Development of a Precast Bent Cap System for Seismic Regions (2011) / Chapter Skim
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From page 9... ...
-- joint reinforcement requirements than the current 2009 LRFD SGS. For example, in contrast the 2006 LRFD RSGS, the 2009 LRFD SGS specifies vertical joint stirrups both inside and outside the joint region, a larger total area of joint stirrups, and a significant increase in bent cap longitudinal reinforcement.
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From page 10... ...
. Bent cap transverse reinforcement outside of the joint region was designed according to the 2006 LRFD BDS (16)
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From page 11... ...
. As the prototype bridge would be expected to exhibit ductile plastic hinging in the column region adjacent to the bent cap due to transverse response, the scaled CIP control specimen -- loaded in the transverse direction under quasi-static force control and displacement control sequences -- was expected to perform similarly.
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From page 12... ...
Joint reinforcement for the CPFD specimen did not include the horizontal J-bars used for the CIP specimen, although two 2-leg construction stirrups were placed within the joint region. Emulative Precast Bent Cap Connections for Seismic Regions: Component Test Report-Grouted Duct Specimen (Unit 2)
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From page 13... ...
Each hybrid detail uses half of the conventional reinforcement as compared to the CIP and emulative specimens connected to the bent cap using a grouted duct connection with a grouted bedding layer joint dimension of 1 in. The conventional hybrid specimen used closely spaced spiral reinforcement at the column end to provide lateral confinement of the concrete compression toe.
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From page 14... ...
or lower inside plastic hinging region, per Eq. 4.9-5 Using concrete shear stress for circular columns with hoops, modified by: where is 2 Minimum Column Shear Reinforcement (Spiral)
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Similar to the concrete filled pipe specimen, the reinforcement extending from the bent cap into the column was terminated following adequate development. The three bent caps for the hybrid specimens were identical and were designed in accordance with the 2006 LRFD RSGS (2)
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The use of stainless steel reinforcement locally across the joint serves to provide added comfort in the durability of these systems during their expected service life. To consider the potential influence of stainless steel reinforcement, a variety of material tests were conducted.
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Grouted Duct Specimen Figures 2.8 through 2.12 show the bent cap rebar cage, cap setting operation, and grouting of the GD specimen. Grout compressive strength was designed to exceed that of the bent cap by at least 500 psi to ensure the connection grout was not a weak link in the system.
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From page 18... ...
After the grout cured several days, the bedding layer form was removed and the bedding layer and the top of the ducts were inspected. Cap Pocket Specimen Figure 2.13 compares the bent cap rebar cage for the CPFD and CPLD specimens.
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From page 19... ...
Once grout had adequate time to cure, the column and bent cap assembly was post-tensioned, inverted, and installed in the testing frame. Concrete Filled Pipe Hybrid Specimen The reinforcing cage and details of the bent cap for the concrete filled pipe and dual shell specimens are the same as those presented in Figure 2.16 for the conventional hybrid specimen.
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From page 20... ...
Comparison of cap pocket bent cap rebar cages during fabrication.
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Dual Steel Shell Hybrid Specimen A view down the region between the internal and external shell for the dual shell specimen can be seen in Figure 2.22. Similar to the concrete filled pipe specimen, in this column detail weld beads were placed on the inside of the column to promote the transfer of forces from the reinforcing bar to the shell.
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From page 22... ...
This grouting will not affect the unbonded nature of the tendon because the bond between the grout and PVC will break easily. 2.2.3 Nonintegral Testing Protocol and Instrumentation Emulative Specimens The specimen test setup, shown in Figure 2.24, included a simply supported inverted bent cap that allowed accurate
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From page 23... ...
Internal strain gages were placed on bent cap, joint, and column reinforcing bars, as well as on corrugated ducts or pipe. In addition to the approximately 100 channels of data, specimen response was also monitored using digital photos, crack markings and measurements, video recording, and notes.
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Specimen in Test Bay with External Instrumentation Shown -- GD Figure 2.24. Test setup for emulative specimens.
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Representative force controlled sequence for emulative specimens.
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In addition to the external instrumentation, many internal strain gages were employed to capture the local response of materials. 2.2.4 Design of Integral Prototype Bridge and Specimen An overall elevation of the prototype bridge is shown in Figure 2.31 with the connection detail shown in Figure 2.32.
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Girder to bent cap prototype connection detail.
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From page 28... ...
Ductility demands for both directions were approximately 5, well below the limit of 8 for multicolumn bent caps. Capacity design principles were applied to the design of the superstructure to ensure that the seismic overstrength demands could be resisted in a nominally elastic manner.
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Lab staff performed the casting of concrete and all activities related to erection of members. The first stage of fabrication consisted of the construction of the reinforcing cages for the girder and reaction block.
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A drain hole was placed in the bottom of the form to allow for draining of excess water (water is used to moisten the faces of the reaction block and girder prior to grouting)
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With the formwork in place, the deck reinforcing cage was fabricated. Similar to casting of the deck and reaction block, the deck was cast using a bucket attached to the overhead crane.
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This instrumentation includes strain gages mounted on rebar and post-tensioning and external gages mounted onto the specimen. External instrumentation consists of linear potentiometers, string potentiometers, and inclinometers mounted on the exterior of the specimen.
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From page 33... ...
response of the column is used to characterize the fundamental performance of the specimen. Displacement decomposition refers to the separation of the column displacement into the components that contribute to the overall lateral displacement of the column (column flexure, fixed end rotation due to plastic hinging and bar slip, bent cap flexibility, and joint shear)
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Column Lateral Force versus Lateral Displacement. The lateral force displacement (hysteretic)
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Bent cap longitudinal bars did not yield, reaching only 46% of yield, even though additional bent cap longitudinal reinforcement (0.245Ast) required by 2009 LRFD SGS was not included (1)
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From page 36... ...
Joint region cracking post test -- emulative specimens.
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′fc Column Lateral Force versus Lateral Displacement. The lateral force displacement (hysteretic)
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The joint shear displacement was minor, contributing 4.9% on average to the overall column displacement, and was consistent with visual observations of minor joint cracking. Column bars were well anchored within the ducts, and although splitting cracks developed between ducts (at the top and bottom of the bent cap as tested)
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From page 39... ...
Actual Envelope - CIP Actual Envelope - GD Actual Envelope - CPFD Actual Envelope - CPLD Figure 2.52. Applied lateral force versus lateral displacement envelopes -- all specimens.
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From page 40... ...
Parameter GD/CIP CPFD/CIP CPLD/CIP CPLD/CPFD Joint Shear Stress 0.95 0.89 1.30 1.47 Principal Tensile Stress 0.94 0.88 1.30 1.47 Principal Compressive Stress 0.92 0.81 1.48 1.86 Angle of Principal Plane 1.00 0.98 1.00 1.01 Joint Rotation 1.15 0.89 1.47 1.66 Change in Panel Area 1.19 0.81 2.82 3.26 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 -28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 M ic ro st ra in Location (in) Mu 1 Mu 1.5 Mu 2 Mu 3 Mu 4 Mu 6 Mu 8 Push Bent Cap North Bent Cap South Joint Yield Strain Yield Strain Figure 2.54.
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From page 41... ...
Column Lateral Force versus Lateral Displacement. The lateral force displacement (hysteretic)
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The joint shear stress-strain 42 Figure 2.57. Lateral force versus lateral displacement -- CPFD.
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Pipe strains were largest at midheight, where principal strains were limited to 37% of yield. Cap Pocket Limited Ductility Specimen CPLD specimen response was characterized by a combination of plastic hinging of the column adjacent to the bent cap and joint shear cracking and deformation (see Figure 2.60, Figure 2.61, Figure 2.47, and Figure 2.46)
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Although column longitudinal bars remained anchored within the pipe, significant bar slip developed. Bottom bent cap longitudinal reinforcement exhibited a pattern similar to the CPFD, reaching yield, but pipe strains were larger for the CPLD.
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Column Displacement Decomposition. Limited ductility emulative bridge bent caps are expected to exhibit flexural plastic hinging, but also are expected to achieve a significantly lower displacement ductility capacity (in the range of µ2)
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The overall performance of the bent cap joint indicated only minor flexural cracking, and small crack widths indicated that a reliable joint design methodology was used. Column Lateral Force versus Lateral Displacement.
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Observed bent cap joint damage following the testing is shown in Figure 2.70 for all hybrid specimens. Figure 2.70a shows that only minor damage occurred within the joint during the entirety of the testing.
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Figure 2.68. Lateral force versus lateral displacement envelopes -- hybrids and CIP.
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Joint region cracking post test -- hybrid specimens. Strain, με H ei gh t A bo ve B en t C ap , i nc he s H ei gh t / C ol um n Di am et er 8 10 0.5 12 -20000 -15000 -10000 -5000 0 5000 10000 15000 20000 0 2 0.1 4 0.2 6 0.3 0.4 0.6 0 Bedding Layer Push (South Gages)
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From page 50... ...
The bent cap responded as anticipated and similarly to the conventional hybrid specimen even with the increase in lateral demand recorded. The overall performance of the bent cap joint indicated only minor flexural cracking, and small crack widths indicated a reliable joint design methodology was used.
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Observed bent cap joint damage following testing of the concrete filled pipe hybrid specimen is shown in Figure 2.70b. Figure 2.70b indicates that only minor damage occurred within the joint during the entirety of the testing, similar to what was observed in the conventional specimen.
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From page 52... ...
Even though the residual drifts were greater than those of the conventional hybrid specimen, the recorded residual drift was significantly less than the residual drift of the CIP specimen, indicating an overall improvement in the post-earthquake performance of the system. Dual Steel Shell Hybrid Specimen Similar to concrete filled pipe hybrid specimen, the primary lateral response of the dual steel shell hybrid specimen was dominated by the localized end rotations at the bedding layer, as shown in Figure 2.77 and Figure 2.78.
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From page 53... ...
The specimen was also observed to have decreased in overall height following seismic testing due to the reduction in bedding layer thickness associated with a reduction in the bearing area of the grout. Column Lateral Force versus Lateral Displacement.
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Observed bent cap joint damage following the testing is shown in Figure 2.70c. Review of this figure indicates that only minor damage occurred within the joint during the entirety of the testing, similar to what was observed in the other hybrid specimens.
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From page 55... ...
Even though the residual drifts are greater than those of the conventional hybrid specimen, in comparison to the CIP specimen, the recorded residual drift is significantly less, indicating an overall improvement in the post-earthquake performance of the system. 2.3.3 Integral Connection The integral experimental specimen (INT)
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From page 56... ...
This reduction in stiffness resulted in the slip between the girder and reaction block at large rotations, as shown in Figure 2.86b. The shear slip was caused by inadequately developed shear reinforcement within the girder end when subjected to flexural joint opening.
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From page 57... ...
The recommended modification to the shear reinforcement detailing at the girder end is expected to alleviate much of this issue and thus result in an increase in the ultimate rotation capacity of the connection. Even with the reduction in ultimate rotation capacity due to the kinking action, the ultimate rotation capacity results in a system that can safely undergo relative settlements between adjacent bent caps in excess of 1 ft for a structure 100 ft long.
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From page 58... ...
Results from this loading indicate that the maximum relative slip between the girder and reaction block is less than four-hundredths of an inch for the entirety of the elastic loading cycles. Interestingly, these results indicate that during the elastic loading cycles, the girder also slipped upwards during many cycles.
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From page 59... ...
Future work is required to verify the benefits of modifications to the grout bedding layer for improving performance of the second and third hybrid specimens. Nonlinear Time History Analyses In presenting a new structural system for use in seismic regions, the potential implications of realized displacement demands during strong ground shaking must be investigated.
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From page 60... ...
The hatched region on the plots represents the region in which the experienced displacement demand results in ductility values in excess of the maximum code limit of 6. Results from these analyses indicate that the hybrid systems investigated have displacement demands similar to those of more conventional systems.
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From page 61... ...
61 Period, seconds In el as tic d isp la ce m en t f ac to r, C R 0.1 0.2 0.3 0.5 0.7 1 2 3 3 3 3 0 2 4 6 8 10 R = 2 - Site Class D HYB Mean EPT Mean μD > 6 Period, seconds In el as tic d isp la ce m en t f ac to r, C R 0.1 0.2 0.3 0.5 0.7 1 2 0 2 4 6 8 10 R = 3 - Site Class D HYB Mean EPT Mean μD > 6 Period, seconds In el as tic d isp la ce m en t f ac to r, C R 0.1 0.2 0.3 0.5 0.7 1 2 0 2 4 6 8 10 R = 4 - Site Class D HYB Mean EPT Mean μD > 6 Period, seconds In el as tic d isp la ce m en t f ac to r, C R 0.1 0.2 0.3 0.5 0.7 1 2 0 2 4 6 8 10 R = 6 - Site Class D HYB Mean EPT Mean μD > 6 Figure 2.93. Hybrid system inelastic displacement modification factor (5)
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