Robert Park

STRESS-STRAIN BEHAVIOR OF HIGH-STRENGTH CONCRETE CONFINED BY ULTRA-HIGH- AND NORMAL-STRENGTH TRANSVERSE REINFORCEMENTS

In situations where high-strength concrete (HSC) is used for reinforced concrete members subjected to seismic loading, it is more difficult to achieve ductile behavior of such members than when normal-strength concrete is used. This paper presents an experimental study of a number of quasi-static axial loading tests on HSC specimens confined by various amounts of transverse reinforcement. A stress-strain relationship for confined HSC is proposed that is found to give reasonably good prediction of the experimental behavior of circular and square specimens with HSC confined by either normal- or ultra-high-yield-strength with various configurations. An empirical formula for the ultimate longitudinal strain of confined HSC corresponding to the first hoop or spiral fracture is also proposed.

CONSTITUTIVE BEHAVIOR OF HIGH-STRENGTH CONCRETE UNDER DYNAMIC LOADS

This paper describes an experimental investigation into the behavior of short reinforced high-strength concrete columns. Thirty reinforced concrete columns, either 240 mm diameter circular or 240 mm square and 720 mm high, containing different confining reinforcement configurations, yield strengths of transverse reinforcement, and concrete compressive strengths, were subjected to concentric loads to failure at different strain rates. Results presented include an assessment of the effect of strain rate, different concrete compressive strength, amount and distribution of longitudinal steel, and amount of distribution of transverse steel. A stress-strain curve for confined high-strength concrete loaded at a high strain rate (comparable with seismic loading) is proposed and compared with the curve based on tests conducted at low strain rates.

DUCTILITY OF REINFORCED CONCRETE COLUMN SECTIONS IN SEISMIC DESIGN

THE DUCTILITY REQUIRED OF ECCENTRICALLY LOADED REINFORCED CONCRETE COLUMN SECTIONS IN SEISMIC DESIGN IS DISCUSSED. A METHOD FOR THE DETERMINATION OF THE AMOUNT OF SPECIAL TRANSVERSE STEEL REQUIRED FOR DUCTILITY IS SUGGESTED. THE METHOD IS BASED ON A THEORETICAL STUDY USING STRESS-STRAIN CURVES FOR CONCRETE CONFINED BY RECTANGULAR HOOPS AND FOR STEEL INCLUDING STRAIN HARDENING. THE METHOD TAKES INTO ACCOUNT THE REQUIRED ULTIMATE CURVATURE, THE LEVEL OF AXIAL LOAD, THE LONGITUDINAL STEEL CONTENT, AND THE MATERIAL STRENGTHS. IT IS FOUND THAT PRESENT CODE RECOMMENDATIONS FOR TRANSVERSE STEEL MAY BE LESS THAN APPROPRIATE IN SOME CASES OF HIGH AXIAL LOAD AND LOW LONGITUDINAL STEEL CONTENT, AND COULD BE RELAXED IN SOME OTHER CASES.

Earthquake Resistant Structures

Much of the surface of the earth is subjected to earthquakes from time to time. An earthquake is a spasm of ground shaking, originating from part of the earth’s crust. The consequences of severe earthquakes are injury and loss of life, the costs of repair of damage to structures, contents, and infrastructure, and the costs of disruption of business and other activities. It is evident that the provision of earthquake resistance in buildings, bridges, and other structures remains a great challenge for engineers and others associated with the design and construction of structures in the seismically active parts of the world. This chapter provides brief introduction on origin of earthquakes, strength of earthquakes, earthquake motion, and analysis on the typical damages caused by earthquakes. It also proposes a ductile design approach for resistance to earthquake including their performance criteria and the future trend in design approach. In addition, the approach for earthquake resistance using base isolation and mechanical energy-dissipating devices are discussed. The concise introduction on seismic assessment and upgrading of old structures are provided. Moreover, the earthquake resistance of lifelines is presented. This chapter can be used for the researchers to know the overall overview on the earthquake, its consequences and earthquake resistant structures.

Flexural Strength and Ductility of High-Strength Concrete Columns

High strength concrete with a specified compressive cylinder strength of up to 70 MPa for ductile elements in seismic design and of up to 100 MPa for other elements is now permitted by the recently revised New Zealand concrete design standard NZS 3101:1995. Also, longitudinal reinforcement with a characteristic yield strength of up to 500 MPa is allowed, and for transverse reinforcement in strength calculations a useable steel stress of up to 500 MPa for shear strength and 800 <Pa for confinement is permitted. For concrete with a concrete compressive cylinder strength greater than 55 MPa the parameters for the equivalent rectangular compressive stress block have been modified to rake into account the stress-strain characteristics of high strength concrete. Also, new design equations for confining reinforcement have been included to better account for the affect of the variation of axial load level. Simulated seismic load tests have been conducted in New Zealand to investigate the behavior of high strength concrete columns confined with normal and very high strength transverse reinforcement. These tests demonstrated that the yield strength of very high strength confining reinforcement may not be attained at the stage when the column reaches the peak flexural strength and that the thickness of concrete cover has an important influence on the behavior of columns.

STATIC AND DYNAMIC LOADING TESTS ON TWO SMALL THREE-DIMENSIONAL MULTISTORY REINFORCED CONCRETE FRAMES

Two identical small scale six storey three dimensional reinforced concrete framed structures were constructed consisting of columns, beams and floor slabs. Each structure had a single bay in each direction and was approximately one fifth of full size. All members were designed for ductility according to the seismic design requirements of ACI 318-71. One structure was subjected primarily to static lateral loading and the other structure was subjected primarily to dynamic loading on a shaking table. The lateral load strength of the statically loaded structure was accurately predicted when the actual properties of the structure were taken into account. Severe stiffness degradation occurred during cyclic loading in the inelastic range. The displacement response of the dynamically loaded structure was compared with the displacement response predicted by a dynamic frame analysis computer program and the accuracy of the predicted response was found to be extraordinarily dependent on the stiffness and damping. The tests revealed that under lateral loading significant torsion is induced into the beams at right angles to the direction of loading and this may lead to severe torsional cracking of those beams and a consequent decrease in stiffness and strength of the framed structure. (A) For the covering abstract see IRRD 853656.

Effects of Increasing Concrete Strength on the Dimension of Beams

A theoretical study was carried out to investigate the effects of increasing concrete strength on the depth of rectangular beams. Two series of beams were investigated. Th first series comprised reinforced concrete beams with spans from 6 to 15 m, and the second comprised prestress concrete beams with spans from 12 to 30 m. The concrete strength ranged from 20 to 120 MPa and from 30 to 120 MPa for the reinforced and prestressed concrete beams, respectively. The results show that for rectangular concrete beams, an increase in concrete strength results in a rather significant reduction in the beam depth, whereas for rectangular prestressed concrete beams no significant reduction in the beam depth is gained from increasing the concrete strength because the deflection governs the design.