Ryota Matsui

Cumulative Cyclic Deformation Capacity of Circular Tubular Braces under Local Buckling

Tubular-section members are commonly used as seismic-resistant braces because they have a higher moment of inertia than open cross sections of equivalent area. However, the cumulative cyclic deformation capacity of tubular sections after buckling is smaller than that of open sections, such as H-shaped sections, because fracture is initiated by local buckling of circular tubular sections. To evaluate the seismic performance of such diagonal braces, it is essential to predict the cumulative cyclic deformation capacity of these braces before any fracturing. In this study, the cumulative cyclic deformation capacity of circular tube braces under local buckling was assessed by performing cyclic loading tests for a range of slenderness and diameter-to-thickness ratios. The mechanism of strain concentration in the tubular braces was studied in various types of analysis, and a method is proposed for assessing the cumulative deformation capacity before fracture based on the entire axial deformation of the braces.

Cumulative Deformation Capacity of Steel Braces under Various Cyclic Loading Histories

AbstractThe postbuckling behavior of seismic-resistant braces in steel frames under a cyclic axial force is often evaluated by time–history analyses; however, brace fracture is seldom considered. The authors previously proposed a physical model for predicting the moment of fracture of circular-tube braces after buckling using phenomenological hysteresis. However, the accuracy of that model was confirmed only against the test results of the gradually increasing amplitude loading protocol, and its applicability under other loading histories has not yet been verified. In this study, cyclic loading tests were carried out until fracture on circular-tube and H-section braces under various loading histories, followed by FEM analyses. The validity of the proposed formulas for evaluating the strain-concentration index under various loading histories was examined. The proposed method was used for predicting the moment of fracture and the cumulative deformation capacity until fracture, and the predictions agreed wel...

Seismic Design of Buckling-Restrained Brace in Preventing Local Buckling Failure

When a buckling-restrained brace (BRB) is composed of a flat steel core encased in a rectangular steel tube with infill mortar, the flat steel core develops high-mode buckling waves within the spaces occupied by compressible debonding layers when BRB is in compression. The wave crests and troughs collide with mortar and then acting outward forces on restrainer. The steel tube wall may bulge out if the restrainer is too weak to sustain the outward forces and the BRB may lose its compression carrying capability. The outward force can be estimated according to steel core high-mode buckling wavelength and the debonding layer thickness. The restrainer capacity in resisting outward forces can be estimated by using the upper bound theory in plastic analysis. The results of 39 BRB tests were compared in order to evaluate the effectiveness of steel tube capacity estimation methods. 24 BRBs exhibiting local bulging failure suggested that the steel tube capacity can be estimated by assuming the bulged wall as a wedge shape with five of its boundary developing flexural strength. The proposed estimation is conservative and can be adopted for BRB design in preventing local bulging failure for severe seismic services.

Collapse Analysis of Steel Frames Considering Fracture of Braces and End of Beams

The 2011 Tohoku Earthquake ruptured multiple faults simultaneously, generating strong ground motions over a long duration at long periods. Such severe events have recently begun to be considered in structural design, requiring explicit calculation of the low cycle fatigue capacity and fracture point. This paper presents a numerical study of dual moment and braced frames, with either conventional or buckling-restrained braces. Both brace and beam end fracture is considered, and incremental dynamic analysis (IDA) used to determine the ultimate failure mechanism under extremely large ground motions. Fragility curves are generated, demonstrating the significant role that member fracture has on the seismic performance of braced frames.

Shake Table Testing of Steel Braced Frame Considering Member Fracture

Fracture of concentric braced frames due to local buckling under cyclic loading often requires computationally intensive 3D shell models. For practical design, the authors have previously proposed a phenomenological stick model that accurately captures local fracture, referred to as the Direct Local Strain Evaluation Method. However, the accuracy of the proposed method has previously only been validated using quasi-static cyclic component tests. This research extends this work, validating the method under dynamic loading conditions using numerical studies and in-frame shake table testing. The method is shown to be in good agreement with the global response and to accurately capture fracture due to local buckling under cyclic dynamic loads.

Flexural Properties of Buckling-Restrained Brace Connections

Buckling-restrained braces (BRBs) achieve large peak and cumulative ductility capacities by restricting yielding to an encased core, while maintaining global stability. However, stability is often governed by the connections and is sensitive to the end fixity provided by the adjacent framing and gusset, and flexural continuity between the neck and restrainer. This paper presents simple analytical methods to determine the flexural properties of these key components. Full-depth gusset stiffeners are found to be highly effective in increasing the out-of-plane rotational stiffness (KRg), equivalent to doubling the thickness. An equivalent connection is proposed to account for the adjacent framing (KRf), but this may be neglected if KRf > 10∙KRg. This is typically satisfied if a diaphragm slab and transverse beam are provided, but may exceed beam torsional bracing requirements. The restrainer end moment transfer capacity is extended to mortar-filled RHS restrainers, confirming that neck insert ratios of Lin/Bn > 2.0 are required for full continuity.

APPLICATION OF SEISMIC ISOLATION BEARINGS FOR RACK WAREHOUSE TO PREVENT GOODS FALLING

The seismic response of rack warehouses is known to be reduced by horizontal sliding of the warehouse contents, which act as mass dampers. However, in past earthquakes business continuity has been interrupted due to damage from spilling, toppling or falling contents. In this paper, response control using seismic isolation is investigated. Because the total rack weight is constantly changing and the rack is often loaded at large eccentricities, Spherical Sliding Bearings (SSB) are proposed, which exhibit natural periods independent of the supported weight. Based on numerical and experimental test results undertaken previously, an analytical model is proposed including pressure and velocity dependent friction values. Using the proposed model, the performance of a seismically isolated rack warehouse with variable weight and eccentricity is studied and compared with conventional rubber bearings.

CYCLIC LOADING TESTS OF SUB-STANDARD RC FRAMES RETROFITTED WITH BUCKLING RESTRAINED BRACES AND ELASTIC STEEL FRAMES

Response control retrofit of existing RC buildings with buckling restrained braces (BRBs) assures immediate occupancy performance level after severe seismic events. This method is widely used in Japan and may improve the sub-standard buildings in overseas countries with high building importance factors e.g. school buildings in Turkey. Implementation of BRBs and elastically designed closed-steel frames in seismically deficient such RC frames would provide a much better damage distribution and mitigate the possible residual displacement after an earthquake. This paper describes near full-scale displacement-controlled cyclic testing of five specimens to meet the performance requirements given by the relevant codes in Japan. Special emphasis has been placed on the composite interaction between the RC frame and added elastic frame. Experimental results including hysteretic curves, dissipated energies, crack patterns on the RC elements, and strain histories are promising for the response control retrofit of sub-standard RC buildings located in seismically vulnerable areas.

OPTIMIZATION OF DAMPER ARRANGEMENT WITH HYBRID GA USING ELASTO-PLASTIC RESPONSE ANALYSIS ON SEISMIC RESPONSE CONTROL RETROFIT

Damper distribution rule with equivalent linearization method on seismic control retrofit for overseas brittle RC buildings was proposed by the authors. However, this result is not necessarily proved as optimal. Recently, Genetic Algorithm (GA) is applied to solve numerous kinds of structural optimization problems, mainly in the range of static analysis. In this paper, the optimization method of damper distribution with GA using elasto-plastic dynamic analysis for multi degree of freedom shear spring model is proposed. By using proposed optimization method, the optimal damper distribution, defined as minimum damper distribution satisfying the target story drift angle for seismic retrofit, is obtained. Finally, comparison of the results with GA and solutions with equivalent linearization method is represented and the effectiveness of proposed damper distribution rule is discussed.

STABILITY CONDITIONS OF BUCKLING RESTRAINED BRACES WITH SERIALLY CONNECTED TUBULAR MEMBERS

A Buckling Restrained Brace (BRB) with serially connected tubular members is one of the solutions for composing BRB over 10m length economically for the seismic design of truss structures or large-span structures. Their overall buckling condition is considered to be sensitive for imperfections or eccentricities comparing to ordinary BRBs; however, their effects are not confirmed by experiments yet. In this paper, the overall buckling conditions for BRBs with serially connected tubular members are discussed as the functions of imperfections or eccentricities. Their effects on the stability are confirmed by cyclic loading experiments until member fractures, and design criteria are proposed.