Tadao Nakagomi

Performance Evaluation of Buckling-Restrained Braces Installed in a Mid-Rise Steel Structure

The authors have continuously studied buckling-restrained braces using steel mortar planks (BRBSM). The performance of energy absorption and fatigue against cyclic loading has been evaluated. Although past studies have clarified the structural performance of BRBSM as single member, it is necessary to study not only the performance of BRBSM as single member but also the performance of BRBSM installed in a building structure. In this paper, a frame model of mid-rise steel structure with BRBSM subjected to earthquake motions with various characteristics is analyzed. Comparing the results of the analysis and the past tests, the seismic behavior of a structure is discussed. Especially, the seismic performance of BRBSM installed in the building structure is evaluated. In addition, the seismic performance of two types of BRBSM; basic and developed high-performance types, is compared and evaluated about cumulative plastic strain energy ratio and cumulative fatigue. As a result, the performance capacities of the both types of BRBSM exceed the required values of BRBSM under severe earthquake motions about cumulative plastic strain energy ratio and cumulative fatigue. The basic-type BRBSM has the fatigue capacity against 2 to 5 times severe earthquake motions. The required values of high-performance-type BRBSM are about a half of accumulated fatigue capacity compared with the basic-type one. The high-performance-type BRBSM is applicable against quite many cyclic loadings of low strain amplitude, and able to be used for long-term service.

Improvement of the deformation capacity of the column-to-beam joint by the reinforce welding method around the scallop bottom

Abstract Improvement of deformation capacity for a column-to-beam joint is an important theme for improving the earthquake-resistance performance of building structures. One of the causes of decreasing deformation capacity is the existence of stress concentration against the beam flange by the scallop (access hole). The non-scallop method is generally recommended to improve this problem, but it has a large disadvantage in that it is not able to apply to the on-site joint process. The reinforce welding method around the scallop bottom has been developed to be able to apply to an on-site joint and obtain better performance than the non-scallop method. The improved mechanism is a combination of: (1) decreasing the stress concentration; (2) increasing effective thickness; and (3) increasing total length of the breaking line. The deformation capacity by the reinforce welding method around the scallop bottom is a maximum 3.2 times that for the conventional scallop method in the reverse repeating bend test using an actual structure with a beam flange of 19 mm thickness and 490 N mm−2 class steel. This improving effect is better than the non-scallop method.

A study on the welding procedure of the first layer of the narrow groove welding for steel frames of building by welding robots

As for steel frames of buildings, the welding of single-bevel T-joints with steel backing is usually used. Recently, there have been some experimental reports concerning 25 degree groove welding by semi-automatic arc welding. However, it is not yet carried out in the actual field. The shortening of welding time leads to a reduction in emissions of greenhouse gas. Therefore, as this is an earth environment improvement, we expect this welding process in the steel frames of buildings to become attractive. In the same way as semi-automatic arc welding, the welding of the first layer is one of the biggest problems for the narrow groove welding in steel frames of buildings by welding robots. In this study, we researched actual assembling precision of T-joints, and we found the relationship between welding condition and crack occurrence. Our results demonstrated the probability of narrow groove welding of T-joints between square steel tubes and diaphragms in which robot welding was used.

Design concepts for building steel frame structures and required characteristic of welds

Structural design of building steel frames in Japan allows for the plastic deformation of members so that the space within structures can be most effectively utilised and the cross sections of members can be also effectively utilised in a dynamic sense and minimised. Beam end and column end are, in most cases, included into the regions assumed to be subject to plastic deformation. Gauge H steel and square steel pipe, circular steel pipe and box columns assembled by welding are employed for columnar material; gauge H section steel is employed for beam material, so plasticisation is expected in the weld zones where a column and a beam are joined. The building steel frame joint is made either by weld bonding or high strength bolted friction joining and weld bonding is mostly employed for beam-to-column connections where plasticization is expected. The use of such welded joints is only seen in steel frames employed in buildings; in structures where welding is employed, such as civil engineering structures, marine vessels, pressure vessels and tanks, the design of the weld zones is not such that plastic deformation is expected; because of this, the required performance expected for building steel frames is unique. Accordingly, in this report, required performance expected for the welded joints of building steel frames is introduced from aspects of design concept and material performance etc.

Experimental study on method of estimating strength of weld metal in steel structure

We conducted the welding experiment using three kinds of test piece, actual size, diaphragm and butt joint. Then, we examined the influence on strength, cooling time and carbon equivalent of weld metal by welding conditions on the different test pieces. We calculated an estimate of cooling time and chemical components. Consequently, we concluded that the strength of weld metal can be estimated by heat input, interpass temperature, carbon equivarent of welding wire and shape of test piece.

On fracture toughness at heat affected zones (HAZs) and microstructures of electric arc furnace steels including tramp elements and nitrogen

The use of recycled goods, for the sake of the global environment, has received much attention in recent years, even in the building sector. Amongst steel materials, electric arc furnace steels require reduced energy, produce low carbon dioxide emissions during the manufacturing process and are made from recycled goods. However, there are few studies on electric arc furnace steels and, though there are some reports on mechanical properties of welds, there are hardly any reports on microstructures known to the authors. The toughness at the heat affected zone (hereafter referred to as HAZ) may be improved by the following means: 1 austenitic (hereafter referred to as g) grain boundary coarsening prevention by making use of precipitates and oxides, 2 inhibition of ferrite formation from g grain boundary, 3 ferrite formation from the inside of g grain, 4 matrix strengthening, 5 reduction of free nitrogen, 6 inhibition of island martensite formation. A report concerning HAZ toughness improvement of electric arc furnace steels, referred to 1 and 3 above but detailed results of investigation were not given. Consequently for this study, by means of a comparative test between electric arc furnace steel and blast furnace steel to which simulated thermal cycling was applied, attention was given to the solid-solution strengthening of ferrite (4) by means of tramp elements, Ni, Cu and Sn On fracture toughness at heat affected zones (HAZs) and microstructures of electric arc furnace steels including tramp elements and nitrogen

Rationalization of steel structure welding for buildings in Japan‐mechanical behaviour of beam‐to‐column welded joints without beam scallops

Abstract Synopsis: The majority of Japanese buildings higher than three storeys are constructed of a steel structure instead of the conventional reinforced concrete structure. However, the number of skilled labourers working with steel structures is not enough to match demand. In addition, the cost of steel fabrication has risen considerably. These factors make rationalization of steel works all the more urgent. The cost ratio of welding to the entire steel works is also quite high. In the design stage, consideration of welded joints details is necessary on the basis of required mechanical performance fitting to structures. On the other hand, in the fabrication stage, highly efficient welding methods or welding robots are needed to save labour. In this paper, an outline of rationalization of steel works in Japan is presented. As a rationalization example a welding method without scallops in beam‐to‐column connections is described in the light of the experimental results.

Study on fracture condition of biaxially loaded connections in steel structure.

It is considered that the fracture of beam-to-column connections in steel structures occurs after the slow crack growth, when structures are subjected to repetitive large strains such as a strong earthquake exciation.The purpose of the present experiments and analysis was to investigate the influences of fracture toughness, temperature and stress distribution on their fracture conditions. This paper deals with the investigation on fracture of subassemblages including beam-to-column connections with notch and lamination under biaxial force. The axial stress ratio (axial stress of column/yield stress) was varied. The model was analyzed by using the elasto-plastic finite element method. The results of the analysis and the experiments are as follows.J-integral of these analytical models was influenced by the axial stress which is parallel to the crack; the higher the compressive axial stress of column, the larger the value of J-integral. The fracture of specimens made of low fracture toughness steel can be explained by JIc fracture criterion. For steel of high fracture toughness, however, it is necessary to consider not only J-integral but also other fracture conditions such as elastic strain energy for brittle fracture after slow crack growth.