Experimental Investigation of the Effects of Concrete Alkalinity on Tensile Properties of Preheated Structural GFRP Rebar

The combined effects of preexposure to high temperature and alkalinity on the tensile performance of structural GFRP reinforcing bars are experimentally investigated. A total of 105 GFRP bar specimens are preexposed to high temperature between 120°C and 200°C and then immersed into pH of 12.6 alkaline solution for 100, 300, and 660 days. From the test results, the elastic modulus obtained at 300 immersion days is almost the same as those of 660 immersion days. For all alkali immersion days considered in the test, the preheated specimens provide slightly lower elastic modulus than the unpreheated specimens, showing only 8% maximum difference. The tensile strength decreases for all testing cases as the increase of the alkaline immersing time, regardless of the prehearing levels. The tensile strength of the preheated specimens is about 90% of the unpreheated specimen for 300 alkali immersion days. However, after 300 alkali immersion days the tensile strengths are almost identical to each other. Such results indicate that the tensile strength and elastic modulus of the structural GFRP reinforcing bars are closely related to alkali immersion days, not much related to the preheating levels. The specimens show a typical tensile failure around the preheated location

Fragility analysis of structures with controlled rocking beamcolumn connections and viscous dampers

Controlled rocking system has been proposed as an alternative design method to improve the structural performance of buildings. The rocking systems allow vertical and horizontal components to rotate freely at boundaries. When integrated in a frame system, rocking columns may reduce the yielding strength of the entire system. The acceleration response of yielding structure is proportional to its own weight, but it is limited by the yield strength. Thus using a rocking system, a limited acceleration response can be achieved. However, the displacements of a structure may become undesirably large due to lower strength, but they can be controlled adding damping. A simplified model of the rocking column was developed and verified through experimental tests conducted at the University at Buffalo. As a case study in this paper, the response of an analytical model of a scaled reinforced concrete frame structure is considered. Synthetic ground motions developed using the Barrier model are used as seismic input of the nonlinear dynamic analysis. Fragility analyses are performed to show the seismic performance of the original structures and the retrofitted structure with the rocking beam-column connections and viscous dampers. The analysis shows that the story accelerations can be limited by using rocking columns, while the story displacements can be controlled by using viscous dampers

Fragility analysis of structures with controlled rocking beamcolumn connections and viscous dampers

Controlled rocking system has been proposed as an alternative design method to improve the structural performance of buildings. The rocking systems allow vertical and horizontal components to rotate freely at boundaries. When integrated in a frame system, rocking columns may reduce the yielding strength of the entire system. The acceleration response of yielding structure is proportional to its own weight, but it is limited by the yield strength. Thus using a rocking system, a limited acceleration response can be achieved. However, the displacements of a structure may become undesirably large due to lower strength, but they can be controlled adding damping. A simplified model of the rocking column was developed and verified through experimental tests conducted at the University at Buffalo. As a case study in this paper, the response of an analytical model of a scaled reinforced concrete frame structure is considered. Synthetic ground motions developed using the Barrier model are used as seismic input of the nonlinear dynamic analysis. Fragility analyses are performed to show the seismic performance of the original structures and the retrofitted structure with the rocking beam-column connections and viscous dampers. The analysis shows that the story accelerations can be limited by using rocking columns, while the story displacements can be controlled by using viscous dampers

Seismic performance of segmental rocking columns connected with NiTi martensitic SMA bars

As well known, the damping or energy dissipation capacity of bridge columns is very important because it is directly related to the design spectrum and affects the seismic performance of whole bridge systems. For this reason, several investigations evaluating the energy dissipation capacity of segmental columns, which has a small energy dissipation capacity itself, have been conducted by adopting various material types of energy dissipating (ED) bars. This paper investigates the damping capacity of post-tensioned (PT) segmental rocking columns connected with large diameter (36.5 mm) martensitic SMA bars at their base as ED bars. Two aspect ratios for the columns are considered: 7.5 for slender and 5.0 for medium size. Moment-curvature relationships and complementary computational tools are adopted to model the behavior of the columns. A bilinear model for the PT tendon and a modified four-spring model for the martensitic SMA bar are used. From the quasistatic cyclic analysis, the martensitic SMA bars leads to an equivalent damping ratio between 10.5% and 12.5% for the different aspect ratios and PT tendon forces. For the sake of comparison, another material type of the SMA bar, which is 25.4 mm diameter superelastic bar, is considered. The damping ratio of the columns with the martensitic SMA bars is much higher than the use of the superelastic SMA bars showing between 5% and 7% damping rati

Seismic performance of industrial sheds and liquefaction effects during may 2012 emilia earthquakes sequence in Northern Italy

On May 20, 2012 at 2: 03 UTC, a Mw 6.1 earthquake occurred in Emilia Region of Northern Italy. The event was preceded by a Ml 4.1 foreshock on May 19, 2012 at 23: 13 UTC, and followed by several aftershocks, twenty of them with a magnitude Mw greater than 4. The epicentral area of the seismic sequence covers alluvial lowland that is occupied by both agricultural and urbanized areas. Liquefaction effects were observed in several villages on the west side of Ferrara which were built upon former river beds such as the Reno River. The Emilia seismic sequence resulted in 27 casualties, several of whom were among the workers in the factories that collapsed during working hours, and there was extensive damage to monuments, public buildings, industrial sites and private homes. Almost no municipalities hit by 2012 earthquake were classified as seismic area before 2003; therefore, most of the existing structures had been designed without taking in account the seismic actions. The main aims of MCEER field mission was to document the emergency response and the most common damage mechanisms of industrial sheds during Emilia earthquake sequence which are shown and discussed in detai

Spectral and fragility evaluations of retrofitted structures through strength reduction and enhanced damping

A retrofit procedure for existing buildings called the "weakening and damping technique" (WeD) is presented in this paper. Weakening of structures can limit the maximum response accelerations during severe ground motions, but leads to an increase in the displacements or inter-story drifts. Added damping by using viscous dampers, on the other hand, reduces the inter-story drifts and has no significant effect on total accelerations, when structures behave inelastically. The weakening and damping technique addresses the two main causes for both structural and nonstructural damage in structures. The weakening retrofit is particularly suitable for structures that have overstressed components and weak brittle components. In this paper, the advantages of the WeD are verified by nonlinear dynamic analysis and simplified spectral approach that has been modified to fit structures with additional damping devices. A hospital structure located in the San Fernando Valley in California is selected as a case study. The results from both analyses show that the retrofit solution is feasible to reduce both structural acceleration and displacement. A sensitivity analysis is also carried out to evaluate the effectiveness of the retrofitting method using different combinations of performance thresholds in accelerations and displacements through fragility analysi