btech

• Foundation


  The lowest division of a building—its substructure—constructed partly or wholly below the surface of the ground
• Function of foundation

  • Support and anchor the superstructure above and transmit its loads safely into the earth
• Loads on foundation


  • Combination of dead and live loads acting vertically on the superstructure
  • Anchor the superstructure against wind-induced sliding, overturning, and uplift
  • Withstand the sudden ground movements of an earthquake
  • Resist the pressure imposed by the surrounding soil mass and groundwater on basement walls
  • Counter the thrust from arched or tensile structures
• Active earth pressure


  Exerted by a soil mass on a basement wall
• Passive earth pressure


  Developed by a soil mass in response to the horizontal movement of a foundation
• Lateral forces


  Can cause the foundation to impose nonuniform pressure on the supporting soil
• Shear resistance


  Provided by the friction between the foundation and the underlying soil
• Settlement is the gradual subsiding of a structure as the soil beneath its foundation consolidates under loading
• Differential settlement


  The relative movement of different parts of a structure caused by uneven consolidation of the foundation soil
• Types of substructures

  • Basements
  • Crawl spaces
  • Concrete slabs-on-grade
  • Grid of independent piers or poles
• Shallow foundations


  Employed when stable soil of adequate bearing capacity occurs relatively near to the ground surface
• Deep foundations


  Employed when the soil underlying a foundation is unstable or of inadequate bearing capacity
• Factors to consider in selecting and designing the type of foundation system

  • Pattern and magnitude of building loads
  • Subsurface and groundwater conditions
  • Topography of the site
  • Impact on adjacent properties
  • Building code requirements
  • Construction method and risk
• Underpinning


  The process of rebuilding or strengthening the foundation of an existing building, or extending it when a new excavation in adjoining property is deeper than the existing foundation
• Methods for providing temporary support during underpinning

  • Needle beams passed through the foundation wall and carried by hydraulic jacks and shores
  • Digging intermittent pits under the existing foundation down to the level of the new footings
  • Constructing piles or caissons on either side of the existing foundation, removing a section of foundation wall, and replacing the section with a reinforced concrete pile cap
• Excavation support systems

  • Sheet piling
  • Tiebacks secured to rock or soil anchors
  • Slurry wall
  • Dewatering
• Spread footings


  The lowest part of a shallow foundation, extended laterally to distribute their load over an area of soil wide enough that the allowable bearing capacity of the soil is not exceeded
• Types of spread footings

  • Strip footings
  • Isolated footings
  • Stepped footings
  • Cantilever or strap footing
  • Combined footing
  • Mat or raft foundation
  • Floating foundation
• Spread footing proportions for light frame construction

  • Thickness (T) of concrete or unit masonry foundation wall supporting two floors: 8" (205) minimum
  • Projection = 1/2 T
  • Thickness of footing = T
  • Width of footing = 2T

  
• Frostline


  The average depth at which soil is frozen or frost penetrates the ground
• Foundation walls


  Provide support for the superstructure above and enclose a basement or crawl space partly or wholly below grade
• Functions of foundation walls

  • Resist active earth pressure
  • Anchor the superstructure against wind and seismic forces
  • Resist the penetration of water and soil gases
  • Control heat flow
  • Accept a range of suitable finishes
  • Accommodate windows, doors, and other openings
• Concrete ground slab


  Part of the foundation wall system
• Foundation walls


  • Resist active earth pressure
  • Anchor the superstructure against wind and seismic forces
• Components of a foundation system

  • Wall system
  • Floor system
  • Positive anchorage required to resist lateral, uplifting, or overturning forces
  • Gravity load from superstructure
• Foundation walls enclosing habitable space

  • Resist the penetration of water and soil gases such as radon
  • Control heat flow
  • Accept a range of suitable finishes
  • Accommodate windows, doors, and other openings
• Cast-in-place concrete or concrete masonry units


  Materials used for foundation walls
• Anchoring foundation wall to footing

  • Steel dowels or mechanical keys
• Expansion joint


  • Premolded filler and sealant
• Footing size


  Based on the foundation wall load and the allowable bearing capacity of the underlying soil
• Extending foundation wall above grade

  • At least 6" (150) when supporting wood construction
• Finish grade slope


  • Minimum 5% to drain surface water away from foundation
• Transferring lateral loads on superstructure to ground

  • Through soil friction on bottom of footings and passive soil pressure on sides of footings and foundation walls
• Crawl space clearance

  • Minimum 18" (455) to bottom of beam or girder
  • Minimum 24" (610) to bottom of joists
• Crawl space ventilation


  • 
    
  • Screen openings to prevent entry of insects and animals
• Concrete foundation wall


  • Minimum 8" (205) thickness
  • Horizontal and vertical reinforcement as required
  • Concrete ground slab
  • Concrete footing
  • Steel dowels anchor wall to footing
  • Key provides additional resistance to lateral slippage
• Concrete masonry foundation wall


  • Modular 8" (205) units laid in running bond
  • Fill cells in top course with grout
  • Vertical reinforcement in grouted cells and horizontal bond beams as required
  • Concrete ground slab
  • Concrete footing
  • Steel dowels anchor wall to footing
  • Full mortar joint on roughened footing
• Preparing top of foundation wall

  • To receive, support, and anchor wall and floor systems of superstructure

  
• Superstructure support options

  • Wood beams
  • Open-web steel joists
  • Wood joists
• Sill plate anchors or holddowns


  • Required to secure wall and floor framing to foundation against uplifting wind or seismic forces