Not long ago when a new building was designed in an urban environment, one of the first steps in the construction process was to clear the proposed site of any existing structures in order to prepare for the new structure. Increasingly, designers and engineers are moving away from this strategy in favor of repurposing an existing structure.
One advantage of this strategy is the reduction of embodied carbon, which not only saves the emissions associated with extracting and installing new materials, but also reduces the waste produced during demolition.
This was the approach in 2017 when the Rice Management Company, stewards of the Rice University endowment, purchased the remaining years of a 99-year ground lease for the Sears department store in Houston’s Midtown. The plan was to redevelop the building into a 266,000-square foot anchor for Houston’s Ion District.
The redeveloped structure, known as The Ion, was designed to support businesses at all stages of the innovation life cycle and provide resources for Houstonians seeking to participate in the local innovation economy. However, the project presented a bevy of engineering challenges during its transformation and expansion from a retail space to a collaborative technology innovation hub for Houston’s entrepreneurial, corporate, and academic communities.
The challenges included a vertical expansion by two floors, a horizontal expansion to increase the structure’s floor space as well as the additions of a new canopy, rooftop terraces, a light well to allow daylight inside the structure, and a glass curtainwall.
“A major challenge of the original building is that it was a dark box,” says Dennis Wittry, senior principal at Walter P Moore. “To make the building inviting to commercial tenants, bringing daylight into the space was critical. The original scope of work anticipated using punched windows, bringing nominal daylight into the space while minimizing demolition of the exterior concrete walls that were serving as the primary lateral system for the building.”
The final solution involved a more significant extent of demolition both on the building exterior and interior, as well as the creation of a light well running through the center of each floor of the building.
The Ion, which completed construction in 2021, retains many features of the original structure including angular corners, glass-block windows, decorative tiles, and a three-sided storefront that opens to the streetscape.
Vertical and Horizontal Expansion
The existing three-story concrete building, constructed in 1939 and modified in the 1960s, was not designed for future expansion. Recognizing the potential value to the project, Walter P Moore, who served as the structural engineer for The Ion, presented the option for vertical expansion during the proposal phase. The project’s building team also included three design and architectural firms—James Carpenter Design Associates, SHoP Architects, and Gensler—while Gilbane served as the general contractor.
After reviewing the proposed designs from the building team, Rice Management Company decided to proceed with the steel framed vertical expansion along with the horizontal expansion to match the existing concrete structure for the lower three levels. The horizontal and vertical expansions each served a different role in the adaptation of the building for its new use.
“The horizontal expansion was imagined to be, in a sense, the completion of the historic structure,” says Anneli Rice, project director at SHoP Architects. “Our understanding of the original design was that the south elevation was not intended to be as cobbled and irregular as it eventually became. The Ion design filled those irregularities to complete the building form, and allow it to be experienced fully in the round. Conceptually and practically, the horizontal expansion made most design sense in concrete, the system of the original structure.”
In order to expand the existing three-story structure vertically, most of the existing spread footing foundations were strengthened although it was not necessary to strengthen the existing columns. Furthermore, the existing roof framing had an insufficient load capacity to serve as an occupied floor, so new steel framing was spanned between the existing columns. In essence, the existing roof was overbuilt as it was inadequate to serve as a new floor.
“In considering options of strengthening the existing roof or over-building the existing roof, an overbuilt solution was found to be the most cost-effective approach,” Wittry says. “This was accomplished by building a new steel floor just over the existing roof.”
There were several advantages to this approach. First, new steel framing at this level did not require fireproofing because the required fire separation was provided by the existing concrete slab below. Second, the 2 ft, 3 in. depth of the overbuild allowed for the required depression at this level to achieve the occupiable terraces with pedestal pavers.
Because Rice Management Company desired terraces on the north and south sides of the floor plans, Walter P Moore cantilevered the new steel overbuild framing past the existing column lines. New beams were framed between the cantilevered beams and new concrete curbs formed adjacent to the existing concrete parapet wall.
A thin, tapered canopy extending around the roof was designed to cover the new terraces. The canopy cantilevers 28 feet on the north side, 19 feet on the south, and 13 feet on the east and west sides. The horizontally expanded south, east, and west sides have vertical sunshades that hang from the canopy.
“The biggest challenge with the tapered roof canopy was controlling deflections to accommodate the vertical sunshades that were hung from it,” Wittry says. “While it would have been easier structurally to support the sunshades from the floor below, this would have put the slender shades in compression, requiring them to be bulkier. We proposed instead to hang the sunshades, which required more detailed studies of the canopies deflection performance, but resulted in a much more aesthetic solution.”
The canopy profile was achieved by tapering the steel beams to a depth of 8-in. at the end. The beams are segmented with the end section welded with an upward camber to achieve the desired upward cant of the canopy. Walter P Moore specified deflections at the head of the glass curtain wall, which occur at varying distances under the canopy.
“We worked very closely with Walter P Moore to minimize the depth at each of the cantilevers,” says Joseph Welker, studio director at James Carpenter Design Associates. “The overhangs created shaded spaces at the fourth floor, similar to a loggia, dependent on the time of day.”
Light Well Introduction
To introduce natural daylight into the former department store, demolition of the existing exterior concrete walls was completed to accommodate the glass curtain wall. A new 26 ft by 92 ft light well was included in the center of the building, and 26 ft by 92 ft holes were implemented throughout all floors of the building to cast the light well’s natural light to the lower levels.
The light well, designed by James Carpenter Design Associates, is key to the building’s daylighting strategy. Functionally, it makes the adaptation of the large floorplates into workspaces possible, providing critical access to daylight and the outdoors.
“We wanted the center of the building to come alive with light and activity, and this central volume created a canvas for these spaces to connect to the sky,” Welker says.
The goal, according to Welker, was to establish a volume of daylight visible from most interior spaces.
“We worked with a punched and perforated material that had been under development in the studio on a couple other projects, and we tailored the size and finish to respond best to Houston light conditions,” Welker says.
Four columns in the middle of the building were demolished to accomplish the full height light well. The light well cut an angled slice out of the building following the path of reflected light from the skylight above. However, it also left a cantilevering slab condition of varying lengths at each level.
“To avoid cracking of the existing slabs, they were ‘overcut’ to the column edge to allow distribution of reinforcement based on the new support conditions,” Wittry says. “This also greatly simplified construction sequencing and erection. A construction sequence was developed for the construction of the light well to avoid overloading existing elements. Additionally erection cambering was used to account for construction sequencing so the finally erected slab edges would be flat after all the floors were installed.”
Walter P Moore hung the remaining slabs at each level using three-inch diameter hanger rods from new concrete transfer girders placed over the existing roof level. Originally, the plan was to extend the hanger rods all the way up to the new roof to support them from deep steel members. However, when it became necessary to overbuild the existing roof, Walter P Moore designed a scheme to discontinue the hanger rods within the depth of the overbuild. This left the upper two floors free of any visual obstructions around the light well. The atrium created by the multi-story opening required 40,000 lb. horizontal fire shutters at Levels 2 and 4.
“Conceptually, The Ion is a platform that brings innovators and the community together, and the light well is the material expression of this vision,” Rice says. “Providing visual access to all levels, and physical access between the communal spaces, the light well gives place to the idea of collaboration.”
On the south face of the atrium, each floor has a direct view up to the skylight and conversely receives indirect light. All areas around the skylight offer immediate visual connection to the sky, which was particularly important for the spaces in the lowest level of the building.
“To mitigate the direct solar penetration over the course of the year, a series of shading elements were positioned within the insulated glass assembly of the skylight,” Welker says. “The angle was optimized to provide some occasional light to the lowest level, while maximizing the wash of light on each face of the atrium.”
The Ion pays homage to the original art deco style of the structure, particularly at the ground-level storefront where the historical façade on the north side was recreated and vertically expanded. The addition of the glass curtainwall on the expanded south, and introduction of daylight by means of large window openings in the east and west concrete exterior walls connect the open interior to the surrounding streetscape—a nod to the window shopping past when the structure was a department store. The large, multistory exterior openings added for new windows in the concrete walls, as well as the two-story vertical expansion, resulted in the need for new concrete shear walls to be introduced near the new stair and elevator cores.
Part of the vision for The Ion was to maintain the aesthetic of the northwest and northeast storefront corners, while extending the height of the curtainwall in the corners by an additional 15 feet. It was a challenge to achieve this while concurrently maintaining the stability of the areas, which supported the existing concrete façade and concrete roof framing.
“Maintaining the storefront aesthetic was extremely difficult from a construction standpoint,” says Taliah McGowan, senior project manager at Gilbane Building Company. “To install the curtainwall and maintain the corner aesthetic, the existing concrete walls needed to be demolished and opened up.”
This required that the existing concrete structure be reinforced with new steel structure, but the reshoring required to carry the existing roof beams—weighing upwards of 100,000 lbs. that were formerly supported by the concrete to be demolished —actually impeded the placement of the new structure to a great extent, according to McGowan.
“The sequencing of this work was extremely challenging, critical, and required careful planning to avoid any structural malfunctions,” McGowan says. “Gilbane worked very closely with Walter P Moore to establish the demolition and installation sequence, and Walter P Moore was actively involved in monitoring the demolition and construction sequencing.”
Shoring was installed to support the center portion of the wall to allow for demo at the outer locations of the wall. After demo, some of the new steel framing was installed in order to support the remaining wall. The center shoring was removed, and rest of the planned demo and steel installation took place.
Retrofitting the structure was also made difficult because nearly all of the renovated structure was designed to remain exposed like the existing structure.
To accomplish this, each retrofitted area has to maintain the aesthetic of the existing exposed 9-½-in. concrete flat slab structure that has 4-½-in. drop panels at each column. Walter P Moore widened existing concrete beams or cast new 4-½-in. deep beam stems doweled into the soffit of the existing slab where existing openings needed to be infilled or existing beams strengthened. The beams were constructed using self-consolidating concrete pumped through new holes created in the existing slabs.
According to Welker, James Carpenter Design Associates worked closely with Gensler and Walter P Moore to continue the concrete structural expression at the south end of the building as well as the columns and slabs. This required integration with the existing conditions, including the foundation.
“Self-consolidating concrete was used in the concrete beams that were added to the existing structure as well as in the new shear walls,” says Aaron Smith, project manager at Encore Concrete Construction. “In an existing structure, when placing new concrete beams underneath an existing floor plate, standard concrete with a 5-in. slump could not be placed from above.”
Large core holes were placed roughly 10-ft apart through the existing slab and the self-consolidating concrete was poured from the top. Smaller holes approximately ½-in. in diameter were drilled approximately 2-ft apart between the larger holes so air could escape and not become trapped between the new self-consolidating concrete and the existing slab and beams.
“Self-consolidating concrete was used as a means to ensure quality,” Smith says.
Additional challenges to repurposing The Ion included infilling the existing stairs, elevators, and escalators while framing new stairs and elevators based upon the ideal circulation patterns. To infill these areas, Walter P Moore incorporated the use of new concrete beams doweled into the soffit of the existing slab. By matching the depth of these beams to the existing drop panels, the beams appear as part of the original construction.
New steel framing to re-support the existing structure around the new stair and elevator openings was concealed within the interior stud walls.
“The Ion design features several moments where the old and new structure combine,” Rice says. “While the lightwell celebrates this intersection, the vertical circulation components—the elevators and stairs—instead emphasized a choreographed user experience through the building.”
As such, the elevator lobbies were envisioned as portals that lead to the open stairs, which are highlighted in a vibrant red plus and minus motif showcasing the ionic charge theme. These design elements are kept simple to seamlessly guide the user up the building by foot, according to Rice.
While reusing and repurposing existing buildings is highly effective at reducing embodied carbon, it poses numerous structural challenges especially in regard to The Ion project. However, repurposing a structure can effectively address these challenges while still delivering a modern, structurally redefined building.
The Ion is celebrated not only for its stunning transformation from aged department store into modern tech hub, but for its success at minimizing the project’s embodied carbon through the reuse of the existing building.
Rachel Calafell, PE, is a principal and project manager at Walter P Moore. She can be reached at [email protected].