EP1236838A2

EP1236838A2 - A floor structure

Info

Publication number: EP1236838A2
Application number: EP02445028A
Authority: EP
Inventors: Jan Strömberg
Original assignee: Plannja AB
Current assignee: Plannja AB
Priority date: 2001-03-01
Filing date: 2002-02-27
Publication date: 2002-09-04
Also published as: SE521850C2; SE0100716D0; SE0100716L; EP1236838A3

Abstract

A vibration-damped floor structure has a supportive floor part in which a false ceiling is resiliently suspended by virtue of the ceiling joists 5 springing at their suspension points 13. Damping means 15, 17 that have viscous characteristics are coupled between the floor part and the ceiling. The damping means include a butyl rubber mass 17 which, in one example, adheres to a part 15 which is rigid with respect to the floor part, and to a part 5 which is rigid with respect to the ceiling and subjected to shear in response to vibratory movement between said floor part and said ceiling.

Description

FIELD OF INVENTION

The present invention relates to a floor structure comprising a load bearing floor part and a ceiling flexibly suspended from said floor part.

DESCRIPTION OF THE KNOWN PRIOR ART

Light floor structures are mainly used at present in single unit dwellings and terraced houses. The floor structures often comprise joists, most often wooden joists although the use of steel joists is also becoming more common. The floor is often comprised of one or more layers of sheet material made from wood or gypsum. In the case of relatively wide joist spacings, profiled metal sheet may also be used on the joists beneath the sheet material, to increase rigidity. The ceiling of the floor structure is comprised of sheets that are either screwed directly to the supporting joists or indirectly via slender cross-joists which, in turn, are fastened to the supporting joists. Sound-damping insulation consisting of mineral wool may be included in the floor structure.
This type of light floor structure has many advantages over traditional concrete floor structures. For example, the light floor structure is dry, i.e. no drying-out period is required as in the case of concrete, this latter lengthening the construction process. Such floor structures are also light in weight, which results in cheaper transportation and lighter loads on other parts of the construction. They are also normally cheaper than concrete floor structures and have less detrimental effect on the environment.
The drawbacks of such light floor structures primarily reside in their acoustic and oscillation properties. Higher acoustic requirements are placed on floor structures that lie between apartments and on office floor structures than on the floor structures of detached houses. A known method of improving air insulation and impact sound insulation so as to fulfil standard requirements for apartment floor structures is to suspend those joists to which the ceiling sheets are fastened in such a manner that they will hang so flexibly as to prevent acoustic vibrations from propagating as readily in the construction from floor to ceiling. This can be achieved by fastening the joist to the floor beams through the medium of a separate resilient suspension arrangement, or by fastening the joist so that it will be subjected to large local elastic deformations at its suspension points in the manner described further on. A floor structure that includes a load-bearing floor part and a flexibly suspended false ceiling will at first swing significantly when the floor is subjected to an impact, for instance to a so-called heel stamp, and thereafter becomes progressively weaker. The time during which such movement can be discerned may vary in relation to a number of factors, such as in relation to the force of the impact, the stiffness of the supporting beams, material properties, the sheet material, the span, etc., and is normally from between a half-second to two seconds. The oscillation normally takes place in two dominating oscillation modes. The one mode causes the floor structure and false ceiling to oscillate in phase, while the other mode causes them to oscillate in counter-phase. Because the amplitudes are added together in the counter-phase oscillation mode, the work carried out by the suspension spring will be greater than in the case of oscillation in the phase mode, in which the oscillations are subtracted.
Another problem is that the floor structures are more flexible or "softer" than similar thick concrete structures. They are particularly flexible transversely to the bearing direction of the bearing joists. Spot loads in respect of light floor structures result in greater deflections than in the case of concrete floor structures. Such deflections can be limited, however, by using stiffer joists and placing them closer together. Steel joists are also preferable, since they can be made more rigid than wooden joists. This problem can thus be adequately overcome by constructive measures. A floor structure that is not excessively rigid is not solely a negative feature. A given degree of softness can be experienced as being comfortable.
The most serious problem and the problem which has prevented the commercial success of light floor structures resides in the risk of vibrations in the floor structure. A vibration is set-up in a floor structure, when it is walked or jumped upon. This vibration is very limited in a concrete floor structure. The prime reason is because this structure is heavy per unit of surface area. According to Newton's law, the greater the mass the smaller the acceleration, which becomes noticeably apparent when jumping on a concrete floor structure. Moreover, the concrete has a high internal damping factor, i.e. the oscillation quickly dies away as a result of the energy being converted to heat in the floor structure. A light floor structure is easily accelerated, and steel beams in particular have a small internal damping property, meaning that the vibrations will be felt over a long period of time, which is troublesome. Attempts have been made to solve this problem with mass damping, i.e. with a heavy mass within the floor structure, which is coupled by a spring to said structure. The spring has a rigidity which is tuned so that the oscillating energy in the floor structure will be transmitted to the mass via the spring. Typical natural frequencies in respect of the first oscillation mode in the floor structure are 7-20 Hz. It has not been possible to obtain damping that functions in practice, since it is difficult to adapt the spring constant to the natural frequency of the floor structure, which, after all, depends on the span of the floor structure and its load. Tuning of the spring must lie close to the natural frequency of the floor structure, in order for the oscillation energy to be transferred. Moreover, the energy returns to the floor structure after a short period and meanders forwards and backwards until it fades away due to internal losses.
It has hitherto been found necessary to limit the span of the floor structure in order to restrict vibrations in light floor structures. These spans normally measure at most 4-6 m, depending upon design. Vibrations can be disturbing, even in the case of such limited spans.

OBJECT OF THE INVENTION AND A SUMMARY OF THE INVENTION

The object of the invention is to provide strong damping of the kinetic energy in the floor, so that initiated vibration will die away more quickly and therewith not felt to be unpleasant. This object is, in principle, fulfilled by viscous damping of the oscillating motion occurring between the suspended ceiling and the floor part. The invention has the characteristic features set forth in the accompanying Claims.
The invention is inexpensive and easily applied. Moreover, it functions within wide limits with respect to span, load on the floor structure, rigidity of the floor structure, and the weight of said floor structure. The invention enables light floor structures to be used for considerably wider spans than was earlier the case, typically spans of up to 6-8 m, and possibly larger, therewith enhancing its area of use and also its popularity.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of a floor structure element according to the present invention.
Figure 2 shows how the floor structure elements according to Figure 1 can be mutually coupled to a floor structure.
Figure 3a shows how the ends of the floor structure elements can be placed on beams.
Figure 3b illustrates another placing method.
Figure 4 is a longitudinal sectioned view of the floor structure element.
Figure 5 illustrates part of the elastic suspension of the false ceiling with viscous damping.
Figures 6 and 7 illustrate how the viscous damper shown in Figure 5 is deformed when false ceiling and floor part of the floor structure swing relative to one another.
Figure 8 shows the results of measurements made on a floor structure element according to Figure 1.
Figures 9-16 illustrate how floor and ceiling swing under different conditions subsequent to floor impact. The figures are based on accurate finite element calculations that have been checked against a series of tests made on actual floor structures and floor structure elements, among other things those that are shown in Figure 8.
Figure 17 illustrates an alternative method of providing viscous damping integrally with the suspension arrangement.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS OF THE INVENTION

The manufacture of floor structure elements for constructing a floor structure is rational from the aspect of assembly. Figure 1 shows a method of achieving this. Reference numerals 1 and 2 identify floor structure beams comprised of folded thin metal sheet, normally spaced 1.2 m apart. The floor structure element thus has a width of 1.2 m. Reference numeral 3 identifies a trapezoidal profiled metal sheet that carries the load between the beams. Reference numeral 4 identifies surface sheets that protect against fire and constitute an underlay for the floor covering. Reference numeral 5 identifies cross joists having a centre to centre distance of 450 mm and supporting the ceiling sheets 6. Reference numeral 7 identifies mineral wool which has a density in the range of 15-30 kg/m³ and whose purpose is to dampen sound.
Figure 2 shows two mutually coupled elements. Reference numeral 8 identifies a U-shaped fitting which fits into the trapezoidal profile and which is screwed firmly to the upper surfaces of said beams. The fitting holds the elements together. It may be practical not to fit the floor sheets before the floor structure elements have been fitted and screwed together.
Figures 3a and 3b respectively show two different methods of placing floor structure elements on a support, for instance on a primary beam or on a wall. Reference numerals 9 and 10 identify alternative cross beams at the ends of the floor structure elements. These beams are screwed to the beams 1 and 2 via end angles 11 and 12.
Figure 4 is a longitudinal sectional view showing cross joists 5 which are folded from thin metal sheet having a thickness of about 0.5-0.7 mm and which support the ceiling sheets 6.
Figure 5 shows a part of one of the joists 5 and its suspension from the beams 1 and 2 of the floor structure. The joist thus has a length of 1.2 m and is suspended at its ends. The screw 13 is placed far from the web of the joist, to provide soft resilient suspension, such that the joist web will sag under the weight of the ceiling sheets. Located at the attachment points of the joists 5 in the beams 1 and 2 is a damper 14 which consists of a short angle 15 comprised of sheet metal having a thickness of about 2 mm and screwed with screws 16 close to its corners, so as to sit rigidly affixed to the undersurface of the beam. Located between the joist web and the angle 15 is a viscoelastic mass of high viscosity. The viscoelastic mass may be butyl rubber, for instance the butyl rubber retailed under the trade name Terostat by Trelleborg AB. The butyl rubber adheres to the metal surfaces. In the illustrated case, the butyl mass measures b*h*t = 15*30*12 mm. When taking measurements, this gives an approximate viscosity Cd = 150 N/(m/s) and a rigidity Kd = 30 kN/m, these values being applicable in the following example unless nothing is said to the contrary. All measurements regarding rigidity and damping apply to each joist, unless nothing is said to the contrary. Joist spacing is 450 mm and the length of respective joists is 1.2 m, meaning that the numbers will be about 1.9 times greater per m². The viscosity Cd is thus about 280 N/(m/s) per m² and the elastic rigidity of the damper is thus about 55 kN/m per m² floor structure surface. The rigidity of the suspension is in the order of magnitude of 50-150 kN/m, in the following example 94 kN/m = about180 kN/m per m² of floor structure surface area, when nothing is said to the contrary. When making tests and calculations, the effect of the suspension spring rigidity and the elastically rigid part of the damper are added together The viscosity is measured and given in N/(m/s), thus as though it is velocity dependent linearly. This is practical, because it simplifies the calculations and can be readily analysed, and tests have shown that the approximation is reasonable. The viscosity may be non-linear in practice. As will be shown further on, the viscous rigidity and the elastic rigidity may vary within wide limits and still provide effective damping. In respect of a floor structure that has an intrinsic weight of between 60 and 300 kg/m², it is favourable if the ceiling weighs between 20 and 50% of the entire weight of the floor structure.
Figures 6 and 7 show how the butyl rubber is deformed by shear in response to relative movements between floor and ceiling.
Figure 8 illustrates frequency response measurements in tests carried out on floor structure elements according to Figure 1 having a length of 7.2 m, partly with 18, partly without 19, with damping 14 according to Figure 5. Each of the bearing joists 1 and 2 has a moment of surface inertia corresponding roughly to 24x10⁶ mm⁴. At a frequency of about 8 Hz there is a mode which causes floor and ceiling to oscillate in phase. This mode is only influenced to a small extent by the damping, since the damper is subjected to only a small degree of movement. In the absence of damping, there occurs at a frequency of 14 Hz an oscillating mode that causes the floor and ceiling to oscillate in counter-phase. When damping is included, the frequency of this mode increases to 17.5 and damping is achieved to a very high degree.
Figure 9 shows how the state of the floor, curve 40, and the state of the ceiling, curve 41, change in response to a floor impact in the absence of damping between the beams of the floor structure and the ceiling joists, although with the internal damping which measurements show is nevertheless present in a similar floor structure. It will be seen that floor and ceiling still oscillate significantly after a lapse of one second, which is perceived as highly disturbing.
Figure 10 shows how the state of the floor, curve 42, and the state of the ceiling, curve 43, change in response to a floor impact in the presence of damping between floor beams and ceiling joists. It will be seen that floor and ceiling oscillate at a significantly smaller amplitude and at a lower frequency, i.e. at a lower velocity.
Figure 11 illustrates the acceleration of the floor in m/s² subsequent to an impact in the absence of a damper, curve 20, and in the presence of a damper, curve 21. The significant influences of the dampers is clearly seen. The effect on acceleration is greater than the effect on displacement. It is the acceleration that is discerned as disturbing, because, according to the laws of motion and according to the laws of kinematics, it produces the force that is felt by the person present on the vibrating floor structure.
Figure 12 illustrates the acceleration of the ceiling in m/s² subsequent to impact in the absence of damping, curve 22, and in the presence of damping, curve 23. It will be seen that the dampers have a significant influence even in respect of the ceiling.
Figure 13 shows the influence on the acceleration of the floor in response to making the elastic suspension spring about three times more rigid, e.g. by moving the screw 13 closer to the web of the joist 5. Curve 24 shows a floor structure that is not dampened, while curve 25 shows a dampened floor structure. Although damping still functions, the effect is not as great. Moreover, the reduction in sound has been impaired by the stiffer suspension. We can perceive this as being an approximate limit in obtaining an almost sufficient effect in respect of this floor structure. The sum of the stiffness of the suspension spring and the elastic part of the stiffness of the damper is, in the illustrated case, about 700 kN/m per m² of floor structure surface. This limit may be different in the case of other weights on suspended false ceilings and in the case of other conditions in general.
Figure 14 illustrates the effect on ceiling acceleration when the elastic spring is made about three times stiffer, for instance by moving the screw 13 closer to the web of the joist 5. Curve 26 illustrates a non-dampened floor structure, and curve 27 illustrates a dampened floor structure. Although damping still functions, the effect is not as great. Moreover, sound reduction has been impaired by the stiffer suspension.
Figure 15 illustrates the effect of an impact on the acceleration of the floor, by virtue of the combined viscosity of the two dampers per joist being reduced from 150 N/(m/s), curve 28, to 50 N/(m/s), curve 30. Curve 31 illustrates a fully undampened embodiment. The curves show that damping has still a great effect down to viscous damping 50 N/(m/s) per joist, but that this effect is nevertheless significantly worse in comparison with the strong damping effect obtained with the described embodiment. 50 N/(m/s) per joist corresponds to about 90 N/(m/s) per m². Let it be said that a reasonable minimum limit for a functioning damping facility is 70 N/(m/s) per m².
Figure 16 shows the effect obtained when solely the five centremost joists of the joists 5 of the floor structure element 15 have viscous damping, curve 32, instead of all the joists being dampened, curve 33, or when no joist is dampened, curve 34. The comparison shows that effect is limited when the dampers on joists outwardly of the centre part are removed. The explanation is because movement is greatest in the centre.
The aforedescribed damping example is one of several possible designs. Another method which was found to function in tests is described in Figure 17. An additional metal sheet 35 of roughly the same thickness as the joist is placed around the attachment, and a layer of butyl rubber 36 is disposed between the mutually contacting surfaces. Shear occurs in the layer of butyl rubber upon deformation of the sheets, therewith giving rise to viscous damping. To prevent the sheets from being pressed together by the pressure forces, these sheets are kept spaced apart, for instance by baking in the butyl rubber hard, spherical particles 37 which function to hold the metal sheets apart.
The scope of the present invention also enables other types of dampers that have viscous characteristics to be used.
The smallest damping that is sufficiently effective and the greatest suspension stiffness that is sufficiently effective without unduly impairing acoustic damping will depend on the weight of the floor structure and its construction, the material included, the span and the requirements placed on the structure. The limit values that have been tested and proposed in the aforegoing may thus be somewhat different in other cases, although still lying within the scope of the invention.
In the illustrated example, the floor structure is built-up from prefabricated elements with floor beams comprised of folded thin metal sheet as main supports. However, the invention will function equally as well with floor structures that are built on site and with floor structures built with welded or rolled steel beams or wooden joists or concrete beams. The supporting construction may also comprise longitudinally extending, trapezoidal-profiled metal sheet having a high profile height, normally above 100 mm. The floor structure may even consist of concrete floor structures cast on site, with a resilient suspended false ceiling that has viscous dampened suspension. The limit values with respect to elastic and viscous rigidities must be adapted to the components and spans included in the structure.

Claims

A floor structure comprising a load-carrying floor part (1-4) and a ceiling (5, 6) suspended softly from the floor part, characterised by a damping means (15-17) which has viscous characteristics and which is coupled between the floor part (1-4) and the ceiling (5, 6), wherein the viscous damping per m² of floor structure surface area is at least 70 N/(m/s); and in that the stiffness of the ceiling suspension including the elastic or resilient part of the damper stiffness is at most 700 kN/m per m² of floor structure surface area.
A floor structure according to Claim 1, characterised in that the floor structure has an intrinsic weight of between 60 and 300 kg/m², of which the ceiling (5, 6) weighs between 20 and 50%.
A floor structure according to Claim 1, characterised in that said structure carries load chiefly in one direction; and in that the ceiling comprises joists (5) which are fastened to supporting parts (1, 2) of the floor part (1-4); and in that ceiling sheets (6) are secured to said joists.
A floor structure according to Claim 3, characterised in that the supporting construction of the floor part (1-4) comprises folded, thin sheet-metal beams (1-2).
A floor structure according to Claim 3, characterised by trapezoidal-profiled metal sheet (3) as supporting elements between the beams (1, 2).
A floor structure according to Claim 3, characterised in that the supporting construction of the floor part is comprised of wooden joists.
A floor structure according to Claim 3, characterised in that the supporting construction of the floor part is comprised of profiled metal sheet.
A floor structure according to any one of the preceding Claims, characterised in that the damper includes a viscoelastic mass (17) as a viscous damping element.
A floor structure according to Claim 8, characterised in that the viscoelastic mass (17) is fastened to a part (15) which is rigidly fastened to the floor part, and also to a part (5) which is rigidly fastened to the ceiling.
A floor structure according to Claim 8 or 9, characterised in that the viscous damper utilises the viscous properties of the viscoelastic mass (17) in shear.
A floor structure according to Claim 10, characterised in that the ceiling (5, 6) includes joists (5) made of folded thin metal sheet, wherein said joists are fastened to the floor part (1-4) in a manner to allow local, elastic deformation of the sheets around their attachment points, so as to form the soft or flexible ceiling suspension, and wherein the viscoelastic mass (17) is fastened to the ceiling by virtue of being fastened in said joists (5).
A floor structure according to any one of Claims 1-7, characterised in that the damping means includes a sandwich element comprised of two metal sheets (5, 35) and an intermediate layer (36) of viscoelastic mass, wherein said sandwich element is coupled so that it will be subjected to shear forces in the event of vibratory movement between the floor part (1-4) and the ceiling (5, 6).
A floor structure according to Claim 12, characterised in that the viscoelastic mass (36) includes particles (37) that function as spacing elements such as to prevent said layer becoming thinner than the size of the particles.
A floor structure according to any one of the preceding Claims, characterised in that the elastic mass is comprised of butyl rubber.