Reinforced Concrete II Hashemite University The Hashem ite University Departm ent of Civil Engineering Lecture Lecture 9 9 – – Footings Footings Dr. Hazim Dwairi Dr Hazim Dwairi Dr Hazim Dwairi Dr. Hazim Dwairi Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Footings Definition Footings Definition • Footings are structural members used to support Footings are structural members used to support columns and walls to transmit and distribute their columns and walls to transmit and distribute their columns and walls to transmit and distribute their columns and walls to transmit and distribute their loads to the soil in such a way that the load loads to the soil in such a way that the load bearing capacity of the soil is not exceeded, bearing capacity of the soil is not exceeded, excessive settlement, differential settlement, or excessive settlement, differential settlement, or rotation are prevented and adequate safety rotation are prevented and adequate safety against overturning or sliding is maintained. against overturning or sliding is maintained. g g g g g g • Since the soil is generally weaker than concrete Since the soil is generally weaker than concrete columns and walls, the contact area between the columns and walls, the contact area between the footing and the soil is much larger than between footing and the soil is much larger than between the supported members and footing. the supported members and footing. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 1
Reinforced Concrete II Hashemite University Ty p es of Footings Ty p es of Footings a) a) Strip or Wall Footing Strip or Wall Footing b) S b) b) Spread Footing (Single or Isolated) b) Spread Footing (Single or Isolated) S d F d F ti ti (Si (Si l l I I l t d) l t d) Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Ty p es of Footings Ty p es of Footings c) c) Spread Footing (Stepped) Spread Footing (Stepped) d) Spread Footing (Tapered) d) d) S d) Spread Footing (Tapered) S d F d F ti ti (T (T d) d) Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 2
Reinforced Concrete II Hashemite University Ty p es of Footings Ty p es of Footings e) e) Pile Cap Pile Cap f) f) f) f) Combined Footing C C Combined Footing bi bi d F d F ti ti Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Ty p es of Footings Ty p es of Footings g) Cantilever or Strap Footing g) Cantilever or Strap Footing Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 3
Reinforced Concrete II Hashemite University Ty p es of Footings Ty p es of Footings h) h) Mat or Raft Footing Mat or Raft Footing Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Distribution of Soil Pressure Distribution of Soil Pressure • When the column load When the column load P is applied on the is applied on the pp pp centroid centroid of the footing, a of the footing, a uniform pressure is uniform pressure is assumed to develop on assumed to develop on the soil surface below the soil surface below the footing area. the footing area. • However the actual distribution of the soil is not However the actual distribution of the soil is not uniform, but depends on may factors especially uniform, but depends on may factors especially the composition of the soil and degree of the composition of the soil and degree of flexibility of the footing. flexibility of the footing. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 4
Reinforced Concrete II Hashemite University Distribution of Soil Pressure Distribution of Soil Pressure Soil pressure Soil pressure distribution in distribution in cohesive soil. (Sand) cohesionless soil. (Clay) Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Design Consid era tions Design Consid era tions Footings must be designed to carry the column Footings must be designed to carry the column loads and transmit them to the soil safely while loads and transmit them to the soil safely while loads and transmit them to the soil safely while loads and transmit them to the soil safely while satisfying code limitations. satisfying code limitations. 1. 1. The area of the footing based on the allowable The area of the footing based on the allowable bearing soil capacity bearing soil capacity 2. 2. Two Two- -way shear or punch out shear. way shear or punch out shear. 3 3. 3. One O One- -way shear way shear h h 4. Bending moment and steel reinforcement 4. Bending moment and steel reinforcement required required Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 5
Reinforced Concrete II Hashemite University Design Consid era tions Design Consid era tions Footings must be designed to carry the column Footings must be designed to carry the column loads and transmit them to the soil safely while loads and transmit them to the soil safely while loads and transmit them to the soil safely while loads and transmit them to the soil safely while satisfying code limitations. satisfying code limitations. 1. Bearing capacity of columns at their base 1. Bearing capacity of columns at their base 2. 2. Dowel requirements Dowel requirements 3. 3. Development length of bars Development length of bars 4. 4. Differential settlement Differential settlement Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Eccentrica lly Loa d ed Footing Eccentrica lly Loa d ed Footing Locate column within the Kern of footing to prevent developing tension developing tension stresses in the soil underneath the footing Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 6
Reinforced Concrete II Hashemite University Gross a nd Net Bea ring Ca p a city Gross a nd Net Bea ring Ca p a city 10.8 kN/m 2 0.6m 15.0 kN/m 2 0.6m γ s =18kN/m 3 25.8 kN/m 2 γ c =25kN/m 3 = − − q q W W net gross s footing = W weight of soil s 10.8 kN/m 10 8 kN/m 2 = W weight of footing 15.0 kN/m 2 footing 25.8 kN/m 2 Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Size of Footing Size of Footing • The area of footing can be determined from the The area of footing can be determined from the actual external loads such that the allowable soil actual external loads such that the allowable soil actual external loads such that the allowable soil actual external loads such that the allowable soil pressure is not exceeded. pressure is not exceeded. Service Load ( ) Total load including selfweight = Area of footing Gross allowable soil pressure q q = FS ult q all FS = Factor of safety in the range of 2.5 to 3.0 Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 7
Reinforced Concrete II Hashemite University Design for Tw o Design for Tw o- -w a y Shea r w a y Shea r • For Slabs and footings, For Slabs and footings, V c is the smallest of a, b is the smallest of a, b and c: and c: and c: and c: Where: b o = perimeter of critical section β = ratio of long side of column to short side < 2 α s = 40 for interior columns, 30 for edge columns and 20 for corner columns. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Design for Tw o Design for Tw o- -w a y Shea r w a y Shea r The shear force The shear force V u acts at a acts at a section that has a length section that has a length section that has a length section that has a length ) or or 2(c b 0 = = 4 4( (c+d c+d) (c 1 +d) + +d) + 2 2(c (c 2 +d) +d) where where d d is the effective depth is the effective depth the section is subjected to a the section is subjected to a vertical downward load vertical downward load P u and and vertical upward pressure q u . vertical upward pressure vertical upward pressure q vertical upward pressure For Square column: = − + 2 V P q ( c d ) u u u For Rectangular column: = − + + V P q ( c d )( c d ) u u u 1 2 Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Reinforced Concrete II Reinforced Concrete II Dr. Hazim Dwairi 8
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