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Fiber reinforced polymer as potential solution for vibration problem in concrete slab for supporting human comfortable

Vibration is a common problem in concrete slab system when designed using ultimate strength capacity which producing slim structure. Although this elemen strong enough to sustain the loads, however another problem emerges because vibration of the element is not anticipated during the design. This study was selected from experimental data conducted on two story buildings system. Thin slab member of 120 mm in two story building showed vibration problem during sevice life. Two sensors were located on slab and middle beam to study behaviour of slab system due to human activity load. Numerical study was elaborated in this research for comparison. Strengthening of the slab using Fiber reinforced polymer (FRP) was applied to the slab system to reduce detrimental effect of vibration on slab. Peak acceleration of slab strengthening using FRP was brought down to 0.018 m/s2 and 0.013 m/s2 for damping of 2% and 3% respectively compared with unstrengthening slab which was 0,09 m/s2 which higher than accepted criteria 0,5% G (0,045 m/s2).


Introduction
Fibre reinforced polymer (FRP) is a composite material made of a polymer matrix reinforced with fibres which are usually glass, carbon, aramid, or basalt. Over the last few decades, application of FRP in construction industry has been developed [1]. FRP structural form can be in form of FRP deck panel, FRP pultrude structural profile for building and bridge construction [2]. It has been more than four decade this material used as strengthening structure because of FRPs is an efficient and affordable solution in replacing steel reinforcing bars in concrete structure due to its corrosion resistance, lightweight, durable, and high strength. In footbridge application, FRP was used for slab which subjected human activities [3]. Vibration measurement has been proven to be promising in Structural Health Monitoring (SHM) [3], [4]. It can be used for studying behavior of structure such as vibration in building, damage in structure by evaluating natural frequency and other dynamic parameters.
This study was to evaluate performance of concrete slab system in two story office building as showed in Figure 2. The slab was subjected to walking load to evaluated whether frequency and acceleration complied with recommendation standard for human comford [4], [5], [6].
The comfort of building occupants and safe building conditions are important requirements in designing building structures. The dynamic response of the building if it is loaded by dynamic loads needs to find the response. One of them is that humans are able to generate strength far greater than their own body weight through energetic activities such as walking or jumping. As mentioned, the load generated by humans is difficult to predict in detail and the amplitude and frequency will vary under different types of induction. Consequently, it is necessary to categorize the various induction caused by humans based on the frequency and amplitude domains.
The frequency and amplitude caused by human activities such as walking, dancing and jumping cause forces on the floor. These forces will cause vibrations in the floor system because the building  This study aims to study behaviour of concrete slab system strengthening with FRP box beam as additional secondary beam in slab system. Two methods used in this study were experimental method and numerical method using Finite Element Analysis software.

Methods
Two methods used in this project were experimental and numerical method. Experimental method was collecting data at location of building, whereas the numerical method was used finite element analysis (FEA) software to modellized the object study.

Experimental Method
This field study was conducted on the second story building at Faculty of Engineering in University of Lampung. The selected slab panel has dimension of 7.2 m length and 5.7 m width with thickness of 120 mm. Slab was supported by four side primary beams with dimension of 350 mm width and 600 mm height. Two secondary cross beams devided slab panel on the middle as shown on the     Figure  4 showed one person weighting about 700N with normal walk pace of 2 Hz [8]. Walking force was characterized by weight of people and pace [8]. In area of buildings, there are people exposed to vibration and having no direct influences on the source of vibration and feel these vibration in a passive way [9]. Two accelerations were used and attached to concrete slab and secondary beam as depicted in Figure 5 to study behaviour of the slab system. Those acceleration recorded vertical acceleration (in Z direction) of the slab system.

Numerical Method
It is necessary to compare the experimental method with other method such as theoretical method or numerical method. Here was used the numerical method to get more insight dynamic characteristic of the slab system such as mode shape, frequency for some modes, and psudo acceleration spectrum which somehow time consuming using theoretical method. The acceptable criteria for floor vibration are to calculate the natural frequency and the peak acceleration of a structure [10]. Lower natural frequency and smaller dumping of floor system, was susceptible to human activities (such as walking, running and jumping) [11]. This annoying vibration caused by human walking was an servicability problem for long floor system [12]. In addition to this, the large span structural layout in modern building floor made vibration serviceability is the governing design criterion [13].
Slab was modelled as single panel which supported by four columns as showed in Figure 6. First model showed slab without strengthening and the second model was slab strengthening using FRP box beam in Y direction.
(a) (b) Figure 6. (a) Slab model without strengthening, (b) Slab model with strengthening with FRP box beam in Y direction. Figure 7 showed results analysis of experimental method. Natural frequency of slab and beam obtained from two sensors located below the slab and the secondary beam were showed in Figure 6(a) and 6(b) respectively. By using Fast fourier Transform (FFT) method which converts a signal from time space to frequency domain, it found natural frequency of slab and beam were 16.67 Hz and 13.61 Hz respectively.

Results and Discussion
(a)   Figure 8 showed frequency of slab due to human walking load in long direction of slab system. The vertical acceleration showed value of -0.06 m/s 2 which was higher than 0.5% G (=0.04905 m/s 2 ) as recommended. This lead to uncomfort for human activity in the building. The frequency of slab due to this walking load was 16.02 Hz which was outside the recommendation range of comfort. Figure 9 showed frequency of secondary beam due to human walking load in long direction of slab system. The vertical acceleration showed value of 0.30 m/s 2 which was higher than 0.5% G (=0.04905 m/s 2 ) as recommended. This lead to uncomfort for human activity in the building. The frequency of beam due to this walking load was 12.65 Hz which was higher than recommendation value.  Figure 10 showed first three shape of slab system which have frequency of 13.417 Hz, 21.464 Hz and 24.624 Hz respectively when period changed to frequency. Natural frequency of this slab system is smaller than experiment which was 16.67 Hz, and similar to the frequency of the beam which was 13.61 Hz. Natural frequency obtained from both results, experimental and numerical were still higher compared with standard recommendation curve for offices building which is between 4 Hz to 8 Hz. Figure 10. First three mode shape of unstrengthening concrete slab system. Figure 11 showed the relationship between Pseudo Spectral Acceleration and frequency of concrete slab that strengthened with FRP box beam. The slab was subjected to a person walking on slab in longitudinal direction. Analysis included damping for 0%, 2% and 3 %. Due to this walking load natural frequency of slab reduced to 12 Hz from previous non-strengthening slab which was 13.417 Hz (-10.56%). With assumption that FRP box beam can absorb the energy with damping 2% to 3%, the peak acceleration of the slab far reduced to 0.0184 m/s 2 and 0.0131 m/s 2 . This acceleration is below the recommendation standard curve. This was good outcome although the natural frequency not reduced significantly using this strengthening system.

Conclusions
From experimental study and numerical study were conducted in this research, it can be concluded as follows: 1) Experiment study for unstrengthening concrete slab system showed natural frequency of beam and slab were 16.67 Hz and 13.61 Hz respectively. Numerical method showed natural frequency of slab system was 13.417 Hz which similar with one of the experimental results. It showed that the numerical method was suited to predict the dynamic behaviour of the slab system. 2) Both experimental and numerical results had natural frequency higher than recommended value for human comford in office which was between 4 Hz to 8 Hz. 3) Natural frequency of strengthening slab using FRP was 12 Hz whereas the unstrengthening slab was 13.417 Hz. The different was only 10.56%. Although this reduction was small, the FRP box system could reduced the vertical peak acceleration significantly below the maximum recommended value. 4) Peak Acceleration in vertical direction (Z-axis) for slab experimentally was 0,06 m/s 2 , it was still higher than recomended which is 0,5% G (0,045 m/s 2 ) for residence/office. However, strengthening with FRP box brought this value down to 0.018 m/s 2 and 0.013 m/s 2 for damping of 2% and 3% respectively. It meant that very light the FRP box system was potential for giving solution in vibration problem in concrete slab. 5) It was still need considered to reduce natural frequency of the slab by making it thicker. Thin slab will give vibration problem on slab.