Why Capacity Assessment?
Structures designed and built to older codes may no longer comply with current standards — not because they were wrongly designed at the time, but because codes evolve as structural knowledge advances, as new hazard data becomes available (revised seismic maps, updated wind speed records), and as occupancy patterns change. A steel tower designed to 1985 wind provisions may be deficient under ASCE 7-22 or NBC 2020 requirements.
Capacity assessment is also triggered by change of use (increased live loads, new equipment added), visible distress (corrosion, deformation, cracking), or as due diligence before purchase or insurance renewal. Whatever the trigger, the assessment must be rigorous, traceable, and honest — clearly stating what the structure can carry, what it cannot, and why.
The Assessment Process
Document Review
The first step is assembling original design drawings, calculations, and specifications where available. These documents establish the intended design — member sizes, material grades, connection types, and the code to which the structure was originally designed. Where original documentation is incomplete or absent, dimensional survey and material testing become necessary.
Site Survey and Condition Assessment
A physical inspection of the structure identifies current condition: corrosion loss in steel members (reducing effective section), concrete deterioration (spalling, carbonation, chloride attack), weld defects visible on exposed connections, foundation settlement or movement, and any modifications made since original construction. Condition findings directly affect the capacity calculation — a corroded W-shape has a reduced section modulus; a foundation with measurable settlement requires differential settlement analysis.
Analytical Model Development
We build a structural model of the existing structure as found, incorporating actual member sizes, identified condition deductions, and current code loads. This model is analysed under the full suite of design load combinations from the applicable current code — not the code under which the structure was built. The results identify which members or connections are overstressed relative to current code requirements, and by how much.
Capacity Rating
The assessment produces a capacity rating for the structure: what loads it can safely carry under current code provisions. For structures subject to repeated dynamic loading (entertainment structures with moving loads, structures subject to vibration from equipment), fatigue assessment may also be required. The rating distinguishes between adequate members that meet current code, members that have insufficient reserve capacity, and critical deficiencies that require immediate action.
Case Study: Ontario Steel Structure Replacement (NBC 2020)
Project P-2022-044 involved the capacity assessment and replacement design for an existing steel tower structure in Ontario, Canada. The structure had been designed to an earlier edition of the National Building Code of Canada and had experienced surface corrosion over years of service in an outdoor environment.
Our assessment found that, while most primary members retained adequate capacity under NBC 2020 wind and snow loads, several connections had been formed with undersized welds relative to what current AISC 360 provisions would require for the same load demands. Additionally, the original design predated the current NBC 2020 seismic provisions for eastern Ontario, and the structure lacked the ductile detailing required for its seismic design category.
The decision to replace rather than retrofit was made jointly with the client based on economics: the number of connection upgrades required, combined with the seismic detailing modifications needed for the existing geometry, exceeded the cost of a new structure designed cleanly to current standards. The replacement was designed with the full benefit of current structural knowledge and software tools, resulting in a lighter and more efficient structure than would have been possible through a piecemeal retrofit of the original.
Retrofit Strategies
When the assessment indicates that specific members or connections are deficient but the overall structure has value worth preserving, retrofit is the appropriate response. Common retrofit strategies include:
- Section reinforcement — Welding additional plates to increase the section modulus of bending members, or adding cover plates to compression flanges to increase lateral torsional buckling resistance.
- Connection strengthening — Adding weld passes or additional bolts to connections found deficient in shear or tension capacity.
- Lateral system addition — Adding bracing bays or shear walls to a structure lacking adequate lateral resistance under current seismic or wind code requirements.
- Foundation underpinning — Increasing foundation bearing area or adding piles to foundations found deficient under current loads or settlement criteria.
- Corrosion protection renewal — Blast cleaning and recoating steel members to restore full section, followed by upgraded corrosion protection systems appropriate for the service environment.
When to Retrofit vs Replace
The retrofit-vs-replace decision is an engineering and economic judgement that we make explicitly and transparently for each assessment. The relevant factors are: the extent of deficiency (few isolated problems versus systemic issues), the cost of intervention (can connections be accessed?), the remaining service life desired, the presence of historic or aesthetic value, and the disruption that each option causes to ongoing operations.
Retrofit recommendations are made without commercial bias. We have no material preference for retrofit or replacement — the recommendation follows the engineering evidence and the client's objectives, stated clearly with supporting analysis.
What Clients Receive
- Capacity assessment report — Structured assessment findings, condition notes, analytical model documentation, and member-by-member capacity ratings versus current code requirements.
- Deficiency schedule — Tabulated list of non-compliant members and connections, with demand/capacity ratios and priority classifications (immediate, short-term, long-term).
- Retrofit or replacement design — Full structural design for the recommended intervention, with calculations and drawings to the same standard as new-build projects.
- Cost-basis recommendation — Engineering assessment of the relative merits of retrofit versus replacement, documented for client decision-making.