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SECTION 305 FOOTINGS

305.1 General

Footings and foundations shall be constructed of masonry, concrete or treated wood in conformance with Chapters 4, 6 and 7. Footings of concrete and masonry shall be of solid material. Foundations supporting wood shall extend at least 150 mm above the adjacent finish grade. Footings shall have a minimum depth as indicated in Table 305-1, unless another depth is warranted, as established by a foundation investigation.

The provisions of this section do not apply to building and foundation systems in those areas subject to scour and water pressure by wind and wave action. Buildings and foundations subject to such loads shall be designed in accordance with approved national standards.

Table 305-1 Minimum Requirements for Foundations 1,2,3

Number of

1 Where unusual conditions are found, footings and foundations shall be as required in Section 305.1.

2 The ground under the floor may be excavated to the elevation of the top of the footing.

3 Foundation may support a roof in addition to the stipulated number of floors. Foundations supporting roofs only shall be as required for supporting one floor.

4 The depth of embedment must always be below potential depth of scour

305.2 Footing Design

Except for special provisions of Section 307 covering the design of piles, all portions of footings shall be designed in accordance with the structural provisions of this code and shall be designed to minimize differential settlement when necessary and the effects of expansive soils when present.

Slab-on-grade and mat type footings for buildings located on expansive soils may be designed in accordance with the geotechnical recommendation as permitted by the building official.

305.2.1 Design Loads

Footings shall be designed for the most unfavorable load effects due to combinations of loads. The dead load is permitted to include the weight of foundations, footings and overlying fill. Reduced live loads as permitted in the Chapter on Loadings are permitted to be used in the design of footings.

305.2.2 Vibratory Loads

Where machinery operations or other vibratory loads or vibrations are transmitted to the foundations, consideration shall be given in the report to address the foundation design to prevent detrimental disturbances to the soil due to vibratory loadings.

Dynamic Soil Properties shall be included where required.

305.3 Bearing Walls

Bearing walls shall be supported on masonry or reinforced concrete foundations or piles or other permitted foundation system that shall be of sufficient size to support all loads.

Where a design is not provided, the minimum foundation requirements for stud bearing walls shall be as set forth in Table 305-1, unless expansive soils of a severity to cause differential movement are known to exist.

Exceptions:

1. A one-story wood or metal-frame building not used for human occupancy and not over 40 m2 in floor area may be constructed with walls supported on a wood foundation plate permanently under the water table when permitted by the building official.

2. The support of buildings by posts embedded in earth shall be designed as specified in Section 305.7. Wood posts or poles embedded in earth shall be pressure treated with an approved preservative. Steel posts or poles shall be protected as specified in Section 306.10.

305.4 Stepped Foundations

Foundations for all buildings where the surface of the ground slopes more than 1 unit vertical in 10 units horizontal (10% slope) shall be level or shall be stepped so that both top and bottom of such foundation are level.

305.5 Footings on or Adjacent to Slopes 305.5.1 Scope

The placement of buildings and structures on or adjacent to slopes steeper than 1 unit vertical in 3 units horizontal (33.3% slope) shall be in accordance with this section.

305.5.2 Building Clearance from Ascending Slopes In general, buildings below slopes shall be set a sufficient distance from the slope to provide protection from slope drainage, Scour erosion and shallow failures. Except as provided for in Section 305.5.6 and Figure 305-1, the following criteria will be assumed to provide this protection. Where the existing slope is steeper than 1 unit vertical in 1 unit horizontal (100% slope), the toe of the slope shall be assumed to be at the intersection of a horizontal plane drawn from the top of the foundation and a plane drawn tangent to the slope at an angle of 45 degrees to the horizontal. Where a retaining wall is constructed at the toe of the slope, the height of the slope shall be measured from the top of the wall to the top of the slope.

305.5.3 Footing Setback from Descending Slope Surface

Footings on or adjacent to slope surfaces shall be founded in firm material with an embedment and setback from the slope surface sufficient to provide vertical and lateral support for the footing without detrimental settlement.

Except as provided for in Section 305.5.6 and Figure 305-1, the following setback is deemed adequate to meet the criteria. Where the slope is steeper than 1 unit vertical in 1 unit horizontal (100% slope), the required setback shall be measured from an imaginary plane 45 degrees to the horizontal, projected upward from the toe of the slope.

305.5.4 Pools

The setback between pools regulated by this code and slopes shall be equal to one half the building footing setback distance required by this section. That portion of the pool wall within a horizontal distance of 2 m from the top of the slope shall be capable of supporting the water in the pool without soil support.

305.5.5 Foundation Elevation

On graded sites, the top of any exterior foundation shall extend above the elevation of the street gutter at point of discharge or the inlet of an approved drainage device a minimum of 300 mm plus 2 percent. The building official may permit alternate elevations, provided it can be demonstrated that required drainage to the point of discharge and away from the structure is provided at all locations on the site.

305.5.6 Alternate Setback and Clearance

The building official may approve alternate setbacks and clearances. The building official may require an investigation and recommendation of a qualified engineer to demonstrate that the intent of this section has been satisfied. Such an investigation shall include consideration of material, height of slope, slope gradient, load intensity and erosion characteristics of slope material.

305.6 Foundation Plates or Sills

Wood plates or sills shall be bolted to the foundation or foundation wall. Steel bolts with a minimum nominal diameter of 12 mm shall be used in Seismic Zone 2. Steel bolts with a minimum nominal diameter of 16 mm shall be used in Seismic Zone 4. Bolts shall be embedded at least 180 mm into the concrete or masonry and shall be spaced not more than 2 m apart. There shall be a minimum of two bolts per piece with one bolt located not Figure 305-1 Setback Dimensions for Building Clearance from Slopes

H/2 but

more than 300 mm or less than seven bolt diameters from each end of the piece. A properly sized nut and washer shall be tightened on each bolt to the plate. Foundation plates and sills shall be the kind of wood specified in Chapter 6.

305.7 Designs Employing Lateral Bearing 305.7.1 General

Construction employing posts or poles as columns embedded in earth or embedded in concrete footings in the earth may be used and designed to resist both axial and lateral loads. The depth to resist lateral loads shall be determined by means of the design criteria established herein or other methods approved by the building official.

305.7.2 Design Criteria 305.7.2.1 Non-constrained

The following formula may be used in determining the depth of embedment required to resist lateral loads where no constraint is provided at the ground surface, such as rigid floor or rigid ground surface pavement.



b = diameter of round post or footing or diagonal dimension of square post or footing, m

d = depth of embedment in earth in m but not over 3.5 m for purpose of computing lateral pressure, m h = distance from ground surface to point of

application of P, m P = applied lateral force, kN

S1 = allowable lateral soil-bearing pressure as set forth in Table 304-1 based on a depth of one third the depth of embedment, kPa

S3 = allowable lateral soil-bearing pressure as set forth in Table 304-1 based on a depth equal to the depth of embedment, kPa

305.7.2.2 Constrained

The following formula may be used to determine the depth of embedment required to resist lateral loads where constraint is provided at the ground surface, such as a rigid floor or pavement.

b

The resistance to vertical loads is determined by the allowable soil-bearing pressure set forth in Table 304-1.

305.7.3 Backfill

The backfill in the annular space around column not embedded in poured footings shall be by one of the following methods:

1. Backfill shall be of concrete with an ultimate strength of 15 MPa at 28 days. The hole shall not be less than 100 mm larger than the diameter of the column at its bottom or 100 mm larger than the diagonal dimension of a square or rectangular column.

2. Backfill shall be of clean sand. The sand shall be thoroughly compacted by tamping in layers not more than 200 mm in thickness.

305.7.4 Limitations

The design procedure outlined in this section shall be subject to the following limitations:

305.7.4.1 The frictional resistance for retaining walls and slabs on silts and clays shall be limited to one half of the normal force imposed on the soil by the weight of the footing or slab.

305.7.4.1 Posts embedded in earth shall not be used to provide lateral support for structural or nonstructural materials such as plaster, masonry or concrete unless bracing is provided.

305.8 Grillage Footings

When grillage footings of structural steel shapes are used on soils, they shall be completely embedded in concrete.

Concrete cover shall be at least 150 mm on the bottom and at least 100 mm at all other points.

305.9 Bleacher Footings

Footings for open-air seating facilities shall comply with Chapter 3.

Exceptions:

Temporary open-air portable bleachers may be supported upon wood sills or steel plates placed directly upon the ground surface, provided soil pressure does not exceed 50 kPa.

SECTION 306