Why Server Rooms Use Raised Access Floors
Raised access floors are a standard component in the design of a server room or datacentre. They provide many benefits including a modular and flexible approach to how equipment is powered, cooled and even placed within the IT space. The construction consists of a floor tile that sits a set of four pedestals which are in turn fixed to a concrete base. The result is an elevated structural floor onto which IT components can be placed including server cabinets, uninterruptible power supplies and cooling systems.
When deciding where to place components on a raised access floor, attention should be given to access, maintenance and point loadings (weights). A typical server cabinet can weigh up to 300Kg for heavier weights it may be necessary to consider a spreader plate or to remove some of the floor tiles and pedestals and have a plinth constructed. The top of the plinth sits flush to the raised floor height and sits directly onto the concrete sub-base.
When it comes to raised access floors there are two types. The most commonly seen is that used within an IT hardware space and is based on the use of standard floor tiles and metal pedestals. The tiles are heavy and are held in place by their weight and lateral location. A lock-down or screw-down floor tile is an alternative format but this is less common in datacentre-type environments as this type of system gives less access and flexibility.
The floor tiles themselves vary in thickness, material and weight dependent on the usage and floor traffic expected. Panel or tile lifters are used to lift and move a floor tie, using two suction cups to affix to the floor tile.
Server room floor tiles are graded from Extra Heavy through to Heavy, Medium and Light grade based on many factors including the Concentrated Loads and Uniformly Distributed Loads they are designed to support. The loading on a floor tile is measured in kilo-Newtons (kN) for Concentrated Loads and N/m2 for Uniformly Distributed Loads.
Floor tiles are themselves made from a mixture of high-density particle board with a galvanised steel casing and vinyl wrap. In a server room or datacentre, the tile may be anti-static coated with the alternative coating being static-conductive or other finish. For non-IT environments, these finishes can include lino (linoleum), rubber, wood, marble, stone and of course carpet. Another key point to consider about floor tiles is their acoustic and fire-retardant capabilities. The floor tiles themselves should have a minimum life of 25years, with supporting components a lifetime of 50years.
From a planning point of view a standard computer room tile is 500×500m and the pedestals range in height from 70-1200mm. When pedestals longer than 600mm are installed, it is common to also use stringers to provide further lateral stability. These are generally 30mm deep and when used they will restrict the amount of space between the sub-floor and raise floor. An in important point to note is that ideally this area (known as the void or plenum) should be relatively clear to allow easy routing for cables (electrical and network) and cooling are.
Aside from the floor, it is also important in the overall design of the server room to ensure that the room itself dry and water-tight to control humidity and air temperature. Ideally the humidity should be less than 75% and the room temperature no higher than 25oC. The sub-floor must also be flat, stable, clean and sealed (no joint lines) to prevent dust being generated and circulated into the room via the air conditioning system. Overall a raised access floor should comply with BSI standard BSEN 12825 and the National Building Specification K41.
So, what are the benefits of a raised access floor? There are several for IT and datacentre managers. The under-floor void provides a space through which to route and contain server room infrastructure, cabling and pipework including: electrical wiring, data and telecoms wiring, security wiring and HVAC/air conditioning system components. The use of a void and standard sized floor tiles also makes it easier when it comes to room layout and future equipment moves or additions. Tiles can be lifted easy to give access to under floor connections and additional ones added or moved to another part of the computer room facility.
One of the principle uses of a raised access floor is for cooling or to be more particular as an air plenum for cooling, general ventilation and even heating. When designing a server room as much attention must be given to the air flow in the plenum as the above floor area air flow. The under floor plenum must be suited to the design air pressure differentials with limited or no leakage at panel joints. These are critical factors in the overall design and quality of the floor installation and the pressure rating (25-30psa high or 7-15psa low) of the air conditioning system installed. In a high-pressure air conditioned environment, neoprene or similar gaskets can be fitted to the edge of the floor tiles to form a tight seal. Air flow from the floor into the room space is typically managed via vented floor tiles with aluminium grills.
Most loads placed onto a raised access floor will be static and rarely, if ever, moved except as part of a swap-out or replacement program. UPS systems and server cabinets are typical examples of hardware systems that will sit inside a server room. Their placement onto the raised access floor should be planned and managed. Where necessary, due to the weights and sizes involved, a specialist logistics company with lifting-gear may be required to lift and site the equipment; especially if there is a plinth or step onto the raised access floor.
The approach to adding equipment to an existing computer room may require additional work than if the datacentre is a new-build. Over time equipment designs and technology generally improves and shrinks in size whilst power requirements rise. Cloud computing and virtualised server environments can reduce the number of servers and server cabinets required but with each server cabinet increasing in power demand from 3kW to 5kW. An alternative example would be that of distributed UPS systems being replaced by a smaller but centralised UPS system that sites on the raised access floor. In either case the overall electrical design of the room may require a re-think and the under-floor electrical cables rerouted to suite the new layout and hardware.
Some of the hardware for siting onto the raised access floor may have castors. These concentrate the total weight onto four points (assuming four castors with one on each corner). Before installation, the actual floor loading/set point loading (measured in Kg/m2) should be reviewed and preferably by a structural engineer. Where necessary, as mentioned a purpose-made spreader plate should be used to distribute the weight from heavier systems over the wider area and preferably over eight or more floor tiles (up to 8×4 pedestals).
Raised access floors are a common component within a server room or datacentre and offer a number of advantages. The use of a standard floor tile size (500×500mm) makes layout of the space far easier with server cabinets installed into specific rows and air conditioning systems placed strategically to support a hot-aisle/cold-aisle arrangement. The well-constructed flooring system should not only assist and tidy up cable management (electrical and network) but also provide a suitable void through which to distribute cooled air into the cold-aisle space.
In a properly sealed and routed raised access floor void, the air flow can be managed in the most thermodynamically efficient way. This in turn leads to improved energy efficiency and lower electricity costs overall. In addition overall server and IT load reliability will improve and possibly to the point that overall working lives can be extended.
