height is 14 to 16 feet without an interstitial space and 20 feet with an interstitial space (Mayer, 1995). Vibration problems are common in buildings with grid lengths longer than about 30 feet. The width of the grid dictates the width of a laboratory module. A width of 22 to 24 feet is divided in half to make a laboratory module (or bay) of 11 to 12 feet. The use of the laboratory module to design the layout of the laboratories is discussed in the section titled "Modular Approach to Laboratory Floor Layout" in this chapter. The traditional laboratory module width of 10 feet may be too narrow for some research activities, especially in instrumentation laboratories. The wider grid provides greater flexibility in laboratory design and for future renovations.

Floor Loading. Some instruments and research equipment are heavy, and their weight exceeds the floor loading of many buildings. Some commonly encountered heavy equipment is listed in Box 3.14. Laboratory buildings should be designed with floor loading of 100 to 150 pounds per square foot to meet both current and future needs.

Frequently, various pieces of equipment require additional floor loading support. Several options are available to address the problem: place the equipment on the ground-level floor grade, strengthen several of the lower floors, or strengthen a wing or defined area of the building. A common problem with the last option is that the defined area of the building will support the equipment, but the circulation corridors and elevators in the building will not have been designed to support the weight.

Placing the heavy equipment at ground level has several advantages: little or no additional building stiffening is required, on-grade equipment is in a low-vibration zone of the building, and elevators do not need the higher load capacity.

Heavy equipment is usually large and bulky, thus requiring wider halls and doors and comers with a wide turning radius. If such equipment is kept in a designated area, then standard-dimension halls and doors can be used in the remainder of a laboratory building for a saving of space and construction costs.

BOX 3.14 Commonly Encountered Heavy Equipment and Instruments

• Nuclear magnetic resonance spectrometers

• Freezers and refrigerators

• Mass spectrometers

• Optics and laser tables

• Large number of medium-weight instruments

• Incubators

• Extruders

• Fabrication equipment

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