Commercial Foundations

1. Commercial Foundations He who has not first laid his foundations may be able with great ability to lay them afterwards, but they will be laid with trouble to the architect and danger to the building. Niccolo Machiavelli (1469 - 1527), The Prince

2. Foundations • Purpose • Considerations • Types of Foundations • Shallow Foundations • Spread Footings • Strip Foundations • Slab-on-Grade and Thickened Slabs • Mat Foundation • Deep Foundations • Piles • Cast-in-Situ Piles • Why Do Foundations Fail?

3. Purpose of Foundations • Provide a level, stable surface to safely support a building • Transfer building loads to soil • Anchor the building from wind, flood, and seismic loads

4. Design Considerations • Loads from the structure • Allowable soil bearing pressure • Frost depth • Flood elevation • Drainage • Costs

5. Loads from the Structure • Foundations must resist • Dead Load • Live Load • Lateral Loads • -- Wind • -- Seismic activity • -- Flood SOIL REACTIONS

6. Allowable Soil Bearing Pressure • Indicates the maximum pressure that a soil may be designed to support • Includes a factor of safety • Dictates the size, depth, and type of foundation • Typically presented in pounds per square foot (psf) • Different types of soils have different allowable soil bearing pressures

7. Soil Information • Local Building Department, Codes and Regulations • Preliminary info: USDA Web Soil Survey http://websoilsurvey.nrcs.usda.gov • Local or State Building Codes • Soil testing/analysis • Site inspection and simple soil testing • Soil borings taken at proposed foundation locations

8. Estimated Allowable Soil Bearing Pressure

9. Frost Depth • Freezing of soil can cause heaving of foundations • Silt or clay soils with a high water table are highly susceptible to frost Defense • Build base of foundation below frost depth or • Provide frost protection for foundation

10. Frost Heave

11. Flood Elevation • Inundation by flood waters should be avoided • Damage to structure • Damage to contents • Height of floors and/or flood proofing is dictated by building codes Courtesy Federal Emergency Management Agency. Photographer Dave Saville.

12. Flood Zones • A Zone – Areas with a 1% annual chance of flooding • V Zone – Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves • BFE(Base Flood Elevation) – The elevation shown on the Flood Insurance Rate Map that indicates the water surface elevation resulting from a flood with a 1% chance of equaling or exceeding that level in any given year

13. Flood Insurance Rate Maps Note: V29 indicates V-zone and BFE = 29 ft

14. A Zone • Residential buildings - lowest floor (including basement) elevated at or above the BFE • Non-residential buildings • Lowest floor (including basement) elevated at or above the BFE OR • Dry-floodproofed to the BFE Dry floodproofing: Building must be designed and constructed to be watertight to floodwaters

15. V Zone All Buildings (residential and non-residential) • Elevated on piles and columns • Lowest horizontal structural member of lowest floor at or above the BFE • Anchored to resist flotation, collapse, and lateral movement • The area below the lowest floor must be • Used only for the parking of vehicles, building access, or storage, • Free of obstruction, OR • Any enclosure must be constructed of breakaway walls

16. V Zone Break-away walls: non-supporting and non-load bearing walls that easily break away from the structure when subjected to lateral flood forces.

17. Types of Foundations • Shallow Foundation: Transfers loads to the soil very near the surface • Spread footing or strip footing • Mat or raft foundation • Slab-on-grade • Deep Foundation: Transfers loads to deeper soil layers • Piles • Cast-in-Situ Piles

18. Shallow Foundations

19. Shallow Foundation • The load from the footing spreads out so that the soil bearing pressure diminishes with depth. • The soil directly under the footing takes the greatest load. LOAD Critical Load Area Bearing Pressure (decreases with depth)

20. Spread (Column) Footing A footing that spreads the load over a broad area which supports one (or a few) load(s) USES Usually used in low-rise buildings COLUMN LOAD PIER (Concrete or Masonry) SPREAD FOOTING (Concrete)

21. Continuous (Strip) Footing A wide strip of reinforced concrete that supports loads from a bearing wall USES • Light frame construction • Under foundation walls LOAD FOUNDATION WALL (Concrete or Masonry) STRIP FOOTING (Concrete)

22. Slab-on-Grade and Thickened Slab Slab-on-Grade – Reinforced concrete floor supported by soil Thickened Slab – A slab on grade with an integral footing created by thickening the slab USES • Residential or light commercial construction • Shallow frost depth or when frost protection is used (instead of strip footing) WALL SLAB-ON-GRADE THICKENED SLAB

23. Mat Foundation A large, heavily reinforced concrete slab placed under the entire building to support loads from several points USES Heavy loads on weak soil MAT FOUNDATION CONCRETE PIER

24. Deep Foundations

25. Deep Foundation LOAD PILE CAP The building LOAD is transferred through frictionon the sides of the piles and/or bearing on the end of the piles Top Soil PILES Friction Force (Resisting Force) Weak Soil Bearing Force (Resisting Force) Strong Soil

26. Pile Foundation PIER (CONCRETE) Pile – Vertical structural member that is driven, jetted, or drilled into the ground in order to gain support from deeper soil layers LOAD PILE CAP (CONCRETE) USES Weak shallow soil with deep satisfactory soils PILE

27. Cast-in-Situ Piles A large diameter cast-in-place concrete pile GRADE BEAM USES Weak shallow soil with satisfactory soils at intermediate depth CAST-IN-SITU PILE LOAD BELL can improve bearing capacity

28. Why Do Foundations Fail? Bending Failure – Foundation fractures due to bending moment Shear Failure – Foundation breaks due to excessive shear Punch Through – Structural member “punches through” concrete foundation Foundation Failure Bending Failure SECTION Punch Through PLAN

29. Why Do Foundations Fail? Settlement – Foundation moves • Weak or compressible soil • Expansion/contraction of soil (moisture) • Frost heave Soil Failure Illustrations courtesy U. S. Marine Corp.

30. Foundations • Purpose • Considerations • Types of Foundations • Shallow Foundations • Spread Footings • Strip Foundations • Slab-on-Grade and Thickened Slabs • Mat Foundation • Deep Foundations • Piles • Cast-in-Situ Piles • Why Do Foundations Fail?