Structural Requirements
Structural Design Requirements and Application Limitations
(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 Construction," or
- (c) be designed according to Part 4 using the loads and deflection and vibration limits specified in, (i) this Part, or (ii) Part 4.
(2)Where floor framing is designed in accordance with Clause (1)(b) or (c) and where supporting wall framing and fastenings, or footings, are designed according to Clause (1)(a), the maximum specified live load on the floor according to Table 4.1.5.3. shall not exceed 2.4 kPa.
(3)Location-specific information for structural design, including snow and wind loads and seismic spectral response accelerations, shall be determined according to MMAH Supplementary Standard SB-1, "Climatic and Seismic Data."
Specified Loads
(1)This Subsection applies to light-frame construction whose wall, floor and roof planes are generally comprised of frames of small repetitive structural members, and where
- (a) the roof and wall planes are clad, sheathed or braced on at least one side,
- (b) the small repetitive structural members are spaced not more than 600 mm o.c.,
- (c) the clear span of any structural member does not exceed 12.2 m,
- (d) the maximum deflection of the structural roof members conforms to Article 9.4.3.1.,
- (e) the maximum total roof area, notwithstanding any separation of adjoining buildings by firewalls, is 4 550 m2, and
- (f) for flat roofs, there are no significant obstructions on the roof, such as parapet walls, spaced closer than the distance calculated by Do = 10(Ho – 0.8 Ss / γ) where Do = minimum distance between obstructions, m, Ho = height of the obstruction above the roof, m, Ss = ground snow load, kPa, and γ = specific weight of snow taken as 4.0 kN/m3 or 0.43Ss + 2.2 kN/m3, whichever is lesser.
(1)Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = CbSs + Sr where S = specified snow load, Cb = 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 roofs, Ss = 1-in-50 year ground snow load in kPa, determined according to MMAH Supplementary Standard SB-1, "Climatic and Seismic Data," and Sr = associated 1-in-50 year rain load in kPa, determined according to MMAH Supplementary Standard SB-1, "Climatic and Seismic Data."
(2)In no case shall the specified snow load be less than 1 kPa.
(3)Bow string, arch or semi-circular roof trusses having an unsupported span greater than 6 m shall be designed in conformance with the snow load requirements in Subsection 4.1.6.
(4)Where the height of a roof step at the intersection of an upper level roof and a lower level roof is greater than 2 m, and the upper level roof has a slope less than 1 in 6 and an area greater than 600 m2, the specified snow load on the lower level roof shall be
- (a) for distances from the roof step that are less than or equal to the drift length, xd, calculated in accordance with Sentence (5), not less than 1.5 times the specified snow load, S, calculated using the formula in Sentence (1) with Cb equal to 0.55, and
- (b) for distances from the roof step that are greater than the drift length, xd, calculated in accordance with Sentence (5), as specified in Sentence (1).
(5)For the purposes of Sentence (4), the drift length, xd, in m, shall be calculated as follows: where h = height of the roof step, in m, and γ = specific weight of snow as specified in Clause 9.4.2.1.(1)(f).
(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.
(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 live load of the ceiling.
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).
| 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 |
| Other than plaster or gypsum board | 1/240 | |
| Plaster or gypsum board | 1/360 | |
| Ceiling joists | Other than plaster or gypsum board | 1/240 |
| Plaster or gypsum board | 1/360 | |
| Floor beams, floor joists and floor decking | All cases | 1/360 |
| Beams, joists and decking for balconies, decks and other accessible exterior platforms | Serving a single dwelling unit | 1/240 |
| Other | 1/360 |
Foundation Conditions
(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 procedures described in Section 4.2. are permitted to be used in lieu of the design procedures in this Subsection.
(3)The design procedures described in Section 4.2. shall be used where,
- (a) deep foundations are used,
- (b) the footing size falls outside the scope of this Section, or
- (c) the foundation is constructed on peat, filled ground or on sensitive clays as described in Article 9.15.1.1.
| Type and Condition of Soil or Rock | Maximum Allowable Bearing Pressure, kPa |
|---|---|
| Dense or compact sand or gravel(1) | |
| Loose sand or gravel(1) | |
| Dense or compact silt(1) | |
| Stiff clay(1) | |
| Firm clay(1) | |
| Soft clay(1) | |
| Till | |
| Clay shale | |
| Sound rock |
(1)Where a soil or rock within a distance equal to twice the footing width below the bearing surface has a lower allowable bearing pressure than that at the bearing surface as shown in Article 9.4.4.1., the design capacity of the foundation shall not be greater than would cause the weakest soil or rock to be stressed beyond its allowable bearing pressure.
(2)In calculating subsurface pressures referred to in Sentence (1), the loads from the footings shall be assumed to be distributed uniformly over a horizontal plane within a frustum extending downward from the footing at an angle of 60° to the horizontal.
(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.
(1)Where a foundation is located in an area where soil movement caused by changes in soil moisture content, freezing, or chemical-microbiological oxidation is known to occur to the extent that it will damage a building, measures shall be taken to preclude such movement or to reduce the effects on the building so that the building's stability and the performance of assemblies will not be adversely affected.
(1)Except where constructed in accordance with Section 9.15., walls supporting drained earth shall be designed
- (a) for a pressure equivalent to that exerted by a fluid with a density of not less than 480 kg/m3 and a depth equal to that of the retained earth, or
- (b) in accordance with Section 4.2. so as to be able to resist the loads and effects described in Article 4.1.2.1.
(2)Walls supporting other than drained earth shall be designed
- (a) for the pressure described in Clause (1)(a) plus the fluid pressure of the surcharge, or
- (b) in accordance with Section 4.2. so as to be able to resist the loads and effects described in Article 4.1.2.1.