Tests on Half-Scale, Two-Story, Two-Bay, Moment-Resisting Hybrid Concrete Frames

ACI Structural Journal, Sep/Oct 2009 by Xue, Weichen, Yang, Xinlei

The hybrid concrete frames investigated in this paper consisted of composite concrete beams (composite beams) and cast-in-place (CIP) concrete columns. Two, two-story, two-bay, hybrid concrete frames were tested under cyclic and monotonic loading, respectively. Behavior of the specimen was evaluated in terms of failure pattern, failure mechanism, stiffness degradation, energy dissipation, and displacement ductility. Test results revealed that both specimens exhibited a mixed sidesway mechanism and exhibited full and stable hysteresis loops. The maximum load-carrying capacity under cyclic loading was approximately equal to that under monotonic loading. Both specimens behaved in a ductile manner, and the displacement ductility under monotonic loading was approximately 10% larger than the corresponding value under cyclic loading. A four-linear restoring force model was developed through analysis of the test results.

Keywords: cyclic loading; ductility; energy dissipation; hybrid concrete frame; restoring force model; stiffness degradation.

INTRODUCTION

Precast concrete could provide high-quality structural elements, construction efficiency, and savings in time and overall cost of investment. However, post-earthquake field investigations of precast concrete structures after large earthquakes, such as the 1996 Adana-Ceyhan Earthquake,1 the 1999 Marmara Earthquake,1 and the 2008 Wenchuan Earthquake, showed that many precast concrete structures were observed to have failed and to have been damaged due to failures of connections.1 The details of connections have significant effects on the seismic behavior of frame structures. Thus far, only a few experimental investigations have been conducted to investigate the behavior of precast concrete moment-resisting frames. Bo et al.2 studied the seismic behavior of precast concrete frames. It was reported that the specimens behaved in a ductile manner, and the maximum load-carrying capacity and the energy dissipation capacity of the precast concrete frames were very similar to those of the control specimen. A test of precast concrete frames under cyclic loading was carried out by Xu et al.3 The results revealed that the precast concrete frame exhibited a mixed sideway mechanism and behaved in a ductile manner. A five-story precast concrete frame was tested at the University of California at San Diego.4 The authors concluded that the seismic behavior of the test structure was very satisfactory and the damage in the frame direction was much less than the expected damage for an equivalent cast-in-place (CIP) concrete structure. A half-scale, two-story precast concrete building incorporating a dual system was investigated in Mexico. It was found that the walls of the test structure controlled the force path mechanism and significantly reduced the lateral deformation demands.5

The existing research on precast concrete structures can be summarized as follows:

* Existing studies mainly focused on behavior of the frames consisting of precast concrete beams and precast concrete columns. However, investigations on the hybrid concrete frames composed of CIP concrete columns and composite concrete beams are very scarce.

* Very few experimental investigations on precast concrete frames under horizontal monotonic load have been carried out.

* There are some general provisions about designing precast concrete frames and no detailed provisions for the frames investigated in this study in IBC 2006,6 EC 8,7 NZS 3101-2006,8 and the Chinese code for seismic design of buildings.9

At present, the frame type investigated in this paper was implemented in some pilot projects in China. Nevertheless, confidence in the wide application of this frame type in regions of high seismicity is mainly based on laboratory testing for the validation of their performance. The objective of this paper is to investigate the behavior of hybrid concrete frames, which were composed of CIP columns and composite beams, under cyclic and monotonic loading, respectively.

RESEARCH SIGNIFICANCE

Precast concrete could provide many advantages, such as good quality control, simple and fast erection, cost effectiveness, reduction of shore, and saving formwork. At present, there are many real estate enterprises wanting to implement this frame type in China. However, there are scarce data available on the behavior of this structure type. The results of this investigation could enrich the data available on the behavior of hybrid concrete frames and contribute to extend the market of the precast concrete structures in seismic zones.

EXPERIMENTAL INVESTIGATION

An experimental program consisting of the tests of half-scale, two-story, two-bay hybrid concrete frames under cyclic and monotonic loading, respectively, was conducted to evaluate the behavior of moment-resisting hybrid concrete frames. Both test specimens were constructed and tested at the Tongji University Structural Engineering Laboratory.

Description of test specimens

The test models were from a prototype frame in Shanghai, China. The prototype building was a rectangular six-story building with plan dimensions of 29.51 x 78.69 ft (9 x 24 m). The column spacing was 13.11 ft (4 m) and the story height was 9.18 ft (2.8 m). The test specimens represented an inner column strip along the S-N direction of the prototype structure (refer to Fig. 1). Both specimens were designed according to the strong column/weak beam seismic design philosophy and the Chinese code for seismic design of buildings.9 Both specimens were against shear failure because shear failure was brittle and there might be little visible warning in comparison with flexural failure. In both the frames, beamcolumn joints had sufficient strength to prevent the joints from failure prior to the beams and columns. In addition, the specimens were detailed in conformance with the requirements of ACI 318-05.10 The prototype frame was reduced to a half-scale model due to experimental space constraints. When the scale factor was 1/2, the size effect was not apparent and the model did not lose important characteristics in the behavior of the prototype. The prototype was scaled according to the geometric similarity law. It is essential to maintain the similitude law in the material properties between prototype and model reinforcement. It was difficult, however, to make the cross section of the model reinforcement conform exactly to the similitude law. Thus, the yield forces, rather than yield stresses, were selected as the target to be achieved. The specimen under cyclic loading was named HCF-C, and under monotonic loading was named HCF-M. Both specimens consisted of composite beams and CIP columns, and the composite beams were composed of precast beams, precast slabs, and CIP slabs. Both specimens had identical dimensions and reinforcement details, as shown in Fig. 2. The layout of precast beams and slabs is depicted in Fig. 3. Both frames were constructed in a vertical position. The detailing and construction technology were in accordance with those of the prototype structure. All precast elements were prefabricated in a large-scale enterprise specialized in manufacturing precast concrete.