Construction of exterior precast shear walls
The seismic retrofit project started in January 2016 and was substantially completed by November 2017. The design team consisted of RJC as the prime consultant and structural engineers, Mitchell Jensen Architects as the architectural consultant, and Piotrowski Consultants Ltd., as the mechanical and electrical engineering consultants. The project was tendered and awarded to Kenalex as the general contractor (GC). The construction value of the project was approximately $14 million.
Due to the relative poor soils and bearing capacities at the site, the 13 shear-wall foundations consisted of a series of deep micro piles with cast-in-place concrete pile caps. Anchors with steel end plates were embedded into the concrete pile caps and would provide the connection for the precast shear walls above.
Each of the 13 shear walls were constructed using reinforced precast concrete, ranging in thickness between 475 and 550 mm (19 and 22 in.). The concrete mix for the precast shear walls was designed as 35 MPa (5000 psi), Exposure Class C-1. Mockups of the mix design were completed prior to full wall fabrication to select the colour pigment and texture (i.e. sandblasting) that best matched the existing precast bands at the exterior building walls. The formed side of the precast shear walls was exposed and received a medium sandblast finish with penetrating sealer to match the roughness and degree of aggregate exposure of the existing precast. All the precast shear walls were installed over a period of two weeks during nights to avoid interfering with court operations, and to mitigate impact of road closures.
Each of the shear walls consisted of three vertically stacked segments to meet handling and shipping requirements from the precast plant to the site. The walls were cast with embedded steel plates to facilitate beam connections to the building structural steel frame, and were strengthened with internal reinforcement and post-tensioned threadbars with couplers to interconnect the vertically stacked wall panels. At the construction joint between the panels, a 75-mm (3-in.) deep recess was formed on the exterior face to accommodate architectural granite panels. The construction joint recess and granite panel covering matched the elevation of the existing building precast bands in between the windows. As one of the main architectural features of the new precast shear walls covering the construction joints and threadbar grout ports, the granite panels also complemented the building esthetics by matching the existing brick veneer.
Interior construction: Phasing and structural components
The interior scope of work for the project was completed between January 2016 and May 2017. Placing the new lateral shear walls outside of the building footprint helped maintain full use and occupancy during construction, however, a significant amount of structural steel reinforcements within the building interior was also required. This work was typically completed during nights/weekends so courtroom sessions and building use was not disturbed during the week. A ‘swing space’ was constructed at an unoccupied floor area within the building and was used for temporary relocation of separate building groups while work within their particular area was completed. In total, the interior scope of work was completed over nine separate phases. In some parts of the building (i.e. stairwells and corridors), work areas were required to be reopened after each night shift and occupied during the day.
The interior structural work generally consisted of reinforcements to the existing steel beam framing that would form a series of ‘drag struts.’ These would form the load paths required to transfer the seismic lateral forces generated within each floor and roof levels out to the external precast shear walls. Retrofit work consisted of steel plate reinforcements to the existing steel beams and upgrades to the beam-column connections along the designated load paths in order to withstand drag strut axial forces. Additional steel diaphragm connections between the steel beams and concrete floor slabs were also installed. At localized areas below the lower roof deck, horizontal cross bracing was installed above the ceiling to transfer lateral loads from the upper roof deck out to the designated drag strut locations.
Within the stairwells, cracked masonry joints were repointed, cracked masonry units were replaced, and an elastomeric coating was applied over the entire wall surface.
Conclusion
In summary, cracking observed within the stairwell masonry walls ultimately led to a complete building seismic retrofit of the North Bay courthouse. Although temporary and permanent relocation of the courthouse services was considered, the final desire of the main stakeholders was to keep courthouse services in the current location. External precast shear walls proved to be the preferred lateral retrofit scheme in combination with strategically phasing the interior structural work. Not only was the primary goal of keeping the courthouse services in the current location achieved, but also building operations were maintained during construction. The implementation of the seismic upgrades was successful, and the precast shear wall design complimented the existing building esthetics.
Tim Van Zwol, P.Eng., is a regional manager/associate with the building science and restoration group at RJC Engineers. Van Zwol has over 15 years of engineering consulting experience in new construction and building restoration, including a wide range of residential, commercial, institutional, and municipal projects. His project management expertise gained from new building construction and renovations enable him to take an active role in the investigation and rehabilitation of aging structures. Van Zwol can be reached via e-mail at tvanzwol@rjc.ca.
