|A few of the obvious project challenges included the facts that the west wall of project was immediately adjacent to medical office building and that the existing campus drive was maintained under the entire length of new tower.
Seton Medical Center is Austin, Texas’ largest medical and surgical acute care center, with nine levels and 455 beds. Established in 1902, they moved to their current location in 1975. With several expansions behind them, they were faced with a shortage of available building space when they wanted to construct a new Women’s Center on their campus.
The Seton Healthcare Network, the owner of the project, decided to locate the building on a sliver of land on the west edge of their property, over an existing campus drive that they wanted to keep. This site selection came with a whole host of restrictions.
The new tower, which paralleled the property line, was oriented 30.2 degrees off the existing building grids, but was in direct contact with the irregular end of the existing hospital over a distance of 200 feet. In addition, the layout of the patient floor required the new addition to be wider than the space available between the existing building and the property line, so the southeast side of the new six-level tower was designed to span 85 feet over a corner of the existing two-level Emergency Department, which had to remain in 24-hour operation throughout construction. Finally, the campus drive slopes 15 feet from Level 1 at the north end to the Basement Level at the south end, and the hospital wanted a patient entry at the Basement Level. This required demolition and reconstruction of a large existing site retaining wall along the property line and additional basement excavation throughout the footprint of the new building.
Despite the many site challenges, the design team successfully configured solutions to every situation.
Foundations — The reconstructed retaining wall, which supports the building columns adjacent to the property line above the first elevated level, has no lateral support there, because the first elevated level of the building was eliminated from most of the building footprint to gain needed vehicle clearance above the roadway at the north end of the building. This also increased the volume and openness of the building entry/drop off area and allowed light into the area through bay openings above the retaining wall along the entire west side of the building. To resist the lateral earth loads, as well as a buckling load component of the building columns above, a cantilevered retaining wall was employed, but because the site soils are quite expansive, the cantilevered wall was supported by a 30-inch-thick mat cast on cardboard carton forms and supported on drilled shafts, rather than a more conventional soil-bearing spread footing. This foundation system presented significant problems with achieving sufficient resistance to sliding. Since the carton-formed void space eliminated the transfer of lateral forces by footing/soil interface friction and there was no continuous concrete floor slab at the basement level, most of the substantial lateral loads had to be transferred into the ground through long drilled shafts confined to the region of the wall footing and surrounded by relatively soft materials near the ground surface. This required careful analysis and larger, more numerous shafts than would otherwise have been necessary to support gravity and overturning loads, alone.
Pedestrian bridge — To maintain the link to the adjacent medical office tower that had previously occurred at grade, a bridge from the remaining portion of the Level 1 floor to the top of the retaining wall on the property line was included. The bridge utilized an upturned tube steel structure to achieve the required vehicle clearance below and still maintain floor alignment.
Tower framing — Although concrete structures are normally used in this area for medical facilities, steel was selected to frame the tower. The lowest elevated level was up to 30 feet above grade, and shoring for cast-in-place concrete would have been very expensive and obstructive. In addition, the floor-to-floor heights of the existing hospital were short, and steel provided additional room for mechanical piping and ductwork above the ceiling (utilizing beam web penetrations for some ductwork). Most importantly, steel structure enabled the design of a story-height truss system between the fifth level and the roof, which supported the east side of those levels as well as the third and fourth floors below, where they extended over the Emergency Department.
The truss system was installed in the south and east walls of the mechanical penthouse, where the story height, and resulting truss depth, were larger, and the truss diagonals would not interfere with patient room windows. This system included a truss in the south wall of the tower that cantilevered about 20 feet to pick up the south end of an 85-foot span truss along the east side. W-shape hangers carried the third and fourth floors, as well as the entire curtainwall, down to just above the existing Emergency Department roof/Level 2 floor. The hanger columns cantilever below the third floor to laterally support the masonry veneer curtainwall that is separated from the existing roof structure by an expansion joint.
Erection sequencing was critical to the successful installation of the trusses. Because nearly half of the relatively narrow south end of the building was hung from the top-level truss system, only two of the bays could be erected to the top floor before the trusses were lifted into place, and only the narrow middle bay could be extended to the penthouse roof since the penthouse did not extend over the westernmost building bay. This left a very narrow diaphragm to brace the top chord of the truss during erection, and the top chord had to be braced back to that diaphragm with roof beams spanning over a 60-foot-tall void until the intervening floors could be placed. The trusses were shipped to the site in smaller sections and assembled on the fifth floor slab adjacent to the mechanical penthouse. They were then lifted into place by a single 300-ton mobile crane sitting at the basement level and reaching over the south end of the six level building. The contractor and the steel fabricator collaborated closely with the structural designers to ensure the efficient erection of the trusses. Special attention was given, early in design and in cooperation with the steel fabricator, to the bolted connections used throughout the trusses. Member sizes were selected not only for their load carrying capacity, but also for their dimensional similarity to simplify the bolted connection fit-up. Erection began early on a Saturday morning and completed late that evening to minimize interference with Emergency Department operations.
|A patient drop-off area was located under the new tower immediately off of the campus drive. natural light is provided through openings in the west wall above a retaining wall. a pedestrian bridge was added to maintain access between the hospital and adjacent Medical office Building.
The trusses were designed to be very stiff to minimize post-erection deflections, which would impact multiple framing levels and the alignment of the new Level 2 floor with the adjacent floor to be placed directly on the existing Emergency Department roof structure. (The existing roof structure was built up with insulation and concrete topping to align with the rest of the tower level). Expansion joints running diagonally, relative to the new tower framing grid, were required to mate the existing Emergency Department building to the new Level 2 tower floor that it projects into.
Where the northern half of the new tower connects to the existing patient tower, the upper levels of the existing building are setback, twice, from the existing floors below. In several places, cantilevered trusses were needed to extend the upper levels of the new tower over the lower level projections of the existing hospital. This necessitated installation of expansion joints which were offset, horizontally, at different levels. This interface also incorporated a large mechanical shaft to accommodate ductwork that served a renovation of the existing hospital from new mechanical units in the penthouse of the new tower.
Links to the existing patient tower were established at upper levels of the new tower by spanning over the existing lower floors to existing 50-foot-span bridges linking the north and south halves of the existing patient tower. To carry the load of the new links, the existing bridges needed to be widened so that the new structure could be sized to support the additional loads. This new wider bridge structure was supported on Teflon slide bearing assemblies bolted to the existing concrete walls, and the new links were, in turn, supported on Teflon slide bearing assemblies welded to the new bridge structure.
|Completed erection of level 5 trusses are designed to support levels 3 and 4 below.
The tower frame is laterally braced with narrow, but heavy, chevron-braced frames, because the opportunities for continuous lateral bracing generally existed only around the elevators and stair wells. To accommodate access to the elevator lobby, the chevrons of the north-south frames adjacent to the elevator could not be connected to a common point, and the brace loads had to be transferred in bending through the horizontal beams that connected them. The relatively small footprint of the diagonal braces resulted in large overturning forces, relative to the size of the building, which had to be resisted by large drilled shafts below the elevator pits. Because of the proximity of the columns to the elevator pit, transfer of the uplift forces from the steel building columns, through the elevator pit walls, to the drilled shafts below was accomplished by extending the W-shape columns through the elevator pit walls and lapping them with the longitudinal shaft reinforcing that projected into those same walls from the drilled shafts. The columns were detailed with octagonal base plates to fit inside the drilled shaft reinforcing cage and provide an upward-bearing surface that could transfer uplift loads into the elevator pit walls. Development of the drilled shaft reinforcing within the depth of the elevator pit walls was accomplished with Lenton bar terminators because hooked bars would have been difficult to install and coordinate with the adjacent pit and the lapping steel columns.
Construction of the new tower displaced a rainwater detention pond which was replaced by a detention tank that was incorporated into the building construction at the north end of the tower. This structure consisted of drilled shaft-supported, cast-in-place concrete slabs and walls with a structured floor to maintain separation from the expansive clays below, and it is roughly level with the Ground Floor, rather than below it. A cast-in-place concrete roof, supported on the perimeter walls and interior columns, covers the tank and creates a plaza at Level 1.
|The new tower is built immediately adjacent to the existing hospital, and floor framing is cantilevered to the existing building face and hung from trusses above in several locations. The bottom floor in this photo is part of the existing hospital.
A clean bill of health
Because of the extensive interface of the new tower with the existing hospital, many modifications of the existing structure were required to achieve the mating of the two structures. In addition, the mechanical system requirements of the new tower and the concurrent renovations of portions of the existing hospital required extensive phased relocation of mechanical units and ductwork. Creative architectural and structural solutions were needed to address all of the objectives and constraints inherent in the project. The high level of cooperation between the owner, the contractor, and the design team allowed this complex project to be completed with a minimum of delay and interference with Seton’s continuous operations. All of the team members are very proud of the resulting facility.
Seton Women’s Center
Contractor and construction manager
|By the Numbers:||Seton Women’s Center|
|Size, shape, and type|
Spotlight: Datum Engineers
Q&A with the SE
Datum Engineers’ Director of Healthcare Design Marty Sloan, P. E. (MS), a principal with the firm, discussed the Seton Women’s Center with Structural Engineer Editor Jennifer Goupil, P.E. (JG).
JG: What was the most challenging aspect of the structural design? How was it solved?
MS: The retaining wall/basement wall design: the wall was cantilevered off of a drilled shaft-supported concrete mat over carton-formed voids to separate it from the expansive clay below.
JG: What was the most unique problem to solve on the project? How was it solved?
MS: The new tower had to bridge over the existing Emergency Department. Floor-height steel trusses were installed at the top floor to span the existing building and hang lower floors.
JG: What new design innovations were employed by the structural design team?
MS: We used steel columns lapped with drilled shaft longitudinal reinforcing, in lieu of anchor bolts, to transfer large uplift forces to the foundation.
JG: Were there any construction considerations that affected the structural design?
MS: First elevated floor height above ground contributed to a decision not to use cast-in-place concrete framing. Erection of trusses over occupied space and limited staging area on site mandated a rapid erection of trusses and use of bolted connections.
JG: What engineering ideas did you implement to save project costs?
MS: We designed a conventional truss at the penthouse level with hanger columns to support the lower floors in lieu of a heavy, moment-resisting vierendeel frame to accommodate patient windows in bridging wall.
JG: Were there any surprises? How did you adapt to them?
MS: There were many surprises. The project evolved as we worked to adapt to the very tight site. Creative solutions by one discipline would require creative reactions from another discipline so that the design could move forward. Several surprises involved discoveries of existing conditions or constraints related to the existing structure that were not apparent at the beginning. I am particularly proud of the project, I think, because of the number of hurdles that were cleared to complete it.
JG: What lessons did you learn from this project that you will apply toward future projects?
MS: You can do more than you think you can. The number of challenges on this project was daunting, and they were not all apparent at the beginning. Because of that, we focused on them individually, where we might have been overwhelmed by all of them, together. Extending that to future projects, tackle problems one or two at a time — focus on them and get them out of the way so that you can tackle the next one.
Marty Sloan, P. E., is a principal with Datum Engineers, and is the director of Healthcare Design for the firm. Throughout his 29-year career with the firm, he has managed a diverse array of technically complex projects spanning several market sectors including healthcare, higher education, civic, historic renovation, performing arts, and high tech. He can be reached at firstname.lastname@example.org.