Development and Seismic Evaluation of Pier Systems w/pocket Connections and Advanced Materials in Plastic Hinges

Transcription

1 Development and Seismic Evaluation of Pier Systems w/pocket Connections and Advanced Materials in Plastic Hinges Test Model: Precast square columns with UHPC and ECC in plastic hinge zone and pocket connection in cap beam and footing Graduate Assistant: Alireza Mohebbi, PhD Candidate Undergraduate Assistant: Alicia Robb PI: Dr. M. Saiid Saiidi Co-PI: Dr. Ahmad M. Itani Department of Civil and Environmental Engineering University of Nevada, Reno April, 2016

2 1- Brief description of the project Accelerated bridge construction (ABC) has piqued interest in recent years due to its proposed revolutionary changes to construction of bridges. The use of prefabricated reinforced concrete members is the cornerstone of ABC, and these precast members have numerous advantages. Because these bridge components do not need to be cast-in-place on-site, traffic delays and road closures can be minimized or eliminated, and construction time and labor costs can be reduced. Careful bridge design in seismically active regions is critical for the structure to withstand an earthquake. Due to this, ABC focuses not only on ease of constructability, but also structural integrity and capacity to resist seismic damage. Various connection types have been researched in recent years to help reduce damage after a seismic event. These connections can be placed in two categories of coupler and pocket connections. Columns can be connected to their cap beams and footings by moment connections and pin connections. This research project concentrates on both connections as they have shown promising results while not violating the current AASHTO ban on couplers. Innovation concepts of this research project are as follows: Precast column, cap beam, and footing with pocket connection Engineered Cementitious Composite (ECC) in plastic hinge zone Ultra-high performance concrete (UHPC) in plastic hinge zone This study focuses on precast square columns that are connected to the precast footing using the pin connection and connected to the precast cap beam using the moment connection. The precast columns are simply inserted into the footing and extended into the cap beam using pocket connections. High strength non-shrink grout are used to fill the space between columns and the pockets at the footing and the cap beam. Specific objectives of the research project are to determine: a) the seismic performance of the precast bent and pocket connections b) the appropriate embedment length of square precast columns in pocket connections c) the performance of engineered cementitious composite (ECC) used in the plastic hinge zone of the square column, d) the performance of ultra-high performance concrete (UHPC) in the plastic hinge zone of the square columns, and e) design considerations and methods for connections, precast columns, and advanced materials in plastic hinges. 1

3 2- Specimen description To accomplish the objectives of the study, a 1/3 scale of a precast two-column bent model was constructed. The novel bent has moment connection at the top and pin connection at the bottom. The plastic hinge zone in each column was fortified with a different advanced material. One column s plastic hinge region was made with ECC, and the other was made with UHPC. The novel columns were inserted into the precast footing, which had pockets for the column s cylindrical pin connection. The embedment length of the columns in the footing was 1.36 times the column dimension. The cap beam was then placed on top of the columns with the columns sliding into the pocket connections. The embedment length of the column in the cap beam was 1.0 times the column dimensions. Figure 1 shows a schematic representation of the test model. The properties of the precast bent are given in Table 1. The 28-day cylinder tests for concrete, UHPC and ECC are given in Table 2. Figures 2-7 give the geometry and cross section of the columns, cap beam details, and footing details. Figure 1. Schematic representation of the test model 2

4 Table 1. Properties of UHPC and ECC Scale factor 1/3 Bent cap dimensions (inch) 19 x 26 x 134 Footing dimensions (inch) 23 x 36 x 132 Column dimensions (inch) 14 x 14 Column clear height (inch) 61.0 Aspect ratio 4.35 Column longitudinal bar 8 - #5 Column longitudinal steel ratio 1.26% Column transverse steel 2.0 Column transverse steel ratio 2% Longitudinal bar at hinge section 6 #5 Longitudinal steel ratio at hinge section 2.36% Transverse steel at hinge section 1.5 Axial load index 7.0% UHPC Length (inch) 21 ECC Length (inch) 21 Embedment length of column at top (inch) 14.0 Embedment length of column at bottom (inch) 19.0 Gap in pocket connection (inch) 1.0 Dead load (kip) Mp Hinge / Mp Col. 40% Base shear (kip) 72 Shear demand (left col., right col.) (kip) 29, 43 Column shear capacity (left col., right col.) (kip) 74, 74 Hinge shear capacity (left col., right col.) (kip) 47, 79 Table days compressive strength of concrete, ECC, and UHPC. Concrete 28-days compressive strength, f'c (ksi) Footing 5.07 Column - outside the plastic hinge zone 4.20 Cap beam 5.07 UHPC Column - in the plastic hinge zone ECC Column in the plastic hinge zone

5 Figure 2. Geometry of the test model Figure 3. Cross section of the columns and hinges 4

6 Figure 4. Cap beam details Figure 5. Cap beam cross sectional details Figure 6. Footing details 5

7 Figure 7. Footing section A-A details 3- Instrumentation Total of 167 channels were used to record data from shake table test. The details of instrumentations are given in Table 3. Numbering of the instruments as well as locations are shown in Figures Table 3. Instrumentation details Instrumentation Type Quantity Channels String Pot UniMeasure 7 7 Accelerometer Triax MEM 3 6 Potentiometer Novotechnik Long. Rebar s Strain Gauges YFLA-2-5L Trans. Rebar s Strain Gauges YFLA-2-5L Cap Beam Strain Gauges YFLA-2-5L Footing Strain Gauges YFLA-2-5L 8 8 Load Cell in the Link 1 1 Total Strain Gauges

8

9

10

11

12

13 4- Testing Procedure The test set up is shown in figure 13. Spreader beams were used to apply 100 kips axial load on the columns. Four concrete mass blocks are used on the platform to include initial force corresponding to the dead load. Figure 13. Shake table test setup This bridge bent was designed for Los Angeles area, Lake Wood, with the latitude and longitude of N, and W, respectively. Seismic properties of this location based on AASHTO 2009 were as follows: As=0.473g SD1=0.637g To=0.11 sec SDS=1.155g Ts=0.552 sec Site class: D The 1994 Northridge earthquake acceleration history recorded at the Sylmar station, RSN1084_NORTHR_SCS052, will be simulated in the shake table test. The time scaled acceleration and velocity histories for this near-fault motion are shown in Figure 14 and Figure 15, respectively. 12

14 Velocity, in/s Acceleration, g Time, sec Figure 14. Time scaled acceleration history for Sylmar Time, sec Figure 15. Time scaled velocity history for Sylmar 52 The maximum acceleration, velocity and displacement were 0.623g, in/s, and 5.15 in, respectively (Pacific Earthquake Engineering Research Center, 2011). Figure 16 shows the scaled design spectrum and response spectrum of the selected ground motion. Figure 17 shows the acceleration history for different runs. Figures 18, 19 and 20 show the predicted response of the novel bent for cumulative runs. The load factors to be multiplied by the acceleration points to produce the desired amplitude are presented in Table 4. 13

15 Response Acceleration, g Tbent = 0.2 sec Scaled Response 122% Sylmar 52 Design Spectrum Period, sec Figure 16. Time scaled response acceleration Figure 17. Acceleration history for cumulative runs 14

16 Figure 18. Displacement history for cumulative runs Figure 19. Base shear history for cumulative runs 15

17 Figure 20. Hysteresis loops for cumulative runs 16

18 References AASHTO. (2014). AASHTO Guide Specifications for LRFD Seismic Bridge Design. Washington, DC: American Association of State Highway and Transportation Officials. Pacific Earthquake Engineering Research Center. (2011, November 8). Peer Groung Motion Database. (University of California, Berkeley) Retrieved December 16, 2013, from PEER: 17

19 Notes:. 18

20 Table 4. Load factors and predicted values Run # Test type Testing protocol Factor PGA (g) %DE Dmax, in Analytical prediction F@Dmax, kip Test Results Dmax, in F@Dmax, kip Notes WN1 White Noise 1 Sylmar % WN2 White Noise 2 Sylmar % WN3 White Noise 3 Sylmar % WN4 White Noise 4 Sylmar % WN5 White Noise 5 Sylmar % WN6 White Noise 6 Sylmar % WN7 White Noise 7 Sylmar 52 WN8 White Noise 8 Sylmar 52 19

21 Base Shear, kips Analytical Force Displacement Response 90 Displacement, in % DE 100% DE 110% DE 150% DE % DE Push-Pushover Anal. Pull-Pushover Anal. Seismic Anal % DE Drift, % 20