Floor Load Capacity

One of the most important issues to be concerned with in the early stages of the data center design is weight. It is important to know how much load will be placed on the raised floor so that a support grid and tiles with an adequate load rating can be ordered. Careful planning at this stage is critical. You want to plan for the weight you'll place on the floor today, and the weight you'll place on the floor in the future. Remember: Once you install the raised floor, it's going to stay there. Changing out a raised floor in an online data center is a monstrous and costly job. Plan for a raised floor stretcher system and tiles with higher than necessary load ratings.

If you know exactly what equipment you'll be putting on the raised floor and where on the floor you'll be placing the equipment, acquiring tiles and the stretcher system with the correct load capacity is straightforward. Part of the strength of a raised floor is in the fact that each stretcher is connected to four other stretchers in different directions. If you have to replace the tiles and stretcher system of a raised floor, the removal of even a portion of the raised floor would cause weakness in the rest of the floor.

Load capacity won't be much of an issue for ramps made of poured concrete, but it will be for raised floors and structural ramps. There are three types of loads you should consider:

• Point load. Most racks sit on four feet or casters. The point load is the weight of a rack on any one of these four points. For example, a Sun Fire 15K server is 2200 pounds with four casters, so the load distribution is 550 pounds per caster. A floor tile must have higher than 550- pound point load, which means that for a 1-inch square area on the tile must be able to support 550 pounds on that 1-inch area without deflection of more than 2 mm.

• Static load. Static load is the additive point loads on a tile. If you have two racks, each with a 400 pound point load, and each rack has one caster on a tile, this tile will have a 800 pound static load. The tile must be rated for at least an 800 pound static load.

• Rolling load. Rolling load should be close to static load and is usually only applicable to perforated tiles. Since it is possible that you might use your cool aisle to also serve as an aisle to move equipment, the perforated tiles will need to support the weight of two point loads of a rack as they are rolled along the aisle. If the perforated tiles cannot accommodate this load, you would have to temporarily replace them with solid tiles. This would prohibit proper air flow from the cool aisle, and adds work every time you need to move a rack.

The load rating of the raised floor will depend on the design and purpose of the room. Historically, most raised floors were constructed out of concrete-filled steel -shelled floor tiles. While solid tiles might be able to support the current and near future load requirements, the perforated tiles cannot. The strength of these tiles rely on the concrete fill, and perforations in the concrete greatly weaken the tile. Sun's Enterprise Technology Centers have switched to aluminum floor tile systems. These tiles can handle a point load of 1,750 pounds even on a perforated grate with 55 percent air flow. The static load of the same tile is 3,450 pounds.

In a pre-existing building, the structural floor must be assessed to determine whether or not it will support the predetermined weight. Areas designed for light duty, such as offices, might not be able to handle the load. This determination should be made by a qualified structural engineer.