Abstracts:This paper presents the response modification factor ‘R’ of the new bracing configuration of concentrically braced frames (CBFs). This innovative bracing system called “hexa-braced frame” is composed of hexagonal pattern of braces in which vertical structural elements connect the V and inverted-V bracings over three stories to form the hexagonal bracing configuration. FEMA P695 provides a global methodology to quantify seismic performance factor for new structural systems. Following this methodology, a set of 4-, 10- and 20-story archetypes representing low-, mid- and high-rise buildings, respectively, were used to evaluate the R factor of the hexa-braced frame. Trial values of R factor were examined through nonlinear static and dynamic analyses to satisfy acceptance criteria of the P695 methodology. The results were compared with the responses of similar X-braced frame models as the benchmark. The iterative process to determine R factor for the hexa-braced frame was performed using values of R factor, 6 and 7. Based on the performance evaluation of hexa- braced frame archetypes by measuring their collapse fragility, the value of R factor, 7 achieved the safety margin against collapse during the earthquakes. As expected, the analysis results confirmed the given value of R factor 6 for X-braced frame system in design codes (ASCE 7).

Abstracts:The present study evaluates the seismic performance of steel moment resisting frames (MRFs) upgraded with different structural protective systems. For this, three 5 storey steel MRFs (Ordinary moment frame (OMF), intermediate moment frame (IMF) and special moment frame (SMF)) and two 10 storey MRFs (IMF and SMF) were studied. As structural protective systems, friction dampers (FDs), base isolation with lead rubber bearings (LRBs), and a combination of them were considered. The structures were modeled using a finite element program and evaluated by the nonlinear time history analyses. In the nonlinear time history analyses, seven natural accelerograms, namely, 1976 Gazlı, 1978 Tabas, 1986 San Salvador, 1987 Superstition Hills, 1992 Cape Mendocino, 1994 Northridge and 1999 Chi-Chi were taken into account. Roof drift, roof absolute acceleration, relative displacement, interstorey drift ratio, base shear, top storey moment, and hysteretic curve were employed to compare the elastic and inelastic responses of all frames. The results clearly highlighted that the application of FDs with LRB had remarkable improvement in the earthquake performance of the case study frames reducing the local/global damages in the main structural systems and satisfied the serviceability (i.e., fully operational, FO and operational, OP) limit states as well.

Abstracts:Beams with web openings represent an attractive system for multi-storey buildings characterized by long spans. Openings in web of steel beams enable building services to be integrated within the constructional depth of the floor, thus reducing the total floor depth. This paper employs the general finite element software ABAQUS to numerically model the behaviour of restrained structural subassemblies of steel beam with web openings to concrete filled tubular column under fire loading scenarios. Steel beams, in this case, are axially restrained which results in a drastically different behaviour when compared to unrestrained beams. In this research, a comprehensive finite element (FE) study of the behaviour of structural subassemblies of steel beams with openings under fire scenarios is conducted. Four sizes of perforated steel sections with seven different opening configurations are modelled and studied in order to understand the significance of their effects. Moreover, the influence of applied load ratios in addition to the level of axial restraint are examined. Consequently, considerations for the performance of axially restrained perforated steel beams under fire scenarios is provided.

Abstracts:The tension-only concentrically braced steel beam-through frame (TCBBF) has the potential to enhance the seismic and post-earthquake performance for low-rise buildings in low to moderate seismic regions. To a better understanding of the dynamic response of TCBBFs, full-scale shake table tests under different seismic hazard levels and aftershocks were conducted on a three-story structural model. From the test results, the damage-control and low-residual-displacement behaviors were verified firstly. Under maximum considered earthquake (MCE), the main frame nearly remained elastic while all the braces yielded and there was almost no residual displacement. Under stronger earthquakes exceeding the MCE, yielding occurred at the beam-column connections, and the main frame demonstrated pinching behavior. Meanwhile, the residual displacement of the structure was still at a very low level. The tension-only braces played a key role in the seismic behavior. The existence of prestress made the braces able to sustain a little compression and thus increased the initial lateral stiffness of the structure. The slackness of the brace would enlarge story drift response and also induce dynamic impact due to the sudden tensioning. This impact effect was effectively controlled because of the high ductility of the braces. Tightening the slack braces was a convenient way to restore the structural behavior if the main frame was not damaged.

Abstracts:This paper examines residual drift demands in steel moment-resisting frames incorporating the influence of degradation and ground motion frequency content. Detailed assessments are carried out using 54 multi-storey framed buildings, with a wide range of structural characteristics, which are designed according to the provisions of Eurocode 8. In order to identify the influence of cyclic and in-cycle degradation effects, the analysis is carried out with and without degradation modelling. Incremental dynamic analysis is employed in order to achieve various limits of lateral strength demand, using a suite of 56 ground motion records. It is shown that residual drifts are markedly higher in degrading models in comparison with non-degrading models, with the differences being more pronounced in relatively short period ranges, when higher rates of cyclic deterioration are employed, and for comparatively high lateral strength demand levels. The residual drift demand is also shown to increase with the increase in number of stories, and is often concentrated in the lower levels when degrading models are used. Overall, significant residual drift demands are observed in the structural systems considered, with a high likelihood of exceeding a 0.5% residual drift limit in most cases. Based on the results, two simplified prediction relationships are proposed to estimate the permanent drifts of multi-storey steel moment framed systems. The first is concerned with the design stage based on the results of elastic analysis, whilst the second is associated with post-earthquake structural assessment based on actual measurements of residual drifts.

Abstracts:The use of modern Carbon Fiber Reinforced Polymer (CFRP) to strengthen and repair steel beams in flexure has been rapidly increased within the past few years. This technique benefits from light-weight and extra-strong CFRP material to enhance the flexural capacity of cross section. To study the reinforcing effect of CFRP, one hundred and seventy-eight models were analyzed to cover six variables representing the common problem parameters; the variables were the slenderness ratio of web (h w /t w ), the mono-symmetric ratio of I-beam (ψ), the area of CFRP (A cfrp ), the modulus of elasticity of CFRP (E cfrp ), the tensile strength of CFRP (F ucfrp ), and the length of CFRP sheet (L cfrp ). The adhesive properties used in parametric analysis were determined from experimental tests conducted for double-strap steel-to-CFRP joints with various bond lengths (50 to 200 mm), and the proposed model constructed using the general finite element program, ANSYS 17, was verified with experimental tests of full-scale steel beams reinforced with CFRP. The parametric study revealed that CFRP sheets were very efficient in reinforcing compact mono-symmetric sections, whereas the enhancement in beams with non-compact sections was very small. CFRP sheets were able to reach its ultimate strength provided that enough bond length was ensured. Analytical procedure to calculate the flexural strength of steel I-shaped beams reinforced with CFRP sheets at tension flange was presented.

Abstracts:This paper presents an experimental investigation on residual mechanical properties of high strength steels (HSS) after exposure to fire. A test program was conducted to study the post-fire mechanical properties of cold-formed HSS. A total of 41 tensile coupon specimens was extracted from cold-formed tubular sections with nominal yield stresses of 700 and 900 MPa. The specimens were exposed to various elevated temperatures ranged from 200 to 1000 °C and then cooled down to ambient temperature before testing. Stress-strain curves were obtained and the mechanical properties of Young's modulus, yield stress (0.2% proof stress), ultimate strength, ultimate strain and fracture strain of the cold-formed high strength steel materials after exposure to elevated temperatures were derived. The post-fire retention factors that obtained from the tests were compared with existing predictive equations in the literature. The influence of heating rate on post-fire mechanical properties was also investigated. New predictive curves for the determination of residual mechanical properties of HSS after exposure to fire are proposed. It is demonstrated that the proposed predictive curves are suitable for both cold-formed and hot-rolled HSS with nominal yield stresses ranged from 690 to 960 MPa.

Abstracts:The design principles for high strength steel (HSS) structures exposed to fire are under development. In this paper, the response of HSS structures in fire is studied through numerical simulation of a beam and a two-bay frame. Geometrical imperfections and residual stresses are introduced into the structural models. Deformation limit criteria are used to compare the critical temperatures of the structures made of HSS and mild steel. The comparisons show that HSS structures have higher strength reserve than mild steel structures. Using the mechanical properties at elevated temperatures from literature sources, it is observed that the deflection behaviour of the studied structures depends on the ratio of strength to elastic modulus. The deflection of the studied beam is sensitive to yield strength reduction factors as the beam fails by plastic hinge mechanism. Whereas, the deflection of the HSS frame is sensitive to the reduction factors of the elastic modulus as the frame fails by inelastic instability. The above-mentioned observations on the studied structures are made using a three-stage mechanism which is developed for interpreting the deformation response.

Abstracts:An approach based on continuum damage mechanics (CDM) is applied to predict the fatigue life of welded joints with artificial corrosion pits. Full penetration load-carrying fillet cruciform welded joints with a 45° inclined angle were constructed, and artificial corrosion tests and fatigue tests of the welded joints were carried out. A new damage variable based on the crack size was defined to assess the stiffness degradation. Material parameters in the damage evolution equations were obtained from the fatigue experimental data. The CDM model combined with numerical simulations was used to describe the fatigue damage evolution process. A comparison between the fatigue life predicted results and the test results was made. The results show that fatigue life decreases with the increase of pit depth, decreasing by approximately 50% from d = 0 mm to 2 mm at the same stress range. The fatigue damage curves can be divided into three stages: the crack initial growth stage (D < 0.3), the crack slow growth stage (0.3 ≤ D ≤ 0.8), and the crack rapid fracture stage (D > 0.8). The fatigue damage curves for different stress ranges are nearly the same under the same pit depth. In addition to the material and loading conditions, the corrosive environment also has an effect on the material parameters in the fatigue damage evolution process. The fatigue life predicted results agree well with the test results, and the maximum relative error is 10.6%. The crack size can be used to describe the fatigue damage evolution of welded joints with artificial corrosion pits.

Abstracts:This research presents a study about the structural behavior of steel cellular beams, focused on the web-post buckling. The main objective of this study is to propose a new formulation to calculate the shear resistance of cellular beams for this phenomenon, based on laboratory tests and numerical analysis. Series of tests were performed in this work, with full-scale steel cellular beams. In these experiments, vertical and lateral displacements were measured, as well as web-post deformations. The steel mechanical properties of these beams were determined by tensile testing. A numerical model was proposed, developed in ABAQUS software, to perform parametric analysis. From these numerical models, processing 597 cases, a new formulation to determine the shear resistance in cellular beams for the web-post buckling was proposed, based on resistance curves. The proposed formulation was verified in several situations of geometry and material properties, presenting compatible results with those obtained numerically, and showing better accuracy than those available in the literature.