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.

Practicing engineers increasingly favor the use of high-strength concrete and reinforcement in their design. The paper included in this CD present results from recent research studies and examples of practical applications and use of high-strength concrete and steel reinforcement in recent projects. This CD consists of 10 papers that were presented at a technical session sponsored by ACI Committee 441 at the ACI Convention in Toronto. Ontario, Canada in October 2012. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-293

The advancement of material technology has led to higher grades of both concrete and steel strengths. High-strength concrete and steel can decrease the size of structural members and increase the available floor area. In addition, it can decrease the consumption of aggregate and steel, promoting environmental sustainability. This research investigates the shear behavior of high-strength reinforced concrete columns under low axial load. The specified compressive strength of concrete is 70 MPa or 100 MPa. The specified yield strengths of longitudinal and transverse reinforcement are 685 MPa and 785 MPa, respectively. Eight large-scale column specimens were constructed and tested in double bending under lateral cyclic load. Test results showed that all specimens had shear failure without yielding of longitudinal reinforcement as expected in design. Higher concrete compressive strength, higher axial load and smaller spacing of transverse reinforcement resulted in higher shear strength. The peak applied load was reached before yielding of transverse reinforcement. The critical shear crack angle was approximately 30° and 20° for columns with 10% and 20% axial load, respectively. The simplified shear strength equation of the ACI 318 code was conservative for columns tested in this research and for high strength columns collected from literature. However, the detailed shear strength equation exhibited non-conservative results for most of the columns examined.

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.