Case Study: Beresford Arms Hotel Earthquake retrofit

The Challenge
This hotel was built on unstable fill and sands and was in danger of collapse in the event of a severe earthquake. A structural analysis showed that the inner core of the building needed to be reinforced with shear walls and underpinned with cast-in-place reinforced concrete piers.

Action:
The inner courtyard was inaccessible except for an 8 ft. tall x 5 ft. wide opening. The height of the build­ing prevented the use of cranes to place equipment or material into the courtyard. Furthermore, since the soils were noncohesive, all piers had to be cased for the top 25 ft.
A small track rig was used that fit through the narrow opening. Groups of 4 piers 40 ft. deep were placed in each of the 4 corners. 18” diameter contin­uous flight augers and steel casing were used for the pier drilling.
The cages were winched through the opening in sections and then assembled to full length with rebar couplers. They were then raised over the piers and lowered in using temporary tube steel pole and winch systems bolted to the sides of the building. The piers were then poured as the casing was re­tracted.

Results:
This initial phase of a multi-phase retrofit program helped bring the Beresford Arms Hotel up to current earthquake standards.

Captions:
Multi-story Hotel, San Francisco
Detail showing rebar couplings
Starting a new pier hole before inserting casing
Overhead shot of pier drilling in corner of courtyard
Raising a reinforcing cage consisting of #8 rebar and spiral ties
Overview of construction site showing continuous flight augers and one set of completed piers at upper left corner.
Placing a reinforcing cage into a pier hole in the corner
Lowering a cage down into a pier hole with the temporary winch system
Picture showing temporary winch systems bolted to building and suspended cage at right side. Access opening visible at center bottom.

RWR Construction Inc. 200 Gary Place, San Rafael, CA 94901 (415) 457-5658 www.RWR-inc.com