Lecture 7 - Foundation Design

transcript

Villanova UniversityDept. of Civil & Environmental Engineering

CEE 4606 - Capstone IIStructural Engineering

Lecture 7 – Introduction to Foundation Design

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 ?

Isolated Footings

Column

Footing (Square)

q, soil pressure

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

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

Eccentric Loading

Column

Footing (Square)

qmin qmax

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 ?

Maximum Allowable Soil Pressure(McCormac, 2001)

Soil Description Allowable

Rock 20% of ultimate

Compact sand or hard clay

Medium stiff or sandy clay

Compact sand and silt mix

Loose sand 3 ksf

Soft sandy clay or clay 2 ksf

Loose sand and silt mix 1 ksf

Muck or mud 0 ksf

Failure Types

• Factor of safety against severe settlement ranges from 2.5 < F.S. < 3.0– This limits bearing

failure

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

Tributary area for one-way shear (hatched)

L/2 – a/2 - d

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

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

d/2 a d/2

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

Note: 11-36 is wrong in the handout

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

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

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

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

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

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

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

Computer Aided Structural Engineering

• Homework #2 and #3 due Friday• Volunteers?• Who wants to be famous?

Lecture 7 - Foundation Design

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