Front view of UTD Student Services Center. Photo: Charles Smith, AIA

Project team:
Owner: UT Dallas OFPC
Construction manager: Hill & Wilkinson
Design architect: Perkins + Will
Structural engineer: JQ
Cladding designer: CDC (Curtainwall Design Consulting)

The University of Texas at Dallas (UTD) has the goal of obtaining Tier One status in higher education. To achieve this goal, the university has incorporated a new construction strategy of developing "best in class" facilities to enhance the reputation of the school, attract the brightest students and gain investment support from the North Texas business community. Earning the LEED Platinum designation while remaining within the established budget presented challenges and opportunities in the design, engineering and construction of U TD's new Student Services Building.

Structural systems
JQ worked closely with the architect early in the design phase to develop a scheme that provided open spaces in office areas and an efficient structural system. The building consists of a four-story concrete frame, steel bar joist roof and steel-framed clerestory in the center of the building to bring natural light to the interior spaces. The bar joist roof structure is supported by a steel drop-in girder system with concrete columns extending up to brace the roof. The elevated floors are comprised of 7-inch conventionally reinforced concrete slabs spanning 15 feet between 22-inch-deep post-tensioned concrete beams and girders. Each floor also supports a terracotta louver shading system hung 3 feet off of the building perimeter with a steel outrigger system. The ground floor is a conventional 5-inch slab-on-grade and the foundations are drilled straight-shaft piers founded in underlying limestone.

Interior entrance lobby. Photo: Charles Smith, AIA

LEED and the structural engineer
Generally, structural engineers do not consider LEED-designed buildings as having a large impact on the structural design. Requirements are often satisfied by simply adding some verbiage to the specifications requiring minimum levels of recycled content, use of regionally produced materials and possibly higher than normal fly ash or slag replacement of Portland cement in concrete mix designs. While this may be true for LEED-certified and Silver projects, a higher level LEED certification can significantly affect the structural design. This project contained the typical LEED requirements mentioned above; however, the desire for a LEED Platinum-certified building created many "collateral" effects on the structural design, including the following:

• A terracotta louver system was designed to hang off the side of the building to provide shading on the curtain wall.
• Architecturally exposed concrete in lieu of floor and ceiling finishes.
• Architecturally exposed structural steel for the grand stair, entries and support of the louver system.
• Due to the tight site, two 20,000-gallon cisterns were located under the loading dock structure. The cisterns collect rain water and are used for irrigation and sewage conveyance.

Structural design challenges
Design challenges abounded over the course of the project. Early identification of the issues and a collaborative approach between the engineer, architect and the construction manager resulted in mutually acceptable solutions that met the architectural intent while being cost-effective.

The first of these was to create large column-free spaces while meeting requirements for higher live loads and future flexibility. The architect proposed a column grid on a 40-by-20-by-40-foot pattern in the short direction and 30-foot bays in the long direction. JQ opted for post-tensioned beams and conventionally reinforced slabs as the structural system that would solve all of the critical issues. The conventionally reinforced slabs span 15 feet and give the owner flexibility to make floor penetrations in the future. The P-T beams allow the structure to easily span the desired column spacing while controlling deflections. In addition, the ability of the P-T system to help control shrinkage cracks in the architecturally exposed concrete floors is a benefit.

The next challenge was a result of the site size and topography. The site was just large enough to fit the building footprint and the existing ground sloped roughly 8 feet from west to east. After re-grading and excavation for a recessed mechanical room, the west wall needed to retain 16 feet of soil. To keep the building columns as small as possible, concrete shear walls were strategically placed in the east-west direction at the lower level to resist the lateral soil pressure. The shear walls then created the next design challenge, since they would also increase the concrete shrinkage produced by the post-tensioned girders and cause unsightly cracking in the exposed floor slab. JQ resolved this issue by detailing a slip joint between the walls and the floor that allowed the slab to slip until it was grouted in place 90 days later (see Figure 1). Additionally, the concrete columns that would normally be cast integrally with the retaining walls were detailed with expansion joints to allow the concrete to shrink under the post-tensioning stresses. These details allowed most of the concrete shrinkage to occur prior to locking the building into the shear walls and minimized the potential cracking had the post-tensioning been restrained.

Figure 1: A shear wall-to-slab shrinkage connection. Figure 2: A detail of a congested joint.

Controlling the force of Texas' abundant sunshine
In studying Texas' climate, the architect realized the need to protect occupants from the blazing sunshine. Perkins + Will studied indigenous designs and the vernacular architecture within early Texas buildings, including some of the earliest aboriginal forms using woven willow screens. The woven willow patterns influenced the design of an exterior terracotta louver system that allowed for different shading strategies for each façade of the building.

The louvers were designed with varying densities to maximize shading in the harshest locations and to capitalize on views in less critical areas. This unusual design feature presented one of the most significant engineering challenges on the project. To expedite the solution, the team hired a special cladding engineer to consult on the overall design of the system. A 2-inch-thick post-tensioned terracotta system was designed to span 60 inches and cantilever up to 24 inches on either side of 5/8-inch diameter stainless steel vertical rods. The supporting vertical rods were tensioned between the building levels to 3,000 pounds. Springs were used to compensate for thermal, live load and long-term deflections. The nested spring assembly had an outside diameter of 3.5 inches and was hidden within a built-up steel plate outrigger system that cantilevered from the building at each level.

The outriggers, spaced at 5-foot intervals, transferred large shear and rotational design forces back to the building's spandrel beams. As these intervals coincided with the vertical mullions for the curtain wall system, many of the attachment points occurred at beam/column joints where congestion of the P-T anchorage devices and beam/column reinforcement was already a considerable construction challenge.

With many elements merging at these joints, it was critical for the design team to map out the locations of the embedment plates to avoid conflicts that could delay the project during installation. These outriggers induced large forces on the embed plates and due to the congestion, a post-installed correction was not an option. Furthermore, due to the critical nature of this element, a 2-inch (+/-) eccentricity was assumed in the design, which resulted in a plate twice as thick as would have been designed otherwise. Additionally, JQ stressed the critical nature of these embeds to the general contractor in the pre-construction meeting and required a mockup of the congested joint. Because of the critical nature of the embeds, the general contractor used a GPS system to verify the embed locations prior to each concrete placement. As a result, all 144 embeds were placed within the 2-inch tolerance. Finally, in order to reduce congestion in the beam-column-embed joint, Lenton "terminators" were detailed for both the beam reinforcing and the embed dowels to eliminate any hooked bars in the joints.

As-built condition of a congested joint. Photo: JQ Dallas

The design team was able to overcome the many design challenges the louver system presented and delivered an architecturally significant element that was designed to minimize solar heat gain while still allowing light to enter deep into the interior spaces.

Architecturally exposed structure
Large portions of the concrete and steel structure were architecturally exposed in order to reduce the amount of ceiling and flooring material used in the building. The open office spaces have exposed concrete soffits, the floors in the public corridors have a ground concrete finish, and the louver support steel and the monumental stair were left exposed.

Mockups were constructed for each concrete condition on the project, and the 2003 AISC guideline specification on architecturally exposed structural steel (AESS) was utilized. Key plans were used to clearly identify all areas of architecturally exposed structure and to define the level of finish desired in each area. JQ worked closely with the architect to determine the level of finish desired while minimizing the impact to the budget.

The concrete columns and soffits were specified to have a Class A finish; however, an ultra high finish was not required by the architect and would have increased the price of the concrete structure unnecessarily. The concrete floors were finished by grinding the top 1/16-inch to 1/8-inch off of the hardened concrete surface. Multiple mockups were produced with different quantities of dark colored aggregate hand broadcasted onto the concrete surface during finishing, producing the desired contrast in the floor finish.

The louver support steel was specified to meet a Category 3 finish since most of the steel would be viewed from a distance of 15 feet or more. The monumental stair was specified to meet a Category 2 finish since it would be viewed from a distance of less than 10 feet but could not be touched. The general contractor hired a separate architectural steel subcontractor to perform the AESS work, and they produced all of the AESS work well above the specified levels, resulting in a very clean structure.

Focus on long-term sustainability
UT Dallas' LEED Platinum building recognized Texas' climate and its impact on the built environment by incorporating the innovative, exterior terracotta louver system that provided a unique energy-efficient shading strategy for the building's inhabitants while reducing solar heat gain and maximizing daylighting and views to the outside.

The project team documented 53 out of 69 LEED credits, ensuring the achievement of LEED Platinum for New Construction, and the project was completed approximately 10 percent under the established construction budget.

John Hoenig, P.E., LEED AP, is a principal with JQ Dallas. Ryan Bragg is a senior associate at Perkins + Will in Vancouver, Canada.