Structural Issues in Healthcare Design
Behind the heart of every medical facility lies its backbone—its structure. Like a human skeleton, it supports the building and protects the circulation systems within. As structural engineers, our basic function is to create structural systems that respond to a project's program and architectural design. Medical facilities pose particularly unique challenges because of their requirements for flexible spaces and their need to accommodate changing medical technology, as well as acoustic and vibration sensitivity.
Being aware of these issues and coordinating the structural design early in the design process with the entire project team—from facility owners and medical planners to architects, consultants, and contractors—can result in a building that better matches the facility's long-term needs and often saves construction time and costs.
Healthcare facilities strive to connect with the communities they serve by maintaining a notable civic presence and providing open and welcoming public spaces. To achieve this, architects may design large, transparent entries and public atria. These meet the design goal but add complexity to the building's structure, because they can require large floor openings; tall, unbraced columns; or multiple column-free spaces.
Structure for column-free space.
A column-free space requires space-planning considerations for a truss or transfer girder on an above floor. For example, the structural design for a new hospital with a conference center and auditorium on the third floor required that 60-foot-long, one-story-deep transfer trusses be located in the seventh-floor mechanical rooms. Hangers suspended from these trusses were used to support the areas above the column-free spaces (figure 1). The trusses were configured to allow human traffic flow, as well as permitting large mechanical ductwork to pass between truss members.
Healing gardens pose another challenge. When they are located on elevated structures, such as terraces, they present special loading requirements and a need for detailing to prevent water infiltration. Structural engineers need to closely collaborate with landscape architects to identify ideal locations for particularly heavy landscaping elements, such as trees and planters. Water infiltration issues must be dealt with by using topping slabs, sloped structures, waterproofing membranes, etc.
Healthcare facilities house a variety of functional areas: patient rooms, intensive care units, surgical suites, treatment areas, doctors’ offices, mechanical and electrical rooms, cafeterias and food service areas, and public spaces. Many of these may be relocated during the building's life to accommodate changing patient needs. Since design loads for these areas may vary, strictly matching the project's initial functions to specific structural designs can limit the overall usefulness of the facility in the long run.
To allow the owner maximum flexibility, simple studies may be performed by structural engineers to develop baseline load assumptions that allow parts of the structure to support more than one function, with minimal impact to the overall structure. For example, the primary structural elements of one hospital were designed so that imaging equipment could be located anywhere on the floor. If the equipment needs to be moved to a different location in the future, only secondary structural elements will have to be added, if any. This approach maximizes the owner's flexibility with minimal impact on cost.
Certain lateral-force–resisting systems, such as steel braces or concrete shear walls, often limit future flexibility. Other structural elements can lend themselves to more nimble space arrangements. In low-rise steel buildings, a moment-frame system (providing rigid joints for beams and columns that add to lateral stiffening) can provide design flexibility at a relatively small premium. For taller steel buildings, using elevator cores as lateral bracing elements or placing steel bracing along the perimeter can yield maximum flexibility while still maintaining an efficient internal structure. In concrete buildings, a beam and slab system can allow for increased column spacing while doubling as a concrete frame to resist lateral loads.
Flexibility in routing of mechanical ducts, electric conduits, and plumbing lines is often a concern. A standard series of openings through the floor beams to allow pass-through of piping and ductwork can be provided in specific areas to reduce expensive field modifications and decrease the overall building height by reducing above-ceiling space requirements for piping and ductwork. Using castellated beams (figure 2) is another option for areas with restricted ceiling space.
Concrete floor slabs in specific areas can be designed to allow for future slab coring for technology, plumbing, or electrical conduits, using what is called “sacrificial” reinforcement (i.e., prospectively beefing up reinforcement in areas that might be used for future piping or conduits). In a recent project, slabs in electrical rooms were designed with this approach to allow coring through the slab without expensive post-penetration reinforcement or additional structural steel.
Accommodating Medical Technology
The need to accommodate medical technology, particularly radiation shielding and sensitive equipment, has a significant impact on structural design. Early planning with structural engineers and radiation physicists can often lead to shielding systems that match the needs of the facility while minimizing the cost and space-loss to the facility.
Radiation treatment rooms require shielding from the inside out. Conventional concrete is often the most cost-effective material for this purpose. However, the thickness of the wall required to provide adequate shielding is often so great that it impedes overall space planning of the facility. Viable alternatives include using lead, steel plates, or heavyweight concrete with high-density aggregates that reduce the overall wall thickness. The structural engineer can assist in evaluating both the cost and constructability of these various options.
Judicious location of radiation treatment areas may also bring additional savings to the construction costs. For example, by locating the linear accelerator room below grade in a corner location of the lower level (figure 3), one hospital saw significant cost savings. That's because the earth's natural barrier eliminated the need for shielding below the rooms and reduced the shielding requirements along the perimeter. The location also allowed construction materials to be delivered and placed more easily than with elevated locations. Also, secondary building costs, such as premiums for providing larger supporting columns and foundations, were minimized.
Reducing floor-to-floor height using castellated beams.
Economic location of linear accelerator room
Imaging rooms may require shielding from the outside in, using built-in steel or lead plates to isolate equipment from the effects of nearby electrical and magnetic interference. The structural design must take into account both the loads related to the shielding materials and the imaging equipment weight. Floor depressions are often required to accommodate shielding from below, and possibly mounting hardware, while keeping the floor level throughout. It is essential from a cost standpoint to provide for these depressions during the design phase, rather than after construction, when the only solution becomes to elevate the floor in these areas.
Managing Acoustics and Vibrations
Noise and vibration may arise from sources inside or outside the healthcare facility. Internal sources may include HVAC equipment, electrical transformers, emergency generators, and plumbing equipment, while external sources might include adjacent building equipment and traffic. Careful planning and anticipation in the early stages of design may help avoid expensive modification, such as acoustic and vibration isolation.
For example, simply distancing mechanical and electrical rooms, public corridors, lavatories, and other sources of sound and vibration from sensitive spaces can minimize need for structural premiums to control these effects. If isolation of a sensitive space is required, however, the building's structural design plays an important role. Using thicker floor slabs can protect against airborne noise, while providing structural stiffening elements and damping devices (to dissipate kinetic energy) can minimize the effect of vibrations. Acoustic isolation slabs (figure 4) may also be used. Structural discontinuities, such as simple joints in the floor slab, can also prevent transmission of sound and vibrations through the structure. The structural designer must work closely with planners, as well as acoustic and vibration consultants, to develop the most desirable environment in the most cost-effective manner.
The structural design must also address the operating conditions of sensitive equipment. Equipment vibration criteria, generally provided by the supplier, specify limits on the vibrations occurring at the equipment supports and consequently on the floor supporting the equipment. Criteria for floors supporting sensitive equipment—such as optics, electron microscopes, and instruments for microsurgery, eye surgery, and neurosurgery—are included in AISC and ASHRAE publications addressing floor vibrations and available to everyone; these can be used in the early stages of a project in lieu of manufacturers’ criteria for specific equipment. Design of the floor framing elements must meet these stringent vibration criteria.
The unique issues facing the healthcare architect and facility owner can have important implications for a facility's structure. Being aware of these issues early in the design process can result in a structural system that is better integrated with the overall facility design, often with significant long-term cost savings to the owner. HD
Faz Ehsan, PhD, PE, is Vice-President and David Weihing, PE, SE, is an Associate with Thornton-Tomasetti Group, Inc., based in Chicago, with 12 offices throughout the United States as well as in Shanghai and Hong Kong.
Toward Evidence-Based Structural Design
Structural engineers may be well advised to develop an evidence-based design process for the engineering of healthcare facilities. The process described by D. Kirk Hamilton, FAIA, FACHA, in “Certification for Evidence-Based Projects” (HealthCare Design, May 2004, p. 37) might be adapted to guide them. The steps outlined would include setting design goals, identifying key design issues, selecting criteria for success or failure, measuring results in the field, and reporting the findings.
Using design for sensitive equipment as an example, it would follow that the design goal would be for the structure to meet the relevant vibration criteria. Key design issues would include selection of building systems (e.g., steel versus concrete), column spacing, etc. Success or failure would be measured by the ability of the built system to meet the criteria. Vibration measurements would be performed in the field to verify the design and the results reported.