Fullscale tests on a onestory steel frame structure with a typical precast cladding system using ambient and free vibration methods are described in detail. The cladding system is primarily composed of ALC (Autoclaved Lightweight Concrete) external wall cladding panels, gypsum plasterboard interior linings, and window glazing systems. Ten test cases including the bare steel frame and the steel frame with addition of different parts of the precast cladding system are prepared for detailed investigations. The amplitudedependent dynamic characteristics of the test cases including natural frequencies and damping ratios determined from the tests are presented. The effects of the ALC external wall cladding panels, the gypsum plasterboard interior linings, and the window glazing systems on the stiffness and structural damping of the steel frame are discussed in detail. The effect of the precast cladding systems on the amplitude dependency of the dynamic characteristics and the tendencies of the dynamic parameters with respect to the structural response amplitude are investigated over a wide range. Furthermore, results estimated from the ambient vibration method are compared with those from the free vibration tests to evaluate the feasibility of the ambient vibration method.
Steel frames with precast cladding systems are very commonly used in both residential and office construction in many parts of the world including North America, Europe, and Japan. According to current design codes, steel frames are required to resist lateral and vertical loads under ultimate and serviceability loading conditions, while precast cladding systems are considered as nonloadbearing components considering only their mass and hence are ignored in the structural design. However, experimental investigations and analytical studies have demonstrated that precast cladding systems can in fact have a significant impact on the stiffness and dynamic response of steel frames. The addition of precast cladding systems to an originally bare momentresisting steel frame may enhance both the lateral stiffness and strength of the steel frames and alter the dynamic response of the overall structural system [
Dynamic behavior is one of the most important design considerations for buildings, and dynamic responses of steel frame buildings under wind or earthquakeinduced loadings are strongly dependent on dynamic parameters such as natural frequencies, damping ratios, and mode shapes. Several experimental studies have shown that precast cladding systems can affect the dynamic characteristics of steel frames. They can cause stiffening of steel frame structures and result in an increase in natural frequencies [
Moreover, from field measurements made over the last three decades, it has been recognized that natural frequencies and damping ratios are nonlinear parameters and may increase with vibration amplitude [
This paper investigates the effects of a precast cladding system on the dynamic characteristics of a fullscale onestory momentresisting steel frame and studies the amplitude dependency of dynamic characteristics. The precast cladding system considered in this paper mainly consists of ALC (Autoclaved Lightweight Concrete) external wall cladding panels, gypsum plasterboard interior linings, and corresponding window glazing systems. Ambient vibration tests (AVT) and free vibration tests (FVT) were conducted on the steel frame to provide a basis for evaluation of dynamic characteristics, and corresponding outputonly system identification algorithms were applied to field acceleration measurements to estimate those dynamic parameters with respect to response amplitude. The outcome of this study is expected to promote understanding of the effects of precast cladding systems on the evaluation of dynamic parameters of steel frame buildings and on the amplitude dependency of dynamic characteristics and to evaluate the adequacy of current design practices.
A fullscale onestory onebay steel frame with a precast cladding system was designed and fabricated, and ambient vibration and free vibration tests were conducted.
The steel frame was designed as one section of an actual lowrise residential house. Figure
Plans and elevations of steel frame (unit: mm).
Plan of 1F
Plan of 2F
Elevation in
Elevation in
The precast cladding system was designed and fabricated following conventional construction procedures. ALC external wall cladding panels, gypsum plasterboard interior linings, and window glazing systems were installed symmetrically on the steel frame. These cladding components are detailed in Table
Details of precast cladding components.
Cladding components  Dimensions  Weight  

ALC external wall cladding panels  2130 × 2870 × 75 mm thick  0.49 KN/m^{2}  
Plasterboard interior linings  1830 × 2620 × 12.5 mm thick  0.20 KN/m^{2}  
Window openings  Small size  610 × 460 mm  — 
Middle size  610 × 1420 mm  —  
Large size  1830 × 1420 mm  — 
A typical “Rocking Installation Method”.
In order to investigate the effects of the precast cladding system in detail, ten cases were tested, and the effects of the ALC cladding panels, plasterboard, and window glazing systems were investigated in detail and separately. The test cases are summarized in Table
Summary of test cases.
Case number  Construction characteristics  Direction 

1  Bare steel frame 

2  Steel frame + ALC claddings 

3  Steel frame + ALC claddings + plasterboard 

4  Steel frame + ALC claddings + small window 

5  Steel frame + ALC claddings + plasterboard + small window 

6  Steel frame + ALC claddings + middle window 

7  Steel frame + ALC claddings + plasterboard + middle window 

8  Steel frame + ALC claddings + large window 

9  Steel frame + ALC claddings + plasterboard + large window 

10  Steel frame + ALC claddings composed of two layers 

Layout of test cases.
Case
Case
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Case
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Case
External views of Case
External views of Case
Internal view of Case
External view of Case
Ambient vibration and free vibration tests were conducted to estimate corresponding natural frequencies and damping ratios for each test case. Ambient vibration tests were adopted due to their advantages of being economical and practical, and experience with this testing procedure has shown its validity [
Free vibration tests were conducted after each ambient vibration test. The free vibration test is a common and simple method for identifying dynamic characteristics, and it is known to be a reliable excitation method to provide accurate mode parameters. In the current study, the free vibration tests were carried out using an electromagnetic shaker. The shaker was placed at the center of the second floor, and its excitation orientation was kept along with the direction of each case. The setup of the shaker was illustrated in Figure
Six servotype accelerometers, marked A1 to A6, were utilized to collect the response acceleration data. Their layout was illustrated in Figure
In ambient vibration tests, only output data can be measured and recorded, while there is no way to obtain input information. Thus, characteristic dynamic parameters can only be identified from output data using outputonly modal identification techniques. In the past several decades, outputonly modal identification techniques have developed fast and there are already a lot of outputonly modal identification techniques available. Moreover, several outputonly mode identification methods have succeeded in estimating dynamic parameters from ambient vibration tests, like frequency domain decomposition (FDD), Random Decrement Technique (RDT), stochastic subspace identification (SSI), eigensystem realization algorithm (ERA), and so forth [
As mentioned in the Introduction, dynamic characteristic parameters are nonlinear with respect to response magnitudes. In order to estimate amplitudedependent parameters, the Random Decrement Technique was adopted in this study to process ambient vibration test data. The Random Decrement Technique is a powerful method for estimating amplitudedependent dynamic characteristics and some researchers have succeeded in using it to identify amplitudedependent dynamic parameters for fullscale structures [
RDT allows an estimation of amplitudedependent dynamic parameters. With a set of gradually increasing trigger values in the range of structure response, a set of RD functions can be extracted from the dynamic response and corresponding dynamic characteristic parameters can be evaluated. Thus, the relationship between dynamic characteristic parameters and trigger values, which is physically the structure response amplitude, can be established. The procedure of processing ambient vibration test data is described in Section
During the processing procedure, ambient vibration test data first passes through a bandpass filter to isolate the contribution of the target mode. Then, RDT is applied to extract RD functions. Finally, natural frequencies and damping ratios with respect to corresponding trigger values are evaluated from free decay functions using the Curve Fitting Technique. The processing procedure is described in the following steps.
The response acceleration data is transformed into the frequency domain using Fourier transformation and then passes through a basspass filter to isolate the contribution of the target mode. Afterwards, the filtered acceleration timehistory data containing the target modal information is derived from inverse Fourier transformation. An appropriate bandpass filter is required in this step, in which the bandpass filter should not be too wide or too narrow. The bandpass filter’s width is determined using a successive approximation approach that the bandpass filter’s width increases gradually in the vicinity of the target natural frequency until the estimated natural frequency or damping ratio stays stable with increasing width.
RD functions corresponding to the prescribed trigger values are extracted from the filtered acceleration data using RDT. Appropriate trigger values and time segment length are two important parameters, because these two parameters determine the number of time segments and the quality of the RD functions. Trigger values can be checked according to RD functions, so that a trigger value should be denied if its corresponding RD function does not well represent the free decay function. Time segment length can be determined such that the relationship between segment length and natural frequency or damping ratio is established first and then the segment length is selected in the range in which natural frequency or damping ratio stays stable. Although high amplitude and long free decay records are anticipated, it should be noted that a high trigger value and a long segment length will lead to an insufficient number of time segments and induce low quality RD functions.
The Curve Fitting Technique is adopted to evaluate natural frequencies and damping ratios from RD functions. The first ten cycles of each RD function are fitted with the theoretical formula of the free decay function, and corresponding natural frequency and damping ratio are identified. The error ratio between RD function and fitting result is restricted to less than 3%. The theoretical formula for the free decay function is written as
Figure
Data processing procedure of ambient vibration test data of Case
Time history of response acceleration data on top floor
Fourier spectrum of response acceleration data
RD function
Curving fitting result
Amplitudedependent dynamic characteristics estimated from ambient vibration tests.
Case
Case
Case
Case
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Case
Case
Amplitudedependent fundamental natural frequencies and damping ratios were estimated from ambient vibration test data using the proposed procedure for the test cases. The response acceleration data at the top/center of the roof was adopted for analysis. The mean values, standard deviations, and variable coefficients of the fundamental natural frequencies and damping ratios are presented in Table
Natural frequencies and damping ratios estimated from ambient vibration tests.
Case  Peak acc. 
Frequency  Damping ratio  



COV (%) 


COV (%)  
1  0.90  3.40  0.001  0.029  0.1  0.01  10.0 
2  0.27  3.92  0.001  0.026  2.0  0.05  2.6 
3  0.50  4.07  0.002  0.049  1.9  0.07  3.7 
4  0.47  3.95  0.001  0.025  1.3  0.04  3.0 
5  0.63  4.42  0.001  0.023  1.5  0.01  0.7 
6  0.86  3.90  0.001  0.026  1.7  0.09  5.3 
7  1.43  4.27  0.007  0.163  0.9  0.06  6.5 
8  1.00  3.59  0.005  0.139  1.2  0.12  9.9 
9  0.85  3.76  0.003  0.080  1.3  0.05  3.9 
10  0.52  4.65  0.006  0.129  1.9  0.02  1.1 
Note:
Figure
Free vibration tests were conducted following the ambient vibration test for each case and three times at the fundamental resonance frequency of each case to obtain more data and reduce errors.
In order to estimate amplitudedependent dynamic characteristics, free vibration test data was processed in the following steps. The response acceleration data was passed through a bandpass filter and the contribution of the target mode was isolated at first. Then, the filtered free vibration signal was decomposed into a set of subsignals, and each subsignal was defined as the oscillation cycle between two successive positive peaks. Afterwards, natural frequencies and damping ratios could be evaluated from subsignals by using Curve Fitting Technique, and corresponding amplitude could be defined as the starting positive peak of each subsignal, so that amplitudedependent dynamic characteristics could be estimated from the free vibration test data.
Figure
Data processing procedure of free vibration test data of Case
Time history of response acceleration data on top floor
Fourier spectrum of response acceleration data
Filtered free vibration data
Curving fitting result
Amplitudedependent natural frequencies estimated from free vibration tests.
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Amplitudedependent damping ratios estimated from free vibration tests.
Case
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Amplitudedependent fundamental natural frequencies and damping ratios estimated from free vibration test data are presented in this section, and Table
Natural frequencies and damping ratios estimated from free vibration tests.
Case  Peak acc. 
Frequency  Damping ratio 




COV (%)  
1  96.2  3.41  0.007  0.21  0.1~0.5 
2  36.1  3.63  0.132  3.64  1.5~3.2 
3  32.5  3.77  0.128  3.40  1.3~3.1 
4  33.5  3.81  0.146  3.83  0.8~3.4 
5  32.1  4.16  0.118  2.84  0.8~3.0 
6  44.9  3.70  0.106  2.86  0.7~2.8 
7  39.8  3.95  0.100  2.53  0.5~2.7 
8  94.0  3.44  0.042  1.22  0.5~2.0 
9  94.3  3.53  0.048  1.36  0.5~2.2 
10  21.9  4.53  0.134  2.96  0.9~3.5 
Note:
Figures
The main objective of the work presented in this paper was to evaluate the effect of precast cladding systems on the dynamic characteristics of steel frame buildings. To improve understanding of the effect of cladding components, this section starts by comparing the natural frequencies and damping ratios provided by ambient and free vibration tests. The mean values of the results estimated from ambient vibration tests are adopted, and scopes of the results estimated from free vibration tests are given for comparison. In addition, the estimated results corresponding to the response amplitudes of 5 × 10^{2 }m/s^{2} and 20 × 10^{2 }m/s^{2} in free vibration tests are presented, which represent the dynamic properties in the low and highamplitude range, respectively.
Figure
Effect of ALC external claddings on dynamic characteristics of steel frame.
Fundamental natural frequency
Fundamental damping ratio
Figure
Effect of plasterboard on dynamic characteristics of steel frame.
Fundamental natural frequency
Fundamental damping ratio
Figure
Effect of Window glazing systems on dynamic characteristics of steel frame.
Fundamental natural frequency
Fundamental damping ratio
The comparisons above demonstrate that the effects of precast cladding systems on dynamic characteristics of steel frames far exceed those that had been anticipated previously. As current design is based on the assumption that the steel frame carries the lateral loads and precast cladding systems are considered as nonstructural components, it can be appreciated that there is a huge discrepancy between design and actual performance of steel frame buildings. For accurate design of steel frame structures, it is essential to take into account the effect of precast cladding systems on dynamic characteristics in current design practice.
The test steel frame with precast cladding systems can be simplified analytically as a single degree of freedom system. Hence, the effective lateral stiffness of the bare steel frame and the steel frame with addition of different parts of precast cladding systems can be determined in accordance with the effective mass and the fundamental natural frequency, based on the idealization of a lightly damped single degree of freedom system using
Given the effective masses and the fundamental natural frequencies measured in free vibration tests, values of lateral stiffness of the bare steel frame and the steel frame with precast cladding systems are evaluated, and the effects of ALC claddings, plasterboard, and window glazing systems on lateral stiffness are illustrated in Figures
Effect of ALC external claddings on lateral stiffness.
Effect of plasterboard on lateral stiffness.
Effect of window glazing systems on lateral stiffness.
The evaluation of lateral stiffness demonstrates that precast cladding systems including ALC external wall cladding panels, gypsum plasterboard interior linings, and window glazing systems contribute significantly to structural stiffness. The significant stiffness contribution indicates that seismic loads may be underestimated while response displacement may be overestimated on condition that the effects of precast cladding systems are ignored in the design of steel frame structures.
Another important objective of this study was to examine the amplitude dependency of dynamic characteristics. The results of free vibration tests illustrated in Figures
Another important observation is the tendency of dynamic characteristics with increase in response amplitude. In order to explain this tendency, Figure
Amplitude dependency of natural frequency and damping ratio of Case
The comparison of dynamic parameters provided by ambient vibration tests with those derived from free vibration tests is another issue that should be addressed. Table
Comparison of dynamic parameters estimated from ambient and free vibration tests.
Case  Natural frequency  Damping ratio  

AVT (Hz)  FVT (Hz)  AVT 
AVT (%)  FVT (%)  AVT  
1  3.40  3.41  1.00  0.1  0.20  0.50 
2  3.92  3.63  1.08  2.0  2.7  0.72 
3  4.07  3.77  1.08  1.9  2.4  0.78 
4  3.95  3.81  1.04  1.3  2.4  0.56 
5  4.42  4.16  1.06  1.5  2.4  0.63 
6  3.90  3.70  1.05  1.7  2.2  0.78 
7  4.27  3.95  1.08  0.9  2.2  0.43 
8  3.59  3.44  1.04  1.2  0.8  1.51 
9  3.76  3.53  1.07  1.3  1.0  1.27 
10  4.65  4.53  1.03  1.9  2.4  0.79 
The differences between the results estimated from ambient vibration test data and those determined by free vibration tests can be explained by the dependence of dynamic parameters with the vibration amplitude. As the response amplitudes involved in ambient vibration studies are significantly smaller than those for the free vibration tests, most contact surfaces between primary structural members and secondary components are still in the stuck condition. Thus, the frequencies determined by ambient vibration tests are higher than those determined by free vibration tests, while damping ratios determined by ambient vibration tests are lower than those determined by free vibration tests. The particular conditions observed for damping ratios of Case
This paper has highlighted the effects of precast cladding systems typically used for Japanese steel buildings on the dynamic characteristics of steel frame structures. Dynamic tests on a fullscale onestory momentresisting steel frame with the addition of ALC external wall cladding panels, gypsum plasterboard interior linings, and window glazing systems have been presented. Ambient vibration tests and free vibration tests were conducted to determine the fundamental characteristics of the test cases. The Random Decrement Technique and the Curve Fitting Technique were employed to estimate natural frequencies and damping ratios with respect to response amplitudes. The findings of the present study may be summarized as follows:
Precast cladding systems provide a big increase in lateral stiffness over a bare steel frame that far outweighs the increase in mass. The increase in natural frequency of the bare steel frame due to the effect of ALC claddings could reach 40% at most, and the stiffness contributions from plasterboard and window glazing systems could increase the natural frequency of the bare steel frame by up to 10% separately.
The addition of precast cladding systems to a bare steel frame greatly improves structural damping. The contributions of ALC cladding panels to structural damping lead to an increase in the damping ratio of a bare steel frame up to 18fold. However, plasterboard attached to ALC claddings restricts their rotational capacity, thus reducing the damping ratio. Window glazing systems also contribute to structural damping, and damping ratio decreases as window size increases.
Amplitude dependency of dynamic characteristics develops with the existence of precast cladding systems. The dynamic parameters show nonlinearity with respect to response magnitude. Natural frequency tends to decrease with increasing amplitude, while damping ratio first increases but eventually decreases with increasing of response amplitude after reaching critical amplitude.
The dynamic parameters derived from ambient vibration measurements are compared with those estimated from free vibration tests. It is found that natural frequencies estimated from ambient vibration tests are higher than those determined by free vibration tests, while damping ratios estimated from ambient vibration tests are generally lower than those determined by free vibration tests. These may be explained by the amplitude dependency of dynamic characteristics.
The findings in this study have some important implications for the design of steel frame structures in many parts of the world. While precast cladding systems are generally considered as nonstructural components in the design stage, interactions between precast cladding systems and primary structural members are inevitable, and the effect of precast cladding systems on dynamic characteristics should be taken into account; that is, the stiffness provided by precast cladding systems should be considered in determining the seismic loads on a structure, and amplitudedependent damping ratio should be considered in predictions of structural dynamic responses. Hence, for accurate evaluation and design of steel frame buildings, it is essential to understand the effects of precast cladding systems on the dynamic characteristics of steel frame structures and to take account of these effects in the design stage.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the funding for this study provided by Asahi Kasei Homes Corporation Co., Ltd., the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through the Global Center of Excellence (GCOE) Program, 2008–2012, and the China Scholarship Council (CSC).