International Concrete Abstracts Portal

Column plays an important role as a vertical and lateral load carrying member in building structures. In order to reduce the dimension of the lower-story columns of high rise buildings, the use of high strength materials has become an effective measure. However, most of the available experimental data on the high strength columns came from the testing of reduced-scale specimens due to the limited testing capacity in laboratory. This research reports an experimental study of 5 full-scale specimens (600×600 mm [23.6×23.6 in.]) of columns subjected to quasi-static cyclic loading under high axial load up to 0.67Ag fc˜ . The main purpose of this research is to study the confinement effect and lateral drift capacity of reinforced concrete columns made of concrete with fc˜ > 80 MPa [12 ksi], longitudinal steel with f y = 685 MPa [100 ksi] and transverse reinforcement with f yt = 785 MPa [114 ksi]. Finally, based on test observations, a design equation on the seismic confinement of column is recommended.

More than 500 high-rise RC buildings with height exceeding 60m (197feet) have been built since early 70's in Japan. The number of stories sometimes exceeds 50. Use of base isolation systems or vibration control devices in high-rise RC buildings has significantly increased since 1995 Kobe Earthquake. Ultra-high-strength materials have also been used in such buildings. The specified concrete strength of 150 MPa (21800psi) or higher is currently practiced and SD685 deformed bars of 685 MPa nominal yield strength are used as the main reinforcing bars. Such buildings are subjected to intensive large axial and lateral loads in case of sever earthquakes and strong winds, particularly at their lower stories where exterior columns experience varying high-axial loads shifting from compression to tension. Furthermore, as concrete strength increases, fire resistance decreases and cracking behavior of RC members changes which affects the structural performance. A lot of experimental studies with regards to such columns and subassemblies have been carried out to investigate their structural performance and to establish appropriate design methods. This paper presents some design issues related to the application of high-strength materials to RC columns or subassemblies. It also emphasizes recent research works and design methods for the application of ultra-high-strength concrete for high rise RC buildings.

This paper experimentally investigates the behavior of high-strength reinforced concrete columns confined using a new cross spiral confinement technique. The new cross spiral confinement technique uses two opposing spirals to confine circular concrete columns enhancing their strength and ductility, and increasing spiral spacing to facilitate the flow of fresh concrete. The new confinement arrangement is experimentally evaluated and compared to the conventional single spiral confinement arrangement. Twenty-one circular high-strength reinforced concrete columns with four different spiral spacings and longitudinal reinforcement ratios were tested under monotonic axial loading. Seven specimens utilized the conventional single spiral confinement, used as control specimens, while the remaining specimens utilized the new cross spiral arrangement. The new arrangement enables an increase in spiral spacing while maintaining the same volumetric confinement ratio as the conventional. Alternatively, doubling the volumetric confinement ratio without violating ACI 318-081 requirement for minimum spiral spacing. The study showed that the new cross spiral arrangement with the same volumetric confinement ratio as the conventional spiral obtained similar ultimate stress values while it attained about a twenty percent increase in ultimate displacement. The cross spiral confinement using twice the volumetric confinement ratio greatly outperformed the conventional spiral in all aspects.

The use of high-strength longitudinal reinforcement—having a specified yield stress between 80 and 120 ksi—in concrete elements has been shown to allow for the use of lower reinforcement ratios leading to reductions in fabrication costs and congestion. This is especially relevant to structures built in seismically active regions in which reinforcement ratios are typically higher than in structures in regions with a lower seismic risk. Recent research initiatives related to the use of high-strength reinforcement have largely been focused on the response of isolated elements instead of the response of building frames. This paper presents results from a suite of numerical analyses designed to investigate the effects of high-strength longitudinal reinforcement on overall building frame response. Using steel with a higher yield stress allows for reductions in reinforcement ratio. Those reductions, in turn, cause a decrease in post-cracking stiffness. To investigate the effects of this relative softening, a series of multiple-degree-of-freedom models were proportioned to represent idealized frames reinforced with high-strength steel. Nonlinear dynamic analyses were conducted to estimate their response to a set of seven strong-motion accelerograms. It is shown that increases in drift demands related to the use of high-strength steel range from negligible to approximately 20 percent, depending on a number of factors including base shear strength, ground motion intensity, and extent of high-strength steel use. This increase in drift demand 1) is modest compared to the uncertainties associated with predicting ground motion intensities and 2) needs to be confirmed through experiments.

In an attempt to provide consistently conservative yet reliable estimations of flexural and axial strengths of concrete columns, various stress block parameters have been proposed within the last two decades. The fact that flexural and axial strengths of many tested high-strength concrete columns were overpredicted by the current ACI 318 stress block parameters is the primary motivation behind all of the proposals for stress block parameters. Chapter 10 (Flexure and axial loads) of ACI 318-11 introduces the concrete stress block parameters and provides design formulas for calculating flexural and axial strengths and bearing strengths. The stress block is also used for various design applications in other chapters of ACI 318. Those chapters include Chapter 18 (Prestressed concrete), Chapter 22 (Structural plain concrete), Appendix A (Strut-and-tie models) and Appendix B (Alternative provisions for reinforced and prestressed concrete flexural and compression members). All ACI 318 design implications stemming from any suggested changes for the concrete stress block parameters needs to be examined holistically. This paper provides a comprehensive examination of various stress block parameters. Flexural and axial strengths predicted by different stress blocks are compared with experimentally-obtained strengths from 224 column tests. Normalized P-M interaction curves are developed for this purpose. In addition, the impact of change of stress block parameters on other design expressions is examined. They include the bonded tendon stress in prestressed concrete and the compressive stress of bottle-shaped struts in strut-and-tie model.