I first discovered the power of architecture in Florida, where my sprawling extended family used to gather for summer beach vacations. The thirty-or-so of us would pack into three or four rented houses for a week of family bonding over card games, beach hangouts, and dinners. Each year a different one of these houses would become The Place to Be.

These gatherings had representatives from every part of the political spectrum, with ages ranging from newborn Baby Liam to “Happy 80th Birthday!” Grandpa Leo. Throw into this mix a few vague, deep-seated grudges based on who bullied whom as children, and you couldn’t get the group to come to consensus on anything. Yet, before Day Three, somehow everyone would have tacitly agreed which house to focus on for that year’s family gathering.  As a kid, I struggled to comprehend what force could have enough power to unify this unruly group.

It was the houses themselves that had this effect. Each year, the group of Zahners had the same goals: be near the action, hang out with your favorite cousins, and play lots of cards.  The house that best accommodate these goals rapidly became The Place.

This simple revelation first opened my eyes to the power of architecture. If the layouts of houses could hold such irresistible sway over a group as vigorously stubborn as my extended family, what other effects did architecture have on our lives?

How Engineers Think about Buildings

I had a structural engineering professor who liked to say, “Engineering design is not ‘How big is the beam?’ but ‘Should there be a beam?’” That is, while a building’s structure is governed by building codes with thousands of prescribed rules, the structural engineer’s main concern is not the mere verification of these regulations. After all, it is much faster and cheaper for computers (or interns) to plow through this litany of requirements. What, then, is left for the structural engineer?

The answer lies in the structural engineer’s most fundamental tool: the concept of a “load path.” The idea is simple: how does a force, exerted somewhere on a structure, make it to the ground without breaking anything? For example, consider the load path for the weight of a Zahner cousin (let’s go with Beth) playing cards in one of those Florida houses. Beth’s chair is sitting on a hardwood floor.  The boards in the floor distribute the weight of her and her chair to the closely spaced wooden beams below. The ends of these beams rest on the basement walls, which in turn sit on the house’s concrete foundations. The foundations bear on the soil below, transmitting the weight of my cousin (and everything else in the house) down to the earth itself. The load path, then, for any force on that floor is:

floorboards

↓

beams

↓

walls

↓

foundation

↓

earth.

Now let us consider a less obvious example: Beth, sitting in the same chair, but this time on the seventh floor of a medium rise building. This building has yet to be designed, so there are many potential load paths that could keep Beth up in the air. She could be sitting in a much taller version of the same wooden house with the same simple load path. Alternately, she could be on a concrete slab which hangs, like the deck of the Golden Gate Bridge, from huge cables supported by even larger steel towers. Both these options, like an infinity of others, are perfectly valid.

However, each choice of load path has its own consequences. There are engineering consequences: a seven story building sees a lot of wind, so what happens in a big storm? Will Beth’s perch behave better in an earthquake if it the floor is lightweight wood or heavier (but also sturdier) concrete? How do you connect one giant beam to another anyways? Then there are architectural concerns: while a seven story tall stone pyramid would be an extremely stable base on which to play Spades, the architect’s vision for the ideal lobby may not call for a blank wall of stone. Finally, monetary cost is almost always a concern to the one footing the bill. How difficult (and thus expensive) will it be to build? Is there a different option that would use less material?

The structural engineer’s job is to select load paths out of this jumble of possibility and consequence. Once a load path is determined, a computer can analyze it and optimize every link in the load-path chain; to use my professor’s terminology, once the engineer has answered “should there?” the computer handles “how big?”.  Thus, the engineer’s primary challenge is developing the inputs: of the infinite number of ways beams, cables, slabs, columns, and trusses could be organized to support Beth, only one load path will best satisfy the owner and architect while still maintaining general compliance with physics. Design, for the structural engineer, is finding that best solution.

Example

Rem Koolhaus and Joshua Prince-Ramus’s Seattle Central Library has evoked a tremendous public reaction since opening in downtown Seattle in 2004. Some hate it (it looks nothing like a traditional library: way too modern) and others love it (it’s very modern: it looks nothing like a traditional library!), but no one doubts that it makes a powerful impression. What role did the structural engineer play in the creation of this Seattle landmark?

With the Seattle Central Library, the design team sought to provide Seattle with a building that, like the best of those Florida houses, would draw people in and provide them with a focused place for interacting with each other. The centerpiece of this strategy of community involvement is “The Living Room.” The Living Room is the first place you come to when you enter the library from 5th Avenue. The architects envisioned the Living Room, like the common rooms of those Florida houses, as the center of the action.  Unlike the common rooms of those Florida houses, this Living Room covers nearly an entire city block, with seven floors packed with books and people above.

However, when you first enter the Living Room, you are oblivious to all this weight. In fact, all you see is open space and natural light from the glass walls and central atrium, with only four or five thin columns scattered around the edge of the building. How was the design team able to achieve this sense of openness and light given the physical mass and weight of the floors above?

The answer lies in creative load paths. Let’s move Beth’s card game to one of the study tables hidden within the stacks of the “book spiral” section of level seven. As before, the weight of her chair goes from floor to beam. However, at this point the load path diverges: instead of simply moving down walls to the foundations, the force in the beams transfers to gigantic four-story trusses ringing the outside of floors six, seven, eight, and nine. These trusses gather up all of the load from these four floors and concentrate it into one or two points on each side. Slender steel columns, sloped to match the facade, support these few points, carrying the concentrated load down through the atrium and past several other floors to the foundations beneath the Living Room. By supporting the whole middle section of the library on those few steel columns, the engineers managed to essentially sneak all of the weight of the shelves above down through the Living Room without hurting the architect’s vision of openness and light.

Developing this unusual load path was an iterative process involving many rounds of collaboration between architects and engineers. The architects knew what they wanted: an airy central space with the stacks hidden above. Engineers are problem solvers: they knew there was a way to turn the architects’ vision into a buildable reality. As the design evolved, the engineers helped inform the architects on the many trade-offs presented by different load paths. For example, the four-story trusses that encircle the middle floors only work if the edges of those floors all match up. These trusses help reduce the number of columns going through the Living Room so, since a minimally obstructed view from the Living Room was one of the architects’ first priorities, they chose to align the middle floors. As the design progressed from rough concepts to its final, ready-for-construction state, the architects and engineers made hundreds of these types of decisions.

This is a gross oversimplification: I didn’t even begin to describe all that thought given to what happens when an earthquake hits and all of those books start shaking. If you’re ever in Seattle, swing by downtown and I’ll give you a full hour-plus tour of how careful engineering helps achieve the architects’ goals for the library, sometimes by hiding the structure and other times by putting it on center stage. Until then, consider this the thesis of this essay: architecture has power, the realization of ambitious architecture often requires talented engineering, and thus the role of the structural engineer is to facilitate the creation of powerful spaces.