Load Combinations — CSA S16 Canadian Standard

CSA S16 / NBCC ultimate and serviceability limit state load combinations. Factored resistance combinations for Canadian structural steel design. Educational use only.

This page documents the scope, inputs, outputs, and approach of the CSA S16 Load Combinations tool on steelcalculator.app. The interactive tool runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

Generating ULS and SLS load combinations per the National Building Code of Canada (NBCC) for use with CSA S16 steel design.
Applying the companion load approach with importance factors and load combination factors.
Identifying the governing combination from the NBCC Table 4.1.3.2.

What this tool is not for

It does not determine characteristic load values (those come from NBCC Part 4 and referenced loading standards).
It does not cover all provincial variations or site-specific seismic hazard determination.
It does not produce combination-specific member forces or load paths.

Key concepts this page covers

NBCC load combination framework (Table 4.1.3.2)
principal and companion load concept
importance factor I for different building categories
ULS and SLS limit states per CSA S16

Inputs and outputs

Typical inputs: dead load D, live load L (with reduction factors), snow load S, wind load W, earthquake load E, and the importance category.

Typical outputs: all applicable ULS and SLS combinations, factored values for each, the governing combination, and the companion load factors used.

Computation approach

The tool applies the NBCC load combination table directly. Each combination multiplies the principal load by its full factor and companion loads by reduced factors (typically 0.5 for wind or earthquake as companion, 1.0 or 0.5 for live/snow). The tool evaluates all specified combinations and returns the maximum factored demand along with the combination identifier.

Frequently Asked Questions

How do NBCC load combinations differ from ASCE 7? NBCC uses an explicit companion load approach where each combination identifies one principal load at its full factored value and other variable loads at reduced companion values. ASCE 7 has a similar structure but uses fixed load factors without the formal companion load designation. The numerical factors differ: for example, NBCC uses 1.25D + 1.5L while ASCE 7 uses 1.2D + 1.6L for the primary gravity combination. Both are calibrated to achieve acceptable reliability, but the target reliability indices and reference periods differ.

What is the importance factor in NBCC? The importance factor I adjusts loads for building importance: I = 0.8 for low-importance structures (farm buildings, minor storage), I = 1.0 for normal importance, I = 1.3 for high importance (schools, community centers), and I = 1.5 for post-disaster facilities (hospitals, fire stations). This factor multiplies the specified wind, snow, and seismic loads in the load combinations to provide higher reliability for buildings where failure consequences are severe.

When does the counteracting dead load factor 0.9D apply? The factor 0.9D is used in combinations where dead load resists (stabilises against) the destabilising effect of wind or earthquake. For example, 0.9D + 1.4W checks whether wind uplift exceeds the restoring effect of self-weight. Using 0.9 instead of 1.25 accounts for the possibility that dead load is less than estimated (overestimating dead load would be unconservative when it is stabilising).

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

The site operator provides the content "as is" and "as available" without warranties of any kind. To the maximum extent permitted by law, the operator disclaims liability for any loss or damage arising from the use of, or reliance on, this page or any linked tools.