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Evaluation of Accuracy of Simplified Methods of Analysis and Design of Buildings with Damping Systems for Near-Fault and for Soft-Soil Seismic Motions
This report assesses the validity of the simplified methods of analysis
and design of buildings with damping systems specified in FEMA’s
National Earthquake Hazard Reduction Program (NEHRP) Recommended
Provisions for Seismic Regulations for New Buildings and Other Structures issued in
2000 and updated for 2003, and the upcoming ASCE-7 Standard for 2005 when
the effects of near-field and soft-soil ground motions are taken into account.
The procedures outlined in these documents are largely based on studies
that excluded these effects. To determine their impact, both single- and
multiple-degree-of-freedom structures with linear and nonlinear viscous
damping devices were studied using two sets of near-field ground motions
and one set of soft-soil ground motions.
The study found that the damping coefficient values are accurate or conservative; the ductility demand for near-field and soft-soil motions are very similar to those previously observed for far-field motions; simplified methods of analysis for single-degree-of-freedom systems produce results on displacement and acceleration that are generally of acceptable accuracy or conservative for near-field or soft-soil motions and are very similar to that previously observed for far-field motions; and their application to steel moment frames with linear and nonlinear viscous damping systems provided conservative estimates of drift and predictions for damper forces and member actions in good overall agreement with the average of results of nonlinear response-history analysis.
Assessment of Geotechnical Issues in Acute Care Facilities in California
This report summarizes the findings from an evaluation of geotechnical
reports submitted as part of the compliance reports required by Senate
Bill 1953 (SB 1953) for all hospitals in California. The geotechnical reports
from 153 of the 470 licensed hospitals in California were reviewed with
the cooperation of the California Office of Statewide Health Planning and
Development (OSHPD). Review of this data indicates that less than half
of the hospital buildings in California in 2001 were considered to be structurally
compliant with the requirements of SB 1953. Almost 40 % were determined
to be at significant risk for structural collapse and a danger to public
safety in the event of a strong earthquake. Over 70 % had basic nonstructural
systems essential to life safety and patient care that were inadequately
anchored to resist earthquake forces. The survey of the geotechnical evaluations
indicated that about 20 % of the hospital sites had a potential for liquefaction
based on the SB 1953 design ground motions.
Scissor-Jack-Damper Energy Dissipation System
This report describes an energy dissipation system configuration that
extends the utility of fluid viscous damping devices to structural systems
that are characterized by small interstory drifts and velocities. The geometry
of the brace and damper assembly is such that the system resembles a jacking
mechanism, and thus the name “scissor-jack-damper energy dissipation
system” is adopted. The system is a variant of the toggle-brace-damper
system, and offers the advantage of a more compact configuration. A theoretical
treatment of the scissor-jack-damper system is presented and its effectiveness
is demonstrated through testing of a large-scale steel framed model structure
under imposed harmonic displacement on the strong floor, as well as dynamic
excitations on the earthquake simulator. Experiments demonstrate that despite
its small size, the scissor-jack system provides a significant amount of
damping while also substantially reducing the seismic response of the tested
structure. Application of the system in a new building in Cyprus is described.
Seismic Retrofit of Bridge Steel Truss Piers Using a Controlled Rocking Approach
The research presented in this report investigates a seismic retrofit
technique for steel truss bridge piers that allows pier rocking by using
passive energy dissipation devices implemented at the anchorage locations
to control the rocking response. Specially detailed hysteretic energy dissipating
elements (buckling-restrained braces) are used to act as easily replaceable,
ductile structural “fuses.” The dynamic characteristics of
the controlled rocking/energy dissipation system are investigated in order
to formulate a capacity design procedure using simplified methods of analysis.
Design constraints are established that attempt to satisfy performance
objectives and nonlinear time history analyses are used to assess the seismic
behavior of the bridge piers retrofitted per this strategy. The retrofit
strategy is shown to be more applicable to slender piers. The methods of
predicting key response values were found to be conservative in most cases
and capacity protection of the existing pier (to the prescribed limits)
was achieved in all cases considered.
Experimental & Analytical Studies of Structures Seismically Isolated with an Uplift-Restraint Isolation System
This report describes the development of a novel uplift-prevention Friction
Pendulum isolator called the XY-FP. It presents the principles of operation
and mathematical model of the XP-FP isolator, describes its mechanical behavior
through testing of a single isolator, and demonstrates its effectiveness
through testing of a quarter-scale steel-frame model structure. The computer
program 3D-BASIS-ME was modified to include an element representative of
the mechanical behavior of the new XY-FP isolator, and the validity and accuracy
of analytical methods to predict its behavior is assessed. The study shows
that the XY-FP isolator provides effective uplift prevention regardless of
the state of displacement in the bearing, allows for decoupling of the bi-directional
horizontal motion along two orthogonal directions, and has the capability
to provide distinct stiffness and energy dissipation along the principal
directions of the bearing. In addition, by encompassing much less structural
material, the isolator offers a lighter and more economical alternative to
the standard Friction Pendulum bearing.
A Versatile Experimentation Model for Study of Structures Near Collapse Applied to Seismic Evaluation of Irregular Structures
This report presents a study of irregular structures near collapse and the
development of an experimental model to study many types of structural systems
in the near collapse state. Many analytical studies have been carried out
to evaluate irregular structures, but few experimental works have been done
on this subject. This study provides an overview of the accuracy of the analytical
methods in predicting the structural response. Equally important in this
research was the design of a structural model for study of structural systems
near collapse. A versatile reconfigurable structural model was developed
to be used and reused with structures undergoing severe damage to sacrificial
elements, thus capable of being repaired and further tested without complete
collapse.The study shows that a separation of lateral and gravity load resisting
systems can produce a stable structure in case of major damage to lateral
system, provided that redundancy exists to control lateral deformations.
Such a system can be implemented when retrofitting structures, by weakening
the connections of gravity columns and providing a redundant external lateral
load resisting system.
Spring 2005
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