Prof. Ing. Carlo Andrea Castiglioni Seismic Behaviour of Steel ...

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Seismic behaviour of steel storage pallet racking systems . 212) Damage limitation requirementIn order to avoid damages and use limitation, whose costs would bedisproportionately high in comparison with the cost of the structure itself, theracking structure shall be designed and constructed to withstand a seismicaction having a larger probability of occurrence than the design seismic action,without local damage and the associate limitations of useServiceability limit states are those associated with damage occurrence,corresponding to states beyond which specified service requirements are nolonger met.SLS for racking systems is defined as elastic response under an earthquake withacceleration equal to 25% of the design value.The movement of stored unit loads is not to be considered as a damage.3) Movement of palletsThe seismic accelerations can cause the sliding of the pallet on the beams, whenthe dynamic horizontal force on the pallet exceeds the static friction forcebetween pallet and beam.Full scale tests showed that this effect occurs also for small ground accelerationsvalues (low intensity earthquake) for ordinary situations (wooden pallets onpainted steel beams), because of the amplification of the seismic forces at thehighest storage levels.The consequences of these phenomena are:• the reduction of the seismic action on the rack, due to the energydissipation, and the limitation of the horizontal action that can betransferred from the pallet to the rack’s structure• the risk of fall of the pallet, that can produce the local or global collapse ofthe rack, when the pallet falls inside the rack between the pallet beams, orinjury to people and damages to the equipment nearby the rack when thepallet falls out of the rack.The modification of the seismic response is deemed in Pr FEM 10.2.08 by means oftwo coefficients that conventionally estimate the effects of typical phenomena of theracking systems, such as energy dissipation due to the pallet-beam friction, palletdamping due to the movement of the stored product, pallet flexibility and otherphenomena:E D1 = design spectrum modification factorE D2 = mass modification factorDefinition of the intensity of the seismic actionWith reference to prEN1998-1 , two elastic spectra are defined in Pr FEM 10.2.08to describe the earthquake motion (called Type 1 and Type 2).
Seismic behaviour of steel storage pallet racking systems . 22Earthquake design return period and importance factor γ IReference importance factors defined in Pr FEM 10.2.08 are reported in Table 1.1.For economical or strategic reasons the end-user may specify a higher importancefactor; in any case the importance factor must be at least equal to the importancefactor specified for the part of the building in which the racks are located.ImportanceClassDescriptionImportance factorreducedreference[Note (2)]IWarehouses with fully automated storage operationsLow warehouse occupancy (1)0.8 0.67II Normal warehouse conditions, including picking areas 1.0 0.84III Retail areas with public access 1.2 n. a.IV Hazardous product storage 1.4 n. a.Notes1. The reduced importance factor can only be used for racking systems not located onstorey of a building and/or not used for retail areas or hazardous product storage.2. The reduced importance factor is based on design life of the rack of 30 years instead of50.Table 1.1 : Importance factors for racksAccording to Pr FEM 10.2.08 the vertical component of the seismic action shallonly be taken into account in the following relevant cases:1) Cantilever components2) Beams supporting columns (for example in order picking tunnels)The effect of the vertical seismic component must be taken into account only for theelements under consideration and their directly associated supporting elements orsubstructures.For normal racking structures the vertical component of the seismic action generallyis not considered.Design parameters for the seismic analysisDesign spectrum modification factorThe design spectrum modification factor E D1 takes into account the effect of theenergy dissipation and damping related to the interaction between the pallets and theracking structure, and can be assumed constant and equal to 0.8.When pallets are blocked on the pallet beams by means of any special system (forexample materials increasing the friction between pallet and beam) E D1 = 1.0 shall beassumed.
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CHAPTER 4Seismic behaviour of steel
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Prof. Ing. Carlo Andrea Castiglioni Seismic Behaviour of Steel ...

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