Buckling analysis of thin-walled cold-formed steel structural members using complex finite strip method

In this paper, a generalised complex finite strip method is proposed for buckling analysis of thin-walled cold-formed steel structures. The main advantage of this method over the ordinary finite strip method is that it can handle the shear effects due to the use of complex functions. In addition, distortional buckling as well as all other buckling modes of cold-formed steel sections like local and global modes can be investigated by the suggested complex finite strip method. A combination of general loading including bending, compression, shear and transverse compression forces is considered in the analytical model. For validation purposes, the results are compared with those obtained by the Generalized Beam Theory analysis. In order to illustrate the capabilities of complex finite strip method in modelling the buckling behavior of cold-formed steel structures, a number of case studies with different applications are presented. The studies are on both stiffened and unstiffened cold-formed steel members

Effectiveness of modified pushover analysis procedure for the estimation of seismic demands of buildings subjected to near-fault ground motions having fling step

Near-fault ground motions with long-period pulses have been identified as being critical in the design of structures. These motions, which have caused severe damage in recent disastrous earthquakes, are characterized by a short-duration impulsive motion that transmits large amounts of energy into the structures at the beginning of the earthquake. In nearly all of the past near-fault earthquakes, significant higher mode contributions have been evident in building structures near the fault rupture, resulting in the migration of dynamic demands (i.e. drifts) from the lower to the upper stories. Due to this, the static nonlinear pushover analysis (which utilizes a load pattern proportional to the shape of the fundamental mode of vibration) may not produce accurate results when used in the analysis of structures subjected to near-fault ground motions. The objective of this paper is to improve the accuracy of the pushover method in these situations by introducing a new load pattern into the common pushover procedure. Several pushover analyses are performed for six existing reinforced concrete buildings that possess a variety of natural periods. Then, a comparison is made between the pushover analyses' results (with four new load patterns) and those of FEMA (Federal Emergency Management Agency)-356 with reference to nonlinear dynamic time-history analyses. The comparison shows that, generally, the proposed pushover method yields better results than all FEMA-356 pushover analysis procedures for all investigated response quantities and is a closer match to the nonlinear time-history responses. In general, the method is able to reproduce the essential response features providing a reasonable measure of the likely contribution of higher modes in all phases of the response

3D time history analysis of RC structures versus common methods with attention to the modeling of floor slabs and near versus far-fault earthquakes

Commercial softwares such as ETABS and SAP, commonly used for the analysis of apartment buildings, assume the slabs as a rigid or semi-rigid membrane and only roughly allow for the slab’s flexural stiffness using the concept of effective width. These assumptions when further simplified adopting a 2D frame method that ignores the torsional effects may produce results that are very different to the full 3D finite element modeling in particularwhen time-history nonlinear dynamic behavior is sought.The errors could be larger in nearfault earthquakes that often excite higher vibration modes. Recent major earthquakes (Northridge 1994, Kobe 1995, Chi-chi 1999 and Bam 2003, etc.) have shown that many near-fault ground motions possess prominent acceleration pulses that result in different structural responses for common medium to high-rise buildings. Incorrect incorporation of the flexural stiffness of slabs can in some cases underestimate the lateral stiffness. It is shown in the current paper that in a wall-frame structure subjected to near-fault earthquakes, the full 3D time history modeling can significantly vary the analysis results and as such is an important consideration in design

Effect of FRP wrapping in seismic performance of RC buildings with and without special detailing - a case study

The results of a numerical investigation into the efficiency of glass fibre reinforced polymers (GFRPs) in improving the seismic performance of an 8-storey moment resisting reinforced concrete building are presented. In order to assess the effect of the transverse reinforcement, the building is detailed with different levels of transverse reinforcement representing well-confined and poorly-confined conditions. Although GFRP wrapping of columns at critical regions is the main retrofitting technique considered in this study, the effect of increasing the beam ductility on the seismic performance of a structure is also evaluated for the code-compliant building. The retrofitting strategy aims to provide both columns and beams with more ductility and energy dissipation instead of increasing the lateral strength. The load–displacement curves obtained from pushover analysis of the frames are then used in the seismic assessment using a capacity spectrum approach (the N2 method). The results confirm that GFRP wraps is capable of improving the seismic performance and ductility of the poorly-confined structure substantially, compared to the original structure. However, it was found that using FRP composites in order to increase the ductility of code-compliant building only, was not that effective

Effect of elaborate plastic hinge definition on the pushover analysis of reinforced concrete buildings

Due to its simplicity, lumped plasticity approach is usually used for nonlinear characterization of reinforced concrete (RC) members in pushover analysis. In this approach, the inelastic force deformation of hinges could be defined as either the nonlinear properties suggested in FEMA-356 and ATC-40 or defined hinges quantified on the basis of the properties of RC members. However, the nonlinear response of RC structures relies heavily on the inelastic properties of the structural members concentrated in the plastic hinges. To provide a comparative study, this paper attempts to show the results of pushover analyses of RC structures modeled on the basis of the FEMA nonlinear hinges and defined hinges. Following the validation of the adopted models, the force–deformation curves of the defined hinges are determined in a rigorous approach considering the material inelastic behavior, reinforcement details and dimensions of the members. For the case studies, two four-story and one eight-story frames are considered in order to represent low-rise and mid-rise buildings with different ductility. Nonlinear responses of both models are elaborated in terms of the inter-story drift, hinging pattern, failure mechanism and the pushover curve. It is confirmed that FEMA hinges underestimate the strength and more importantly the displacement capacity, especially for the frame possessing low ductility

Reliability of ductility requirements in concrete design codes

Ductility is an important limit state for the design of reinforced concrete beams. Its implementation varies considerably between design codes. This is investigated using reliability-based assessment with ductility defined by strain ratio. The modelling uncertainty for the ductility limit state typically is much greater than that for structural strength limit state. This is reflected in the corresponding reliability indices of limit state defined for ductility. Some of these could be considered unacceptably low