static loading tests and seismic performance ...traditional wooden houses, static loading tests,...
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STATIC LOADING TESTS AND SEISMIC PERFORMANCE EVALUATION OF TWO-STORIED TRADITIONAL WOODEN FRAMES Yasuhiro Hayashi1, Mina Sugino2, Saki Ohmura3, Satomi Tokuoka4
ABSTRACTS: There are a lot of traditional wooden houses forming the historical townscape in Japan. It is very important to develop more rational seismic performance evaluation methods and to carry out the seismic retrofit of traditional wooden houses in order to preserve these houses against severe earthquakes. In this paper, to develop a seismic performance evaluation method considering the interaction effects of the first and second stories by through columns passing straight through two stories, we have carried out the static loading tests of two-storied traditional wooden frames and its simulation analyses. As the experimental results, asymmetry property of the restoring force characteristics due to the uplift and pull-out of columns was observed. Then, we have demonstrated that the experimental results are simulated by the frame analysis model and our simple proposed evaluation method by practical precision.
KEYWORDS: Traditional wooden houses, Static loading tests, Seismic performance evaluation, Analysis model 1 INTRODUCTION 123 There are a lot of traditional wooden houses forming the historical townscape in Japan. On the other hand, there are many wooden buildings collapsed in the Hyogo-ken Nambu, Kobe earthquake in 1995. Therefore, it is very important to develop more rational seismic performance evaluation methods and to carry out the seismic retrofit of traditional wooden houses in order to preserve these houses against severe earthquakes. In this paper, to develop a seismic performance evaluation method considering the interaction effects of the first and second stories by through columns passing straight through two stories as well as the breakage of through columns, we have carried out the static loading tests of two-storied traditional wooden frames and its simulation analyses.
1 Yasuhiro Hayashi, Kyoto Univ., C2-308, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto, Japan. [email protected] 2 Mina Sugino, Kyoto Univ., C2-316, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto, Japan. [email protected] 3 Saki Ohmura, Kyoto Univ., C2-316, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto, Japan. [email protected] 4 Satomi Tokuoka, Kyoto Univ., C2-316, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto, Japan. [email protected]
2 OUTLINE OF EXPERIMENTS 2.1 SPECIMENS Two types of two-storied specimens Cw and Cn are used for the static loading tests as shown in Fig. 1. The hight of the first and second stories of the specimens is 3,055mm and 2,670mm, respectively. The two specimens have ‘through columns’ on both sides which pass straight through two stories. The specimen Cw have walls which are made of dry mud panels [1] at both the first and second stories, although the Cn have a wall only at the second story. Walls are installed asymmetrically for both specimens. Thickness of dry mud panel is 26mm and dry mud panels are fixed to the crosspieces by screws. Figure 2 shows the details of mortise and tenon joints in the specimens. The properties of woods in this experiment are listed in Table 1. These two specimens are used for shaking table tests beforehand [2]. Namely, the beams and columns are reused for the specimens for static loading tests because all members are not damaged seriously except for the pins at beam-column joints in the shaking table tests as shown in Section 3. But, the damaged pins at all the beam-column joints are replaced by new ones. As shown in Fig.1, the shear deformation angle of the whole specimen R is the horizontal displacement of the top of the specimen divided by the height 5,725mm. The shear deformation angle of the first story R1 and that of the second story R2 are the relative horizontal displacement between the beam and the column base of the specimens divided the relative height 3,055mm and that between the top and the beam divided the relative height 2,670mm, respectively.
Table 1: Properties of woods
Wood species Dimension Young's *
modulus(mm) (N/mm2)
Column Japanese cypress 120x120 9000Beam Oregon pine 270x120 10000Joist Oregon pine 120x120 10000Pin Oregon pine 15x15 10000* Young's modulus is evaluated from the past study
(a) Specimen Cn (b) Specimen Cw
Figure 1: Two types of specimens (Unit : mm)
(a) Specimen Cn (b) Joint A (c) Joint B
(d) Joint C (e) Joint D
Figure 2: Joints in the specimens (Unit : mm)
2.2 SHAKING TABLE TESTS [2] In the shaking table tests, specimens are composed of the same two parallel two-storied wooden frames combined by binding beams, horizontal structural plywood and vertical stainless steel brace as shown in Fig. 3. The constant vertical weight on the first and second floor and ceiling are about 3.7kN, 19.6kN, and 19.6kN, respectively. To calculate the inertial force, the mass of the first story and second story are calculated by the summation of the constant vertical weight, the mass of the wooden frame and the walls per one structural plane. The mass of the first and second story of Cn are 1.24ton, 1.20ton. The mass of the first and second story of Cw are 1.25ton, 1.20ton. Multiplying the mass by the
acceleration of each story gives the inertial force of each story. Shear force at the first story Q1 is calculated by the summation of the inertial force of the first story and second story. The Natural frequency of the specimen Cn and Cw before shaking tests are 1.3Hz and 2.5Hz, respectively. The shaking table is excited by one direction and controlled by displacement. Figure 4 shows a sinusoidal pulse, a Ricker wavelet and an artificial earthquake ground motion used in this experiment as the input motions. The acceleration of the sinusoidal pulse is made by one cycle of a sine wave and the pulse period Tp is the period of one cycle. The Tp of the Ricker wavelet is the peak of its Fourier spectra. The Dp is the maximum displacement of the input waves. In contrast, the artificial earthquake ground motion which continues for a long time compared to the sinusoidal pulses and the Ricker wavelet is also used in this experiment. The artificial earthquake ground motion continues in 165 seconds and made by using the random phase to be suitable for the standard acceleration response spectra (damping ratio 5%) on the free engineering bedrock at the safety limit (level 2) in Building Standard Law of Japan.
Figure 3: Schematic view of shaking table tests
(a) Sinusoidal pulse
(b) Ricker wavelet
(c) Artificial earthquake ground motion
Figure 4: Displacement of input waves
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2.3 STATIC LOADING TESTS To allow the uplift and sliding at the bottom of column, the columns are just set on the flat stone and not fixed at all. Constant vertical weight 28.8kN, which corresponds to fixed load and live load, are applied at the top of specimen. Then, cyclic loading tests with gradually increasing displacement amplitude are conducted. The shear deformation angle R of specimens are defined as the horizontal displacement of the top beam devided by the height of through column 5.575m. The half of the maximum amplitude of displacement at the roof level is about 1m to make the maximum shear deformation angle up to 0.2 rad. To apply such a large deformation, the following loading system has been employed as shown in Fig.4 [3].
Figure 5: Schematic view of static loading tests
The loading frame consists of two steel plane frames. Each frame has two pillars and two horizontal beams and is connected each other so that their movements should be same. The pillars are supported by pin joints at the bottom, and the horizontal beams are also connected to the pillars by pin joints. Therefore, the loading frame is unstable but flexible in the in-plane direction. To apply the horizontal displacement to a specimen, the specimen is connected to the loading frame through load cells at roof level. In-plane horizontal displacement of a specimen is controlled by a hydraulic jack, whose stroke is 1m, connected to the pillars of the loading frame. We perform the loading by amplifying displacement using the principle of the lever. The positive and minus deformation angles are loading in the right and left directions, respectively. Horizontal load to the specimens are measured by the load cells set between the top beam of the specimens and the loading frame. 3 RESULTS OF EXPERIMENTS 3.1 SHAKING TABLE TESTS [2] Figure 6 shows the maximum deformation of specimens. Figure 6 is calculated from the deformation of each story and the rotation of each joint excited by the Ricker wavelet Tp=0.7s, Dp=250mm (Cn), -250mm (Cw). The uplift and pull out of columns are shown in the figure. There was no wall at the first story but the specimen Cn deformed equally between at the first and second story. The rotation of the wall at the first story of Cw without shear failure caused the uplift of the column base about 65mm at the right column and the pull out of
the column more than 36mm at the center column, and broke the pin at the center column. The pin of the left end of the joist was broken and the wall at the first story cracked at the upper part. However, no through column broke in the shaking table tests because of the performance limit of the shaking table.
(a) Specimen Cn (b) Specimen Cw
Figure 6: Maximum deformation of specimens excited by Ricker wavelet Tp=0.7s, Dp=250mm (Cn), -250mm (Cw) (Unit : rad, mm)
We also investigate the relationship between the horizontal movements of the column bases after each excitation because the column bases are not fixed at all. However, the horizontal movements of the column bases for the specimens Cn and Cw were very small for the sinusoidal pulses and the Ricker wavelets. The horizontal movement of the column base was caused by uplift of the column base and the sliding of the specimen was not observed. This is because the strength of the specimen was not so large that the base shear force does not exceed the sum of the static frictional force at the column base. Figure 7 shows the relation between the shear force Q1 and the deformation angle R1 at the first story for the specimen Cw. In Fig. 7, the Ricker wavelet Tp=0.7s, Dp=-250mm excitation is shown in the solid line. The shear force is calculated as one structural plane. As Fig. 7 indicates, the restoring force characteristics were asymmetrical. In negative, the shear force did not increase until in positive and only the deformation angle increaseed because of the uplift of the column.
Figure 7: Hysteresis characteristics of the first story of Cw
PinSteel beam
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3.2 STATIC LOADING TESTS Each static loading test finishes when the horizontal load vanishes when the R increases. Figure 8 shows the final deformation and damage status of the two specimens. Figure 9 shows the restoring force characteristics of the two specimens. As for the specimen Cw, no through column was broken, but the dry mud panels at the first and second stories were broken as shown in Fig.8(a). The uplift of the right-side through column at the bottom (see Fig.8(c)) and the pull-out of the center column from the beam at the top of the first story were observed. But the breakage of the through columns for the specimen Cn was observed at the R of -0.1rad as shown in Fig. 8(b).
(a) Specimen Cw (R=-0.15rad)
(b) Specimen Cn (R=-0.1rad)
(c) Uplift of the right column (Specimen Cw)
Figure 8: Final deformation mode
From the restoring force characteristics shown in Fig. 9, an asymmetry property due to the uplift and pull-out of columns is observed. In addition, the big difference in the restoring force characteristics of the two loading directions is also observed. This difference is caused by
the interaction effects of the through columns and the multi-story wall, since the second story is same between the specimen Cn and Cw. Then, the difference cannot be explained by the current seismic performance evaluation method [4] in Japan that does not consider the effects of through columns.
(a) Specimen Cw
(b) Specimen Cn
Figure 9: Restoring force characteristics
4 SIMULATION ANALYSIS 4.1 ANALYSIS MODEL The frame analysis model which can consider the structural properties of traditional wooden houses such as uplift of column base and pull out of column is employed on the basis of the previous studies [5-12]. The frame analysis model of the specimen Cw is shown in Fig. 10. The beams and columns are simulated by elastic line element. The stiffness is determined by the cross section and Young’s modulus of woods. The Young’s modulus of the Japanese cypress and Oregon pine are 9.0x103N/mm2 and 1.0x104N/mm2, respectively [5]. The column base has an axial spring which is elastic in a compression side and free in a tension side on the pin joint to express uplift. The stiffness in a compression side is determined to be the same Young’s modulus with the columns. Elastoplastic axial and rotational springs are placed at each joint [6-12]. The moment resistance of each joint consists of compressive force inclined to the grain, compression frictional force and resistance force of a pin.
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The restoring force characteristic of rotation is a tri-linear slip type which is the superposition of moment resistance calculated from the compressive force inclined to the grain, the compression frictional force and the resistance force of a pin. The restoring force characteristic of the axial force is a bi-linear slip type which is the compressive force inclined to the grain in a compression side and the resistance force of a pin in a tension side. The walls made of dry mud panels are expressed by the braced model which has the same lateral strength as the past experimental data of the dry mud panels. The restoring force characteristics of the braces are modelled as a tri-linear slip type.
Figure 10: Analysis model for specimen Cw
4.2 SIMULATION OF STATIC LOADING TESTS Push-over analyses are conducted to simulate the static loading tests using the frame analysis model. In the simulation analysis, P∆ effects are also taken into account. The restoring force characteristics of the second stories for the two specimens are shown in Fig. 11. The asymmetry property and the difference between the two specimens is well simulated by using the analysis model. Next, the uplift of the right column base in comparison with the experimental value for the specimen Cw is shown in Fig. 12. Except the tendency that the uplift of the column base decreases as the shear deformation angle R increases and the damage of dry mud panels progresses, the analysis result almost agrees with the experimental value. Finally, the relationship between the shear deformation angle R and the bending stress of the column at the joint is shown in Fig.13. The bending strength obtained from a material test is also shown in the same figure. From this figure, the through column is not broken in the analysis while the column was broken in the static loading test. This may be due to the underestimation of the strength of dry mud panels in the analysis and low shear force of the column. Therefore, it is very important to estimate the strength of dry mud panels in order to simulate the results of the static
loading tests including the failure mode by using the frame analysis model.
Figure 11: Restoring force characteristics of the second stories for the two specimens
Figure 12: Uplift of the right column base in comparison with the experimental values for specimen Cw
Figure 13: Bending stress of column at joint for specimen Cn
Then, the shear force of the through columns is studied using the frame analysis model. Figure 14 shows the total shear force of the through columns at the first and second stories. Although the structure of the specimen Cn and Cw is same at the second story, the shear forces
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is completely different from each other. As for the specimen Cn, shear force at the second story is acting opposite to the direction of that at the first story. As for the specimen Cw, the shear force at the first and second stories is acting to the same direction. In addition, the shear force in Cw is very small compared to that in Cn. Considering that the restoring force of Cw is larger than that of Cn (see Fig. 11), the large restoring force is born by dry mud panels and the shear force of the through columns is drastically reduced by dry mud panels.
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Figure 14: Relationship between total shear forces of through columns and shear deformation angles R1 and R2
Finally, the effects of constrain condition at the column base on the restoring force characteristics are studied for specimen Cw. Figure 15 shows the relationship between the horizontal load and the shear deformation angle R. Push-over analyses are carried out using the analyses model with or without the column base uplift. The restoring force characteristics do not depend on the consideration of uplift of the column base for the loading in the right direction. But the restoring force characteristics for the loading in the left direction is strongly dependent on uplift of the column base. Therefore, the asymmetrical property of the restoring force characteristics is due to the uplift of the column base.
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Figure 15: Restoring force characteristics obtained by frame analysis of specimen Cw
5 SEISMIC PERFORMANCE EVALUATION
5.1 METHODS FOR EVALUATION The maximum values of the shear deformation angles R, R1 and R2 is calculated from the following approximate methods based on a capacity spectrum method [4, 13-15]. a) Push over analyses are conducted to determine a Q-
δ curve for each story by using the frame analysis model. External force distribution is determined based on Ai distribution defined in the Building Standard Law in Japan. Generally speaking, the restoring force characteristics varies according to a direction. Therefore, push over analyses are conducted for right and left directions.
b) After the 2-DOF system are transformed into a S-DOF system [15], the maximum response of the S-DOF system is calculated by using a capacity spectrum method [4, 13-15]. We assume big one the maximum from the maximum response values calculated for the left and right loading directions.
5.2 VALIDITY OF EVALUATION To demonstrate the accuracy of the evaluation method, the simulation analyses of the shaking table tests are carried out. Sinusoidal pulses and Ricker wavelets, whose Tp is 0.5s to 3.0s, are used as input motions. Since the input motions are pulse like waves, the following
reduction factor Fh(h) of displacement response spectra due to damping factor h is used [14, 16].
( ) ( )ππ hhFh ++= 1/05.01)( (1)
Figures 16 show the comparison of the maximum shear deformation angle of the first story R1max and the maximum uplift displacement of the columns between the evaluated values and the experimental results. When the R1 becomes large, the accuracy of the evaluation of the R1 is not high. However, the evaluated values of the maximum uplift displacement of the columns show good correspondence to the experimental results with satisfactory precision. Additionally, we have confirmed that we can evaluated the maximum responses of the two storied frame under random input waves with comparable precision.
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Figure 16: Comparison between the evaluated values and the experimental results
Figure 17 shows the comparison of the maximum shear deformation angle Rmax of the specimen Cw for the left and right loading directions. Figure 18 shows the damage condition of the specimen Cw at the maximum response under the sinusoidal input of Tp=0.5s, Dp=200mm. Even if the restoring force characteristics changes by the loading direction as shown in Fig. 15, the difference in the maximum R is not so large. This is because the pulse like motions are used as the input motions [2]. However, the damage conditions are
completely different by the loading direction. Namely, the large uplift and pull-out of the column is evident in the right direction but the yield of the wall of dry mud panels occurs in the left direction. Therefore, we should set a safe limit according as the loading direction.
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Figure 17: Comparison of shear drift angles between evaluated shear drift angles of right and left directions (Cw)
(a) Right direction (b) Left direction
Figure 18: Comparison of evaluated damage in the right and left directions for specimen Cw under sinusoidal pulse of Tp=0.5s, Dp=200mm
6 CONCLUSIONS Based on the static loading tests and simulation analyses, of two-storied traditional wooden frames in Japan, the following conclusions have been drawn: a) Asymmetry property of restoring force
characteristics due to the uplift and pull-out of columns can be observed.
b) It is very important to consider the interaction effects between the first and second stories by through columns and multi-story walls.
c) We have demonstrated that experimental results are simulated by a frame model and our simple proposed evaluation method by practical precision.
AKCNOWLEDGEMENTS A part of this research was supported by Grants-in-Aid for Scientific Research (A) (No.22246072). We would like to express gratitude to Mr. Toya Nakanishi for his
Yield of joint
UpliftPull out
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11mm
cooperation in conducting static loading tests and simulation analyses. REFERENCES [1] Sugiyama, R., Suzuki, Y., Gotou, M. & Murakami,
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[3] Y. Hayashi, A. Nakagawa, N.Takiyama, S.Hirosue : Experimental Study on Two Storied Traditional Wooden Houses in Japan, 15th WCEE, Lisbon, 2012. Agency for Cultural Affairs: Implementation Guidance for Basic Seismic Assessment of Important Cultural Properties (Buildings). http://www.bunka.go.jp/seisaku/bunkazai/hogofukyu/pdf/kokko_hojyo_taisin13.pdf (accessed at 2015.9.30)
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