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A retrofitting framework for pre-Northridge steel moment-frame buildings

Citation

Bjornsson, Arnar Bjorn (2014) A retrofitting framework for pre-Northridge steel moment-frame buildings. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05302014-102237282

Abstract

In the 1994 Mw 6.7 Northridge and 1995 Mw 6.9 Kobe earthquakes, steel moment-frame buildings were exposed to an unexpected flaw. The commonly utilized welded unreinforced flange, bolted web connections were observed to experience brittle fractures in a number of buildings, even at low levels of seismic demand. A majority of these buildings have not been retrofitted and may be susceptible to structural collapse in a major earthquake.

This dissertation presents a case study of retrofitting a 20-story pre-Northridge steel moment-frame building. Twelve retrofit schemes are developed that present some range in degree of intervention. Three retrofitting techniques are considered: upgrading the brittle beam-to-column moment resisting connections, and implementing either conventional or buckling-restrained brace elements within the existing moment-frame bays. The retrofit schemes include some that are designed to the basic safety objective of ASCE-41 Seismic Rehabilitation of Existing Buildings.

Detailed finite element models of the base line building and the retrofit schemes are constructed. The models include considerations of brittle beam-to-column moment resisting connection fractures, column splice fractures, column baseplate fractures, accidental contributions from ``simple'' non-moment resisting beam-to-column connections to the lateral force-resisting system, and composite actions of beams with the overlying floor system. In addition, foundation interaction is included through nonlinear translational springs underneath basement columns.

To investigate the effectiveness of the retrofit schemes, the building models are analyzed under ground motions from three large magnitude simulated earthquakes that cause intense shaking in the greater Los Angeles metropolitan area, and under recorded ground motions from actual earthquakes. It is found that retrofit schemes that convert the existing moment-frames into braced-frames by implementing either conventional or buckling-restrained braces are effective in limiting structural damage and mitigating structural collapse. In the three simulated earthquakes, a 20% chance of simulated collapse is realized at PGV of around 0.6 m/s for the base line model, but at PGV of around 1.8 m/s for some of the retrofit schemes. However, conventional braces are observed to deteriorate rapidly. Hence, if a braced-frame that employs conventional braces survives a large earthquake, it is questionable how much service the braces provide in potential aftershocks.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Pre-Northridge; Steel Moment-Frame; Retrofit; ASCE-41
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Civil Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hall, John F.
Thesis Committee:
  • Heaton, Thomas H. (chair)
  • Hall, John F.
  • Beck, James L.
  • Graves, Robert W.
Defense Date:15 May 2014
Funders:
Funding AgencyGrant Number
FulbrightUNSPECIFIED
Housner FellowshipUNSPECIFIED
Hellwig FellowshipUNSPECIFIED
USGSG09AC00150
USGSG13AP00061
Record Number:CaltechTHESIS:05302014-102237282
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05302014-102237282
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8440
Collection:CaltechTHESIS
Deposited By: Arnar Bjornsson
Deposited On:30 May 2014 23:51
Last Modified:30 Jun 2014 21:06

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