Richard N. White

SEISMIC FRAGILITY OF LRC FRAMES WITH AND WITHOUT MASONRY INFILL WALLS

This paper summarises the first phase of the fragility analyses of generic (representative) buildings in the area of Memphis, Tennessee, USA. The study was conducted at Cornell University as a part of the project Loss Assessment of Memphis Buildings (LAMB) for the National Center for Earthquake Engineering Research (NCEER). In this study, the fragility analyses focus on low-rise Lightly Reinforced Concrete (LRC) frame buildings with and without infill walls. The obtained fragility curves are compared with those of ATC-13 for different facility classes. Based on the obtained fragility curves, it is concluded that adding masonry infill walls to low-rise LRC frame buildings significantly reduces the likelihood of seismic damage.

AUTOMATIC GENERATION OF TRUSS MODEL FOR OPTIMAL DESIGN OF REINFORCED CONCRETE STRUCTURES

This paper presents a new approach to the computer-aided design of distributed regions of concrete structures. An optimization approach is used to define the topology of the equivalent truss structure needed to provide a specified load capacity while requiring an absolute minimum volume of reinforcement. The algorithms include 2 new features: 1) a practicality factor to avoid the generation of impractical reinforcement layout; and 2) a stress redistribution factor related to the total plastic energy required in developing the capacity of a given truss. Comparisons between alternate optimal solutions and between theory and experiment are also provided.

Implications of Experiments on the Seismic Behavior of Gravity Load Designed RC Beam‐to‐Column Connections

This paper summarizes recent experimental research at Cornell University conducted on the behavior of gravity load designed reinforced concrete building frame components subjected to reversing cyclic loads (simulated seismic effects). Reinforced concrete framing systems, designed primarily for gravity loads, with little or no attention given to lateral load effects, are typically characterized by non-ductile reinforcing details in the joint regions and in the members. The seismic response of connection regions for gravity load design (GLD) frames has received relatively little attention in earlier studies, thus making it difficult to reliably evaluate GLD frames and to properly plan repair or retrofit strategies. Thirty-four full scale bare interior and exterior beam-to-column joints have been tested under reversed cyclic bending to identify the different damage mechanisms and to study the effect of critical details on strength and deformations. The discussion of test results focuses on the definition of joint shear strength factors for GLD frames to complement those provided by ACI-ASCE Committee 352 for frames designed with better details.

Shear-Critical Cracking in Slender Reinforced Concrete Beams

This paper describes an experimental investigation into the cause of critical-shear cracking in slender reinforced concrete beams, using 20 simply supported reinforced concrete beams loaded to failure. The basic approach used was that the test beams were specially designed and fabricated to artificially isolate or add the effect of a certain factor on the critical-shear cracking process. These test results were then compared with results from ordinary control beams, and the differences were analyzed to deduce the major cause for the initiation and the propagation of flexural shear cracking. The results indicated that the initiation of flexural shear cracking was strongly associated with the bond between concrete and reinforcement. It was found that the propagation of the critical shear crack depended exclusively on the intensity of horizontal cracking. Some results were incompatible with the concept of shear strength of critical sections that form the basis of current shear design provisions. A more rational hypothesis of bond-induced shear failure mechanism is derived based on these experimental results.

RESPONSE OF INFILLED FRAMES USING PSEUDO-DYNAMIC EXPERIMENTATION

SUMMARY An accurate and practical testing technique to study seismic performance of multi-storey infilled frames is formulated. This technique is based on the pseudo-dynamic method which can provide an acceptable approximation of the dynamic performance of structures under the influence of earthquake excitation. The pseudo-dynamic experimental technique is outlined and applied for testing a two-bay, two-storey gravity load designed steel frame infilled with unreinforced concrete block masonry walls. From the discussion of the results, the dynamic performance of the tested structure is assessed. ( 1998 John Wiley & Sons, Ltd. Pseudo-dynamic experimentation is a testing procedure in which the dynamic response of the structure is calculated and the obtained displacements are statically applied to the structure in an on-line procedure. This technique is essentially identical to traditional time domain analysis but rather than idealizing the non-linear sti⁄ness characteristics of the structure, the static restoring forces are directly measured from the specimen as the experiment proceeds. Computation of displacements is based on numerical integration of the governing second-order di⁄erential equations of motion of a system with assumed mass and damping properties and with a forcing function corresponding to a selected dynamic loading. During the test, actual displacements and restoring forces are measured using equipment normally used for static experiments. These measured quantitites are utilized in subsequent calculations. In this way, both dynamic e⁄ects and progressive damage of the specimen are included in the imposed displacements, and the procedure allows for an in-depth monitoring of the performance of the structure for the entire duration of realistic earthquake excitation. Infilled frames have been investigated experimentally by many researchers, most often with monotonic (see e.g. Reference 1) or quasi-static cyclic loading (see e.g. Reference 2), and in a few cases with actual dynamic

Behavior of Gravity Load Design Reinforced Concrete Buildings Subjected to Earthquakes

Two small-scale reinforced concrete building models were tested on the Cornell University shake table. The models were a 1/6 scale two-story office building and a 1/8 scale three-story one-bay by three-bays office building. Both structures were designed to resist purely gravity loads without regard to lateral loads (wind or earthquake forces). The reinforcement details were based on typical reinforced concrete frame structures constructed in the central and eastern United States over the past 50 to 60 years, as characterized by (a) low reinforcement ratio in teh columns, (b) discontinuous positive moment reinforcement in the beams at the column locations, (c) little or no confining reinforcement in the joint regions, and (d) lap splices located immediately above the floor level. Both models were tested using the time-compressed Taft 1952 S69E ground motion scaled to increasingly large peak ground accelerations. Test results indicated that gravity load design (GLD) reinforced concrete buildings without walls will experience very large deformations associated with a considerable stiffness degradation during a moderate earthquake. The high flexibility produced significant P-Δ effects in the three-story building model. Although the nonseismic details associated with the gravity load design philosophy forms a source of damage, the experiments indicate that these details will not necessarily lead to collapse or to a complete failure mechanism. Comparison with analytical results indicated that inclusion of the slab contribution to beam flexural strength is a vital step in the assessment of the performance of GLD reinforced concrete structures since it has the potential of altering the relatively ductile strong column-weak beam mechanism to a more brittle soft-story mechanism.

ENHANCED CONTACT MODEL FOR SHEAR FRICTION OF NORMAL AND HIGH-STRENGTH CONCRETE

Shear friction is the term used to describe the mechanism of shear transfer along a concrete-to-concrete interface subjected to simultaneous shear and normal compression. The aim of this paper is to suggest a rational, yet simple, methodology for obtaining shear friction capacity of both normal and high-strength concrete. The approach considers the following factors: interface morphology (roughness), concrete strength, amount of reinforcement crossing the interface, and normal stress applied to the interface. The interface roughness is probabilistically idealized according to the contact density model developed by Li and Maekawa. The relation between interface roughness and concrete strength is established based on a fracture energy approach. Finally, the suggested methodology is compared with American Concrete Institute (ACI) 318-95, Mau and Hsu, and the Canadian Code.

Arch Action in Reinforced Concrete Beams—A Rational Prediction of Shear Strength

A rational expression, developed to predict the shear strength of reinforced concrete beams, is derived from the relationship between shear and the rate of change of bending moment along a beam coupled with experimental findings for the arch action. Eight reinforced concrete simple span beams without web reinforcement were tested statically up to failure to investigate quantitatively the arch action. Variables included four shear span-to-depth ratios (2, 2.5, 3, and 4) and two longitudinal steel ratios (1% and 2%). On the basis of the experimental findings, an equation is proposed to predict the internal moment arm length, which then leads to a shear strength equation that combines beam action and arch action. The proposed ultimate shear strength equation, arising from analytical premises and then calibrated with experimental data, is a similar form to the American Concrete Institute (ACI) 318 equation derived mainly from an empirical approach. The proposed ultimate shear strength equation applied to existing test data and the results were compared with those predicted by the ACI 318 equation and the Zsuttys equation.

Strength and stiffness of reinforced concrete containments subjected to seismic loading: Research results and needs☆

Abstract The primary purpose of this paper is to present results of an experimental investigation on the strength and stiffness of reinforced concrete subjected to combined biaxial tension and simulated seismic forces. The test specimens represent a section of a wall of a containment structure carrying combined pressurization and seismic loading. Shear stiffness and strength, and their degradation with shear cycling, are given, along with simple expressions for predicting strength and extensional stiffness. The secondary purpose of the paper is to discuss research needs for improved prediction of the response of containment structures to seismic effects.

Consideration of compression stress bulging and strut degradation in truss modeling of ductile and brittle corbels

The paper presents a newly introduced modification of the strut-and-tie model that takes into account compression stress bulging of the strut with proper attention to its degradation as tensile strains increase. The method is capable of discriminating between brittle and ductile modes of failure, thus eliminating the gross underestimation of capacity often observed in applying the truss model to brittle corbels. The proposed method provides improved corbel-capacity prediction in comparison to the provision of three design codes.