Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 1 CEE Capstone II Structural Engineering Lecture 7 – Introduction to Foundation Design

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 2 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 ?

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 3 Isolated Footings Column Footing (Square) q, soil pressure

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 4 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 5 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 6 Eccentric Loading Column Footing (Square) q min q max

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 7 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 ?

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 8 Maximum Allowable Soil Pressure (McCormac, 2001) Soil DescriptionAllowable Rock20% of ultimate Compact sand or hard clay8 ksf Medium stiff or sandy clay6 ksf Compact sand and silt mix4 ksf Loose sand3 ksf Soft sandy clay or clay2 ksf Loose sand and silt mix1 ksf Muck or mud0 ksf

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 9 Failure Types Factor of safety against severe settlement ranges from 2.5 < F.S. < 3.0 –This limits bearing failure

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 10 Design of Square Isolated Footings Two types of shear must be considered  Beam shear, V U1 (One-way)  L – length of footing  d – distance to reinforcing  a – column width d L a d

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 11 Tributary area for one-way shear (hatched) L / 2 a d L/2 – a/2 - d

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 12 V u <  (V c +V s ) ACI Similar to a beam V c = 2 f’ c bd V s = 0 if there is no web reinforcing d L a d

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering Punching Shear (Two-way) Critical section is d/2 from the column face V c is the smallest of 3 values ACI Equations –11-35 –11-36 –11-37 L / 2 d/2 a d/2

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 14 Punching Shear where,  c = ratio of long side of the column to the short side b 0 = perimeter of the critical section  s = 40 for columns in the center of the footing Note: is wrong in the handout

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 15 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 16 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 17 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 18 P/A M P My/I y

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 19 Limitations M = P e, where e = eccentricity Maximum e for this to be a valid analysis is when q min = 0 Load must be within the kern e k = L/6 L L/6

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 20 References Handout – Chapter 16 of Macgregor, J. G., Reinforced Concrete, Mechanics and Design, 3 rd Edition, Prentice-Hall,1997. ACI code McCormac text, Chapter 11 CONCAD, McCormac software

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 21 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 22 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

Villanova University Dept. of Civil & Environmental Engineering CEE Capstone II Structural Engineering 23 Computer Aided Structural Engineering Homework #2 and #3 due Friday Volunteers? Who wants to be famous?