First constructed in 1963, Poly Canyon’s Blade Structure is the oldest structure originally built in the canyon. Completed by students Steve Gilmore, Mark Haselton, Dan McMann, and Ken Minor, it was one of the first structures on campus to test post-tensioning methods. After many years of deterioration, the blade structure was reconstructed in 2003 by students Ben Green, Robert Pacheco, Elley Arinez, Susan Smilanich, Jeff Messana and Jon Voorhies, with the assistant of Architectural Engineering Professor Nick Watry. Design work required the students to modify calculations to abide by the updated building code and create a more structurally sound project. Three of the original students who worked on the project returned to Cal Poly to help them achieve this, providing the students with advice, funding, and knowledge from experience. In 2006, the Post Tensioning Institute awarded the design team with an Award of Excellence for their design of the new Blade Structure.
The blade structure has gone by many names over its sixty-year existence, including the petal structure, the Cal Poly memorial, “earth forms,” the concrete flower, and a sculptural study in pre-stressed concrete. Originally constructed in 1963 by students Steve Gilmore, Mark Haselton, Dan McMann, and Ken Minor,this structure tested newly-discovered post-tensioning methods. After a few decades in the canyon, the ‘petals’ deteriorated and collapsed. Nearly forty years after completion, the blade structure was rebuilt by students and Professor Nick Watry, as well as three of the students who worked on the original structure. Some of the original fractured petals still remain around the new structure today as a reminder of the original design.
Pre-stressing, Pre-tensioning, and Post-tensioning
The blade structure is the oldest structure originally built in Poly Canyon, as well as one of the first structures on the Cal Poly campus to utilize pre-stressed concrete. Other campus buildings that employ pre-stressing techniques are the Julian A. McPhee University Union and the Housing South parking structure.
Pre-stressing is used to force concrete into a constant compression state by putting an axial compressive load on the given member. Tension cables are used where beams or slabs are expected to be under a high tensile load, often times the bottom of the member, to “pull” the member together into compression. This creates a cambered member, enabling it to resist higher flexural loads by limiting the tension experienced on it. Concrete naturally has a higher capacity in compression than in tension which allows this method to support larger loads.
Pre-stressing concrete, or creating an axial compressive load in the member, can be done two ways. The first, pre-tensioning, involves pouring concrete around stretched, taught cables, then releasing the tension in the cables once the concrete has fully cured. The cables “snap” back to their original shape, similar to a rubber band, and pull the concrete together to form an internal compressive force within the beam or slab. The second option, post-tensioning, consists of casting concrete with small, tunnel-like cavities that will be threaded with cables once the concrete has cured. The cables are kept taught by anchors at each end of the “tunnels.” This method also forces the concrete into compression to withstand greater loads. The Blade Structure in Poly Canyon utilizes post-tensioning methods.
Rumor has it that cattle used the petals on the Blade Structure as a rubbing post, eventually leading the project to deterioration. The exact year the petals collapsed is unknown, but they likely fell independently over time. This is largely due to the original design not accounting for these upward loads. When the students first post-tensioned this structure, they only placed reinforcing steel on the exterior curve of the blades to account for gravity, wind, and potentially seismic loads. As cattle push up against this structure, they create a force in the opposite direction of the loading condition mentioned, essentially mirroring the designed loads. To resist this force, there equally would have to be mirrored reinforcement, or reinforcing steel along the interior curve of the blades as well.
Redesigning the Blade Structure
In 2003, students Ben Green, Robert Pacheco, Elley Arinez, Susan Smilanich, Jeff Messana and Jon Voorhies, along with Architectural Engineering Professor Nick Watry, began the redesign of the blade structure. Three of the alums who worked on the original project, including Steve Gilmore, Mark Haselton, and Ken Minor, returned to help with the reconstruction.
Using more accurate post-tensioning information and new materials, the group revised and edited the earlier calculations. Unlike the original structure, the new blades were designed as a catenary arch, combining both structural performance and artistic quality. A catenary arch is the unique shape an arch takes when supporting only its self-weight (one of the most famous examples of this is the Gateway Arch in St. Louis, Missouri). This curvature enables the petals in the blade structure to be in a constant compressive state without any internal tension forces. However, the design team still included post-tensioning methods in the renovation of the structure for safety purposes and to keep the vision of the original design. In 2006, the students won an Award of Excellence at the Post Tensioning Institute Awards for their design of the new Blade Structure.