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Annex C (informative)Load combinations and load factors in the National Building Code of Canada, 2005

Notes: (1) This Annex is not a mandatory part of this Standard.(2) This Annex provides an adapted version of portions of Subsection 4.1.3 from an unpublished draft of the National

Building Code of Canada, 2005 (NBC). Those portions deal with load factors and load combinations. This material has been adapted in accordance with CSA editorial requirements and is included for information only.

(3) The load factors and load combinations presented in this Annex are discussed in Part 4 of the NBC User’s Guide and should be used in conjunction with the resistance factors specified in Clause 8.4.

(4) The NBC defines the following classes of loads:(a) permanent loads such as dead loads, D, and effects of prestress, P;(b) variable loads due to use and occupancy, L, wind loads, W, and snow loads, S;(c) rare loads such as earthquake loads, E; and(d) imposed deformations, T (see Clause 8.2.2).

(5) The equations used to compute the loads S, W, and E for snow, wind, and earthquake in the NBC include importance factors Is , Iw, and Ie , which are a function of the use and occupancy of the building.

(6) The following symbols are used in this Annex:D = permanent loads due to dead load, or related internal moments and forcesE = earthquake loads, or related internal moments and forcesH = load due to lateral earth pressure, including groundwater, and related internal moments and forcesL = variable load due to intended use and occupancy, including loads due to cranes and pressure of liquids in

containers, or related moments or forcesP = effects of prestress, including secondary moments due to prestressR = nominal resistance of a member, connection, or structure based on the dimensions and on the specified

properties of the structural materialsS = variable loads due to ice, rain, and snow (including associated rain)T = effects of imposed deformations due to moisture changes, shrinkage, creep, temperature, and ground

settlement, or combinations thereofW = variable loads due to wind, or related internal moments and forcesφ = resistance factor applied to a specified material property or to the resistance of a member, connection, or

structure, which for the limit state under consideration takes into account the variability of dimensions and material properties, quality of work, type of failure, and uncertainty in the prediction of resistance

C.1 Limit states designNote: See Appendix A of the NBC.

C.1.1 DefinitionsThe following definitions apply in this Annex:

Companion load — a specified variable load that accompanies the principal load in a given load combination.

Companion-load factor — a factor that, when applied to a companion load in the load combination, gives the probable magnitude of a companion load acting simultaneously with the factored principal load.

Effects — forces, moments, deformations, or vibrations that occur in the structure.

Factored load — the product of a specified load and its principal-load factor or companion-load factor.

Factored resistance — the product of nominal resistance, R, and the applicable resistance factor, φ.

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Importance factor — a factor applied in Subsections 4.1.6 to 4.1.8 of the NBC to obtain the specified load, to account for the consequences of failure as related to the limit state and the use and occupancy of the building.

Limit states — those conditions of a building structure in which the building ceases to fulfill the function for which it was designed.Note: Those states concerning safety are called ultimate limit states (ULS) and include exceeding the load-carrying capacity, overturning, sliding, and fracture. Those states that restrict the intended use and occupancy of the building are called serviceability limit states (SLS) and include deflection, vibration, permanent deformation, and local structural damage such as cracking. Those limit states that represent failure under repeated loading are called fatigue limit states.

Principal load — the specified variable load or rare load that dominates in a given load combination.

Principal-load factor — a factor applied to the principal load in the load combination to account for the variability of the load and load pattern and analysis of its effects.

Specified loads (D, E, H, L, P, S, T, and W) — the loads specified in Note (6) of the preliminary Notes to this Annex.

C.1.2 Strength and stability

C.1.2.1 A building and its structural components shall be designed to have sufficient strength and stability so that the factored resistance, φR, is greater than or equal to the effect of factored loads, where the effect of factored loads shall be determined in accordance with Clause C.1.2.2.

C.1.2.2 The effect of factored loads for a building or structural component shall be determined in accordance with the load combinations specified in Table C.1 and the provisions of Clause C.1.2, the applicable combination being that which results in the most critical effect. (See Appendix A of the NBC.)

C.1.2.3 Where the effects due to lateral earth pressure, H, restraint effects from prestress, P, and imposed deformation, T, affect the structural safety, they shall be taken into account in the calculations, i.e., H with a load factor of 1.5, P with a load factor of 1.0, and T with a load factor of 1.25. (See Appendix A of the NBC.)

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Table C.1Load combinations for ultimate limit states

(See Clauses 8.3.2, C.1.2.2, and C.1.2.4 to C.1.2.8.)

C.1.2.4 Except as provided in Sentence 4.1.8.16.(1) of the NBC, the counteracting factored dead load, 0.9D in the load combinations specified in Cases 2 to 4 of Table C.1 and 1.0D in the load combination specified in Case 5 of Table C.1, shall be used when dead load acts to resist overturning, uplift, sliding, and failure due to stress reversal, and to determine anchorage requirements and factored member resistances. (See Appendix A of the NBC.)

C.1.2.5 The principal-load factor 1.5 for live load, L, in Table C.1 may be reduced to 1.25 for liquids in tanks.

C.1.2.6 The companion-load factor 0.5 for live load, L, in Table C.1 shall be increased to 1.0 for storage occupancies, and for equipment areas and service rooms in Table 4.1.5.3 of the NBC.

C.1.2.7 The load factor 1.25 for dead load, D, for soil, superimposed earth, plants, and trees in Table C.1 shall be increased to 1.5, except that when the soil depth exceeds 1.2 m, the factor may be reduced to 1 + 0.6/hs , but not less than 1.25, where hs is the depth of soil in metres supported by the structure.

C.1.2.8 Earthquake load, E, in the load combination specified in Case 5 of Table C.1 includes horizontal earth pressure due to earthquake determined in accordance with Sentence 4.1.8.16.(4) of the NBC.

Case

Load combination*†

Principal loads Companion loads

1 1.4D —

2 (1.25D‡ or 0.9D§) + 1.5L** 0.5S†† or 0.4W

3 (1.25D‡ or 0.9D§) + 1.5S 0.5L††,‡‡ or 0.4W

4 (1.25D‡ or 0.9D§) + 1.4W 0.5L‡‡ or 0.5S

5 1.0D§ + 1.0E§§ 0.5L††,‡‡ + 0.25S††

*See Clause C.1.2.2.†See Clause C.1.2.3.‡See Clause C.1.2.7.§See Clause C.1.2.4.**See Clause C.1.2.5.††See Article 4.1.5.5 of the NBC.‡‡See Clause C.1.2.6.§§See Clause C.1.2.8.Notes: (1) This Table corresponds to Table 4.1.3.2 of the NBC.(2) The factored load combinations in this Table each include one or more

permanent loads, one principal variable load that dominates a given load combination, and one or more companion variable loads that have a magnitude likely to occur in combination with the given principal variable load when that principal variable load acts on the structure.

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C.1.2.9 Provision shall be made to ensure adequate stability of a structure as a whole and adequate lateral, torsional, and local stability of all structural parts.

C.1.2.10 Sway effects produced by vertical loads acting on the structure in its displaced configuration shall be taken into account in the design of buildings and their structural members.

C.1.3 ServiceabilityA building and its structural components shall be checked for serviceability limit states as defined in Clause 4.1.3.1.(1)(a) of the NBC under the effect of service loads for serviceability criteria specified or recommended in Articles 4.1.3.5 and 4.1.3.6 of the NBC and in the Standards listed in Section 4.3 of the NBC.