Raised floor support

Raised floor supports are used to achieve the distance between the raised floor level and the bare floor in raised floor systems. These are available in a wide variety of shapes and materials. Since fire protection requirements are often placed on raised floors , supports made of materials with a low melting point (such as aluminum) are less suitable.

description

A distinction is essentially made between low, medium and high supports. Supports are at least in two parts (they consist of a head and a foot). Metal supports usually also have a hexagon nut for height adjustment.

to form

Basically, a distinction can be made between

Cylindrical with multiple concentric half-thread.

Mineral support type LNA II

Such supports were only offered by a few manufacturers (and only as low supports) and are currently no longer available on the market.

Base with bolt, tube with head (welded, pressed or deep-drawn).

This shape is typically used primarily for low supports. Realizable construction heights range from approx. 80 mm to approx. 150 mm. The foot bolt is usually made of M16 . The height is adjusted using a nut or internal thread in the head.

Steel support with tube 24 x 2

Head with bolt, tube welded to base plate.

This shape is mainly used for installation heights of up to approx. 1 m (possibly even more). A typical head bolt is M20 (with nut), suitable for the inner pipe diameter of 20 mm.

Head with bolt, tube with foot cap.

This is a typical "control room support". Construction heights of 1 m to 2 m (and more) are realistic. The head shape is optimized for the assembly of the "control room profiles" resting on it. The static system of the control room can be clamped at the top and articulated at the bottom, which leads to a shortening of the buckling length (in this version Euler case 3 applies with β = 0.7).

Head with bolt, tube with sleeve tube.

In the case of high control rooms with large loads, a so-called "sleeve tube" (in addition to the regular support tube) is used for reinforcement. Is the regular pipe e.g. B. a precision tube 26 x 3, then the 1 ″ -over tube has only little play.

materials

Mineral

Anhydrite , gypsum or cement mortar in a plastic cover (as permanent formwork).

aluminum

Die - cast aluminum for the head and foot, the tube is made of extruded aluminum.

steel

Heads and feet made of are made of S235JR or special steel in deep-drawn quality (for heads with molded bolt mounts). The pipes are made of E235 as a precision steel pipe or (in the case of the push-on pipe) as an "inch water pipe" (pipe 33.7 x 3.25).

Electrical Properties

Not conductive

Mineral supports are not electrically conductive in the sense of the VDE , but can be electrostatically dissipative.

Conductive

Steel and aluminum supports are electrically conductive, but should (or must) be earthed.

Insulating

Since the support heads are provided with plastic washers, a steel support can also have an insulating effect. In order to achieve electrostatic dissipation, the support washers must be made of a special, electrostatically conductive plastic.

Static properties

Cylindrical mineral supports

These can take very large loads (also eccentrically). However, reliable figures can only be determined through destructive tests .

Aluminum supports

Here the permissible stress of the special aluminum alloy of both the die-cast and the extruded material determines which loads can be achieved.

Steel supports

For raised floors with an installation height of approx. 400 mm up to "walkable" raised floors, supports made of galvanized precision steel tubing have become established. The question of the pipe cross-section arises, particularly for large construction heights and high loads.

Raised floor supports are to be dimensioned for a planned eccentric load. The amount for the eccentricity is given by the eccentricity e . In the case of a pipe support with the outside diameter D (assuming that the force F acts on only one quadrant of the pipe) an eccentricity of

${\displaystyle e={\frac {D}{2\cdot {\sqrt {2}}}}}$.

By introducing the omega (ω) factor, DIN 4114 enables a simplified proof of buckling safety in the form

${\displaystyle \sigma _{\omega }=\omega \cdot {\frac {F}{A}}}$.

For the case of the off-center load, the standard provides an extended formula:

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

In this case, is in the bending moment M , the load F with the distance (eccentricity) e . So we can summarize:

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

The expression can be evaluated numerically and is fairly accurate , so the formula can be simplified to ${\displaystyle {\frac {0{,}9}{2\cdot {\sqrt {2}}}}}$ ${\displaystyle {\frac {1}{3{,}14}}}$

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

A raised floor support can be dimensioned with this formula if the buckling length l _k and the load F are known. ω is a function of the slenderness λ . The slenderness ratio λ is calculated from the buckling length l _k and the radius of gyration i :

${\displaystyle i={\sqrt {\frac {I}{A}}}}$and , where ${\displaystyle \lambda ={\frac {l_{k}}{i}}}$

${\displaystyle A={\frac {D^{2}-d^{2}}{4}}\cdot \pi }$ and

${\displaystyle I={\frac {D^{4}-d^{4}}{64}}\cdot \pi }$is ( D is the outside diameter and d is the inside diameter of the pipe). Björnstjerne Zindler provides us with a very good approximation to calculate ω :

If lambda is less than or equal to 115 then applies

${\displaystyle \omega =0{,}99+{\frac {\lambda }{728}}+{\frac {\lambda ^{2}}{153^{2}}}+{\frac {\lambda ^{3}}{143^{3}}}}$. If lambda is greater than 115 but less than 250 applies

${\displaystyle \omega ={\frac {\lambda ^{2}}{76{,}95^{2}}}}$ (Slenderness degrees greater than 250 are generally not permitted).

Here is an example calculation for a 1.25 m high pipe support made of pipe 24 x 2 with a load of 3000 N

in the event that the support above and below can be regarded as articulated:

Created with Mathcad Express

The calculated stress is less than the permissible stress for E235 (140 MPa). The support is kink-proof.

ATTENTION: Simple verifications according to Euler or Tetmayer do not provide the correct result because they do not take the eccentricity into account. ${\displaystyle \left(\sigma _{k}={\frac {\pi ^{2}\cdot E}{\lambda ^{2}}}\right)}$ ${\displaystyle \left(\sigma _{k}=310-1,14\cdot \lambda \right)}$

Individual evidence

1. ↑ 1.4 What do the terms “conductive”, “dissipative” and “insulating” mean? - BG RCI. Retrieved April 2, 2020 . 
2. ↑ Federal Association of System Floors (publisher): Application guidelines for raised access floors according to DIN EN 12825 . Dusseldorf.
3. ↑ DIN 4114-1 - 1952-07 - Beuth.de. Retrieved March 26, 2020 . 
4. ↑ Björnstjerne Zindler: Approximation formula. Retrieved March 26, 2020 . 
5. ↑ Instead of the tables published in DIN 4114
6. ↑ Euler case 2 with l = l _k i.e. β = 1.