Seismic retrofit of historic courthouse with precast concrete shear walls

By Tim Van Zwol, P.Eng.

Photos courtesy RJC Engineers
Photos courtesy RJC Engineers

Cracking within existing concrete masonry unit (CMU) wall construction is common, and repairs will depend on the existing assembly, function, and condition of the mortar joints and CMUs.  However, when wall cracks were observed within the stairwells of an Ontario courthouse, a building condition assessment by RJC Engineers (RJC), led to a seismic retrofit and construction of a new lateral force-resisting system (LFRS). For this seismic retrofit, a unique solution was required to maintain building operations during construction, and to ensure the new lateral system would complement the existing building esthetics.

Overview of the North Bay courthouse building

The North Bay Provincial Courthouse building, located in North Bay, Ont., was constructed in two phases. The first phase consisted of a courtroom and lock-up erected in 1888 as a two-storey brick building. In circa 1989, the second phase of the structure was built wrapping above and around the north, east, and west sides of the original building. The current building consists of a partial basement area and four stories above grade with a height of approximately 16 m (53 ft). In total, the interior floor area is approximately 9300 m2 (100,000 sf) across all building levels.

The existing building foundation consists of reinforced cast-in-place concrete interior pad foundations and perimeter strip footings with perimeter concrete foundation walls, and a concrete slab-on-grade at the basement level. Columns are generally spaced on a 6 x 6-m (19.7 x 19.7-ft) grid. The building superstructure is constructed with conventional structural steel column and beam framing with a 150-mm (6-in.) thick concrete slab on steel deck. The concrete floor slabs are non-composite with the steel deck acting as a formwork surface only. The roof consists of an upper level near the centre of the building, and a lower level around the outside perimeter with a sloped transition in between. The roof structure consists of steel beam framing with metal roof deck. The building envelope consists of a brick veneer cladding system with horizontal precast bands located in between the windows. Five interior stairwells are located throughout the building footprint, and the walls surrounding the stairwells consist of CMU unit construction. The stairwell masonry walls do not provide vertical support for the adjacent floor slabs.

Condition survey assessment and remedial concepts

An overview of masonry cracking within the stairwell.
An overview of masonry cracking within the stairwell.

Cracking within the interior stairwell wall mortar joints and CMU was initially observed by building management staff. Prior to RJC’s involvement with the building condition assessment, third-party investigations concluded there were no geotechnical or building foundation concerns at the site. RJC was initially retained in 2014 to complete a building structure condition assessment to review and document the masonry wall cracking within the stairwells. The scope of work included:

  • a visual site review of the as-built conditions;
  • assessment of the available construction drawings and geotechnical reports;
  • a lateral structural analysis of the building; and
  • the development of potential remedial concepts.

Based on the findings of the structural condition assessment, it was determined the building had no clearly defined LFRS, and the existing beam-to-column steel connections did not have sufficient lateral capacity to meet the requirements of the Ontario Building Code (OBC). In lieu of a well-defined bracing system, it appeared, incidental lateral utilization of the concrete masonry walls within the stairwells was occurring, even though they were not originally intended to be part of a LFRS. Lateral engagement of masonry walls and the calculated lateral deflections under wind and seismic loading would result in cracking. Further, it was resolved the building structure was at risk of severe damage or partial collapse if subjected to a design seismic event.

An equivalent seismic static analysis was completed using the following criteria (the seismic loading was calculated to govern above the calculated wind loading):

  • Rd (ductility-related seismic force modification factor) equals 1.5; Ro (overstrength-related force modification factor) equals 1.3 (shear wall conventional construction);
  • Building period equals 0.4 seconds;
  • Site Class ‘D’– poor soils; and
  • Seismic base shear equals approx. 11 per cent of building weight.

The next phase of the investigation was to review potential remedial concepts to address the lateral stability of the existing building. Requirements for a successful retrofit scheme included ongoing operation of the facility during construction and maintaining overall building function and esthetics once the project was complete. Although temporary relocation of the courthouse services and construction of a new building at the site was considered, the preferred approach of the main stakeholders was to retrofit the existing building. However, this is not a frequent or common solution. Four schematic seismic retrofit options for the existing building were considered and are outlined below.

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