Experimental testing of joints for seismic design of lightweight structures. Part 3: gussets, corner joints, x-braced frames

An experimental campaign on joints of x-braced frames is presented. Tests are performed on strap-gusset joints and lower and upper corner joints. Many different phenomena and failure modes are observed in these tests. However, it is possible to see that only one failure mode can be accepted if a good seismic performance of the frame needs to be guaranteed: the net-section failure of the diagonal strap after yielding. The results of the tests also allow to establish a set of recommendations for seismic design of joints. The effectiveness of these recommendations is verified by means of two final tests on complete x-braced frames. A full explanation on the behaviour of the joints and frames tested is included, together with a description of the special test set-up developed for the experimental campaign.Peer ReviewedPostprint (author’s final draft

Experimental testing of joints for seismic design of lightweight structures: part 1: screwed joints in straps

Connections in x-braced shear walls play a crucial role in the seismic performance of lightweight structures: they should be strong enough to allow energy dissipation via plastification of the diagonal straps. An extensive experimental research on tensile screwed joints between straps is reported here. Two dominant failure modes are identified: (1) tilting and net section failure (T+NSF) and (2) tilting, bearing and pull out (T+B+PO). The analysis in terms of strength and ductility shows that T+NSF joints are suitable for seismic design, because the straps yield before the connections fail. T+B+PO joints, on the contrary, are not suited because the connection fails before the straps can yield. The influence of various design parameters (strap thickness, number and diameter of screws, steel grade) in the failure mode is studied, and design criteria to induce a T+NSF response are given.Peer ReviewedPostprint (author’s final draft

Influence of the cold work effects in perforated rack columns under pure compression load

This paper analyses the influence of residual stresses and strain hardening due to the cold rollforming process on the load carrying capacity of perforated rack columns. First of all, a residual strain distribution of the manufacturing process is obtained via a finite element analysis and then it is introduced in the model as initial state to carry out a nonlinear buckling analysis. Two different methodologies to introduce the residual strain pattern are presented in order to reproduce the local stress effects because of perforations. Moreover, the changes of the mechanical properties of the material by strain hardening are evaluated through experimental tests and its influence on the rack column behaviour is investigated. The results obtained agree well with experimental tests and show a method to predict the load carrying capacity of perforated rack column taking into account the effects of the cold roll-forming process.Postprint (author’s final draft

Distortional influence of pallet rack uprights subject to combined compression and bending

A pallet rack is a structure in which the beams and uprights are cold-formed steel sections. The beam-upright connections are normally clipped in order to select the beam levels required and for ease of assembly. For this reason, the uprights contain holes and perforations distributed along their length. The complexity of these components, with local, distortional and torsional/flexural buckling behaviour, represents a challenge in structural design. The loads acting on the structure are induced by the weight of pallets to be stored, and transmitted to the uprights by the semi-rigid connection system. Therefore, the uprights are thin-walled singly symmetrical perforated open crosssections bearing axial load and bending moment simultaneously. The main objective of this paper is to analyse the influence of the bending moment on the load bearing capacity of rack uprights subject to axial load together with bending moment. Bending is induced by means of axial forces eccentrically applied. The influence of bending on the strength capacity of rack uprights is analysed through finite element analysis, where residual stresses and strength enhancement induced during cold forming of sections are also considered. Experimental tests have been performed reproducing the load conditions in order to validate the FEA results. The ultimate load has also been calculated according to European standards and compared to experimental resultsPostprint (author's final draft

Simulation of the Roll-Forming Process for the Determination of the Relevant Residual Stresses of Cold-Formed Steel Members

The uprights for pallet rack systems are usually thin-walled open cold-formed steel sections. Their behaviour under compression can be analysed experimentally, analytically or by finite element simulation. In the finite element simulation usually material non linearity, large displacements and initial geometric imperfections are introduced. But until now, the effect of the residual stresses due to the forming process, is usually non considered. With the aim of introducing these stresses in the finite element simulation, our group has started to simulate the roll-forming of open steel sections. In this paper we present our first experiences in the simulation of roll-forming of U-, C- and trapezoidal sections, some comparisons with experimental data available in the literature and the first results of the forming of a medium load upright section.Postprint (published version

Elaboració de material docent avançat per a l'ensenyament de l'estructura metàl·lica

El Departament de Resistència de Materials i Estructures a l’Enginyeria està elaborant un material docent per a l’ensenyament pràctic de l’estructura metàl•lica, que està composat de tres parts principals: un llibre en format paper amb exemples de càlcul i informació d’aplicació pràctica, un programari d’anàlisi i comprovació d’estructures i uns models gràfics en format electrònic d’un conjunt d’estructures reals construïdes recentment. En aquesta presentació s’inclou una explicació general sobre el projecte complert i sobre l’experiència en l’ús dels models d’estructures exemple en classes amb alumnes amb nivells de coneixements diferents. S’ha comprovat que aquests models son una eina docent potent que es pot fer servir com a material per a l’aprenentatge individual i com a material audiovisual en classes expositives convencionals

Experimental study of the compression behavior of mask image projection based on stereolithography manufactured parts

The article presents the results of a series of compression tests on samples manufactured by means of the mask image projection based on stereolithography additive manufacturing technique (MIP-SL). Recent studies demonstrate the orthotropic nature of the MIP-SL materials. A research is initiated by the authors to attempt to predict the degree of anisotropy from the manufacturing parameters of the MIP-SL parts. The article focuses mainly on the development of the experimental compression tests of the first stage of the research. Special attention is paid to the four methods used to obtain the stress-strain curve of the material: strain gages, 2D Digital Image Correlation, extensometer measurements and crosshead displacement measurements. The article shows the advantages and limitations of each method. Finally, the anisotropic behaviour is verified and a testing procedure is set to obtain the constitutive parameters of the MIP-SL tested materialsPeer ReviewedPostprint (published version

Investigation on the down-aisle ductility of multiple bay pallet racks by means of pushover analyses

This paper presents numerical pushover analyses on multiple bay pallet racks, aiming to quantify variations in global ductility when using different beam-to-upright connections. The connections differ in the layout of the beam-to-endplate welding, being one of them a technological novelty. They are modeled with envelopes of monotonic and cyclic moment-rotation curves obtained from component tests and presented in previous research. Moreover, the influence of the level height is studied throughout two different rack configurations. A single-column model for multiple bay racks, made with 3D beams and shells, is presented and compared with simpler 2D models to quantify the influence of 3D effects and upright perforations. Results exhibit that the novel connection improves the capacity to absorb energy, but an inappropriate rack configuration can lead to a soft-story mechanism, thus not taking full advantage of its ductility.Peer ReviewedPostprint (published version

State-of-the-art review on adjustable pallet racks testing for seismic design

This paper presents a state-of-the-art review on seismic tests of adjustable pallet rack systems: on particular components of racks, subassemblies, full racks and stored goods. The tested particular components are the most critical connections: beams-to-uprights, braces-to-uprights, and floor-to-uprights; subassemblies also include beam-to-upright connections, among other components. Tests on full racks can be static (pushover), pseudo-dynamic, or dynamic (pullout and shaking table). Finally, tests on stored goods are sliding (aimed to identify the friction coefficient with the supporting members) and tilting (to check their confinement). The examination of the discussed experiments provides relevant conclusions and allows identifying research needs.Peer ReviewedPostprint (published version

Linear buckling analysis of perforated cold-formed steel storage rack columns by means of the generalised beam theory

The investigation attempts to adapt a be am finite element procedure based on the Generalised Beam Theory (GBT) to the analysis of perforated columns. The presence of perforations is taken into account through the use of two beam elements with different properties, for the non- perforated and perforated parts of the member. Each part is meshed with its corresponding finite element and, afterwards, they are linked by means of constraint equations . Linear buckling analyses on steel storage rack columns are carried out to demonstrate how the proposed procedure should be applied. Some practical issues are discussed, such as the GBT deformation modes to be included in the analyses, or the optimum finite element discretiz ation. The resulting buckling loads are validated by comparison with the values obtained in anal yses performed using shell finite element models. Finally, it is verified that the buck ling loads produced with the propo sed method are rather accuratePeer ReviewedPostprint (author's final draft