VIBRATION SUSCEPTIBILITY OF MULTI-SPAN LVL-CONCRETE COMPOSITE FLOORS

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1 VIBRATION SUSCEPTIBILITY OF MULTI-SPAN LVL-CONCRETE COMPOSITE FLOORS Nor Hayati Abd. Ghafar 1, Bruce Deam 2, Massimo Fragiacomo 3 ABSTRACT: Vibrations on floor system are caused from rotating machinery, human activity, traffic, etc. Human activities such as walking or jumping can produce uneasiness feeling which mainly comes from resonance of acceleration rather than from amplitude of the vibration. This issue was studied more than 3 years ago and many approaches were published to provide remedies to decrease the vibration effect on the floor system. Various factors were investigated such as the structural mass, flexibility, type of material as well as boundary condition. The boundary condition is one of the most important parameter, which affects the dynamic properties of timber-concrete composite floor systems. Unfortunately, the influence of the boundary conditions is very difficult to investigate numerically and analytically. Thus, a reduced scale (one third) model of a four-span LVL-concrete beam with continuous concrete slab and notched connection was constructed at University of Canterbury. LVL blocks were positioned at the end of each span to model the main beam supporting the LVL joist and to provide rigid support conditions. Modal testing was carried out using an electrodynamic shaker to provide sinusoidal excitation and instrumented impact hammer was used to generate impact force. Modal shapes, natural and resonant frequencies were then evaluated using the experimental results. The four-span continuous specimen was then separated in a three-, two- and single-span specimen by cutting the concrete slab above the inner supports. The specimens were then tested again with the aims to investigate the propagation of the waves from adjacent spans. The paper discusses the wave propagations which happen during impact test and shows the significant effect of the concrete continuity on the dynamic performance of the actual floor strip. KEYWORDS: multi-span, vibration, timber-concrete composite, boundary condition 1 INTRODUCTION 123 High impact activities such as jumping to music and sudden crowd movements are well known problem, but more common activities such as walking and running can produce discomfort for occupants with uncontrolled vibration magnitudes, and compounded by the growth excitation sources such as those from running train or dancing classes. These serviceability issues were discussed over 3 years ago. But, recently, floor vibrations have become a significant design consideration for engineers due to 1 Nor Hayati Abd Ghafar, PhD student, Dept of Civil Engineering, University of Canterbury, Christchurch, New Zealand. nhb24@student.canterbury.ac.nz 2 Bruce Deam, Leicester Steven EQC Lecturer, Dept. of Civil Engineering, University of Canterbury, Christchurch, New Zealand. bruce.deam@canterbury.ac.nz 3 Massimo Fragiacomo, Associate Professor of Structural Design, Dept. of Architecture, Design and Urban Planning, University of Sassari, Alghero, Italy. fragiacomo@uniss.it lightweight and slenderness of the modern floor systems. Ian Smith [1] categorised two kinds of vibration serviceability sources: continuous and transient. Periodic forces from human activities (dancing or jumping) and rotary motion from machinery produce the continuous vibration of the floor. The resonance will appear when the amplitude of resultant motion is considerably increased by a periodic force synchronised with a structural frequency. On the other hand, impact load from human (e.g walking) will cause transient vibration with a decay motion in proportion to the available damping of the structural system. However, most of the studies only focused on singlespan floors. Multi-span floor systems can be particularly susceptible to these vibration problems. Activities on one of the floors of multi-storey buildings can affect the occupant on upper or lower level depending on the range of vibration, type of structural materials, and the length of the time to decay. Longinow and Hannen [2] studied five different cases of building which had a vibration problem on the floor.

2 They concluded that to design a building busy with human activities, it is important to be aware of possible internal and external activities which may produce floor vibrations. If should be noted that not only vibration causes discomfort to occupants, but sometimes can also damage the structural systems. Building vibration propagation was investigated by Hughes et al [3] using a four-story laboratory model which was compared with a mathematical model. Boundary conditions changed the resonance frequencies and material damping was found to be relatively insensitive to the boundary conditions. As mentioned before, the type of materials has a strong influence on serviceability issues. Timber is more flexible than concrete and steel due to the lower Young s modulus. A combination of timber and concrete can markedly improve the performance of the composite system. A timber floor upgraded to a timberconcrete composite floor is stronger and stiffer than a traditional timber floor, has larger thermal mass and better acoustic separation. This system requires shear connectors to transfer the forces from the concrete slab to the timber joist. By connecting a concrete layer on the compression side to a timber beam on the tension side it is possible to exploit the best properties of these materials in term of strength and stiffness. In this way an effective structure characterised by relative low weight can be obtained [4]. Modern technologies with engineered wood products such as metal-connected trusses, I-joists and laminated veneer lumber (LVL) have improved significantly traditional joisted floor systems, and permitted construction of lightweight and long-span timber floors. These systems can compete with traditional steel, precast concrete and steel-concrete composite floors. However, the high strength to Young s modulus ratio of LVL makes the serviceability limit state the most critical design criterion in most circumstances, and composite action with a stiffer slab is necessary for long span floor systems. Dolan [5] suggested that the requirements of design codes provide a safe structure, but often fail to provide a serviceable structure. An alternative solution for long-span timber floors is to use timber-concrete composite structures. LVL is an engineered wood-based material assembled from rotary peeled timber veneers that are glued with a durable adhesive and laid up with parallel grain orientation to form a continuous billet, up to 12 m long. Sawn or glulam timber have larger coefficients of variation of both the strength and stiffness compared to LVL, which is very strong (almost three times the strength of sawn timber), more reliable and with a higher modulus of elasticity (about 1.5 times the MOE of sawn timber). Therefore, a semi-prefabricated LVL-concrete composite floor system was recently developed at the University of Canterbury, New Zealand. In this system, the composite action is obtained by cutting notches in the LVL beam which are then filled by concrete. A series of LVL-concrete composite beam specimens were built to investigate the static and dynamic behaviour of the system. The experimental programme included shear tests on small composite blocks [6], short-term tests to failure [7], and long-term tests of full-scale beams to characterise their performance under sustained load [8]. The preliminary results of dynamic tests performed on full scale TCC beams are reported in [9] and [1]. 2 THEORETICAL BACKGROUND The mathematical model used to predict the mode shapes were calculated using Biggs [1] continuous beams theory. Such theory is based on the assumption of a uniform mass distribution and stiffness., The characteristic shape of the n th mode and the s th span can be obtained using the equation in the following: (1) where cot, cosec and with, l s, m s, w n the maximum modal bending moment (taken as the moment at the interior support), span length, beam mass and fundamental frequency in rad/s, respectively. For a two-span beam, equation (1) reduces to the following equations for the natural frequencies for first two modes:, 3.92, and for the characteristic mode shapes:, where w is the natural frequency, and, E, I, m and l are mode shapes for first and second span, modulus elasticity, moment of inertia, total mass of the beam and span length, respectively. By combining the two modes with the modal static deflection, from equation (6) the dynamic deflection at the centre of the left span is: (2) (3)

3 .95 (4) mass shaker and the dynamic deflection at the centre of the right span is:.95 where the DLFs, which are dynamic load factor depend on the load-time function f (t) and are evaluated in the usual way as for a one-degree-of-freedom system, and F 1 is the impulse force, which is the static load onto one of the beam. The modal static deflection is taken as:, 287 mm.95 3 BEAMS CONFIGURATION The vibration behaviour of multi-span LVL-concrete composite floor system was investigated through a onethird reduced scale simply supported beam specimen as illustrated in Figure 1. The joist was 287 mm long with cross section of 25 x 45 mm. The beam was connected to a 3 x 65 mm timber block at both ends using standard joist hangers. Six rectangular 1 x 45 mm notches were cut in the top surface of the LVL joist to create the shear connection to the concrete topping. Two 6 mm diameter coach screws were installed in each notch to improve the performance of the connection system. 7 mm plywood sheathing was nailed to the LVL joist as permanent formwork for the 5 x 25 mm concrete topping. 45 mm Double 6 mm coach screw 1 mm Figure 1: Simply supported LVL beam prior to the concrete placement (5) (6) Four simply supported LVL specimens were then connected by screwing the end blocks together so as to to create a continuous 4-span beam, see Figure 2. The concrete topping was poured continuous across the tops of the simply-supported LVL beams. After the first dynamic tests, the four-span continuous LVL-concrete composite beam was then separated in a three- and two-span beam by cutting the concrete slab above the inner supports to investigate the vibration behaviour for different numbers of span. Concrete Figure 2: 4-span LVL-concrete composite beam with continuous concrete topping 4 VERTICAL FORCED TEST A shaker was used to apply nominally sinusoidal excitation forces with frequencies ranging from 18 Hz to 25 Hz. The frequency was incremented in.1 Hz intervals and the test was run long enough at each frequency to produces a steady-state response that was then processed to provide the frequency response of the beams. Accelerometers were attached along the LVL joist to record their vertical movement as illustrated in Figure 2. They were sampled at 1 khz frequency. Approximate natural frequencies and damping ratios were obtained by fitting sine waves to the acquired data using least squares minimisation. Figure 3 shows natural frequencies (ω n = 2.14 Hz) and proportions of critical damping (ξ =.4 %) estimated for a singlespan beam LVL joist ω n = 2.14 Hz ξ =.4 % Measured value Accelerometers Natural Frequency (Hz) Fitted response Figure 3: Fitted graph for a single-span beam Timber block support LVL blocks were placed at the end of the beam to provide rigid boundary condition. However, the LVL blocks were placed on the ground without additional restraint to provide rigid connection to the ground. Thus, a movement of the support was recorded during the modal testing as shown in Figure 4, which shows a peak amplitude different by zero at both ends of the beam.

4 Peak Acceleration (mm) Distance (m) Figure 4: Mode shape of single-span beam Before the experimental testing, the characteristic mode shapes of the two-span beams were calculated using numerical analysis. The maximum force of the shaker (F=113N) was applied on the left side beam and the maximum deflection were recorded as.31 mm and.9 mm for the left and right spans, respectively. Figure 5 shows the comparison between predicted (numerical analysis) and actual (experimental) mode shapes for the two-span beam. A rigid support condition was assumed (boundary condition equal to zero) while analysing the beam. However, the joist hanger gave some flexibility to the continuous beams system, which can explain the significant difference between predicted and actual maximum deflection, although the mode shape pattern was similar. The envelope mode shapes for both four-span beams showed a significant discontinuity at the beams ends. The metal joist hangers were expected to be flexible, but relative accelerations and corresponding deformations were larger than expected. Nonlinear behaviour of these connections was also thought to cause significant but unexpected 2 nd harmonic responses observed within the measured accelerations (only odd harmonics were expected because of the boundary conditions). The four-span continuous beam was then reduced to a three-span beam by cutting the concrete topping above the inner support as illustrated in Figure 7. The dynamic response is shown in Figure 6 (c) and (d). Again, when one of the outer beam was excited, the middle beam had a smaller response compare to the distant beam. Tests carried outby re-arranging the shaker position in the middle beam gave almost similar peak amplitude in the two beams nearby and a slightly difference due to damping of the beams. Concrete slab was cut 3-span beams Peak Amplitude (mm) Predict.31 Actual Distance (mm) Figure 5: Predicted and experimental mode shapes for the two-span beam Figure 6 illustrates the envelope mode shapes estimated by interpolation between the accelerations measured at different locations along the span when the resonance was reached, with different number of spans. Peak amplitudes of each span are depicted in Figure 6 (a) and (b) for continuous four-span beams. When the outer span was excited, reduced accelerations were recorded in the outer span at the other end of the structure, and the two inner spans had even smaller accelerations. There was less difference between the adjacent inner and outer span responses when the inner span was excited, however the response of the more distant outer span was significantly more attenuated. Figure 7: Four-span beam reduced to three-span beam An accelerometer was attached at the mid span of the disconnected beam, while the shaker excited the threespan beams. The result shows that even though the beam was disconnected, it was still excited by the beam nearby as illustrated in Figure 6 (f). The more distant beam seems to show higher response than the beams adjacent to the excitation soruce. The energy seems to separate evenly to the left and right beams, and be trasferred even to the disconnected beam through the ground. Natural frequencies and proportions of critical damping were measured for each beams as illustrated in Figure 6. From the overall view, the position of shaker on the excited beams shifts the natural frequencies compared when reduced the number of spans.

5 Mode Shapes Natural Frequencies.2 A1 A2 A3 A ω n = 22.1 Hz ξ =.2 % (a) Test T1.2.1 A1 A2 A3 A ω n = 23.9 Hz ξ =.2 % (b) Test T A1 A2 A ω n = 21.7 Hz ξ =.2 % Peak Deflection (mm) A1 A2 A (c) Test T ω n = 23.6 Hz ξ =.3 % (d) Test T4.2 A1 A ω n = 23.5 Hz ξ =.1 % (e) Test T5.2 A.23 A1.2 ω n = 21.7Hz ξ =.2 % Distance (m).1 (f) Test T6 ω n = 22.9Hz ξ =.1 % Frequencies (Hz) Figure 6: Dynamic behaviour of the continuous beams

6 However, the natural frequencies decreased to.4 Hz and.3 Hz after reducing the four-span beam to a three-span beam when the shaker was located on one of the outer beam and on the inner beam, respectively. All beams gave similar values of damping ratios,.2%, except for test T4 (Figure 6 (d)) which had.3% damping ratio. 5 IMPACT FORCE TEST An impact hammer test was carried out to study the human-induced vibrations. An instrumented impact hammer with soft tip was stroke on the mid span of one of the outer beam; meanwhile the remaining beams were monitored with accelerometers (with maximum acceleration of gravity is 5 m/s 2 )to record the beam response. The test layout is illustrated in Figure 8. Figure 8: Hammer test layout More than one acceleration peak was found in beams A2 and A4. Although the second or third peak was less than.1g, it still shows that wave propagation was happen during the impact test. It is because the vibration energy was travelled along the beams and, somehow, a number of waves returned back due to the effect of the timber block supports. However, the acceleration time history for beam A3 (refer to Figure 9 (b)) shows that no turning back wave activity happened on that beam. The impact force energy was past through the inner beams as the maximum peak acceleration was recorded in beam A4. x A Hammer x x x A2 A3 A Time (s) (a) Response of Beam A (b) Response of Beam A3 (c) Response of Beam A4 Figure 9: Acceleration beam time-history 6 CONCLUSIONS Time (s) TIme (s) A four-span continuous beam on a third reduced scale was built to predict the behaviour of a LVL-concrete composite floor system with continuous concrete topping. After dynamic testing, the beam was reduced to a three-, two- and single-span beam by cutting the concrete topping. Instrumented hammer and electrodynamic shaker were used to perform the impact and vertical forced vibration test, respectively. The primary conclusions from the experimental results are reported herein after. Unexpected wave propagation was recorded, where the inner beams transferred the vibration energy to the distant beam and one third of the excitation maximum response was recorded. Beams which side by side of excited beam had similar peak amplitude, however, the number slightly differed due to the structural damping and flexibility of pressed-metal joist hanger.

7 The flexibility of joist hangers was expected, but the relative accelerations and corresponding deformations were larger than expected. Natural frequencies and damping ratios did not change significantly when the number of spans was reduced. Vibration wave propagation transfer through the ground from the excited beam to the disconnected beam was monitored during the test. ACKNOWLEDGEMENT The technical support from Structural Timber Innovation Company (STIC), Carter Holt Harvey (Mr. Warwick Banks and Mr. Hank Bier) is gratefully acknowledged, together with the financial contribution provided by Carter Holt Harvey and by the New Zealand government through the FIDA funds. Special thanks are also to Mr. David Carraddine and Mr. John Maley for their hard work and dedication to this research project. Submitted for possible publication to Materials and Structures, RILEM. [9] Abd Ghafar N.H et al.: Susceptibility to Vibrations of LVL-Concrete Composite Floors. In VII Workshop Italiano Sulle Strutture Compose, Italy, 28 [1] Abd Ghafar N.H., Deam, B. and Fragicomo, M.: Dynamic Measurement of LVL_Concretre Composite Floor. In 13 th Asia Pacific Vibration Conference, November 29, Christchurch, New Zealand [11] Biggs, J.:Introduction to Structural Dynamics. McGraw-Hill, Unites States of America, REFERENCES [1] Ian Smith. :Vibration of Timber Floors: Serviceability Aspects. Timber Engineering, First Edition, Centrum Hout, The Netherlands, 1995, pp [2] Longinow, A., and Hannen, W. R. (29). Floor Vibrations in Buildings. Practice Periodical on Structural Design and Construction, 14(4), [3] Hughes et. Al. Experimental Validation of Building Vibration Propagation Using a Four Story Laboratory Model. In 28 Structures Congress, April 24 26, 28, Vancouver, BC, Canada [4] Stojic, D. and Cvetkovic, R. (21). Analysis of a Compsite Timber-Concrete Structures According to The Limit States. Journal of Architecture and Civil Engineering. Vol 2, No 2, Facta Universitatis, pp [5] Dolan J. D. et. al. Preventing Annoying Wood Floor Vibrations. Journal of Structural Engineering, Vol. 125, No. 1, 1999, pp [6] Yeoh, D., Fragiacomo, M., De Franceschi, M., and Buchanan, A. Experimental tests of notched and plate connectors for LVL-concrete composite beams. Submitted for possible publication to Journal of Structural Engineering, ASCE. [7] Yeoh, D., Fragiacomo, M., and Deam, B. Experimental behaviour of LVL-concrete composite floor beams at strength limit state. Submitted for possible publication to Engineering Structures. [8] Yeoh, D., Fragiacomo, M., and Deam, B. Longterm behaviour of LVL-concrete composite connections and beams under sustained load.