Showing papers on "Earthquake resistant structures published in 2012"

Unbonded Prestressed Columns for Earthquake Resistance

Abstract: Modern structures are able to survive significant shaking caused by earthquakes. By implementing unbonded post-tensioned tendons in bridge columns, the damage caused by an earthquake can be significantly lower than that of a standard reinforced concrete bridge column by reducing the amount of residual displacement. Reducing the amount of damage will allow for speedy repairs on the bridge and minimal closure time. The objective of this research was to investigate new construction methods for unbonded post-tensioned bridge columns that will reduce the amount of damage caused by an earthquake. Two 0.4-scale columns containing unbonded tendons were selected for testing. The two columns are identical except for the amount of longitudinal reinforcement crossing the joint between the column base and footing. For the specimens to be true scale models, the amount of post-tensioning required in a full scale column was taken into consideration. The large amount of prestress needed in a full-scale column requires separate tendons being spread out around the center of the column cross section. Following an earthquake, the tendons may need to be inspected or replaced. Anchoring the tendons to the side of the footing as opposed to the base of the footing was introduced for ease of replacement. The introduction of the unbonded tendons showed a significant reduction in residual displacements.

An Introduction to the Methodology of Earthquake Resistant Structures of Uniform Response

Abstract: Structures of Uniform Response are special earthquake resistant frames in which members of similar groups such as beams, columns and braces of similar nature share the same demand-capacity ratios regardless of their location within the group. The fundamental idea behind this presentation is that seismic structural response is largely a function of design and construction, rather than analysis. Both strength and stiffness are induced rather than investigated. Failure mechanisms and stability conditions are enforced rather than tested. Structures of Uniform Response are expected to sustain relatively large inelastic displacements during major earthquakes. A simple technique has been proposed to control and address the gradual softening of such structures due to local/partial instabilities and formation of plastic hinges. In structures of uniform response, the magnitude and shape of distribution of lateral forces affects the distribution of story stiffness in proportion with story moments, therefore affecting the dynamic behavior of the system as a whole. Simple closed form formulae describing the nonlinear behavior of moment frames of uniform response have been proposed. While the scope of this contribution is limited to moment frames, the proposed method can successfully be extended to all types of recognized earthquake resisting systems.

Numerical Insights into Liquefaction-Induced Building Settlement

Abstract: The effective design of earthquake resistant structures and liquefaction mitigation techniques requires an improved understanding of the consequences of liquefaction. To provide insight, a series of four centrifuge experiments were performed, which involved structural models founded atop a layered soil deposit that included a liquefiable layer. Results from these centrifuge experiments were utilized to evaluate the predictive capabilities of a state-of-the-practice numerical tool. Numerical simulations with the UBCSAND model implemented in FLAC-2D captured building settlements measured in these experiments reasonably well, mostly within a factor of 0.7 and 1.8. The soil model captured the overall contribution of deviatoric displacement mechanisms (i.e., SSI-induced building ratcheting and partial bearing failure) and localized volumetric strains during partially drained cyclic loading. The model was not able to capture all aspects of the soil's response to earthquake loading. In these experiments, it overestimated the extent of soil softening and building displacement for slower rates of earthquake energy buildup.

Taquezal Buildings in Nicaragua and Their Earthquake Performance

Abstract: Taquezal is a common earthen building type in Nicaragua It is constructed by building a wood frame and then packing the frame with mud to create thick earthen walls The wood frame allows the structure to be constructed without the formwork (which is required for rammed earth buildings) and without first constructing blocks (which is generally required in adobe construction) The wood frame also allows a thinner wall than other earthen building types Commonly, taquezal roofs are made of timber framing and heavy clay tile roofs Taquezal buildings are not engineered and therefore are difficult to analyze with modern structural engineering methods During the 1972 Managua earthquake, nearly 10,000 people died, and most of them were in taquezal buildings This paper discusses taquezal as a structural system and applies engineering methods to this nonengineered structure It was found that taquezal buildings perform well during low-to-moderate earthquakes if well maintained However, if the wood is

Numerical and experimental exploration of the fundamental nonlinear dynamics of self-centring damage resistant structures under seismic excitation

Abstract: Current elasto-plastic design has succeeded in reducing the number of casualties, during large seismic events, while economic losses have grown significantly. These financial losses have emphasized the need for more elaborate damage resistant structures. These structures are capable of exhibiting large deformations while still remaining elastic (although nonlinear), in contrast with current earthquake resistant structures which go plastic. Precast concrete frames with post tensioned tendons connecting elements are a typical example of this class of structure. A physical model of the aforementioned class of structure was tested both statically and dynamically. Through nonlinear system identification techniques, a simplified numerical representation of the physical model was generated and then validated against its physical counterpart. The physical and numerical models confirmed the existence of nonlinear dynamic phenomena, such as coexisting solutions over a wide frequency range, as well as sensitivity to harmonic and seismic excitation parameters, within both models.

Three Dimensional Displacement Response Study of a Rubble-House Using a 3D Laser Scanner

Abstract: After the devastating earthquake that hit Haiti in January 2010, a number of non-profit organizations started building cost effective replacement homes for the needy using the rubble from collapsed buildings. Rubble-Houses are environmentally friendly structures with walls comprised of recycled loose rubble placed in welded wire baskets. Rubble-Houses are assumed to be earthquake resistant structures due to their improved damping and ductility characteristics arising from the rubble and welded wire basket, respectively. However, the response of such structures under static and dynamic loads has not been studied in detail. In order to have a better understanding of its behavior, a full-scale rubble-house was built in the middle of Southern Polytechnic State University campus and subjected to a series of in-plane and out-of-plane static loads. Wall displacements were recorded using a 3D laser scanning technique in addition to total station and displacement gauge measurements. 3D laser scanning is an effective and efficient approach for precise and dimensionally accurate as-built documentation. This paper presents and discusses the use of 3D laser scanning technique in measuring the displacement response of a rubble-house under static loads.

A Quantitative Method for Assessing Seismic Collapse-Resistant Capacity for RC Frame Structures

Abstract: A quantitative method for evaluating the seismic collapse-resistant capacity for reinforced concrete structures is proposed. Based on the push-over analysis, pushing a building to collapse can obtain the maximum value of base shear force, corresponding to collapse critical point. The maximum shear force is divided by the total mass of the structure, and a horizontal structural acceleration called SA which represents the structure response is obtained. Then the maximum earthquake affecting coefficient can be calculated by SA and the characteristic period Tg. Finally peak ground acceleration (GA) is obtained, which can be a performance index to evaluate the seismic collapse-resistant capacity. The method is verified by two frame structure models, indicating that the method can compare the seismic collapse-resistant capacity of different buildings.

An Integrated Model to Design Earthquake Resistant Structures

Abstract: Selection of the material and design of earthquake resistance structures are an important issue today. Many people die every year due to inappropriate design and selection of the materials. There are several software to be used for structural design of buildings, however they just design the structure based on some limited standards. There is a need to develop a computer-based earthquake resistant design model to integrate the current market’s software with different design standards of different countries. The objective of this study is to propose a model to integrate the local structural design standards/codes with available market’s programs. To achieve this objective, Microsoft Excel was used as the core of the model to be integrated with one of the market’s program. Then, the model was developed in three phases. To test the model, the Iranian design standard (Code 2800) was used to design a 7-story apartment. The results show that the model can be fully integrated with those market’s programs which support Microsoft Excel. The result of Phase 1 of the model is useful to select the optimum selection of the material while Phase 2 and 3 contribute to design of the earthquake resistant structure.

Performance-based Seismic Evaluation of Wuhan Center

Abstract: This paper discusses the analysis procedure adopted for the performance evaluation of Wuhan Center in Wuhan City, China. The 88-story, 438-meter tower will become the tallest building in central area China upon completion. The selected structural system consists of a composite concrete core, five exterior belt trusses, three sets of steel outriggers and 16 concrete filled steel pipe super columns. Given the scale and importance of this building, a rather sophisticated evaluation of seismic performance during a severe earthquake (2% probability in 50 years or 2475-year return period) was performed. Two synthetic and five recorded ground motions were selected for the nonlinear time-history analysis. These ground motions were scaled to meet China Seismic Code provisions BG50011-2010. The performance levels for immediate occupancy, life safety and collapse prevention were set based on ASCE 41-06, “Seismic Rehabilitation of Existing Buildings” (2006). Complex 3-D computer models with geometric and material nonlinearities were generated for nonlinear dynamic time-history analyses. The detailed performance assessments of main structural elements such as coupling beams, core walls, super columns, outriggers and belt-trusses are presented in this paper.

Collapse Behavior and Ultimate Earthquake Resistance of Weak Column Type Multi-Story Steel Frame with RHS Columns

Abstract: In seismic design, moment resisting frame is designed to form the overall sway mechanism under severe earthquake. However, it is possible to form the weak column mechanism with the rise of strength of beams and panel zone due to strain hardening. The causes to collapse the weak column type frame are strong bi-axial seismic force, P-A effect and deterioration of restoring force. In this study, a series of response analyses of weak column type multi story steel frames under bi-axial ground motion are carried out. In the analysis, realistic hysteresis model of story including deteriorating range is used. From analytical results, collapse behavior and ultimate earthquake resistance of steel frames governed by deterioration of columns are evaluated.

Seismic-Resistant Connections between Precast Concrete Columns and Drilled Shafts

Abstract: In most areas of the country traffic is becoming more congested, and delays, more common. Highway construction, and especially construction that requires lane closures, exacerbates the delays, and imposes costs that can be measured in dollars, wasted fuel, carbon emissions, safety and personal stress levels. Methods of constructing bridges that require less time on site are therefore being developed in order to address these costs, and are referred to collectively as Accelerated Bridge Construction (ABC). One approach to ABC involves prefabricating concrete elements of the bridge off-site, and connecting them onsite. This procedure saves the time needed to erect formwork, assemble reinforcement cages and wait while the concrete gains strength. The elements are most conveniently cast and transported if they are line elements, such a straight beams and columns. In non-seismic regions, this presents no special problems. However, in seismic regions, precasting line elements means that the connections must be made at the intersections between them, which is where the moments are highest and the inelastic cyclic deformations the largest. Designing connections that are sufficiently robust to withstand severe seismic loading and are at the same time simple to complete is a challenge. This report described a method of connecting a precast concrete column to a drilled shaft foundation in a way that is suitable for use in a high seismic region. The method was adapted from the existing way of constructing a cast-in-place drilled shaft-to-column connection, and involves embedding the precast column into the top of the cast-in-place drilled shaft. The quantity and detailing of the reinforcement in the transition region, where the embedment occurs, is critical to achieving good seismic performance. Two scale specimens were tested in the laboratory, and the test results provided a lower bound on the amount of spiral reinforcement needed in the transition region. Analytical models were developed to describe the behavior of the connection.