Maria Koliou

Distributed Yielding Concept for Improved Seismic Collapse Performance of Rigid Wall-Flexible Diaphragm Buildings

AbstractRigid wall-flexible diaphragm (RWFD) buildings are a common type of single-story construction in North America, Europe, and New Zealand that incorporate rigid in-plane concrete or masonry walls and flexible in-plane wood, steel, or hybrid roof diaphragms. RWFD buildings have shown poor seismic performance during past earthquake events. In particular, it has been observed that the global seismic response is dominated by the response of the diaphragm, which is mainly attributed to large in-plane diaphragm displacements that significantly exceed the displacements of in-plane walls. In this study, the concept of distributed yielding in the flexible diaphragm by weakening certain intermediate diaphragm zones is explored as a cost-effective means to improve the seismic collapse capacity of RWFD buildings and mitigate their seismic vulnerability. A two-dimensional numerical framework was developed specifically for analyzing RWFD buildings and was used to evaluate the proposed concept. Results of nonlinea...

Seismic Response of High-Voltage Transformer-Bushing Systems Incorporating Flexural Stiffeners II: Experimental Study

High-voltage bushings have exhibited vulnerability during past earthquakes. The good performance of bushings mounted on a rigid base observed during shake table testing does not correlate well with their performance in the field. It is suspected that the seismic performance of high-voltage bushings is improved when mounted on a rigid base, as opposed to when mounted on more flexible cover plates of transformers. In this first of two companion papers, the seismic response of bushings was investigated numerically for various mounting conditions. The addition of flexural stiffeners on the transformer cover plates was explored as a means to stiffen the base of the bushings and mitigate their seismic vulnerability. Linear dynamic analyses conducted on four transformer-bushing system models showed that the simple approach of stiffening the cover plates of transformers is beneficial to the seismic response of high-voltage bushings.

Buildings with Rigid Walls and Flexible Roof Diaphragms. I: Evaluation of Current U.S. Seismic Provisions

AbstractBuildings having rigid walls and flexible roof diaphragms (RWFD) are a type of building construction widely used for light industry in the United States; they incorporate rigid in-plane concrete or masonry walls and flexible in-plane wood, steel, or hybrid roof diaphragms. In this first of two companion papers, the probability of collapse of this type of building designed to current code provisions in the United States, given an MCE earthquake event, is evaluated according to the FEMA P695 methodology for a large set of representative building archetypes. The results of the study indicate that current code provisions for this type of building do not satisfy the collapse objective requirements of FEMA P695 under maximum considered earthquake ground motions. This is because the analysis provisions are based on assumed yielding of the walls rather than the roof diaphragm. Also, current code provisions considerably underestimate the period of RWFD buildings. To assist with creating provisions that tak...

Buildings with Rigid Walls and Flexible Roof Diaphragms. II: Evaluation of a New Seismic Design Approach Based on Distributed Diaphragm Yielding

AbstractA seismic collapse evaluation study of buildings with rigid walls and flexible roof diaphragms (RWFD) presented in the companion paper indicates that this type of structure, as designed to current seismic design provisions in the United States, does not satisfy FEMA P695 performance criteria for Risk Category II Buildings. The seismic performance of RWFD buildings is often characterized by large deformations and yielding in the roof diaphragm rather than in the vertical elements of the seismic force–resisting system (SFRS). In this paper, a new seismic design approach is proposed to account for flexible roof diaphragm response. The proposed approach relies on distributed yielding in the roof diaphragm as the predominant inelastic response under extreme ground shaking. This is obtained by strengthening the end diaphragm regions, thereby allowing yielding to spread deeper into the diaphragm. The basic steps of the design approach and its limitations are described. A validation study of the proposed ...

Performance Assessment of Tilt-Up Big-Box Buildings Subjected to Extreme Hazards: Tornadoes and Earthquakes

AbstractThis paper investigates the response of tilt-up (referred to as “big-box”) buildings subjected to two extreme hazards to which they have been observed to be susceptible—high winds (tornadoe...

Assessing Modeling Complexities on the Seismic Performance of an Instrumented Short-Period Hospital

AbstractLow-rise, short-period buildings dominate the construction in the United States including residential, recreational, and critical, including hospital, facilities. Previous analytical studies have indicated that these structures behave poorly under seismic excitation compared to buildings with longer fundamental periods; however, such trend has not been observed in previous earthquake events. In this paper, a case study is considered to better evaluate the dynamic response of short-period light-frame wood buildings and help identify the modeling parameters and level of complexity required to accurately capture their dynamic response. For this purpose, a portion of the Twin Cities Community Hospital, located in Templeton, California (called herein the Templeton Hospital), was considered. This one-story light-frame wood building with a large roof diaphragm layout did not suffer any structural damage during the 2003 San Simeon earthquake despite being subjected to intense ground shaking. The Templeton...

Development of wood and steel diaphragm hysteretic connector database for performance-based earthquake engineering

Performance-based earthquake engineering (PBEE) considers certain metrics to assess the seismic response of buildings, which integrate economic losses into the design process. PBEE requires the development and use of reliable nonlinear response analysis models to simulate the seismic performance of structures through collapse. The structural damage is assessed by evaluating physical damage caused by engineering demand parameters (EDPs), while the nonlinear numerical models are used to conduct dynamic analyses for varying levels of seismic intensity to compute the values of the representative EDPs. Accurate representation of structural members’ stiffness and strength deterioration (hysteretic) parameters plays an important role into simulating dynamic response through collapse. These parameters’ values are usually calibrated to a large number of experimental data. The development of a hysteretic parameter database for wood and steel diaphragm connectors is presented in this paper. The wood diaphragm connectors are commonly used in light-frame wood building construction for shear walls or roof diaphragms. The steel diaphragm connectors are used for building structures that incorporate steel frame roof diaphragms. The experimental data were used for quantifying the hysteretic parameters of two well-known nonlinear models considered into structural modeling as well as evaluating their energy dissipation properties. Case studies on the collapse performance assessment of a light-frame wood wall system and a low-rise building incorporating a steel roof system were conducted to demonstrate the usefulness of the diaphragm connector database.

Development of seismic design methodologies for rigid wall-flexible diaphragm structures

Evidence indicates that the dynamic behavior of Rigid Wall – Flexible Diaphragm (RWFD) structures is dominated by the diaphragm’s response instead of the walls’ response, and this is a significant departure from the underlying assumptions of the widely used equivalent lateral force method in current building codes. RWFD buildings are common in North America and other parts of the world, and incorporate rigid in-plane concrete or masonry walls and flexible in-plane wood or steel roof diaphragms. With the use of a numerical computer modeling framework developed specifically for this type of building, this study sets out to investigate the seismic response of a variety of building archetypes with the intent to develop a simpler, more rational approach to the engineering design of RWFD buildings. A representative list of building archetypes is developed accounting for a variety of common parameters found in North America involving the building size, shape, diaphragm material, and diaphragm connections. Archetype designs are developed under ASCE/SEI 7-10 and this study’s proposed approach to develop design methodologies uses the FEMA P-695 methodology to evaluate building performance. In addition, two separate seismic force levels were utilized during the assessment representing both moderate and high seismic exposures to evaluate the impact of these parameters.