Raised floor (construction)

Area	Soil types
title	Raised floors
Latest edition	2002-04
ISO

Lifting a plate

Under a raised floor

Server room with raised floor, some with ventilation panels

A double floor is a second floor above the actual floor of the room. It differs from the hollow floor in that every point of the room below the double floor remains accessible at all times. This makes it particularly suitable for rooms with frequent changes to the installations and when the installations should be quickly accessible at all times.

General

In construction, a double floor is a system floor design as an elevated floor construction consisting of industrially manufactured modular elements, essentially panels and supports. The raised access floor panels are usually made of wood (high-density flat pressed panels), cement fiber , anhydrite , gypsum (fiber-reinforced plasterboard), steel or aluminum . These plates rest on double floor supports made of steel or aluminum. The supports can be connected to one another at their heads by means of grid rods.

All installations for e.g. communication, electricity, water can be integrated into the cavity created in this way. In addition, especially in clean rooms, the raised floor is often used to convey the exhaust air out of the room through countless small holes and thus a very large flow cross-sectional area, so that a laminar air flow from the ceiling (supply air also via large cross-sections) to the floor in the room adjusts. The entire cavity represents an air duct, so to speak.

A so-called control room floor (or control room double floor) is used especially for electrical rooms or for areas in which high demands are placed on lateral stability and load absorption . This system consists of heavy-duty supports on which continuous steel C-profiles are screwed in one direction and fittings in the other. In the walking area, raised floor panels are laid on this profile grid sub-construction; in the area of the control cabinets, however, frames are made with higher C-profiles. This means that all panels can be removed when switching cabinets are set up, and subsequent installations can be carried out easily.

In server rooms or data centers , the raised floor can also be used for active air conditioning in the hot or cold aisle . There are active water-cooled double floor climate control panels for this purpose; these panels are available with air ducts directed downwards or upwards.

Load information for the raised floor

The load specification is the maximum permissible load for a raised floor. In contrast to a ceiling (for which one can hold the raised floor), the load is not as distributed load in kg / m or N / m but as a single load is declared (point load) in Newton.

The reason for this is that the supporting layer of the raised floor consists of individual (mostly 60 cm × 60 cm) raised floor panels, which rest on raised floor supports at their four corners . If one were to apply an evenly distributed surface load to this construction and assume the deflection of the plate with the usual limit of 1/300 of the span as the load limit, then extremely high load specifications (20,000 N / m² and more) would result if they were erroneously If traffic loads are interpreted, this leads to the incorrect assessment that the raised floor can carry more load than the concrete ceiling on which it stands. One therefore chooses a different way of determining this important key figure:

A raised access floor panel is placed on rigid supports at its corners and loaded with a test stamp of 25 mm × 25 mm (usually hydraulic). One looks for the point at which the plate has its greatest deflection. In the case of panels made of homogeneous material, this can be the center of the edge or a point on the edge that is approximately one panel thickness away from the support. In the case of non-homogeneous panels (e.g. ventilation panels), this can also be any other point.

Once you have found the weakest point of the plate element, you increase the load until the deflection at this point reaches 1/300 of the span (with a grid dimension of 60 cm, i.e. 2 mm). The value of this load is the maximum permissible load for the raised access floor panel. Depending on the construction, this can be 3000 N, 4000 N, 5000 N or more. With special constructions (control rooms, heavy-duty floors), significantly higher values can be achieved.

But with that you only have the value for the plate level at first. In addition, the column is at risk of buckling with increasing height, so the raised floor is to be constructed so that the required nominal load in both Newtons

at the weakest point of the plane of the plate as well
as the eccentric buckling load of the column

is proven.

Usually, the floor is tested on behalf of the manufacturer in a neutral institute (MPA) and assessed and certified with a general building authority test certificate. Regardless of this, however, it is basically possible to prove the raised floor load mathematically.

The certificates (ABP) can be viewed at the Bundesverband Systemböden eV: https://www.systemboden.de/abp-zentralregister/. Mathematical evidence should only be prepared by specialists who are very familiar with the matter.

Statics (supports)

The proof of performance for these floors is based on EN 12825 for raised floors. The standard applies in Germany as DIN standard DIN EN 12825. Conformity certificates are issued for floor systems that comply with the standard and the application guideline .

The pillars of raised floor systems usually consist of steel tubes, the statics of which can be verified via:

proof

{\ displaystyle \ sigma = \ omega \ cdot {\ frac {F} {A}} + {\ frac {F \ cdot D} {\ pi \ cdot W}} \ leq \ sigma _ {perm}}

The factor ω is given below for the material E235. The steel tubes used in raised floor supports are typically made from this. The permissible stress is then 140 MPa. ${\ displaystyle \ sigma _ {perm}}$

Rating

With the area and the section modulus for a pipe ${\ displaystyle A}$ ${\ displaystyle W}$

{\ displaystyle A = {\ frac {D ^ {2} -d ^ {2}} {4}} \ cdot \ pi \ qquad \ qquad W = {\ frac {D ^ {4} -d ^ {4} } {32 \ cdot D}} \ cdot \ pi}

this gives the geometrical moment of inertia and the radius of gyration . ${\ displaystyle I}$ ${\ displaystyle i}$

{\ displaystyle I = {\ frac {D ^ {4} -d ^ {4}} {64}} \ cdot \ pi \ qquad \ qquad i = {\ sqrt {\ frac {I} {A}}}}

Here is the outside diameter of the pipe, the inside diameter and the point load. Now the slenderness can be calculated using the buckling length , ${\ displaystyle D}$ ${\ displaystyle d}$ ${\ displaystyle F}$ ${\ displaystyle 20 \ leq \ lambda \ leq 250}$ ${\ displaystyle l_ {k}}$

{\ displaystyle \ lambda = {\ frac {l_ {k}} {i}}}

which in turn defines the buckling number . ${\ displaystyle \ omega}$

{\ displaystyle \ omega = {\ begin {cases} 0 {,} 99 + {\ frac {\ lambda} {728}} + {\ frac {\ lambda ^ {2}} {152 ^ {2}}} + {\ frac {\ lambda ^ {3}} {143 ^ {3}}} & {\ text {if}} \ lambda \ leq 115 \\ {\ frac {\ lambda ^ {2}} {76 {,} 95 ^ {2}}} & {\ text {if}} \ lambda> 115 \ end {cases}}}

Alternatively, tables are available in DIN 4114 for determining . ${\ displaystyle \ omega}$

Derivation

The proof given above is tailored for the pillars of the raised floor. For example, DIN 4114 generally specifies the existing stress when an eccentric buckling load is applied: ${\ displaystyle \ sigma _ {\ omega}}$

{\ displaystyle \ sigma _ {\ omega} = \ omega \ cdot {\ frac {F} {A}} + 0 {,} 9 \ cdot {\ frac {M} {W}}}

Whereby the bending moment is composed of the force and the eccentricity . The size of itself is composed of the outer radius and the secant of a quadrant of the pipe. This has an effective distance of . This can be used in the verification: ${\ displaystyle M = F \ cdot e}$ ${\ displaystyle F}$ ${\ displaystyle e}$ ${\ displaystyle e}$ ${\ displaystyle {\ frac {D} {2}}}$ ${\ displaystyle {\ frac {1} {\ sqrt {2}}}}$ ${\ displaystyle M = F \ cdot {\ frac {D} {2}} \ cdot {\ frac {1} {\ sqrt {2}}}}$

{\ displaystyle \ sigma _ {\ omega} = \ omega \ cdot {\ frac {F} {A}} + {\ frac {0 {,} 9} {2 \ cdot {\ sqrt {2}}}} \ cdot {\ frac {F \ cdot D} {W}}}

.

The simplification of the term results can be written so that: ${\ displaystyle {\ frac {0 {,} 9} {2 \ cdot {\ sqrt {2}}}}}$ ${\ displaystyle {\ frac {1} {3 {,} 14 ...}} \ approx {\ frac {1} {\ pi}}}$

{\ displaystyle \ sigma = \ omega \ cdot {\ frac {F} {A}} + {\ frac {F \ cdot D} {\ pi \ cdot W}}}

Further use of the term

The term double floor is also used in shipbuilding and boat building.

For shipbuilding see: Raised floor (shipbuilding)
For boat building: These are two floors one above the other that are laminated together . There is air between the two floors.
Also a formation of the chart analysis is used as a double floor designated as an interpretable reversal rate pattern about the course of a W describes.

literature

Jörn Krimmling: Facility Management: Structures and Methodical Instruments , Fraunhofer IRB Verlag publishing house, 2008, ISBN 3816774989 , p. 177 f. ( Google Books ).
Dipl.- Ing.Björnstjerne Zindler, M.Sc. The Omega procedure
Federal Association of System Floors, download area for information sheets and general information

Web links

Further information on raised floors from the Federal Association of System Floors

Individual evidence

↑ Federal Association of System Floors : application guidelines for DIN EN 12825 raised floors. Retrieved February 18, 2020 .
↑ Björnstjerne Zindler: The Omega method according to DIN 4114. (pdf) In: nadirpoint.de. Retrieved on February 18, 2020 (German).

[1] Federal Association of System Floors : application guidelines for DIN EN 12825 raised floors. Retrieved February 18, 2020 .

[2] Björnstjerne Zindler: The Omega method according to DIN 4114. (pdf) In: nadirpoint.de. Retrieved on February 18, 2020 (German).