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

Seismic behaviour of steel storage pallet racking systems . 11structural failures were a consequence of the fall of the pallets and of the impact ofthe goods on the beams at the lower levels, creating a progressive dynamic collapse.Moreover no clearly established data and statistics exist related to the actual loadingconditions of storage racks, in particular related to the “occupancy rate” of the rackduring normal service, to different kinds of storage equipment, to differentenvironments where racking systems are located and to different goods to be stored.This is due to the practically infinite possibilities of different applications occurringin the real practice.In this context it shall also be mentioned that any observed damage to a rackcomponent due to lift truck collisions, in general means loss of store capacity andflexibility over a certain period, so it requires an immediate repair.1.1.1 BackgroundThe use of steel single selective pallet storage racks in areas accessible to the publicis a relatively recent development. Since being first introduced on a significant scaleabout 25 years ago, the number of big box stores utilizing storage racks in publicareas has increased dramatically – especially during the past 15 years.Although relatively few damaging earthquakes have occurred during this period oftime, and vary little information is available, it is interesting to analyse steel storagerack damage and contents spillage during several earthquakes in California, (i.e., the1987 Whittier, 1989 Loma Prieta, 1992 Landers, 1994 Northridge, and 2003 SanSimeon earthquakes) and one in Washington (i.e., the 2001 Nisqually earthquake),as reported in (FEMA 2006).To assess storage rack earthquake performance, it is important to determine theground motions that occurred at the site, the design capacity of the rack, the actualloading of the rack, rack design details, and any other extenuating condition thatwould affect the rack performance. When an earthquake occurs, the actual groundmotions at a given site are function of many factors. These factors include themagnitude of the earthquake, the depth of the earthquake, the distance of the sitefrom the earthquake source and the soil conditions at the site. Of particular interestare sites where ground motion was equal to or exceeded the design level groundmotion on which seismic code is based. Because of the nature of storage racks,another important issue is the actual pallet loading at the time of the earthquakerelative to the rated capacity of the storage rack. Thus, during an earthquake,damage to a particular rack might not occur because the rack was lightly loaded orbecause the ground motion at the site was not severe. It is difficult to assess theperformance of racks that have not really been subjected to their design limit.The information publicly available concerns only a very small fraction of thefacilities that have experienced earthquakes, in any case, according to (FEMA2006), “less than 1 percent of the total reports and inspections made over almost 20years in these stores describe unsatisfactory performance, failure or life safety