Ontario Steel Structure Replacement

Background and Context

Project P-2022-044 began as a capacity assessment commission for a steel structure in Ontario, Canada that had been in service for a number of years and had developed visible surface corrosion on several members. The owner required an engineering assessment to determine whether the structure was safe to continue operating as-is, whether repairs were sufficient, or whether replacement was necessary.

The National Building Code of Canada 2020 (NBC 2020) governs structural design for Ontario. NBC 2020 Part 4 covers structural loads and resistance, and the 2020 edition introduced updated seismic provisions, revised wind load methodology, and enhanced snow load maps for eastern Canada. Structures originally designed to earlier NBC editions may not satisfy current NBC 2020 requirements — the assessment process must evaluate the structure against the current standard, not the one under which it was built.

Capacity Assessment Process

Document Collection and Review

Original design drawings were obtained for the structure. These identified the design code in effect at the time of construction, the intended member sizes and grades, and the original connection configurations. The drawings were compared against field measurements to verify that the structure had been built as designed, and to identify any field modifications made during or after construction.

Condition Assessment

A detailed visual inspection was conducted, supplemented by thickness measurements on corroded members using ultrasonic testing equipment. The inspection identified surface corrosion on several secondary members, with measurable section loss at the base of columns where water had pooled over years of service. The corrosion was classified: light surface rust requiring cleaning and recoating at some locations, and more significant section loss at column bases requiring structural consideration in the capacity calculation.

Structural Analysis

A STAAD.Pro model of the existing structure was built using the actual member sizes and grades from the original drawings, with reduced section properties applied to the corroded members based on the measured section loss. This as-found model was loaded with NBC 2020 Part 4 loads: gravity loads per NBC 2020 Table 4.1.5.3, wind loads per NBC 2020 Clause 4.1.7, and seismic loads per NBC 2020 Clause 4.1.8.

The NBC 2020 wind load methodology applies the reference velocity pressure from the NBC 2020 climatic data tables for the project location in Ontario, combined with exposure factor, gust factor, and pressure coefficient derivations consistent with Part 4. The wind load demands from this analysis were compared against the design resistance of the existing members — reduced by the corrosion section loss where applicable.

Assessment Findings

The assessment identified three categories of deficiency:

• Minor deficiencies — Surface corrosion on secondary members without structural significance. Remediation by abrasive blast cleaning and recoating was sufficient.
• Moderate deficiencies — Column base section loss reducing axial capacity below the NBC 2020 demand by calculated margins ranging from 8% to 22%. These columns retained adequate capacity for current loading but had insufficient reserve for future load increases.
• Significant deficiencies — Several connections that were formed with undersized welds relative to AISC 360 requirements for the load demands, predating the adoption of current weld design standards. The weld deficiency was not a failure risk under current loading but would become one if loads increased or if the structure experienced seismic loading above the original design level.

Additionally, the original structure predated the current NBC 2020 seismic requirements for eastern Ontario. The 2020 seismic maps reflect improved probabilistic seismic hazard data that increases design demands in parts of eastern Canada relative to earlier editions. The existing structure's lateral system — a single plane of bracing — was insufficient for the current NBC 2020 Clause 4.1.8 seismic demands and would have required a complete lateral system upgrade to achieve compliance.

Replacement Decision

The combined effect of the column base deficiencies, the weld connection issues, and the seismic lateral system non-compliance made retrofit economically unfeasible. A complete replacement would be more cost-effective than the sequence of column plating, weld reinforcement, and seismic system addition that retrofit would require — and the replacement would produce a lighter, more efficient structure benefiting from current design knowledge and software tools.

The replacement structure was designed as a new commission per NBC 2020, using a more efficient structural configuration than the original, with improved corrosion protection detailing at vulnerable areas (column bases raised on galvanised base plates above the standing water level, drainage provisions in the foundation design).

NBC 2020 Replacement Design

The replacement structure was designed as a concentrically braced frame per NBC 2020 seismic provisions. The bracing configuration was optimised for both seismic resistance and efficient member utilisation — using a chevron (inverted-V) bracing pattern that distributes seismic forces symmetrically and avoids the unbalanced force condition that plagued single-diagonal configurations in earlier design practice.

Snow loads in Ontario are significant — the 1-in-50-year ground snow load for the project site from NBC 2020 climatic data significantly affects roof member design. The roof framing was designed for the combined rain-on-snow, balanced snow, and unbalanced drift conditions specified in NBC 2020 Clause 4.1.6, with the unbalanced drift case often governing beam design at eave conditions.