SEISMIC PERFORMANCE OF FULL-SCALE REINFORCED CONCRETE BEAM-COLUMN JOINTS

Transcription

1 SEISMIC PERFORMANCE OF FULL-SCALE REINFORCED CONCRETE BEAM-COLUMN JOINTS Yoshiko Tsunehara Home institution: University of California, Los Angeles REU host institution: University of California, San Diego REU faculty advisor: Professor Tara C. Hutchinson PhD mentor: Barbara Chang

2 Abstract Testing full-scale models of reinforced concrete beam-column joints requires several months of tremendous labor in order to complete the construction of each specimen. Powell Structural Laboratories provide a state-of-the-art center for full-scale research, including the equipment and skilled labor necessary for complex, nonstandard projects. In addition to the beam/column subassembly, multiple other projects are simultaneously ongoing in the laboratories. A Pankow/Industry sponsored testing series on full-scale planar models of reinforced concrete beam-column joints is the primary project for Powell Structural Systems Laboratory. The models represent typical interior joints of mid to high-rise office buildings under seismic loading. Each specimen consists of a fifteen-foot column and two beams, with varying materials and connection designs. Other projects undertaken at Powell Laboratories include seismic testing of laminated glass panels, generation of a ground truth image, and static testing of a composite reinforced box girder. Frame assembly and adequate placement of instrumentation is essential for accurate testing of the glass panels, which will occur via uniaxial shake table tests. A ground truth image represents damage to structures shown in satellite imagery taken before and after a major seismic event has occurred. The composite reinforcement testing requires an incredible labor force to construct a large scale, two-cell box girder. 2

3 Reinforced Concrete Beam-Column Joints: Overview The Pankow/Industry sponsored testing series on full-scale planar models of reinforced concrete beam-joints is being conducted in order to provide experimental data for the support of a rational design procedure incorporating high strength concrete. Initially, designs for three specimens were developed, but due to serious deficiencies in the testing of the first specimen, a fourth specimen has been planned. The primary design considerations for the initial three specimens are summarized in Table 1. Multiple details were added and altered as construction occurred, in order to economically test various systems. Table 1. Summary of primary design considerations for each specimen. Test No. Design 1 High strength column; DDC (ductile rod) system; cast in-place 2 High strength column; DDC (ductile rod) system combined with post-tensioning; pre-cast 3 High strength (MMFX) steel; cast in-place The first two specimens have already been completed and tested, as shown in Figures 1 and 2. Construction of the third specimen is currently underway. Figure 1. Pankow #1 test assembly prior to testing. 3

4 Reinforced Concrete Beam-Column Joints: Materials & Assembly Specimen assembly involves considerable time and labor. All specimens consist of a full-scale interior t-joint beam-column configuration, tested in a simply-supported frame, shown clearly in Figure 2. A heavy pin above the support footing allows the column supports to rotate but not translate. Each specimen is approximately 14.5 feet in length and extends 11.3 feet in either direction. The construction of Pankow #3 has been on going from mid-june and will continue through August. Several steps are involved, including placement and removal of formwork, positioning and tying of rebar, strain gage application, and concrete pouring. One of the unique features of Pankow #3 is that the connection between beams involves a continuous coupler joint system, while the previous specimens employed the use of threaded Dywidag rods. The ends of the six longitudinal bars on each beam are threaded so as to screw into a 5'8" connecting piece on either side. The centerpiece of the coupler includes a 5" thick extension to provide bearing capacity, and theoretically creates a continuous connection between beams. The rebar insertion after swaging covers a length of 5 to 6 inches. The actual assembly of the coupler requires intense physical strength and tools. A diagram of the coupler is shown in Figure 3. Figure 2. Pankow #2 test assembly prior to testing. 4

5 Figure 3. Detailing of joint coupler between beams. In terms of materials, while the first two specimens called for high strength 10,000 psi concrete, only 6,000 psi concrete was poured for Pankow #3. Alternatively, high strength MMFX steel was used for both longitudinal and transverse rebar. High strength steels are not only three times stronger but also more ductile and rust resistant than traditional rebar. In order to encourage their development, this test should demonstrate that high strength steels allow for a considerable improvement in behavior. The connection of the joints went as follows. The rebar skeleton for the column, shipped from an outside manufacturer, lay inside its formwork with an already tied beam on one side. The longitudinal rebar for the other beam was then inserted into the formwork; the coupler was screwed onto the tied beam, then the singular bars were screwed into the coupler. Difficulties came with fitting the rebar into the small circular openings in the formwork, and with the considerable strength required to screw in the rebar. The stirrups were then tied onto the remaining beam. Strain gage application came after completing the rebar cage. Wires at the end of the gage must be prepped by bending, twisting, and soldering the ends. The rebar area for gage application was ground, cleaned, and roughened. 60 gages were used, with 5 mm length to accommodate large strains while maintaining accuracy. Each gage was glued and protected with m-bond and mastiff tape, though one hour must be allowed between layers of m-bond, and eighteen hours before the tape is placed. After the gages are all in place, they must then be labeled and routed along the rebar with zip-ties in order to protect the wires from the vibrator during the concrete pour. Thus 5

6 gage application is a tedious and time-consuming effort that must be taken into consideration when scheduling the project. Concrete pouring is perhaps the most grand and observable part of the construction process; it requires many hands yet only takes a few hours. The project investigator and industry officials arrived for the column pour, but unfortunately a stirrup was misplaced. The correction caused the pour to be offset by several days. Such set-backs are typical, and this one resulted from the fact that the diagram showing the stirrup placement appeared to show the stirrup on top of the longitudinal beam rebar; the staff was not aware that the side was important to the design. Thus checks of the design transferred to the construction are emphasized. The properties of the concrete include 6,000 psi strength as previously mentioned, with 1 inch aggregates and a water/cement ratio of 38%. Admixtures of WRDA 64, fly ash, and 1% air are also used. The slump test, standard at any concrete pour whether in research or actual construction, was designed for a 4-inch slump and was satisfied with an actual slump of 3 ¾ inches, as shown in Figure 4. Figure 4. Slump test for specimen column. In addition to the slump test, twenty-three 6 inch diameter cylinders were prepared for strength testing. Groups of three were planned to be placed under compressive testing at specified intervals, in order to generate a curve of time verses strength. Each cylinder is filled in thirds, 6

7 then tapped down with a rod twenty-five times between each later, with the excess concrete rolled off of the top so that a labeled sealing cap can be placed on. The column itself was accessed directly by the concrete mixing truck, and the concrete was poured and carefully vibrated into the formwork. For aesthetic and orderly reasons, the final step was to trowel the surface until smooth. The formwork remained in place for several days for security, before being removed. Because of the pace of construction, during actual building construction formwork is often removed the day after the pour. The process was repeated several weeks later for the beams, though pouring occurred after the specimen was erected. Removing the formwork was an intensely laborious but self-rewarding event. During the testing, every crack will be outlined in red or black marker; to facilitate this, the concrete surface was painted white. This required wearing protective harnesses and riding the man lift, as the erect specimen stands far over ten feet, the greatest height before a harness is required for safety. Testing for Pankow #3 was scheduled for September 18th, and was completed on September 19th. For the previous specimen, testing took both fifteen-hour and eleven-hour days, since testing must be paused each time new cracks are incurred so that they may be marked. This same process was repeated for Pankow #3, as seen in Figure 5. A goal of late October has been set for the fourth specimen, though funding must still be acquired. Ultimately, both industry and research benefit greatly from this and future projects running in similar coordination. Figure 5. Pankow #3 after testing. 7

8 Conclusions Despite the difficulties involved in construction and testing of massive specimens, full-scale models provide accurate reactions and confident analytical results. Undergraduate and graduate students greatly benefit from the unique experience of working on actual construction level specimens, by combining theory and design with hands-on application. Aiding in construction for a ten-week period was a rewarding and insightful experience, and an excellent application of knowledge gained through academia. Special thanks to Professor Tara Hutchinson and Barbara Chang for facilitating the PEER 2007 internship at the University of California, San Diego, and providing the enriching exposure to full-scale research. References Englekirk, B. and Hutchinson, T.C. and Chang, B. (2007). "Pankow Report: Pankow Sponsored Beam/Column Subassembly Test Specimen #1 (Pankow #1)." University of California, San Diego, April