As in many other cities, the rebirth in the 1990s of the downtown Dallas residential market began largely with the renovation of old buildings. Turn-of-the-century multistory warehouses, offices and department stores were converted to economical and often trendy apartments and condominiums. While the supply of suitable older buildings dwindled, the demand for economical downtown housing did not. To meet the continuing need, developers have now turned their attention to new projects that simulate the character of the old buildings-a character that frequently includes exposed structure and minimal interior finishes.
As is frequently the case, the challenge facing designers and builders is to wring every ounce of efficiency out of structural, mechanical and architectural systems to keep the unit costs as low as possible. In the case of two new residential projects in downtown Dallas, the design concept integrated the structural framing with the layout of residential units using architecturally exposed concrete beams and slabs. The construction team used only conventional concrete materials, placement and finishing techniques, combined with a column-mounted table forming system. And, while the terms 'architecturally exposed concrete' and 'economical' do not always go hand in hand, a somewhat 'rough' finished appearance of the concrete was preferred to approximate the look of older concrete buildings.
Cases in point
Work had barely been completed on the first tower of this kind at 2011 Cedar Springs-designed by the Dallas office of architecture firm Corgan Associates Inc. and developed by Addison, Texas-based CLB Partners-when ground broke at the nearby McKinney Avenue Lofts project. The McKinney project was designed by Dallas-based A/E firm HKS Inc. and developed by OLY/CLB LP, a newly formed partnership based in Dallas. El Paso-based C.F. Jordan LP served as the general contractor for both projects, and the concrete subcontractor was Capform Inc., Carrollton, Texas.
At 15 stories, 2011 Cedar Springs has 45 two-story loft units with one level of covered parking. The project was completed last year for a total construction cost of about $11 million. The McKinney Avenue Lofts project is larger, designed to stand at 25 stories with 62 two-story units and three levels of parking. The new building is expected to be ready for occupancy by spring of 2002, and the total construction cost is estimated at $19 million, not including land.
Integral to the project design concept for both new buildings was the choice of cast-in-place concrete as the construction material. Cast-in-place concrete would most readily serve the desire of the developer and designers to recreate the unique character of older concrete and brick industrial and warehouse buildings that had been successfully converted into residential lofts. The selection of the structural concrete system used on these projects came out of early structural design and construction cost studies, a joint effort requiring the experience and expertise of the general contractor and the structural engineer, L.A. Fuess Partners Inc., based in Dallas.
Framing and forming
The projects required a concrete framing system that would accommodate one- and two-story clear heights within each residential unit, as well as a spacing of concrete columns that matched the spacing and layout of the residential units. Also fundamental to the design were large window or balcony openings on all four sides of the towers and supported brick on the exterior surfaces (see details, this page).
Some framing systems that have been a frequent sight during the recent boom in residential construction were not considered. Tunnel-formed concrete, for instance, with its uniform slabs and walls was not a good fit. Pan-formed concrete joists, with their longer slab-span capability, were not needed for the relatively short floor spans and its ribbed look was not favored by the building team. Other framing systems-like post-tensioned flat-plate floors with shear walls-were evaluated but found to not readily accommodate the large window openings and open design of the lofts.
The study converged on a conventionally reinforced beam-and-slab system-without shear walls-that met all of the project requirements. Led by the structural engineer and the architect, the building team selected 6-in.-thick slabs and a grid of 24-in.-wide columns and girders. Girder depths of 24 inches were used at 2011 Cedar Springs, which has no upper level parking. McKinney Avenue Lofts, which is conceived with parking on the first three levels, has been designed with a wide column spacing and 30-in. girder depths. The rigidity of the beam-and-column framework was designed to provide the lateral load resistance that might otherwise have been provided by shear walls.
Although there was a desire on the part of the developer and architect to end up with a relatively rough, unpolished look in the finished concrete surfaces, there was no desire to have irregular geometry or wavy formwork. Surface blemishes were considered acceptable, but straight, plumb and square formwork-resulting in precisely dimensioned beam and slab edges-was a project requirement.
The project team considered several shoring and forming systems, including ordinary 'stick' forming and standard tables with an aluminum truss system, but finally settled on a column-mounted table system (see 'Shoring and framing the loft look,' below).
While the recent building boom has limited the availability of materials in some areas, concrete supply was never an issue on either of these projects. The compressive strength of concrete used for the design of these projects was typically 4,000 pounds per square inch (psi), with 5,000-psi concrete used at some lower columns on the McKinney Avenue Lofts project. A steady supply of this concrete was available throughout the work at 2011 Cedar Springs, and the same is expected for the second project.
With its downtown location, though, concrete deliveries have generally been scheduled between 2:00 a.m. and 3:00 a.m. to avoid delayed concrete deliveries caused by heavy traffic. While some of the concrete was pumped, most of it was lifted in 2-yard buckets.
To speed up the forming and pouring schedule, the concrete strength was boosted slightly to achieve a higher early strength. About 70 percent of the design strength was achieved in three days, permitting stripping of forms and moving of the tables in rapid succession up the building.
Antique look, modern result
The 2011 Cedar Springs units were fully sold upon completion, attesting to the robust Dallas housing market and the recent successes of retrofitted older buildings in the downtown area.
Not surprisingly, the building team's architect and structural engineer had been involved in numerous adaptive-reuse projects of Dallas warehouse buildings in the 1980s and 1990s. Some of the manufacturing buildings from the early 1900s featured concrete frames and brick exteriors, like 2220 Canton Lofts, a five-story structure that reopened in 1995 with 70 loft units, and the Adam Hats Building, a four-story building that opened with 60 units in 1997. These projects-like many others-are marked by the reawakening of outdated, underutilized or abandoned structures. The efforts helped lead the way to broader revitalizations of their neighborhoods, just as 2011 Cedar Springs and the
McKinney Avenue Lofts projects hope to continue to foster new urban development in their own neighborhoods.
Mark Peterman and Ann Piazza are registered engineers and principals with L.A. Fuess Partners in Dallas.