Behavior and Capacity of Headed Reinforcement

ACI Structural Journal, Jul/Aug 2006 by Thompson, M Keith, Jirsa, James O, Breen, John E

Results from two studies of headed reinforcement have been combined to develop design recommendations for headed reinforcement. Important aspects of headed bar anchorage are summarized, and a model for determining the anchorage capacity of headed reinforcement is presented. The model includes two components that contribute to the total bar stress, head bearing, and bond, each calculated separately. The proposed model is compared to data from previous headed bar studies. Proper use of the model and areas for additional study are discussed.

Keywords: anchorage; lap splice; reinforcement.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

The Texas Department of Transportation (TxDOT) funded a program to study the feasibility of headed reinforcement in bridge structures. Under this project, two basic anchorage conditions for headed reinforcement were studied: 1) anchorage at compression-compression-tension (CCT) nodes; and 2) anchorage in lap splices. Results from these test programs have provided the basis for a general design approach for ensuring adequate detailing of headed reinforcement.

RESEARCH SIGNIFICANCE

Data from test programs of CCT nodes and lap splices are compared. Similarities in observed behavior support the proposal of a single design model for headed bar anchorage. Recommendations for calculating the anchorage capacity of headed reinforcement have been developed by combining the results of the two studies. These recommendations were used to analyze results from previous studies of headed reinforcement and the understanding of previous test results was enhanced.

TEST PROGRAMS

CCT nodes

A typical CCT node specimen is shown in Fig. 1. The basic CCT node configuration was unconfined (no local reinforcement aside from the tie bar) and anchored only a single tie bar, thus reducing the number of parameters influencing behavior of the node. The major variables of the test program included head size (indicated by relative head area, A^sub nh^/A^sub b^ = 0.0 to 10.4), bar size (25 or 36 mm diameter bars), and strut angle (30, 45, or 55 degrees). The test program also included a few companion tests with hooked bars or stirrup confinement. Further details of this test program are documented in References 1 and 2.

Lap splices

A typical lap splice specimen is shown in Fig. 2. As with the CCT nodes, the lap splice configuration was kept simple in order to reduce the number of influencing parameters. Lap zones in most specimens were unconfined and the basic arrangement of the lapped bars was varied little from test to test. The main variables of the test program were head size (A^sub nh^/A^sub b^ = 0.0 to 4.7) and splice length (L^sub s^ /d^sub b^ = 3.0 to 14.0). Additional parameters, which were studied in only a few tests, included contact versus non-contact lap arrangements, bar spacing (6d^sub b^ or 10d^sub b^), and confinement details. Further details of this test program are documented in Reference 3.

In both test programs, three basic head types were used, representing the variety in headed reinforcement that was commercially available at the time of testing. The three basic head types are shown in Fig. 3. Within the scope of the test programs, there was little observed difference in behavior between head types when equal bearing areas were provided. The recommendations contained in this paper are intended to apply to each of these head types.

TEST RESULTS

Stages of anchorage

Headed bar anchorage is provided by a combination of head bearing and bond. Initial anchorage is carried primarily by bond. As additional stress is applied to the bar, bond achieves peak capacity and begins to decline. As the process of bond deterioration occurs, bar anchorage is transferred to the head, causing a rise in head bearing. The anchorage capacity at failure is provided by a combination of peak head bearing and reduced bond. This pattern of behavior was observed in both CCT node and lap splice tests (Fig. 4). Using this understanding of headed bar anchorage, a model for anchorage capacity was developed based on separate models for the head bearing and bond components.

Model for bearing capacity

The proposed model for head bearing capacity is based on existing ACI4 code equations for bearing strength and side blowout capacity. A database including more than 500 tests of anchor bolts, headed bars, and rigid bearing plates was used to create the model. Development of the model is documented in Reference 2. The equations used in the model are listed below

... (1)

... (2)

The n^sub 5%^ term adjusts the model such that only 5% of tests from the database had a capacity less than that calculated by the model. A value of 0.7 is recommended. Without the n^sub 5%^ term, the model calculates the mean capacity of the test data. The model does not apply if the anchorage length L^sub a^ becomes too short. A minimum anchorage length of 6d^sub b^ is suggested.3

Equation (1) can be reformatted to calculate the bar stress provided by the head f^sub s,head^