Elevated floors serve a purpose in construction

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Many of us remember from our college days the room housing the mainframe computer. It usually had a glass wall, was air conditioned and, as you walked in, you had to ascend a step or two. That floor was raised so that the interconnecting wiring could be run underneath it. Removing the floor panels provided easy access to the wiring and made it possible to make wiring changes easily while having a room neat in appearance and safe from accidental tripping hazards.

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A small industry was founded that specialized in what is now called raised-access flooring.

Semiconductor plants soon adopted this flooring concept so that now the vast majority of their fabrication facilities feature raised-access flooring. Besides wiring, the bulk of the chemical piping for wafer fab equipment is run underneath the floor. The standards of cleanliness required in  microchip facilities rely on a continuous stream of filtered air. The logical way to provide the clean, laminar stream of air is to have the air flow down from the ceiling and out through the floor. This way gravity helps pull contaminating particulates away from critical work areas. The air passes through either a grating or perforated-style floor panel to be subsequently filtered and recirculated.

A number of manufacturers produce raised access flooring, both in the United States and abroad. The two main components of a raised access flooring system are the floor panels and the pedestals upon which they rest.

Floor panels

In this country, the standard floor panel is a 24-inch square aluminum die-casting. Abroad, the standard size is 600 millimeter(23.62 inches) which allows the using the same die-casting. A typical panel is about two-inches thick. By using rib designs of various styles, panels that weigh only about 20 pounds are capable of supporting substantial loads. The edges of the panels are machined to be even and to bring the panel into square. The corners of panels can also be machined to provide the same height at the corners. These machining operations are important for a neat, tight-fitting, and flat floor. For an extremely smooth floor, the top surface of the die-casting can be ground flatter than the 0.020-inch surface roughness typical of a die-casting of this size.

For loads above 1,200 pounds (350 pounds per square foot), welded steel or aluminum panels provide about any loading desired. Some panels are designed for loads of 6,000 pounds or more. However, as the structural capacity increases, so typically does the weight of the panel thereby precluding easy access to the components below the flooring.

The top surface of the floor panels can be finished in several ways. The two most common finishes are epoxy-powder paint or a vinyl-tile laminate. Both provide reasonably good resistance to chemicals, wear and abrasion. Carpeting and other common flooring materials can also be laminated to the panels for applications other than clean rooms. Paint, vinyl and even carpeting are usually specified to provide a given range of electrical resistance to provide the electrostatic discharge properties that are important to the protection of microchips and delicate instrumentation.

Pedestal designs

The design of the under-structure for a raised-access floor varies considerably for different facilities. In general, it must provide different finished floor heights, clearances and structural requirements. Common to under-structure designs is the adjustable pedestal. The volume of contemporary raised-access floors allows makers to use aluminum die-cast bases and pedestal heads. The pedestal head has a screw thread to provide precise height adjustment. The thread is long enough to provide at least one-inch of elevation adjustment. This accommodates installations in which the sub-floor is not flat or level. A locking nut fixes the pedestal height once the raised access floor is leveled. Floor height is typically designed to be anywhere from 6 to 48 inches. Simply cutting the aluminum pedestal tube establishes the correct height.

The most common under-structure design is a grid of pedestals on 24-inch centers. Each pedestal then supports the corners of four floor panels or, looking at it another way, four pedestals support each panel.

For some facilities, the two-foot pedestal spacing may not provide sufficient room for the piping, wiring, and access space needed.

Using heavier pedestals overcomes this—typically by means of 48-inch centers. Pre-fabricated stringers interconnect the pedestals just below the floor panels. Each stringer has a pre-punched hole at its midpoint to allow the adjustable pedestal heads to seat properly. A diagonal stringer provides support for the pedestal at the center of each four-foot square. Proposed designs extend this concept to six-foot on center waffle-slabs. It is only necessary to beef up the pedestals and stringers in such designs to minimize deflections. An additional advantage of the stringer designs is that they allow easy attachment of pipe or wiring hanger assemblies using commonly available hardware.

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