Design Of Strap Footing With Full Detail
Design of Strap footing
Introduction
When two columns are reasonably close, a combined footing is designed for both columns as shown in below. When two columns are far apart, a strap is designed to transfer eccentric moment between two columns as shown in below. The goal is to have uniform bearing pressure and to minimize differential settlement between columns.
Design assumptions
- Strap does not provide bearing.
- Strap is ridge enough to transfer moment from one footing to the other.
Design procedure
Service load design:
- Determine the length of exterior footing and its eccentricity, e.
- Determine eccentric moment, M = Pa*e.
- Determine shear force, V = M / L
- Determine footing reaction, Ra = Pa+V, and Rb = Pb-V
- Determine footing sizes for both A & B.
Structural analysis
- Calculate factored column loads, Pua & Pub
- Calculate factored eccentric moment Mu = Pua*e
- Calculate factored shear, Vu = Mu / L
- Determine factored reactions, Rua & Rub.
- Perform structural analysis, determine factored shear and moment on footings and strap.
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Reinforced concrete design
- Design exterior footing. Check shear stresses and design flexural reinforcement.
- Design interior footing. Check shear stresses and design flexural reinforcements.
- Design footing strap as a reinforced concrete beam.
Service load design:
Design procedure:
- Determine the length of exterior footing and its eccentricity, e.
- Determine eccentric moment, M = Pa*e.
- Determine shear force, V = M / L
- Determine footing reaction, Ra = Pa+V, and Rb = Pb-V
- Determine footing sizes for both A & B.
Determine sizes of strap footing
Given:
Column information:
Column A: Live load = 40 kips, Dead load = 50 kips
Column B: Live load = 80 kips, Dead load = 100 kips.
Distance between two columns: 22 ft.
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Footing information:
Allowable soil bearing capacity; 3000 psf
Distance from column A to edge of footing: 1 ft.
Allowable soil bearing capacity = 3000 psf
Weight of soil above footing = 120 psf
Depth of footing= 24”
Depth of soil above footing = 12”
Requirements: Determine the size of footing A & B.
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Solution:
Assume a footing width of 6 ft, the eccentricity of footing A is e = 6/2-1=2’.
The distance between footing reaction, L = 22-2=20’
The eccentric moment is M = (40+50)*2=180 ft-kips
The shear produced by M is, V = 180/20=9 kips
Reaction at footing A = 40+50+9 =99 kips
Net soil bearing capacity = 3000-2*150-120=2580 psf
Required footing area of A = 99/2.58=38.4 ft2.
Use 6’ by 6.5’ footing, area = 39 ft2.
Reaction at footing B = 100+80-9=171 kips
Required footing area = 171/2.58=66.3 ft2.
Use 8’ by 8.5’ footing, A = 68 ft2.
Structural analysis of strap footing
Procedures
- Calculate factored column loads, Pua & Pub
- Calculate factored eccentric moment Mu = Pua*e
- Calculate factored shear, Vu = Mu / L
- Determine factored reactions, Rua & Rub.
- Determine factored footing reactions.
- Perform structural analysis; determine factored shear and moment
on footings and strap.
Determine moment and shear in a strap footing
Given: The strap footing in
Requirement: Determine maximum factored shears and moment in the footings and strap.
Solution:
Factored column load of A = 1.4*50+1.7*40=138 kips
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Factored column load of B = 1.4*100+1.7*80=276 kips
Factored eccentric moment, Mu = 138*2=276 ft-kips
Factored shear, Vu = 276/20=13.8 kips
Factored footing reaction at A = 138+13.8=151.8 kips
Factored footing pressure per linear foot of A = 151.8/6=25.3 k/ft
Factored footing reaction at B = 276-13.8=262.2 kips
Factored footing pressure per linear foot at B = 262.2/8=32.8 k/ft.
Shear diagram:
At point 1: Vu = 25.3*1.5-138= -100.1 kips
At point 2: Vu = 25.3*6-138=13.8 kips
At point 3: Vu = 25.3*6-138=13.8 kips
At point 4: Vu = 13.8+32.8*3.5= 128.6 kips
At point 5: Vu = 32.8*-3.5=-114.8 kips
Moment diagram:
At point 1: Mu = 25.3*1.52/2-138*0.5= -40.5 ft-kips
At point 2: Mu = 25.3*62/2-138*5= -234.6 ft-kips
At point 3: Mu = 25.3*6*(6/2+13)-138*(5+13)=-55.2 ft-kips
At point 4: Mu = 25.3*6*(6/2+13+3.5)-138*(5+13+3.5)+32.8*3.52/2=194 ft-kips
At point 5: Mu = 32.8*3.52/2=200.9 ft-kips
Reinforced concrete design of strap footing
Design procedure:
- Design exterior footing. Check shear stresses and design flexural reinforcement.
- Design interior footing. Check shear stresses and design flexural reinforcements.
- Design footing strap as a reinforced concrete beam.
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