Stringers are introduced for various reasons each with their own specific design.
Snap on stringers. These snap onto the pedestal head and are used to provide additional lateral support to the raised floor. They are normally introduced at floor heights of 24" and above or for use with floor panels complete with factory bonded finishes. Snap on stringers are normally designed to increase the structural performance of the raised floor.
Bolt on stringers. These are screwed into the pedestal head and are designed as structural components and as such increase the structural performance of the raised floor system. They will also provide increased lateral stability.
Air plenum stringers. These stringers are designed only as a means of providing an air seal to the panel joints through the use of a gasket strip. They do not provide any increase to the lateral stability or structural performance of the floor.
Perimeter stringers. These provide additional support to cut panels around the perimeters if required by the project specification
It is important that consideration is given to the intended use of the building as part of the evaluation process. Raised access floors are used in a wide range of buildings which includes the following:
General modern offices, any office environment where there is a need for a significant level of computer/telecom equipment.
Call Centers, office environment set up to handle large scale customer enquires thereby requiring significant levels of computer/telecom equipment.
Data processing centers. Large scale computer rooms set up for the processing of electronic data i.e. customer information, financial information etc.
Telecom switch centers. Old mechanical telephone exchanges now replaced by electronic switch facilities. Also new mobile technology requires new electronic switch facilities.
Distribution centers. These facilities distribute a vast range of fast moving consumer goods with order processing and such activities handled in a modern office environment.
Educational facilities, raised flooring used in specific learning areas in schools, universities etc. Also used in library and major archive areas.
Retail facilities such as major department stores increasingly using raised floors surfaced with special finishes.
The type of area in which the raised access floor will be used will help to define the structural performance required of the raised floor and also the specific type of finish required to the floor surface.
The anticipated use of the basic space in which the raised access floor is to be used will determine the projected level of power, data, telecom, HVAC and other services that will be located under the raised access floor. This information can then be used to determine the cavity depth required under the raised floor and hence the finished floor height which will then be used in specifying the raised access floor system.
It is important at an early stage in the consideration of a raised access floor that a detailed assessment is made of the likely loadings that will be imposed on the floor surface. These loadings need to be assessed in terms of:
. Uniformly distributed loads
. Point loads
. Vehicle configuration and weight
. Areas of high traffic need to be determined.
. This information can then be used to determine the structural requirements of the raised access floor.
In August 2001 a European Standard was issued after several years of consultation between all the European manufacturers and was adopted as a British Standard in November 2001, BS EN 12825 . The use of this standard is increasing as it provides specifiers with increased flexibility. This specification classifies raised flooring products by their structural performance. Installation issues are clarified by the Access Flooring Association Code of Practice and further clarification will occur when the National Building Specification K41 adopts this standard.
Other standards regularly in use are:-
CISCA. The North American specification which often travels with either the American client or American architects and is regularly used internationally. This is an industry driven specification and test method which is based upon individual component testing.
The following range of floor finishes is available:
Here the floor panel will not be finished with a surface covering. The surface of the panel will normally be the epoxy powder coated steel top sheet. This bare finished raised floor will normally be covered on site by the application of loose lay carpet tiles.
Factory applied finishes
The following finishes can be supplied factory bonded to the appropriate floor panel:
. Anti static vinyl
. Static conductive vinyl
. Wood in various forms
. Stone, ceramic tiles.
. Other finishes may be available after evaluation.
Using standard pedestals finished floor heights from 3" to 48" are achievable. Bespoke solutions for lower and higher options are available.
As a general rule above a finished floor height of 24" stringers will be introduced to provide additional lateral stability.
The finished floor height (FFH) is defined as "The nominal vertical dimension from the specified sub floor level to the specified finished floor level".
In certain circumstances the available space between the underside of the raised floor panels and the sub floor surface (known as the air plenum) will be used as part of the building's heating and ventilation system. In these instances a pressure differential will be created between the air within the plenum chamber i.e. under the floor, and the air above the floor in the office environment.
In these situations there are certain attributes of the raised access floor that become important. The leakage of air through panel joints and at perimeters will need to be kept within specific parameters for the required air pressure differential. The air leakage rate through the panel joint line is dependant on the size of gap between the panels, which is dependant upon the manufactured quality of the panel edge detail and also the quality of the raised floor installation. The air leakage at perimeters is also dependant upon the specific detail design and the quality of the installation.
Air leakage rates
A typical air leakage rate for a raised floor system without a finish i.e. carpet tiles, would be in the region of 0.20 - 0.24 cfm/sq.ft. at a pressure differential of 0.12 inches of water.
Where the permitted air leakage rate through the raised access floor complete with the required floor covering is very low, or the specified air pressure differential is high, the raised floor in its standard form may not suffice. In these instances the raised floor panels may be fitted with neoprene or similar gaskets to their edges in order to form a seal when installed against other similar panels. Alternatively stringers fitted with gasket seals may be installed to seal the panel joint lines.
The air leakage rate through a raised access floor can be determined by laboratory testing in order to give indicative air leakage rates through both the panel joint lines and perimeter detail. This can be undertaken at a project specific pressure differential or across a range of pressures.
Alternatively the raised floor can be tested in its entirety on site, as part of the installation process. This on site testing can determine not only the air leakage through the raised floor but also the leakage through the floor void via other means e.g. incorrectly sealed service penetrations through the sub floor slab.
Air outlets and distribution
The conditioned air within the floor plenum is delivered into the area above the floor via a range of outlets. These are usually aluminium grilles of the same module size as the floor panels. Dampers may be fitted to allow control of the airflow through individual grilles in order to achieve localised control to suit specific local requirements. Perforated floor panels can be used again fitted with dampers if required as an alternative to grilles. A further alternative is use of smaller circular air diffusers fitted into the floor panels, again fitted with dampers to allow adjustment locally to suit specific requirements. All of these air outlets are readily relocated within the raised floor area as requirements change.
Carpet location systems are where the carpet tile is fitted with small plastic location pips (normally four) which locate into location holes in the panel top surface. The carpet tile will be of the same module size as the floor panel and will be laid on the same grid as the raised floor panels (major air leakage). This has the benefit that in order to gain access to the under floor services the lifting of carpet and floor panels is kept to a minimum. However many experts would say that the quality of the finished carpet tile installation is inferior to a conventional off grid (minimal air leakage) carpet tile installation. Also the choice of carpet tile may be significantly limited and incur additional costs.