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Nonlinear time-history analysis conducted as part of a performance-based seismic design approach often require that the ground motion records are selected and then scaled to a specified level of seismic intensity. In such analyses, besides an adequate structural model, a set of acceleration time-series is needed as the most realistic representation of the seismic action. In this paper, the effects of scaling procedure on seismic demands of steel frames are investigated. To this, two special steel moment-resisting frames with considerable higher mode effects, and two sets of ground motions, including near-fault and far-fault motions are considered. Moreover, three scaling procedures are introduced for performing nonlinear dynamic time-history analysis of structures. Among different demands, lateral roof displacement and interstory drift are selected as seismic demands. The height-wise distribution of demands shows that the inelastic seismic demands of the near-fault pulse-like ground motions differ considerably from those of far-fault ones. These results show that the story drifts are mostly larger for far-fault motions in the upper story levels in comparison to near-fault records and in the lower floors, the reverse is true. Thus, the scaling procedures directly affect the results of seismic demands and choosing different methods would result in varying responses. Moreover, a low-cost and fairly effective procedure is proposed to estimate the target displacement demands of buildings from response-spectrum analyses, considering near-fault effects. The precision of this method is verified by nonlinear time-history analysis results, as the benchmark solution, and acceptable improvements have been achieved. Key words: record scaling procedures, pulse-like ground motion, near-fault earthquakes, record selection, nonlinear time-history analysis, pushover analysis, target displacement, story drift. L'analyse du diagramme devolution non lineaire effectuee dans le cadre d'une approche de conception sismique basee sur le rendement exige souvent que les enregistrements de mouvement du sol soient selectionnes et ensuite mis a l'echelle a un niveau precis d'intensite sismique. Dans de telles analyses, outre un modele structurel adequat, un ensemble de series chronologique d'acceleration est necessaire en tant que representation la plus realiste de l'action sismique. Dans cet article, les effets de la procedure de mise a l'echelle sur les contraintes sismiques des charpentes en acier sont etudies. Pour ce faire, deux charpentes speciales en acier resistant aux moments avec des effets de mode considerablement plus eleves, et deux ensembles de mouvements de sol, y compris des mouvements pres et loin de la faille sont consideres. En outre, trois procedures de mise a l'echelle sont introduites pour effectuer une analyse dynamique non lineaire du diagramme devolution des structures. Parmi les differentes contraintes, le deplacement lateral du toit et la derive inter-etages sont choisis comme contraintes sismiques. La repartition en hauteur des contraintes montre que les contraintes sismiques inelastiques des mouvements du sol semblables a des impulsions a proximite de faille different considerablement de celles des mouvements provenant de celles distantes. Ces resultats montrent que les derives d'etages sont generalement plus grandes pour les mouvements provenant de faille distante dans le cas des etages superieurs par rapport aux enregistrements de faille proche, et pour les etages inferieurs, l'inverse est vrai. Ainsi, les procedures de mise a l'echelle ont un effet direct sur les resultats de contraintes sismiques et le choix de differentes methodes entrainerait des reponses variables. En outre, une procedure peu couteuse et assez efficace est proposee pour estimer les contraintes de deplacement cible des batiments a partir des analyses du spectre de reponse, en tenant compte des effets de faille proche. La precision de cette methode est verifiee par des resultats d'analyse de diagramme d'evolution non lineaire, comme solution de reference, et des ameliorations acceptables ont ete obtenues. [Traduit par la Redaction] Mots-cles : procedures de mise a l'echelle des enregistrements, mouvement du sol semblable a une impulsion, faille proche, selection des enregistrements, analyse du diagramme d'evolution non lineaire, analyse de renversement, deplacement cible, derive d'etages., Introduction Nonlinear time-history analyses (NL-THAs) conducted as part of a performance-based seismic design approach often require that the ground motion records are selected and then scaled to a specified level [...]

Summary: On January 19, 2017, the 16‐story Plasco tower located in Tehran, the capital city of Iran, collapsed in several stages, due to a fire lasting 3.5 h. This paper presents the results of numerical investigations on how the structure behaved during that incident. In this regard, initially, a part of floor system including main truss girders, secondary truss beams, tie beams, and reinforced concrete slab is simulated to study its deformed shape and collapse modes. Then 2D and 3D models of the building are developed to evaluate the global behavior of the structure under fire. The results show that at elevated temperatures, deflection of the floor system, main trusses, and out‐of‐plane bending of the beam‐to‐column gusset plates become excessive, leading to partial collapse of floor, in the location of fire initiation. Also, 2D models show that thermal expansion and catenary action of heated truss induce large lateral displacement and bending moment in perimeter column that cause failure of this column at about 650°C. Finally, 3D models indicate that general collapse of the building is initiated by buckling of the external columns located in the southern perimeter of the building. This result is in good agreement with videos and photos showing actual collapse of the structure. [ABSTRACT FROM AUTHOR]

In this paper, a fairly effective procedure called dynamic load pattern (DLP), is proposed to account for the effects of near-fault ground motions in estimating the seismic demands of structures from pushover analyses. The seismic demands are obtained by enveloping the results of single-run conventional first-mode and single-run DLP pushover analyses. Improving the estimation of target displacement is another objective, implemented by performing response-spectrum analysis. Three special steel moment-resisting frames are considered and the seismic demands resulting from DLP are compared to those from the nonlinear time-history analysis as a benchmark solution, as well as to those predicted from modal pushover analysis. [ABSTRACT FROM AUTHOR]

The built-up special concentrically braced frames (BSCBFs) containing double-angle and double-channel braces are among the common steel structural systems resisting lateral loads. In this paper, the parameters influencing the BSCBF seismic behaviours are investigated under cyclic loading. For this purpose, nonlinear finite element analyses of a single-bay single-story frame are employed, and the results show that increasing the number of stitches or decreasing their distances along the length of the built-up members may not necessarily improve the behaviour of the braced systems. That is, inelastic deformation concentration probably occurs in individual elements between stitches, resulting in earlier failure of braces. Moreover, the failure results of the built-up brace members display that the usage of vertical stitch connectors (parallel to the webs) should be avoided in all BSCBF systems in comparison with lateral-stitch braces (parallel to the flanges). In addition, the failure results of double-channel back-to-back BSCBFs are much better than those of similar face-to-face ones. The results of the double-angle diagonal braces also reveal that the failures of the back-to-back brace sections occur earlier in comparison with similar face-to-face or toe-to-toe braces. [ABSTRACT FROM AUTHOR]

Summary: This study is carried out to evaluate the progressive collapse of steel buildings under fire event. To this end, a 15‐story steel structure with moment‐resisting system and composite floors is considered. The effects of various parameters such as beam section size, gravity load ratio, vertical irregularity of resisting system, and location of fire compartments on collapse modes are investigated numerically. Different temperature‐time curves are defined across the composite floors according to the Eurocode 4. It is found that local collapse of the frames at the ground floor fire is triggered by the buckling of the interior heated columns at approximately 540°C. The redistributed loads by floors delay the global collapse at least 45 min. Increasing gravity loads accelerates the global collapse of the frames significantly. The heated columns of the middle floor buckle at higher temperature compared to the ground floor heated columns and no global collapse occur due to this scenario. In general, the potential of collapse of the regular and irregular frame due to fire in the edge bay is higher compared to the fire in the middle bay. It is also found that the local and global collapse of regular frames occur earlier than irregular frames. [ABSTRACT FROM AUTHOR]

Summary This study is carried out to evaluate the response modification, overstrength, and deflection amplification factors for steel diagrid system. To quantify these factors, the rational procedure introduced in Federal Emergency Management Agency (FEMA) P695, which is based on low probability of structural collapse and encompasses nonlinear static and dynamic analyses, is used. To this end, a series of diagrid structures with various slopes of external braces are designed. Nonlinear static analyses are performed to obtain system overstrength and period‐based ductility. Incremental dynamic analyses are then performed to assess collapse margin ratio (CMR) of the archetypes. The effects of spectral shape of different ground motions are considered in CMR values to obtain an adjusted CMR (ACMR), for each archetype. The values of calculated ACMRs are compared with the acceptable values of FEMA P695 to assess reliability of presumed seismic performance factors of diagrid systems. The results show that values of calculated ACMRs for steel diagrid systems designed using the response modification factor equal to 4.50 are acceptable. It is also observed that as the slope of the peripheral diagonal elements increases, the overstrength and the collapse margin ratios decrease, and the ductility of system increases. [ABSTRACT FROM AUTHOR]