Report - PEER - University of California, Berkeley

In general, these types of bench-mountedscientific equipment are short and rigid, thus,imposed seismic excitation results in a slidingdominatedresponse, rather than a rockingdominatedresponse. An example of a typicalscience laboratory bench-shelf system, withequipment mounted onto the surface of the bench,is shown in Figure 1. Upon sliding, there is concernthat the equipment may be damaged either byfalling from the bench-top surface or throughimpact with neighboring equipment or surroundingsidewalls. The probability that either potential limitstates will be exceeded is often expressed in theform of a seismic fragility curve. A seismicfragility curve associates the probability ofexceedance of a defined limit state (a damagemeasure, DM) with an engineering demandparameter (EDP). An EDP may be considered aninput parameter to the fragility curve, for example,maximum floor acceleration or maximum interstorydrift.Since the sliding of unrestrained rigid equipment is initiated when theacceleration at the top of the supporting element overcomes the resistance due tofriction between the two surfaces of contact, considering the accelerationamplification due to a support element (such as a furnishing element) in the fragilitycurve development is very important. Science laboratory benches often have uni-strutrailing systems providing a pinned support at the floor and ceiling to anchor thebench, creating a system with some flexibility. The result may be that the naturalfrequency of the laboratory bench system lies within the acceleration sensitive zone ofthe input floor response spectrum, and may therefore experience accelerationamplification. However, since the sliding response is nonlinear, it is not possible todetermine the response of the equipment by simply scaling the input acceleration toaccount for the bench amplification. This has also been observed by other researchers[e.g., Shao and Tung (1999) and Garcia and Soong (2003)].1.1 Background and Previous WorkFigure 1. Typical benchmountedequipment within aScience Laboratory (Photocourtesy of Mary Comerio).Research has been conducted to understand the toppling and sliding behavior ofunrestrained rigid equipment under seismic excitation. Perhaps the first analyticalformulation describing the fundamental equations of motion for rigid unattachedbodies was presented by Shenton and Jones (1991). In later work, Shenton (1996)investigates the criteria for sliding and rocking and sliding-rocking of rigid bodymodes. Shao and Tung (1999) cast the problem into a statistical formulation, studying198