林 和宏

Abstract This paper focuses on the ultimate state of three‐story wood dwellings with high aspect ratios, which are increasing in Japan's urban areas. Using shaking table test results from the 2019 full‐scale shaking table test, a comprehensive study is conducted on the accuracy of evaluating ultimate state through the story shear failure mode at the first story, the tension fracture mode at the wall base of the first story, and foundation sliding mode on the soil. Methods evaluating the dynamic response behaviors of the building systems are also investigated. In the test, the current Japanese seismic design guidelines were applied, and two Grade‐3 buildings were prepared. One adopted the Post‐and‐Beam structure (A‐building), and the other the Shear‐Wall structure (B‐building). A series of tests planned very different physical boundary conditions surrounding their reinforced concrete (RC) mat foundations. The sills, column bases and wall bases of the upper wood structures were anchored to the RC foundations by steel anchor bolts, according to the current Allowable Stress Design (ASD) requirements. In the first stage, A‐building equipped a Base‐Isolation system, while B‐building represented a generic foundation constructed on a 1.5 m‐height real soil ground by preparing a rigid soil box (Foundation‐Soil system). In the second stage of A‐building and B‐building, the foundation was firmly fixed (Fixed‐Foundation system), and shaking table motions were fully applied to the foundations. The entire test system was setup on the large shaking table facility at E‐Defense, and a series of tests were conducted using JMA‐Kobe motion and JR‐Takatori motion recorded in the 1995 Kobe earthquake as Maximum‐Considered‐Earthquake motions. Confirmed was the change in the structural mechanism due to the upper structural systems and the foundation boundaries. Regarding the upper wood structure performance in the Fixed‐Foundation system, a story shear failure mode was observed at the first story in A‐building, while a tension fracture mode at the base of the first story in B‐building. This difference of failure mode is difficult to determine with ASD. The maximum strength were more than four times higher than the ASD base shear force. Tension fracture capacity at the wall base was mainly enhanced by the presence of the steel anchor bolts. Regarding the foundation performance in Foundation‐Soil system of B‐building, a horizontal displacement up to 240 mm was observed between the foundation and soil when JMA‐Kobe 100% was applied. A response reduction effect was observed in the upper wood structure, similar to the Base‐Isolation system of A‐building. The initial friction and cyclic friction strength capacities between the foundation and soil were quantitatively evaluated considering the horizontal two‐directional sliding. The representative test results were converted to the corresponding SDOF systems based on the first mode response assessment. In the Fixed‐Foundation system, the dynamic response characteristics of the upper wood structures were properly represented using Ibarra‐Medina‐Krawinkler pinching model in the equivalent SDOF system.

A naturally buckling brace (NBB) is a steel brace that consists of a high-strength and low-yielding steel channel arranged in parallel with eccentricity acting along the brace length. To enhance the feasibility of NBB fabrication, three connecting design details of the NBB are proposed: (1)gusset plate (GP) connections for connecting the brace to the steel frame; (2)bolting connections to tie the two channels together; and (3)steel block, knife plate, or angle connections to apply eccentricity along the brace length at the NBB ends. Four brace specimens were subjected to cyclic quasi-static loading to evaluate the proposed configurations. Although GPs provided some rotational restraint, the new design maintained all the advantages of the original concept, especially until 2% story drift, whereas rupture of neither the brace nor the GP occurred up to 4% story drift. A simple design equation for estimating the postyielding stiffness is proposed. The equation is reasonable for the range of slenderness between 40 and 70 and for the ratio of strength between the two channels from 3 to 5. (C) 2017 American Society of Civil Engineers.

New high-strength structural steel, named H-SA700, is utilized to achieve a structural system that enables continuous use after very large earthquakes. Columns are built up of H-SA700 plates using bolts only, and bolted connections are adopted extensively for easier replacement or reuse of structural components. An analytical study is performed to predict three distinctive local buckling modes of the column and estimate its elastic flexural stiffness, yield strength, and maximum strength subjected to combined flexure and compression. Cyclic loading behavior of the column is examined through five half-scale specimens, which vary with axial force, bolt pitch, and loading direction (about major or minor axis). The columns achieve a very large elastic rotation, approximately 0.02 rad at the axial force ratio of 0.2, and a maximum bending strength surpassing the plastic moment of the reduced section accounting for bolt holes without fracture. The proposed equations predict the local buckling modes well and present a reasonably conservative estimation on the column strength subjected to combined flexure and compression. (C) 2016 American Society of Civil Engineers.

The steel slit shear wall has attracted much attention as a lateral force-resisting system. However, issues such as fractures formed at the slit ends and pinched hysteresis reduce energy dissipation. To address these issues, the authors have developed a steel slit shear wall made from low yield point steel that has a low yield stress and large ductility and strain hardening. Steel slit shear walls made from low-yield-point steel dissipated energy at small lateral drifts, shear deformation was evenly distributed among all rows, fracture was eliminated, and fat hysteresis without the requirement for out-of-plane constraints was feasible. By adjusting dimensions of the link (segment divided by slits) and the number of rows of links while maintaining the required shear strength and stiffness, a small width-to-thickness ratio for the links was achievable to ensure the in-plane behavior of links and thus good energy dissipation. The combined hardening model developed using commercially available software simulated well the large strain hardening of low-yield-point steel. A proposed design procedure that ensures good energy dissipation was given.

Concrete-filled steel tube (CFT) columns offer significant advantages over columns made of either steel or concrete alone, such as large energy dissipation and increased strength and stiffness. To further improve the seismic performance of these columns, an experimental investigation was conducted into CFT columns using ultrahigh-strength steel. More specifically, seven square and circular specimens made with high-strength and conventional steel were subjected to constant compressive axial load and cyclic flexural load protocols with two and 20 cycles imposed at each drift level. Based on the test results, the influence on the CFT's cyclic behavior of the high-strength steel, cross-sectional shape, axial load, and number of cycles in lateral loading history was studied. In comparison with the conventional steel specimens, larger elastic deformation, higher strength, and delay of local buckling were observed in the high-strength steel specimens, while compared with the circular specimens, the square specimens sustained larger drift angles without fracture of their steel tubes because of the development and progress of serious local buckling. Furthermore, a simple analytical model based on the concept of the superposed strength method was proposed. The accuracy of this model was confirmed with the experimental results.

The performance of a single-headed stud and group of studs embedded in steel-fiber-reinforced cementitious composites (SFRCC) were investigated by conducting push-out tests. A total of 16 specimens were tested with various parameters including diameter of studs, numbers of studs, gauge lengths, pitch lengths, and fiber volume fractions of SFRCC. The test results showed that the headed studs embedded in SFRCC ensured the failure mode of stud fracture even without using steel rebar in the slab. The studs densely arranged with the smallest pitch length permitted for installation provided more than 90% of the full shear strength of a single stud. Using nine studs densely arranged in an area allowing for only four studs in the current design code, the overall shear resistance was twice that of the conventional design. When the fiber volume fraction of SFRCC directly affects the ductility of the stud, it was found using a fiber volume fraction of 6% is most suitable. (C) 2015 American Society of Civil Engineers.

This paper is the second part of a two-part report describing the behavior of headed stud-SFRCC (steel-fiber-reinforced cementitious composite) shear connection. The first paper presented an experimental study of headed stud-SFRCC shear connections, whereas this paper focuses on the strength evaluation of those shear connections. This paper first presents a finite-element model of the connections, which was validated using the experimental results. An updated empirical equation considering the contribution made by the weld collar is proposed to estimate the shear strength of a single headed stud. Finite-element models were developed to evaluate the shear strength of each stud in a densely arranged group of headed studs. The results reveal that the interaction between headed studs mitigated the confining effect on studs, and the studs thus sustained less shear strength than those with larger stud spacing. Studs placed in different positions sustain different shear forces, and this difference is greater when smaller stud spacing is used. Formulas with which to estimate the reduction in the strength of headed studs in SFRCC are suggested according to a parametric study of the finite-element analysis. (C) 2015 American Society of Civil Engineers.

Steel concentrically braced frames inherently provide great strength and stiffness, and are widely used for seismic resisting systems in buildings. These include conventional buckling braced frames and buckling restrained braced frames. Although the latter can prevent brace buckling and provide ductile behavior, both types of braces provide no hysteretic damping at small drift levels and offer very limited post-yielding stiffness. This study proposes a new type of steel brace with a novel mechanism-the naturally buckling brace (NBB). The design combines high-strength and low-yield steels arranged in parallel with a specified initial eccentricity along the brace length, providing ductile seismic behavior. Six tests of various NBB models subjected to cyclic loading were conducted to examine the seismic performance of the proposed NBB. Two specimens out of the six achieved the characteristics intended for NBB. The test results showed that the NBB specimens with appropriate design parameters could achieve early yielding, or hysteretic damping, from around 0.11% story drift and prevent local buckling as well as deformation concentration up to a very large story drift (greater than 3%). A single NBB would provide an asymmetric hysteretic behavior, a large post-yielding stiffness in tension, and a ductile performance with stable energy dissipation. Further systematic studies of NBBs are needed to comprehensively evaluate the capacities and limitations of the NBBs, including the reliability of performance with repeated tests. (C) 2015 American Society of Civil Engineers.