Lecture note on Footing Design, Lecture notes of Structural Analysis

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Lecture 26 – FootingsLecture 26 – Footings

November 2, 2012

CEN 479

Dr. Md. Monirul Islam

Lecture GoalsLecture Goals

Footing Classification

Footing Design

Footing Examples

Types of FootingsTypes of Footings

Wall footings are used to support structural walls that carry loads for other floors or to support nonstructural walls.

Types of FootingsTypes of Footings

Isolated or single footings are used to support single columns. This is one of the most economical types of footings and is used when columns are spaced at relatively long distances.

Types of FootingsTypes of Footings

Cantilever or strap footings consist of two single footings connected with a beam or a strap and support two single columns. This type replaces a combined footing and is more economical.

Types of FootingsTypes of Footings

Continuous footings support a row of three or more columns. They have limited width and continue under all columns.

Types of FootingsTypes of Footings

Pile caps are thick slabs used to tie a group of piles together to support and transmit column loads to the piles.

Distribution of Soil PressureDistribution of Soil Pressure When the column load P is applied on the centroid of the footing, a uniform pressure is assumed to develop on the soil surface below the footing area. However the actual distribution of the soil is not uniform, but depends on may factors especially the composition of the soil and degree of flexibility of the footing.

Design ConsiderationsDesign Considerations Footings must be designed to carry the column loads and transmit them to the soil safely while satisfying code limitations. The area of the footing based on the allowable bearing soil capacity Two-way shear or punch out shear. One-way bearing Bending moment and steel reinforcement required

Design ConsiderationsDesign Considerations Footings must be designed to carry the column loads and transmit them to the soil safely while satisfying code limitations. Bearing capacity of columns at their base Dowel requirements Development length of bars Differential settlement

Two-Way Shear (PunchingTwo-Way Shear (Punching Shear) Shear) For two-way shear in slabs (& footings) V c is smallest of long side/short side of column concentrated load or reaction area < 2 length of critical perimeter around the column where,  c = b 0 = V f c b 0 d ACI 11- c c

 When  > 2 the allowable V c is reduced.

Design of two-way shearDesign of two-way shear Assume d. Determine b 0

b 0 = 4(c+d) b 0 = 2(c 1 +d) +2(c 2 +d)

for square columns where one side = c for rectangular columns of sides c 1 and c 2

Design of two-way shearDesign of two-way shear Allowable Let V u

=V

c

4.  V^ c^ ^4  f c b 0 d c 0 u 4 f b V d   If d is not close to the assumed d, revise your assumptions

Design of one-way shearDesign of one-way shear For footings with bending action in one direction the critical section is located a distance d from face of column V f b d c c 0   2 