Precast Bent System for High Seismic Regions: Laboratory Tests of Column-to-Drilled Shaft Socket Connections

SUMMARY AND CONCLUSIONS

Summary

A new system is proposed for connecting drilled shafts to precast columns in bridge bents. It is adapted from the column-to-footing "socket" connection proposed for spread footing by Haraldsson, and it consists of embedding a precast column into the cast-in-place transition region of the drilled shaft.(10) The purpose is to facilitate rapid on-site construction through the use of prefabricated elements. The system is suitable for use in seismic regions.

Two drilled shaft specimens were fabricated and tested at the University of Washington. This section of the document summarizes the construction procedure, the design methodology, the test specimens and performance, and analytical models used to study the connection.

Construction Sequence

The field construction sequence is as follows:

A precast column is cast. The surface is roughened in the region where the column will be embedded in the cast-in-place drilled shaft.
The hole for the shaft is bored, the reinforcing cage is placed, and the shaft is cast up to the bottom of the transition region (approximately 10 feet below grade).
The precast column is positioned, leveled, and braced in the drilled shaft.
The transition region (approximately the top 10 feet of the shaft) is cast around the precast column.

Connection Design

A shaft and column system prototype was designed according to the AASHTO LRFD Bridge Design Specifications, AASHTO Seismic Guide Specifications, and the WSDOT BDM.(11,12,13) Reduced-scale test specimens were then developed from the prototype.

The prototype precast column was designed to have a reinforcement ratio of approximately 1 percent. The transverse reinforcement in the column was defined by requirements for shear and confinement of the concrete core.

The column longitudinal reinforcement was equipped with anchor heads, instead of using the conventional detail of bending the longitudinal bars outwards into the foundation. This configuration has several benefits. It reduces the development length of the reinforcement and, thus, indirectly reduces the embedment length of the column in the shaft. It has better seismic performance, because the force transfer between the column and shaft is more direct. This design also makes fabrication, transportation, and erection safer and easier.

The surface of the column was roughened where it was to be embedded in the drilled shaft. The details of the roughening were the same as those specified in the WSDOT BDM for roughening the ends of prestressed concrete girders. Load transfer at the interface was designed using the AASHTO LRFD shear-friction design procedure. In the embedded region, the column section was an octagon, circumscribed within the circular section of the main column. This was done to facilitate the forming of the roughened surface, which used wooden strips.

The embedment length of the column in the drilled shaft was defined by requirements for splicing the shaft and column bars. The splice is by definition a non-contact splice, for which the WSDOT BDM provides design requirements. The column bars were larger than the shaft bars, so they would normally control the splice length.(5) However, the heads on the column bars improve their anchorage so that the shaft bars, which had no such heads, in fact controlled the splice length.

The shaft was designed as a capacity-protected element to ensure that the hinge would form in the column and not in the shaft. The scale of the system was chosen so that the ratio of shaft diameter/column diameter was as small as possible according to the WSDOT BDM, so that test specimens represented the most critical conditions. That choice led to a prototype system of a 6-foot-diameter column and a 9-foot-diameter shaft, which were represented at 28 percent scale in the laboratory specimens by a 20-inch-diameter column and a 30-inch-diameter shaft.

The shaft spiral was designed according to WSDOT BDM requirements for non-contact lap splices in conventional cast-in-place drilled shafts. The spiral was terminated by three turns of the spiral.

Test Specimens

Two test specimens were built. The scale factor (1/3.6, or 28 percent) was chosen to match the 20-inch-diameter of column specimens tested by previous researchers.(10,14)

The only difference between specimens DS-1 and DS-2 was the amount of spiral in the column-to-shaft transition region, which was reduced by half in DS-2. The goal was to promote failure in the shaft transition region in specimen DS-2, to develop an understanding of the load transfer mechanism there.

In both specimens, a cast-in-place base was built monolithically with the transition region to attach the specimen to the testing rig. The specimens were heavily instrumented.

Test Performance

Quasi-static, cyclic lateral load tests were performed to evaluate the seismic performance of the two specimens.

In each test, the specimen was subjected to a constant, unfactored axial dead load value of 159 kips, accompanied by a cyclic displacement-controlled lateral excitation. The displacement history was a modification of a loading history for precast structural walls recommended in NEHRP.(19)

The response and mode of failure of each specimen was the same as had been anticipated during design. Specimen DS-1 was controlled by column behavior. Failure occurred by plastic hinging in the column, while the connection region in the shaft remained largely undamaged. Testing was stopped when almost all the column longitudinal reinforcement had fractured. By contrast, specimen DS-2 was dominated by deformations of the shaft. The failure mode was prying action of the shaft in the transition region. Testing was stopped when all the spiral reinforcement in the shaft had broken.

Response Data

The measured data included loads and displacements, deflection and rotation of column and shaft, strain in the column and shaft longitudinal reinforcement, and strain in the shaft spiral. The measured data confirmed the observed responses and modes of failure.

Analytical Model

Load transfer within the transition region was modeled using a three-dimensional strut-and-tie mechanism. This model is necessarily a simplification of the true behavior, but it provides reasonable agreement with the experimental results and helps to identify the probable load paths.

Conclusions

From the results obtained in these tests, the following conclusions can be drawn for the behavior of the column-to-shaft connection:

Provision in the precast system of the amount of spiral reinforcement required for conventional drilled shafts of the same dimensions protects the shaft and causes failure to occur by plastic hinging of the precast column, as desired. Because the test specimens had the smallest possible shaft/column diameter ratio, and the shortest possible embedment length, this conclusion will hold for all permissible shaft and column combinations.
Use of half of the conventional amount of shaft spiral causes failure to occur in the shaft, by prying action of the concrete shell surrounding the precast column. The test specimens contained no external confining steel shell. If one were used, it might provide some of the benefits of additional spiral and force the failure back into the column.
Mechanical anchor heads are needed at the ends of the column longitudinal reinforcement to ensure hinging in the column without anchorage failure, especially if the large bar system proposed by Pang et al. is used.(5) The need for heads on the shaft bars is apparent from the strut-and-tie model, but it was not clear from the test results.
The spiral at the very top of the shaft is subjected to high tension during lateral loading. The strut-and-tie model shows that the spiral resists prying failure of the shaft, and that conclusion is supported by the high strains recorded in the spiral in specimen DS-1 and the fractured spiral in specimen DS-2. However, the spiral was in both cases distributed uniformly up the transition region, with additional turns at the termination point at the top. Further testing is needed to determine whether a non-uniform distribution of the spiral, with the majority at the top, would provide better response.
The WSDOT requirements for non-contact lap splices do not provide satisfactory agreement with the experimental results.
Two questions are raised to obtain a consistent strut-and-tie model.
- What is the value of inclined angle, , of struts which transfer tensile force from column reinforcement to shaft reinforcement? This value has a large effect on the amount of spirals required in the splice. A bigger value of leads to a smaller amount of spirals. However, the size of these struts is limited by the small space between the column reinforcement and shaft reinforcement. Thus, a large is not plausible.
- What role, if any, does the tensile strength of the shaft concrete play in the confinement of the splices? It appears to have played an important role in the tests.
If the tension strength of the shaft concrete is excluded, the analytical model shows that the amount of spiral steel must be increased by a factor of 2.5.

Recommendations for Further Research

This study demonstrated the fundamental behavior of the column-to-shaft socket connection. However, further work is needed in the following areas:

Two more tests are recommended to determine the optimal distribution of the transverse reinforcement at the top of the shaft. One test specimen should be the same as specimen DS-1 except that the shaft spiral should be anchored in such a way that additional spiral turns are not used. Welding is a possibility. The other test specimen should be designed with no distributed spiral in the shaft and a concentrated ring of spiral at the top of the shaft. If the first specimen fails in the shaft and the second one fails in the column, the top of the shaft will have been proven to be the optimal location for the spiral.
Experiments should be conducted to determine the need for mechanical anchorages or hooks on the top of the longitudinal shaft bars. The strut-and-tie model suggests the need for anchorage devices of some struts, but the test specimens performed well without them.
The WSDOT requirements for confinement of non-contact splices in drilled shafts should be re-examined. In particular, the k-factor of 0.5 should be evaluated critically.
The contribution of the tensile strength of concrete in the confinement of non-contact splices should be examined. One test specimen should be the same as specimen DS-1 except that the transition concrete is separated in two parts. Two plastic pieces can be placed on the north side and the south side to isolate two parts of the transition concrete so the confinement would be based on only spirals.
Experiments should be conducted to determine the three-dimensional force transfer mechanism between reinforcing bars in the splice. The bars should be heavily gauged. Based on the force transfer mechanism, a consistent value of inclination angle of the struts should be proposed for design.