Post on 22-Nov-2014
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
CEE 4606 - Capstone IIStructural Engineering
Lecture 7 – Introduction to Foundation Design
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Types of Footings
1. Wall (strip) footings2. Isolated footings3. Combined footings4. Mat (raft) footings (not the same
as a slab on grade)5. Pile cap
What did you learn last semester ?
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Isolated Footings
Column
Footing (Square)
q, soil pressure
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Soil Pressures
• At the ground surface we assume a uniform pressure distribution, as long as the load above is applied at the C.G. of the footing.
• Does this reflect reality?– No, but it’s not a bad assumption
• Factored loads• Simplifies calculations
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Non Uniform Soil Pressure
• If the load is applied eccentrically with respect to the C.G. we assume a uniform variance in proportion to the moment
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Eccentric Loading
Column
Footing (Square)
qmin qmax
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Allowable Soil Pressures
• Structural engineers usually rely on geotechnical engineers for this information – Test borings– Load tests– Other experiments
We have none of these!
What can we do ?
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Maximum Allowable Soil Pressure(McCormac, 2001)
Soil Description Allowable
Rock 20% of ultimate
Compact sand or hard clay
8 ksf
Medium stiff or sandy clay
6 ksf
Compact sand and silt mix
4 ksf
Loose sand 3 ksf
Soft sandy clay or clay 2 ksf
Loose sand and silt mix 1 ksf
Muck or mud 0 ksf
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Failure Types
• Factor of safety against severe settlement ranges from 2.5 < F.S. < 3.0– This limits bearing
failure
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Design of Square Isolated Footings
• Two types of shear must be consideredBeam shear, VU1
(One-way)L – length of
footingd – distance to
reinforcinga – column width
d
L
a d
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Tributary area for one-way shear (hatched)
L / 2
a d
L/2 – a/2 - d
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Vu < (Vc +Vs)
• ACI 11.12.1.1• Similar to a beam
• Vc = 2 f’c bd
• Vs = 0 if there is no web reinforcing
d
L
a d
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
2. Punching Shear (Two-way)
• Critical section is d/2 from the column face
• Vc is the smallest of 3 values
• ACI Equations– 11-35– 11-36– 11-37
L / 2
d/2 a d/2
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Punching Shearwhere,c = ratio of long
side of the column to the short side
b0 = perimeter of the critical section
s = 40 for columns in the center of the footing
dbf
dbfb
d
dbf
c
cs
cc
0
00
0
4
2
42
11-35
11-36
11-37
Note: 11-36 is wrong in the handout
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Designing for Bending Moments
• Bending moment in a square footing is the same about both axes due to symmetry
• What about column width, a?– Yes for a square column– No for a rectangular column
• Generally use smallest value of a for analysis
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
ACI 15.4 – Moments in footings
• Location of critical sections– At the face of the column for
concrete columns (our case)– Masonry wall– Steel column
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Stresses Under a Footing
• P = vertical load (+ compression)• A = area of contact surface between the soil
and the footing• I = moment of inertia of this area• M = moment about the centroidal axis• y = distance from the centroidal axis to the
point where you want to calculate the stresses
• This assumes that the loads are at or near the center of the footing
I
My
A
Pq
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
P/A
M
P
My/I
y
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Limitations
• M = P e, where e = eccentricity
• Maximum e for this to be a valid analysis is when qmin = 0
• Load must be within the kern
• ek = L/6 L
L/6 L/6
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
References
• Handout – Chapter 16 of Macgregor, J. G., Reinforced Concrete, Mechanics and Design, 3rd Edition, Prentice-Hall,1997.
• ACI code• McCormac text, Chapter 11• CONCAD, McCormac software
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Interim Assignment #3
• Link on the website to a personality indicator test– Fill it out, print out the summary sheet, put your
name on it, and turn it in on Tuesday– We will be using the results, so take it seriously
• If you haven’t paid your $40 course fee by Tuesday, we will deduct points off of your next group progress report
Project managers are responsible
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Schedule for Progress Report #2
11:30 – 11:40 En Fuego Engineering 11:45 – 11:55 Ameriduras Ingeniros12:00 – 12:10 Comma Engineers 12:15 – 12:25 Los Amigos de Amigos de Jesus12:30 – 12:40 HyperStructo-CyberTronics
Engineering and Associates
Keep the setup and presentation portion < 10 minutes
Work on improving animation and timing
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Villanova UniversityDept. of Civil & Environmental Engineering
CEE 4606 - Capstone IIStructural Engineering
Computer Aided Structural Engineering
• Homework #2 and #3 due Friday• Volunteers?• Who wants to be famous?