Rehabilitation of Nonductile RC Frame Buildings Using Steel Joint Jackets and Encasement Plates

Vitelmo V. Bertero
Professor Emeritus
Department of Civil and Environmental Engineering
University of California, Berkeley

During an earthquake, the joints and joint area are among the most critical areas of a nonductile reinforced concrete (RC) moment frame structure. Inelastic deformations may develop here and/or in the beams. To ensure against collapse, the reinforced concrete at the joints needs to be toughened by providing additional reinforcement and, particularly, by improving its confinement. An attractive means of doing this is through the use of a steel jacket. The jacket covers all the exposed faces of the joint and extends into the beam a distance approximately equal to the beam depth and into the column a distance equal to the largest dimension of the column. In typical buildings, the dimensions of the cross sections of RC members remain almost constant. The advantage of this is that the steel joint jackets can be prefabricated in sections, installed, prestressed to the RC joint, and welded together-all at the site.

In a research project under way at PEER, a jacket is being used in combination with steel encasement plates to strengthen the beam outside the jacket. The encasement plates on the top and bottom of the beam are attached to the concrete by prestressed bolts and welded to the jacket. The encasement plates strengthen the beam away from the joint with minimal disruption to the floor slab. The jacket can also be used in combination with other retrofit procedures including (1) diagonal, prestressed cross bracing; (2) supplemental energy-dissipation devices; (3) fiber wrapping outside the joint region (column); and (4) base isolation. The segmental plates greatly facilitate postearthquake evaluation because they can easily be removed for inspection of the concrete.

Analytical studies are in progress to verify the applicability of the proposed rehabilitation approach to an older existing prototypical RC building that can be expected to experience ground motions varying from impulsive to harmonic.

The studies take into account various combinations of steel jackets and encasement plates to indicate the strength and deformation demands placed on an upgraded structure.

Future work will include cyclic load tests on large-scale specimens representing the nonductile components of older RC frame buildings needing retrofit. Under the umbrella of performance-based engineering, this project will be coordinated with a multifaceted, innovative system for the upgrading of existing RC buildings having nonductile moment frames.