Elevated floors serve a purpose in construction

April 19, 2006

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.

The typical raised-access floor installs over a concrete sub-floor or waffle-slab. There are designs, however, that mount the pedestals directly to steel girders and stringers laid on the two-foot on center spacing. Although the standard floor design is limited to about 4-inches high, special designs can be used for much greater elevation. For example, one application required the floor to be 10-feet high so that it could be mounted in an existing basement yet leave the access floor level with the ground level flooring. One proposal for this situation was the use of pre-fabricated four-foot square structural steel towers. Spacing the towers at four or six feet and interconnecting them with the pre-fabricated stringers offered a fast under-structure erection time that still met the local seismic conditions.

Design considerations

There are design limitations to be considered in any new or retrofit access flooring installation. The first is the design of the under-structure to meet stability and vibration requirements. This is especially important if seismic requirements are important. Of course, floor panels themselves have load capabilities that must be considered, especially when moving equipment and permanently placing heavy loads on the floor.

The Ceilings and Interior Systems Construction Association publishes a document, "Recommended Test Procedures for Access Floors," that is widely used and cited. This document covers six test procedures as shown in Table 1. Although manufacturers use the CISCA procedures to rate their product and others use them as a basis for writing specifications for raised-access flooring, there are limitations to the procedure that should be noted by those interested in new designs.

Note that only the last test is a test-to-failure. The norm on the other tests uses a sample size of only three. For example the ultimate loading test requires three randomly selected panels to be tested—one at the center, another at the center of an outer edge and the third at the "weakest point." If the weakest point is not the center or the edge, the test can be passed on the basis of a single sample at what the manufacturer sets as the ultimate load. One can intuitively feel that a sample of one is not statistically valid for a material such as die-cast aluminum commonly used in floor panels.

The loads are applied though a 1 inch square steel indentor but this usually does not represent how equipment is supported in the real world where distribution plates often are used to spread the load, especially over grated panels. Material handling equipment for moving equipment may not use the same wheels as used in CISCA or it may use multiple wheel dollies. For this reason, the manufacturer should provide the end user with details not covered by CISCA. Details of the test data, like sample size and actual failure numbers, should be available as well as information relating to how the factor of safety for the design loads has been determined.

When on shakey ground

Another important design consideration wherever there is earthquake potential is whether the access floor meets the seismic requirement of the local building code and the Uniform Building Code. The 1997 edition of the Uniform Building Code gives formulas and tables to calculate lateral loads as a function of seismic zone, degree of hazard posed by a failure, location within the building and other relevant factors. The structural engineer using these load calculations should check three possible failure points:
  • the connection to the sub-floor which is usually made by epoxy cement often augmented by concrete screws,
  • the capability of the pedestal base to withstand the overturning moment, and
  • the ability of the pedestal tubes to withstand the combined stress of the overturning moment and the vertical compression loads.

Greater floor heights in more active seismic areas often demand that the pedestals be braced. One or more lines of lateral bracing transmit the seismic lateral loads directly to the pedestal bases. It is a good idea to use corner lock screws on floor panels in active seismic zones. The flathead screws attach the four corners of each panel to the four pedestal heads.

Corner locking

In recent earthquakes, some of the panels popped out when they were not locked at the corners. Corner locking and bracing also provide more protection against weakening caused by vibration. In concept, the raised access floor is a series of cantilevered columns that support a mass at the tip. Thus, each floor has a natural frequency in the lateral direction that can contribute to problems when sensitive equipment is placed on the floor. For example, in the clean room, sensitive optical microscopes often need vibration isolation because walking in the vicinity of a wafer inspection station interferes with precise microscopic measurements. Adding bracing near sensitive equipment often solves the vibration problem. In extreme cases the tool or equipment is put on a separate base isolated from the floor.

Final considerations

The cost of raised-access flooring is, of course, significantly higher than that of a standard concrete slab floor and bracing make it difficult to generalize about the total cost of raised access flooring. However, typical prices would run in the $30 to $40 per square foot (1998 prices) for panels and pedestals, exclusive of installation costs.

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