Gilbert A. Hegemier

Model of FRP-Confined Concrete Cylinders in Axial Compression

This paper introduces a dilatancy-based analytical model of the response of an axially loaded concrete cylinder, confined with a fiber-reinforced polymer (FRP) composite jacket. Model construction is based on the experimentally based observation that the relation between axial secant stiffness and the lateral (dilatancy) strain is effectively unique for cylinders with the same unconfined concrete strength, although the confinement levels may differ. Model development incorporates strength degradation of the concrete with dilatancy (lateral dilation); this feature allows one to demonstrate that the performance of FRP-confined concrete is consistent with the strength envelope obtained from triaxial tests. Model validation is accomplished by comparisons with existing test database and the new results on large-scale concrete cylinders. The results of the validation reveal good agreement with key response functions and parameters. The present study illustrates basic constitutive equations to model FRP-confined...

Experimental Performance of Concrete Columns with Composite Jackets under Blast Loading

Measures to prevent progressive collapse of structures include protection of critical elements such as columns. In support of this goal, nine tests were conducted to assess the as-built performance of typical columns and the effectiveness of carbon fiber jackets in improving their performance. A quasi-static load protocol was developed to replicate in the laboratory the damage patterns observed in blast testing in the field. Load-deflection curves (resistance functions) and jacket strain were measured. Jackets were observed to change the failure mode from brittle shear to ductile flexure and to increase the load and displacement capacities of the column. Variations in jacket strain are discussed and experimental results are used to assess predictive models for shear capacity and resistance functions. The data support the use of these models for design but identify some limitations in the resistance functions.

Analytical Model for Fiber-Reinforced Polymer-Jacketed Square Concrete Columns in Axial Compression

This study develops a new analytical-based model for the response of square fiber-reinforced polymer (FRP)-jacketed concrete columns in axial compression. Jacket membrane strain and the maximum jacket flexural strains at the centers of the sectional side and corner are approximately evaluated by assumed parabolic functions. The proposed model reveals the effects of sectional shape and jacket strength on the specimen performance. Using test data on six large-scale square column specimens, validation of the proposed model is conducted via the comparison with the experimental curves of axial stress-strain, axial stress-transverse strain, and axial stress-jacket hoop strain. These results indicate that the model is accurate in estimating the influences of corner radius and jacket properties on the axial stress-strain relation. A parametric study shows that the axial strength and deformation capacity of the columns with high side length to corner radius ratios may not be significantly enhanced by increasing jacket thickness.

Simulated Seismic Laboratory Load Testing of Full‐Scale Buildings

Full-scale building systems have been tested to-date in Japan, the United States, and Europe under controlled laboratory conditions with simulated seismic loads, to determine behavior and design limit states and to calibrate predictive analytical and design models. Seismic load simulation for these tests consisted of increasing cyclic load/deformation patterns with predetermined load distribution or, where possible, of loading patterns derived experimentally from the measured building response in conjunction with updated displacement time-histories through pseudo-dynamic testing. Difficulties in the pseudo-dynamic testing of stiff multi-story buildings due to the tight coupling between individual actuators, stability problems with the numerical integration alorithms, measurement errors and error growth, as well as the control of undesirable torsional modes, were addressed with innovations in the testing hardware and in the actuator control alorithms in the first US full-scale building test of a 5-story reinforced masonry building.

Homogenization of plain weave composites using two-scale convergence

Abstract In this paper, we homogenize a plain weave composite using convergence results developed for periodic functions. The two-scale convergence scheme is discussed and its results are applied to the elasticity problem of modeling composite weaves. Numerical implementation is discussed and parameter studies are given.

Development of a mixture model for nonlinear wave propagation in fiber-reinforced composites

Abstract A method for constructing dispersive, nonlinear mixture models for unidirectionally fiber-reinforced composites is described. System nonlinearities in the treated example result from nonlinear material properties of the constituents. The proposed model is a nonlinear generalization of the linear model developed by Murakami and Hegemier [ Journal of Applied Mechanics . Vol. 53, pp. 765–773 (1986)] for elastic constituents. Model construction is based upon a homogenization technique which employs multivariable asymptotic expansions in conjunction with certain weighted residual procedures. The methodology furnishes the equations of motion, the appropriate initial and boundary conditions, and a set of consistent rate constitutive relations. Model validation for linear and nonlinear dynamic responses is accomplished by comparing predicted results for wave-guide and wave-reflect problems with available experimental data or data obtained by use of a detailed finite element (FE) analysis. The validation studies reveal that the derived continuum model provides good simulations of complex wave phenomena and furnishes an economical alternative to detailed, explicit FE, models. The studies performed reveal the importance of wave dispersion and attenuation phenomena in nonlinear as well as linear wave propagation in the composites.

Carbon Fiber Composite Jackets to Protect Reinforced Concrete Columns against Blast Damage

An elevated risk of deliberate attacks on high-profile and critical facilities underscores the need to protect buildings against blast damage. Under blast loading the structure may be vulnerable to progressive collapse, which begins when a load-bearing member suddenly loses its ability to carry the loads above it. Once this happens, the failure can propagate through much or all of the structure if there is insufficient redundancy and continuity. Preventing progressive collapse involves designing appropriate alternative load paths and hardening critical elements to withstand blast loading. The experimental and computational work herein addresses the latter strategy. The experimental portion shows the efficacy of carbon fiber reinforced polymer (CFRP) wraps as an effective retrofit method and provides data for calibration of computational models. The computational portion assesses current modeling capabilities. Areas in need of improvement are identified.

Experimental and analytical study of the dynamic response of low‐porosity brittle rocks

Motivated by an inadequate understanding of large differences observed in particle velocities and displacements in in situ large-scale tests in wet and dry granite, a combined experimental and analytical study was undertaken to investigate the influence of porosity, crack population, pore water content and pressure, and confining pressure on the characteristics of high-amplitude stress wave propagation in a low-porosity (0.8–1.0%) brittle rock under well-controlled laboratory conditions. A unique porosity enhancement technique, using a gas fracturing process, was developed to provide a controlled, homogeneous, isotropic increase in porosity to 2.4–2.9% (a factor of 2.5–3.5) in Sierra White granite. Combined with a technique to independently control confining and pore pressures, the experiments provided measured radial particle velocities, calculated displacements, and peak velocity and displacement attenuations. In the experiments, key parametric effects due to fracture-induced porosity increase combined with pore water content were observed. In particular, a significant porosity (and accompanying fracture) increase combined with fully saturated, undrained condition (no effective stress) resulted in a substantial decrease in peak velocities and increased pulse durations, leading to greatly increased particle displacements. Reducing the pore pressure in enhanced-porosity rock produces substantially smaller displacements because of narrower pulses. It was concluded that in the region where deviatoric response is important, anisotropic damage evolution through compression-induced fracture formation rather than the resulting porosity increase itself appears to dominate material response. A phenomenological plastic damage model allowing damage accumulation under compression-induced (splitting) cracking was used to numerically simulate the key experimental observations.

SHEAR LOADING IN TWO-COLUMN BRIDGE BENTS

The seismic performance of two-column bridge bents was investigated by quasi-static testing two one-third-scale bridge bents replicating the Dumbarton Bridge in the San Francisco Bay Area. Both tall flexurally dominated and short shear-dominated specimens were represented. For both tests, experimental activity focused on lateral force distribution between the two columns and how the distribution relates to lateral capacity of the bents. A three-dimensional (3-D) finite element analysis computer model, validated by experimental results, was used to perform a parametric study of a variety of bent configurations to determine shear distribution between columns and to establish the specific column that governs shear design. Results showed that the column under compression was more vulnerable to shear degradation and, prior to first yield, the compression column resisted a larger portion of lateral force than the tension column in two-column bridge bents.

ANALYSIS METHODS FOR CFRP BLAST RETROFITTED REINFORCED CONCRETE WALL SYSTEMS

A blast retrofit technique for concrete structures using carbon fiber-reinforced polymer (CFRP) layers was investigated for use in large infrastructure systems with the overarching goal of preventing against major loss of life and considerable damage that would require extensive repair. Large-scale experiments were conducted and the retrofit behavior was investigated for application on relatively large reinforced concrete walls subjected to blast-like loadings. The experimental program utilized the University of California San Diego (UCSD) Blast Simulator. The Blast Simulator is able to induce various blast-like shock waves to the test specimen in a controlled laboratory environment. The performance of this blast retrofit was tested and then analyzed using SDOF and finite element modeling methods. A finite element model was created using LS-DYNA and utilized contact algorithms for the CFRP-concrete interface. Results and comparisons between the two analysis methods are given.