International Concrete Abstracts Portal

This Symposiuml Publication includes 48 papers from the ACI Fifth International Confrence on Innovation in Design with Emphasis on Seismic, Wind, and Environmental Loading, Quality Control, and Innovation in Materials/ Hot-Weather Concreting, held in December 2002 in Cancun, Mexico. Topics include the behavior of flared-column bents under seismic loading, marine exposure of high-strength light-weight concrete, and seismic strengthening of a nonductile concrete frame building. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP209

This paper examines the effectiveness of nodinear static procedures for seismic response analysis of buildings. Nonbm static procedures are recom- mended in FEMA 273 (Federal Emergency Management Agency-Guidelines for Seismic Rehabilitation of Buildings) for assessing the seismic performance of buildings for a given earthquake hazard representation. Three nonlinear static procedures specified in FEMA 273 are evaluated for their ability to predict defonnation demands ia terms on inter-story drifts and potential failure mechanisms. Two steel and two reinforced concrete buildings were used to evaluate the procedures. Strong-motion data recorded during the Northridge,earthquake are available for these buildings. Tfie study shows that nonlinear static procedures are not effective in predicting inter-story drift demands compared to nonlinear dynamic procedures. Nonlinear static procedures were not able to capture yielding of columns in the upper levels of one of the sekted buildings. This inability can be a sienificant source of concern in identifying local upper story failure mechanisms.

Cracking in concrete is fundamentally altered by the addition of reinforcing fibers. A combination of microfibers (less than 22 pi [0.0oO9 in.] in diameter) and macrofibers (500 p.m 10.0197 in.] in diameter) that contribute in comple- mentary ways to performance, is presented as a means for controlling cracking and improving the lifecycle behavior of concrete. In previous work, a hybrid blend of these fibers in a mortar matrix demonstrated better mechanical performance and lower cracked permeability than was seen with a single fiber type. The research presented in this paper attempts to realize the potential of such blends in concrete. A mixture proportioning method that achieves good workability and cohesion in concretes containing microfibers was used to produce a cast concrete. The mechanical performance and shrinkage cracking resistance of this material were evaluated. In the hybrid reinforced concrete, the microfibers delayed the development of macrocracks and so the composite demonstrated greater strength and cracking resistance than a similar matrix reinforced with macrofibers only. However, this influence was less pronounced than was observed with a mortar matrix and was confined to smaller crack openings.

The benefits derived from the ability of the fibers to control crack propagation have been recognized for many years. In addition, the development of high-perfurmance concretes has enhanced this situation as the increases in strength lead to a more brittle behavior of the material. The introduction of steel-fiber rein- forcement in these concretes is probably the best way to improve the performance of concrete when higher tenacity is required. This paper shows the contribution of fiber reinforcement in both conventional and high-strength concretes exposed to temperatures up to 500°C. Conuutes with diffemnt types and content of fibers are analyzed, mainly regarding the failure mechanism and tenacity. The post-peak behavior under conpressive and flexural loads is studied using a close loop system. NDT was also used to evaluate the damage. The residual mechanical properties of fiber-reinforced concretes qre affected in a similar way thau those corresponding to plain concrete. Nevertheless, it can be seen that the residual parmeters tend to increase as the strength increases when high carbon-steel fibers bstead of low carbon-steel fibers are used, and when fiber reinforcement is introduced.

The basic problem in beam-column design is to establish the proportions of a reinforced concrete cross-section whose design strength is just adequate enough to support the factored axial load and moments. Since the stress distribution due to the axial load and moment depends on the cross-section’s proportions, which are initially unknown, column design cannot be carried out directly. Instead, the proportions of a cross-section must be estimated and then investigated to determine whether its design capacity is adequate for the factored loads and moments. The dimensions of a beam-column cross-section and the area of reinforcing steel required to support a specific combination of axial load and moment can be established by using the column design interaction curves, where an interaction curve represents all possible combinations of axial load and moment that produce failure of the cross-section. The bending resistance of an axially loaded column about a particular skewed axis due to biaxial moments can be determined through itera- tions and lengthy calculations. These extensive calculations are multiplied when optimization of the reinforcing steel or column cross-section is required. This pa- per investigated the suitability of an Artificial Neural Network (ANN) for model- ing a preliminary design of reinforced concrete beam-column. An ANN back- propagation model has been developed to design a beam-column which predicts column cross-section and reinforcing steel requirements for a given set of inputs which are concrete compressive strength, reinforcing steel strength, factored axial load and moment. The trained ANN back-propagation model has been tested with several actual design data, and a comparative evaluation between the ANN model predictions and the actual design has been presented.