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Design and Construction of CONFLUENCE PARK

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By Andrew Kudless

Photo – Casey Dunn – Views of the competed main pavilion at Confluence Park.

In March 2018, Confluence Park opened to the public in San Antonio, Texas. The product of over 10 years of development by the client, the San Antonio River Foundation, the project represents an important case study in design and construction innovation, especially in regards to tilt-up concrete construction. After several years of development by the client focused on site selection and land acquisition, early design explorations with other teams, and fundraising, the final design team started in late 2014 and was tasked by the client to develop an educational park for the local community. The client’s mission centered on creating a civic space that was both inspirational and aspirational while educating the community on issues of sustainability, water conservation, and the ecology of the San Antonio River watershed.

The core design team was made up of my design firm Matsys (lead designer on pavilions), Lake | Flato Architects (architect of record), Rialto Studio (landscape architect), and Architectural Engineering Collaborative (structural engineer). Working intensely and collaboratively during the initial concept and schematic design phases of the project, this team established the design direction based on critical input with the San Antonio River Foundation. In fact, it was the client’s lighthearted suggestion that the park be so alive when it rained that people would still want to visit it that provided the primary design inspiration to the team. The client wanted the park to celebrate the critical importance of water to the region while also creating a civic space that was also a work of art.

Airborne Aerial Photography / Nearing completion in January 2018, the main pavilion can be seen as a series of undulation arches

Cade Bradshaw and Stuart Allen / Lifting one of the concrete petals into place.

The final design for the park consists of 3.5 acres of native planting, a 2000 square foot multi-purpose building, a 6000 square foot main pavilion, and three smaller satellite pavilions distributed through the park. This article will focus on the design and construction of the concrete pavilion and satellite pavilions. The purpose of the pavilions are to create shaded spaces for school groups to gather throughout the day while they are being led in educational activities by the park rangers. In addition, the pavilions serve the wide public on evenings and weekends in providing shelter for a wide-variety of events such as yoga classes, movies nights, and weddings.

Based on the client’s desire for the project to integrate their educational mission focused on water conservation, the design of the pavilions emerged out of a close study of how many plants in the region form structural efficient doubly-curved fronds or petals that capture rainwater and dew and direct it back to their rootstem. Fairly early in the design phase, the team developed a series of funnel-shape forms that opened to the sky and directed rainwater into a subterranean cistern for use throughout the project’s restrooms and irrigation system.

An additional concern of the client also had a serious impact on the design of the pavilion. After several false-starts on the project prior to the current design team’s hiring, the client wanted the new team to complete the design phase and start construction as soon as possible. Although high-design and fast and efficient construction are often at odds with each other in many other projects, the client’s demand for creating a one-of-a-kind artwork that could also be designed and constructed quickly and cost-efficiently led the design team to explore various modular construction systems where formal innovation could be accomplished through an irregular repetition of identical parts. The team explored a variety of material systems to create the funnel shapes such as engineered wooden curved beams or a curved steel frame and tensile fabric skin, however, through the experience of Chuck Naeve, the lead structural engineer, the team started to explore a modified tilt-up concrete system. Tilt-up concrete provided the modularity that we needed for both speed and cost-efficiency while also simplifying the visual aesthetics and maintenance of the materials and increasing the durability. Although tilt-up concrete panels are traditionally flat, we were confident that we could angle the petal during casting such that minimal top-forms were needed. This reduced the overall amount of formwork while also eliminating the need to rebuild the formwork in a new location each time as would be required in site-cast concrete.

Photo – Casey Dunn – Views of the competed main pavilion at Confluence Park.

A modular series of “petals” were developed that collected rainwater while also providing a significant amount of shade. The goal of geometry was to create a repetitive pattern that used a very limited number of molds while also appearing non-repetitive to the viewer. The design developed from a mathematical tiling pattern called the Cairo tile: a five-sided polygon that tessellates in a way that was far less obvious that traditional triangular, rectangular, or hexagonal tilings. Each of these pentagonal base tiles was further subdivided into five triangles, one for each side of the pentagon. Of these five new pentagons, there are only three unique ones which we named A, B, and C. Petals A and B are asymmetric but are mirrors of each other while C is smaller and equilateral. One of the interesting relationships within this pattern is that pairings composed of either A-B or C-C petals make up structural arches. This relationship allowed for only three concrete molds to make all 28 concrete petals of the main and satellite pavilions. From a structural standpoint, the arches represented a classic three-pinned arch and what looks like a structurally complex form was actually quite simple and straightforward.

Matsys / Digital simulation of waterflow on the roof of the concrete petals. This simulation was used to inform the shape of the pavilion as well as the direction of the broom finish on the concrete.

At this point in the process, the general contractor for the project, SpawGlass, was hired and the design and construction team began to think more intensively about specifically how to fabricate the formwork for the concrete. While SpawGlass initially wanted to use traditionally made wooden formwork, the initial estimates with this technique was far outside of the project budget. The high estimates for wooden formwork were most likely due to the unusualness of the geometry and fabricators being unsure of they would be able to provide formwork that would meet our needs for accuracy and durability. As each mold would be used up to ten times, wooden molds would probably not be up to the task and deform too much through consecutive use.

In my role as an architecture professor at the California College of the Arts, I had previously collaborated on academic projects with Kreysler & Associates, a fiberglass composites fabricator with a long history of working on unusual art and architecture projects. Their estimate for fabricating the molds from a composite made up of fiberglass layers with an interior 2” core of balsa wood was not only within the project budget but was far more accurate and durable than traditionally fabricated wooden formwork. In addition, having worked on many innovative and non-standard projects such as the recent addition to the San Francisco Museum of Modern Art which consisted of an 11-story undulating fiberglass composite facade, Kreysler was well-versed in digital design and fabrication workflows. Using a parametric model I developed, the team worked through several iterations of the final petal geometry in relation to aesthetic, structural, and fabrication concerns. Using a variety of 5-axis and 7-axis robotic milling machines, foam positives could be directly milled from our digital model creating a highly accurate physical copy of the three concrete petals. The fiberglass and balsa were then cast against these patterns to create the final molds were then reinforced with a steel truss substructure which would facilitate the transport and on-site assembly of the molds. Due to the scale of the molds (roughly 27’ high and 29’ wide at their largest), each was broken down into three primary bottom parts as well as several smaller side parts. This facilitated their transport on flatbed trucks from California to Texas as well as the demolding of the concrete parts.

Cade Bradshaw and Stuart Allen / The formwork being assembled onsite after delivery from California.

After arriving on site, the composite formwork was assembled on-site and supported by a timber falsework that held it at the proper casting angle. At this point the rebar was positioned within the formwork in a way that transited from the column behavior of the lower half to the plate behavior of the upper half. Within this mesh of rebar were inserted a field of PVC pipes which acted as the formwork for the small 1” – 2” diameter micro-skylights in the petals. As the thickness of the petal varied from 16” at the column base to 4” at the peak, the PVC pipes had the added benefit of acting as a thickness guide. Stainless Steel embeds were inserted at top and bottom of the mold for eventual connections to the foundation beams and top pin connectors. In addition, four lifting points were inserted and connected to the main structural rebar. A concrete mix was developed that provided the necessary flow and compaction into the lower column area (which also had a fiberglass backform) and the necessary stiffness on the upper angled plate areas to resist sliding down the form. The exposed areas of concrete were then broom finished such that the grain of the finish was aligned with the downward direction of water flowing on the surface.

Matsys / The composite mold showing after adding the rebar.

After about one week of curing, the fiberglass composite formwork was demolded and lifted by crane into either the petal’s final position or a temporary position until its arch pair was ready. Each petal was then connected via an embedded steel plate to the foundation beams at the bottom and to its paired petal at the top via a pin connection. Both of these connection allowed the contractor to adjust the position of each pair of petals in order to produce the desired 4” top gaps between all adjacent petals. Once the final positions of all the petals were established, grout was inserted between the bottom steel plates and the ground slab was cast over the foundation beams with an expansion joint separating the slab from the columns.

Although the fabrication went surprisingly well for a project this non-standard, the design and fabrication teams did encounter a couple of challenges which should be addressed. First, developing the appropriate concrete mix that could both flow into the deep column volume without producing excessive voids while simultaneously being thick enough to resist sliding off of the higher sloped areas on the exposed forms took some experimentation. Luckily, the design of the satellite pavilions provided the perfect test area as they were both smaller and less precious than the main pavilion. After several petals the mix and various other workflows were established to the satisfaction of the client, contractor, and designers.

The other major challenge was the seal between the bottom and edge formwork. Although the formwork was incredibly durable, overtime the joint between these two surfaces gain more gaps and thus some small water leakage which affected the surface finish near some of the lower edges. With more time, a more robust joint could have been developed to further minimize these leakage however the team was satisfied with the overall quality of the concrete. In particular, the contrast between the smooth interior of the concrete (which had been cast against the fiberglass) and the rough exterior (which had been broom finished) created an ideal contrast between the interior and exterior of the structure.

This project represents an exemplary case study in design and construction innovation where all team members worked hard to meet the client’s vision of a one-of-a-kind civic space. Instead of relying on traditional design or construction techniques, each team member rose to the challenge and worked together to develop innovative solutions. The use of a modified tilt-up concrete construction technique resulted in a process that was far less expensive than traditional site-cast concrete and produced a unique geometry and surface quality. Although only open to the public for the last six months, it has already become an iconic destination for the local and regional community.
Project Credits
Architecture: Matsys + Lake|Flato Architects
Landscape Architecture: Rialto Studio
Structural Engineering: Architectural Engineering Collaborative
Mechanical Engineering: CNG Engineering
Lighting: Mazzetti
General Contractor: SpawGlass
Concrete Subcontractor: Urban Concrete
Petal Formwork: Kreysler & Associates
Client: San Antonio River Foundation
Conceptual Master Plan: Ball-Nogues Studio and Rialto Studio

Cade Bradshaw and Stuart Allen / The final petal being lowered into place and connected with its neighbor to create the structural arch.

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