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Welcome
to the HOT
TOPIC in Earthquake Engineering # 1
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MODAL
PERFORMANCE FACTOR TESTING PROCEDURE FOR BASE ISOLATION SYSTEMS
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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. |
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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. |
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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 Kn
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. |
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