CONTROLLING CROSS-LAMINATED TIMBER (CLT) FLOOR VIBRATIONS: FUNDAMENTALS AND METHOD

Transcription

1 CONTROLLING CROSS-LAMINATED TIMBER (CLT) FLOOR VIBRATIONS: FUNDAMENTALS AND METHOD Lin Hu 1 and Sylvain Gagnon 2 ABSTRACT: Cross-Laminated Timber (CLT) is proving to be a promising solution allowing wood to compete in building sectors where traditionally steel and concrete have predominated. The dynamic behaviour of CLT floor systems differ from that of the traditional lightweight wood-joisted floors and heavy concrete slab floors. The existing standard vibration-controlled design methods for lightweight and heavy floors may not be applicable to CLT floors. A new design method was developed based on the understanding of the fundamentals of floor vibrations and laboratory study of CLT floors with various construction details. The design method predicted vibration performance of CLT floors well and matched with the subjective ratings. The new design method is simple that the vibration-controlled spans can be directly calculated from CLT stiffness and mass. The vibration-controlled spans of CLT floors predicted by this new design method were almost the same as the spans determined by CLTdesigner software that was developed in Austria. It is concluded that the proposed design methodology to determine vibration-controlled maximum spans of CLT floors is promising. KEYWORDS: CLT floor, Vibration, Normal Walking, Design Method 1 INTRODUCTION 123 Cross-Laminated Timber (CLT) is proving to be a promising solution allowing wood to compete in building sectors where traditionally steel and concrete have predominated. Figure 1 shows a typical crosssection of a CLT floor. Previous studies conducted at FPInnovations have found that bare CLT floor systems differ from traditional lightweight wood-joisted floors that have a typical mass around 20 kg/m 2 and a fundamental natural frequency greater than 15 Hz. Figure 2 illustrates the conventional North American lightweight wood joisted floor built with joists and 15mm-18mm thick wood composite panels. Various toppings and ceilings are common in multi-family wood buildings. Figure 1: Cross-section of a bare CLT floor 1 Lin Hu, FPInnovations, 319, rue Franquet, Quebec, QC G1P 4R4, Canada. lin.hu@fpinnovations.ca 2 Sylvain Gagnon, FPInnovations, 319, rue Franquet, Quebec, QC G1P 4R4, Canada. sylvain.gagnon@fpinnovations.ca Figure 2: Conventional North American Lightweight wood floor built with joists and subfloor

2 Furthermore, even if non-joisted CLT floor looks similar to the concrete slab floor, the CLT floor systems differ from heavy concrete slab floors in terms of construction details. The typical concrete slab floors have a mass greater than 200 kg/m 2 and a fundamental natural frequency less than 9 Hz. Based on FPInnovations test results, bare CLT floors were found to have a mass varying between approximately, 30 kg/m 2 to 150 kg/m 2, and a fundamental natural frequency greater than 9 Hz. Due to CLT floor s unique dynamic behaviour, the existing standard vibration-controlled design methods for lightweight and heavy floors may not be necessarily applicable to CLT floors. Many of the manufacturers recommend using a uniform distribution load (UDL) deflection method for CLT floor control vibrations by limiting the static deflections of the CLT panels under a UDL. Using this approach, success in avoiding excessive vibrations in CLT floors relies mostly on the engineer s judgement. A new design methodology is needed to determine the vibration-controlled spans for CLT floors. This paper describes the development of a new design method to control CLT floor vibrations. 2 METHOD 2.1 KNOWLEDGE OF THE FUNDAMENTALS OF FLOOR VIBRATIONS The new design method was developed based on the understanding of the fundamentals of floor vibrations. FPInnovations previous study [1] on wood-joisted floors found that for floors with a fundamental natural frequency above 9 Hz, the vibrations induced by normal walking exhibit a transient nature. The transient vibrations can be controlled through controlling the combination of floor stiffness and mass. Simply by controlling the combination of the fundamental natural frequency and 1 kn static deflection, it is possible to successfully control the vibration of wood-joisted floors. This led to the proposal of a new design method to control wood joisted-floor vibrations. SINTEF s extensive field CLT floor vibration study further proved this understanding of the fundamentals of wood floor vibrations [2]. SINTEF found that with the FPInnovations new design method using 1 kn static deflection and fundamental natural frequency as design parameters to control wood joisted-floor vibrations, predictions of the field CLT floor vibration performance was in alignment with occupants expectations. 2.2 LABORATORY STUDY Laboratory tests and subjective evaluations were conducted on CLT floors with certain variables such as CLT element thickness, floor spans, type of the betweenelement joints, connections and support conditions CLT Floor Specimens The floor specimens were built using three individual pieces of 2.0 m wide CLT panels with different thicknesses (i.e. 230 mm, 182 mm and 140 mm). The spans varied from 8.0 m to 4.5 m and with two types of joint details for connecting panels together. The joint details are showed in Figures 3 and 4 below. Figure 3: Step joint (half-lapped) Cross-laminated LVL Figure 4: Spline joint For the step joint detail, 8 mm diameter Würth screws were used to connect two CLT panels at a spacing of 320 mm o.c. In the case of the spline joint detail, normal wood screws No. 10 (diameter of 4.83 mm.) were used to connect the continuous strip of cross-laminated-lvl to the CLT panels. The spacing was 200 mm o.c. The ends of each CLT floor assembly were supported on 190 mm thick and 685 mm high Glulam walls connected using Würth screws. The floor assemblies were tested under two types of supporting conditions at the floor edges, i.e. free and simple support along the longitudinal direction of the panels. When the floor edges were supported, the 38 mm x 89 mm wood stud wall panels of 2.0 m long were used as supports. The wall panels were spaced at 2.0 m or less. The measured performance parameters were natural frequencies, modal damping ratios, static deflection under 1 kn static load, velocity and accelerations due to a 1 N-S impulse using the test protocols developed at FPInnovations [3]. This study was to identify the constructions and design parameters that significantly affected the CLT floor vibration performance measured by the natural frequencies, static deflections, velocities and accelerations, and human perceptions.

3 2.2.2 Subjective Evaluation The key objective of the subjective evaluation of vibration performance is to define the maximum annoying vibration level that can be acceptable to the majority of occupants of residential floors. To more closely mimic normal living conditions during subjective performance evaluations, the CLT floor assemblies were carpeted and furnished with a cabinet and two vases filled with water and flowers, as shown in Figures 5 and 6. The reason for decorating the china cabinet using flowers and water is because these objects are good indicators of floor excessive vibrations. If the floor is vibrating at high level when a person is walking by the cabinet, he/she will notice the flower and the water moving. In Figure 6, you also can see some kind of china cabinet with glassware. It has been observed that the glassware in the china cabinet is another good indicator of excessive vibrations. Typically, with poor floors, one can easily notice the rattling of glassware in the room. In these Figures, you could also see a chair located in the centre of the floor. During the subjective evaluation, if the person is sitting on the chair and feels the floor vibrating when someone is walking by the chair, then the floor vibration is most likely not acceptable. Figure 6: Subjective evaluation while the evaluator is sitting, feeling, and observing the floor movement The evaluator was then asked to sit on the chair, while another person would walk on the floor according to a pre-designated pattern (Figure 6). The walking pattern is such that the person walks at least two times along the two diagonal directions from one corner of the floor to the other, then walks at least two times along the middle lines in the parallel and perpendicular directions of the subfloor panels. Again the evaluator was observing the three clues, i.e. seeing, hearing and feeling regarding floor vibration performance. Immediately after the evaluation, the evaluator is asked to fill out a questionnaire which provided an overall performance rating for the floor as well as a score for the three key performance-related clues Static Concentrated Load Test This test was conducted to determine the maximum static deflection of the floor under a 1 kn concentrated static load. This is a measure of the entire stiffness of the floor. Figure 5: Subjective evaluation while the evaluator is walking, feeling, and observing the floor movement At least twenty persons were asked to evaluate the performance of a floor subjectively. Only one evaluator was allowed on the floor at a time. He or she first walked freely on the floor while observing clues related to floor performance (Figure 5). The clues included movement of the flowers and the water, rattling of the china cabinet, and feeling the movement of the floor. The basic elements needed to measure static deflection under a concentrated load are: 1) a stable reference from which to measure floor movement, 2) an accurate and sensitive deflection measuring device, and 3) a mobile loading system. In this study, the concrete ground floor was used as the reference. Two electronic gauges having a resolution of mm were used as the deflection measuring devices as shown in Figure 7. The deflection gauges were mounted to the free ends of two rods in contact with the concrete ground floor surface. The end of one deflection gage was fixed to the bottom of the middle point of the centre CLT panel to measure the static deflection of the floor centre. The end of the second deflection gauge was fixed to the bottom of the middle point of the joint of two CLT panels to measure the static deflection of the joint. The concentrated static load was applied by a person standing over each CLT panel s centre, floor edges and the joints in turn while recording the measurement at the gauge location. Figure 8 shows the static loading process using the tester s weight. The deflection profile of the floor was generated from a complete set of measurements. Three

4 measurements were taken at each loading location to ensure that stable results were obtained. The average of three sets of the deflection profiles were used to plot the deflection profile of the test floor under the person s weight. The deflection measurements were normalized to 1 kn load. to determine the natural frequencies, modal damping ratios and mode shapes. Figure 9: Modal test on a CLT floor specimen Figure 7: Static deflection measurements using two deflection gages under the CLT floor Figure 8: Static loading using the tester s weight Modal Test Modal test was conducted to determine the natural frequencies, modal damping ratios, and vibration modes of the CLT floor specimens. Modal testing followed a standard procedure specified in FPInnovations protocols [3]. Hammer excitation was selected because of its simplicity and reliability. The hammer impact was applied at the top of the floor by a person sitting on a beam supported on the ground so that the tester s weight was not added to the floor. The hammer impact was located on the side panel and was offset from the mid-span of the test floor areas. At such a location, it was unlikely that a nodal point of the first three modes would occur. The floor vibration was measured on each panel at one quarter of the span of the test floor areas. Figure 9 shows a typical modal test setup and the locations for the hammer and accelerometers. The force and acceleration signals were recorded by a multi-channel analyzer. The signals were post-processed Forced vibration test Forced vibration testing was conducted to determine the dynamic responses including acceleration, velocity, and displacement responses of the CLT floors to an impulse similar to the heel impact force of the foot steps of human normal walking. Forced vibration testing followed the procedure described in the FPInnovations test protocols [3]. The excitation was triggered by dropping a 5 kg medicineball on a force plate instrumented with the force transducer. The ball drop impact was performed by a tester while sitting on a stool. The ball was caught when it rebounded to avoid multiple impacts. The ball drop force was applied at the floor centre. An accelerometer was also located at the impact location to measure the maximum response of the floor. The impact force and acceleration signals were recorded by the multi-channel analyzer. The acceleration signals were then integrated to obtain velocity or displacement responses. The impact force signal was processed to determine the impulse, which was further used to normalize the dynamic responses to a 1 N-s impulse. Figure 10 shows the ball drop impact test and the measurement setup. Figure 10: Forced vibration test on a CLT floor specimen using the ball drop as the excitation

5 3 RESULTS 3.1 KEY CONSTRUCTION AND DESIGN PARAMETER Based on data analysis, it was found that the combination of fundamental natural frequency with 1 kn static deflection, or with acceleration, or with velocity was well correlated to human perception. It is understood that the fundamental natural frequency is mainly controlled by the longitudinal stiffness and the mass, while the 1 kn static deflection is determined by the entire stiffness of the CLT floor in both the longitudinal and lateral directions. The acceleration or velocity is determined by the excitation, the entire stiffness and by damping ratio. It has been wellrecognized that determining the damping ratio accurately and to a certain reliability level of is not easy. Reproducibility is another issue. Besides, damping is largely controlled by the floor constructions details such as the joints, connections, and support conditions, and the non-structural components such as potions, flooring, toppings, insulation materials, furniture, etc. Moreover, the modal test results showed that, for the bare CLT floors tested, the measured damping ratio did not vary from one assembly to another as it was quit constant with a value of around 1%. Based on the laboratory study results, it has been found that in meeting the safety requirements for the supports and joints, the vibration performance of CLT floors were largely controlled by the CLT stiffness along the longitudinal direction and by the mass. The type of joints between the CLT panels did not significantly affect the measured fundamental natural frequencies, the 1 kn static deflections, and the subjective ratings. Therefore, it was decided to use the stiffness in the longitudinal direction and the mass as the key parameters in the design method to control CLT floor vibrations through a combination of fundamental natural frequency and 1 kn static deflection. 3.2 PROPOSED DESIGN METHOD The proposed design method to control CLT floor vibrations consists of a design criterion and the relevant equations to calculate the design parameters Scope At this point, the scope of the proposed design method to control vibrations of CLT floors covers the following 1. bare floors with finishing, partitions and furniture, but without heavy topping, 2. vibrations-induced by normal walking, 3. well-supported floors, 4. well-jointed CLT panels, and 5. inclusion of the self weight of CLT panels only (i.e. without live load). cover various types of toppings and ceilings, and other floor design options Advantages The proposed design method is focused on target features, which include, among others 1. simple for hand calculation, 2. user-friendly, 3. mechanics-based using the design values CLT panels available in producer s specification, 4. reliable to prevent CLT floors from excessive vibrations induced by normal walking Design Criterion Based on the understanding of the fundamentals of floor vibrations and the special features of CLT floor vibrations, and following the laboratory test results, a proposed simple design criterion using fundamental natural frequency and 1 kn static deflection of a simple 1-m wide CLT panel as design parameters has been developed. The design criterion is expressed in Equation (1). f d Or 1.43 f d 39 where f = fundamental natural frequency calculated using Equation (2) in Hz, and d = 1 kn static deflection calculated using Equation (3) in mm Equations to Calculate the Design Parameters f l EI 1m eff A (1) (2) where, f = fundamental natural frequency of 1m CLT panel simply supported in Hz, l= CLT floor span in m, EI 1 m eff = effective apparent stiffness in the span direction which is published by the producers for 1m wide panel in N-m 2, = density of CLT in kg/m 3, and A = crosssection area of 1-m wide CLT panel, i.e. thickness*1m width in m 2. The static deflection under 1 kn load can be calculated using Equation (3) below Pl (3) d 1m 48EI eff where, d = static deflection at mid-span of the 1m wide simply supported CLT panel under 1 kn load in mm, and P = 1000 N. However, because of the mechanics-based feature, it is possible to expand its scope to include other construction details. A study has been planned to extend the scope to

6 1kN static deflection calculated using Eq.2 (mm) Verification Figure 11 shows a comparison of predictions using the proposed new design methodology to predict the CLT floor vibrations and corresponding subjective ratings by evaluators Predicted CLT floor vibration performance vs. subjective ratings criterion ( f/d^0.7>13.0) Unacceptable Marginal Acceptable Fundamental Natrual frequency calculated using Eq. 1 (Hz) Figure 11: Predicted CLT floor vibration performance by the proposed design method vs. subjective rating by participants Simple Form of the Design Method Inserting Equations (2) and (3) into the design criterion, i.e. Equation (1), it is possible to obtain a simple form of the design method which can be expressed by Equation (4) as given below. l ( EI 1m eff ( A) ) (4) Using Equation (4), it is possible to determine the vibration controlled spans for CLT floors directly from the effective apparent stiffness in the span direction, density and cross-section area of 1m wide CLT panels Impact Study The vibration controlled CLT floor spans determined using the proposed design method were compared with the spans determined by CLTdesigner developed at university of Graz in Austria [4]. Table 1 provides the comparison. Table 1: Vibration controlled CLT floor spans determined using the new design method vs. the spans determined by CLTdesigner CLT Thickness (mm) Span Determined by the Proposed Method (m) Span Determined by CLTdesigner for 1% Damping As given in Table 1, the vibration controlled spans of bare CLT floors predicted by this new design method were very close to those spans determined by CLTdesigner. 4 CONCLUSIONS It is concluded that the proposed design methodology to determine vibration-controlled maximum spans of bare CLT floors is promising. This methodology uses only the design values of CLT mechanical properties. The method is simple, user-friendly, and reliable. Wide acceptance of the proposed design method relies on the use and evaluation of the method by productmanufacturers and designers. FPInnovations is open to feedback and ready to adopt and further refine the design method according to the needs of the producers and designers. The current form of the design method is for CLT floors without heavy topping. A study of the effect of heavy topping on the vibration performance of CLT floors is under way. ACKNOWLEDGEMENT FPInnovations would like to thank its industry members and Natural Resources Canada (Canadian Forest Service) for their financial support of this work. The authors wish to thank KLH for providing CLT panels for this study and the guidance on CLT floor construction. Thanks are also extended to Mr. Thomas Orskaug of Moelven Massivtre AS (Now of KLH Solid Wood Scandinavia AB) and Dr. Anders Homb of SINTEF Byggforsk for sharing their experience of massive wood slab non-joisted floor systems with us and for providing the opportunity to visit CLT buildings in Norway. Thanks are also extended to Dr. Gerhard Schickhofer of Graz Institut für Holzbau und Holztechnologie, Austria for conducting the comparison given in Table 1.

7 REFERENCES [1] Hu J.: Design guide for wood-framed floor systems. Final report No.32 for Canadian Forestry Service. FPInnovations, Quebec, [2] Homb A.: Vibrasjonsegenskaper til dekker av massivtre (in Norwagien). Report of Prosjektrapport 24, Sintef Byggforsk, Norway, [3] Hu J.: Protocols for field testing of wood-based floor systems. Appendix V in Report of Serviceability design criteria for commercial and multi-family floors. Report No. 3 for Canadian Forest Service, FPInnovations, Quebec, [4] Schickhofer G.: Comments on FPInnovations new design method for CLT floor vibration control. Private provided the link to access to CLTdesigner,2010: ignertestversion.jnlp