An Alternative Approach for the Design of Chevron-Braced Frames

The design of steel chevron-braced frames as per Eurocode 8 is based on the idea that only the braces should buckle and yield during ground motions, while other members should remain elastic. The elastic design of the braced frames is also allowed. However, in both cases, the seismic performance of the frame may be compromised because of premature yielding/buckling of columns. This paper proposes an alternative design procedure that promotes yielding of beams in addition to yielding of braces. This mitigates the vertical unbalanced force transmitted by compressive and tensile braces to the beam and in turn reduces the internal forces of the columns. The result is the overall improvement of the seismic performance owing to the reduction of the number of cases in which failure of the columns occurs before full exploitation of the ductility capacity of the dissipative members. The proposed design procedure is validated by incremental dynamic analyses performed on a set of chevron-braced frames. In particular, the peak ground accelerations of the frames designed by the proposed procedure at the attainment of Significant Damage and Collapse Prevention limit states are determined and compared to those of frames designed according to Eurocode 8. Furthermore, frames designed according to the Eurocodes and to the proposed method are compared in terms of structural cost.

Assessment on the deflection amplification factor of steel buckling-restrained bracing frames

Researchers in the field of earthquake engineering are always looking for new ways to improve the seismic behavior of structures. The buckling-restrained brace (BRB) is one of these exciting innovations that are employed to increase the ductility capacity of traditional steel braced frames. Understanding the nonlinear response of these novel systems in estimating maximum displacements due to an earthquake has been of significant importance for structural designers. Accordingly, this research is carried out to study of deflection amplification factor ( C d) in BRBs, which have recently been presented in seismic design provisions as one of the seismic lateral-resisting systems. To this end, five 3-, 5-, 7-, 10-, and 15-story BRBs are modeled in the software framework of OpenSees. Ground motion simulation is performed by selecting several scaled earthquake records, and the values of elastic and ultimate displacements of structural systems are computed through pushover and nonlinear time-history analyses. The results showed that the deflection amplification factor suggested within famous building codes (such as ASCE-7-16) compared to the obtained values is, in some cases, for certainty; conversely, it is underestimated under some conditions. In fact, the findings indicate that the magnitude of C d in these systems is strongly related to the height of the building.

Quasi-static tests on RC building columns strengthened with CFRP

To explore the possibilities and benefits of using CFRP (Carbon Fibre Reinforced Polymers) in strengthening RC building columns, quasi-static tests (compression and bending) were carried out at the Institute of Earthquake Engineering and Engineering Seismology - IZIIS, Skopje by variation of concrete class, reinforcement percentage and by using various strengthening technologies. Some recommendations and outcomes regarding the approach, technology and conclusions drawn from practical application of these materials, are given. Based on the analysis of values obtained from nonlinear static and nonlinear time history analyses, it can be concluded that the ductility capacity for displacement of model strengthened with CFRP is greater by 60 %, while its strength capacity is greater by 7.7 % when compared to the values obtained for the model without CFRP. It can generally be concluded that CFRP systems are a very practical tool for strengthening and retrofitting concrete structures, as they can extensively improve flexural strengthening, shear strengthening, column confinement, and ductility.

Confinement Effect of Reinforced Concrete Columns with Rectangular and Octagon-Shaped Spirals

Conventional spiral-type transverse reinforcement is effective at increasing the ductility and the maximum strength of reinforced concrete (RC) columns because it confines the inner concrete and the longitudinal reinforcement. However, when arranging crossties in a RC column with spirals, problems such as mutual interference with longitudinal reinforcement, overcrowding of reinforcement, and deterioration of constructability occur. Furthermore, the loosening of 90 and 130-degree standard hooks due to the lateral expansion of concrete causes buckling of the longitudinal reinforcement. This paper describes the ability of a newly developed spiral-type transverse reinforcement with various yield strengths to confine RC columns subjected to cyclic lateral load and constant axial load. The ductility capacity, energy dissipation, and effective stiffness of RC columns confined by the developed spiral-type transverse reinforcement were compared with those of RC columns confined by typical rectangular reinforcement. The experimental results showed that RC column specimens with the developed spiral-type transverse reinforcement have better performances in terms of ductility capacity and energy dissipation, even though the amount of reinforcement used for the specimens decreased by about 27% compared with the specimen with typical rectangular reinforcement.

Experimental study on the flexural behavior of flat steel-concrete composite beam

In this study, a new type of composite beam named flat steel-concrete composite beam is suggested for the rapid development of assembly structure. The experiment of seven flat steel-concrete composite beams and one traditional steel-concrete composite beam are performed to investigate their flexural behaviors. The failure mode of bending-shear fracture for the flat steel-concrete composite beam is found while the traditional steel-concrete composite beam fails by compression-bending fracture. The plane cross-section assumption is applicable to the flat steel-concrete composite beam. Compared with the traditional steel-concrete composite beam, the flat steel-concrete composite beam exhibits higher flexural stiffness, higher moment capacity, and higher ductility capacity. Based on the design theory of the traditional steel-concrete composite beam, simplified models for estimating the flexural stiffness and ultimate moment capacity of the flat steel-concrete composite beam are proposed, and they agree well with test data.

Simplified Data-Driven Model for the Moment Curvature of T-Shaped RC Shear Walls

Sectional deformation quantities, such as curvature and ductility, are of prime significance in the displacement-based seismic design and performance evaluation of structural members. However, few studies on the estimates of curvatures at different limit states have been performed on asymmetric flanged walls. In this paper, a parametric study was performed for a series of T-shaped wall cross-sections based on moment-curvature analyses. By investigating the effects of the axial load ratio, reinforcement content, material properties, and geometric parameters on curvatures at the yield and ultimate limit state, we interpret the variation in curvature with different influencing factors in detail according to the changes of the neutral axis depth. Based on the regression analyses of the numerical results of 4941 T-shaped cross-sections, simple expressions to estimate the yield curvature and ultimate curvature for asymmetric flanged walls are developed, and simplified estimates of the ductility capacity including curvature ductility and displacement ductility are further deduced. By comparing with the experimental results, we verify the accuracy of the proposed formulas. Such simple expressions will be valuable for the determination of the displacement response of asymmetric flanged reinforced concrete walls.

Response of Mid-Rise Reinforced Concrete Frame Buildings to the 2017 Puebla Earthquake

The response of mid-rise reinforced concrete (RC) buildings in Mexico City after the 2017 Puebla Earthquake is assessed through combined field and computational investigation. The Mw 7.1 earthquake damaged more than 500 buildings where most of them are classified as mid-rise RC frames with infill walls. A multinational team from Colombia, Mexico, and the United States was rapidly deployed within a week of the occurrence of the event to investigate the structural and nonstructural damage levels of over 60 RC buildings with 2–12 stories. The results of the study confirmed that older mid-rise structures with limited ductility capacity may have been shaken past their capacity. To elucidate the widespread damage in mid-rise RC framed structures, the post-earthquake reconnaissance effort is complemented with inelastic modeling and simulation of several representative RC framing systems with and without masonry infill walls. It was confirmed that the addition of non-isolated masonry infills significantly impacts the ductility capacity and increases the potential for a soft-story mechanism formation in RC frames originally analyzed and designed to be bare systems.

Effects of Soil-Structure Interaction on the Inelastic Seismic Response of Continuous Bridges on Piled Foundation

Effect of soil-structure interaction (SSI) on seismic response of bridges is not clearly understood, and in general practice, bridge design is carried out ignoring its effect. This is due to the general consensus that fixed base leads to a more conservative design despite the fact that many researchers indicate that ignoring soil-structure interaction may lead to underestimation of seismic response. The current paper aims to investigate the effect of SSI on the nonlinear seismic behavior of 9-span continuous bridge supported on pile foundation penetrating sandy soils. Three types of soils were investigated representing medium to stiff sandy soil. Both pushover analysis and nonlinear time history incremental dynamic analysis are carried out using Opensees to investigate the effect of SSI on the seismic response parameters (namely, fundamental period, pushover curves, foundation rigid body motions, global ductility capacity and demand, and maximum drift ratio) of the bridge and to compare it to fixed base assumption (i.e., SSI ignored). The results indicate that although SSI increases the flexibility of the structure (accordingly increasing fundamental period), the seismic demand of the bridge increases. This increase is more pronounced as the soil becomes softer.