Chapter 5: Overhead or Under-Floor Installation?

Cisco Press

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The second option, making the top of the raised floor flush with other floor surfaces, requires the Data Center subfloor to be set into the ground. In this configuration, what is normally called the height of the raised floor becomes its depth. The more space needed for the subfloor, the deeper the cavity below the surface this space must be. Constructing a sunken Data Center floor in this manner is more expensive, but does provide certain advantages.

A major benefit from this arrangement is that no ramps or lifts are needed to transport equipment into the Data Center. The floor surface never changes elevation between the Data Center and adjoining rooms or corridors, so items can be rolled in or out of the room without assistance. Eliminating these mechanisms from a Data Center conserves floor space and adds convenience for users. Materials and equipment can be brought in to the room from multiple access points—any door that is wide enough—rather than only by way of specially equipped entrances. Depending upon how the areas are designed and where they are located in relation to the Data Center, a level floor may enable you to forgo otherwise-needed construction of raised floors in adjacent support areas such as a Build Room or storage area—recouping at least some project costs due to the sunken floor.

If you choose to have a viewing area into your server environment, perhaps for a control center or to facilitate tours, the uniform floor surface height also enables you to avoid making the choice between having to look up into the elevated server environment or building up the height of the viewing area.

Because this decision can affect the building's foundation or require greater heights between stories, depending on what level the Data Center is located on, it should be made as early in the design process as possible.

Ramps and Lifts

Assuming that your Data Center's raised floor surface is elevated, there are two mechanisms for bringing equipment into the room and level with the floor—ramps and lifts.

Ramps are the most popular choice. They enable either people or materials to be brought into the Data Center and have no moving parts to malfunction. Their length is determined by two factors—the height of the raised floor and the slope used to reach that height. In the Unites States, Data Center ramps feature a 1 in 12 incline—a 1 inch increase in height for every 12 inches in length. Such a slope is considered acceptable under the Americans with Disabilities Act of 1990, which is designed to make all workplace areas accessible to those with disabilities. Using such an incline, a room with a 15-inch high raised floor needs a ramp that is 15 feet long, while one with a 24-inch raised floor needs a ramp 24 feet long. (Although the formula doesn't translate as gracefully in the metric system, that is 4.6 meters long for a 38.1-centimeter high floor, or 7.3 meters for a 61-centimeter high floor.)

Although this requirement doesn't apply in other countries, still consider a 1:12 incline. A shallower incline requires more space for the ramp, which doesn't help your design. A steeper incline means a shorter ramp, but makes it significantly more difficult to steer heavy equipment in or out of the Data Center. Full server cabinets can be quite heavy, and if you have ever had to push an object up an incline—say a stalled car or a piece of heavy furniture—you know that it is much harder to move it up a steep rise than a shallow one.

Data Center ramps are ideally going to be 6 feet (1.8 meters) wide and have landings at both ends that are 6 feet (1.8 meters) square. These dimensions enable sufficient room for maneuvering large equipment in or out of the Data Center. When designing your Data Center ramp, keep in mind the advantages of having floor space serve multiple functions. It is completely acceptable to have the bottom landing for the server environment located in the building's hallway, for example, as long as there is sufficient clearance.

Building codes in many regions require some form of handrail on both sides of the ramp, to prevent a person or piece of equipment from accidentally slipping off a ramp edge. For the same reasons, it is advisable to install slip-resistant tread on the ramp. Depending upon the size of your Data Center and the wording of regional building codes, you may be required to have more than one ramp.

The alternative to having a ramp for your Data Center is a lift, an adjustable platform that enables you to raise an item up to the height of the raised floor or down to the level of the regular floor in the adjoining room or corridor. A lift occupies less floor space than a ramp, but is more expensive. As such, it is best reserved for sites where floor space is at a premium.

Weight Bearing Ability

Arguably the most important element of any Data Center floor, whether it includes a raised floor system or not, is its weight-bearing ability. The more weight your Data Center floor can support, the more equipment, large and small, that can potentially be installed in the room. Ideally, your Data Center floor should be able to support 1500, 2000, or more pounds (680.4, 907.2, or more kilograms) per server cabinet location. Those were considered exorbitant weight loads a few years ago, but current design trends for servers and other Data Center devices are producing heavier and heavier equipment. The most prudent approach is to assume those trends are to continue and to design your server environment accordingly.

Ultimately, the overall weight-bearing ability for a Data Center floor is limited by the structural makeup of the building—the thickness and integrity of the building's concrete slab if the room is on the bottom floor and the same characteristics of its steel skeleton if the server environment is above the ground.

Specify in your design package how much weight you want your raised floor system to bear. You want the system to have essentially the same strength as the subfloor. Lesser weight-bearing ability doesn't take full advantage of the building's strength, while greater ability increases construction costs without providing any benefit.

Overall weight isn't the only characteristic that must be specified for your Data Center floor. There are several types of load that are applied to the floor, and they fall into two overall categories:

  • Static loads, which involve constant weight:

  • Concentrated or point load—Weight applied on a small area, such as where the pegs or casters of a fully loaded server cabinet touch the floor. For example, if a cabinet is filled with equipment and weighs 2000 pounds, the load is distributed evenly among its four casters, 500 pounds each. To support this cabinet, floor tiles must be rated for a point load of at least 500 pounds.

    Uniform or static load—Weight distributed over a larger area. Common occurrences in a Data Center are heavy boxes or large equipment that sits flush on the ground and possesses no support pegs or wheels. This load — can also be an accumulation of point loads. If two of the casters from the previous example rested upon the same floor tile, that tile would have a static load of 1000 pounds upon it. To successfully support such weight, the panel must be rated for a static load of at least that amount.

  • Dynamic loads, which involve the movement of weight over an area:

  • Rolling load—Weight rolled over an area from passing equipment. This load is defined not only by the weight of the object but also by the size and hardness of the wheels that make contact with the tile and how many passes are made. Practically anything transported across a Data Center floor—server cabinets on casters, supplies moved by way of a pallet jack, backup media transported by mobile cart, even a person sliding across the floor in an office chair with wheels—is a rolling load.

    Affect load—The force put on a raised floor by a dropped object. This is defined by the weight of the item and distance it falls. Although items aren't regularly dropped or tipped over in a Data Center, the floor must be able to withstand such force in the event that an accident does occur.

    Ultimate load—The minimum weight that exceeds a floor tile's ability to support a load. This is essentially the breaking point of a floor panel, past which an object crashes through to the subfloor. This may be represented as an independent weight or as a multiple of a panel's concentrated load.

Although concentrated and uniform loads place ongoing pressure on the raised floor, the dynamic loads have the most potential to cause damage in the Data Center. Rolling items over the same floor tiles can cause wear and tear over time, while dropping an item on the floor may cause small cracks in the floor system as well as damage panel surfaces.

When specifying the weight load requirements of your Data Center, apply them to whatever ramps or lifts the room is equipped with as well. It does little good for your server cabinet locations to be able to support thousands of pounds each if the equipment can't make it past the entrance because of a weaker lift or ramp.


Note - Consider setting the structural capacity of the Data Center so that all cabinet locations can support at least 20 percent more than the heaviest item you ever expect to house. So, if the heaviest fully loaded server cabinet will be 1500 pounds (680.4 kilograms), specify 1800 pounds (816.5 kilograms) per tile location. This provides a buffer to accommodate unanticipated equipment weights and density in the future.


If you are constructing a server environment in an existing building, obtain a map that indicates the weight-bearing abilities of the structure. Such information can typically be acquired through the architectural firm and can help you lay out the Data Center space most efficiently.

Types of Floor Tiles

Three types of floor tiles are involved in a raised floor system:

  • Blanks

  • Perforated

  • Notched

These floor tiles come in one standard size—2 feet (61 centimeters) square—and are typically made of steel, with wood or concrete at the core, or cast aluminum. Tiles can be ordered in various colors, or even made transparent, although beige and light gray are the most common.

Blank or solid tiles are the most abundant in any Data Center. They have no openings and are capable of supporting the most weight, and therefore are traditionally placed in all aisle locations, directly under Data Center equipment, and on the room's ramp.

Perforated or vented tiles have dozens of tiny holes in them, enabling air to flow through. These panels are most often placed immediately in front of server rows, although they may be located anywhere in the room depending upon how you want to direct airflow. Where specifically to place them in your Data Center is explained in Chapter 8, "Keeping It Cool." Perforated tiles can be configured with a sequence of adjustable metal plates on the bottom. Adjusting a single control mechanism on the tile, usually with a hexagonal wrench, shifts the position of the plates. In this way you can restrict as much or little airflow through a perforated tile as you want.


Note - So, if perforated tiles enable you to better control airflow in the Data Center and it is possible to completely close them, why not put them in the ramp and aisles, too, and not bother with any blank tiles? It is because perforated tiles are weaker than blanks due to all of the holes in them. Installing perforated tiles throughout the room would reduce the weight-bearing ability of your raised floor.


Notched or cut tiles are similar to blanks except that they have one or more openings for patch cords and power cables to be threaded through. You can have the openings made to practically any size or shape and placed anywhere on the tile. You want the openings to be large enough to accommodate an average-sized bundle of power and data cables routed through your Data Center, but not so large that air from the floor can escape. The more air that escapes out of your Data Center's cut tiles, the less that comes out of your perforated tiles, which are where you want the cooling.

There are multiple products available to help prevent air from leaking out of oversized tile cut-outs. Some consist of foam padding or wiry bristles that enable cables to be threaded through a tile opening but prevent air from passing through. They work reasonably well but, over time, might become weighted down or frayed and no longer function as effectively. Others are caps of wood, metal, or plastic that rest in tile cut-outs. These can be adjustable in size, enabling cables to pass through while occupying the remainder of the opening. These continue to function for years, but are vulnerable to human nature. Data Center users often remove and set tile caps aside, requiring additional time from the room's support staff to track down and replace them. The caps, no matter what they are made of, may also give people a false impression about how much weight they can hold. Data Center users transporting cabinets or equipment across the raised floor are usually conscientious enough to steer around notched tiles with open cut-outs, but may not think twice about rolling heavy equipment over one with a cap. The result is a broken or damaged cap and possibly a toppled server cabinet.

It is important to maintain air pressure in a server environment so that air can circulate properly through the room and thereby regulate ambient temperatures humidity. Data Center air pressure is affected by how floor tiles are deployed—the more openings in the floor from perforated and notched panels, the harder it is to maintain pressure. Your proposed deployment of floor tiles in the Data Center should therefore be reviewed by the project vendor responsible for the room's cooling. They may recommend a limit for the total number of perforated tiles in your Data Center.

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