Under The Guidance Of,Mr. AvinashAssistant Professor Dept. Of Civil Engineering Manipal Institute Of Technology

Seismic Retrofitting Of Reinforced Concrete And Steel Framed Structures Using Buckling Restrained Bracing Systems

Presented by,

Dhanush S Ganig

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Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures.

Exterior steel infill shear truss, University of California, Berkeley

Buckling restrained braces (BRBs) are composed of a slender steel core continuously supported by a concrete casing in order to prevent buckling under axial compression. The core and the casing are decoupled to prevent interaction between them.

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Abstract Existing buildings designed for gravity loads only Earthquake code have been formed/updated after the

completion of structure Old and existing structures are vulnerable to earthquake

forces, which can have devastating effect on them Comparative analysis of concrete frames, steel framed

structures and RC structures are done Study the effects of bracing systems in decreasing the

lateral drift and reduce the base shear of structures Plastic design based approach of bracing systems

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Use Of Steel Bracing In Reinforced Concrete Frames 1

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Bracing systems for seismic retrofitting of steel frames 2

Concentrically Braced

Mega Braced

Spectral Acceleration

Interstorey Drifts For Mbfs With Ordinary (Top) And Unbonded (Bottom) Braces: Normal Yield Steel (Left) And Low-yield Diagonals

(Right).

Interstorey Drifts For CBFs With Ordinary (Top) And Unbonded (Bottom) Braces: Normal Yield Steel (Left) And Low-yield Diagonals

(Right).

Seismic retrofitting with buckling restrained braces: Analytical studies 3

Plan views of study buildings (a) 3-story (b) 6-story (c) 9-story

BRBFs and their brace - beam column connections (a) 3-story (b) 6-story (c) 9-story

Interstorey drift response a)DBE and b) MCE

Conclusions The test results indicate a large increase in the in-plane shear

strength of a concrete frame due to only one diagonal brace (acting either in compression or tension).

A substantial increase in the shear resisting capacity of concrete frames can be achieved using diagonal steel X-bracing. The strength of the X-braced model frame was measured at four times that of the unbraced frame.

The behaviour of the X-braced frame under loading shows the dominance of the tension brace as it carries larger portion of the load.

The present analytical work showed that moment resisting frames (MRFs) with insufficient lateral stiffness can be retrofitted with diagonal braces. The results of the performed inelastic analyses demonstrate that MBFs are the most cost-effective. The reduction of inter-storey drifts with respect to the original MRF is on average equal to70%. Maximum lateral drifts in MBFs are 45%–55% lower than CBFs; the reductions in global deformations depend, however, on the characteristics of earthquake ground motions, especially frequency content.

Mega-braces can be installed without business interruption within the building thus preventing loss of use (downtime) caused by the structural retrofitting strategy.

The mean values of inter-storey drifts are well within the target drift values that were predetermined for DBE.

References

1. M.R. Maheri And A. Sahebi ‘Use Of Steel Bracing In Reinforced Concrete Frames’ Journal Of Engineering Structures Vol.19, No.12, Pp 1018-1024, 1997

2. L. Di Sanro And A. S. Elnashai ‘Bracing Systems For Seismic Retrofitting Of Steel Frames’ Journal Of Constructional Steel Research, 2008

3. Dipti R. Sahoo, Shih-ho Chao ‘Seismic Retrofitting With Buckling Restrained Braces: Analytical Studies’ Journal Of Engineering Structures, 2010

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