Geotechnical and Structural Engineering Congress 2016
Dynamic Testing of a Laboratory Stadium Structure Osama Abdeljaber1; Adel Younis2; Onur Avci, Ph.D., P.E., F.ASCE3; Necati Catbas, Ph.D., P.E., F.ASCE4; Mustafa Gul, Ph.D.5; Ozan Celik6; and Haiyang Zhang7 1
Research Assistant, Dept. of Civil and Architectural Engineering, College of Engineering, Qatar Univ., P.O. Box 2713, Doha, Qatar. E-mail:
[email protected] 2 Research Assistant, Dept. of Civil and Architectural Engineering, College of Engineering, Qatar Univ., P.O. Box 2713, Doha, Qatar. E-mail:
[email protected] 3 Assistant Professor, Dept. of Civil and Architectural Engineering, College of Engineering, Qatar Univ., P.O. Box 2713, Doha, Qatar (corresponding author). E-mail:
[email protected] 4 Professor, Dept. of Civil, Environmental and Construction Engineering, Univ. of Central Florida, 12800 Pegasus Dr., Orlando, FL 32816-2450. E-mail:
[email protected] 5 Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Alberta, 7-203 Donadeo Innovation Centre for Engineering 9211-116 St. NW Edmonton, AB, Canada. Email:
[email protected] 6 Ph.D. Candidate, Dept. of Civil, Environmental and Construction Engineering, Univ. of Central Florida, 12800 Pegasus Dr., Orlando, FL 32816-2450,. E-mail:
[email protected] 7 Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of Alberta, 7-203 Donadeo Innovation Centre for Engineering 9211-116 St. NW Edmonton, AB, Canada. Email:
[email protected]
Abstract Studies with large physical models are a vital link between the theoretical work and field applications provided that these models are designed to represent real structures where various types and levels of uncertainties can be incorporated. While comprehensive analytical and laboratory joint studies are ongoing at Qatar University, University of Central Florida and University of Alberta, this paper presents the initial findings of dynamic testing at Qatar University. A laboratory stadium structure (grandstand simulator) has been constructed at Qatar University. Capable of housing thirty spectators, Qatar University grandstand simulator is arguably the largest laboratory stadium in the world. The structure is designed in a way that several different structural configurations can be tested in laboratory conditions to enable researchers to test newly developed damage detection algorithms. The study presented in this paper covers the finite element modeling and modal testing of the test structure.
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Introduction Popular sport events, like soccer, are now considered as a major industry attracting an increasing number of people around the world. The growing interest in such events has necessitated construction of new stadia with non-traditional and innovative designs. In order to satisfy architectural and structural requirements, engineers have been employing lightweight materials, slender members, and longer spans in the design of modern stadia. However, flexible stadium structures with smaller damping and lighter masses are relatively more vulnerable to dynamic loading induced by environment conditions and/or human activities. While vibrations serviceability has been pronounced as a major concern in building type of structures over the years (Avci, 2012; Avci, 2015; Davis and Avci, 2015; Avci and Davis, 2015), it is also a limit state for the design of modern stadia. Vibrations can become highly significant especially when a large number of spectators participate in a synchronized crowd motion in stadia. Therefore, it is important to develop a complete structural health monitoring (SHM) framework (Mansouri et al., 2015) that is capable of providing real-time information on the vibration levels in stadia during major sport events in order to avoid excessive vibrations resulting from both random and coordinated human-induced excitations. Also, the SHM system should incorporate smart structural damage detection systems with the ability to detect, locate, and quantify damage (Gul and Catbas, 2013). Researchers from Qatar University, University of Central Florida and University of Alberta are currently conducting comprehensive analytical and laboratory joint studies related to structural health monitoring and vibrations serviceability of stadia. These studies aim at developing new structural damage detection methods optimized for monitoring of modern stadia as well as investigating the effects of humanenvironment-structure interaction on the dynamic response of such structures. The expected outcomes of this research project include the following: 1. Novel signal processing algorithms to detect and quantify damage. 2. Comprehensive database of forcing functions that emulate human induced load due to walking or jumping. 3. Deep understanding of human-environment-structure interaction effects on the dynamic response of grandstands. This includes modeling the complex effects of occupants/spectators on the natural properties of grandstands such as modal frequencies and damping. Before the routine application of the SHM framework under investigation to the reallife stadia, the aforementioned models and algorithms should be verified by laboratory experiments under relatively more controlled environments. Laboratory studies with large physical models are a vital link between the theoretical work and field applications provided that these models are designed to represent real structures where various types and levels of uncertainties can be incorporated. Therefore, the research team has decided to construct a grandstand simulator that will serve as a test bed for the newly developed models and algorithms. As will be discussed later, the grandstand simulator is in the first stages of construction at Qatar University. This paper introduces the general guidelines and considerations followed to design this laboratory structure. Additionally, the early findings of the initial
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analytical and experimental tests on the modal properties of the grandstand simulator are presented. Previous Grandstand Simulators Studies on design and implementation of grandstand simulators are scarce. The only relevant previous work in this field was conducted by Comer et al. who designed and constructed a grandstand simulator to study human-structure interaction effects on grandstands (Comer et al. 2007, Comer et al. 2010). This test structure was designed as a racked grandstand capable of hosting 15 occupants. Built-in force plates were provided to measure the dynamic load induced by the human subjects due to jumping or bobbing on the grandstand. Additionally, the grandstand simulator was supported on air springs and driven electric actuators. A novel structural control method was employed to drive the electric actuators allowing the structure to respond to humaninduced loads as a dynamic system with user-defined damping and frequency. Tunable dynamic properties allowed the researchers to simulate the response of a wide-range of modern grandstands. Moreover, the details of the grandstand simulators such as the seating arrangement and spacing, dimensions of risers and treads, and racking angle of the deck were selected based on the current trend in new stadia. In another study, the same authors used this grandstand simulator to identify the response of grandstands under the load induced by coordinated crowd bobbing (Comer et al. 2013). Qatar University Grandstand Simulator As mentioned in the introduction, a grandstand simulator is currently under construction in Qatar University (QU). The main purpose of this structure is to serve as a test bed for human-structure interaction and human-induced load modeling studies. In addition, QU grandstand simulator will be used for testing the performance of several damage detection algorithms. While the previously discussed grandstand simulator designed by Comer et al. is ideal for studying human-structure interaction, it is unsuitable for studies related to structural damage detection because such studies requires a test structure with variable connections and removable members. Besides, the grandstand designed by Comer et al. is rather small and has limited seating capacity. QU grandstand simulator is designed to host a total of 30 spectators with footprint dimensions of the 4.2m×4.2m. The steel deck is racked with an angle of 20° which falls within the range recommended for grandstands. Several considerations were taken into account while designing this test structure to ensure its safety and compatibility with the specifications of modern stadia. Figure 1 illustrates the structural design of the steel frame. The frame consists of 8 main beams and 25 filler beams supported on 4 columns. The 8 main beams are 4.6 m long, while the length of the 5 filler beams in the lower portion is about 1m and the length of the remaining filler beams is 77 cm. Previous studies have shown that steel grandstands are slender and flexible structures, which means that their fundamental natural frequencies are low and closely spaced. Therefore, in order to reduce the fundamental natural frequencies of QU grandstand simulator as much as possible, the main girders as well
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as the second dary beamss were assigned by slender steeel sections (IPE120). Add ditionally, as a shown in Figure 1, th he lower porrtion of the deck is canntilevered in ord der to introd duce furtherr flexibility to the struccture and reepresent thee cantilever actiion many real stadium m structure has. h The 255 filler beam ms are rem movable and inteerchangeablee which makes this testt structure iideal for dam mage detecttion studies sincce many dam mage scenariios can be siimulated eithher by looseening the bollts at beams to girder g connections or by replacing so ome of the fi filler beams w with damageed ones. Thee test structu ure is still in the initial ph hase of consstruction. So far, as show wn in Figure 2, only o the maain steel fram me has been n constructedd. Non-strucctural elemeents such as the risers, treaads, handraails, and seeats will bee installed in the nexxt stage off con nstruction. Beffore proceed ding with thee constructio on of the QU U grandstandd, the researcch team has deccided to utilizze the test sttructure in itts current forrm (Figure 22) to conducct a series off stud dies related to modal testing and strructural dam mage detectioon. This papper presents the analytical and a experimental resultss of modal teesting conduucted on thee main steel fram me of the QU U grandstand d.
Fiigure 1. The structural design of the main steel fframe of QU U grandstand simulator.
Figure 2. QU Q grandstan nd simulatorr under consttruction.
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Fin nite Elementt Modeling and Simula ation Bassed on the design calcu ulation and CAD draw wings, a detaailed three-ddimensional finiite element model of th he main steeel frame of QU grandsttand was created using Abaaqus 6-14 (Abaqus, 2014). This FE model is shown in F Figure 3. All members within the steel deck weree modeled ass C3D20R eelements (200-node quaddratic brick, reduced integraation). A very fine mesh h was used to model thhe members in order to enh hance the acccuracy of the FE model. The coonnections bbetween thee main and secondary beam ms were mo odeled in Abaqus A as tiie constraintts. The steeel deck was assu umed to be pin-supporteed over the locations off the four coolumns. All elements in the FE model were w assigneed by 210 GPa G Young’ s modulus, 0.3125 Poissson’s ratio, 3 and was used tto identify the natural d 7850 Kg//m density. Lanczos eigensolver e freq quencies and d the mode shapes of th he first 4 bennding modess of the FE model. The resu ults of this numerical n mo odal analysiss are provideed in Figure 4 and Tablee 1.
Fiigure 3. Abaaqus finite ellement modeel of the testt structure.
Figure F 4. Th he first 4 ben nding modess of the test sstructure.
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Table 1. The natural frequencies of the first 4 bending modes of the FE model. Natural Frequency Bending Mode (Hz) 1 16.00 2 16.58 3 24.48 4 48.34 Experimental Modal Analysis Dynamic impact test was carried out in order to verify the modal properties computed by finite element analysis and estimate the modal damping ratios. The FE analysis results presented in Table 1 reveal the existence of two closely coupled modes around 16 Hz. To detect these modes experimentally, it is recommended to conduct a multiple reference roving impact test (Schwarz and Richardson, 1999). This test is conducted by hitting the structure vertically at multiple points using an impact hammer while measuring the acceleration response using a set of accelerometers. The test structure was instrumented with 15 PCB 393B04 accelerometers (Figure 5) installed to measure the acceleration in the vertical direction at the locations shown in Figure 6. The structure was hit vertically at the six locations shown in Figure 6 using Brüel & Kjær modal sledge hammer (Figure 7). The accelerometers and the modal hammer were connected to a 16-channel data acquisition device (DT9857E-16, shown in Figure 8). ME’ScopeVES 6.0 software was used for data acquisition, signal-processing, FRFs computations, and modal analysis (Vibrant Technology, Inc., 2003). The required data was collected at a sampling frequency of 200 Hz. Exponential window was applied to reduce the leakage in the spectrum of the acceleration responses (Ahn et al. 2004). The resulting multi-input multi-output data set consists of 6 sets of impact forces and 15 sets of accelerometers measurements. The 6×15=90 FRFs corresponding to MIMO dataset were computed. An example FRF is shown in Figure 9. After that, these FRFs were processed by the multi-reference CMIF algorithm in order to compute the modal properties of the test structure. Table 2 shows the natural frequencies and damping ratios computed experimentally. A comparison between the finite element and experimental modal frequencies is presented in Table 3. This comparison indicates an excellent agreement between the actual frequencies measured experimentally by impact testing and the frequencies predicted analytically by finite element analysis especially for the first two bending modes. Therefore, it can be concluded that the finite element model represents the fundamental modes of the structure accurately even without applying finite element model updating techniques.
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Figuree 5. PCB model 393B04 accelerometter.
Figure 6. Thee instrumenttation plan sh howing the ddistribution oof accelerom meters and the im mpact locationns.
Figure 7. B&K B modall sledge ham mmer (model 8210).
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Figuree 8. 16-channel data acqquisition deviice 2 25
FRF Magnitude
2 20 1 15 1 10
5 0 0
10
20
30
40
50 Frequency (Hz)
60
70
80
90
100 0
F of the respo onse at accellerometer AC C12 due to iimpact appliied at point Figure 9. FRF 07. Table 2. Th he experimental natural frequencies and dampinng ratio of the first 4 bend ding modes.. Naturral Frequency Damping Ratio Bending Mode (Hz) (%)) 1 15.7 0.2255
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16.4
0.3866
3
25.2
0.3277
4
50.6
0.5411
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Table 3. Comparison between the finite element and the experimental natural frequencies. Finite element Experimental Error Bending Mode frequency frequency (%) (Hz) (Hz) 1 16.00 15.7 1.91 2
16.58
16.4
1.10
3
24.48
25.2
-2.86
4
48.34
50.6
-4.47
Conclusions A grandstand simulator capable of hosting thirty spectators is currently under construction at Qatar University. This structure is to serve as a test bed for studies related to structural damage detection and human-structure interaction. This paper presented the results of preliminary analytical and experimental modal analysis of the main steel frame of the grandstand simulator. A detailed FE model of the frame was created and analyzed using Abaqus to predict the modal properties of the structure. After that, experimental impact tests were carried out to identify the actual natural frequencies and damping ratios. The results indicate that the finite element model represents the fundamental modes of the structure with an excellent degree of accuracy. For future works, additional analytical and experimental testing will be conducted on the grandstand including human-structure interaction, damage detection and finally vibrations serviceability of the spectators. Acknowledgements The financial support for this research was provided by Qatar National Research Fund, QNRF (a member of Qatar Foundation) via the National Priorities Research Program (NPRP), Project Number: NPRP 6-526-2-218. The statements made herein are solely the responsibility of the authors.
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References Abaqus/CAE version 6.14-1. Providence, Rhode Island, Dassault Systèmes, 2014. Ahn, S. J., Jeong, W. B., and Yoo, W. S. (2004). "Unbiased expression of FRF with exponential window function in impact hammer testing." Journal of Sound and Vibration, 277(4-5), 931-941. Avci, O. (2012). Retrofitting Steel Joist Supported Footbridges for Improved Vibration Response. Structures Congress 2012: pp. 460-470. http://dx.doi.org/10.1061/9780784412367.041 Avci, O. (2015). "Modal Parameter Variations due to Joist Bottom Chord Extension Installations on Laboratory Footbridges." J. Perform. Constr. Facil., 10.1061/(ASCE)CF.1943-5509.0000635 , 04014140. http://dx.doi.org/10.1061/(ASCE)CF.1943-5509.0000635 Avci, O. and Davis, B. (2015). “A Study on Effective Mass of One Way Joist Supported Systems”. ASCE Structures Congress 2015, April 23-25, 2015, Portland, Oregon, USA. http://dx.doi.org/10.1061/9780784479117.073 Comer, A. J., Blakeborough, A., and Williams, M. S. (2007). "Human–structure interaction in cantilever grandstands—design of a section of a full scale raked grandstand." Proc., Proceedings of the IMAC XXV, Conference on Structural Dynamics. Comer, A. J., Blakeborough, A., and Williams, M. S. (2010). "Grandstand simulator for dynamic human–structure interaction experiments." Experimental Mechanics, 50(2010), 825834. Comer, A. J., Blakeborough, A., and Williams, M. S. (2013). "Rhythmic crowd bobbing on a grandstand simulator." Journal of Sound and Vibration, 332(2), 442-454. Davis, B. and Avci, O. (2015). "Simplified Vibration Serviceability Evaluation of Slender Monumental Stairs." Journal of Structural Engineering, ASCE, 10.1061/(ASCE)ST.1943541X.0001256 , 04015017. http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0001256 Gul, M., and Catbas, F. N. "A Review of Structural Health Monitoring of a Football Stadium for Human Comfort and Structural Performance." Proc., Structures Congress 2013, 24452454. Mansouri, M., Avci, O., Nounou, H. and Nounou, M. (2015). “Iterated Square Root Unscented Kalman Filter for Nonlinear States and Parameters Estimation: Three DOF Damped System”. Journal of Civil Structural Health Monitoring (Springer). http://link.springer.com/article/10.1007/s13349-015-0134-7 Schwarz, B. J., and Richardson, M. H. (1999). "Experimental modal analysis." CSI Reliability Week Orlando, FL. Vibrant Technology, Inc. (2003). "ME’ScopeVES 4.0", Scotts Valley, California.
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