Concrete footings are the unsung backbone of Alberta's multifamily and residential construction sector. Among the most critical design criteria is the management of footing projections relative to thickness. NBC 2019 (Alberta), specifically 9.15.3.8.(2), addresses this issue where excessive projection poses a direct risk to shear and flexural performance. Familiarity with these stipulations, their practical implications, and engineering best practices is essential to achieve both code compliance and durable, cost-effective foundations in Alberta's unique conditions.

Footing Projections and Thickness: A Technical Breakdown

The horizontal projection of a footing is the part extending outward from the base of the supported wall or column. This projection enables the spread of structural loads over a broader soil area, thereby reducing bearing pressure and, when proportioned appropriately, minimizing the risk of bearing and shear failures.

Key requirements under NBC 9.15.3.8.(1) establish that the minimum thickness of a strip or pad footing must be the greater of 100 mm or the width of the projection measured from the edge of the supported element. For multifamily podium slabs, grade beams abutting foundation walls, and typical residential strip footings, this detail becomes an immediate design consideration as plans evolve from standard 20 cm wall+footing to larger block or heavily loaded wall foundations.

Example Calculations:

  • A 200 mm wide concrete wall with a total footing width of 600 mm yields a projection of 200 mm on each side. The minimum thickness is therefore 200 mm, per NBC 9.15.3.8.(1).
  • If the design increases projection to 250 mm per side, the thickness escalates to at least 250 mm, unless footings are reinforced - at which point, section 9.15.3.8.(2) comes into play.

Structural Risks with Large Footing Projections

Unreinforced concrete excels in compression but is inherently weak in tension and susceptible to diagonal shear when projections are too large. When a footing projection exceeds half its thickness, a significant portion of the flexural and shear resistance must be carried by reinforcement to prevent failure along a potential sliding plane.

The governing failure mode theorized for this scenario is a diagonal shear crack propagating from the edge of the supported element down through the footing, at an angle near 45 degrees. If the projection is allowed to grow unchecked without compensation, the risk is brittle failure: the footing could shear off at the projection, compromising both load transfer and alignment of the structure above.

Real-World Failures

  • Historic records in Prairie construction show multiple instances where unreinforced strip footings with excessive projections experienced catastrophic rotation and settlement, especially in variable clay soils common in southern Alberta.
  • Investigations of multi-unit residential complexes have revealed that frost heave and differential soil movement often concentrate stresses near footing projections, magnifying the need for reinforcement even in non-codespecified cases.

The Implication of NBC 9.15.3.8.(2): When is Reinforcement Required?

While 9.15.3.8.(2) does not lay out a tabulated formula for reinforcement, it clearly signals that projections exceeding half the footing thickness cannot rely on unreinforced concrete alone. The specification identifies the need for reinforcement as projections exceed this critical boundary, stating in effect: if projection (p) > 0.5 x thickness (t), reinforcement is required, and a reduction in overall thickness may be permitted based on engineered design.

Actual application involves structural analysis per CSA A23.3. This standard references methods of determining minimum rebar area, bar placement, spacing, and cover, considering bending moment and shear induced at the critical section just outside the wall/column footprint. Contemporary Alberta practice typically defaults to the following:

  • Rebar placement parallel to the footing length, centered near the tension face (bottom 50-65 mm from concrete bottom), and extending full width for uniform load distribution.
  • Bar sizing and spacing determined by design moment, with a frequent baseline minimum of 1-2 #15M or #20M bars for moderate loads absent a full engineer’s stamp.
  • Ensuring lateral continuity at re-entrant corners and transitions, especially for stepped grade beams or L/T-shaped footings at building perimeters.

Designing Reinforced Footings: CSA A23.3 and Beyond

Once reinforcement is required, footing design advances from empirical to engineered. The primary steps involve:

  1. Identifying Loading Conditions:
    • Service and ultimate loads due to supported elements, including dead, live, snow, and, where applicable, seismic or wind loads.
    • Occasional consideration of temperature and shrinkage forces, especially with large or heavily projected footings in exposed environments.
  2. Determining Critical Sections:
    • Analysis of flexure at the face of the supported element and shear along critical planes extending from the element edges.
    • Computation of maximum design moments and shears for projection beyond supported element.
  3. Specifying Reinforcement:
    • Minimum cross-sectional steel required, calculated based on calculated bending moments and the yield strength of the rebar (typically 400 MPa).
    • Minimum bar diameter and distribution to control crack spacing and ensure continuous load transfer across the footing projection.
  4. Detailing for Constructability:
    • Minimum 50 mm clear cover to exterior formwork or earth, per CSA A23.1 for durability.
    • Lap splices and bar bends at intersections must be engineered to avoid stress concentrations and accommodate site formwork constraints.
    • Consideration for increased cover in aggressive environments, e.g., de-icing salts or sulphate soils common to some Alberta locations.

Typical Alberta Reinforcement Practices

In multifamily basement shells and higher-load strip footings where projections regularly exceed 0.5t, reinforcement details frequently adopted include:

  • 1-2 continuous 15M or 20M bottom bars, spaced at 300 mm OC across width or length based on projection length and load.
  • Additional transverse bars (stirrups) when projections or soil conditions justify increased restraint against lateral overturning.
  • Closed ties or hooked ends at corners to resist diagonal splitting and maintain bar position during pour.

Standardized details, reviewed by local structural engineers, are common in design-build packages, but custom analysis for each job is increasingly demanded as bay spans widen and building footprints dictate atypical foundation layouts.

Strategic Cost, Schedule, and Quality Implications

Balancing Thickness against Reinforcement

While boosting concrete thickness can offset the need for rebar, construction realities in Alberta's urban settings often constrain total depth. Deep footings conflict with frost line, complicate below-grade mechanical routing, and escalate excavation and hoarding costs in winter. As soil bearing capacity and geotechnical conditions vary between Calgary, Edmonton, and adjacent municipalities, over-thickening brings diminishing returns as concrete volumes rise, with aggregate delivery and forming logistics multiplying at scale.

Strategically incorporating reinforcement can:

  • Permit reduced overall footing thickness without sacrificing load capacity or NBC compliance.
  • Optimize excavation volumes, especially important in cohesive soils with slow progress and costly dewatering.
  • Enable more predictable crack control and structural behavior under concentrated and dynamic loads, as in elevator shaft footings or high-load party wall footings.

Coordination with Other Trades and Envelope Detailing

Rebar in footings must coexist with embedded utilities, slab dowels, insulation, and waterproofing details. Early, deliberate design for reinforcement location and orientation streamlines coordination with mechanical and envelope trades, reducing costly late-stage revisions. Reinforced thin footings may also allow for more straightforward tie-in with foundation walls via dowels, improving overall connectivity and seismic resilience.

Practical Examples and Lessons from the Field

Case Study 1: Podium Slab Footings in Mixed-Use Developments

Large-format mixed-use developments often couple underground parking/support with grade-level commercial and residential superstructures. Foundations may require 600 mm+ wide footings to bear facade walls or transfer columns, with wall footprints as narrow as 200 mm. In these cases, projections easily reach 200 mm, at or above half the footing thickness for a typical 300 mm deep pad.

Exposed, unreinforced projections in early value-engineered projects led to observable diagonal cracking and latent movement over several seasons. Subsequent design reviews specified a minimum of 2-15M bars in the bottom third, continuous across the length of each footing, markedly reducing settling and repair liabilities. The upfront investment in reinforcement was recouped by simplified crack repairs and minimized warranty claims over the first decade of occupancy.

Case Study 2: Townhome Strip Footing Upgrades Following Inspection

A townhome development in Calgary, intended for simple strip footings, was flagged during inspection as projections exceeded the permitted 0.5t threshold due to field changes in wall alignment. Rather than rebuilding wider, deeper footings, the contractor incorporated 20M rebar, with engineered lapped splices at transitions. The design enabled the use of shallow footings without trench deepening, keeping the project within schedule and reducing over-excavation.

Subsequent monitoring during the first three freeze-thaw cycles indicated acceptable performance, with minor, expected shrinkage cracks but no diagonal shear or significant differential settlement at the footing edges.

Frost, Heave, and Soil Movement: Alberta-Specific Considerations

Reinforced footings are especially valuable in regions subject to seasonal frost and expansive clay subsoils. NBC minimums assume a uniform soil response, but Alberta's actual conditions rarely match this ideal. Reinforcement provides crucial redundancy against unexpected soil uplift, settlement, or edge loading, limiting the development of full-thickness cracks that compromise both structural and building envelope performance.

Detailing and Placement: Ensuring Code Compliance on Site

Reinforcing bars must be installed with attention to spacing, cover, lap joints, and intersection integrity. Construction oversight is critical; Alberta field data shows a high incidence of misplaced or under-covered rebar in residential footings, particularly when projected widths push beyond 0.5t. Critical craft practices include:

  • Chair supports, spacers, or brick used to maintain minimum cover above mudslab or undisturbed soil.
  • Proper tying to prevent float or displacement during concrete placement.
  • Consistent end bearing and bar continuity, especially at cutouts for columns or service penetrations.
  • Site records and as-built documentation to evidence compliance for later warranty or forensic investigation.

Inspection Regimes

Building Officials and consultants expect detailed bar placement consistent with submitted drawings, particularly in high-profile or warranty-backed projects. Photographic documentation before concrete placement is an industry norm. Failure to meet minimum reinforcement in these key projections is among the most common triggers for non-conformance and remedial orders in field audits throughout Alberta municipalities.

Design Optimization: Software and Analytical Aids

Advances in structural modeling enable designers to rapidly iterate different footing sizes and reinforcement layouts. Tools such as SAFE, RISA, and ETABS allow for elastic and ultimate load assessment along potential shear planes and can optimize the steel area needed to bring NBC 9.15.3.8.(2) footings back within code. These analyses are especially common in high-density urban infill and custom multifamily projects, yielding substantial material savings compared to conservative all-unreinforced designs.

Parametric Impact Assessment

Software-driven iteration can:

  • Quantify how small increases in projection dramatically raise moment and required rebar area.
  • Compare alternative bar sizes and spacings for lowest cost/cubic meter installed.
  • Predict minimum thickness necessary if site restrictions prohibit reinforcement (e.g., shallow rock profiles or high groundwater).

Integration with Geotechnical Data

Many advanced builds in Alberta now pair analytical footing design with geotechnical assessment, using soil modulus and movement predictions to select optimal reinforcement profiles. These geotechnical links become essential where developer risk tolerance or insurance conditions require performance-based foundation guarantees.

Code Evolution, Interpretation, and Local Authority Interaction

The NBC 2019 (Alberta Edition) indirectly evolved the treatment of excessive projections by signaling reinforcement as a means of code compliance rather than a strict maximum projection limit. Local Authorities Having Jurisdiction (AHJs) in Calgary, Edmonton, and smaller centers interpret 9.15.3.8.(2) variously, with some municipalities requiring sealed schedules for any footings with high projections, and others accepting prescriptive details provided by experienced concrete contractors or design-build teams.

Building strict code compliance regimes around reinforcement - instead of merely scaling up unreinforced thickness - has tangible benefits for insurance, liability, and warranty management. Record drawings must clearly identify which footings are reinforced, the steel layout, and connections to adjacent elements, as these become reference points in the review and approval process and any future dispute resolution.

Best Practice Recommendations for Alberta Projects

  • Always assess the projection-to-thickness ratio during initial foundation layout to avoid costly redesign or field changes later. Err on the side of reinforcement when margins are close to the 0.5t threshold, especially with unknown or variable soil support.
  • Engage with structural engineering support at concept stage when projections are likely to exceed half the intended thickness or when custom-shaped footings are required for tight urban footprints.
  • Document all reinforcement details, including bar size, spacing, cover, and lap, in the construction drawings and update as-built records to conform to actual field conditions for future traceability.
  • Provide training and clear field direction for site personnel placing rebar, with periodic checks for cover and placement before pour.
  • Maintain open, documented communication with building officials and inspectors regarding all deviations from standard minimums, and be prepared with engineer-sealed details as required by individual AHJ policy.

Adhering to these best practices, in accordance with NBC 9.15.3.8.(2) and CSA A23.3, establishes a durable and code-compliant foundation system that underpins lasting structural integrity and reduces lifecycle risk across Alberta's rigorous construction environment.

Cumulative Benefits of Properly Reinforced Footings

Proper reinforcement of widely projected footings goes beyond strict code compliance; it extends the service life of the entire structure and facilitates lower maintenance costs. Expertly detailed and executed reinforcement mitigates the risk of performance claims, refines schedule certainty by reducing remedial rework, and supports premium leasing or sale valuations by underlining construction quality.

For projects in Calgary, Edmonton, Red Deer, and throughout Alberta, alignment with NBC 9.15.3.8.(2) via engineered reinforcement is now part of the expected standard, not just for regulatory approval, but for market differentiation, risk management, and project liability containment.

Summary

Footings with projections beyond half their thickness occupy a critical structural and regulatory boundary within Alberta's residential construction sector. NBC 9.15.3.8.(2) establishes the necessity for reinforcement in these cases, directing practitioners towards CSA A23.3's engineered approaches rather than unreinforced prescriptive minimums. Understanding, detailing, and executing proper reinforcement ensures that footing strength, stability, and longevity are maintained even as architectural demands and site complexities push foundations to their limits.

Compliance with these standards - through meticulous bar placement, accurate documentation, and coordinated trade execution - builds both code conformity and lasting value. Drawing on decades of foundation expertise, Kingsway Builders delivers multifamily concrete solutions that exceed expectations for structural safety and long-term asset performance in Alberta.