Seismic Behavior Of Frames Research Articles

Seismic Behavior of Frames with Special-Shaped Concrete-Filled Steel Tubular Columns and H-Shaped Steel Beams

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Reliability assessment of RC frames rehabilitated by eccentrically braces having vertical shear link

A considerable number of existing reinforced concrete (RC) structures need to seismic rehabilitation due to several reasons such as being designed just based on gravity loading and/or having an unsatisfactory level of ductility. One of the types of steel bracing can be referred to eccentrically braced frames with a vertical link. This system has some advantages such as an increase in ductility, stiffness and lateral strength, the ability to adapt to the architecture, and also the minimum weight added to the structure. In this study, reliability analysis assessment of two existing 3-, and 9-story RC frames in two cases including original and rehabilitated with an eccentrically braced frame having a vertical link is presented. Two limit states are defined as: maximum roof displacement and maximum inter-story displacement. The seismic behavior of frames was assessed by nonlinear static pushover analysis with finite element program OpenSees in two performance levels, including collapse prevention and life safety. Five random variables represented the variability in resistance of concrete material, bars and steel profiles yield stress, beams height, columns dimension, and also bars cross-section. Sensitivity analysis was carried out to recognize the effect of random variables on the reliability index. The reliability analysis was performed by two different methods: Hasofer–Lind and Monte Carlo with 25, 100, 1000, 10,000 and 100,000 simulated samples by considering two distributions including Normal and Log-Normal. Finally, a comparison between two common reliability methods was carried out in order to select the most appropriate method for performing the best seismic performance reliability analysis of RC frames rehabilitated by the proposed system.

Seismic behavior of frames with weak bolted end plate connections rehabilitated by posttensioned tendons under near‐/far‐field earthquakes

SummaryPosttensioned tendons can be used for rehabilitation of frames as well as for providing self‐centering ability. The optimum selection of retrofit parameters is carried out based on their effect on the seismic behavior of a structure. The results of 2D numerical modeling of the frames in this paper revealed that the rehabilitation of a frame with weak end plate bolted connection by adding posttensioned tendons improves the seismic response of the weak frame to be close to that of the reference frame, whose connections are designed according to AISC code. It was observed that the roof displacements were reduced by 5%, and the maximum story drifts in near‐ and far‐field earthquakes were reduced by 8% and 2% in the rehabilitated frames on average compared to those of the reference frame. The results of 3D substructure numerical models, verified by experimental results, indicated that the changes in the tendon diameter from 12 to 18 mm increased the moment capacity and initial rotational stiffness by 38% and 60%, respectively, compared to those of the reference model. The increase in tendon distance from 18 to 22 cm resulted in 29% rise in moment capacity compared to that of the reference frame.

Cyclic behavior of lightly reinforced concrete moment frames with partial- and full-height masonry walls

Existing lightly reinforced concrete (RC) moment frames are vulnerable to earthquakes. The seismic behavior of these frames could be affected by the presence of masonry infill walls. The objective of this study was to investigate the seismic behavior of gravity-designed RC frames having partial- and full-height masonry infill walls. For this purpose, experimental and numerical studies were conducted. Three one-story and one-bay gravity-designed RC moment frames with and without partial- and full-height masonry infill walls were made and tested under cyclic lateral loads. Numerical models for RC moment frames and masonry walls were proposed based on test data. Nonlinear static and incremental dynamic analyses (IDAs) were conducted for three-story RC moment frames with and without partial- and full-height masonry infill walls using the numerical models. Both experimental and numerical studies demonstrated that the masonry-infilled RC frames had larger lateral strength and stiffness than bare RC frames, whereas their drift capacity was less than that of bare frames. The partial-height masonry-infilled RC model frame had the least collapse strength among the frames.

Seismic Behavior of Frames with Bolted End Plate Connections Rehabilitated by Welded Haunches Under Near- and Far-Fault Earthquakes

A retrofit method for bolted end plate connections is the use of haunches. This method needs no change or replacement of connection components. The optimum selection of rehabilitation parameters like thickness or haunch-beam angle is of great importance for designers. Besides, one of the important factors for designers to select this retrofit method, is the efficiency of retrofitting by haunches and the effectiveness of retrofit parameters on behavior of retrofitted frames in near- and far-field earthquakes. The results of 2D numerical modeling of the frames shows that rehabilitation of the weak end plate connections using haunch improves the seismic behavior of the weak frame relative to the reference frame with connections designed according to AISC code. For example, the roof displacements are reduced by 8% averagely and the maximum story drifts in near and far-field earthquakes are reduced by 22% and 12% in the rehabilitated frames compared to those of the reference frame. The results of 3D substructure numerical models, after verification by experimental results, showed that the increase of haunch thickness from 12 to 18 mm did not significantly increase the moment capacity and initial rotational stiffness. However, the pinching in moment-rotation hysteresis is decreased. The increase of haunch angle from 30 to 60 resulted in shift of plastic hinge to haunch internal area and 100% reduction of energy absorption and initial rotational stiffness compared to those of the reference specimen. Since the change of retrofit method parameters results in change of stiffness and natural time period of the structure and the affecting parameter on structure dynamic behavior is “the ratio between structure main time frequency to earthquake predominant time period”, the selection of the first haunch thickness with an angle of 30 degree toward the beam that satisfy the requirements of FEMA350 will be economic and optimized.

Seismic behavior of frames with innovative energy dissipation systems (FUSEIS1-2)

Modern seismic codes allow for inelastic deformations in dissipative zones during design earthquakes, accepting damage to a certain extent in the relevant structural parts. In the frame of a European Research Program “FUSEIS” two innovative seismic resistant systems were introduced and relevant design guides developed. The first system, named FUSEIS1, consists of a pair of closely spaced strong columns joined together by a number of fuses. This system is further subdivided in FUSEIS1-1 where the dissipative fuses are beams and in FUSEIS1-2 where the fuses are short pins. This paper reports on the seismic performance of buildings with FUSEIS1-2 systems. It discusses results of nonlinear static and dynamic analyses that were based on element properties as derived by the tests and provides design recommendations and appropriate behavior factors. The results confirm the good seismic performance of the system under seismic loading and its capacity to develop self-centering properties under certain conditions.

Experiment on Seismic Behavior of Frame with Specially Shaped Columns Used Wide Flange in the First Floor and General on the above

Studies show that the stiffness of the first floor column about concrete frame with general specially shaped column is insufficient, the column bottom will be compression damaged easily, the author advice using wide flange specially shaped column in the first floor to enhance the stiffness, so this paper designed a 1/3 scale RC frame with specially shaped columns used wide flange in the first floor and general on the above, furthermore the height of the first floor was higher than other floors. Then low frequency cyclic loading was experimented to the concrete frame, the seismic behavior of this kind frame such as damage characteristic, failure mechanism, bearing capacity, deformation capacity, hysteresis curve and storey drift were researched. It was shown that the seismic behavior of this structure was generally good, the method that changed the first floor to wide flange specially shaped column for concrete frame with specially shaped columns could enhance the stiffness of the first floor clearly, meanwhile the seismic behavior of the frame were improved.

Seismic behavior of frames with innovative energy dissipation systems (FUSEIS 1-1)

After strong earthquakes conventional frames used worldwide in multi - story steel buildings (e.g. moment resisting frames) are not well positioned according to reparability. Two innovative systems for seismic resistant steel frames incorporated with dissipative fuses were developed within the European Research Program "FUSEIS" (Vayas et al. 2013). The first, FUSEIS1, resembles a vertical Vierendeel beam and is composed of two closely spaced strong columns rigidly connected to multiple beams. In the second system, FUSEIS2, a discontinuity is introduced in the composite beams of a moment resisting frame and the dissipative devices are steel plates connecting the two parts. The FUSEIS system is able to dissipate energy by means of inelastic deformations in the fuses and combines ductility and architectural transparency with stiffness. In case of strong earthquakes damage concentrates only in the fuses which behave as self-centering systems able to return the structure to its initial undeformed shape. Repair work after such an event is limited only to replacing the fuses. Experimental and numerical investigations were performed to study the response of the fuses system. Code relevant design rules for the seismic design of frames with dissipative FUSEIS and practical recommendations on the selection of the appropriate fuses as a function of the most important parameters and member verifications have been formulated and are included in a Design Guide. This article presents the design and performance of building frames with FUSEIS 1-1 based on models calibrated on the experimental results.

Experimental and Nonlinear Finite Element Analysis on Seismic Behavior of SRC Column and Steel Beam Planar Frame

Based on experiment of two mixed frame with SRC columns and steel beam under low-reversed cyclic loading, hysteresis behavior, ductility, energy dissipation are studied. The test results show that the mixed frame with SRC columns has better seismic performance. The nonlinear finite element analysis on the seismic behavior of the frame specimen was conducted using Openness developed by the Pacific Earthquake Engineering Research (PEER) Center. The calculated results are much coincided with the tested results.