Supporting a Community at Asteri Ithaca

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On the south shore of Cayuga Lake lies the city of Ithaca, New York. The home of Cornell University, this city faces the best of times—a high quality of living and steady growth. With this growth comes a need for more parking, more meeting spaces, more amenities, more housing, and like many cities across the nation, more affordable housing. With these challenges in mind, BW Architecture and Engineering partnered with the City of Ithaca to develop a three-phase project which consisted of adding affordable housing to the area, the construction of a new conference center, and the renovation of the existing Green Street Parking Garage.

STAND Structural Engineering teamed with BW Architecture & Engineering and delivered a three-story conference center with nine stories of affordable housing stacked on top. The lower three stories house a 55,000 square foot conference center with multiple large meeting rooms, a two-story ballroom, full commercial kitchen, management offices, and a ground floor retail space. The upper nine stories consist of 181 affordable housing units, 40 of which remain reserved for people in need of supportive services per the New York State Empire State Supporting Housing Initiative (ESSHI). The residential tower also houses a full community room, fitness space, outdoor patio, and conference room on the top floor with a full balcony.

The design team initially performed an extensive system selection exercise to determine and select the best structural system for the project. Early on in this process, the team determined that the main challenge was laying out a column grid so that the columns would have a minimal impact within the ballroom spaces. The primary options explored were:

Option 1: Utilize a conventional composite steel framing system. Although a natural place to start, the downside of this system is that to maximize the spans, a deeper structural depth is required in the residential tower portion of the building. This deeper structural depth would result in eliminating an entire floor since the overall building had already been near the code specified height limit.

Option 2: Utilize precast hollow-core planks with upset support beams. This would entail creating custom T-shaped members that would span the units for the precast planks to bear on. This option minimized the depth of structure needed, but it did not maximize the spans and overall created a heavier system as compared to other evaluated systems.

Option 3: Design a four-story composite steel podium designed to clear-span over the ballrooms below. This hybrid system uses 4 stories of composite steel framing, paired with 8 stories of load-bearing cold-formed walls stacked above. This structural scheme would allow for the individual requirements of the two main zones of the building to each be met. Throughout the conference center the composite steel system allows for the flexibility to design around a non-repetitive layout, as compared to a cold-formed wall system above which better suits the repetitive nature of the apartment units.

After evaluating the three systems, as well as additional options not discussed, the design team chose to move forward using Option 3—the Hybrid Steel Podium.

After selecting the overall general structural system for the building, designing an element to clear span the ballroom and support eight stories of residential space above presented the next challenge. The design team chose to explore various custom steel truss designs that would span between columns located along the exterior edges of the ballrooms.

One of the primary concerns with the truss design was deflection throughout the construction of the project. If the trusses deflected too much during the construction process, the cold-formed strap bracing in the shear walls could buckle and require refastening or the gypsum wall sheathing could crack as the lower levels begin finishing.

The design team made two critical design decisions at this point. First, they reduced the overall weight of the residential tower by using lightweight concrete for the construction of the tower floors rather than conventional normal weight concrete. Even by doing this, since the design live load for residential spaces is 40 psf, roughly 75% of the load applied to the trusses consisted of dead load. Second, they designed the trusses to a very stringent deflection limit of L/600 for the total load deflection and a max deflection of 3/4 inches for dead load deflections. With these two decisions in mind, the team decided on a one-story deep wall truss.

As seen in Figure 1, the wall trusses spanned an entire floor, with the 4th floor being the bottom chord and the 5th floor being the top chord. At the southern portion of the truss outside of the ballroom area, the truss deepened from 15 feet deep to 23 feet deep. The overall length for each truss measured 115 feet with a maximum clear span between supports of 75 feet. Four of these trusses were used spaced at 30 feet on-center with composite floor framing spanning between them.

For the fabrication and erection of the trusses, the steel fabricator responsible for the trusses and all steel throughout the project, JPW Companies out of Syracuse, New York, elected to shop fabricate the wall trusses in four separate sections for easier transport. Per standard practice in the New York area, all connections on the trusses were to be bolted connections. Once on site, the crane would hoist the sections of the truss into location and workers would field bolt them into place.

For the lateral force resisting system, the team used a combination of cast-in-place concrete shear walls, light gauge cold-formed shear walls, and steel braced frames. Between all these systems, the full height concrete shear walls making up the stair towers and elevator cores function as the lateral element that resists most of the lateral load from the residential tower. The light gauge shear walls primarily resist the wind loading from the nearly 20-foot-tall parapet and screen walls. And finally, the steel braced frames act as the main lateral element for the steel podium for the lower five stories.

During the bidding process, contractors Vecino Construction and Welliver elected to bring Vulcraft on board with its Vulcraft RediCor system in place of the previously designed cast-in-place concrete shear walls. By making this change, the stair towers and elevator cores would be pre-manufactured in modules at an offsite facility that would then be transported to the site and could quickly be erected. One of the benefits of switching to the RediCor system included the immediate stair access to all floors once workers installed the modules. At the time of construction, this project utilized the tallest RediCor stair core to date at a height of 152’-6” and the tallest four-sided elevator shaft RediCor had produced to date at a height of 145’-6”.

The project site provided its fair share of challenges. Three sides of the building contained utility easements, sewers, or electrical and telephone vaults located within 12 inches of the face of the pile caps. Due to the location of these elements, the building needed to incorporate cantilevers into the design since foundations could not be installed in these areas. These conditions led to the development of one of the signature focal elements of the building—the double skewed columns at the Southeast corner of the building as seen in Figure 4.

The foundation design of the building also posed considerable hurdles. Even though the existing building located across the street bears directly on bedrock, preliminary borings at the start of the project revealed that rock was not located within 100 feet of finished grade on our site. With the columns that support the main trusses supporting loads upwards of 2,000 kips, the team decided to use 100-ton steel H-piles as the primary foundation system. During the bidding process, the initially designed foundation system switched to the Menard USA’s proprietary controlled modulus columns. These controlled modulus columns are 18-inch diameter vertical grouted elements which displace soils laterally, producing little spoils. With this being a drilled system versus the driven H-piles, the disturbance to the surrounding building and neighborhood was greatly reduced, as well. That decision alone saved the project nearly $400,000.

The Asteri Ithaca project overcame many challenges throughout its design including design constraints and economic hurdles, not to mention a pandemic sweeping the nation at the start of construction. Upon its completion this project now stands as a pillar to help support the City of Ithaca, be it through the city’s new conference center and its ability to bring new commerce and events to the city or the residential tower and the opportunity it brings to help house more of the ever-growing community of Ithaca, NY. ■

Cody Gibbens is an Associate Principal at STAND Structural Engineering. (cgibbens@stand-sei.com)