Welcome to the  HOT  TOPIC  in Earthquake Engineering # 1 !
MODAL PERFORMANCE FACTOR TESTING PROCEDURE FOR BASE ISOLATION SYSTEMS
      Performance tests have been conducted on full-sized and scale models for years. However, those tests have two major drawbacks:
     They are extremely expensive. 
     ~  Their results relate to the particular structures the experiments were performed on.
     An alternative approach exercised in the ASCE Draft Standard for Testing Seismic Isolation Systems (1997) is based on the assumption that there is no need to submit a proof of usefulness of base isolation irrespective of a structural type or a type of isolation hardware: the isolation effectiveness is taken for granted just by definition! The only worry is that the isolators themselves will not survive an earthquake. For this reason, all isolators should undergo 12 basic property tests.
       This approach also assumes that the properties of isolators identified in the process of quasi-dynamic testing are reliable, that analytical methods engaged by engineers are adequate and, therefore, expected performances of the structures put on those isolators can be predicted quite accurately. Unfortunately, the recorded performances of base isolated buildings at the last California earthquakes did not support expectations. This was a primary incentive for developing a testing protocol called Modal Performance Factor Testing Procedure (MPFTP)*.
  MPFTP  is a budgetary procedure intended to provide a preliminary verification of whether the seismic isolator under consideration is the best choice for a specific project.  It provides an earthquake performance criterion for comparison of different competing technologies.  MPFTP  incorporates two proven methods: the analytical modal participation factor technique and shake table testing of objects put on base isolators.
      Purpose: MPFTP  is conducted to determine ability of isolators to reduce horizontal inertia forces in essentially elastic superstructures under a standardized sequence of selected harmonic excitations induced with a shake table.
       Sequence: Groups of horizontal harmonic excitations are resonant to all corresponding modal frequencies of the building. Each group consists of one, two and three cycles run at the following peak velocity increment: 20, 40, 80, 100, 120, 140 and 160 cm/sec. Tests shall be conducted in the order listed, with the vertical load equal to a fraction of the weight Pd  proportional to the participation factor value corresponding to the running mode of excitation.
       Procedure: Place four specimens on a shake table (see above) and secure to the supports and loading slab. Apply the correspondingly factored vertical load to each specimen and allow the loads to stabilize. Tests should be run consistently, with sufficient pauses between cycles and changes in velocity increments to allow the heat developed during  previous test to dissipate. 
      Criteria: Modal Performance Factor (MPF) is specified as a ratio of an absolute value of the maximum accelerations of a non-isolated structure averaged along its height (computed) to the value of  maximum acceleration of an isolated solid block which represents the superstructure (recorded) at a particular resonant modal frequency. The story stiffness  K is assumed to be equal to 1500 (Pd/ng),  where  n  is a number of stories, and the damping ratios x=0.02. 
     Special Requirements:  Report of results shall include the values of   MPF  for the targeted number-of-story building, for each resonant modal frequency of it, and for each increment of the peak velocity.  MPF = 1 means the isolated structure will have no advantages at an earthquake. MPF < 1 means the isolated structure is worse than the non-isolated one. MPF > 1 is a proof of the anticipated effectiveness of the tested isolation system.
   COMMENTARY One of the biggest challenges for an isolation system is a superstructure, in particular, its ability to oscillate resonantly with the frequency of the ground. This ability, among other things, depends on the number of stories which determines the actual spectrum of natural frequencies of the structure. However, there is no need to conduct testing of multistory models if the  Modal Performance Factor Testing Procedure  (MPFTP)  is used. 
       MPFTP  is based on the well known fact that only a portion of the total weight of a multistory building is effective in producing a base shear during any particular mode of vibration. Accordingly, one can avoid testing of the multitude of stories and deal just with a one-story model of varying stiffness Kp and mass Mp equal to the participation mass value, providing the shake table vibrates resonantly with the corresponding natural mode of the structure.  It should be understood, however, that MPFTP  is not intended to replace traditional testing of structural systems mounted on seismic isolators or to adequately substitute much more realistic time-history ground inputs. Neither are they meant for a modal superposition. 
     * Shustov, V., 2001,  "Modal Performance Factor Testing Procedure for Base Isolation Systems", http://www.seaint.org/seaocconvention/convention1999/Proceedings/MODAL.pdf
Your questions to the author of this page may be emailed to: valentin.shustov@csun.edu.
You may also visit Dr.Shustov's Home Page  or  CE&AM research Web Page  or  "HOT TOPICS".
Our address is: CE&AM, 18111 Nordhoff Street, Northridge, California 91330-8347