M. H. El Naggar

Nonlinear analysis for dynamic lateral pile response

An analysis of pile lateral response to transient dynamic loading and to harmonic loading is presented allowing for nonlinear soil behavior, discontinuity conditions at the pile-soil interface and energy dissipation through different types of damping. Furthermore, the effect of neighbouring piles is taken into account for piles in a group. The validity of the approach was examined and a reasonable agreement with field tests and more rigorous solutions was found. Equivalent linear stiffness and damping parameters of single piles and interaction factors for approximate nonlinear analysis are presented.

Nonlinear lateral interaction in pile dynamics

Abstract A model for pile lateral response to transient dynamic loading and to harmonic loading is presented allowing for nonlinear soil behaviour, discontinuity conditions at the pile-soil interface and energy dissipation through different types of damping. The approach is used to establish equivalent linear stiffness and damping parameters of single piles as well as dynamic interaction factors for approximate nonlinear analysis of pile groups. The applicability of these parameters to the pile-group analysis was examined, and a reasonable agreement with the direct analysis was found. The superposition technique may be used to analyze the response of small pile groups. Also, the dynamic stiffness of pile groups is greatly affected by both the nonlinear behavior of the soil and the slippage and gapping between the pile and soil. For a basic range of soil and pile parameters, equivalent linear stiffness and damping parameters of single piles and interaction factors for approximate nonlinear analysis are provided.

PREDICTING PERFORMANCE OF SELF-COMPACTING CONCRETE MIXTURES USING ARTIFICIAL NEURAL NETWORKS

This paper shows that artificial neural networks (ANN) can be used to effectively predict the performance of self-compacting concrete (SCC) mixtures. Inspired by the internal operation of the human brain, the ANN method has learning, self-organizing, and auto-improving capabilities. Thus, it can capture complex interactions among input/output variables in a system without any prior knowledge of the nature of these interactions, and without having to explicitly assume a model form. Indeed, such a model form is generated by the data points themselves. The database assembled, the architecture of the network selected, and the training process of the ANN model used are described. Initial tests show that the ANN method can accurately predict the slump flow, filling capacity, segregation, and compressive strength test results of SCC mixtures. A model for the acceptance or rejection of SCC mixtures based on knowledge of their mixture proportions is proposed and may be used after sufficient development of a more comprehensive database on an industrial scale for the proportioning of SCC with tailor-made properties.

Seismic Vulnerability Assessment of Modular Steel Buildings

Contemporary seismic design is based on dissipating earthquake energy through significant inelastic deformations. This study aims at developing an understanding of the inelastic behavior of braced frames of modular steel buildings (MSBs) and assessing their seismic demands and capacities. Incremental dynamic analysis is performed on typical MSB frames. The analysis accounts for their unique detailing requirements. Maximum inter-story drift and peak global roof drift were adopted as critical response parameters. The study revealed significant global seismic capacity and a satisfactory performance at design intensity levels. High concentration of inelasticity due to limited redistribution of internal forces was observed.

Frequency dependent dynamic properties from resonant column and cyclic triaxial tests

The resonant column (RC) and cyclic triaxial (CT) devices are commonly used for the measurement of soils’ dynamic properties. The results of these tests do not agree when extrapolated to similar strain levels. The main objectives of this paper are to evaluate the effect of excitation frequency on the dynamic properties of soils, and to provide a methodology to reconcile shear modulus values obtained from RC and CT tests. The effect of frequency on the dynamic properties is evaluated using the new non-resonance (NR) method in the RC device and CT tests. Sand specimens with varying percentages of bentonite–water mixture and a clay specimen are tested. The results obtained from RC tests utilizing the NR method indicate significant change in shear modulus with frequency. The extrapolation of shear modulus from the conventional RC results to shear strains used in CT is significantly overestimated. The extrapolations improved when the results were corrected for frequency effect inferred from the NR method. & 2010 The Franklin Institute. Published by Elsevier Ltd. All rights reserved.

Seismic Overstrength in Braced Frames of Modular Steel Buildings

The seismic behavior factor, R, is a critical parameter in contemporary seismic design. In the 2005 edition of the National Building Code of Canada, the R factor consists of ductility related force modification factor, Rd, and overstrength-related force modification factor, Ro. The choice of these factors for design depends on the structural system type. In this investigation, typical braced frames of Modular Steel Buildings (MSBs) are designed and modeled. Nonlinear static pushover analyses are conducted to study the inelastic behavior of these frames. Structural overstrength resulting from redistribution of internal forces in the inelastic range, design assumptions, and strain hardening behavior of steel and displacement ductility are evaluated and their relationships with some key response parameters are assessed. The results show that the reserve strength of MSB-braced systems is greater than that prescribed by the Canadian code for regular braced systems. It also appears that R depends on building height, contrary to what has been prescribed in many seismic design codes. It is concluded that some unique detailing requirements of MSBs need to be considered during design to eliminate undesirable seismic response.

Interpretation of Lateral Statnamic Load Test Results

The Statnamic device has been used to test piles in the lateral direction. This paper describes a dynamic analysis approach for the prediction of the static lateral behavior of piles using the results of the Statnamic lateral test. The objectives of this analysis are twofold: to simulate the pile head displacement-time history observed during the test using the measured load time history at the pile head and varying the soil parameters along the pile shaft until a satisfactory match is achieved and to predict the static lateral load-deflection curve of the pile using the soil parameters established through the matching process. The dynamic analysis was used to analyze some Statnamic lateral tests, and satisfactory agreement between the computed and measured displacement-time histories was achieved. The predictions of the proposed approach in the given case studies were in good agreement with the static lateral load test results. The results obtained thus far suggest that the Statnamic lateral test and the described model are successful in the prediction of the lateral load-deflection behavior of piles. Both the Statnamic lateral test and the one-dimensional analysis are simple, fast, and inexpensive, and the approach promises to be very useful in the prediction of the lateral load-deflection behavior of foundations.

Characterization of Glyben for Seismic Applications

Glyben is an artificial clay prepared by mixing sodium bentonite powder and glycerin. It is used for lab tests and scale modeling for geotechnical applications. The mechanical properties of glyben depend on the bentonite and glycerin mix proportions. In this research, the shear strength, dynamic shear modulus, damping ratio, and Poisson’s ratio were evaluated for glyben samples prepared with different glycerin/bentonite ratios. Vane shear tests, T-bar tests, hammer tests, and resonant column tests were also conducted on glyben specimens and the shear strength and dynamic properties were evaluated considering a wide range of strain values and confining pressures. The measured glyben properties were compared with properties of natural cohesive soils to verify the range of applicability of glyben as a test bed material. It was found that glyben has the same range of strength and dynamic properties as soft to medium stiff clay. The trend of variation of the shear modulus and damping ratios of glyben is similar to that of natural clays. It is noted, however, that the damping ratio of glyben is higher than that of natural clays for shear strains below 0.01%. It was concluded that glyben can reasonably model the nonlinear behavior of natural soil under strong dynamic excitation.

Cyclic Response of Axially Loaded Tapered Piles

Tapered piles have a substantial advantage over straight-sided wall piles with regard to their load-carrying capacity in the downward frictional mode; however, their behavior under cyclic axial loading has not been investigated. In this study, the characteristics of the cyclic response of tapered piles were established from experimental investigation. A large laboratory facility for testing model piles was developed. In this facility, the soil was contained in a steel chamber and pressurized using an air bladder to model the confining pressure. Three instrumented steel model piles with different degrees of taper were installed in loose sand and subjected to two-way cyclic axial load tests. The results of this study indicated that the pile stiffness under cyclic loading increased with an increase in the confining pressure for all piles examined in this study. Moreover, the pile stiffness increased through cyclic loading due to the densification of the sand surrounding the pile. The amplitude of the cyclic load had a significant effect on the performance of the piles. As a result, it is recommended that the amplitude of the cyclic load be limited to 25% of the static axial capacity and 75% of the static uplift capacity to ensure satisfactory performance of tapered piles. This requirement is readily satisfied in the design of most piles. In this case, the performance of tapered piles under cyclic axial load was found to be superior to that of straight-sided wall piles.

Effects of installation disturbance on behavior of multi-helix piles in structured clays

Helical piles and anchors are installed by applying torque to the pile head. Their application as a foundation option has gained popularity in recent years because of their intrinsic advantages of rapid installation with minimal vibration and noise, and the development of powerful hydraulic driving heads. In spite of extensive research that investigated the behavior of helical piles and anchors, discrepancies between predictions and actual observations of axial behavior of helical piles installed in clay still exist. This is, in large part, because much of previous research involved installation of helical pile models in remolded (reconstituted) cohesive materials rather than natural soil deposits. Since the strength of the remolded materials does not change significantly, the effects of installation cannot be distinguished in remolded materials. In this study, full scale uplift and compression load tests data are analyzed and different failure patterns have been investigated for helical piles and anchors installed in structured clays. The research findings indicate that the behavior of the helical piles and anchors is significantly affected by the degree of soil disturbance induced by penetration of pile shaft and helices. In addition, back-calculated undrained shear strength mobilized by different sections of pile revealed that, for helical piles and anchors installed in structured clay, the undrained shear strength should be reduced to account for installation disturbance.