Andrew S. Whittaker

Performance of reinforced concrete buildings during the August 17, 1999 Kocaeli, Turkey earthquake, and seismic design and construction practise in Turkey

Abstract A large number of reinforced concrete buildings collapsed or were heavily damaged during the 7.4 magnitude earthquake that struck northwestern Turkey on August 17, 1999. Recorded peak ground accelerations were relatively low (0.3 g–0.4 g) compared to the magnitude of the structural damage, and the elastic acceleration response spectra from the recorded motions were comparable with the elastic design spectra specified in the current Turkish seismic code. Seismic code requirements are discussed and compared with observed details. Many structural deficiencies were highlighted by the earthquake damage, including: reinforced concrete columns with insufficient confinement and transverse reinforcement, 90-degree hooks at the end of column ties, poor detailing in beam-column joint regions, strong-beam and weak-columns, soft and weak stories, and poor quality construction. Buildings with shear wall structural elements generally performed well.

Investigation of air-blast effects from spherical-and cylindrical-shaped charges

Although the distributions of peak incident overpressure and impulse generated from spherical charges and cylindrical charges of the same weight can differ greatly close to the point of detonation, spherical charges are assumed for nearly all explosive-effects computations per modern standards for blast-resistant design such as UFC-3-340-02 and the soon-to-be published ASCE Standard for the Blast Protection of Buildings. A blast-testing program was performed using a reinforced concrete slab as the target to investigate the reflected peak overpressure and impulse distributions as a function of charge shape, orientation, and scaled distance. The charge shapes were cylindrical and spherical, and the charge mass varied from 0.24 to 8.0 kg. Nine pressure transducers were installed on the surface of the slab to record the distribution of pressure histories over the face of the target. A finite element model of the explosive and the target was validated using the experimental data. The validated model was then used to undertake a parametric analysis to more broadly study the effects of detonation point, ratio of charge length to charge diameter, charge orientation and standoff distance on the distribution of reflected overpressure. Numerical results are compared with predictions of UFC-3-340-02. For cylindrical charges, the ratio of charge length (L) to diameter (D), the orientation of the longitudinal axis of the charge, and detonation point within the charge affected the distributions of reflected peak overpressure and impulse in the immediate vicinity of the explosive. The UFC-3-340-02 underpredicts substantially the reflected peak overpressure and impulse on a target aligned with the vertical axis of a cylindrical charge with an aspect ratio of 1.0.

Shear Strength of Squat Rectangular Reinforced Concrete Walls

Squat reinforced concrete walls (height less than twice the length) are important structural components of both conventional and nuclear safety-related structures. Predictive equations are available in the literature to compute the shear strength of squat walls but the scatter in the results for a given set of design variables is large. The utility of five predictive equations is evaluated using data from tests of 120 rectangular walls. The equation proposed by Wood in 1990 resulted in a median ratio of the predicted to measured strengths close to 1.0 with a small coefficient of variation. Test data are also used to quantify the loss of strength with repeated cycling. The inter-cycle drop in strength and stiffness is significant, with the largest reductions observed for walls with aspect ratios less than 0.5.

BIDIRECTIONAL MODELLING OF HIGH-DAMPING RUBBER BEARINGS

High-damping rubber (HDR) bearings are used in seismic isolation applications for buildings and bridges, although no models are currently available for the accurate description of the shear force-deformation response under bidirectional loading. A strain rate-independent, phenomenological model is presented which effectively represents the stiffness, damping, and degradation response of HDR bearings. The model decomposes the resisting force vector as the sum of an elastic component in the direction of the displacement vector and a hysteretic force component parallel to the velocity vector. The elastic component is obtained from a generalised Mooney—Rivlin strain energy function, and the hysteretic component is described by an approach similar to bounding sur-face plasticity. Degradation is decomposed into long term (“scragging”) and short term (“Mullins effect”) components. Calibration is carried out over a series of bidirectional test data, and the model is shown to provide a good match of slow strain-rate experimental data using a unique set of material parameters for all tests. A testing protocol and calibration of the model for use in design of structures with HDR bearings are discussed.

Seismic Analysis of Conventional and Isolated LNG Tanks Using Mechanical Analogs

The seismic response of a conventional and an isolated vertical, cylindrical, Liquefied Natural Gas (LNG) tank is computed using a mechanical analog and a finite element code to judge the utility of the analog for preliminary design and of the effectiveness of seismic isolation. Data reported and statistically sorted include base shear, global overturning moment, and wave height in the tank. The results obtained from the two numerical models are in good agreement and demonstrate that the mechanical analog can be used with confidence for the preliminary analysis and design of conventional and isolated LNG tanks that have similar dimensions to the sample tank of this study. The base shear and overturning moment in the seismically isolated LNG tank are 10% to 15% of the values computed for the conventional tank; the wave heights are unaffected by the introduction of a seismic isolation system.

Response of Base-Isolated Nuclear Structures for Design and Beyond-Design Basis Earthquake Shaking

ASCE 43-05 presents two performance objectives for the design of nuclear structures: 1) 1% probability of unacceptable performance for 100% Design Basis Earthquake (DBE) shaking, and 2) 10% probability of unacceptable performance for 150% DBE shaking. To aid in the revision of the ASCE 4-98 procedures for the analysis and design of base-isolated nuclear power plants (NPPs) and meet the intent of ASCE 43-05, a series of nonlinear response-history analyses was performed to study the impact of the variability in both earthquake ground motion and mechanical properties of isolation systems on the seismic responses of base-isolated NPPs. Computations were performed for three representative sites (rock and soil sites in the Central and Eastern United States and a rock site in the Western United States), three types of isolators (lead rubber, Friction Pendulum™ and low-damping rubber bearings), and realistic mechanical properties for the isolators. Estimates were made of 1) the ratio of the 99%-ile (90%-ile) response of isolation systems computed using a distribution of spectral demands and distributions of isolator mechanical properties to the median response of isolation systems computed using best-estimate properties and 100% (150%) spectrum-compatible DBE ground motions; 2) the number of sets of three-component ground motions to be used for response-history analysis to develop a reliable estimate of the median response of isolation systems. Only the results for the rock site in the Central and Eastern United States are presented. The results of this study provide the technical basis for the revision of ASCE Standard 4-98.Copyright

In-plane seismic behavior of rectangular steel-plate composite wall piers

AbstractAn experimental study investigated the behavior of large-scale steel-plate composite (SC) walls subjected to cyclic lateral loading. The testing program involved four rectangular SC wall specimens with an aspect ratio (height-to-length) of 1.0. The specimens were anchored to a concrete basemat with a pretensioned bolted connection that was designed to be stronger than the walls. The design parameters considered in the investigation were wall thickness, reinforcement ratio, stud spacing, and tie bar spacing. The pretest analyses, global force-displacement responses, contributions of the steel faceplates and infill concrete to the lateral resistance, load transfer between the faceplates and infill concrete, and damage to the face plates and infill, are documented. The four SC walls failed in a flexural mode characterized by tensile cracking of the concrete, tensile yielding of the steel plates, crushing of concrete at the toes of the wall, outward local buckling of the steel faceplates, and fracture...

A Rate Dependent Stress-Strain Relationship Model for Normal, High and Ultra-High Strength Concrete

High and ultra-high strength concrete are becoming popular for many applications, including critical infrastructure subjected to high strain rate loading such as blast and impact. A strain rate dependent material model that is applicable to a range of strengths, varying from normal strength to ultra-high strength concrete, is presented in this paper. The results from a comprehensive experimental study conducted to investigate the strength and deformation capacity of concrete cylinders under high-velocity impact loading using a Split Hopkinson Pressure Bar (SHPB) test setup is reported. Unconfined 50 mm diameter concrete cylinders with compressive strengths varying from 32 MPa (4640 psi) to 160 MPa (23 200 psi) were tested to derive the dynamic properties of concrete at strain rates up to 300 s−1. The SHPB test data were analysed to obtain the stress-strain relationships and strength dynamic increase factors (DIFs) for these concrete specimens under dynamic axial compression.

Peak Shear Strength of Squat Reinforced Concrete Walls with Boundary Barbells or Flanges

Squat reinforced concrete walls are important structural components of both conventional and safety-related nuclear structures because they may provide much or all of a structures lateral strength and stiffness to resist earthquake and wind loadings. Equations are provided in the literature to compute the peak shear strength of squat walls. The utility of five such equations is evaluated using data from tests of 247 solid squat walls with barbell or flange boundary elements. Of the five equations studied, the use of the ASCE/SEI 43-05 equation resulted in a median ratio of predicted to measured strength slightly less than 1.0 with a relatively small coefficient of variation, although this equation does not include variables to account for the contribution of the barbells (flanges) to the peak strength. Importantly, the ASCE/SEI 43-05 equation underpredicts or overpredicts peak shear strength by a wide margin for many of the test specimens. Test data are also used to quantify the loss of maximum wall resistance with repeated cycling. The intercycle drop in strength and stiffness is significant, with the largest reductions observed for walls with aspect ratios less than 0.5.

Seismic Evaluation and Retrofit of the Ataturk International Airport Terminal Building

The new three-story reinforced concrete terminal building at the Ataturk International Airport was damaged during the 1999 Izmit earthquake. Conventional and innovative retrofit strategies were developed for the building to meet higher levels of performance than that specified for such construction by the Turkish seismic code. The retrofit scheme selected by the owner included seismic isolation of the spaceframe roof, jacketing and strengthening of existing reinforced concrete columns, and elimination of expansion joints between the pods that formed the building. The existing and retrofitted buildings were evaluated using the nonlinear static procedures of FEMA 273. The performance of the retrofitted building was further evaluated by nonlinear dynamic analysis.