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Methods of timber floor construction for minimising vibrations
Methods of Timber Floor Construction for Minimising Vibrations Y H Chui and A R Abbott
Timber Research and Development Association (TRADA). Hughenden Valley, High Wycombe, UK.
Abstract Excessive vibration of timber floors has been a problem for a number of years. Not until recently, however, has any research been undertaken in the UK to investigate this problem. A description of the cause and extent of floor vibration is given in this paper. Recent research work carried out at TRADA in this field is also briefly described Finally, construction measures for improving the vibrational performance of timber floors are proposed
Introduction For centuries timber has been the traditional material used for domestic floor construction, due primarily to its ready availability, ease of handling and above all, its high strength-toweight ratio. A conventional timber floor is constructed by laying a suitable flooring material onto a series of timber joists whose ends bear onto either external walls or intermediate bearers. The flooring is connected to the joists by means of nails or glue, or a combination of both. Historically, the most commonly used flooring material in the UK was nailed softwood boarding. The advent of timber framed housing construction in the UK in the late 1960s and early 1970s coincided with a significant move away from the use of softwood boarding in favour of sheet materials such as plywood and chipboard. Apart from reducing the materials costs, considerable cost savings may also be achieved by using sheet materials because of their ease of handling and laying. The size of joist section required used typically to be determined by the 'rule-of-thumb' approach whereby
the approximate depth in inches of 2inch thick joists spaced at 15-inch centres may be found by dividing the span in feet by 2 and adding 2 to the quotient ~. Thus, for example, an 8foot span would require 6 i n c h x 2inch joists. Timber floors are currently designed on the basis of calculated stresses and deflections of a single joist under design loading. The stresses and deflection must not exceed the permissible values given in the design code, BS 5268: Part 2: 1984 ~2~.In this 'working stress' design approach no account is taken of the contribution of the flooring to the overall stiffness and strength of the floor system. Thus, the choice of flooring material does not affect the design solulJon and, for each material, the thickness required is governed only by the joist spacing and the use to which the floor is to be put. The 'working stress' approach to design generally produces floors with acceptable vibrational performance. But recently the number of complaints about floor vibration has steadily increased. While s o m e of these cases can be attributed to poor workmanship or improper material
CONSTRUCTION & B U I L D I N G MATERIALS Vol. 1 No. 1 M A R C H / A P R I L 1 9 8 7
quality control on site, the majority occur in floor systems designed and constructed in accordance with current practice. This clearly indicates that floors produced these days are more flexible. This greater flexibility has been brought about by the refinement of design methods, use of higher moduli of elasticity and new construction techniques. Current design practices, which treat floor systems as one dimensional beam structures, do not include requirements specifically for vibrational serviceability, although the static deflection limits do provide some indirect control over the vibration problem. For example, BS 5268: Part 2: 1984, specifies a deflection limit of 0.003 x span or 14 mm, whichever is smaller, for a single joist under design loading. In some cases this is not regarded as adequate as the vibrational performance of a timber floor depends very much on the properties of the flooring, especially mass per unit area and the overall bending stiffness of the floor system in the direction perpendicular to the joist length. In general, lack of design guide-
51
lines is the main cause of the problem, although i m p r o v e d construction t e c h n i q u e s m a y help solve the problem to s o m e extent.
Research into floor vibration TRADA's interest in floor vibration can be dated back to 1969. In conjunction with the Princes Risborough Laboratory (formerly known as the Forest Products Research Laboratory), of the Building Research Establishment, vibration tests on timber floor panels and a limited survey of householders' reaction to vibration were carried out <3~. Since then, until recently, no active research in this field has been undertaken in the UK. In 1984, TRADA embarked upon a two year research project to investigate the problem of vibration in timber floors following an extensive literature survey~4L This project was 50% funded by the Department of the Environment (DOE). Full sized floor vibration tests (Figure I) were conducted in the laboratory ~5' to investigate how vibrational performance was affected by changes in construction variables, such as joist spacing, flooring material, type of fasteners, etc. Vibration was initiated by an impact and the responses at various locations were measured. The data were then analysed to obtain the vibrational characteristics. The test and data analysis procedures are illustrated in Figure 2. A limited n u m b e r of tests were also carried out in occupied houses. This enabled a comparison between laboratory and field data to be made. The experimental work was supplemented by a theoretical study in which a mathematical model was developed. Given the materials properties of a floor, the model would predict the vibrational characteristics. This enabled additional construction variables to be studied without the necessity of carrying out further tests. From the results of these studies, m e a s u r e s for improving vibrational performance were derived. These are described in the next section. Previous studies of human reaction to floor vibration showed that the m o s t important factors concerning acceptibility are the frequency and the m e a n magnitude of vibration. In the research project, the threshold values for these two parameters were evaluated ~6~ and design equations
52
Vibration testing of floor systems in the laboratory
Force
framsducer
// I Digitisingof signal
Data logging ]
I Analysisof data J
utput of vibrati0nal~ characteristics J
Fig 2
Floor test and data analysis procedures
were also derived for predicting these two parameters'". The use of these equations enables a designer to predict whether a particular design is likely to have vibration problems. At present, no vibrational serviceability requirements are specified in BS 5268: Part 2. In TRADA's Final Report ~"~ to the DoE, recommendations of how current codes of practice can be modified to take vibrational perofrmance into account are given. The eventual inclusion of these recommendations in design codes will, hopefully, lead to the production of new floors which are consistently acceptable to their users.
Improving vibrational performance Before discussing m e a s u r e s for im p proving floor p e r f o r m a n c e , it is worthwhile explaining what is meant by an improvement in vibrational performance. As described before, h u m a n reaction to floor vibration is influenced principally by the frequency and m e a n magnitude of the vibration. In general, the degree of discomfort increases with decreasing frequency or increasing mean magnitude. Hence improvement in performance is attained if the frequency can be raised or the m e a n magnitude of the vibration reduced. Each of the measures described in this section is aimed at achieving at least o n e of these objectives.
C O N S T R U C T I O N & B U I L D I N G M A T E R I A L S Vol. 1 No. 1 M A R C H / A P R I L 1987
Support of perimeter joists The_ current design code does not require that all four sides of a floor be supported. Economic reasons dictate that most domestic floors only have two sides where the joist ends meet the walls supported. It is desirable, from the point of view of reducing the magnitude of vibration, to support the other two sides as well .This may be achieved by resting the first and the last joists of a floor onto brick piers, timber columns or by supporting the joists on metal brackets. The latter seems to be more feasible as the presence of piers would reduce the area of room beneath the floor and in addition, the costs of constructing piers are relatively high.
Insertion of adequate betweenjoists strutting The addition of between-joists strutting has two objectives: to increase the bending stiffness of a floor system in the diretion perpendicular to joist length and to spread out any point or impact loading so that a higher proportion of the force is carried by the adjacent joists. Although this practice has not effect upon frequency, it does r e d u c e significantly the magnitude of vibration. Present construction practice is that if the joist span is less than 2.5 m, no intermediate strutting is required; for spans between 2.5 m to 4.5 m, one line of strutting is installed. For spans over 4.5 m, usually two lines of strutting are provided. It is recomm e n d e d here that the number of lines of strutting adopted should be as follows.
Joist span (m)
Unes of Strutting
Less than 2.5 m 1 2.5 m - 4.5 m 2 More than 4.5 m 3 The values quoted above may be regarded as the minima. Designers are advised to specify as great a number of lines as they feel may be necessary, especially if the floor area is large. The two most c o m m o n forms of strutting are herring bone and solid timber blocking. Herring-bone strutting comprises pairs of inclined pieces of timber which are tightly fitted between the joists (Figure 3). This form of strutting has the advantage of remaining effective, even if the joists shrink in both width and thickness. Shrinkage in depth re-
Fig 3
Details of traditional herring.bone strutting
duces the angle of inclination of the struts with a corresponding tightening at the joints. More recently, herringbone strutting using thin metal strip nailed to the slides of joists has b e c o m e more popular. The f'~ng of solid timber blocking usually requires greater effort as every piece needs to be cut squarely to the precise length in order to maximise its effectiveness. However, timber blocking suffers from loosening if the joists shrink in width, which directly reduces its efficiency. There is some scope for ingenuity here to produce ways of preventing this occuring.
Use of stiff flooring materials The use of stiff flooring material as against a more flexible one would have the same effect as installing extra between joists strutting. The most c o m m o n types of flooring used today are plywood and chipboard. Plywood flooring generally has a higher bending m o d u l u s in the across joists direction than an alternative chipboard flooring. However, for a given joist spacing, the minimum thickness required for plywood flooring is less than for chipboard. As a consequence, it is possible for chipboard flooring to have a higher bending stiffness than plywood of equivalent capacity since bending stiffness is proportional to the cube of the thickness. Tests carried out at TRADA have shown that a floor built with plywood flooring performed better than a
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 MARCH/APRIL 1987
corresponding one built with chipboard. It must also be mentioned that the plywood costs about twice as much as the chipboard, lfa change to a stiffer flooring is considered, it may be more economical to use a thicker chipboard than to use a plywood of the required minimal thickness. In such cases, designers must ensure that the greater unit weight of chipboard does not adversely affect the frequency, as natural frequency is inversely proportional to the mass of the floor.
Adoption of optimum joist span direction In traditional construction practice, joists have always spanned in the shorter dimension of a floor area. At present, it is an increasingly c o m m o n practice to design joists to run in the longer direction in order to reduce material costs. This is considered to be bad practice and leads to poor vibrational performance. As frequency of floor vibration is inversely proportional to the square of the span, spanning the joists in the longer dimension considerably lowers the frequency of vibration. To improve floor performance, joists should also be spanning perpendicular to any internal Ioadbearing partition beneath the floor as this effectively divides the floor into two floors with smaller spans. It is recommended that floors be designed to span perpendicular to intermediate support and in the
53
shorter direction. It is s o m e t i m e s not possible to satisfy both conditions as the position of internal partitioning is normally not changeable. The design method presented in reference ~7'may be used to determine the optimum span direction.
Use of damping materials The damping capacity of a floor system is a measure of its ability to damp out a certain motion; the higher the damping capacity, the smaller the m e a n magnitude of vibration. The damping capacity of a timber floor can be enhanced significantly by the use of artificial damping materials, such as rubber pads. These materials are s o m e t i m e s placed between the flooring and the joists to reduce the amount of vibration, as in
54
floating floor construction. This is a very effective method of reducing vibration magnitude, but, owing to its relatively high cost, may only be adopted for remedial work and for special cases where even minor floor disturbances are unacceptable to the occupants.
References
]. McKay. W B Carpentry, 5th ed 1974, Harlow, Longman Group Ltd. p.75. 2. British Standards Institution. Structural use of timber: Part 2. Code of Practice for permissible stress design, materials and
workmanship. British Standard BS 5268: Part 2: London, BSI. 1984. 3. Curry, W T and Brown, B, Exercise in Joint Research. Timber Trades Journal, March 1969.
4. Whale, L R J Vibration of timber floors A literature review. TRADA Research Report 2/83, Hughenden Valley, TRADA, 1983 5. Chui, Y H Evaluation of vibrational per formance of lightweight wooden flool~ Determination of effecL~ of changes in construction variables on vibrational characteristics. TRADA Research Report 2,,/86, Hughenden Valley, TRADA, 1986 6. ChuL Y/-f Evaluation of uibrational pe~ formance of light-weight wooden floor~: Method of assessment of floor vibration due to human footsteps. TRADA Research Report 3/86. Hughenden Valley, TRADA. 1986.
7. Chui, Y H Evaluation of vibrational pel [ormance of light- weight wooden floor.~: Design to avoid annoying vibrativn~ TRADA Research Report ! 5/86, Hughenden Valley, TRADA, 1986. 8. Chui, Y H Design aspects of floor vibration - Final Report to DOE. TRADA Research Report (unpublished). Hughenden Valley, TRADA~ 1986.
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 MARCH/APRIL 1987
Timber Research and Development Association (TRADA). Hughenden Valley, High Wycombe, UK.
Abstract Excessive vibration of timber floors has been a problem for a number of years. Not until recently, however, has any research been undertaken in the UK to investigate this problem. A description of the cause and extent of floor vibration is given in this paper. Recent research work carried out at TRADA in this field is also briefly described Finally, construction measures for improving the vibrational performance of timber floors are proposed
Introduction For centuries timber has been the traditional material used for domestic floor construction, due primarily to its ready availability, ease of handling and above all, its high strength-toweight ratio. A conventional timber floor is constructed by laying a suitable flooring material onto a series of timber joists whose ends bear onto either external walls or intermediate bearers. The flooring is connected to the joists by means of nails or glue, or a combination of both. Historically, the most commonly used flooring material in the UK was nailed softwood boarding. The advent of timber framed housing construction in the UK in the late 1960s and early 1970s coincided with a significant move away from the use of softwood boarding in favour of sheet materials such as plywood and chipboard. Apart from reducing the materials costs, considerable cost savings may also be achieved by using sheet materials because of their ease of handling and laying. The size of joist section required used typically to be determined by the 'rule-of-thumb' approach whereby
the approximate depth in inches of 2inch thick joists spaced at 15-inch centres may be found by dividing the span in feet by 2 and adding 2 to the quotient ~. Thus, for example, an 8foot span would require 6 i n c h x 2inch joists. Timber floors are currently designed on the basis of calculated stresses and deflections of a single joist under design loading. The stresses and deflection must not exceed the permissible values given in the design code, BS 5268: Part 2: 1984 ~2~.In this 'working stress' design approach no account is taken of the contribution of the flooring to the overall stiffness and strength of the floor system. Thus, the choice of flooring material does not affect the design solulJon and, for each material, the thickness required is governed only by the joist spacing and the use to which the floor is to be put. The 'working stress' approach to design generally produces floors with acceptable vibrational performance. But recently the number of complaints about floor vibration has steadily increased. While s o m e of these cases can be attributed to poor workmanship or improper material
CONSTRUCTION & B U I L D I N G MATERIALS Vol. 1 No. 1 M A R C H / A P R I L 1 9 8 7
quality control on site, the majority occur in floor systems designed and constructed in accordance with current practice. This clearly indicates that floors produced these days are more flexible. This greater flexibility has been brought about by the refinement of design methods, use of higher moduli of elasticity and new construction techniques. Current design practices, which treat floor systems as one dimensional beam structures, do not include requirements specifically for vibrational serviceability, although the static deflection limits do provide some indirect control over the vibration problem. For example, BS 5268: Part 2: 1984, specifies a deflection limit of 0.003 x span or 14 mm, whichever is smaller, for a single joist under design loading. In some cases this is not regarded as adequate as the vibrational performance of a timber floor depends very much on the properties of the flooring, especially mass per unit area and the overall bending stiffness of the floor system in the direction perpendicular to the joist length. In general, lack of design guide-
51
lines is the main cause of the problem, although i m p r o v e d construction t e c h n i q u e s m a y help solve the problem to s o m e extent.
Research into floor vibration TRADA's interest in floor vibration can be dated back to 1969. In conjunction with the Princes Risborough Laboratory (formerly known as the Forest Products Research Laboratory), of the Building Research Establishment, vibration tests on timber floor panels and a limited survey of householders' reaction to vibration were carried out <3~. Since then, until recently, no active research in this field has been undertaken in the UK. In 1984, TRADA embarked upon a two year research project to investigate the problem of vibration in timber floors following an extensive literature survey~4L This project was 50% funded by the Department of the Environment (DOE). Full sized floor vibration tests (Figure I) were conducted in the laboratory ~5' to investigate how vibrational performance was affected by changes in construction variables, such as joist spacing, flooring material, type of fasteners, etc. Vibration was initiated by an impact and the responses at various locations were measured. The data were then analysed to obtain the vibrational characteristics. The test and data analysis procedures are illustrated in Figure 2. A limited n u m b e r of tests were also carried out in occupied houses. This enabled a comparison between laboratory and field data to be made. The experimental work was supplemented by a theoretical study in which a mathematical model was developed. Given the materials properties of a floor, the model would predict the vibrational characteristics. This enabled additional construction variables to be studied without the necessity of carrying out further tests. From the results of these studies, m e a s u r e s for improving vibrational performance were derived. These are described in the next section. Previous studies of human reaction to floor vibration showed that the m o s t important factors concerning acceptibility are the frequency and the m e a n magnitude of vibration. In the research project, the threshold values for these two parameters were evaluated ~6~ and design equations
52
Vibration testing of floor systems in the laboratory
Force
framsducer
// I Digitisingof signal
Data logging ]
I Analysisof data J
utput of vibrati0nal~ characteristics J
Fig 2
Floor test and data analysis procedures
were also derived for predicting these two parameters'". The use of these equations enables a designer to predict whether a particular design is likely to have vibration problems. At present, no vibrational serviceability requirements are specified in BS 5268: Part 2. In TRADA's Final Report ~"~ to the DoE, recommendations of how current codes of practice can be modified to take vibrational perofrmance into account are given. The eventual inclusion of these recommendations in design codes will, hopefully, lead to the production of new floors which are consistently acceptable to their users.
Improving vibrational performance Before discussing m e a s u r e s for im p proving floor p e r f o r m a n c e , it is worthwhile explaining what is meant by an improvement in vibrational performance. As described before, h u m a n reaction to floor vibration is influenced principally by the frequency and m e a n magnitude of the vibration. In general, the degree of discomfort increases with decreasing frequency or increasing mean magnitude. Hence improvement in performance is attained if the frequency can be raised or the m e a n magnitude of the vibration reduced. Each of the measures described in this section is aimed at achieving at least o n e of these objectives.
C O N S T R U C T I O N & B U I L D I N G M A T E R I A L S Vol. 1 No. 1 M A R C H / A P R I L 1987
Support of perimeter joists The_ current design code does not require that all four sides of a floor be supported. Economic reasons dictate that most domestic floors only have two sides where the joist ends meet the walls supported. It is desirable, from the point of view of reducing the magnitude of vibration, to support the other two sides as well .This may be achieved by resting the first and the last joists of a floor onto brick piers, timber columns or by supporting the joists on metal brackets. The latter seems to be more feasible as the presence of piers would reduce the area of room beneath the floor and in addition, the costs of constructing piers are relatively high.
Insertion of adequate betweenjoists strutting The addition of between-joists strutting has two objectives: to increase the bending stiffness of a floor system in the diretion perpendicular to joist length and to spread out any point or impact loading so that a higher proportion of the force is carried by the adjacent joists. Although this practice has not effect upon frequency, it does r e d u c e significantly the magnitude of vibration. Present construction practice is that if the joist span is less than 2.5 m, no intermediate strutting is required; for spans between 2.5 m to 4.5 m, one line of strutting is installed. For spans over 4.5 m, usually two lines of strutting are provided. It is recomm e n d e d here that the number of lines of strutting adopted should be as follows.
Joist span (m)
Unes of Strutting
Less than 2.5 m 1 2.5 m - 4.5 m 2 More than 4.5 m 3 The values quoted above may be regarded as the minima. Designers are advised to specify as great a number of lines as they feel may be necessary, especially if the floor area is large. The two most c o m m o n forms of strutting are herring bone and solid timber blocking. Herring-bone strutting comprises pairs of inclined pieces of timber which are tightly fitted between the joists (Figure 3). This form of strutting has the advantage of remaining effective, even if the joists shrink in both width and thickness. Shrinkage in depth re-
Fig 3
Details of traditional herring.bone strutting
duces the angle of inclination of the struts with a corresponding tightening at the joints. More recently, herringbone strutting using thin metal strip nailed to the slides of joists has b e c o m e more popular. The f'~ng of solid timber blocking usually requires greater effort as every piece needs to be cut squarely to the precise length in order to maximise its effectiveness. However, timber blocking suffers from loosening if the joists shrink in width, which directly reduces its efficiency. There is some scope for ingenuity here to produce ways of preventing this occuring.
Use of stiff flooring materials The use of stiff flooring material as against a more flexible one would have the same effect as installing extra between joists strutting. The most c o m m o n types of flooring used today are plywood and chipboard. Plywood flooring generally has a higher bending m o d u l u s in the across joists direction than an alternative chipboard flooring. However, for a given joist spacing, the minimum thickness required for plywood flooring is less than for chipboard. As a consequence, it is possible for chipboard flooring to have a higher bending stiffness than plywood of equivalent capacity since bending stiffness is proportional to the cube of the thickness. Tests carried out at TRADA have shown that a floor built with plywood flooring performed better than a
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 MARCH/APRIL 1987
corresponding one built with chipboard. It must also be mentioned that the plywood costs about twice as much as the chipboard, lfa change to a stiffer flooring is considered, it may be more economical to use a thicker chipboard than to use a plywood of the required minimal thickness. In such cases, designers must ensure that the greater unit weight of chipboard does not adversely affect the frequency, as natural frequency is inversely proportional to the mass of the floor.
Adoption of optimum joist span direction In traditional construction practice, joists have always spanned in the shorter dimension of a floor area. At present, it is an increasingly c o m m o n practice to design joists to run in the longer direction in order to reduce material costs. This is considered to be bad practice and leads to poor vibrational performance. As frequency of floor vibration is inversely proportional to the square of the span, spanning the joists in the longer dimension considerably lowers the frequency of vibration. To improve floor performance, joists should also be spanning perpendicular to any internal Ioadbearing partition beneath the floor as this effectively divides the floor into two floors with smaller spans. It is recommended that floors be designed to span perpendicular to intermediate support and in the
53
shorter direction. It is s o m e t i m e s not possible to satisfy both conditions as the position of internal partitioning is normally not changeable. The design method presented in reference ~7'may be used to determine the optimum span direction.
Use of damping materials The damping capacity of a floor system is a measure of its ability to damp out a certain motion; the higher the damping capacity, the smaller the m e a n magnitude of vibration. The damping capacity of a timber floor can be enhanced significantly by the use of artificial damping materials, such as rubber pads. These materials are s o m e t i m e s placed between the flooring and the joists to reduce the amount of vibration, as in
54
floating floor construction. This is a very effective method of reducing vibration magnitude, but, owing to its relatively high cost, may only be adopted for remedial work and for special cases where even minor floor disturbances are unacceptable to the occupants.
References
]. McKay. W B Carpentry, 5th ed 1974, Harlow, Longman Group Ltd. p.75. 2. British Standards Institution. Structural use of timber: Part 2. Code of Practice for permissible stress design, materials and
workmanship. British Standard BS 5268: Part 2: London, BSI. 1984. 3. Curry, W T and Brown, B, Exercise in Joint Research. Timber Trades Journal, March 1969.
4. Whale, L R J Vibration of timber floors A literature review. TRADA Research Report 2/83, Hughenden Valley, TRADA, 1983 5. Chui, Y H Evaluation of vibrational per formance of lightweight wooden flool~ Determination of effecL~ of changes in construction variables on vibrational characteristics. TRADA Research Report 2,,/86, Hughenden Valley, TRADA, 1986 6. ChuL Y/-f Evaluation of uibrational pe~ formance of light-weight wooden floor~: Method of assessment of floor vibration due to human footsteps. TRADA Research Report 3/86. Hughenden Valley, TRADA. 1986.
7. Chui, Y H Evaluation of vibrational pel [ormance of light- weight wooden floor.~: Design to avoid annoying vibrativn~ TRADA Research Report ! 5/86, Hughenden Valley, TRADA, 1986. 8. Chui, Y H Design aspects of floor vibration - Final Report to DOE. TRADA Research Report (unpublished). Hughenden Valley, TRADA~ 1986.
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 1 MARCH/APRIL 1987