SEISMIC PERFORMANCE OF BRIDGE SLAB-COLUMN JOINTS WITH HEADED REINFORCEMENT

Transcription

1 Eleventh U.S. National Conference on Earthquake Engineering Integrating Science, Engineering & Policy June 25-29, 2018 Los Angeles, California SEISMIC PERFORMANCE OF BRIDGE SLAB-COLUMN JOINTS WITH HEADED REINFORCEMENT V. Papadopoulos 1, J. Murcia-Delso 2 and P.B. Shing 3 ABSTRACT This paper presents results of an experimental and analytical investigation on the seismic performance and design of slab-column joints using headed bars as longitudinal column reinforcement. Three full-scale slab-column assemblies were tested under cyclic lateral loading to required to develop the full capacity of the longitudinal bars. Each of the specimens had a 2-ft. diameter, 12-ft. tall column and a 16-in. thick slab. The columns were connected to the slabs with headed bars with embedment lengths varying between 8.7 and 11 times the bar diameter. These lengths are shorter than the development length required by ACI 318. The reinforcement details for the slabs were based on Caltrans specifications. The results of the tests have indicated that with sufficient joint reinforcement, an embedment length equal to 11 times the bar diameter is sufficient to develop the full capacity of the headed bars and ensure an adequate seismic performance of the slab-column joint. Test specimens with shorter embedment lengths were able to develop the moment capacity of the columns but resulted in moderate to severe cracks in the top face of the slabs due to the punching action of the headed bars. The experimental results were complemented by numerical simulations using nonlinear finite element models, which confirmed that the anchorage performance is governed by the punching failure of the slab. 1 Civil engineer, PhD, Archirodon N.V., Athens, Greece ( v.papadopoulos@archirodon.net) 2 Assistant Professor, CAEE Department, University of Texas at Austin, TX ( murcia@utexas.edu) 3 Professor, Dept. of Structural Eng., University of California, San Diego, CA ( pshing@ucsd.edu) Papadopoulos V, Murcia-Delso J, Shing PB. Seismic Performance of Bridge Slab-Column Joints with Headed Reinforcement. Proceedings of the 11 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA

2 Seismic Performance of Bridge Slab-Column Joints with Headed Reinforcement V. Papadopoulos 1, J. Murcia-Delso 2, and R. Wright 3 ABSTRACT This paper presents results of an experimental and analytical investigation on the seismic performance and design of slab-column joints using headed bars as longitudinal column reinforcement. Three full-scale slab-column assemblies were tested under cyclic lateral loading to required to develop the full capacity of the longitudinal bars. Each of the specimens had a 2-ft. diameter, 12-ft. tall column and a 16-in. thick slab. The columns were connected to the slabs with headed bars with embedment lengths varying between 8.7 and 11 times the bar diameter. These lengths are shorter than the development length required by ACI 318. The reinforcement details for the slabs were based on Caltrans specifications. The results of the tests have indicated that with sufficient joint reinforcement, an embedment length equal to 11 times the bar diameter is sufficient to develop the full capacity of the headed bars and ensure an adequate seismic performance of the slab-column joint. Test specimens with shorter embedment lengths were able to develop the moment capacity of the columns but resulted in moderate to severe cracks in the top face of the slabs due to the punching action of the headed bars. The experimental results were complemented by numerical simulations using nonlinear finite element models, which confirmed that the anchorage performance is governed by the punching failure of the slab. Introduction Reinforced concrete (RC) slab bridges are commonly used for bridges with short spans because they are economical to construct. In the event of a major earthquake, columns in these bridges may develop plastic deformations. However, the damage of the slab should be prevented. For this to happen, the columns can be pin-connected to the deck slab, or the column longitudinal reinforcement must have a sufficient embedment length in the slab to develop its full tension capacity in the plastic-hinge region. Very often, the slab thickness required to carry the service loads may not provide a sufficient embedment length to develop the longitudinal reinforcement anchored with standard hooks. The use of headed deformed bars can significantly reduce the required embedment length and also the congestion introduced in the joint region by hooked bars. This paper summarizes the results of an experimental and analytical investigation on the seismic performance and design of slab-column joints using headed bars as longitudinal column reinforcement [1]. Three slab-column assemblies were tested under cyclic lateral loading to required to develop the full capacity of the longitudinal bars. The columns were connected to the 1 Civil engineer, PhD, Archirodon N.V., Athens, Greece ( v.papadopoulos@archirodon.net) 2 Assistant Professor, CAEE Department, University of Texas at Austin, TX ( murcia@utexas.edu) 3 Professor, Dept. of Structural Eng., University of California, San Diego, CA ( pshing@ucsd.edu) Papadopoulos V, Murcia-Delso J, Shing PB. Seismic Performance of Bridge Slab-Column Joints with Headed Reinforcement. Proceedings of the 11 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA

3 slabs with headed bars with embedment lengths shorter than the development length required by ACI 318 [2]. The experimental results were complemented with numerical simulations using nonlinear finite element (FE) models. Experimental study Three full-scale slab-column assemblies were tested under cyclic lateral loading in the laboratory. The slab-column assemblies were tested in an upside-down position with two edges of the slab sitting on hinge supports and the lateral force applied to the top of the column, as shown in Figure 1. The test specimens had a 2-ft diameter column with a height of 12 ft measured from the top surface of the slab in the specimen to the point at which the lateral load was applied. The compressive strength of the concrete in the column and the slab was approximately 5 ksi when the specimens were tested, and the reinforcement was Grade 60 complying with ASTM A706. The main differences among the three specimens were the size of the column longitudinal bars and the thickness of the slab at the slab-column joint. Specimen 1 had No. 9 bars with headed ends for the column longitudinal reinforcement. The thickness of the slab was 16 in. The embedment length provided to the headed bars extending into the slab was 11 in., or 9.8 times the bar diameter (db), as shown in Figure 2. This length was the maximum possible considering the space required for the concrete cover and longitudinal reinforcement of the slab. The minimum development length required by ACI 318 [2] for this bar would be 14db. The slab reinforcement complied with Caltrans requirements in MTD 20-7 [3], which requires a minimum amount of vertical stirrups and ties in the column-slab joint. The column of Specimen 2 had No. 10 headed bars as longitudinal reinforcement. The thickness of the slab was kept to 16 in., which resulted in an embedment length of 8.7 db for the column longitudinal bars. The reinforcement of the slab was the same as that of Specimen 1, except that it had four additional vertical stirrups adjacent to the column reinforcement. The column dimensions and reinforcement for Specimen 3 were identical to those of Specimen 2. The slab had also a thickness of 16 in. but a 3-in. drop cap was added in the slab-column joint region. As a result, the total embedment length of the headed bars was increased to 11db. Section AA Clear Cover 2" C1 A 8 #9 bars with HRC 150 head A C1 8 #9 bars with HRC 150 head C2 #5 3.5" o.c. (butt welded) C2 #5 3.5" o.c. (butt welded) Figure 1. Test specimen and setup Figure 2. Column reinforcement for Specimen 1 The three slab-column specimens presented a ductile behavior with a plastic hinge forming at the base of the column. Figure 3 shows the lateral load-vs.-displacement response of Specimens

4 Force (kips) 2 and 3. The tensile strains measured in the column longitudinal bars indicate that the embedment length of these bars was sufficient to develop not only their yield strength, but also reach higher stresses caused by strain hardening. In Specimen 1, only moderate damage was observed in the slab in terms of radial splitting cracks and punching cracks in the bottom face of the slab. The cracks at the bottom face were caused by the punching of the headed bars in compression. Concrete cover spalling was observed at the bottom of the slab of Specimen 2, as shown in Figure 4. Severe spalling occurred at a column drift of 8%, which was followed by a significant drop of the lateral load resistance, as depicted from Figure 3. The test was stopped at that point. The spalling of concrete was caused by the severe punching action of the headed bars in compression. As shown in Figure 3, the lateral load-vs.-displacement behavior of Specimen 3 was similar to that of Specimen 2, except that the hysteresis curves are less pinched due to the higher embedment length provided by the drop cap, which contributed to reducing the damage in the slab to mild punching cracks. 40 F = 35 kips Spec. #2 Spec. # μ=6 (Spec. #2) Figure 3. Drift (%) μ=7 (Spec. #3) Lateral force-vs.-drift ratio curves for Specimens 2 and 3 Figure 4. Damage at the bottom face of the slab of Specimen 2 Nonlinear finite element analysis A three-dimensional FE model was developed in Abaqus to simulate the behavior of the slabcolumn assemblies, and to further investigate the anchorage mechanism of the headed bars and the damage in the slab-column joints. The FE model was verified with the results of the column-slab assembly tests. To investigate the influence of the concrete cover below the bar head (in the specimen configuration) on the punching resistance, a FE analysis has been conducted. In the FE model, the embedment length of the headed bars in Specimen 2 is reduced to 6.7db to increase the concrete cover by 2db. As shown in Figure 5, the increased concrete cover significantly reduces the punching damage. In spite of the smaller embedment length, the lateral load capacity and ductility of the column were not compromised, and the hysteresis curves were less pinched as compared to Specimen 2. This indicates that punching resistance is the controlling factor and the development length required to develop bar tension can be as low as 6.7db.

5 Lateral load (kips) (a) Specimen #2 (Caltrans MTD 20-7) at max. drift Specimen #2 (FEA) -40 Specimen #2B (FEA) Drift (%) (c) Lateral load-drfit curves (b) Specimen #2B (headed bars moved 2d b upwards) at max. drift Figure 5. Variation of concrete cover thickness below the headed bars. Conclusions The results of the tests have indicated that with sufficient joint reinforcement, an embedment length equal to 11db can develop the full capacity of the headed bars and ensure an adequate seismic performance of the slab-column joint. Test specimens with shorter embedment lengths were able to develop the moment capacity of the columns but resulted in moderate to severe cracks in the slabs due to the punching action of the headed bars. The experimental and numerical results have shown that the anchorage performance is governed by the punching action of the bar heads on the cover concrete in the slab rather than the breakout failure of concrete induced by bars in tension. The finite element results have shown that the development length can be further reduced without jeopardizing the anchorage performance when the thickness of the cover concrete over the bar heads (in an actual deck slab) is increased. Acknowledgments This study was supported by the California Department of Transportation. The authors appreciate the technical input provided by Caltrans engineers throughout this study. However, the opinions expressed in this paper are those of the authors and do not necessarily reflect those of the sponsor. The authors would also like to express their sincere gratitude to the laboratory staff of the Powell Laboratories at the University of California at San Diego where the tests were conducted. References 1. V. Papadopoulos, J. Murcia-Delso, P. B. Shing, Development Length for Headed Bars in Slab-Column Joints of RC Slab Bridges, Report No. SSRP 15/09, Department of Structural Engineering, University of California, San Diego, ACI, Building Code Requirements for Structural Concrete (ACI ), Farmington Hills, MI, Caltrans, Seismic Design of Slab Bridges, Memo to Designers (MTD) 20-7 (October 2014), California Department of Transportation, Sacramento CA, 2014.