This tip is the second in a series on choosing a raised floor for a data center. Read part one about considering...
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a raised floor in a data center.
Let’s say you’ve decided to use a raised floor in a data center. What are the most important factors to consider? There are really three: structural strength, airflow and leakage if you’re using it for cooling and static dissipation. Selecting and properly specifying a raised access floor involves more than we can cover here, but this tip explains how to evaluate raised floors and panels based on a variety of factors, including ratings, floor surface materials and airflow.
Evaluating raised floors and panels
Raised access floors come in many types and flavors. Early panels were wood, wood composite or hollow steel. Most were not particularly strong, nor did they need to be. Modern raised floors for data centers are usually made of cement-filled steel or cast aluminum. For easy access, we need “lay-in” panels that can be easily removed, rather than the screw-down type that are bolted to the pedestals at each corner. And because cabinets in today’s data centers are getting heavier, we need the strength and stability of a “bolted stringer” understructure, rather than panels that just self-lock to the pedestals at the corners.
Selecting a panel to meet the structural needs of a data center can be confusing. Panels have historically been labeled and marketed for their “concentrated load” ratings. This is the maximum load that can be applied to the weakest one square inch of the tile without deforming it by more than a specified amount. But different manufacturers provide other ratings, including uniform load (the average weight per square foot the panel can support when weight is evenly distributed across its four square-foot surface), yield point (where the panel permanently deforms) and ultimate load (where the concentrated load actually causes the panel to collapse or break).
One major manufacturer prefers to specify “design load,” which is essentially the concentrated load (multiplied by a safety factor) measured with the panel on its actual understructure rather than supported on four corner “test blocks,” as is the usual standard testing method. Design load may relate better to real world use, but it makes comparisons with products rated on concentrated load more difficult. Either way, it is the concentrated load or design load, not the uniform load, we care about most, since cabinets usually sit on small leveling feet or casters, not solid full-size bases. Uniform load is meaningless in a data center.
Understanding floor ratings
Floors typically come with Design or Concentrated Load ratings between 1,000 and 3,000 pounds. So what strength do we really need? There is a tendency to use the highest load rating simply because cabinets are getting heavier, but is that really necessary? Consider that a 2,500-pound cabinet with its weight equally distributed on its four feet or casters could put 625 pounds on one square inch of the tile. If the cabinet was 24 inches by 24 inches, we would need a tile rated to support 2,500 pounds. Today, cabinets are usually larger than that, so all four cabinet support legs may be on different tiles. In that case, tiles rated at only 1,000 pounds would support each leg. But equipment weight is never uniformly distributed within the cabinet, with more weight usually concentrated toward the front. Moreover, cabinet rows will always have some tiles that support the legs of two cabinets. So if the unequal loading results in 850 pounds on each of the front legs, and both legs are on one tile, that’s 1,700 pounds, which requires either a floor rated to support 2,000 pounds or extra pedestals under tiles that are rated to support 1,500 pounds. Why not just be safe, and call for a 2,500-pound rated floor? In making this decision, consider several other things.
First, stronger floors usually weigh more, which, depending on the building slab rating, may reduce the useful cabinet weight that a raised floor can support. This is a reason to compare the weights of similarly rated panels, and is also why some designers advocate cast aluminum floors despite their much higher cost. Second, although some cabinets may weigh 2,500 pounds or more, others will probably weigh less. If you only have several heavy cabinets, extra pedestals under them may be fine, but if you have too many heavy cabinets, extra pedestals could be forgotten with resulting damage to the floor. Another important rating is "rolling load,” because we need to get cabinets across the floor and into position. Here the numbers can be misleading.
Examining panel ratings
Panels are rated for the loads they will sustain for 10 wheel passes, as well as the loads they will sustain for 10,000 passes, so it’s important to know what kinds of loads you’ll be moving over the floor and how often, or to have cover plates available to lay over your path while you’re moving equipment (plywood is too soft; heavy masonite or aluminum plating is necessary). You could also install stronger panels in your delivery paths, so long as the panels with different strengths are interchangeable in the floor structure. But some strange numbers have appeared on data sheets for rolling loads, with the explanation well hidden in the documentation. There are panels that list 1,500 pounds for 10 wheel passes, and 2,000 pounds for 10,000 wheel passes! Did they really get stronger when more weight was rolled across them thousands of times? Of course not! The two tests were run with different wheel sizes, because the panels would fail before 10,000 passes if the smaller wheels were used with the same weight.
Clearly, it’s important to carefully read manufacturers' data sheets and to question anything that looks incongruous. Since not all floor manufacturers test and specify the same way, it’s also good to know how tests were run to compare ratings and determine whether they were done by independent testing labs. There are many other raised floor structural ratings, but these are the most commonly examined in areas that are not concerned about frequent earthquakes.
Floor surface material
We should also be concerned about the antistatic characteristic of a floor material. There are two types of floors that are often confused: conductive and static dissipative. Technical definitions classify static dissipative as a particular type of conductive floor, but manufacturers of raised floor products for data centers and clean rooms will generally identify them separately. Conductive flooring is typically used in clean rooms, where people are handling microchips. This type of flooring has a lower resistance to ground than static dissipative products. Conductive flooring is not needed, nor recommended, for data centers.
In data centers, we need static dissipative floors that will conduct static charges of more than 100 volts away from our bodies and clothing and through the floor tile to the ground. This requires a surface material to have the necessary static dissipative qualities, and a grounded understructure that prevents the generation of static electricity. The understructure should also conduct electrical charges away so that they are not harmful to our equipment. A wrist strap should still always be worn when working inside of equipment! Ratings should be based on the resistance from any point on the panel surface to the pedestal, which also needs to be properly grounded to work.
The surface material on a computer room floor should be a zero-maintenance product. It should never need to be waxed or buffed, as wax accumulates dirt and must be removed with liquids, and buffing creates dust. The material must also be hard enough for equipment to roll over and sit on without denting or deforming. This rules out rubber and vinyl materials. And, of course, carpet of any kind should never be used, as it both creates and traps particulates. The most commonly used surface covering in data centers is known as high-pressure laminate (HPL). It can be made with the necessary static dissipative qualities and also has the hardness and maintenance characteristics needed. It should also be made so the laminate edges are not easily damaged.
Floor tightness for airflow
If the floor plenum is going to be used for airflow, it’s important that it is installed with air tightness in mind. The panels must be square and tight to minimize air leakage, and every edge next to walls and air conditioners and around pipe penetrations must be sealed. Cable cutouts should also be equipped with air-seal grommets, of which several types are now available. Some manufacturers have entire specifications for the installation of air plenum raised floors. Adherence to such documents should be required in the architect’s specifications. The selection of airflow panels is also an important matter, with too many options and considerations to examine in this article.
The important thing to remember is that while adjustable dampers on tiles can be very valuable for balancing air delivery among cabinets, adding a damper to any tile effectively reduces its “open percentage” and its airflow, even when the damper is fully open. For example, a Tate 25% open perforated panel with no damper will pass 746 cfm of air at 0.1 inches of static pressure under the floor. Simply adding a damper and leaving it fully open reduces this to 515 cfm. That’s the equivalent of only 17.4% open. With a 56% open grate tile, the difference is even more dramatic--2,096 cfm with no damper and only 1,128 cfm with a fully open damper (a reduction from 56% to 30.5% open). An examination of manufacturers’ airflow characteristics and images, with and without dampers, will quickly reveal this effect.
In short, a raised access floor is an important part of many data centers, and should be chosen and installed with as much care as is the equipment sitting on it.
ABOUT THE AUTHOR: Robert McFarlane is a principal in charge of data center design for the international consulting firm Shen Milsom & Wilke LLC. McFarlane has spent more than 35 years in communications consulting, has experience in every segment of the data center industry and was a pioneer in developing the field of building cable design. McFarlane also teaches the data center facilities course in the Marist College Institute for Data Center Professional program, is a data center power and cooling expert, is widely published, speaks at many industry seminars and is a corresponding member of ASHRAE TC9.9 which publishes a wide range of industry guidelines.