Room-within-a-Room / Floating Floors

GERB Spring Elements for Low-Frequency Floating Floor and Room-in-Room Structures


Range of Application


Floating floor and room-in-room structures represent the state-of-the-art in efficiently reducing the transmission of vibration and structure-borne noise within buildings. They are mainly used to decouple sensitive performance centres like TV, broadcasting or recording studios, recital and rehearsal rooms, theatres as well as gymnasia, but also discotheques and HVAC areas dynamically and acoustically from the surrounding structure. Special applications of floating slabs can be, for example, helicopter landing pads and seismic slabs for sensitive equipment in power stations.

For several decades GERB has been developing and supplying spring elements for floating floor and room-in-room structures based on helical coil springs. They guarantee highest attenuation values by extremely low system natural frequencies. Designed to achieve system eigenfrequencies of 2.5 to 7 Hz, GERB spring elements represent the most efficient solution to protect highly sensitive areas safely and permanently from mechanical vibration as well as structure-borne noise. The low system frequency as well as the ability of easy adjustments make them far superior to any elastic pad material like rubber, cork or cellular foam.

Embedded Spring Elements type PF
The PF type floating floor spring element system (European Patent No. 0610634 dated 13.01.1999) offers the following features:

  • A range of different springs is available to provide system natural frequencies as low as 2.5 to 7 Hz. The choice depends on the respective requirements in terms of vibration attenuation, slab stability, and function.
  • The springs are accessible from above making subsequent levelling of the floating slab as well as the replacement of springs with springs of other load capacities possible.
  • In order to achieve uniform spring deflections, elements of different type and loading capacity can be combined in a system layout to allow for irregular slab loadings.
  • Slab construction and the installation of the springs is very simple. Apart from a bond-breaking plastic layer, there is usually no need for bottom formwork. The slab is lifted directly from the substructure after the concrete has reached its design strength.
  • The elements can be equipped with a damping system in order to stabilize the floating slab and to further improve the attenuation capacity at higher frequencies.
  • The jack-up and adjustment facilities are an integral part of every element.
  • Low construction height, from just 125 mm slab thickness (plus air gap).
  • In addition, acoustic pads arranged below the steel springs can provide a further improvement in the audible range.
Fig. 1: Type PF spring element, cross section

Fig. 1: Type PF spring element, cross section


The thickness of the floating slab is usually established based on the structural requirements, the load conditions, and the intended use. However, a minimal slab thickness H given in the table below is necessary. The air gap between the floating slab and the substructure should have a minimum depth of 30 to 40 mm to ensure that the air spring effect will have no noticeable influence on the vertical system natural frequency. In special cases, usually on demand of the acoustics consultant, a gap of 100 mm or more is also possible. In order to further improve the attenuation of structure-borne noise, a suitable acoustic layer can additionally be arranged within the air gap.

Selecting elements

In order to meet the requirements of a wide range of applications, GERB has developed a series of elements based on the standard types PF1 - PF4. The elements come with height-adjustment as standard, and can be equipped with a damping system.

Fig. Table: Properties of each basic type of PF element equipped with the respective standard springs.

Fig. Table: Properties of each basic type of PF element equipped with the respective standard springs.


Elements with other properties are available on request.

Arrangement of Spring Elements
 
The slab thickness needs to be chosen to suit the dynamic and static requirements, according to the intended use of the structure. In order to achieve the optimum spring element arrangement, all permanent loads including wall and ceiling loads must be known at design stage. The expected actual live loads need to be estimated as precisely as possible, areas of high load concentrations must be known as well as those where no spring elements can be placed.

Fig. 2: Example of a spring arrangement plan

Fig. 2: Example of a spring arrangement plan


Larger slabs and especially slabs with a non-rectangular shape, e.g. as regularly used for recording studios, usually require detailed calculations.

Fig. 3: Arrangement of type PF spring elements underneath a heavy wall of a room-in-room structure

Fig. 3: Arrangement of type PF spring elements underneath a heavy wall of a room-in-room structure


The reinforcement of the concrete slab may be determined by the contractor or a structural engineer on the basis of the spring element arrangement plan.

Elements Type K and F

Spring elements type K and F are simply arranged between the isolated and the non-isolated structure. Their height usually determines the acoustic gap below the floating slab. These low-cost elements have no provision for later adjustment, the final load distribution therefore has to be precisely known at design stage already. They offer the following features:

  • System natural frequencies as low as 2.5 to 7 Hz provided by a range of different types of springs. The choice depends on the respective requirements in terms of vibration attenuation and slab stability.
  • The elements can be equipped with a damping system in order to stabilize the floating slab and to improve the attenuation capacity at higher frequency levels.
Fig. 4: Type K elements

Fig. 4: Type K elements

Fig. 5: Type K elements with metal deck floor

Fig. 5: Type K elements with metal deck floor

Fig. 6: Type F elements

Fig. 6: Type F elements


Elements Type GP and GPNV

Spring elements type GP and GPNV are designed for high, concentrated loads. They are placed underneath the isolated structure, and have to be accessible for initial adjustment after installation. They feature:

  • System natural frequencies between 3.5 and 7 Hz.
  • The spring elements can be adjusted for precise levelling of the floating slab as well as replaced with spring elements of other load capacities.
  • The elements are equipped with the GERB Coil Resonance Damping System as standard, ensuring improved attenuation capacity at higher frequency levels.
  • High load bearing capacity makes them an ideal choice for heavy room-in-room solutions, heliports, etc.
Fig. 7: GPNV elements supporting TV production studio (showing temporary struts during construction)

Fig. 7: GPNV elements supporting TV production studio (showing temporary struts during construction)

Fig. 8: Spring elements in a helipad structure

Fig. 8: Spring elements in a helipad structure

Fig. 9: Arrangement of spring elements underneath TV studio

Fig. 9: Arrangement of spring elements underneath TV studio


Scope of GERB Services

  • Consultation with GERB expert engineers
  • Vibration measurements and surveys
  • Selection and arrangement of spring elements, in consultation with the architect and structural engineer
  • Preparation of layout drawings
  • Design, manufacture and supply of spring elements and accessories
  • (adjustment shims, adhesive pads, etc.)
  • Complete installation or just supervision
  • Inspections