00078 Lin Hu - Timber Design Society

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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. 2.2.1 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.
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. 2.2.3 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 0.001 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
Page 1: CONTROLLING CROSS-LAMINATED TIMBER
Page 5 and 6: 3 RESULTS 3.1 KEY CONSTRUCTION AND
Page 7: REFERENCES [1] Hu J.: Design guide

00078 Lin Hu - Timber Design Society

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