Part 9 structural requirements, excavation, footings and foundations

Application items

Several articles in 9.1.1 just point you elsewhere:

Buildings on flood plains must resist the expected vertical and horizontal hydrostatic pressures, and must keep exits usable during flooding OBC Div. B 9.1.1.8.OBC Div. B 9.1.1.8. Building in Flood Plains(1) Buildings constructed on flood plains shall, (a) be designed and constructed in accordance with good engineering practice to withstand anticipated vertical and horizontal hydrostatic pressures acting on the structure, and (b) incorporate floodproofing measures that will preserve the integrity of exits and means of…. A factory-built building is deemed to comply in two cases OBC Div. B 9.1.1.9.OBC Div. B 9.1.1.9. Site Assembled and Factory-Built Buildings(1) Except as provided in Sentence (2) and in Sentence 12.2.1.2.(2), a manufactured building is deemed to comply with this Code if it is designed and constructed in compliance with, (a) CSA Z240.2.1, "Structural requirements for manufactured homes," if the building is constructed in sections not wider than 4.88 m, or…:

• it meets CSA Z240.2.1, for sections not wider than 4.88 m; or
• it meets CSA A277.

Site installation and components built off the factory site still follow the Code.

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Structural design routes and loads (9.4)

A Part 9 structural member may follow any of three routes OBC Div. B 9.4.1.1.OBC Div. B 9.4.1.1. General(1) Subject to the application limitations defined elsewhere in this Part, structural members and their connections shall (a) conform to requirements provided elsewhere in this Part, (b) be designed according to good engineering practice such as provided in the CWC 2014, "Engineering Guide for Wood Frame…:

1. The Part 9 prescriptive rules and tables.
2. Good engineering practice, such as the CWC Engineering Guide for Wood Frame Construction.
3. A Part 4 design, using Part 9 or Part 4 loads and limits.

Mixing routes has a limit. If a floor is engineered but the walls or footings below it come from the Part 9 tables, the floor live load may not exceed 2.4 kPa. Climatic and seismic values always come from SB-1.

Part 9 snow load OBC Div. B 9.4.2.2.OBC Div. B 9.4.2.2. Specified Snow Loads(1) Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = C_bS_s + S_r where S = specified snow load, C_b = basic snow load roof factor, which is 0.45 where the entire width of a roof does not exceed 4.3 m and 0.55 for all other…. This is simpler than Part 4:

S = C_b S_s + S_r, where C_b is 0.45 if the whole roof width is 4.3 m or less, and 0.55 for every other roof.

There is no importance factor, and S may never be less than 1 kPa. Bowstring and arch trusses spanning more than 6 m go back to 4.1.6. A roof step over 2 m below a large, flat upper roof gets a drift surcharge.

Other Part 9 load rules:

• Decks and balconies serving a single dwelling unit are designed for the roof snow load or 1.9 kPa, whichever is greater OBC Div. B 9.4.2.3.OBC Div. B 9.4.2.3. Platforms Subject to Snow and Occupancy Loads(1) Balconies, decks and other accessible exterior platforms intended for an occupancy and subject to snow loads shall be designed to carry the specified roof snow load or 1.9 kPa, whichever is greater, where the platform, or each segregated area of the platform, serves a single dwelling unit. (See Note A-9.4.2.3.(1)).
• Ceilings in attics of limited access are designed for a total load of 0.35 kPa OBC Div. B 9.4.2.4.OBC Div. B 9.4.2.4. Attics and Roof Spaces(1) Ceiling joists or truss bottom chords in residential attic or roof spaces having limited accessibility that precludes the storage of equipment or material shall be designed for a total specified load of not less than 0.35 kPa, where the total specified load is the sum of the specified dead load plus the specified….

Deflection limits come from Table 9.4.3.1.OBC Div. B Table 9.4.3.1. Maximum Deflections — Forming Part of Sentence 9.4.3.1.(1)Table 9.4.3.1 — Maximum Deflections — Forming Part of Sentence 9.4.3.1.(1) Structural Members | Type of Ceiling Supported | Max. Allowable Deflection as an Expressed Ratio of the Clear Span Roof rafters, roof joists and roof beams | No ceiling | 1/180 Roof rafters, roof joists and roof beams | Other than plaster or…. Dead load is ignored when checking them OBC Div. B 9.4.3.1.OBC Div. B 9.4.3.1. Deflections(1) The maximum deflection of structural members shall conform to Table 9.4.3.1. (2) Dead loads need not be considered in computing deflections referred to in Sentence (1)..

Bearing pressures OBC Div. B Table 9.4.4.1.OBC Div. B Table 9.4.4.1. Allowable Bearing Pressure for Soil or Rock — Forming Part of Sentence 9.4.4.1.(1)Table 9.4.4.1 — Allowable Bearing Pressure for Soil or Rock — Forming Part of Sentence 9.4.4.1.(1) Type and Condition of Soil or Rock | Maximum Allowable Bearing Pressure, kPa Dense or compact sand or gravel[1] | 150 Loose sand or gravel[1] | 50 Dense or compact silt[1] | 100 Stiff clay[1] | 150 Firm clay[1] | 75 Soft…. Some values: 150 kPa for dense sand or gravel and stiff clay, 75 kPa for firm clay, 40 kPa for soft clay and 500 kPa for sound rock. If the water table under gravel, sand or silt is within one footing width of the bearing surface, use 50% of the table value OBC Div. B 9.4.4.3.OBC Div. B 9.4.4.3. High Water Table(1) Where a foundation bears on gravel, sand or silt, and the water table is within a distance below the bearing surface equal to the width of the foundation, the allowable bearing pressure shall be 50% of that determined in Article 9.4.4.1.. Section 4.2 is required for deep foundations, and for foundations on peat, fill or sensitive clay OBC Div. B 9.4.4.1.OBC Div. B 9.4.4.1. Allowable Bearing Pressures(1) Footing sizes for shallow foundations shall be (a) determined in accordance with Section 9.15., or (b) designed in accordance with Section 4.2. using, (i) the maximum allowable bearing pressures in Table 9.4.4.1., or (ii) allowable bearing pressures determined from subsurface investigation. (2) The design….

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Excavation (9.12)

Before you build:

• Remove topsoil and vegetable matter under the building.
• Keep excavations free of standing water and the bottom from freezing.
• Carry foundations down to undisturbed soil OBC Div. B 9.12.2.1.OBC Div. B 9.12.2.1. Excavation to Undisturbed Soil(1) Excavations for foundations shall extend to undisturbed soil..

Minimum depth comes from Table 9.12.2.2.OBC Div. B Table 9.12.2.2. Minimum Depths of Foundations — Forming Part of Sentence 9.12.2.2.(1)Table 9.12.2.2 — Minimum Depths of Foundations — Forming Part of Sentence 9.12.2.2.(1) Forming Part of Sentence 9.12.2.2.(1) Type of Soil | Min Depth of Foundation Containing Heated Basement or Crawl Space[1] With Good Soil Drainage | Min Depth of Foundation Containing Heated Basement or Crawl Space[1] With Poor Soil…. Read it by three things in turn: soil type, whether the space is heated, and how well the soil drains.

There is a trap here. A foundation insulated so it cuts heat flow to the soil under the footings counts as no heated space OBC Div. B 9.12.2.2.OBC Div. B 9.12.2.2. Minimum Depth of Foundations(1) Except as provided in Sentences (4) to (7), the minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2. (2) Where a foundation is insulated in a manner that will reduce the heat flow to the soil beneath the footings, the foundation depth shall conform to that required for…. Some small structures need no minimum depth:

• non-masonry accessory buildings of 1 storey, up to 55 m², with no more than 600 mm from ground to the underside of the joists;
• similar small decks that do not support a roof.

Backfill within 600 mm of the foundation must be free of deleterious debris and of boulders over 250 mm in diameter OBC Div. B 9.12.3.3.OBC Div. B 9.12.3.3. Deleterious Debris and Boulders(1) Backfill that is within 600 mm of the foundation shall be free of deleterious debris and boulders larger than 250 mm diam. (See Note A-9.12.3.3.(1)) (2) Except as permitted in Sentence (3), backfill shall not contain pyritic material or material that is susceptible to ice lensing in concentrations that will damage…. Grade backfill so water drains away from the foundation after settling.

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Footings (9.15.3)

Section 9.15 needs stable soil with a bearing pressure of at least 75 kPa OBC Div. B 9.15.1.1.OBC Div. B 9.15.1.1. General(1) Except as provided in Articles 9.15.1.2. and 9.15.1.3., this Section applies to (a) concrete or unit masonry foundation walls and concrete footings not subject to surcharge (i) on stable soils with an allowable bearing pressure of 75 kPa or greater, and (ii) for buildings of wood-frame or masonry construction, (b)…. On fill, peat or sensitive clay, footings go to Section 4.2. The footing table also needs joist spans of 4.9 m or less and floor live loads of 2.4 kPa or less OBC Div. B 9.15.3.3.OBC Div. B 9.15.3.3. Application of Footing Width and Area Requirements(1) Except as provided in Sentence 9.15.3.4.(2), the minimum footing width or area requirements provided in Articles 9.15.3.4. to 9.15.3.7. shall apply to footings, where (a) the footings support (i) foundation walls of masonry, concrete, or flat insulating concrete form walls, (ii) above ground walls of masonry, flat….

Table 9.15.3.4.OBC Div. B Table 9.15.3.4. Minimum Footing Sizes — Forming Part of Sentence 9.15.3.4.(1)Table 9.15.3.4 — Minimum Footing Sizes — Forming Part of Sentence 9.15.3.4.(1) Forming Part of Sentence 9.15.3.4.(1) Number of Floors Supported | Minimum Width of Strip Footings Supporting Exterior Walls[2], mm | Minimum Width of Strip Footings Supporting Interior Walls[3], mm | Minimum Footing Area for Columns Spaced… gives the base sizes by number of floors supported:

Swipe sideways to see the whole table

Add to the exterior widths for each storey of OBC Div. B 9.15.3.5.OBC Div. B 9.15.3.5. Adjustments to Footing Widths for Exterior Walls(1) The strip footing widths for exterior walls shown in Table 9.15.3.4. shall be increased by (a) 65 mm for each storey of masonry veneer over wood-frame construction supported by the foundation wall, (b) 130 mm for each storey of masonry construction supported by the foundation wall, and (c) 150 mm for each storey…:

• masonry veneer over wood frame: 65 mm;
• solid masonry: 130 mm;
• flat ICF: 150 mm.

Column areas change in proportion to the column spacing. If the water table under sand, gravel or silt is closer than one footing width below the bearing surface, double the width or area OBC Div. B 9.15.3.4.OBC Div. B 9.15.3.4. Basic Footing Widths and Areas(1) Except as provided in Sentences (2) and (3) and in Articles 9.15.3.5. to 9.15.3.7., the minimum footing width or area shall comply with Table 9.15.3.4. (2) Where the supported joist span exceeds 4.9 m in buildings with light wood-frame walls, floors and roofs, strip footing widths shall be determined according to….

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Foundation walls (9.15.4, 9.15.5)

For unreinforced walls, Table 9.15.4.2.-AOBC Div. B Table 9.15.4.2-A. Maximum Height of Finish Ground Above Basement Floor or Crawl Space Ground Cover, mTable 9.15.4.2.-A. — Maximum Height of Finish Ground Above Basement Floor or Crawl Space Ground Cover, m Forming Part of Sentence 9.15.4.2.(1) Solid concrete and concrete core in flat wall insulating concrete forms[3], 15 MPa min. strength Minimum Thickness of Concrete or Concrete Block, mm | Maximum Height of Finish… gives the maximum height of finished ground above the basement floor. Read it by material and strength, then by wall thickness, then by whether the wall is laterally supported at the top and how tall it is. For example, a 200 mm, 15 MPa concrete wall supported at the top and up to 2.5 m high may retain 2.15 m of earth. The same wall unsupported at the top retains only 1.2 m.

A wall counts as laterally supported at the top in four cases OBC Div. B 9.15.4.3.OBC Div. B 9.15.4.3. Foundation Walls Considered to be Laterally Supported at the Top(1) Sentences (2) to (4) pertain to lateral support for walls described in Sentence 9.15.4.2.(1). (2) Foundation walls shall be considered to be laterally supported at the top if (a) such walls support a solid masonry superstructure or flat insulating concrete form wall, (b) the floor joists are embedded in the top of…:

• it carries solid masonry or a flat ICF wall;
• the joists are embedded in it;
• the floor is anchored to it with anchor bolts;
• it stands no more than 300 mm above grade with backfill on both sides.

The part of the wall under an opening wider than 1.2 m is unsupported unless it is reinforced. So is the wall where openings total more than 25% of its length. Exterior foundation walls rise at least 150 mm above finished ground OBC Div. B 9.15.4.6.OBC Div. B 9.15.4.6. Extension Above Ground Level(1) Exterior foundation walls shall extend not less than 150 mm above finished ground level.. Walls over 25 m long need crack control joints at no more than 15 m. Hollow masonry under joists is capped, and beams on masonry sit on at least 190 mm of solid masonry OBC Div. B 9.15.5.2.OBC Div. B 9.15.5.2. Support of Beams(1) Not less than a 190 mm depth of solid masonry shall be provided beneath beams supported on masonry. (2) Where the beam referred to in Sentence (1) is supported below the top of the foundation walls, the ends of such beams shall be protected from the weather..

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Floors-on-ground and reinforced slabs

A floor-on-ground that carries loads from the superstructure must be designed to Part 4 OBC Div. B 9.16.1.2.OBC Div. B 9.16.1.2. Structural Floor Slabs(1) Floors-on-ground that support loads from the superstructure shall be designed in conformance with Part 4.. Where groundwater may push up on it, it must be a poured concrete slab designed for that pressure OBC Div. B 9.16.3.2.OBC Div. B 9.16.3.2. Hydrostatic Pressure(1) Where groundwater levels may cause hydrostatic pressure beneath a floor-on-ground, the floor-on-ground shall be (a) a poured concrete slab, and (b) designed to resist such pressures.. Wood floors-on-ground follow CSA S406 OBC Div. B 9.16.5.1.OBC Div. B 9.16.5.1. Wood-Frame Floors(1) Floors-on-ground constructed of wood shall conform to CSA S406, "Specification of permanent wood foundations for housing and small buildings.".

Section 9.40 covers a reinforced slab over a basement cold room. The slab may span no more than 2.5 m on its short side and must have no interior supports OBC Div. B 9.40.1.1.OBC Div. B 9.40.1.1. Application(1) This Section applies to (a) reinforced concrete slabs that are suspended over cold rooms in basements, and are supported by foundation walls along the perimeter of the slab with no additional interior supports, and (b) slabs in which the clear span between supporting walls is not more than 2.5 m along the shortest…. Vehicle-bearing slabs and anything larger go to Part 4. The prescriptive slab is OBC Div. B 9.40.1.4.OBC Div. B 9.40.1.4. Slab Construction(1) Concrete shall be cast against formwork in accordance with CSA A23.1, "Concrete materials and methods of concrete construction." (2) The slab shall be not less than 125 mm thick. (3) The slab shall be reinforced with 10 M bars spaced not more than 200 mm o.c. in each direction, with 30 mm clear cover from the…:

• at least 125 mm thick;
• 10M bars at no more than 200 mm each way, with 30 mm bottom cover;
• at least 75 mm of bearing;
• 10M bent dowels at 600 mm.

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