Stephen A. Mahin

Seismic demands on steel braced frame buildings with buckling-restrained braces

Some results are highlighted in this paper from a research effort being undertaken to identify ground motion and structural characteristics that control the earthquake response of concentrically braced steel frames and to identify improved design procedures and code provisions. The focus of this paper is on the seismic response of three and six story concentrically braced frames utilizing buckling-restrained braces. A brief discussion is provided regarding the mechanical properties of such braces and the benefits of their use. Results of detailed nonlinear dynamic analyses are then examined for specific cases as well as statistically for several suites of ground motions in order to characterize the effect on key response parameters of various structural configurations and proportions.

A Simple Pendulum Technique for Achieving Seismic Isolation

An innovative seismic isolation system, the Friction Pendulum System (FPS), offers improvements in strength, versatility and ease of installation as compared to previous systems. Moreover, the approach offers several inherent performance benefits not available before. The FPS uses geometry and gravity to achieve the desired seismic isolation results. It is based on well known engineering principles of pendulum motion, and is constructed of materials with demonstrated longevity and resistance to environmental deterioration. The desirable isolation characteristics exhibited by FPS components hold the promise of an effective and practical system for significantly increasing the seismic resistance of new and existing buildings. This paper summarizes results of a comprehensive research and testing program to assess the technical performance of the FPS. In addition, an example building design using the FPS is given.

Seismic evaluation of existing reinforced concrete building columns

Past earthquakes have emphasized the vulnerability of reinforced concrete columns having details typical of those built before the mid-1970s. These columns are susceptible to axial-flexural, shear, and bond failures, which subsequently may lead to severe damage or collapse of the building. Research was undertaken to investigate the lateral and vertical load-resisting behavior of reinforced concrete columns typical of pre-1970s construction. Eight full-scale specimens were constructed and were loaded with constant axial load and increasing cyclic lateral displacement increments until failure. Test data are presented and compared with behavior estimated by using various evaluation methods.

Lessons from damage to steel buildings during the Northridge earthquake

One of the important overall surprises of the Northridge earthquake of 17 January 1994, was the widespread and unanticipated brittle fractures in welded steel beam-column connections. The economy, versatility and presupposed high plastic deformation capacity of welded steel moment-resisting frame (WSMF) buildings led to their common usage in Los Angeles and elsewhere in the US. No casualties or complete collapses occurred during the Northridge earthquake as a result of these connection failures, and WSMF buildings in areas of moderate shaking were not damaged at all. However, a wide spectrum of brittle connection damage did occur, ranging from minor cracking observable only by nondestructive testing to completely severed columns. This paper reviews the performance of steel buildings during the Northridge earthquake, and examines some of the results of studies undertaken as part of a project initiated by US Federal Emergency Management Agency (FEMA) to reduce the earthquake hazards posed by steel moment-resisting frame buildings. The objective of this project is to develop and verify reliable and cost-effective methods for the inspection, evaluation, repair, and rehabilitation of existing steel frame buildings and the construction of new ones. The project is conducted by the SAC Joint Venture, consisting of the Structural Engineers Association of California, the Applied Technology Council and the California Universities for Research in Earthquake Engineering.

Nonstationary ARMA modeling of seismic motions

Abstract Discrete time-varying autoregressive — moving average (ARMA) models are used to describe realistic earthquake ground motion time histories. Both amplitude and frequency nonstationarities are incorporated in the model. An iterative Kalman filtering scheme is introduced to identify the time-varying parameters of an ARMA model from an actual earthquake record. Several model verification tests are performed on the identified model. Applications of these identification and verification procedures are given and show that the proposed models and identification algorithms are able to capture accurately the nonstationary features of real earthquake accelerograms, especially the time-variation of the frequency content. The well-known Kanai-Tajimi earthquake model is covariance equivalent with a subset of the low order ARMA(2,1) model. Using the results and methodology of this study, the parameters of a time-varying Kanai-Tajimi earthquake model can be estimated from a target earthquake record or they can be directly associated with characteristic earthquake features such as predominant frequency and frequency bandwidth.

Vertical Response of Twelve Structures Recorded during the Northridge Earthquake

This paper summarizes the results of a study on vertical response of twelve instrumented structures recorded during the Northridge earthquake. The selected structures include buildings ranging in height from 2 to 14 stories, located within a distance range of 8 to 71 km from the causative fault of the Northridge earthquake. They include four steel buildings, five concrete structures and three base-isolated buildings. The recorded peak vertical structural accelerations are found to be larger than those at the base by a factor of 1.1 to 6.4. The identified lowest vertical frequencies of the structural members and systems of all the twelve structures are within the frequency range of 3.9 to 13.3 Hz (period range of 0.075 to 0.26 sec). For the structures located in the near-source region, this period range may correspond to a range of high vertical spectral accelerations.

Two new implicit algorithms of pseudodynamic test methods

Abstract Either measured displacements or calculated displacements are required in the available implicit algorithms. Two new formulations of the implicit algorithms will be presented herein. Neither measured displacements nor calculated displacements are used in the new algorithms. Hence, propagation errors can be significantly reduced in performing pseudodynamic tests. A series of simulations have been performed to study the error propagation characteristics of the algorithms. For the sake of comparisons, the error propagation characteristics of the implicit algorithm originally proposed by Thewalt and Mahin and that suggested by Shing and Manivanna are also considered in this study.

A SOFTWARE FRAMEWORK FOR HYBRID SIMULATION OF LARGE STRUCTURAL SYSTEMS

Hybrid simulation is a versatile, powerful and economically viable experimental method. In particular, through the use of numerical partitioning concepts, it should be possible to test portions of a large structure in one of more laboratories, and to simulate the remainder of the structure/foundation system numerically. Considerable efforts have been expended to develop such capabilities, but their adoption has been hampered by the absence of a common framework for conducting hybrid simulation across various laboratories and computing environments. Therefore, based on object-oriented software design methodologies, a set of interrelated software classes is described herein, which in their unity form a network-enabled framework for integrating experimental testing with standard structural analysis frameworks. In this paper, the open-source Framework for Experimental Setup and Control (OpenFresco) is used in combination with the object-oriented finite element software framework OpenSees (Open System for Earthquake Engineering Simulation) and the graphical user interface OpenSees Navigator. This enables an unlimited number of users to connect multiple laboratories throughout the world, to define specialized experimental setups, to implement advanced integration operators and to utilize high performance control methodologies, in a highly modular and scalable fashion. By utilizing both the object-oriented experimental and finite element software frameworks, response features that are frequently encountered in large structural systems (e.g., geometric nonlinearities, three-dimensional effects, multiple support excitation and soil-structure interaction) can be investigated by incorporating them into the analytical model. One of the most advantageous applications to incorporate geometric nonlinearities into the analytical model is in large displacement tests where a structure is tested until collapse. Contrary to shaking table testing, no large physical masses have to be present in the experimental part of a hybrid simulation. Another example presented illustrates the ability to account for multiple support excitations in long span structures. By using a standardized framework for communication, simulation and control, such tests can be easily set up in various laboratories to test large structures not possible by other means.

Strongback System: A Way to Reduce Damage Concentration in Steel-Braced Frames

AbstractThis paper examines a newly developed seismic force-resisting system: the strongback system (SBS). To achieve improved seismic performance, this system combines aspects of a traditional concentric braced frame with a mast to form a hybrid system. The mast acts like a strong back to help resist the tendency of concentric braced frames to concentrate damage in one or a few stories during severe seismic excitations. The purpose of the strongback system is to promote uniform story drifts over the height of a structure. Three SBS prototypes were designed and analyzed considering a variety of earthquake excitations. Computed responses are compared with responses for three other braced frame systems. Results of quasi-static inelastic analyses, both monotonic and cyclic, are presented to demonstrate differences in the fundamental hysteretic behavior of the braced frame systems considered. A series of nonlinear dynamic response history analyses were then performed to compare the global and local dynamic re...

Seismic Evaluation and Retrofit of Asymmetric Multi-Story Wood-Frame Building

The presented research focuses on large-scale seismic testing under multi-directional ground motion of a three-story high, wood-frame residential building representing late 1960s California construction. Earthquake lateral resistance is provided by plywood shear walls around the perimeter of the building with an open front in the first story for tuck-under parking. Accordingly, the as-built structural configuration is asymmetric in plan and discontinuous in elevation with tendency to twist about a vertical axis and to form a weak story mechanism. The test results confirm this tendency. They also reveal the sensitivity of the response to multi-direction ground motion. Asymmetric damage patterns are induced by the multi-component motions in the walls oriented perpendicular to the open front for the as-built test structure, with or without finish materials. However, the observed damage remained noncritical as far as structural integrity is concerned even for ground accelerations exceeding 120% of that recorded during Northridge earthquake. This is viewed as a consequence of the better construction of the test building compared to actual construction. Investigated retrofit includes adding a welded moment resisting steel frame around of the garage opening and strengthening the diaphragm to header beam connections. The study indicates that the retrofit significantly reduced the maximum story drift in the open front. Moreover, the finish material and the retrofit greatly reduce the maximum rotation of the building about the vertical axis.