CEE 4606 - Capstone II Structural Engineering

CEE 4606 - Capstone IIStructural Engineering Lecture 7 – Introduction to Foundation Design

Types of Footings 1. Wall (strip) footings 2. Isolated footings 3. Combined footings 4. Mat (raft) footings (not the same as a slab on grade) 5. Pile cap What did you learn last semester ?

q, soil pressure Isolated Footings Column Footing (Square)

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

qmin qmax Eccentric Loading Column Footing (Square)

We have none of these! Allowable Soil Pressures • Structural engineers usually rely on geotechnical engineers for this information • Test borings • Load tests • Other experiments What can we do ?

Maximum Allowable Soil Pressure(McCormac, 2001)

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

Two types of shear must be considered Beam shear, VU1 (One-way) L – length of footing d – distance to reinforcing a – column width Design of Square Isolated Footings d a d L

Tributary area for one-way shear (hatched) a d L/2 – a/2 - d L / 2

ACI 11.12.1.1 Similar to a beam Vc = 2 f’c bd Vs = 0 if there is no web reinforcing Vu<f(Vc +Vs) d a d L

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 L / 2

where, bc = ratio of long side of the column to the short side b0 = perimeter of the critical section as = 40 for columns in the center of the footing 11-35 11-36 11-37 Punching Shear 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

P M y P/A My/I

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 Limitations L/6 L/6 L

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 Ingeniros 12:00 – 12:10 Comma Engineers 12:15 – 12:25 Los Amigos de Amigos de Jesus 12:30 – 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?

CEE 4606 - Capstone II Structural Engineering