Experimental Research On Static And Dynamic Working Performance Of Steel Drive-in Racks

As the high density storage devices, drive-in racks are widely used in the warehousing industry in recent years. For the sake of high storage density of goods pallet, the bracings in the down aisle direction cannot be installed in the interior of drive-in racks. As a result, the weak rack stability in this direction is the key problem for the design method of the carrying capacity of drive-in racks. Due to the problem complexity, chinese racking design specification does not provide the theoretical design method of the stability bearing capacity of drive-in racks, and does not mention the content about seismic design of drive-in racks. In view of those facts above and to further improve the theoretical design method of drive-in racks, some experimental tests, finite element numerical analysis and corresponding theoretical analysis were conduted on the static and dynamic properties of members, connections and complete structure of drive-in racks. The main contents and results in the paper are as follow:Three groups of rack connection tests were carried out respectively, including a group of hook-in beam-to-column connections, a group of bolted beam-to-column connections and a group of column base connections. For hook-in connections, there are three kinds of tests, which are subjected to bending, shear or tensile force. According to the test failure phenomena that the hooks in tension side cut into the columns web, the simplified calculation model and the calculation formulas, calculating the moment resistance and initial rotational stiffness of connectors, were proposed. For bolted connections, the bending tests under monotonic and cyclic loading were carried out. The test results indicate that changing the relative position of beams and connecting plates has influence on initial stiffness, ultimate capacity and deformation behaviour of connections. The gap between the bolt and the bolt hole allows the slippage of bolt, which makes turning points arise in the momentrotation curves of specimens. The moment-rotation hysteretic curves of specimens from cyclic loading test show Z-shaped. For column base connections, the bending tests of these connections rotating around the column section’s symmetry axis were carried out. The influence of column axial force on the bending property of connections was taken into consideration. Test results reveal that column axial force is a positive factor for initial stiffness and ultimate moment of column base connections. From subsequent finite element numerical analysis results, the conclusion can be drawn: with the increase of column axial force, the influence of axial force on the initial stiffness of column base connection becomes weaker.The static tests of drive-in racks under the single point horizontal force were conducted. The experimental results show that both top plan bracing and back spine bracing can change the load path through the rack structure and strengthen lateral stiffness of racks. By acting as the horizontal bracings of adjacent columns and reinforcing the rotational stiffness of column base connections, the pallets are also the beneficial factors for improving the lateral stiffness of complete racks. Based on the special mechanic characteristics of tested racks, several simplified models and calculation formulas were proposed for the lateral stiffness of the tested racks in the down-aisle direction. For considering the influence of local deformations in bracing member connectors, sectional area reduction factors of bracing members are introduced into the simplified calculation method.The static tests of drive-in racks were carried out to study the ultimate bearing capacity and failure modes of rack columns. The column damages occur near the corbel connection in the columns which are bearing larger bending, and the damaged columns appear S-shaped. Using the elastoplasticshell element, a row of middle columns under bigger loading in the down-aisle direction are simulated for the finite element analysis. And the FEA result can conservatively estimate the ultimate strength of the racks.Basing on the experiment phenomena, one simplified column model and one line of columns model were purposed to calculate the rack column length factor in the down-aisle direction. In the simplified column mode, a top displacement spring is used to simulate the constraint of rack bracing components, and a bottom bending spring is used to simulate the constraint of column base connections. Using the equilibrium method, the calculation formulas of elastic buckling load were derived. And then the elastic buckling load of models about one line of columns was studied using the finite element analysis. Basing on the numerical results, the simplified calculation formulas for the rack column length factor were gotten.Basing on the results of linear analysis(LA), geometric nonlinear analysis(GNA), linear buckling analysis(LBA) and geometric and material nonlinear analysis(GMNA), and combining with the in-plane stability formula of beamcolumns in technical code of cold-formed thin-wall steel structures(GB50018-2002), three computational methods for load carrying capacity of rack columns were gotten.Through initial displacement method, the free vibration tests were carried out to investigate the dynamic properties of steel drive-in storage racks. The test results indicate that tested racks vibrate with the first mode shape. There are errors for rack damping ratios which are calculated using the logarithmic decrement method, and the calculated damping ratios have poor regularity. Top plan bracings have no influence on the natural frequencies of racks, but adding back spine bracings can increase this dynamic parameter. Two complete rack numerical models were analyzed using static force elastic-plastic Pushover analysis and time-history dynamic analysis. By comparing bottom shear – top displacement results of Pushover and time history analysis, Pushover analysis can be an approximate method to examine the deformation capacity of drive-in racks. In the analysis process, rack columns keep elastic, but beam-to-column connectins, column base connections and spine diagonal bracings occur the plastic deformation.