. Image: Rhode Partners

A plan for the layout of the Independent’s 34th-floor amenity truss, with its position in relation to the building’s central core visible. Image: DCI Engineers

Perhaps the most interesting thing about these rods and struts is their prominence in the design of the building’s interior. With many towers, structural engineers are the unsung heroes, their innovations hidden beneath concrete slabs and tucked away on mechanical floors while the architecture they hold up hogs the spotlight.

That’s not the case here. More so than perhaps any other tower in the city, no effort is taken to disguise or otherwise diminish the visibility of these engineering features — in fact, they’re kind of on center stage, and probably help reassure the tower’s occupants of the building’s structural integrity. In the renderings below, you can see the prominence of the rods and struts, plainly visible in the design of the 34th floor amenity truss:

Left: A diagram of the tension rods and compression struts on the Independent’s 34th-floor amenity truss. Right: A rendering of that same amenity truss, with the rods and struts visible in the design of the room. Images: DCI Engineers / Rhode Partners

A panoramic view of the Independent’s amenity truss under construction, with the tension rods and compression struts seen in the other images above visible here as well. Image: DCI Engineers

A rendering of the amenity truss, with rods and struts visible. Image: Rhode Partners

But these features are also visible in the interior design of the residential units in the tower’s cantilevered sections:

In the residential units of the Independent’s cantilevered sections, the building’s tension rods and compression struts are embraced in the interior design. Image: DCI Engineers / Rhode Partners

Keeping these features visible feels reassuring to me — if I lived in one of the cantilevered units, I think I’d appreciate the constant reminder that a team of engineers designed these features with multiple redundancies to keep the building strong. Jenga, this ain’t.

Putting a Damper on It

The second engineering feature I find particularly fascinating in this tower is located at the crown in its mechanical section, right at the top of the central core connected to those outrigger trusses we talked about earlier. Inside the top of the core is a 50,000 gallon water tank used to supply the building’s fire suppression system. The tank is made from modular fiberglass panels reinforced with a structural steel frame — according to DCI, these panels make the tank lightweight, easier to construct, and provide better resistance against corrosion over materials like reinforced concrete.

A diagram of the Independent’s crown and mechanical section, which includes both the damper tank and outrigger struts. Image: DCI Engineers

But this particular tank’s pulling double duty as a tuned liquid damper — also known as a “slosh tank” — that absorbs a certain portion of the energy placed upon the building from heavy winds that might otherwise cause it to sway beyond the comfort level of its occupants. It’s the same principle behind the better-known tuned mass damper that uses the weight of a large concrete block or other heavy object to compensate for sway in the same manner.

Without the damper, under high winds people might notice the motion of the building — it could even make them seasick! Of course, since the tank’s contents might be needed at any time for the tower’s fire suppression system, it being full is actually not a critical feature — structurally, the building would be completely fine if it were drained, though the occupants might not feel so great.

A scale model of the Independent undergoing wind tunnel tests. Image: DCI Engineers / Windtech

Though the building is downtown Austin’s tallest at the moment, new challengers are already on the horizon. As our skyline gets taller, these structures will demand increasingly clever solutions from the engineers behind the scenes — and for now, the Independent might be the smartest building in town.

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