Seismic cyclic loading test of SRC columns confined with 5-spirals

October 29, 2008

This study is reported in "Science in China Series E: Technological Sciences, Vol. 51, No. 5, pp. 529-555, May, 2008." ( This research project was sponsored by Ruentex Construction Group, Taiwan, China. Professor Frank C.C. Weng with the "Civil Engineering Department of National Chiao Tung University" acted as the project leader.

As shown in Figure 1, the "5-spirals steel cage" is a device of five interconnected spirals with a large spiral at the center and four small ones at the corners of the rectangular column. The innovative concept was put forward by Dr. Samuel Y. L. Yin, CEO of the Ruentex Group.

Traditionally, the lateral reinforcements for SRC columns are largely the rectangular hoops, as shown in Figure 2. As it takes much time and work to bond the hoops, the use of spirals will substantially reduce the time and manpower. In addition, spirals are continuous, unlike the hoops which are separate and need hooks for fixing at the corner. So the spirals enjoy better confining effect and save a considerable amount of steel as they do not need hooks.

However, although the confining effect and economic benefit of the conventional spirals are both superior to the rectangular hoops, the spirals fail to provide effective confinement to concrete at the four corners of the rectangular columns. That is why the conventional spirals are seldom used for the rectangular column in practice.

The 5-spiral effectively solves the problem of unconfined concrete at the corners of the rectangular columns. The innovative configuration breaks through the application restriction of the spirals to circular cross-sectional columns, overcomes the shortcomings of spiral application to rectangular columns and facilitates sound confinement of spirals to concrete at the corners of the column. Moreover, as the 5-spirals are manufactured with automatic machines in the factory, they are expected to substantially reduce manpower and shorten the construction period. Being most "cost-effective", the 5-spirals are suitable for precast construction.

The research team had previously completed a series of axial compression tests on 5-spiral SRC short columns and found that the 5-spirals had sound concrete confinement, could improve the strength and toughness of the columns. In an attempt to discuss the seismic resistance of the 5-spiral SRC column, the research team conducted cyclic loading tests on four full-scale SRC columns (as shown in Figure 3).

The design of SRC structures is aimed at effectively combining the structures of steel (S) and reinforced concrete (RC) so that it may have the advantages of the two structures. In a eugenic perspective, a properly designed SRC structure is like a "eugenic baby" as an outcome of the "marriage" of the S structure and the RC structure.

In addition, this paper also proposed a new design equation called the "Weng's Formula" for the design of the lateral reinforcements of SRC columns. Weng's Formula considers "the contribution of the flanges of steel section in the SRC column to concrete confinement" which significantly relaxed the consumption of confining steel in the SRC columns.

As shown in Figure 4, for the SRC columns which underwent seismic cyclic loading tests, their hysteretic loops were very rich, the drift angles of the four SRC columns reached 6.0% radian, indicating rectangular SRC columns confined with 5-spirals have excellent energy dissipation and seismic resistance.

Finally, observations from the tests indicated that the strengths of the SRC columns were able to sustain and stay high without tangible decline trend throughout the entire cyclic loading process. This phenomenon suggests that the SRC columns confined with 5-spirals are capable of effectively resisting the horizontal shear force caused by strong earthquake without rapid collapse. This feature is of obvious significance to the safeguarding of lives and properties at the outbreak of major earthquakes.

 Weng, C.C., Yin, Y.L., Wang, J.C. and Liang, C.Y., "Seismic Cyclic Loading Test of SRC Columns Confined with 5-Spirals," Science in China Series E: Technological Sciences, Vol. 51, No. 5, pp. 529-555.

Science China Press

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