A STEP AHEAD: Well-coordinated structural drawings are critical to project success today and in the future.
Design of the structure begins with knowing the suitability of the building site for construction, the construction materials (e.g., structural steel or concrete), and the wind and seismic loads that are to be used in the design. Several studies should be authorized early in the life of the project to assist the structural engineer as design begins.
First, a geotechnical report is required to provide foundation and earthwork recommendations for the project. A geotechnical investigation consists of a series of borings done across the building footprint as well as in parking areas to ascertain the subsurface soil conditions. Boring depths may vary from just a few feet to more than 100 feet. From these borings, samples are taken, laboratory tests are done, calculations are made, and a report is generated. This report describes the types of soil on the site, the suitability of the site for construction, how the existing soils need to be prepared for construction, and potential foundation types.
The soil preparation, or earthwork recommendations, may include proof rolling, undercutting the native soil, or other ground improvement methods. Proof rolling typically consists of driving a loaded dump truck or scraper over the site in each of two perpendicular directions, a minimum of two times, to compact the soil. Undercutting involves removing the upper layers of existing native soils, which are unsuitable for construction, and replacing them with compacted engineered fill or other approved fill materials.
Foundation types may consist of shallow or deep options. Shallow options include isolated spread footings, which are independent footings that are provided for each column, and strip footings, which feature a continuous strip of concrete that spreads the weight of a load-bearing wall across an area of soil. These footings are typically located a couple feet below the first floor of the building. Deep foundation options are used when the upper strata of soil are not suitable for shallow foundations. Options include drilled piers (large-diameter, concrete-filled shafts with a cage of reinforcement) or auger cast piles (smaller-diameter shafts installed with high-strength cement grout using a hollow stem auger). Auger cast piles are typically used in groups at each column and interconnected with a concrete pile cap.
The second valuable pre-design investigation is an underground site utility survey, especially on expansion projects where workers must contend with existing underground utilities that need to be either relocated or accommodated in the design. A site utility survey will assist the design team in identifying which underground utilities must be addressed since as-built information is often incomplete, inaccurate, or nonexistent. When an unexpected underground storm or sanitary line is discovered during construction, it can lead to redesign and construction delays.
Third, if necessary, a shielding study for linear accelerator vaults is required to ensure the size and layout of the vault is adequate to limit radiation exposure to staff, patients, and the public. A shielding study provides shielding barrier recommendations, such as the wall and roof thicknesses required. Shielding materials can consist of lead, normal weight concrete, heavyweight concrete, steel plates, composite materials, or other options. This study is performed by a physicist and should be authorized early to allow time for coordination with the vendor, physicist, and design team.
Finally, obtaining any available existing drawings early in the design is important. Healthcare projects are often additions to existing facilities. Reviewing the existing drawings allows the engineer to check the existing structure for any new loads that willbeaddedtothe structure and identify any potential conflicts with the existing structure. Some hospital facilities may have a dedicated storage room for drawings, while others may have digital copies or none at all. When existing drawings are not available, site visits maybe necessary before design begins.
Columns are vertical structural members that support each floor of a building. Coordinating column sizes and locations in the beginning stages of the project can lead to a more economical design while maintaining the flow of the space. For new additions, keeping the column layout in mind when laying out programming space reduces the need to shoehorn columns into undesirable locations after the owner has already signed off on the floor plans. Typical hospital column spacing may range from 30 to 35 feet, but longer spans are also possible. Using modular bay spacing and keeping columns on the same grid lines as much as possible have proven to be more cost-effective design solutions.
Because many healthcare projects are designed for future growth, identifying the number of potential future floors and areas of expansion, including operating rooms, mechanical rooms, and penthouses, is important to ensure the columns and foundations have enough capacity to handle those additions. The structural drawings should include a key plan indicating the anticipated growth.
In the case of vertical expansions or renovations, the column locations are already set by the existing structure, but it's important to recognize that the actual column locations that were built won't exactly match the theoretical column locations on the existing drawings because of permissible construction tolerances, which allow for inherent variances in construction materials and workmanship skills. In some cases, the actual column locations may differ by several inches in multistory hospitals, which could lead to a column potentially encroaching into a new corridor.
The American Institute of Steel Construction (AISC) Code of Standard Practice requires the contractor to survey locations of existing columns prior to detailing or fabricating steel for a vertical addition. The new column will need to be placed as close to the as-built column line as possible because the existing columns typically cannot support the load from the new columns when they are offset by more than one-quarterto one-half an inch.
On horizontal additions, the adequacy of the perimeter beams, columns, and foundations must be checked if the new structure imposes additional loads on the existing structure. Using expansion joints for a horizontal addition will isolate the new gravity and lateral loads from the original hospital. When locating columns at an expansion j oint, it's beneficial to hold the columns back 8 to 10 feet from the existing building and cantilever the framing to the expansion joint to avoid potential conflicts between new and existing column footings or piers.
After the columns are set, the beams, or horizontal structural members supporting the weight of the floor, can be located. Beam depths may vary considerably on a project depending on the distance between columns and the loading that must be supported. The design team should discuss the anticipated structure depth so the floor-to-floor heights and ceiling heights provide adequate clearance for mechanical ductwork, piping, and electrical conduit in above-ceiling spaces.
Patient room layouts also should be reviewed in relation to the beam locations. Toilet, sink, and shower drains that penetrate the floor must not pass through a steel beam or narrow concrete joist because the structural capacity would be undermined. Floor penetrations in concrete beams can sometimes be accommodated by widening the beam or adding supplemental reinforcing bars in the beam. If floor penetrations cannot be moved, beams might need to be respaced or the beam orientation may need to be revised.
The size of elevator shafts and clearances required for the elevator equipment can also affect beam sizes and locations. These dimensions can be estimated by reviewing preliminary information from several different elevator suppliers; however, final elevator shop drawings are needed before construction or fabrication and detailing of the beams. Elevator guiderails, which define the path along which the elevators ride, typically need support tubes because the rails cannot span the floor heights without an intermediate support. The elevator hoistway should be sized and beams located to allow the guiderail support tube to run continuous vertically and not be interrupted by the slab edge. Otherwise, the tubes will interrupt the shaftwall.
It's also important to consider floor vibrations in elevated slabs framed with beams and columns. Structures vibrate when people or equipment move across the floor, which can induce floor vibrations in other areas. Excessive floor vibrations can also create problems in medical equipment such as lights, booms, and suspended monitors. AISC Design Guide 11, "Vibrations of Steel-Framed Structural Systems Due to Human Activity," and Facility Guidelines Institute's Guidelines for Design and Construction are two resources that provide information on acceptable vibration requirements in structures. Different criteria are used if the occupied space is an administrative area, patient room, or operating room. For example, ORs require stiffer floors than typical patient rooms due to the sensitivity to movement of the OR equipment. Larger beam sizes, tighter beam spacing, and a thicker floor slab are some of the techniques used to help address these issues. MRIs on elevated slabs require special consideration, too, and the vibration requirements must be carefully reviewed with the vendor. Thicker slabs, additional columns, and heavy beam sizes are typically necessary, which can impact the project budget. For renovation projects, in-situ testing may be necessary to evaluate the vibration characteristics of the existing floors and ensure compliance with requirements.
Discussing structural-related coordination items such as site conditions, column sizes, and beam placement early in the project will assist the structural engineer in providing a cost-effective plan for the owner. This step can minimize change orders, RFIs, and construction delays, which can lead to cost savings and contribute to the project's overall success.
By Jeremy Salmon
Jeremy Salmon, PE, SE, LEED AP, is a principal at Structural Design Group in Nashville, Tenn. He can be reached at JEREMYS@SDG-STRUCTURE.COM.
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|Date:||Apr 1, 2019|
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