Abbas Moustafa

Optimal placement of viscoelastic dampers and supporting members under variable critical excitations

The concept of performance-based design has recently been introduced and is well accepted in the current structural design practice of buildings. In earthquake-prone countries, the philosophy of earthquake-resistant design to resist ground shaking with sufficient stiffness and strength of a building itself has also been accepted as a relevant structural design concept for many years. On the other hand, a new strategy based on the concept of active and passive structural control has been introduced rather recently in order to provide structural designers with powerful tools for performance-based design.

Critical earthquake loads for SDOF inelastic structures considering evolution of seismic waves

The damage of structures induced by earthquake ground motions depends primarily on three parameters: (1) the characteristics of earthquake source properties and ground motions (magnitude, epicentral distance, duration, frequency content, amplitude and local soil type), (2) the properties of the structure (natural frequencies, mode shapes, damping properties, material of construction, structural system and ductility capacity), and (3) how close the structure’s fundamental natural frequency to the dominant frequency of the ground motion. The 2011 off the Pacific coast of Tohoku earthquake has demonstrated these facts clearly. In general, the ground motion characteristics involve large inherent uncertainties and cannot be controlled while the structure’s properties have smaller variability and can be managed to some extent in general. For instance, the material and members of construction can be selected and the seismic-resistance of the structure can be improved to fairly high levels through member detailing for enhancement of ductility capacity.

Modeling Critical Ground-Motion Sequences for Inelastic Structures

The specification of earthquake loads as inputs to engineering structures is a crucial task in earthquake engineering. There are cases, however, where the site under consideration has limited or scarce seismic data, making this process a difficult task. Meanwhile, structural engineers are often concerned with the worst-case scenario that can happen to the structure during its service-life under possible future earthquakes. Repeated ground-motion sequences occurring after short intervals of time, resulting from mainshock-aftershock earthquakes, have been observed in many parts of the world. Such ground motion is capable of creating severe damage in the structure due to accumulation of inelastic deformations from multiple sequences before any structural repair is possible. This paper models ground motions of multiple sequences that produce the maximum damage in the structure. The ground acceleration is represented as Fourier series, with unknown amplitudes and phase angles, modulated by envelope functions. The unknown parameters are optimized to produce the maximum damage in the structure while the ground motion is constrained to the available seismic data at the site. The resulting inverse nonlinear dynamic problem is tackled using optimization techniques, nonlinear time-history analysis and damage indices. Numerical illustrations on modeling critical earthquake sequences for inelastic frame structures are provided. It is shown that critical repeated acceleration sequences produce larger structural damage compared to single critical earthquakes.

Earthquake Response Bound Analysis of Uncertain Base-Isolated Buildings for Robustness Evaluation

An efficient methodology is explained to evaluate the robustness (variability of response) of an uncertain base-isolated building under various ground motions. It is well known that base-isolated buildings have large structural uncertainties due to wide variability of structural properties of base-isolation systems [1, 2]. The variability resulting from temperature and frequency dependences, manufacturing errors, and aging effect may be a representative one.

Response of Nonlinear SDOF Structures to Random Acceleration Sequences

In performance-based design, the structure is designed to behave linearly elastic without damage under a moderate frequent earthquake and to undergo repairable damage under a rare strong earthquake. Design earthquakes are specified in current seismic codes as single events. However, the structure may experience repeated accelerations in a short period of time. Ground accelerations of multiple sequences could result in more damage to the structure than a single ordinary event.

Critical Characterization and Modeling of Pulse-Like Near-Field Strong Ground Motion

Pulse-like (also known as resonant, cycloidal pulses, or impulse-like) ground motion has been observed in near-field (also near-fault or near-source) records with directivity focusing or fling effects. This class of ground motion is significantly influenced by the rupture mechanism, substantially different from ordinary ground motion records, and can be characterized by the following features [1, 2, 3, 4, 5, 6, 7, 8, among others]: (1) long period and large amplitudes, (2) high peak ground velocity (PGV)/peak ground acceleration (PGA) and peak ground displacement (PGD)/PGA ratios, (3) unusual response spectra shapes, and (4) concentration of energy in one or very few pulses.

Use of Deterministic and Probabilistic Measures to Identify Unfavorable Earthquake Records

The robust design of structures toward earthquake loads is the key for the mitigation of the structure damage against earthquake hazards. The structural engineer aims to design structures that are safe against possible future earthquakes and economic at the same time. The selection of suitable design earthquake loads for structural design is the first step to achieve this goal. The use of accurate nonlinear model describing the inelastic behavior of the structure is the second step to achieve this goal. Earthquake loads can be specified for seismic design of structures using the response spectrum method, using recorded accelerograms, or using the random vibration theory. The method of the critical excitation has been developed in the literature for specifying mathematical earthquake loads on structures.

Critical Correlation of Bidirectional Horizontal Ground Motions

The ground motion is a realization in space and simultaneous consideration of multiple components of ground motion is realistic and inevitable in the reliable design of structures . It is often assumed practically that there exists a set of principal axes in the ground motions. It is well recognized in the literature that the principal axes are functions of time and change their directions during the ground shaking. In the current structural design practice, the effect of the multi-component ground motions is often taken into account by use of the SRSS method (square root of the sum of the squares) or the CQC3 method (extended Complete Quadratic Combination rule).

Earthquake Resilience of High-Rise Buildings: Case Study of the 2011 Tohoku (Japan) Earthquake

Accumulated data and experiences are very important in the reliable seismic design of structures. However, it is also true that theoretical expectations and predictions are also of significance for the design of extremely important structures and facilities which are influential for the society and wide district. This was demonstrated in the past earthquakes which are very rare from the viewpoint of return period in the same area. The most devastating earthquake in Japan after the 1923 Great Kanto earthquake hit eastern Japan in the afternoon of March 11, 2011. The moment magnitude 9.0 earthquake is one of the five most powerful earthquakes in the world since modern recordkeeping began in 1900. It was made clear afterwards that the recording system for low-frequency and large-amplitude ground motions was not sufficient in Japan and the first preliminary Japan Meteorological Agency (JMA) magnitude was smaller than 8 (7.9 exactly). The JMA magnitude was updated immediately as 8.4. Records of earthquake ground motions outside Japan were then used to determine the exact moment magnitude of 9.0 (intermediate announcement was 8.8). The earthquake resulted from the thrust faulting near the subduction zone plate boundary between the Pacific and North America Plates.

Earthquake Response Bound Analysis of Uncertain Passively Controlled Buildings for Robustness Evaluation

The structural control using passive dampers has a successful history in the field of mechanical and aerospace engineering. This may result from the characteristic that these fields usually deal with predictable external loading and environment with little uncertainty. On the other hand, in the field of civil engineering, it has a different situation and history [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Building and civil structures are often subjected to severe earthquake ground motions, wind disturbances, and other external loading with large uncertainties [12]. It is therefore inevitable to take into account of these uncertainties in their structural design and application to actual structures.