Engineering the Future in L.A.

los angeles 12.23.15, 9:30AM  BY puja patel

One of America’s most famous and iconic bridges, the Sixth Street Viaduct, was an engineering feat in its day and its replacement known as “The Ribbon of Light” is proving to be the same with its inter-related series of geometrically complex forms to create the superstructure.

The original Sixth Street Viaduct was constructed over 80 years ago, in 1932, using state-of-the-art concrete technology of the time and an on-site mixing plant. However, just 20 years after its completion, the concrete began to disintegrate due to a chemical reaction known as Alkali Silica Reaction, causing significant deterioration of the structure. Despite the various attempts at restoring it, a seismic vulnerability study showed that the viaduct in its current state of material deterioration and lack of structural strength was a hazard in the event of a major earthquake. A replacement was called for and it had big shoes to fill.

The bridge spans close to 3,500 feet across the Los Angeles River, the Santa Ana Freeway, the Golden State Freeway, and several railroad tracks and streets. It acts as a vital connection between the growing Arts District on the west side of Los Angeles River and the historic neighborhood of Boyle Heights on the east side. But the bridge is more than just a physical connection; it is an icon. At the time it was built, the bridge was a badge of the city’s aspirations to speed off into the future. Now, featured in hundreds of films, television episodes, music videos, and commercials, the Sixth Street Viaduct, an elevated roadway with its signature set of arches, is a celebrity and a symbol of Los Angeles.

There were varying opinions on what the future of the bridge should look like. Preservationists were still hoping to find a fix for the current state of the structure, others called for the new to be a replica of the original or at the least design that fit in with the language of the other bridges, and others still were looking for something entirely new. To decide on a laudable replacement design, in 2012, the City of Los Angeles opened a competition. The winning design by HNTB and Michael Maltzan Architecture features swooping arches along the entirety of the bridge that create a dramatic effect. It is translating that dramatic effect to buildable geometry, where CW Keller + Associates' capacity to build robust 3d models comes into play.

In addition to the improved roadways, the new viaduct will improve travel by providing safer access for pedestrians and bicyclists travelling in and out of downtown Los Angeles. But the bridge was designed to be more than just a means of travel from point A to point B- it was meant to be a destination. This is achieved by not only adding supplemental programs around the bridge, but by making the architecture of it an attraction with the eye catching arches that connect from one to the next in smooth flowing motions. The design, from Maltzan’s perspective, represented a new way of thinking about the city. This visual spectacle heightened the importance of preserving the integrity of the desired geometric details, including the transitions from one section to the next. With the concept and ambitions in mind, a matchless aid to achieve the physical outcome with minimal discrepancies in the build, within the desired budget, and keeping a reasonable timeline revolved around the generation of a precise, high end 3D model.

CWKA took the effort to build a precise model of the bridge one step further by creating a data driven model using Rhino and a popular parametric modeling plug-in, Grasshopper. This method of modeling gave them an upper hand by providing additional benefits. One being able to not only generate geometry, but having the ability to verify the output against provided data.

After prioritizing the geometric constraints of the project, CWKA set up the parametric model to be built on the set of data in the form of spreadsheets and mathematical relationships provided by HNTB drawings. The Excel tables that drive the model are streamed straight into Rhino, meaning the model is directly correlated to the provided numbers. This method of delivering information to the generated the model allowed CWKA to be confident that the discrepancies in the model were not a result of human error, but due to conflicting input data and predefined geometric relationships.

These relationships were defined by both linking native grasshopper nodes and by creating custom grasshopper nodes. A node has defined inputs that are used to generate the specified output geometry. These nodes are used to fulfill modeling steps in the sequence they are linked in. Each step can be as simple as defining points and planes in space or get more complex to generating specific arcs or compound curves. The ability to generate custom nodes allowed CW Keller and Associates to define portions of the geometry that were very specific to the project. These defined geometries went on to compose relationships that compose the solid geometry of the structural and architectural concrete of the bridge.

In addition to generating geometry, the nodes in this type of parametric model also allow one to analyze the created geometry. This has been of great use in comparing expected outputs to actual outputs to find issues that wouldn’t have been apparent until it was built. It provides the benefit of minimizing the effects of these challenges in early stages before it is a larger problem in the field.

The bridge, located in a highly urbanized area just east of downtown Los Angeles, has specific constraints on its relationship to its surrounding landscape. In addition, the dramatic geometry had very specific constraints to maintain structural integrity. These types of constraints allowed CWKA to prioritize geometric relationships within the model.  

Through understanding these relationships, the project team hoped to find opportunities to streamline engineering and manufacturing of the concrete formwork to help the project fall within budget. A first step to achieve this was the need to minimize variation between the components. This would result in the need for less individual concrete forms and in turn lower the cost. To do this, CWKA will be able to use the set up relationships to find a balance between the original geometry and optimized geometry. By adjusting control points and relationships through the spreadsheets, the changes to the overall form will propagate through the entire model. Furthermore, the extracted information will then help understand the repercussions of the change to maintain performance requirements.

An iterative modeling process to find the ideal final geometry can be time consuming and tedious. This method of modeling opens the door to implementing quick changes and understanding the resulting relationships because any updates to the information results in a real time update of the model. This method of modeling is an ongoing process that is going to allow for multiple quick and effective iterations of the overall form and generation of the concrete formwork. In turn, this allows for the bridge be built in a condensed period of time and with minimal error. 

As an estimated 48,000 cubic yards of concrete, 1,245 tons of structural steel, and 4,200 tons of rebar are being hauled away into history, while the replacement, exceeding minimal requirements of functionality, is coming to form through the use of robust engineering tools and resources.