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Proposal for design method of the composite steel-timber structure using buckling-restrained knee braces
Abstract
In the field of building construction, mass consumption of wood materials contributes to reforestation as well as to reduce environmental burden. At present, a composite steel-timber structure has been used in several building structures and is expected to be used even more widely in the future. The authors proposed the composite steel-timber structure which system is based on a damage-controlled structure. This structural member is composed of steel and glued laminated timber. In this paper, a design method of the composite steel-timber structure using buckling-restrained knee braces is proposed on the previous experimental tests. A trial design is conducted where applicability of the design method is shown.
... Le bois n'est alors plus considéré comme une simple protection en cas d'incendie mais également comme un composant structural à part entière. Des travaux similaires se développent par la suite, toujours en considérant des profilés laminés ou soudés utilisés pour des poutres [196,33,207,23,24,25], des poteaux [208,209], ou des systèmes poteaux-poutres complets [210,211,212,34], qui peuvent inclure des planchers CLT [213]. ...
Timber-steel hybridization has great potential, because steel and timber component can reinforce each other, timber can be used to protect steel from fire, and the non-combustibility of steel can be used in an advantageous way. However, this form of hybridization is not widespread despite recent developments in the use of timber for multi-story buildings. Therefore, it is proposed to study composite beams made from timber and steel combined in such a way that the best possible performances are achieved, in normal and fire situations.
Firstly, behavior of steel and timber is described in normal and fire situations. A description of wood combustion is proposed to better understand what underlies the temperature dependence of its properties. A focus is made on the mass transfer that occur into timber as it burns. Then, the timber-steel hybridization is addressed through an overview. Afterward, a literature review is made on a specific configuration type, which is assembled by inserting timber beams between the flanges of a hot-rolled “I” profile, while ensuring that this profile is protected from fire by timber.
The description of the elastic behavior of studied beams in normal situation is achieved using the gamma method. Bending tests on hybrid beams and their components corroborate this analytical model, but an unexpected composite behavior is observed when steel yielding begins. Thus, a significant strength gain results from the combination of timber and steel. We manage to simulate this behavior by increasing the yield point of the modeled steel compared to the measured value, as well as the tensile strength of timber.
Then, fire tests on unloaded specimens are performed. On this occasion, we confirm that correct temperature measurements into timber require orienting thermocouples parallel to isotherms. Many configurations are compared, which allows to understand in detail the effectiveness of the fire protection provided by wood to steel profiles. Mass transfers that occur into timber appear to have a significant effect on temperatures measured on protected steel profiles. The comparison of measured and simulated temperatures allows to highlight the importance of tightness of assembly joints during exposure to fire. Wood combustion and steel temperatures are observed after the end of the fire exposure, and the behavior of hollow configurations is contrasted with that of the timber filled configurations.
Finally, fire tests on mechanically loaded beams show that a steel profile protected using 45 mm thick timber components can resist fire for 81 min. Thus, R60 is exceeded with relatively thin protection. Results show that the loading has an impact on steel temperatures, because of an opening of the assembly joints. Numerical simulations show that timber gives fire resistance of the composite beam both thermally and mechanically, by protecting the steel profile, but also by relieving its load.
This work shows the effectiveness of steel-timber composite beams, in normal and fire situations, and contributes to the understanding of their behavior. However, proposals for improvement and new challenges are formulated, opening prospects for the study and use of these composite beams.
p>In light of global environment issues, the authors proposed a building system comprising steel and timber structure (Hereafter referred to as CSTS), which consist of rolled-H section steel and timber. The CSTS is assumed to adapt for mid-rise story building steel structures. It is a design method of the CSTS that uses the concept of a damage-controlled structure characterized by using buckling-restrained braces as a seismic response control member. It is assumed to use materials such as a cross laminated timber(CLT) for floor structure of the CSTS. The hysteresis model of the CSTS is established based on the available experimental data. Assuming practicality, bending test of the composite steel-timber beam members jointed by bolts is conducted. Behaviour of the composite steel-timber beam members affected by different intervals of bolted joints is evaluated.</p
p>In light of the global environment issues, the authors proposed a building system comprising steel and timber structure (Hereafter referred to as CSTS) and its design method, which consist of rolled section steel and timber. The CSTS is assumed to be mid-rise story building steel structures. This design method use concept of a damage controlled structure with a buckling-restrained brace as a seismic response control member and its analytical model is established based on the available experimental studies. The CSTS is aiming to expand to be used from main structure to floor structure in the building of the non-residential sector and assumed to use materials such as a cross laminated timber for floor structure. This cross laminated timber has a high stiffness and strength because it is joined by multi-layer panels.
In this study, we propose floor structure adapting for the CSTS using cross laminated timber. In- plane shear test of the joint which are consisted of unheaded stud-shear connectors for joint of the CSTS beam and floor structure is conducted. Structural performances of joint of the CSTS beam and floor structure are evaluated. The findings obtained from tests indicate that joint of the CSTS beam and floor structure has a high stiffness and strength.</p
In light of the global environment, the authors proposed a building system comprising steel and timber (Hereafter referred to as CSTS), which consist of rolled section steels and timbers. A design method of the CSTS uses the concept of a damage controlled structure and its mechanical model is established based on the previous tests. In this study, we propose floor structure adapting for the CSTS using cross laminated timber. In-plane shear test of the joint which are consisted of unheaded studs or bolts for joint of the CSTS beam and floor structure is conducted. Structural performances of joint of the CSTS beam and floor structure are evaluated. The findings obtained from tests indicate that the joint of the CSTS beam and the floor structure has a high stiffness and strength.
The basic concept of a damage-controlled (DC) structure is that by clearly separating the main frame from the energy absorption members, the main frame is kept within the elastic range and the seismic energy force is collectively absorbed by the members. The uniquely developed structural system embodying this concept is defined in this paper as a DC structure. Even when exposed to large earthquakes, the beam-to-column connections of the DC structure, being equipped with rotation capacity, allow seismic force to be absorbed by buckling-restrained knee braces designed to function as seismic-response-controlled members. Thus, the main frame can continue to be used by replacing only earthquake-damaged knee braces where necessary and the structural members can also be reused. In this paper, we discuss the designing process of the DC structure. First, the design flow is presented, as well as methods for designing the beam-to-column connection and buckling-restrained knee braces developed based on the results of previous tests. Next, a trial design of the DC structure is implemented based on the proposed designing methods.
The authors developed a buckling-restrained brace that enables increased design freedom at both ends of the core plate and strict quality control while providing stable hysteresis characteristics even under high strains. The buckling-restrained brace can be formed by welding a core plate covered with unbonded material to a pair of mortar-filled channel steels (steel mortar planks) as a restraining part. The use of this approach enables visual confirmation of the status of the mortar filling and also facilitates standardizing structural members and member-by-member quality control. Specimens of a buckling-restrained brace with different steel mortar plank heights are fabricated to adjust the restraining force, along with specimens with different core plate width-to-thickness ratios. The tests were conducted to reveal the hysteretic characteristics of the braces, as well as their cumulative plastic strain energy, elastoplastic properties, and stiffening properties. A performance evaluation formula as well as a buckling-restrained brace design method using the test results is proposed. Copyright © 2006 John Wiley & Sons, Ltd.