Today's medical equipment is a significant investment and a critical component of a healthcare project-it can account for up to 50% of the first cost of today's healthcare facility and generates much of the life cycle revenue. In extreme cases, the cost of equipment housed within a structure can exceed the construction cost of the building.
With heavy weights, strict vibration-control requirements, and the need for protective radiation shielding, this considerable asset has a significant impact on, and greatly relies on, the structural system's design. The cost of not properly planning early in design for the structural systems for healthcare equipment is high.
To meet the challenges of supporting medical equipment, structural engineers have considerations ranging from the truly weighty to the extremely narrow:
The magnet in each magnetic resonance imaging (MRI) unit weighs about the same as a typical yellow school bus filled with a varsity football team. The floor must support this weight in a smaller footprint.
The required protective radiation shielding of a linear accelerator vault can weigh as much as a 20-foot-deep swimming pool and can take up four to eight feet of solid concrete floor depth.
Operating room microscopes are so sensitive to vibration that the structure is commonly required to move less than the width of a human hair in a second.
Although these examples seem extraordinary, they represent only a fraction of the unique challenges of supporting medical equipment. Without proper planning for equipment's impact on the structural system, the results can limit function. For example, deep structural elements limit plenum space, closely spaced columns prevent an efficient architectural layout, and downstream strengthening of a structural element can impact use of space on adjacent floors. The architect, equipment planner, and structural engineer must collaborate during the design phase to ensure that structural systems accommodate both initially planned and future medical equipment and can avoid intrusive and costly structural modifications in the future.
Challenging equipment demands
Heavy live loads. Structural engineers classify the weight of medical equipment, as well as furniture and building occupancy weights, as live loads. Healthcare facilities are required to accommodate live loads between 40 pounds per square foot (psf) in patient rooms to 80 psf in corridors. The weight of commonly used pieces of equipment often exceeds code minimum design loads, such as MRIs with 3.0 Tesla magnets (each weighing 25,000 pounds). When distributed over the footprint of the magnet, this equates to a 300 psf uniformly distributed live load.
The path that equipment will travel during installation to its intended location must also be considered during design (figures 1 and 2).
Equipment's travel path within a facility must be taken into consideration
Without a feasible interior travel path, equipment can be brought in through an exterior wall
Shielding. Protective shielding in a healthcare facility keeps out unwanted interference and isolates potentially harmful radiation and magnetic fields (figure 3). The electromagnetic waves generated by electronic devices may negatively affect or cause malfunctions in other, similar, electronic devices. Such effects are called Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI).
Lead brick walls serve as protective radiation shielding
MRI magnets generate strong magnetic fields and impose strict limits on magnetic material close by. And, with structural steel and steel-reinforced concrete being the two primary construction materials for floor systems, this presents a challenge. Magnetic fields can also disturb the function of adjacent equipment, and can be affected by nearby elevators or electrical transformers.
Several types of shielding exist and all have impacts on the structural design. Typical EMI/RFI shielding consists of thin copper sheets enclosing the room. Although the weight of the copper sheets is typically not a significant structural load, the extent and details of the enclosure must be coordinated to avoid conflicts with structural framing elements and provide adequate structural support.
Magnetic field shielding can either be passive, such as steel plate lining the walls and ceiling of a room, or more commonly, active shielding built into the magnet. New 7.0 Tesla magnets can require passive shielding consisting of steel plates up to 12 inches thick. This presents challenges not only in supporting the weight of the steel, but in designing the steel enclosure to be constructible, given site constraints.
Radiation therapy equipment requires large-scale shielding, such as a vault with concrete floor and wall thicknesses of several feet, and this must be accounted for early in design. Lead bricks, high-density concrete, or steel plate can also be used to reduce the required thickness of shielding through increased material density. In any event, the shielding material must be specified by the owner's certified health physicist, with a keen eye on material pricing, constructability, and facility layout.
The need for future access, repair, or replacement of the equipment through an access hatch or removable concrete panels must also be considered.
Floor and ceiling vibration. A structure's vibrations wreak havoc on research activities using microscopes and other sensitive equipment. Although vehicular traffic and mechanical systems can be sources of vibration, foot traffic in the corridor is often the primary culprit (figure 4) Because equipment sensitive to vibrations can be hung from the ceiling or floor-mounted, vibrations must be considered both in the floor structure above and below.
Uniting surgery and imaging can lead to strict vibration limits
Structural vibration is quantified as velocity measured in micro inches per second (mips). A typical office floor can generally accommodate velocities of 4,000 to 6,000 mips. For healthcare facilities, velocity requirements between 2,000 mips for operating rooms and 500 mips for sensitive microscopes or imaging are common. With early consideration, structural floor framing can economically accommodate common vibration performance requirements. Stricter velocity limits require more extreme structural measures, such as deep beams and girders, close column spacing and the input of a vibration consultant.
Structural system types
As mentioned, the two primary construction materials for floor systems are structural steel and reinforced concrete. Each has an impact on both initial and future healthcare equipment installations. Beam-and-slab systems can accommodate new openings in the slab between beams. Post-tensioning in slabs and girders can reduce the required depth of floor framing but can limit future modifications to the structure requiring cutting or core drilling. While steel floor systems can be modified by field welding new structural elements to frame new floor openings or to support additional loads, changes in framing during construction after initial document issue can have larger cost and schedule implications than with concrete. Because the structural capacity and geometry of cast-in-place concrete framing is established as formwork, is set, and rebar is placed on-site, last-minute modifications can be more easily and cost-effectively accommodated.
Case study
The University of Texas M. D. Anderson Cancer Center is currently constructing its new six-story, 315,000-square-foot Center for Advanced Biomedical Imaging (CABIR). The center will house a variety of state-of-the-art imaging equipment-from an MRI unit and a combination positron emission tomography (PET)/computed tomography (CT) unit, to a cyclotron. In collaboration with P&W Architects, Walter P Moore took an active role in the early planning stages of this facility and designed a structure that economically satisfies the building's performance requirements.
The most vibration-sensitive equipment and that requiring heavy shielding will be located on the ground floor to avoid costly elevated structural framing solutions. Initially, the elevated structure was to be designed for 500 mips, which would have meant an architecturally inefficient column layout. In collaboration with the architect, equipment manufacturers, and vibration specialists, we suggested locating vibration-sensitive equipment close to columns and designed the floor framing for 2,000 mips. This resulted in a cost-effective structural system that will allow proper function of the equipment and efficient use of space.
Planning for tomorrow
Healthcare design teams have numerous factors to consider, further complicated by trends that can't be predicted-for example, blending diagnostic imaging and treatment using equipment such as intra-operative MRI scanners. For this and other reasons, equipment locations outside of traditional departmental boundaries are becoming more common. This requires a balance between freedom to place heavy, sensitive equipment anywhere and restricting the equipment to the confines of a narrowly defined room.
With preliminary design criteria developed acknowledging these parameters, the project team can determine structural system, floor-to-floor heights, and column grid. Both vibration requirements and heavy live loads must be reflected in structural depth and column spacing. Also, since heavy shielding absorbs not only radiation but valuable real estate with unusually thick structural elements, these should be shown on the plans for proper coordination. Electrical distribution pathways and required equipment depressions can be accommodated using raised-access flooring or through trenches and steps in the floor. Decisions such as these should be coordinated early due to the weight of access floor or topping slab and the potential impact to structural geometry.
Conclusions
As programming is defined, the architect, owner, and structural engineer should consider medical equipment and its impact. Early identification of challenges, clear communication, and creative problem-solving will help create a facility that provides the necessary flexibility to respond to medical equipment and its evolution for years to come. HBI
Nathaniel Ryan Seckinger, PE, is based in Walter P Moore's Washington, D.C., office and can be reached at rseckinger@walterpmoore.com. Kurt Young, PE, LEED AP, is the leader of Walter P Moore's Healthcare Community of Practice and can be reached at kyoung@walterpmoore.com. Muhammad Cheema, PE, has designed and managed structural engineering services for healthcare projects ranging from $1 million to $800 million in construction cost and can be reached at mcheema@walterpmoore.com. Healthcare Building Ideas 2009 Spring;6(2):29-31
