STRC03 Foundations 0116

Structural Engineering Review Course

Foundations and Retaining Structures

Foundations and Retaining Structures Structural Engineering Review Course

STRC ©2015 Professional Publications, Inc.

© 2016 Professional Publications, Inc.

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Lesson Overview Foundations and Retaining Structures • Strip Footing • Isolated Column with Square Footing • Isolated Column with Rectangular Footing • Combined Footing • Strap Footing • Cantilever Retaining Wall • Counterfort Retaining Wall



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Learning Objectives You will learn • strip, square, rectangular, combined, and strap footing design for flexure and shear • cantilever and counterfort retaining wall design • how to deal with iterative procedures on the SE exam



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Prerequisite Knowledge You should already be familiar with • statics • mechanics of materials • structural analysis • basic reinforced concrete design • basic foundation terminology



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Referenced Codes and Standards • Building Code Requirements for Structural Concrete (ACI 318, 2011) • International Building Code (IBC, 2012)



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Strip Footing nomenclature length of footing parallel to wall B e

P

eccentricity of wall load with respect to the centroid of the footing eccentricity of wall load with respect to edge of footing, L/2 – e length of footing perpendicular to wall applied wall load

q

soil pressure due to unfactored loads

e´ L



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Strip Footing pressure distribution • unfactored loads • self‐weight of footing • applied load from wall • For footing with length B (parallel to wall), pressure distribution is as shown on next slide. • Size footing so that maximum soil pressure is less than allowable soil pressure. STRC ©2015 Professional Publications, Inc.


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Strip Footing Figure 2.1 Net Pressure Distribution on a Footing



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Example: Strip Footing Example 2.1



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Example: Strip Footing



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Strip Footing factored soil pressure • factored loads • force acting on footing (net pressure) • Self‐weight of footing produces equal and opposite soil pressure (cancels out and is not included in net pressure). • soil pressure due to applied wall load only • used to determine flexure and flexure shear



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Example: Strip Footing For the strip footing shown, the applied loads PD and PL include the weight of the wall. Determine the maximum factored net pressure.



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Example: Strip Footing Pu  1.2 PD  1.6 PL kips  kips     1.2   20  1.6 10      ft  ft     40 kips ft M u  40 ft-kips ft ft-kips M ft  1 ft e  kips P 40 ft 40

L 12 ft   2 ft 6 6 L 6

e Since ,

 6e  P 1   L q  BL kips     6 1    40    1  ft    12 ft     1 ft 12 ft   5 kips ft 2 STRC ©2015 Professional Publications, Inc.


 max  14

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Strip Footing: Critical Sections nomenclature width of wall c d

effective depth of footing

h

depth of footing



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Strip Footing: Critical Sections Figure 2.2 Critical Sections for Flexure and Shear



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Strip Footing flexure shear design same as for reinforced concrete beams, per ACI 318 Sec. 11.2 flexure design same as for reinforced concrete beams, per ACI 318 Sec. 10.2, with exceptions • ρmin = 0.0018 [for grade 60 bars] • main reinforcement, smax ≥ max [18 in, 3h] • main reinforcement, Ø  development length ≤ anchorage length • distributed reinforcement, smax ≥ max [18 in, 5h]



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Example: Strip Footing Example 2.3



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Isolated Column with Square Footing reinforcement details • maximum spacing

• minimum clear spacing between parallel bars in a layer

• principal reinforcement s = 3h ≤ 18 in • distribution reinforcement s = 5h ≤ 18 in

• minimum reinforcement ratio for distribution steel



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Isolated Column with Square Footing reinforcement details (continued) minimum cover • cast against earth = 3 in • exposed to earth or weather • 1.5 in for bars ≤ no. 5 • 2 in for bars > no. 5



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Poll: Isolated Column with Square Footing Which of the following is true about the reinforcement detailing requirements of concrete beams and footings? (A) The maximum spacing of reinforcement for concrete beams is smaller than for footings. (B) The minimum cover for footings is less than that of reinforced concrete beams.



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Poll: Isolated Column with Square Footing Which of the following is true about the reinforcement detailing requirements of concrete beams and footings? (A) The maximum spacing of reinforcement for concrete beams is smaller than for footings. (B) The minimum cover for footings is less than that of reinforced concrete beams.

Solution Detailing requirements are different for beams and footings. More space is allowed between reinforcement for footings, so option A is true. More cover is needed for footings than beams, so option B is false. The answer is (A).



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Isolated Column with Square Footing nomenclature bo

length of the critical perimeter

β

ratio of long side to short side of column s

shear constant



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Isolated Column with Square Footing Figure 2.3 Critical Perimeter for Punching Shear



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Isolated Column with Square Footing design for punching shear (continued) • for bo > 20d

• shear strength • for square columns

ACI Eq. 11‐32 ACI Eq. 11‐33

•

• where

when β > 2 ACI Eq. 11‐31



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Isolated Column with Square Footing design shear axial load only • shear force

• axial and moment

• shear stress

• max shear stress



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Example: Isolated Column with Square Footing Example 2.4



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Example: Isolated Column with Square Footing



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Isolated Column with Square Footing design for flexure shear and flexure

Figure 2.4 Critical Sections for a Footing with Steel Base Plate

• critical section • concrete and masonry (same as strip footing) • steel (as shown) • capacity (same as strip footing)



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Example: Isolated Column with Square Footing Example 2.5



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Isolated Column with Square Footing Figure 2.5 Bearing on Footing Concrete



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Isolated Column with Square Footing transfer of force at base of column • bearing capacity of the column concree

• bearing capacity of the footing concrete

• Bearing capacity is governed by the lesser of the two equations. • A2 is the area at the base of the column where the lines intersect. (See Fig. 2.5.) • This area may be governed by the length of the footing. • Instead of c + 4d, the length is B.



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Isolated Column with Square Footing transfer of force at base of column • When bearing capacity is exceeded, provide dowels or extended longitudinal bars to carry excess load with

• minimum steel across interface



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Example: Isolated Column with Square Footing CSCO Example 6.8



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Example: Isolated Column with Square Footing CSCO Example 6.8



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Isolated Column with Rectangular Footing Figure 2.6 Rectangular Footing: Reinforcement Areas



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Isolated Column with Rectangular Footing design for flexure • Determine critical sections in longitudinal and transverse directions. • Calculate moments in each direction. • Distribute longitudinal reinforcement uniformly. • Distribute transverse reinforcement in bands as shown in Fig. 2.6.



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Example: Isolated Column with Rectangular Footing Example 2.8



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Example: Isolated Column with Rectangular Footing Example 2.8



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Example: Isolated Column with Rectangular Footing



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Example: Isolated Column with Rectangular Footing



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Combined Footing pressure distribution

Figure 2.7 Combined Footing with Applied Service Loads

• Adjust footing length until footing centroid aligns with centroid of service loads (results in uniformly distributed soil pressure). • Adjust width of footing for allowable bearing pressure.



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Combined Footing pressure distribution • Design footing based on factored loads. (Do not include footing weight.)

• reinforcement required in band under column no. 1

• Design as a beam continuous over two supports • max. negative moment at 0 shear location • max. positive moment at column no. 2 outside face



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Combined Footing Figure 2.8 Combined Footing with Applied Factored Loads



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Example: Combined Footing Example 2.9



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Example: Combined Footing



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Combined Footing design for punching shear

Figure 2.8 Combined Footing with Applied Factored Loads

• similar to isolated column footing method • footing design for factored loads • critical perimeter for interior column • critical perimeter for end column



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Combined Footing design for flexural shear


• similar to isolated column footing method • footing design for factored loads



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Combined Footing design for flexure


• similar to isolated column footing method • footing design for factored loads • reinforcement required in band under column no. 1



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Strap Footing Figure 2.9 Strap Footing with Applied Service Loads

pressure distribution • two pad footings connected by strap • soil pressure under strap negligible • strap and pads form rigid body • size footings for uniform pressure under pads • adjust pad sizes for equal pressure under both pads



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Strap Footing pressure distribution (continued) To design a strap footing with equal soil pressure, consider • loads

• soil pressure

• reactions

• resulting lengths



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Strap Footing pressure distribution (continued) • align footing 2 so that R2 is below p2

• take moments about footing 2 and solve for R1

• sum vertical forces and solve for R2



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Strap Footing pressure distribution (continued) To design a strap footing with equal soil pressure, also consider the weight of strap beam.

• estimate R2 the solve for A2 and W2

You should • estimate R1 and solve for A1 and W1

• substitute values into equilibrium equations and iterate the process to convergence



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Example: Strap Footing Example 2.13



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Example: Strap Footing



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Example: Strap Footing



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Poll: Strap Footing An afternoon (depth) problem contains iterative steps, such as how to design a strap footing. Through how many iterations should you go? (A) 1 (B) 3 (C) as many as it takes to reach an exact solution



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Poll: Strap Footing An afternoon (depth) problem contains iterative steps, such as how to design a strap footing. Through how many iterations should you go?

Solution Demonstrate that you understand the procedure, and then move on. The answer is (A).

(A) 1 (B) 3 (C) as many as it takes to reach an exact solution



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Strap Footing design of strap beam for shear • factored loads

• summing vertical forces

• summing moments about pad no. 2

• shear at left and right end of strap



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Strap Footing Figure 2.10 Factored Forces on Strap Footing



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Strap Footing design of strap beam for flexure factored moments •

left end of strap

• right end of strap



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Cantilever Retaining Wall nomenclature w

live load surcharge

hT

total height of retaining wall

WB

weight of base

hw

stem thickness

Wk

weight of key

LB

length of base

WL

weight of surcharge

LH

length of heel

WS

weight of backfill

LT

length of toe

Ww

weight of stem

Lw

Length of stem

hB

depth of base

HA

resultant of active earth pressure

hK

height of shear key

HL

resultant of pressure due to live load surcharge



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Cantilever Retaining Wall nomenclature

symbols

HP

resultant of passive earth pressure

φ

angle of internal friction (of soil)

pA

active pressure

μ

friction coefficient

pL

pressure from live load surcharge

∑W

pp

passive pressure

total weight of retaining wall, and backfill and live load surcharge

q

earth pressure under base

γs

density of soil



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Cantilever Retaining Wall Figure 2.11 Cantilever Retaining Wall with Applied Service Loads



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Cantilever Retaining Wall pressure distribution • as shown in Fig. 2.11

• resultant of live load surcharge

• resultant of active earth pressure (Rankine) alternatively, represented by equivalent height of fill



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Cantilever Retaining Wall pressure distribution (continued) • resultant of passive earth pressure

• required factors of safety sliding

•

• overturning

• friction beneath base

(A factor of safety of 1.5 is required for overturning about the toe.)



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Example: Cantilever Retaining Wall Example 2.16



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Example: Cantilever Retaining Wall



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Cantilever Retaining Wall reinforcement details • minimum reinforcement in stem walls • bars greater than no. 5  vert  0.33%  hor  0.25%

• Walls greater than 10 in thick require two layers of horizontal reinforcement.

• bars no. 5 and smaller  vert  0.33%  hor  0.20%



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Cantilever Retaining Wall reinforcement details (continued) • vertical reinforcement in earth face (governed by flexure) • vertical reinforcement in air face (supply enough so that earth face plus air face is ρ 0.0018) • vertical and horizontal reinforcement spacing (not more than 3 times wall thickness or 18 in)



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Example: Cantilever Retaining Wall Assuming no. 5 bars, what is the minimum required reinforcement spacing for an 8 in thick wall?



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SCL10


Example: Cantilever Retaining Wall Solution For no. 5 bars at 18 in (maximum spacing), As = 0.207 in²/ft.

For no. 5 bars, ρmin,vert = 0.0033 and ρmin,hor = 0.002.

Therefore, use no. 5 at 18 in for both directions.

Ag   8 in 12 in   96 in 2 ft

   min A in    0.3168 in 2 ft  vert   0.0012   96  ft   in 2    0.192 in 2 ft  hor   0.002   96  ft   2



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Slide 90 SCL10

Updated rho(min, vert) to 0.0033 and updated calculation. OK? Sierra Cirimelli-Low, 6/25/2015




Cantilever Retaining Wall design for shear and flexure • Recalculate soil pressure using factored loads. • locations of critical section • flexure in stem located at base of the stem • flexure in toe located at front face of stem • flexure in heel located at rear face of stem



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Counterfort Retaining Wall design of stem and base For 0.5 ln/lc 1.0 • design stem for pressure value at 0.6l

c

• horizontal span moments •

qln2 at counterfort supports, 11

•

qln2 at between counterforts, 16

• cantilever moment at base, 0.035 p l

3

A c



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Counterfort Retaining Wall nomenclature ln

clear span between counterforts

lc

clear height of counterfort



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Counterfort Retaining Wall Figure 2.12 Details of Counterfort Retaining Wall



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Example: Counterfort Retaining Wall Example 2.18



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Example: Counterfort Retaining Wall



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Counterfort Retaining Wall design of counterforts • earth pressure resisted by couple as shown in Fig. 2‐12 elevation B‐B • thrust from earth pressure acting on rear face of stem resisted by horizontal ties



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Counterfort Retaining Wall Figure 2.12 Details of Counterfort Retaining Wall



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Example: Counterfort Retaining Wall



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Lesson Overview Foundations and Retaining Structures • Strip Footing • Isolated Column with Square Footing • Isolated Column with Rectangular Footing • Combined Footing • Strap Footing • Cantilever Retaining Wall • Counterfort Retaining Wall STRC ©2015 Professional Publications, Inc.


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Learning Objectives You have learned • footing design • retaining wall design • how to avoid pitfalls on the SE exam • tricks to increase problem solving speed during the exam



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STRC03 Foundations 0116

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