Division B
Acceptable Solutions

Part 4 — Structural Design

Section 4.1. Structural Loads and Procedures

4.1.1. General

(See Appendix A) (See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.1.1. Scope

(See Appendix A.)

1) The scope of this Part shall be as described in Subsection 1.3.3. of Division A.

4.1.1.2. Definitions

1) Words that appear in italics in this Part are defined in Article 1.4.1.2. of Division A.

4.1.1.3. Design Requirements

1) Buildings and their structural members and connections, including formwork and falsework, shall be designed to have sufficient structural capacity and structural integrity to safely and effectively resist all loads, effects of loads and influences that may reasonably be expected, having regard to the expected service life of buildings, and shall in any case satisfy the requirements of this Section.

2) Buildings and their structural members shall be designed for serviceability, in accordance with Articles 4.1.3.4., 4.1.3.5. and 4.1.3.6. (See Appendix A.)

3) All permanent and temporary structural members, including the formwork and falsework of a building, shall be protected against loads exceeding the specified loads during the construction period except when, as verified by analysis or test, temporary overloading of a structural member would result in no impairment of that member or any other member.

4) Falsework, scaffolding, and formwork shall be designed in conformance with

a)CSA S269.1, “Falsework for Construction Purposes,”

b)CAN/CSA-S269.2-M, “Access Scaffolding for Construction Purposes,” or

c)CAN/CSA-S269.3-M, “Concrete Formwork.”

5) Precautions shall be taken during all phases of construction to ensure that the building is not damaged or distorted due to loads applied during construction.

4.1.1.4. Structural Drawings and Related Documents

1) Structural drawings and related documents shall conform to the appropriate requirements of Section 2.2. of Division C. (See Subsection 2.2.4. of Division C.)

4.1.1.5. Design Basis

1) Except as provided in Sentence (2), buildings and their structural members shall be designed in conformance with the procedures and practices provided in this Part.

2) Provided the design is carried out by a person especially qualified in the specific methods applied and provided the design demonstrates a level of safety and performance in accordance with the requirements of Part 4, buildings and their structural components falling within the scope of Part 4 that are not amenable to analysis using a generally established theory may be designed by

a)evaluation of a full-scale structure or a prototype by a loading test, or

b)studies of model analogues.

4.1.2. Specified Loads and Effects

(See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.2.1. Loads and Effects

1) Except as provided in Article 4.1.2.2., the following categories of loads, specified loads and effects shall be taken into consideration in the design of a building and its structural members and connections:

where

a) load means the imposed deformations (i.e. deflections, displacements or motions that induce deformations and forces in the structure), forces and pressures applied to the building structure,

b) permanent load is a load that changes very little once it has been applied to the structure, except during repair,

c) variable load is a load that frequently changes in magnitude, direction or location, and

d) rare load is a load that occurs infrequently and for a short time only.

2) Minimum specified values of the loads described in Sentence (1), as set forth in Subsections 4.1.4. to 4.1.8., shall be increased to account for dynamic effects where applicable.

3) For the purpose of determining specified loads S, W or E in Subsections 4.1.6., 4.1.7. and 4.1.8., buildings shall be assigned an Importance Category based on intended use and occupancy, in accordance with Table 4.1.2.1. (See Appendix A.)

4.1.2.2. Loads Not Listed

1) Where a building or structural member can be expected to be subjected to loads, forces or other effects not listed in Article 4.1.2.1., such effects shall be taken into account in the design based on the most appropriate information available.

4.1.3. Limit States Design

(See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.3.1. Definitions

1) In this Subsection, the term

a)limit states means those conditions of a building structure that result in the building ceasing to fulfill the function for which it was designed (those limit states concerning safety are called ultimate limit states (ULS) and include exceeding the load-carrying capacity, overturning, sliding and fracture; those limit 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; and those limit states that represent failure under repeated loading are called fatigue limit states),

b)specified loads (D, E, H , L, P, S, T and W) means those loads defined in Article 4.1.2.1.,

c) principal load means the specified variable load or rare load that dominates in a given load combination,

d) companion load means a specified variable load that accompanies the principal load in a given load combination,

e) service load means a specified load used for the evaluation of a serviceability limit state,

f) principal-load factor means a factor applied to the principal load in a load combination to account for the variability of the load and load pattern and the analysis of its effects,

g) companion-load factor means 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,

h) importance factor, I, means a factor applied in Subsections 4.1.6., 4.1.7. and 4.1.8. to obtain the specified load and that takes into account the consequences of failure as related to the limit state and theuse and occupancy of the building,

i)factored load means the product of a specified load and its principal-load factor or companion-load factor,

j)effects refers to forces, moments, deformations or vibrations that occur in the structure,

k) nominal resistance, R, of a member, connection or structure, is based on the geometryand on the specified properties of the structural materials,

l)resistance factor, Φ, means a factor applied to a specified material property or to the resistance of a member, connection or structure, and that, for the limit state under consideration, takes into account the variability of dimensions and material properties, workmanship, type of failure and uncertainty in the prediction of resistance, and

m)factored resistance, ΦR, means the product of nominal resistance and the applicable resistance factor.

4.1.3.2. Strength and Stability

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, which shall be determined in accordance with Sentence 4.1.3.2.(2).

2) The effect of factored loads for a building or structural component shall be determined in accordance with the load combination cases listed in Table 4.1.3.2. and the requirements of Article 4.1.3.2., the applicable combination being that which results in the most critical effect. (See Appendix A.)

3) Where the effects due to lateral earth pressure, H, restraint effects from pre-stress, P, and imposed deformation, T, affect the structural safety, they shall be taken into account in the calculations, with load factors of 1.5, 1.0 and 1.25 assigned to H, P and T respectively. (See Appendix A.)

4) Except as provided in Sentence 4.1.8.16.(1), the counteracting factored dead load, 0.9D in load combination cases 2, 3 and 4 and 1.0D in load combination case 5, shall be used when the dead load acts to resist overturning, uplift, sliding, failure due to stress reversal, and to determine anchorage requirements and the factored resistance of members.

5) The principal-load factor 1.5 for live load, L, in Table 4.1.3.2. may be reduced to 1.25 for liquids in tanks.

6) The companion-load factor 0.5 for live load, L, in Table 4.1.3.2. shall be increased to 1.0 for storage areas, and equipment areas and service rooms referred to in Table 4.1.5.3.

7) The load factor 1.25 for dead load, D, in Table 4.1.3.2. for soil, superimposed earth, plants and trees 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/h_s but not less than 1.25, where h_s is the depth of soil in metres supported by the structure.

8) Earthquake load, E, in load combination case 5 of Table 4.1.3.2. includes horizontal earth pressure due to earthquake determined in accordance with Sentence 4.1.8.16.(4).

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

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.

4.1.3.3. Fatigue

1) A building and its structural components, including connections, shall be checked for fatigue failure under the effect of cyclical loads, as required in the standards listed in Section 4.3. (See Appendix A.)

2) Where vibration effects, such as resonance and fatigue resulting from machinery and equipment, are likely to be significant, a dynamic analysis shall be carried out.

4.1.3.4. Serviceability

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

4.1.3.5. Deflection

1) In proportioning structural members to limit serviceability problems resulting from deflections, consideration shall be given to

a)the intended use of the building or member,

b)limiting damage to non-structural members made of materials whose physical properties are known at the time of design,

c)limiting damage to the structure itself, and

d)creep, shrinkage, temperature changes and pre-stress.

2) The lateral deflection of buildings due to service wind and gravity loads shall be checked to ensure that structural elements and non-structural elements whose nature is known at the time the structural design is carried out will not be damaged.

3) Except as provided in Sentence (4) , the total drift per storey under service wind and gravity loads shall not exceed 1/500 of the storey height unless other drift limits are specified in the design standards referenced in Section 4.3. (See Appendix A.)

4) The deflection limits required in Sentence (3) do not apply to industrial buildings or sheds if experience has proven that greater movement will have no significant adverse effects on the strength and function of the building.

5) The building structure shall be designed for lateral deflection due to E, in accordance with Article 4.1.8.13.

4.1.3.6. Vibration

1) Floor systems susceptible to vibration shall be designed so that vibrations will have no significant adverse effects on the intended occupancy of the building.

2) Where the fundamental vibration frequency of a structural system supporting an assembly occupancy used for rhythmic activities, such as dancing, concerts, jumping exercises or gymnastics, is less than 6 Hz, the effects of resonance shall be investigated by means of a dynamic analysis.

3) A building susceptible to lateral vibration under wind load shall be designed in accordance with Article 4.1.7.2. so that the vibrations will have no significant adverse effects on the intended use and occupancy of the building.

4.1.4. Dead Loads

4.1.4.1. Dead Loads

1) The specified dead load for a structural member consists of

a)the weight of the member itself,

b)the weight of all materials of construction incorporated into the building to be supported permanently by the member,

c)the weight of partitions,

d)the weight of permanent equipment, and

e) the vertical load due to earth, plants and trees.

2) Except as provided in Sentence (5), in areas of a building where partitions other than permanent partitions are shown on the drawings, or where partitions might be added in the future, allowance shall be made for the weight of such partitions.

3) The partition weight allowance referred to in Sentence (2) shall be determined from the actual or anticipated weight of the partitions placed in any probable position, but shall be not less than 1 kPa over the area of floor being considered.

4) Partition loads used in design shall be shown on the drawings as provided in Clause 2.2.4.3.(1)(d) of Division C.

5) In cases where the dead load of the partition is counteractive, the load allowances referred to in Sentences (2) and (3) shall not be included in the design calculations.

6) Except for structures where the dead load of soil is part of the load-resisting system, where the dead load due to soil, superimposed earth, plants and trees is counteractive, it shall not be included in the design calculations. (See Appendix A.)

4.1.5. Live Loads Due to Use and Occupancy

(See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.5.1. Loads Due to Use of Floors and Roofs

1) Except as provided in Sentence (2), the specified live load on an area of floor or roof depends on the intended use and occupancy, and shall not be less than either the uniformly distributed load patterns listed in Article 4.1.5.3., the loads resulting from the intended use, or the concentrated loads listed in Article 4.1.5.10., whichever produces the most critical effect.

2) For buildings in the Low Importance Category as described in Table 4.1.2.1., a factor of 0.8 may be applied to the live load.

4.1.5.2. Uses Not Stipulated

1) Except as provided in Sentence (2), where the use of an area of floor or roof is not provided for in Article 4.1.5.3., the specified live loads due to the use and occupancy of the area shall be determined from an analysis of the loads resulting from the weight of

a)the probable assembly of persons,

b)the probable accumulation of equipment and furnishings, and

c)the probable storage of materials.

2) For buildings in the Low Importance Category as described in Table 4.1.2.1., a factor of 0.8 may be applied to the live load.

4.1.5.3. Full and Partial Loading

1) The uniformly distributed live load shall be not less than the value listed in Table 4.1.5.3., which may be reduced as provided in Article 4.1.5.9., applied uniformly over the entire area or on any portions of the area, whichever produces the most critical effects in the members concerned.

4.1.5.4. Loads for Occupancy Served

1) The following shall be designed to carry not less than the specified load required for the occupancy they serve, provided they cannot be used by an assembly of people as a viewing area:

a)corridors, lobbies and aisles not more than 1 200 mm wide,

b)all corridors above the first storey of residential areas of apartments, hotels and motels, and

c)interior balconies and mezzanines.

4.1.5.5. Loads on Exterior Areas

1) Exterior areas accessible to vehicular traffic shall be designed for their intended use, including the weight of firefighting equipment, but not for less than the snow and rain loads prescribed in Subsection 4.1.6.

2) Except as provided in Sentences (3) and (4), roofs shall be designed for either the uniform live loads specified in Table 4.1.5.3., the concentrated live loads listed in Table 4.1.5.10., or the snow and rain loads prescribed in Subsection 4.1.6., whichever produces the most critical effects in the members concerned.

3) Exterior areas accessible to pedestrian traffic, but not vehicular traffic, shall be designed for their intended use, but not for less than the greater of

a)the live load prescribed for assembly areas in Table 4.1.5.3., or

b)the snow and rain loads prescribed in Subsection 4.1.6.

4) Roof parking decks shall be designed for either the uniformly distributed live loads specified in Table 4.1.5.3., the concentrated live loads listed in Table 4.1.5.10., or the roof snow load, whichever produces the most critical effect in the members concerned.

4.1.5.6. Loads for Dining Areas

1) The minimum specified live load listed in Table 4.1.5.3. for dining areas may be reduced to 2.4 kPa for areas in buildings that are being converted to dining areas, provided that the floor area does not exceed 100 m² and the dining area will not be used for other assembly purposes, including dancing.

4.1.5.7. Floor Loads Due to Intended Use

1) Equipment areas and service rooms, factories, storage areas and warehouses shall be designed for the live loads due to their intended use but not for less than the specified loads listed in Table 4.1.5.3.

4.1.5.8. More Than One Occupancy

1) Where an area of floor or roof is intended for 2 or more occupancies at different times, the value to be used from Table 4.1.5.3. shall be the greatest value for any of the occupancies concerned.

4.1.5.9. Variation with Tributary Area

1) An area used for assembly occupancies designed for a live load of less than 4.8 kPa and roofs designed for the minimum loading specified in Table 4.1.5.3. shall have no reduction for tributary area.

2) Where a structural member supports a tributary area of a floor or a roof, or a combination thereof, that is greater than 80 m² and either used for assembly occupancies designed for a live load of 4.8 kPa or more, or used for storage, manufacturing, retail stores, garages or as a footbridge, the specified live load due to use and occupancy is the load specified in Article 4.1.5.3. multiplied by

where A is the tributary area in square metres for this type of use and occupancy.

3) Where a structural member supports a tributary area of a floor or a roof, or a combination thereof, that is greater than 20 m² and used for any use or occupancy other than those indicated in Sentences (1) and (2), the specified live load due to use and occupancy is the load specified in Article 4.1.5.3. multiplied by

where B is the tributary area in square metres for this type of use and occupancy.

4) Where the specified live load for a floor is reduced in accordance with Sentence (2) or (3), the structural drawings shall indicate that a live load reduction factor for tributary area has been applied.

4.1.5.10. Concentrated Loads

1) The specified live load due to possible concentrations of load resulting from the use of an area of floor or roof shall not be less than that listed in Table 4.1.5.10. applied over an area of 750 mm by 750 mm located so as to cause maximum effects, except that for occupancies not listed in Table 4.1.5.10., the concentrations of load shall be determined in accordance with Article 4.1.5.2.

4.1.5.11. Sway Forces in Assembly Occupancies

1) The floor assembly and other structural elements that support fixed seats in any building used for assembly occupancies accommodating large numbers of people at one time, such as grandstands, stadia and theatre balconies, shall be designed to resist a horizontal force equal to not less than 0.3 kN for each metre length of seats acting parallel to each row of seats, and not less than 0.15 kN for each metre length of seats acting at right angles to each row of seats, based on the assumption that these forces are acting independently of each other.

4.1.5.12. Crane-Supporting Structures and Impact of Machinery and Equipment

1) The minimum specified load due to equipment, machinery or other objects that may produce impact shall be the sum of the weight of the equipment or machinery and its maximum lifting capacity, multiplied by an appropriate factor listed in Table 4.1.5.12.

2) Crane runway structures shall be designed to resist a horizontal force applied normal to the top of the rails equal to not less than 20% of the sum of the weights of the lifted load and the crane trolley (excluding other parts of the crane).

3) The force described in Sentence (2) shall be equally distributed on each side of the runway and shall be assumed to act in either direction.

4) Crane runway structures shall be designed to resist a horizontal force applied parallel to the top of the rails equal to not less than 10% of the maximum wheel loads of the crane.

4.1.5.13. Bleachers

1) Bleacher seats shall be designed for a uniformly distributed live load of 1.75 kN for each linear metre or for a concentrated load of 2.2 kN distributed over a length of 0.75 m, whichever produces the most critical effect on the supporting members.

2) Bleachers shall be checked by the erector after erection to ensure that all structural members, including bracing specified in the design, have been installed.

3) Telescopic bleachers shall be provided with locking devices to ensure stability while in use.

4.1.5.14. Helicopter Landing Areas

1) Helicopter landing areas on roofs shall be constructed in conformance with the requirements contained in the “Canadian Aviation Regulations – Part III,” published by Transport Canada.

4.1.5.15. Loads on Guards

1) The minimum specified horizontal load applied inward or outward at the top of every required guard shall be

a)3.0 kN/m for means of egress in grandstands, stadia, bleachers and arenas,

b)a concentrated load of 1.0 kN applied at any point for access ways to equipment platforms, contiguous stairs and similar areas where the gathering of many people is improbable, and

c)0.75 kN/m or a concentrated load of 1.0 kN applied at any point, whichever governs for locations other than those described in Clauses (a) and (b).

2) Individual elements within the guard, including solid panels and pickets, shall be designed for a load of 0.5 kN applied over an area of 100 mm by 100 mm located at any point in the element or elements so as to produce the most critical effect.

3) The loads required in Sentence (2) need not be considered to act simultaneously with the loads provided for in Sentences (1) and (4).

4) The minimum specified load applied vertically at the top of every required guard shall be 1.5 kN/m and need not be considered to act simultaneously with the horizontal load provided for in Sentence (1).

5) For loads on handrails, refer to Sentence 3.4.6.4.(9).

4.1.5.16. Loads on Vehicle Guardrails

1) Vehicle guardrails for storage garages shall be designed for a concentrated load of 22 kN applied horizontally outward at any point 500 mm above the floor surface. (See Appendix A.)

4.1.5.17. Loads on Walls Acting As Guards

1) Where the floor elevation on one side of a wall, including a wall around a shaft, is more than 600 mm higher than the elevation of the floor or ground on the other side, the wall shall be designed to resist the appropriate lateral design loads prescribed elsewhere in this Section or 0.5 kPa, whichever produces the more critical effect.

4.1.5.18. Firewalls

1) Firewalls shall be designed to resist the maximum effect due to

a)the appropriate lateral design loads prescribed elsewhere in this Section, or

b)a factored lateral load of 0.5 kPa under fire conditions, as described in Sentence (2).

2) Under fire conditions, where the fire-resistance rating of the structure is less than that of the firewall,

a)lateral support shall be assumed to be provided by the structure on one side only, or

b)another structural support system capable of resisting the loads imposed by a fire on either side of the firewall shall be provided.

4.1.6. Loads Due to Snow and Rain

(See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.6.1. Specified Load Due to Rain or to Snow and Associated Rain

1) The specified load on a roof or any other building surface subject to snow and associated rain shall be the snow load specified in Article 4.1.6.2., or the rain load specified in Article 4.1.6.4., whichever produces the more critical effect.

4.1.6.2. Specified Snow Load

1) The specified load, S, due to snow and associated rain accumulation on a roof or any other building surface subject to snow accumulation shall be calculated using the formula

where

2) The basic roof snow load factor, C_b, shall be 0.8, except that for large roofs it shall be

a) 1.0 - (30/l_c)², for roofs with C_w = 1.0 and l_c greater than or equal to 70 m, or

b) 1.3 - (140/l_c)², for roofs with C_w = 0.75 or 0.5 and l_c greater than or equal to 200 m,

where

3) Except as provided for in Sentence (4), the wind exposure factor, C_w, shall be 1.0.

4) For buildings in the Low and Normal Importance Categories as set out in Table 4.1.2.1., the wind exposure factor given in Sentence (3) may be reduced to 0.75, or to 0.5 in exposed areas north of the treeline, where

a)the building is exposed on all sides to wind over open terrain as defined in Clause 4.1.7.1.(5)(a), and is expected to remain so during its life,

b)the area of roof under consideration is exposed to the wind on all sides with no significant obstructions on the roof, such as parapet walls, within a distance of at least 10 times the difference between the height of the obstruction and C_bC_wS_s/γ metres, where γ is the unit weight of snow on roofs (see Appendix A), and

c)the loading does not involve the accumulation of snow due to drifting from adjacent surfaces.

5) Except as provided for in Sentences (6) and (7), the slope factor, C_s, shall be

a)1.0 where the roof slope, α, is equal to or less than 30°,

b)(70° − α)/40° where α is greater than 30° but not greater than 70°, and

c)0 where α exceeds 70°.

6) The slope factor, C_s, for unobstructed slippery roofs where snow and ice can slide completely off the roof shall be

a)1.0 where the roof slope, α, is equal to or less than 15°,

b)(60° − α)/45° where α is greater than 15° but not greater than 60°, and

c)0 where α exceeds 60°.

7) The slope factor, C_s, shall be 1.0 when used in conjunction with shape factors for increased snow loads as given in Clauses (8)(b) and (e).

8) The shape factor, C_a, shall be 1.0, except that where appropriate for the shape of the roof, it shall be assigned other values that account for

a)non-uniform snow loads on gable, arched or curved roofs and domes,

b)increased snow loads in valleys,

c)increased non-uniform snow loads due to snow drifting onto a roof that is at a level lower than other parts of the same building or at a level lower than another building within 5 m of it,

d)increased non-uniform snow loads on areas adjacent to roof projections, such as penthouses, large chimneys and equipment, and

e)increased snow or ice loads due to snow sliding or meltwater draining from adjacent roofs.

4.1.6.3. Full and Partial Loading

1) A roof or other building surface and its structural members subject to loads due to snow accumulation shall be designed for the specified load given in Sentence 4.1.6.2.(1), distributed over the entire loaded area.

2) In addition to the distribution mentioned in Sentence (1), flat roofs and shed roofs, gable roofs of 15° slope or less, and arched or curved roofs shall be designed for the specified uniform snow load indicated in Sentence 4.1.6.2.(1), which shall be calculated using C_a = 1.0, distributed on any one portion of the loaded area and half of this load on the remainder of the loaded area, in such a way as to produce the most critical effects on the member concerned.

4.1.6.4. Specified Rain Load

1) Except as provided in Sentence (4), the specified load, S, due to the accumulation of rainwater on a surface whose position, shape and deflection under load make such an accumulation possible, is that resulting from the one-day rainfall determined in conformance with Subsection 1.1.3. and applied over the horizontal projection of the surface and all tributary surfaces. (See Appendix A.)

2) The provisions of Sentence (1) apply whether or not the surface is provided with a means of drainage, such as rainwater leaders.

3) Except as provided in Sentence 4.1.6.2.(1), loads due to rain need not be considered to act simultaneously with loads due to snow. (See Appendix A.)

4) Where scuppers are provided and where the position, shape and deflection of the loaded surface make an accumulation of rainwater possible, the loads due to rain shall be the lesser of either the one-day rainfall determined in conformance with Subsection 1.1.3. or a depth of rainwater equal to 30 mm above the level of the scuppers, applied over the horizontal projection of the surface and tributary areas.

4.1.7. Wind Load

(See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.7.1. Specified Wind Load

1) The specified external pressure or suction due to wind on part or all of a surface of a building shall be calculated using the formula

where

2) The net wind load for the building as a whole shall be the algebraic difference of the loads on the windward and leeward surfaces, and in some cases, may be calculated as the sum of the products of the external pressures or suctions and the areas of the surfaces over which they are averaged as provided in Sentence (1). (See Appendix A.)

3) The net specified pressure due to wind on part or all of a surface of a building shall be the algebraic difference of the external pressure or suction as provided in Sentence (1) and the specified internal pressure or suction due to wind calculated using the following formula:

where

4) The reference velocity pressure, q, shall be the appropriate value determined in conformance with Subsection 1.1.3., based on a probability of being exceeded in any one year of 1 in 50.

5) The exposure factor, C_e, shall be

a) (h/10)^0.2 but not less than 0.9 for open terrain, where open terrain is level terrain with only scattered buildings, trees or other obstructions, open water or shorelines thereof, h being the reference height above grade in metres for the surface or part of the surface,

b) 0.7(h/12)^0.3 but not less than 0.7 for rough terrain, where rough terrain is suburban, urban or wooded terrain extending upwind from the building uninterrupted for at least 1 km or 10 times the building height, whichever is greater, h being the reference height above grade in metres for the surface or part of the surface,

c) an intermediate value between the two exposures defined in Clauses (a) and (b) in cases where the site is less than 1 km or 10 times the building height from a change in terrain conditions, whichever is greater, provided an appropriate interpolation method is used, or

d)if a dynamic approach to the action of wind gusts is used, an appropriate value depending on both height and shielding.

6) The gust effect factor, C_g, shall be one of the following values:

a)for the building as a whole and main structural members, C_g = 2.0,

b) for external pressures and suctions onsmall elements including cladding, C_g = 2.5,

c)for internal pressures, C_gi = 2.0 or a value determined by detailed calculation that takes into account the sizes of the openings in the building envelope, the internal volume and the flexibility of the building envelope or

d)if a dynamic approach to wind action is used, C_g is a value that is appropriate for the turbulence of the wind and the size and natural frequency of the structure.

4.1.7.2. Dynamic Effects of Wind

1) Buildings whose height is greater than 4 times their minimum effective width, which is defined in Sentence (2), or greater than 120 m and other buildings whose light weight, low frequency and low damping properties make them susceptible to vibration shall be designed

a)by experimental methods for the danger of dynamic overloading, vibration and the effects of fatigue, or

b)by using a dynamic approach to the action of wind gusts.

2) The effective width, w, of a building shall be calculated using

where the summations are over the height of the building for a given wind direction, h_i is the height above grade to level i, as defined in Sentence 4.1.7.1.(5), and w_i is the width normal to the wind direction at height h_i; the minimum effective width is the lowest value of the effective width considering all possible wind directions.

4.1.7.3. Full and Partial Loading

1) Buildings and structural members shall be capable of withstanding the effects of

a)the full wind loads acting along each of the 2 principal horizontal axes considered separately,

b)the wind loads as described in Clause (a) but with 100% of the load removed from any portion of the area,

c)the wind loads as described in Clause (a) but considered simultaneously at 75% of their full value, and

d)the wind loads as described in Clause (c) but with 50% of these loads removed from any portion of the area.

4.1.7.4. Interior Walls and Partitions

1) In the design of interior walls and partitions, due consideration shall be given to differences in air pressure on opposite sides of the wall or partition which may result from

a)pressure differences between the windward and leeward sides of a building,

b)stack effects due to a difference in air temperature between the exterior and interior of the building, and

c)air pressurization by the mechanical services of the building.

4.1.8. Earthquake Load and Effects

(See User's Guide - NBC 2005, Structural Commentaries (Part 4 of Division B).)

4.1.8.1. Analysis

1) The deflections and specified loading due to earthquake motions shall be determined according to the requirements in this Subsection, except that the requirements in this Subsection need not be considered in design if S(0.2), as defined in Sentence 4.1.8.4.(6), is less than or equal to 0.12.

4.1.8.2. Notation

1) In this Subsection

4.1.8.3. General Requirements

1) The building shall be designed to meet the requirements of this Subsection and of the design standards referenced in Section 4.3.

2) Structures shall be designed with a clearly defined load path, or paths, that will transfer the inertial forces generated in an earthquake to the supporting ground.

3) The structure shall have a clearly defined Seismic Force Resisting System(s) (SFRS), as defined in Article 4.1.8.2.

4) The SFRS shall be designed to resist 100% of the earthquake loads and their effects.

5) All structural framing elements not considered to be part of the SFRS must be investigated and shown to behave elastically or to have sufficient non-linear capacity to support their gravity loads while undergoing earthquake-induced deformations calculated from the deflections determined in Article 4.1.8.13.

6) Stiff elements that are not considered part of the SFRS, such as concrete, masonry, brick or pre-cast walls or panels, shall be

a)separated from all structural elements of the building such that no interaction takes place as the building undergoes deflections due to earthquake effects as calculated in this Subsection, or

b)made part of the SFRS and satisfy the requirements of this Subsection.

7) Stiffness imparted to the structure from elements not part of the SFRS, other than those described in Sentence (6), shall not be used to resist earthquake deflections but shall be accounted for

a)in calculating the period of the structure for determining forces if the added stiffness decreases the fundamental lateral period by more than 15%,

b)in determining the irregularity of the structure, except the additional stiffness shall not be used to make an irregular SFRS regular or to reduce the effects of torsion, and

c)in designing the SFRS if inclusion of the elements not part of the SFRS in the analysis has an adverse effect on the SFRS.

8) Structural modelling shall be representative of the magnitude and spatial distribution of the mass of the building and of the stiffness of all elements of the SFRS, including stiff elements that are not separated in accordance with Sentence 4.1.8.3.(6), and shall account for

a)the effect of cracked sections in reinforced concrete and reinforced masonry elements,

b)the effect of the finite size of members and joints,

c)sway effects arising from the interaction of gravity loads with the displaced configuration of the structure, and

d)other effects that influence the lateral stiffness of the building.

4.1.8.4. Site Properties

1) The peak ground acceleration (PGA) and the 5% damped spectral response acceleration values, S_a(T), for the reference ground conditions (Site Class C in Table 4.1.8.4.A) for periods T of 0.2 s, 0.5 s, 1.0 s, and 2.0 s, shall be determined in accordance with Subsection 1.1.3. and are based on a 2% probability of exceedance in 50 years.

2) Site classifications for ground shall conform to Table 4.1.8.4.A and shall be determined using V¯_s except as provided in Sentence (3).

3) If average shear wave velocity, V¯_s, is not known, Site Class shall be determined from energy-corrected Average Standard Penetration Resistance, N¯₆₀, or from soil average undrained shear strength, s_u, as noted in Table 4.1.8.4.A, N¯₆₀ and s_u being calculated based on rational analysis.

4) Acceleration- and velocity-based site coefficients, F_a and F_v, shall conform to Tables 4.1.8.4.B and 4.1.8.4.C using linear interpolation for intermediate values of S_a(0.2) and S_a(1.0).

5) To determine F_a and F_v for Site Class F, site-specific geotechnical investigations and dynamic site response analysis shall be performed.

6) The design spectral acceleration values of S(T) shall be determined as follows, using linear interpolation for intermediate values of T:

4.1.8.5. Importance Factor

1) The earthquake importance factor, I_E, shall be determined according to Table 4.1.8.5.

4.1.8.6. Structural Configuration

1) Structures having any of the features listed in Table 4.1.8.6. shall be designated irregular.

2) Structures not classified as irregular according to Sentence 4.1.8.6.(1) may be considered regular.

3) Except as required by Article 4.1.8.10., in cases where I_EF_aS_a(0.2) is equal to or greater than 0.35, structures designated as irregular must satisfy the provisions referenced in Table 4.1.8.6.

4.1.8.7. Methods of Analysis

1) Analysis for design earthquake actions shall be carried out in accordance with the Dynamic Analysis Procedure described in Article 4.1.8.12., except that the Equivalent Static Force Procedure described in Article 4.1.8.11. may be used for structures that meet any of the following criteria:

a)in cases where I_EF_aS_a(0.2) is less than 0.35,

b)regular structures that are less than 60 m in height and have a fundamental lateral period, T_a, less than 2 s in each of two orthogonal directions as defined in Article 4.1.8.8., or

c)structures with structural irregularity, of Type 1, 2, 3, 4, 5, 6 or 8 as defined in Table 4.1.8.6., that are less than 20 m in height and have a fundamental lateral period, T_a, less than 0.5 s in each of two orthogonal directions as defined in Article 4.1.8.8.

4.1.8.8. Direction of Loading

1) Earthquake forces shall be assumed to act in any horizontal direction, except that the following shall be considered to provide adequate design force levels in the structure:

a) where components of the SFRS are oriented along a set of orthogonal axes, independent analyses about each of the principal axes of the structure shall be performed,

b) where the components of the SFRS are not oriented along a set of orthogonal axes and I_EF_aS_a(0.2) is less than 0.35, independent analyses about any two orthogonal axes is permitted, or

c) where the components of the SFRS are not oriented along a set of orthogonal axes and I_EF_aS_a(0.2) is equal to or greater than 0.35, analysis of the structure independently in any two orthogonal directions for 100% of the prescribed earthquake loads applied in one direction plus 30% of the prescribed earthquake loads in the perpendicular direction, with the combination requiring the greater element strength being used in the design.

4.1.8.9. SFRS Force Reduction Factors, System Overstrength Factors, and General Restrictions

1) The values of R_d and R_o and the corresponding system restrictions shall conform to Table 4.1.8.9. and the requirements of this Subsection.

2) When a particular value of R_d is required by this Article, the corresponding R_o shall be used.

3) For combinations of different types of SFRS acting in the same direction in the same storey, R_dR_o shall be taken as the lowest value of R_dR_o corresponding to these systems.

4) For vertical variations of R_dR_o, excluding penthouses whose weight is less than 10% of the level below, the value of R_dR_o used in the design of any storey shall be less than or equal to the lowest value of R_dR_o used in the given direction for the storeys above, and the requirements of Sentence 4.1.8.15.(3) must be satisfied.

5) If it can be demonstrated through testing, research and analysis that the seismic performance of a structural system is at least equivalent to one of the types of SFRS mentioned in Table 4.1.8.9., then such a structural system will qualify for values of R_d and R_o corresponding to the equivalent type in that Table.

4.1.8.10. Additional System Restrictions

1) Except as required by Clause 4.1.8.10.(2)(b), structures with a Type 6 irregularity, Discontinuity in Capacity - Weak Storey, as described in Table 4.1.8.6., are not permitted unless I_EF_aS_a(0.2) is less than 0.2 and the forces used for design of the SFRS are multiplied by R_dR_o.

2) Post-disaster buildings shall

a)not have any irregularities conforming to Types 1, 3, 4, 5 and 7 as described in Table 4.1.8.6., in cases where I_EF_aS_a(0.2) is equal to or greater than 0.35,

b)not have a Type 6 irregularity as described in Table 4.1.8.6., and

c)have an SFRS with an R_d of 2.0 or greater.

3) For buildings having fundamental lateral periods, T_a, of 1.0 s or greater, and where I_EF_vS_a(1.0) is greater than 0.25, walls forming part of the SFRS shall be continuous from their top to the foundation and shall not have irregularities of Type 4 or 5 as described in Table 4.1.8.6.

4) In cases where I_EF_aS_a(0.2) is equal to or greater than 0.35, for buildings constructed with 5 or 6 storeys of continuous combustible construction as permitted by Article 3.2.2.45. and having any fundamental lateral period, T_a, walls forming part of the SFRS within the continuous combustible construction shall not have irregularities of Type 4 or 5 as described in Table 4.1.8.6.

[Rev. 4, B.C. Reg. 1/2009.]

4.1.8.11. Equivalent Static Force Procedure for Structures Satisfying the Conditions of Article 4.1.8.6.

1) The static loading due to earthquake motion shall be determined according to the procedures given in this Article.

2) The minimum lateral earthquake force, V, shall be calculated using the following formula:

except that V shall not be less than

and for an SFRS with an R_d equal to or greater than 1.5, V need not be greater than

3) The fundamental lateral period, T_a, in the direction under consideration in Sentence (2) shall be determined as:

a)for moment-resisting frames that resist 100% of the required lateral forces and where the frame is not enclosed by or adjoined by more rigid elements that would tend to prevent the frame from resisting lateral forces, and where h_n is in metres:

i)0.085 (h_n)^3/4 for steel moment frames,

ii)0.075 (h_n)^3/4 for concrete moment frames, or

iii)0.1 N for other moment frames,

b) 0.025h_n for braced frames where h_n is in metres,

c) 0.05 (h_n)^3/4 for shear wall and other structures where h_n is in metres, or

d)other established methods of mechanics using a structural model that complies with the requirements of Sentence 4.1.8.3.(8), except that

i) for moment-resisting frames, T_a shall not be taken greater than 1.5 times that determined in Clause (a),

ii) for braced frames, T_a shall not be taken greater than 2.0 times that determined in Clause (b),

iii) for shear wall structures, T_a shall not be greater than 2.0 determined in Clause (c), and

iv) for the purpose of calculating the deflections, the period without the upper limit specified herein may be used.

4) The weight, W, of the building shall be calculated using the following formula:

5) The higher mode factor, M_v, and its associated base overturning moment reduction factor, J, shall conform to Table 4.1.8.11.

6) The total lateral seismic force, V, shall be distributed such that a portion, F_t, shall be assumed to be concentrated at the top of the building, where F_t is equal to 0.07 T_aV but need not exceed 0.25 V and may be considered as zero where the fundamental lateral period, T_a, does not exceed 0.7 s; the remainder, V - F_t, shall be distributed along the height of the building, including the top level, in accordance with the following formula:

7) The structure shall be designed to resist overturning effects caused by the earthquake forces determined in Sentence (6) and the overturning moment at level x, M_x, shall be determined using the following equation:

where

where

8) Torsional effects that are concurrent with the effects of the forces mentioned in Sentence (6) and are caused by the following torsional moments shall be considered in the design of the structure according to Sentence (10):

a)torsional moments introduced by eccentricity between the centres of mass and resistance and their dynamic amplification, or

b)torsional moments due to accidental eccentricities.

9) Torsional sensitivity shall be determined by calculating the ratio B_x for each level x according to the following equation for each orthogonal direction determined independently:

where

10) Torsional effects shall be accounted for as follows:

a)for a building with B ≤ 1.7, by applying torsional moments about a vertical axis at each level throughout the building, derived for each of the following load cases considered separately:

i)T_x = F_x(e_x + 0.10 D_nx), and

ii)T_x = F_x(e_x - 0.10 D_nx)

where F_x is the lateral force at each level determined according to Sentence (6) and where each element of the building is designed for the most severe effect of the above load cases, or

b)for a building with B > 1.7, in cases where I_EF_aS_a(0.2) is equal to or greater than 0.35, by a Dynamic Analysis Procedure as specified in Article 4.1.8.12.

11) Where the fundamental lateral period, T_a, is determined by Clause 4.1.8.11.(3)(d) for buildings constructed with 5 or 6 storeys of continuous combustible construction as permitted by Artlcle 3.2.2.45. and having an SFRS of nailed shear walls with wood-based panels, the lateral earthquake force, V, as determined in Sentence (2) shall be multiplied by 1.2

[Rev. 5, B.C. Reg. 145/2009.]

4.1.8.12. Dynamic Analysis Procedure

1) The Dynamic Analysis Procedure shall be in accordance with one of the following methods:

a)Linear Dynamic Analysis by either the Modal Response Spectrum Method or the Numerical Integration Linear Time History Method using a structural model that complies with the requirements of Sentence 4.1.8.3.(8), or

b)Nonlinear Dynamic Analysis, in which case a special study shall be performed.

2) The spectral acceleration values used in the Modal Response Spectrum Method shall be the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4.(6).

3) The ground motion histories used in the Numerical Integration Linear Time History Method shall be compatible with a response spectrum constructed from the design spectral acceleration values, S(T), defined in Sentence 4.1.8.4.(6).

4) The effects of accidental torsional moments acting concurrently with the lateral earthquake forces that cause them shall be accounted for by the following methods:

a)the static effects of torsional moments due to (± 0.10 D_nx)F_x at each level x, where F_x is determined from Sentence 4.1.8.11.(6) or from the dynamic analysis, shall be combined with the effects determined by dynamic analysis, or

b)if B, as defined in Sentence 4.1.8.11.(9), is less than 1.7, it is permitted to use a three-dimensional dynamic analysis with the centres of mass shifted by a distance of - 0.05 D_nx and + 0.05 D_nx.

5) The elastic base shear, V_e, obtained from a Linear Dynamic Analysis shall be multiplied by the importance factor, I_E, as determined in Article 4.1.8.5., and shall be divided by R_dR_o, as determined in Article 4.1.8.9., to obtain the base shear, V_d.

6) Except as required by Sentence (7) or (10), if the base shear, V_d, obtained in Sentence (5) is less than 80% of the lateral earthquake design force, V, of Article 4.1.8.11., V_d shall be taken as 0.8 V.

[Rev. 5, B.C. Reg. 145/2009.]

7) For irregular structures requiring dynamic analysis in accordance with Article 4.1.8.7., V_d shall be taken as the larger of the V_d determined in Sentence (5) and 100% of V.

8) Except as required by Sentence (9), the values of elastic storey shears, storey forces, member forces, and deflections obtained from the Linear Dynamic Analysis shall be multiplied by V_d/V_e to determine their design values, where V_d is the base shear.