On the mechanical behavior of steel rack connections and its influence on the seismic response of industrial storage systems

Steel storage pallet racks are commonly used worldwide to store goods on pallets and represent complex and challenging structures to design. The main racking system is denoted as “selective steel storage pallet rack”. These racking systems are one pallet deep and are separated by aisles, allowing for each pallet, stored on horizontal beams, to be always accessible. Selective racks behave like bracing system in cross-aisle direction, with uprights connected by diagonal bracings, while in down-aisle direction, bracings are usually not installed to make palletised goods always accessible. Therefore, in down-aisle direction racks behave like moment resisting frames (MRFs) whose stability and seismic resistance depend on the performance of beam-to-column connections. This Thesis is motivated by the need to increase the knowledge about the mechanical behavior of rack joints, investigating how it is affected by structural details and design parameters, and its effect on the global seismic response of rack systems. This is not only a very interesting and challenging problem from a scientific point of view, but it can also have a very large economic impact. The proposed goal is achieved through: experimental tests, carried out on full-scale boltless rack joints to identify their non-linear moment-rotation curve under monotonic and cyclic loading; the development of a theoretical model to simulate experimental curves of joints; probabilistic analyses to highlight the influence of uncertainties in material strength and geometrical features on the mechanical performance of joints; and finally the development of a numerical model, capable to describe the pinching in hysteresis loops of connections and its effect on the seismic response of industrial storage systems. To obtain the moment-rotation curve of rack connections and to evaluate how it is affected by structural details, a set of full-scale joints are tested at the Structures and Material Testing Laboratory of the Department of Civil and Environmental Engineering of the University of Florence. In the first part of the Thesis, results of monotonic and cyclic tests are presented. Some joints are also equipped with additional bolts, which could represent an effective solution to improve the seismic response of steel storage pallet racks. Experimental testing is useful to get information about the semi-rigid behavior and ductility of beam-column joints. Nevertheless, despite the success and popularity of experimental testing, tests can be expensive and time-consuming, therefore current state-of-art models for traditional steel joints are based on the Component Method (CM). The CM approach can be applied to any kind of connections because the joint is modeled theoretically as an assembly of components with an elasto-plastic or rigid force-displacement relationship. A mechanical model based on the CM is developed and used to analytically evaluate the non-linear structural response of rack beam-column joints. The accuracy of the proposed approach is checked by the comparison with experimental results. To explore the impact of the component structural details and the uncertainty in steel mechanical properties and geometrical features, a Monte Carlo simulation of rack connection assemblies is also performed. For the development of simulations, statistical properties of material random variables are assumed on experimental results, the variability in geometric tolerances are assumed in accordance with current standard code requirements and the structural response of rack joints is modeled using the proposed mechanical model based on the CM. Finally, experimental tests showed a non-negligible pinching in the cyclic moment-rotation curve of rack connections, with a reduction of energy dissipation. This structural behavior is expected to influence the seismic response of rack systems and it is investigated in the last chapter of the Thesis. A simplified Finite Element (FE) numerical model is proposed for the analysis of steel storage pallet racks under cyclic loads, considering the pinching in the joint hysteresis loop. The effectiveness of the proposed model is its fast tuning and easily implementation in commercial software packages, commonly used for non-linear seismic vulnerability analyses. For a deeper understanding of the pinching effect, a case-study is discussed, comparing two models of joints differing in the modeling of the degradation of the rotational stiffness. Results highlight that a non-liner dynamic analysis with the proposed pinching model, which describes the effective non-linear elasto-plastic behavior of rack joints, is suggested to obtain a seismic vulnerability assessment of industrial storage systems on the safe side.