Post on 15-Mar-2018
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Design Codes for Reinforced Concrete Foundations
• ACI Committee, American Concrete Institute, & International Organization for Standardization, “Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary,” American Concrete Institute, Detroit, 2011.
• BS 8110. “Structural use of Concrete: Part 1: Code of Practice for Design and Construction,” British Standard Institution, London, 1997.
ACI 318 – 11 Design Philosophy
• Ultimate Limit State Design (USD),
– Various load combinations factored to obtain the most critical load case representing ultimate limit state.
• Load factors mainly account for; accuracy of the estimated design loads, variability of the design loads during the design life of the structure, variability of the results of various analysis methods.
– Material strength reduction factors used to involve a safety factor on the available strength.
– The main principle is such that;
ACI 318 – 11 Design Philosophy
• Serviceability Limit State (SLS),
– Although it is not explicitly mentioned, serviceability check is required to ensure adequate performance of structural members at service load levels.
– This can be achieved by;
• estimation of settlement of foundations for overall performance, and,
• also by estimation of crack width of the designed structure under the impact of the bending moment to satisfy the material performance.
– SLS check also involves checking of the deflection of the foundation system along the footprint of the building structure (discussed in Chapter 2 of the class notes).
Spread (Single, Pad) Foundation Design
• This type of foundation carries load applied by a single column.
• The material used in the construction of spread footings is almost always concrete.
• Could be reinforced or unreinforced.
• Usually only consists bottom reinforcement, sometimes bent up along the sides for better anchorage
Source: http://www.strukts.com/2012/06/types-of-shallow-foundation_94.html
ACI 318 – 11 Background Information
• Spread footings are designed for flexure (bending), shear and bearing.
• Assuming a compressive force is applied via the foundation column, the top side of the foundation becomes under compression and the bottom side will be under tension. Considering a typical section through the foundation;
ACI 318 – 11 Load Factors (9.1 and 9.2)
• Service loads (Design loads) are factored and converted to ultimate loads using the following load combinations;
U: required strength (ultimate load), D: dead load, L: live load, S: snow load, R: rain load, Lr: reduced live load, W: wind load, E: earthquake load, H: soil, soil water or water load.
ACI 318 – 11 Material Strength Factors (9.3)
• The nominal design strength is factored to account for; uncertainty of material strength, variations from designs, quality of construction, accuracy of modelling and analyses, other deviations from design or material behaviour assumptions.
Based on ACI318-86, 9.3
ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)
Bending
• Considering the foundation section as a beam, the assumed stress distribution in foundation section can be approximated as;
max = 0.75 b , min= 0.002 , b = (0.85 1 fc / fy) (600 / (fy + 600) ) ,
where 1 = 0.85 for fc < 27MPa and 1 = 0.85 – 0.05 (fc - 27) / 7 for fc > 27MPa, whilst 1 should not be less than 0.65.
ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)
Shear
• Critical sections;
• Shear stress check governs the thickness design for foundations.
ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)
• Critical sections for moment and shear for foundations;
ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)
• Diagonal tension shear check for foundations;
ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)
• For wide beam shear check use d obtained from DTSC to calculate the applied shear on the critical section as;
Bwbs = B / 2 – (x/2 + d), then
V = Bwbs qu , and V should be < vc for WBS.
Should the above criterion is satisfied, then
it is said that DITSC governs the design for
The foundation thickness, d.
Otherwise, the estimated vc can be
equated to V/bd and the equation solved
For d once again. In that case it is said that
WBS check governs.
ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)
• A typical shallow foundation design involves but are not necessarily limited to the following steps ;