National Science Foundation

Behavior and Design of Cast-in-Place Anchors
under Simulated Seismic Loading (NEES-Anchor)
Phase V: Anchors in Plastic Hinge Zones of RC Columns



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Description:  Anchors installed in plastic hinge zones

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   The well-established design procedures for headed anchors, such as those stipulated in ACI 318-11 and in CEB design guidelines, do not apply to the anchors installed in concrete that likely be substantially damaged during an earthquake. Plastic hing zones of concrete elements such as beams, columns, walls and slabs may develop high levels of cracking and cruching under seicmic loading. It is recommended that the anchor reinforcement must be provided for the anchors installed in plastic hinge zones.  

   The test in Phase IV (to be completed at the MUST-SIM facility at the University of Illinois) of the NEES-Anchor project will provide experimental data regarding the above recommended practice. Specifically, sufficient anchor tension reinforcement is provided per the requirements of ACI 318-08 to two anchor connections located in the plastic hinge zones of a reinforced concrete wall.  The concrete wall will be subjected to cyclic loading on the top while the anchor connections will be simuously load to the same level of loads. 


Phase IV specimen
   The tests in Phase II and Phase III of the NEES-Anchor project indicated that the key role of reinforcement, in addition to carrying the forces from the anchors, is to protect concrete around the anchors from splitting, breaking out, and crushing. This understanding has led to alternative designs and detailing for the anchor reinforcement. The focus of the Phase V tests are to 1) identify the key parameters for the desired performance of anchors in plastic hinge zones; 2) observe the beahavior of anchors in plastic hinge zones with local confinement; and 3) verify the implemented anchor reinforcement detailing.
Phase III Experimental Program:

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   Phase V tests have six specimens: three for anchors in tension and the other three for anchors in shear. The specimen design is shown on the right. The column specimen has a cross section of 12 x 12 in. and a height of 61 in. Nine No. 5 bars (Grade 60) are provided as the longitudinal reinforcement. The test anchors, installed 8 in. from the base of the columns, are 3/4 in. diameter ASTM A193 Grade B7 threaded rod (fy=105 ksi and fut=131 ksi) and a plate washer (1.5x1.5 in.) and a hex nut welded to the end.  Ready-mixed concrete targeting at 4000 psi was used, and cylinder tests using three batches of three 48 in. cylinders tested at 28 days showed an average compressive strength of 5700 psi.    
  The test anchor, if fully developed, will take an ultimate tension load of 43 kips. Two anchors are loaded in shear specimens, resulting in a similar ultimate load, which was designed within the loading capacity of the hydraulic actuator. The required anchor reinforcement for the 3/4-in. anchors was provided using four No. 4 bars, and implemented using two No. 4 closed stirrups in the column located 2-in. from the test anchor.  In addition to the longitudinal bars in the column, four No. 4 U-shaped hairpins were placed near the anchor in the vertical plane. These hairpins are expected to confine the concrete from flexural cracking.  
Phase V Specimens
    The test setup and the instrumentation plan is shown below. Seven strain gages were installed on the longitudinal bars of the columns, especially near the test anchor. These strain gages will define the actual plastic hinge zones developed in the column.  Column displacements are measured using string pots while the anchor displacements are measured using spring-loaded LVDT's.
Phase V test setup
Finite Element Analyses:

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   Finite element analyses have been conducted to verify the loading protocols for the columns.  The column will be subjected to reversed cyclic displacement at 0.2 in. (elastic behavior), 0.4 in. (first yielding), 0.8 in., 1.2 in., 1.6 in., 2.4 in., and 3.2 in.  For the test of Specimen T1, the test anchor will be subjected to monotonic tension till failure after the column loading is completed. Depending upon the observed behavior, the test will be repeated using Specimen T2, and the test anchor in Specimen T3 will be subjected to cyclic loading.  If concrete near the test anchors is damaged significantly such that the full anchor steel capacity is not developed, the rest two specimens will be tested with the columns to be subjected to lower levels of cyclic displacements.
Phase-V column behavior
Test Summary:

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T1 (accidental high-speed loading)
(Front face) 02/08/12
T2 (monotonic tension, at zero position)
(Side face, anchor on the left) 02/23/12
T3 (cyclic tension, at -8Dy)
(Side face, anchor on the left) 03/03/12
S1 (monotonic shear, at zero position)
(Side face, back anchor broken) 03/10/12
Anchor shaft was exposed after the 1.5-in. cover spalled, leading to combined bending and shear on the anchor.
S2 (cyclic shear, with column loaidng)
(Side face, front anchor broken) 03/15/12
Again, anchor shaft was exposed after the cover spalled, leading to combined bending and shear on the anchor.
S3 (with FRP wrapping)

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