In the early years of Datum Engineers Inc.’s engineering practice, designing buildings for “people comfort” was the primary design criteria. We were charged with meeting all code structural loading criteria along with simply providing comfort for the occupants of the building. In 1989, however, this all changed when we were selected to design the Medical Research Campus for the University of Texas (UT) Southwestern Medical Center in Dallas.
At the start of the project, then UT Southwestern Medical Center President Kern Wildenthal, M.D., Ph.D., summoned the design and construction team to his office for one of the most meaningful owner team meetings we had ever attended. He wanted us to know that we were an important member of the university team and that we were all going to have a major role in the creation of the most important medical research school in the country. Our charge was to design the best possible laboratory facilities that would contribute to attracting world-class researchers.
It immediately became apparent that this first undertaking needed to be a special building integration of architectural design with the engineering concept — designing for equipment comfort, meeting the building codes, and achieving the economics of construction that would be required to deliver a facility capable of operating at the highest level in the conduct of sensitive medical research activities.
Although this occurred 22 years ago, our research quickly identified the need to design for equipment comfort at a time when there was not much knowledge or information about the vibration sensitivity of existing medical research equipment. And certainly there was very little expectation about the potential vibration sensitivity of future research equipment.
We also recognized the need to create a structural system that would easily accommodate present and future floor and ceiling penetrations for services to the wet labs. It was extremely important to integrate a structural system that worked with the laboratory module which would provide penetration flexibility through the floor and ceiling structure. With this thought in mind, we adopted a policy to straddle the layout of the laboratory benches with the structural beams. This would allow known and future unknown penetrations to be located under the benches without penetrating main concrete beams.
These new requirements did not override the need to design an economical and fireproof structure that complied with the local building codes.
Choosing the structural system
For this campus, reinforced concrete construction was selected for several reasons that met the critical demands of a special building. Reinforced concrete structural systems respond favorably to the need for vibration mitigation because of greater internal damping compared with steel systems. Damping is extremely beneficial for reducing a floor system’s vibration response. It not only reduces the vibration velocity, but also the length of time the floor system vibrates following a footfall.
Reinforced concrete structural systems also incorporate effective fire protection without the need for additional spray fire coating that could compromise the facility’s relatively clean room environment.
Another advantage is that cast-in-place reinforced concrete construction has traditionally been an economical building system in the Dallas area.
Growth through two decades
During the last 20 years, we successfully completed four phases of medical research laboratories, comprising approximately 1.7 million square feet in buildings that ranged from eight to 14 stories. All laboratory buildings are connected to a two-level, below-grade garage that accommodates more than 5,000 cars.
These buildings are all long, rectangular buildings with the laboratories located on the long faces of the building. The structural system has 32-inch-deep reinforced concrete beams spanning 30 feet and spaced at 10 feet on center, which straddle the lab bench module. A 5-inch-thick slab spans between the beams. This concrete system met the strict vibration criteria required for the research activities in the labs.
The structural column layout assisted in meeting the specified vibration criterion. Smaller column spacing in areas with vibration-sensitive equipment is beneficial if the layout of the labs can accommodate it. We were accustomed to using 40-foot spans in office buildings where the vibration design criteria was people comfort. We started with 40-foot spans and found these long spans to be impractical with the strict vibration criteria required. For this reason, we changed interior column spacing to 30 feet.
Change in plans for Phase 5
When it came time to construct Phase 5 of the North Research Campus, a new master plan had been developed around the next seven planned buildings. These structures are square, rather than rectangular, to distinguish them from the existing research campus. Also, it was decided to increase the laboratory bench module to 10 feet, 6-inches from the 10 feet used in the first four phases, to better comply with the latest laboratory layout concepts.
This required us to stop and re-evaluate the structural system. The square plan had laboratories on all four sides of the building. The laboratory bench module had changed to 10 feet, 6 inches. But more importantly, laboratory equipment had become progressively more sensitive to vibrations as magnification levels increased. This new vibration criteria required that we spend more time on research with the professors and equipment suppliers to understand the requirements of the building and to provide laboratory floor systems with vibration characteristics that allow sensitive equipment to function without loss of resolution.
Although Phase 5 has a square configuration with laboratories on all four sides, we wanted to continue a similar structural concept to the one we developed for the first four buildings, a system that created a consistent relationship between the bench modules and the structural floor system above and below with beams that straddled the benches. This led us to consider a reinforced concrete, waffle slab floor system. We were pleased to find how efficiently the two-way waffle slab met this project’s strict vibration criteria. As a result, a waffle slab with 2-foot, 8-inch deep ribs at 10 feet, 6 inches on center each way was selected as the structural floor system. The beams straddled the benches and the slab is 5 inches thick (see Figure 1).
The columns around the perimeter of the building were spaced at 21 feet on center and centered on every other laboratory bench. This short span from column to column for the spandrel beams created a very stiff spandrel beam that contributed to improving the vibration performance of the floor system in the laboratories. The short span also allowed the soffit of the spandrel beam to be cast flush with all of the beams in the waffle slab. Having the soffit of all beams at the same elevation was a major benefit to the cost and speed of the concrete forming.
The wind forces were resisted with stiff concrete shear walls to limit the sway of the building. Adding shear wall was the appropriate wind-bracing system for a building designed to accommodate vibration-sensitive equipment.
At completion of construction, JEA Acoustics, the design team’s vibration consultant, performed field vibration measurements within the building. Their analysis confirmed that the field-measured vibration levels in the building were similar to those predicted by calculation during design.
The foundation was complicated by the presence of an old landfill and the need to excavate a future two-level, below-grade garage adjacent to the new structure. We worked around these challenges by implementing the following concept revisions:
Ground floor slab — We decided to create a cast-in-place concrete ground floor with a crawl space at ground level to avoid depending on an old, uncontrolled landfill to support a more economical concrete slab-on-grade floor system. Although there was some added expense associated with this decision, and we were fighting against a tight budget, the risk of a potentially non-performing ground floor was not a negotiable risk.
While optical microscopes and other sensitive laboratory equipment are utilized within the tower’s elevated levels, the electron microscopes are positioned on the ground floor. This imaging space is contained within a one-story building that is adjacent to, but completely isolated from, the tower structure due to concern for vibrations generated by the mechanical equipment in the adjacent space.
Additionally, the structural slab supporting the imaging equipment is depressed from the floor elevation to permit the installation of isolation equipment along with raised flooring. The foundation for the equipment in the imaging space is supported by drilled piers and also is isolated from the surrounding building floor construction. Vibration excitations created by nearby road traffic — as well as the thermal energy plant 500 feet away — were concerns that were field-measured by JEA Acoustics prior to the design’s completion.
Battered pier foundation — The next phase of construction will require a two-level excavation for a future parking garage adjacent to the structure. The original four laboratory buildings have parking directly under the towers as well as adjacent to the towers. We evaluated the cost of adding two levels of garage below the footprint of the building at this phase, but the high cost and limited number of cars it would accommodate made us decide against it. So, we added 95-foot-long battered piers on each corner of the shear walls on the ground floor that extended 70 feet into shale. These piers take all of the lateral forces on the building directly to the shale and the structural stability of the building does not depend on the soil around the building, which will be removed at a later date.
The Phase 5 laboratory, completed under UT Southwestern President Daniel K. Podolsky, M.D., included a pedestrian bridge across a drainage channel and a major utility tunnel below the drainage channel to the existing campus.
Successful design of a sophisticated building such as UT Southwestern Medical Center Phase 5, with so many sophisticated issues to address, required the support and guidance of the owner, researchers, equipment suppliers, design professionals, and construction team. The success of this project demonstrates the benefit of such a team effort and affirms that the president’s challenge of 22 years ago has been fulfilled.
Thomas Taylor, P.E., is principal design engineer, and Stephen Price, P.E., is director of laboratory design, Datum Engineers Inc.