Hitoshi Kuwamura

Application of wavelets to analysis and simulation of earthquake motions

A method of applying wavelet transform to earthquake motion analysis is developed from the viewpoint of energy input structures, in which relationships between wavelet coefficients and energy input, namely energy principles in wavelet analysis are derived. By using the principles, time–frequency characteristics of the 1995 Hyogoken-Nanbu earthquake ground motions are analysed and time histories of energy input for various ranges of frequencies and epicentral distances are identified. Furthermore, a technique to simulate earthquake ground accelerations by wavelet inverse transform is developed on the condition that target time-frequency characteristics are specified. Structural responses to the simulated accelerations are compared with the target time–frequency characteristics, which shows satisfactory correlations between wavelet coefficients and energy responses in both time and frequency domains. Copyright

Ductile Fracture Simulation of Structural Steels under Monotonic Tension

This paper aims to predict ductile fracture of mild steel under monotonic loading only from the test results of notchless tensile coupons. A simple fracture model based on the concept of a damage index with only one model parameter is proposed to predict ductile fracture of structural steels. The model is based on an idea of a combination of the void growth model and Miner’s rule in incremental form. Moreover, a new method to modify the true stress-true strain data after necking initiates is proposed, and it is found that the hardening modulus of several structural steels after necking initiates is approximately the same. Finally, ductile fracture of smooth and notched steel specimens is numerically simulated for three types of structural steels, which proves simplicity and acceptable accuracy of the fracture model and the modification method of the true stress-true strain.

Brittle fracture under repeated high stresses

Abstract Experimental evidence of brittle fracture in cold press-braked square tube members is shown in this paper. The brittle fracture was preceded by fibrous cracks starting at the toes of the butt weld of the press-braked member connected to a diaphragm under reversals of bending moment beyond the elastic limit. To find out the causes of the brittle fracture, mechanical properties in the press-braked corner undergoing plastic strain and ageing were investigated. Sizeable decreases in ductile elongation capability and considerable reductions in notch toughness in the press-braked corner were observed.

Experimental and Numerical Study of Postbuckling Ductile Fracture of Heat-Treated SHS Stub Columns

AbstractPrevious studies have proposed a cyclic ductile fracture model by applying the void growth model and an established rule in incremental form, in which only a monotonic tensile coupon test is required to calibrate the parameters of the fracture model and the corresponding plasticity models. The model parameters were deduced from small-scale hourglass steel coupons, but not from large-scale specimens. In practice, however, ductile fracture after the occurrence of local buckling has been observed in steel members during recent huge earthquakes. This paper aims to establish a simulation method for the postbuckling cracking process on the basis of formerly proposed fracture and plasticity models. Experimental results demonstrate that the ultimate behaviors of heat-treated square hollow section (SHS) stub columns associated with plate buckling and ductile fracture under cyclic loading can be simulated with favorable accuracy.

Probabilistic advantage of vibrational redundancy in earthquake-resistant steel frames

Abstract Steel frames are, in general, indeterminate and thus they do not necessarily fail when some elements constituting the system are subjected to fracture or buckling during a severe earthquake. The purpose of this paper is to assess the advantage of such redundancy inherent in indeterminate structural systems in a probabilistic sense. Two structural models are introduced to clarify the benefits of vibrational redundancy. One is a dual system composed of two parallel subsystems which have different natural periods. The other is a single system composed of a single subsystem. The former is exemplified by a moment-resisting frame associated with bracing, while the latter by a bracing system. It is demonstrated numerically that the dual system can survive with a higher degree of safety than the single system under the uncertainty condition that earthquake ground motions which hit the systems have random spectral properties.

DISCUSSION ON RESEARCH ACTIVITY PUBLISHED AS “FAILURE CRITERION OF BLOCK SHEAR STRENGTH OF TIMBER PARALLEL TO THE GRAIN”

Experimental and analytical achievements on the block shear strength of wood in the above paper are discussed about the following items: 1) The proposed failure criterion seems inadequate, because the compressive stress does not act on both sides of the failure section. 2) The elastic modulus for FE analysis seems inadequate, because the Youngs modulus in the transverse direction is too small. 3) The proposed equation for bearing deformation seems inadequate, because it is contradictory to the proposed strength criterion. 4) The test specimens seem improperly arranged, because the directions of the grain are inclined or curved to the bearing force.

DISCUSSION ON RESEARCH ACTIVITY PUBLISHED AS“MANUAL CALCULATION METHOD OF EMBEDMENT STIFFNESS OF PARTIAL COMPRESSION PARALLEL TO THE GRAIN AND ITS STRENGTH PROPERTIES”

Experimental and analytical achievements on the bearing stiffness and strength of wood in the above paper are discussed about the following items: 1) The proposed formula for bearing stiffness is based on an improper model of stress distribution. 2) The equation for calculating local stiffness in the vicinity of the contact region seems improper. 3) The bearing stiffness calculated from the proposed formula shows improper relations with bearing width as well as specimen height. 4) The size effect in bearing strength seems illogical.