DYNAMIC TESTING OF POST-TENSIONED CONCRETE WALLS

K M TWIGDEN & R S HENRY

University of Auckland SUMMARY Post-tensioned precast concrete walls have previously been shown both experimentally and analytically to provide excellent seismic resilience. However, only a limited number of experimental tests have been conducted to investigate the dynamic response of such rocking wall systems. This paper presents the preliminary results of an experimental program that was conducted to further understand the dynamic characteristics of post-tensioned rocking concrete walls. The experimental program investigated the response of single post-tensioned concrete wall panels that were subjected to a range of different loading conditions, including, pseudo-static and high speed cyclic loading, free vibration, and forced-vibration. INTRODUCTION Following the recent Christchurch earthquakes there is an increased public awareness of the damaged caused to ductile concrete structures during earthquakes. The use of unbonded post-tensioned precast concrete elements is quickly becoming a well-recognised method of constructing buildings that will sustain low levels of damage and remain in service following a large earthquake. Unbonded post-tensioned precast concrete walls have previously been shown both experimentally and analytically to demonstrate excellent seismic performance (Kurama et al. 1999; Priestley et al. 1999). The use of precast concrete allows for dry connections that accommodate inelastic demand through the opening and closing of an existing crack, introducing a rocking mechanism. The unbonded post-tensioning is designed to remain elastic during a design-level earthquake and provides a self-centering restoring force, increasing the stability of the rocking system against overturning. The combination of precast elements and unbonded post-tensioning generates a response that undergoes inelastic deformations with minimal damage. However, due to the minimal damage sustained from a seismic event, post-tensioned precast concrete members have limited energy dissipation compared to a traditional reinforced concrete structures. Despite extensive previous research, only a limited number of experimental tests have been conducted to investigate the dynamic response of precast concrete members with unbonded post-tensioning. This paper presents the results of an experimental test program that has been conducted to better understand the dynamic characteristics of post-tensioned rocking concrete walls. Background The concept of connecting precast concrete elements together with unbonded post-tensioning was investigated extensively during the Precast Seismic Structural Systems (PRESSS) research program conducted in the 1990’s. A jointed wall system was developed during the PRESSS research program and was tested in a five storey prototype building by Priestley et al. (1999). Following the PRESSS research program several different rocking wall systems have been developed with a significant focus placed on additional energy dissipating elements (Kurama & Shen 2004; Restrepo & Rahman 2007; Sritharan et al. 2008; Marriott 2009).

Unbonded post-tensioned precast concrete wall systems have been subject to numerous pseudo-static lateral load tests and extensive analytical using nonlinear time history analysis to simulate the response of unbonded post-tensioned wall systems to earthquake excitation (Kurama et al. 2002; Perez et al. 2007; Marriott 2009; Ma 2010; Henry 2011). However, only a limited number of experimental tests have been conducted to investigate the dynamic response of post-tensioned wall systems. Understanding the dynamic behaviour of unbonded post-tensioned rocking wall systems is critical to assessing their seismic performance because dynamic characteristics can be neglected during pseudo-static testing. The damping in an unbonded post-tensioned precast concrete system with no additional energy dissipating elements consists of contact damping and inherent equivalent viscous damping. Contact damping is the energy loss that occurs during rocking impacts between the wall and the foundation (Ma 2010). Contact damping is velocity dependant and therefore neglected during pseudo-static testing and can thus only be captured during a true dynamic loading when the effect of the impact is allowed to propagate. Dynamic testing of precast concrete unbonded post-tensioned rocking walls has been limited to shake table testing performed by Marriott (2009) who conducted tests on a total of five walls. Four of the tested walls had various types of additional energy dissipating elements and the fourth wall relied on contact and inherent damping. Based on the experimental results, Marriott et al. quantified the total damping between the wall and foundation to between 1-3% Equivalent Viscous Damping (EVD) using the post-tensioned only wall. EXPERIMENTAL PROGRAMME A series of experimental tests were conducted to investigate the cyclic and dynamic behaviour of unbonded post-tensioned rocking walls without additional energy dissipating elements. Four unbonded post-tensioned precast wall panel ends were subjected to a series of four different lateral loading conditions, including pseudo-static reverse cyclic, high-speed reverse cyclic, snap-back free vibration, and forced vibration. Construction Detail The precast concrete wall test specimens were designed to be 3 m high, 1 m long and 0.12 m thick, a cross-section of the specimen is shown in Figure 1. Because only minor damage was expected in the wall toes, the specimen had a symmetrical design that allowed it to be flipped and retested on the other end, thus providing four wall ends from two wall panels. The post-tensioning provided the flexural strength and so only minimum longitudinal reinforcement was included to satisfy the requirements of the New Zealand Concrete Standard (2006). Due to the high strains expected in the wall toe, each wall panel had specifically designed confinement reinforcement that consisted of 6 mm hoop stirrups spaced at 40 mm centers over the bottom 200 mm of the wall, with the spacing increased to 80 mm for a further 300 mm of wall height, as shown in Figure 2a. The confinement reinforcement was designed using the confined concrete model presented by Mander et al. (1988) using the maximum expected concrete compressive strain calculated from the simplified analysis method developed by Aaleti et al. (2009). A steel angle base frame constructed from 25×25×5 equal angle was cast into each precast wall end for additional confinement and protection of the edge, as shown in Figure 2b. Two 40 mm drossbach ducts were cast into each wall to accommodate one 15 mm steel bar each. The tendons had a measured yield strength of 900 MPa, an ultimate strength of 1100 MPa, and a modulus of elasticity of 215 GPa. The precast concrete walls had a compressive strength of 35 MPa based on standard testing. The wall was seated on a foundation block designed with a 30 mm steel lined recess to enable grout to be removed easily following a test, allowing for the foundation to be reused for multiple tests (See Figure 2c).

Figure 1. Cross-section of the Precast Wall Panel

(a) Confinement Detail of Wall End (b) Steel Angle Base Frame (c) Foundation

Figure 2. Wall and Foundation during Construction Test Setup

A photo of the test setup is shown in Figure 3b. The wall panel was seated on a foundation that was post-tensioned to a strong floor. Stability struts, which are shown in red in Figure 3, were used to prop the wall during construction to ensure that the wall remained vertical. While the wall was propped, grout was placed between the wall-to-foundation interface to provide an even bearing surface for the specimen. A high strength, high flow grout was used and purposely flowed from one side of the wall to the other to prevent the formation of air pockets. Following grouting a 1237 kg concrete mass block was attached to the top of the wall; this mass block served multiple purposes, including providing anchorage for the tendons, seismic mass for the dynamic testing, a loading beam for the actuator to be attached, and an anchorage/platform for the Eccentric Mass Shaker (EMS) during forced vibration testing. A further one or two mass blocks were added when additional mass was required.

(a) Instrumentation Setup (b) Photo of Test Setup

Figure 3. Wall Instrumentation and Test Setup

For both the pseudo-static and high speed reverse cyclic testing a ± 300 kN capacity hydraulic actuator was used to apply the cyclic lateral displacements to the top of the wall. The actuator was mounted to a strong wall 3.1 m above the wall-foundation interface and attached to the wall by means of the mass block. A lateral support frame provided out-of-plane restraint which prevented twisting and buckling of the wall to ensure an in-plane response was achieved. A gap of 5 mm was left between the lateral support frame and the specimen during testing to allow small movements and to reduce the influence of friction on the sliding interface.

For the forced vibration testing the actuator was removed to allow the structure to be loaded by the EMS pictured in Figure 3b. For all tests the EMS remained in place to provide a consistent additional mass of 812 kg. Free vibration testing was undertaken using the setup shown in Figure 4. A quick release mechanism was secured to the wall, connected to a chain, 100 kN actuator, and load cell. The quick release shackle could be remotely triggered to release the wall from a specified lateral displacement.

Figure 4. Setup for Free Vibration Testing

Instrumentation

Extensive instrumentation was employed to monitor the behaviour of the walls during each of the tests which is detailed in

Figure 3a. For all four tests a Linear Variable Displacement Transducer (LVDT) and two string potentiometers (SP) were used to measure the in-plane lateral displacement at the top of the wall. Portal Gauges (PG) were used to measure uplift at the wall-to-foundation interface, as well as slip at the wall-to-foundation. Load Cells (LC) were used to measure the tendon force and the lateral force. During the cyclic loading the panel deformation was also measured using portal gauges. The concrete compressive strains were measured at each wall corner using both Surface mounted Strain Gauges (SSG) and Internal Strain Gauges (ISG). For the dynamic testing accelerometers (A) were placed horizontally at the same height as the top LVDT and vertically at each wall toe.

Loading Conditions Four identical wall ends were tested which had the same post-tensioning arrangement and prestress, as detailed in Table 1. The total prestress in each tendon at the start of all tests was approximately 240 MPa. With the exception of the loading conditions, the only other parameter that was modified during testing was the inertia mass at the top of the wall, as shown in Table 1.

Table 1. Test Matrix

Specimen: Wall/End

Mass(kg) Pseudo-static cyclic

High speed cyclic

Pre-shaker free vibration

Forced vibration (Shaker)

Post-shaker free vibration

1/1 2482 --

1/2 2482 -- -- --

2/1 3203 -- --

2/2 3949 --

The loading history applied to the wall for both the pseudo-static and high speed reverse cyclic tests was based on ACI recommendations (2008). The wall was subjected to three full reverse cycles at each drift up to maximum lateral drift of ±2% to limit damage and prevent the tendons from reaching yield. For the loading history was applied at a rate of 0.5 mm/s and 200 mm/s for the pseudo-static and high-speed tests respectively. The free vibration tests were conducted by applying a predetermined lateral displacement to the wall using the setup previously shown in Figure 4. At the desired drift the quick release mechanism was triggered and the wall was allowed to undergo a controlled rocking motion and decay. Free vibration testing was conducted for lateral drifts of 0.15%, 0.5%, 1% and 2%. For each drift the test was performed three times to investigate the repeatability of the test. Additionally, the free vibration tests were performed both before and after the forced vibration testing to investigate the effect that any increase in damage in the wall toe had on the free vibration response. An EMS was used to excite the wall specimen into the rocking mode. The EMS consists of two eccentrically weighted flywheels that rotate in opposite directions at a desired frequency resulting in a steady-state sinusoidal in-plane force excitation. The force output for a given frequency can be adjusted by varying the amount of eccentric mass attached to each flywheel. The maximum mass that can be attached to each flywheel is 128kg which consists of 8, 16kg weights, leading to maximum of 16 weighs combined from both flywheels. The total number of weights used during testing ranged from 2-16 16kg weights and small weights. The EMS shaker loading was applied by increasing the input frequency until the lateral displacement of the wall exceeded predetermined limits or when the highest frequency of interest was achieved.

RESULTS AND DISCUSSION Pseudo-static Cyclic Testing and High Speed Reverse Cyclic Testing The purpose of the pseudo-static cyclic testing was to identify and confirm the lateral force-displacement behaviour of the wall. The measured lateral load response of wall end 1/1 subjected to pseudo-static cyclic loading is shown in Figure 5a. A comparison is made between the experimental response and the response calculated using a Simplified Analysis Method (SAM) developed by Aaleti and Sritharan (2009). It can be seen that the SAM predicted the wall response with reasonable accuracy. In Figure 5b the back bone of both the pseudo-static (PS) and high speed (HS) cyclic tests are presented showing a typical bilinear hysteresis. The high speed test resulted in a lower lateral force at higher drifts in comparison to the pseudo-static test.

(a) Full Cyclic Lateral Response (b) Backbone Lateral Load Response

Figure 5. Results from cyclic testing Free Vibration The free vibration response of a structural system is very useful in identifying the damping components of a system. Due to space limitations only the 2% drift pre-shaker free vibration test is presented here with the displacement decay and corresponding EVD shown in Figure 6a and b respectively. The EVD was calculated using Equation (1) from Marriott (2009) where the logarithmic decay formula is used but adjusted to account for the elongated period of the system by using energy in place of displacement and the cumulative average circular frequency. A good trend is evident in Figure 6 which shows 2.5% EVD at high drifts and increased damping at low drifts as rocking is no longer fully initiated.

(

) (1)

Where = circular frequency, t = time, = energy at t=0, and = energy left at t.

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Figure 6. Pre-shaker #1 Test released from 2% lateral Drift

Forced Vibration

Free vibration test results provide a convenient way of analysing the damping in a system, but the lack of input excitation means that the dynamic response of the system to an input force and frequency is not investigated. For this reason, the wall system was also subjected to forced vibration testing using an EMS. Brief observations and details of the testing performed on wall end 2/2, which supported a total mass of 3949 kg, are presented herein. Following the pre-shaker free vibration testing wall end 2/2 was subjected to six different loading combinations by the EMS. The different loading combinations were applied by changing the mass attached to each flywheel of the shaker. The flywheel mass was varied through the following total number of weights: 16, 12, 8, 4, 2, and small weights. The applied frequency and corresponding force for the two weight load combination is shown in Figure 7a with the resulting displacement response of the wall shown in Figure 7b. For all the tests performed the wall was observed to have a very low displacement amplitude response until a certain frequency was attained and then a large increase in response was observed. For the two weight combination this behaviour can be seen in Figure 8b which plots the maximum displacement amplitude versus input frequency, it is clear from this figure that a large increase in displacement amplitude occurred at 3.5 Hz. The maximum amplitude response is highlighted by section A in both Figure 7b and Figure 8b for which a ‘beat’ resonance was observed, a close up of this phenomenon is shown in Figure 8a. The region highlighted in Figure 8b shows the ‘beat resonance’ with a variable amplitude response at the same frequency. As the frequency was increased further the amplitude of response decreased to a constant amplitude harmonic motion. The maximum amplitude response versus input frequency for all six tests in shown in Figure 9. It is clear that with decreasing shaker mass the maximum response tends to occur at higher frequencies. It is important to realise that with increasing frequency a lower force is required to attain the sudden large increase in displacement response.

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Figure 8. Detailed Displacement Response for Two Weight Load Combination

Figure 9. Displacement Response of Wall for Different Force/Frequency Combination

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CONCLUSIONS AND FUTURE WORK Four unbonded post-tensioned precast concrete wall ends were subjected to four different loading conditions to investigate both the cyclic and dynamic response. Pseudo-static and high speed cyclic testing was conducted on one wall end and both tests matched well with a simplified analytical method previously proposed. The free vibration decay was used to determine the system damping which was estimated to be 2.5% at high drifts and 4% at close to zero drift. An EMS was used to excite the precast concrete walls into the rocking mode for a range of eccentric masses. A ‘beat’ resonance response was observed and the frequency of this response increased with decreasing eccentric mass. More extensive data analysis and investigation is currently in progress. Following this first series of tests, further testing will be carried out on wall specimen with additional energy dissipating devices to investigate the interaction of the hysteretic damping and contact damping and what effect this has on the overall response of the wall system. ACKNOWLEDGEMENTS The authors would like to acknowledge the generous support and materials donated by Stresscrete (Papakura) and Mapei. Additionally the assistance of Dan Ripley, Jeffrey Ang, Mark Byrami, and Sam Corney during the experimental programme is greatly appreciated. REFERENCES Aaleti, S. and Sritharan, S. (2009). A simplified analysis method for characterizing unbonded post-tensioned precast wall systems. Engineering Structures Vol. 31 No. 12: 2966-2975 ACI Innovation Task Group 5. (2008). Acceptance criteria for special unbonded post-tensioned precast structural walls based on validation testing and commentary : an ACI standard. Farmington Hills, Mich.: American Concrete Institute. Henry, R. S., (2011) Self-centering precast concrete walls for buildings in regions with low to high seismicity. PhD--Civil Engineering, University of Auckland. http://hdl.handle.net/2292/6875 Kurama, Y., Sause, R., Pessiki, S. and Lu, L. W. (1999). Lateral load behavior and seismic design of unbonded post-tensioned precast concrete walls. ACI Structural Journal Vol. 96 No. 4: 622-632 Kurama, Y. C., Sause, R., Pessiki, S. and Lu, L. W. (2002). Seismic response evaluation of unbonded post-tensioned precast walls. ACI Structural Journal Vol. 99 No. 5: 641-651 Kurama, Y. C. and Shen, Q. (2004). Posttensioned hybrid coupled walls under lateral loads. Journal of Structural Engineering Vol. 130 No. 2: 297-309 Ma, Q. T. M., (2010) The mechanics of rocking structures subjected to ground motion. PhD--Civil and Environmental Engineering, University of Auckland. http://hdl.handle.net/2292/5861 Mander, J. B., Priestley, M. J. N. and Park, R. (1988). THEORETICAL STRESS-STRAIN MODEL FOR CONFINED CONCRETE. Journal of structural engineering New York, N.Y. Vol. 114 No. 8: 1804-1826 Marriott, D., (2009) The Development of High-Performance Post-Tensioned Rocking Systems for the Seismic Design of Structures. Civil Engineering, University of Canterbury Christchurch. New Zealand Standard (2006). Concrete Structures Standard NZS 3101.

Perez, F. J., Sause, R. and Pessiki, S. (2007). Analytical and experimental lateral load Behavior of unbonded posttensioned precast concrete walls. Journal of Structural Engineering-Asce Vol. 133 No. 11: 1531-1540 Priestley, M. J. N., Sritharan, S. S., Conley, J. R. and Pampanin, S. (1999). Preliminary results and conclusions from the PRESSS five-story precast concrete test building. PCI Journal Vol. 44 No. 6: 42-67 Restrepo, J. I. and Rahman, A. (2007). Seismic performance of self-centering structural walls incorporating energy dissipators. Journal of Structural Engineering Vol. 133 No. 11: 1560-1570 Sritharan, S., Aaleti, S., Henry, R. S., Liu, K. Y. and Tsai, K. C. (2008). Introduction to PreWEC ad key results of a proof of concept test. M.J. Nigel Priestley Symposium, North Lake Tahoe, California, August 4-5, 2008