This paper briefly reviews the development of earthquake resistant design of buildings. Measurement of ground acceleration started in the 1930s, and response calculation was made possible in the 1940s. Design response spectra were formulated in the late 1950s to 1960s. Non-linear response was introduced in seismic design in the 1960s and the capacity design concept was generally introduced in the 1970s for collapse safety. The damage statistics of reinforced concrete buildings in the 1995 Kobe disaster demonstrated the improvement of building performance with the development of design methodology. Buildings designed and constructed using out-dated methodology should be upgraded. Performance-based engineering should be emphasized, especially for the protection of building functions following frequent earthquakes.
Large shake table test and subsequent numerical analysis correlation were conducted to develop and validate a suitable nonlinear FEM model for the seismic performance evaluation of underground RC structures, enhancing the existing skeleton and hysteresis rules for RC members. A trilinear nonlinear RC member model that represents the effect of reinforcing bar pullout was developed and validated through numerical correlation analysis using past static loading tests. The shake table test results demonstrate that the deformation of model RC structure is fully governed by ground deformation both in the elastic and inelastic ranges. The FEM model developed here derives estimations that show good correlation with test results in terms of such parameters as structural deformation, shear stress distribution on the upper slab surface and concrete cracks and reinforcing bars yielding events, because of the successful parameter identification of nonlinear soil and RC member models.
This paper reports the results of experimental and analytical investigation on the response behavior of reinforced concrete piles under ground. From the experimental results, it was clarified that the axial load at pile head affects the restoring force degradation and the maximum damage point is dependent on the relative stiffness between the pile and surrounding soil. From the analytical study using 3-dimensional FEM analysis, the experimental behavior could be adequately simulated by the applied method. Further investigations on the shapes or areas of hysterisis loops will be needed for the future application of this method to seismic performance evaluation of the entire structure-pile foundation-soil system.
The sustainability of a vital RC highway, which was constructed prior to the implementation of stringent seismic design codes, was assessed. The RC columns of the highway frames were strengthened by steel jacketing. Based on a subsequent evaluation of the performance of the strengthened frames, it was pointed out that members of some frames might be incapable of resisting the same seismic loads, which the strengthened columns may sustain. An experimental investigation that consisted of four small-scaled models was carried out employing reversed cyclic loading tests. Two specimens, which represent critical prototype frames were tested. The RC beams experienced shear failures at low response ratio levels. Accordingly, strengthening of the RC beams was shown to be of a significant importance. Testing of another two specimens enabled evaluating the positive features of wrapping the shear-deficient RC beams with Aramid Fiber Reinforced Polymer (AFRP) sheets as well as enabled providing practical recommendations. Moreover, analytical modeling of the highway frames utilizing FEM was performed. The numerical simulation resulted in a satisfactory accuracy of the predicted behavior. The calibrated base-line FE model was employed to quantitatively identify which of the RC beams of the prototype highway frames require wrapping with AFRP sheets.
Since the Hyogo-Ken Nanbu Earthquake in 1995, seismic retrofitting of existing RC columns of railway structures has been carried out. This paper first describes conventional seismic retrofitting methods and introduces two new seismic retrofitting methods that can be easily applied in narrow spaces. An outline of experimental results for these new seismic retrofitting methods is provided and the relevant design methods are described. In the first method, called the RB method, retrofitting bars are arranged so as to keep the value of γi·Vyd/Vmu (γi: safety factor, Vyd: shear strength, Vmu: shear at flexural strength) above 1.5. In the second method, called the single-face method, retrofitting bars and retrofitting plates are arranged so as to keep the values of γi· Vyd/Vmu and γi· Vyd/Vmu above 2.0 and 1.4, respectively.
The strut and tie models have been widely used as an effective tool for designing reinforced concrete structures. The concrete is considered to carry only compressive forces through, while the tension forces are carried by reinforcing steel. The strut and tie model is effective for designing disturbed regions, however, it is essential that the designer should have a minimum level of experience to assume optimum trusses. In this study, a generalization of the strut and tie model is introduced through the micro truss model, in which, small isotropic truss members are used and the macro strut and tie model are automatically obtained. Both material and geometrical nonlinearity are introduced. The proposed model can be used for both design and checking the nonlinear response of reinforced concrete structures. The model has been verified through published experimental results. Rational steps of design have been incorporated and examples of design have been illustrated.
This study investigates the dependence of the mechanical behavior of concrete, such as strength, stiffness, and deformation capacity on the damage caused by freezing and thawing cycles (FTC). A stress-strain model for concrete damaged by freezing and thawing prior to the application of mechanical loading was proposed based on plasticity and fracture of concrete elements. The FTC fracture parameter was introduced to explain the degradation in initial stiffness of concrete resulting from freezing and thawing damage. Based on experimental data, the FTC fracture parameter was empirically formulated as a function of plastic tensile strain caused by freezing and thawing with the assumption that the plastic strain was caused by the combined effects of FTC and mechanical loading damage. The stress-strain relationships obtained by the proposed model were compared with the experimental data.
This paper presents a numerical analysis of fracture modes in a plain concrete beam with multiple discrete cracks, focusing on crack interaction and localization. It contains three parts: justification of the strategy used to solve multiple-crack problems using a discrete approach, derivation of the coefficient of interaction, and numerical studies. The coefficient of interaction is defined based on the nodal force components at the tip of a crack, and its usefulness in analyzing various cracking behaviors is demonstrated through numerical studies. Under the given loading conditions, the cracking behavior of the plain concrete beam is shown to depend on the initial configuration of notches and their sizes. It is found that the intensity of crack interaction and its effect on crack localization can vary. As the crack localization progresses, the stress concentration at the dominant crack intensifies, and the crack interaction diminishes quickly.
The effect of three pre-drying treatments on time dependent deformation due to drying of cement paste conditioned at (96 ± 2) % relative humidity and temperature of 22 ± 2 °C was investigated using a.c. impedance techniques. The treatments comprised methanol and isopropanol exchange (followed by vacuum heating at 37°C) and vacuum drying at 37°C alone. Re-saturation with synthetic pore fluid was performed prior to the drying experiment. Real-time changes in microstructure were followed using impedance spectroscopy. Cement paste specimens were in the form of “T-shaped” columns with a minimum thickness value (for the web and flanges) of less than 1.2 mm. The impedance spectra for the untreated (control) specimens and the specimens dried at 37°C indicated the presence of an intermediate arc. This is discussed in terms of interfacial phenomena associated with collapse of C-S-H structure on drying.
The microstructure of AAC changes during carbonation, resulting in degradation such as cracking. AAC panels under working conditions (Field-AAC) aged 5 to 33 years and AAC blocks carbonated under accelerated conditions (Labo-AAC) were subjected to microstructural analysis. The degree of carbonation increased with time and approached 60% both under working and accelerated conditions. Changes of micro-level structures such as surface and crystal structures were more significant under accelerated conditions than under working conditions. Meso-level structures such as the interparticle pore volume were similar regardless of the carbonation conditions. The increase in drying shrinkage was more significant under accelerated conditions than under working conditions because it originated not only from the moisture characteristics but also the micro-level structures.