Retrofitting Existing Buildings in Ontario: Seismic, Structural Upgrades & OBC Compliance

When Structural Retrofitting Is Required in Ontario

Structural retrofitting in Ontario is triggered by three primary drivers:

• Renovation-triggered OBC Part 11 requirements: When an existing building is renovated and the renovation requires increased structural loads or alters the structural system, OBC Part 11.5 requires the full load path to be verified. If verification reveals the existing structure is inadequate for the proposed post-renovation loads, strengthening is required as part of the renovation scope.
• Property Standards enforcement: Municipalities can issue Property Standards Orders under the Building Code Act or municipal property standards bylaws requiring correction of unsafe structural conditions identified during inspections. These orders typically set a deadline for remediation with potential for municipality to proceed with work and charge back cost if owner fails to comply.
• Voluntary institutional risk management: Hospitals, school boards, post-secondary institutions, and government building owners sometimes commission proactive structural vulnerability assessments and implement prioritized retrofit programs independent of any immediate regulatory trigger.

Ontario does not currently have a province-wide mandatory seismic retrofit program for private buildings. Retrofit triggers are generally tied to renovation scope, unsafe conditions, or owner-driven risk management rather than a standalone retrofit ordinance.

OBC Part 11 and the Retrofit Framework

OBC Division B Part 11 contains the framework for all work on existing buildings. For retrofit work specifically:

• The retrofit must not make the building less compliant than before
• The retrofitted elements must comply with current OBC structural requirements (Part 4 for Part 4 buildings)
• New structural elements added as part of the retrofit (new shear walls, new steel frames) must comply with current OBC as if they were new construction
• Connections between new retrofit elements and the existing structure require careful engineering — the existing structure's material strength (which may be lower than current code minimums) governs the connection capacity

OBC Part 11 does not require that the entire building be brought up to current code as a condition of the retrofit — only that the retrofitted portions comply with current requirements without making the non-retrofitted portions worse.

Structural Assessment Before Retrofit

Before any retrofit can be designed, the engineer must develop a reliable understanding of the existing building's structural system, material properties, and condition. Assessment activities:

• Document review: Original structural drawings, soil reports, historical material certifications. For pre-1970s Ontario buildings, assume original drawings are unreliable or unavailable until confirmed otherwise.
• Material sampling and testing: Core samples from concrete members for compressive strength testing; Vickers hardness testing (or coupon sampling) of steel for yield strength; wood member grading and moisture content measurement. At minimum, concrete strength and steel yield strength assumptions must be supported by testing for any significant retrofit design.
• Condition assessment: Detailed documentation of all structural element conditions — concrete spalling, rebar corrosion, steel section loss, wood decay, masonry cracking, foundation settlement. The effective section of corroded or deteriorated members governs the retrofit analysis.
• Load path tracing: Document the entire load path for gravity loads (dead + live) and lateral loads (wind + seismic) from roof to foundation. Identify all connections, bearing conditions, and any load transfer mechanisms within the existing structure.

Seismic Assessment in Ontario

Ontario is in a moderate seismic hazard zone — not as high as British Columbia or the Ottawa River Valley, but with non-trivial seismic risk in Southwestern Ontario (Windsor-Toronto corridor) and Eastern Ontario near the Ottawa Valley seismic zone. NBCC 2020 provides updated seismic hazard data for all Ontario locations.

A seismic assessment follows the NBCC 2020 structural analysis provisions and evaluates:

• Site class determination: From geotechnical investigation (shear wave velocity measurement or equivalent); site class affects seismic spectral acceleration significantly
• Base shear calculation: Using NBCC 2020 Clause 4.1.8.7 equivalent static force procedure (ESFP) for most buildings, or dynamic analysis for buildings with significant irregularities
• Lateral system adequacy: Comparing existing lateral system capacity (determined from material testing and structural analysis) against the required seismic demand from NBCC 2020
• Diaphragm behavior: Whether the floor and roof diaphragms can transfer seismic forces to the lateral system; pre-1970s Ontario buildings with cast-in-place concrete slabs and precast planks often have inadequate diaphragm connections
• Connection adequacy: Beam-column connections, column-to-foundation anchorages, and wall-to-diaphragm connections are often the critical deficiency in pre-modern-code Ontario buildings

Gravity Load Upgrades

Gravity load upgrades are required when a change of use or renovation imposes higher dead or live loads than the existing structure was designed for. Strategies include:

• Steel plate reinforcing of beams: Welding steel plates to the bottom flange of steel beams to increase section modulus and flexural capacity (plate girder addition). Used when the existing steel framing is accessible and the additional load is modest.
• Post-tensioning of existing concrete slabs: Adding unbonded post-tensioning tendons to an existing concrete slab to increase its span capacity without adding significant dead load. Rarely practical due to the difficulty of anchoring PT tendons in existing structure, but used in some renovation projects.
• Adding intermediate columns: Reducing beam spans by adding intermediate columns, supported on new footings or pile caps below. Eliminates overstress in the existing beams by reducing tributary area and span length.
• Carbon FRP flexural strengthening: Bonding carbon fiber reinforced polymer (CFRP) laminates to the soffits of concrete beams and slabs to increase flexural capacity. CSA S806 governs FRP strengthening of concrete structures in Canada. Cost-effective for moderate capacity increases on accessible concrete structure.

Lateral Load Retrofit Strategies

Seismic and wind retrofit strategies for Ontario buildings:

Concrete Structure Retrofitting

Pre-1975 Ontario concrete buildings (particularly mid-rise apartment buildings from the 1950s–1970s construction boom) were designed under earlier codes with lower seismic demands, reduced ductility requirements, and lower material strength minimums. Common deficiencies and retrofit approaches:

• Flat plate punching shear deficiency: Many 1960s–1970s Ontario residential buildings used flat plate slabs without drop panels or shear reinforcing, creating vulnerability to punching shear failure at columns under seismic loading. Retrofit: through-slab steel rod installed with head plates to add punching shear resistance (similar to the stud rail approach in new construction).
• Weak column-strong beam: Pre-ductile-design concrete frames with columns weaker in flexure than the beams framing into them develop collapse mechanisms under seismic loading. Retrofit: FRP jacketing of columns to increase flexural capacity and confinement; or significant structural reconfiguration.
• Inadequate lap splices: Reinforcing bar splices in column verticals designed under older standards may be shorter than current CSA A23.3 requirements. Retrofit: FRP jacketing of the splice zone to supplement bond transfer.

Unreinforced Masonry Retrofitting

Unreinforced masonry (URM) buildings from the pre-1970s Ontario construction era — often factory buildings, warehouses, early multi-residential — are among the highest structural risk buildings in moderate seismic zones. URM retrofits in Ontario typically involve:

• In-plane wall strengthening: Vertical post-tensioning of masonry walls (drilling through-holes, installing P.T. rods anchored at top and bottom) to provide compression prestress that enhances seismic lateral capacity
• Out-of-plane wall anchoring: Adding through-wall anchors between URM walls and floor diaphragms to prevent out-of-plane collapse of walls during seismic shaking — the most common failure mode of URM in earthquakes
• Adding a new internal frame: Constructing a new steel or concrete lateral force-resisting system inside the building that can carry seismic loads independently of the URM perimeter walls

Foundation Capacity Upgrades

Foundation retrofits for Ontario buildings typically use underpinning, helical piles, or micropiles:

• Underpinning (pit method): Extending existing spread footings to deeper, stronger bearing strata by excavating in alternating bays and casting new concrete below existing footing. Traditional method, slower but widely accepted by Ontario building departments
• Helical piles: Screw piles installed adjacent to existing footings with new transfer structure to redirect loads from the original footing to the piles. Fast, low vibration. Widely used in Ontario for both retrofit and new construction on soft soils.
• Micropiles: High-capacity small-diameter drilled piles (typically 150–300 mm diameter). Can be installed in restricted access conditions and high-load applications. Expensive but effective for buildings where high loads must be transferred in limited space.