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Seismic Design of a Self-Centering Brace for Concentrically Braced Frames
Gerard O’Reilly 1 & Jamie Goggins 2
Department of Civil Engineering, National University of Ireland, Galway, Ireland.
1 gerard.oreilly@nuigalway.ie 2 jamie.goggins@nuigalway.ie
Abstract
Concentrically braced frames (CBF’s) are a popular
lateral loading system used in seismic design of steel.
The bracing members in the CBF act as the energy
dissipative elements while other elements remain
elastic. This study focuses on minimizing the residual
displacements in the CBFs after seismic loading. A self
centering (SC) brace will be developed to achieve this
and will be modeled for a variety of designs used with
CBFs. These models can then be used to formulate a
displacement based design (DBD) approach for these
systems.
1. Introduction
Prior to the 1994 Northridge Earthquake in the US,
moment resisting frames (MRF’s) had become a
popular arrangement in steel framed buildings. Some
laboratory testing had shown that the MRF’s exhibited
good ductility under cyclic loading, making them an
ideal choice for seismic design, although further testing
on these systems showed poor hysteretic behaviour [1].
The connection detail was simple and easy to produce,
so this further contributed to its increased popularity
among designers. After the 1994 earthquake, many of
these MRF’s were noted to have performed poorly
under seismic loading, and an investigation [2]
afterwards showed that over 130 buildings had
experienced failures during the earthquake.
Another mechanism that has become popular since
the observations during the Northridge earthquake is the
CBF (Figure 1).
Figure 1: CBF arrangement and mechanism.
Building design codes now tend to require that the
beams and columns in the CBF remain elastic during
seismic loading to maintain the gravity loading system,
and that the diagonal bracing member(s) behave
inelastically, acting as the energy dissipative members
in the CBF [3].
2. Self-Centering CBF (SC-CBF) Brace
In a recent paper on the DBD of a CBF [4], one of
the findings was the inability of analytical models to
accurately predict the displacements of the frame,
whereas accelerations were. This was due to the
residual deformations present in the bracing members
after their inelastic behavior during seismic loading.
The principle aim of this project is to develop a SC
mechanism for these CBF’s that will minimise these
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residual drifts that are found in CBF’s. This will be
achieved by placing post-tensioned (PT) strands of high
strength steel inside the tubular bracing members. This
will reduce the residual drifts in the CBF by the elastic
restoring forces in the PT strands, while the tubular
members will behave inelastically and provide the
energy dissipative mechanism in the CBF.
3. Research Outcomes
3.1 Analytical Models
Using this arrangement within the CBF, testing will
be carried out and an analytical model will be
developed to accurately predict the response of the
bracing member under seismic loading. Software such
as Ruaumoko and OpenSEES will be examined to
establish the most suitable modeling software.
3.2 Parametric Study of SC-CBF’s
Once an analytical model has been established to
accurately predict the response of this SC bracing
member, a number of parametric studies will be
conducted. These will expand on the single brace
element to single SDOF and MDOF systems, dual
systems and vertically irregular systems.
3.3 Direct Displacement-Based Design
Once analytical models for the parametric studies
outlined above have been verified, it is envisaged that
that a procedure for the DBD of these SC-CBF’s can be
developed similar to SDOF CBF’s, as outlined by [4].
4. Conclusions
A SC-CBF brace element will be developed and
verified by fitting and analytical model to laboratory
results. A parametric study on numerous structure
arrangements will be modeled and methods of DBD
developed for these SC-CBF’s.
5. Acknowledgements
Irish Research Council for Science, Engineering and
Technology (IRCSET).
6. References
1. Ricles, J.M., et al., 2001. Posttensioned Seismic-Resistant
Connections for Steel Frames. Journal of Structural
Engineering. 127(2): p. 113-121.
2. Youssef, N.F.G., et al. 1995 A Survey of Steel Moment-
Resisting Frame Buildings Affected by the 1994 Northridge
Earthquake. NISTIR 5625,National Institute of Standards
and Technology, Gaithersburg, MD.
3. Tremblay, R., 2002. Inelastic seismic response of steel
bracing members. Journal of Constructional Steel Research.
58(5-8): p. 665-701.
4. Goggins, J. & Sullivan, T. 2009 Displacement-Based
Seismic Design of SDOF Concentrically Braced Frames. in
Proceedings of STESSA 2009. Philadelphia, Pennsylvania,
USA.