ae303 footing design presentation part 1

8/18/2019 AE303 Footing Design Presentation Part 1

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Footing Design procedure

Definition

Footings are structural members used to

support columns and walls and distribute

their loads to the soil in such a way that:

1) The load bearing capacity of the soil is

not exceeded,

2) Excessive settlement, differential

settlement, and rotation are prevented

3) The footing itself can carry the shears

and moments applied to it.

Footing – Bearing Capacity

Every soil has a bearing capacity.

These capacities are either

assumed “presumptive” or

calculated by the geotechnical

engineer. A geotechnical engineer

typically does soil borings to find

the subsurface properties. The

recommendations from this

investigation are presented in a

“Soils Report”.

Soil bearing capacity and settlement areaffected by the type of soil

(cohesive, cohesionless), the

density of the soil, and the location

of the water table.


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Sometimes, especially for small jobs, the structural engineer will elect to use

presumptive bearing capacities. There are many sources for these, here is

one example..

Footing – Bearing Capacity The bearing capacity and settlement characteristics of the soil can be improved by compaction

and surcharging. This is most useful for shallow foundations (those near the surface).


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The decision as to the type of foundation to use is a decision jointly made by

the client, the structural engineer, and the geotech. Some risks may be

taken to avoid costly deep foundation work.

On Shallow Footings

On Deep Foundations (Piles)

Footing – Bearing Capacity All footings are sized (length and width) based on the actual loads and

the soil’s bearing capacity.

For example: Assume the load from a column is 40K Dead Load and 60 K

Live Load, and that the allowable soil bearing capacity is 2.5 K/SqFt.

Assuming there is no moment, the footing area would have to be:

100 K / 2.5 Ksf = 40 Sq Ft.

Based on this area, we could design a footing 6’-4” long by 6’-4” wide,

or 10’ long by 4’ wide or any other combination of length and width

that gives a bearing area of 40 Sq.Ft or more.

The important thing to remember is that the loads were not factored for thisstep. When it comes time to “design” the footing (calculate thickness

and rebars) we need to factor the loads.

footingof area

P actualallowable ≤q footingof area

P factoredultimate =q


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Differential Settlement

To reduce differential settlement, size all footings

proportionally to carry the full dead load and an equalpercentage of live load

Distribution of Soil Pressure

When the column load P is

applied on the centroid of the

footing, a uniform pressure is

assumed to develop in the soil

surface below the footing area.

If a moment is applied, or the

column is not centered on the

footing, the soil pressure will not

be uniform.

The maximum soil pressure

must be less than the allowable.

The minimum soil pressure

must be greater than zero.


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Footings can have eccentric loads and moments due to errors in

construction and / or unequal column loads, this is especially true forfootings that carry multiple columns (combined footings).

Sizing of Footings, for Axial load and Moment

Strength design requirements

3^**12 / 1

2 / *

Lb

L M

b*L

Pq

factored factored factored +≤

3^**12 / 1

2 / *allowable

Lb

L M

b*L

Pq actualactual +≤

These equations are only valid up to condition c,

For condition d, two simultaneous equations

must be solved.


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Types of Footings

Isolated or single spread

footings are used to support

single columns. This is one

of the most economical

types of footings and is

used when columns are

spaced at relatively long

distances. A square footing

is normally the most cost

effective from a concrete

and reinforcing bar point of

view.

Types of FootingsCombined footings support

two or more columns.

Combined footings are used

when columns are so close

that single footings would

overlap. They are also used or

when one column is located at

or near a property line. They

can be rectangular ortrapezoidal in shape.

The goal is to have the

center of the footing at the

centroid of the loads.


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Types of FootingsCombined footings support

two or more columns. Thegoal is to have the center of

the footing at the centroid of

the loads.

Types of Footings

Cantilever or strap footings consist of two single footings connected with a

beam or a strap and support two single columns. The strap reduces

differential settlement and also adds support for walls.


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Types of Footings

Wall footings are used to support wallsthat carry loads from upper floors.

They can also act as part of a retaining

wall base.

Types of Footings

Raft or mat foundation consists of one footing usually placed

under the entire building area. They are used when soil bearing

capacity is low, column loads are heavy so that single footings

cannot be efficiently used. Differential settlement is reduced.


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Types of Footings

Pile caps are thick footings used to tie a group of piles

together to support and transmit column loads to the piles.

Footing Design Considerations

Once the footings are sized, they must be designed to carry the

column loads and transmit them to the soil safely while

satisfying code limitations.

One and two way shear strength of the concrete

Moment capacity of the section

Bearing capacity of columns to the footing

Dowel requirements

Development length of bars


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Design for two-way shear

( the following examples assume uniform soil pressure )Assume d.

Determine the shear perimeter b0:

b0 = 2(c1+d) +2(c2+d) for

columns of sides c1 and c2.

1

2

3 The shear force Vu acts at a

section that has a length

b0 and a depth d; the section is

subjected to a vertical downward

load Pu and vertical upward

pressure qu.( )( ) )consistentunits(keep 21uuu d cd cqPV ++−=


For two-way shear in footings (and slabs) Vc is the smallest of:

long side/short side of column concentrated load or

reaction area

length of critical perimeter around the column

where, βc =

b0 =

d b f V 0c

c

c 4

2

+=

β

When β >2 the allowable Vc is reduced.

d b f

b

d V 0c

o

sc 2

+=

α

d b f V 0cc 4= αs is 40 for interior, 30 for edgeand 20 for corner column


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VuV

VnVc

Set

c =∴

=

φ

Vc

V d

φ u

=

If the required d is not less than or

equal to the assumed d, pick a larger d

and repeat the calculations.

Two way shear will normally control

for square footings.

Design for one-way shear

For footings, the critical section is

located a distance d from the face

of the equivalent column.

If the column is not concrete, the “face”

location must be calculated. It is half

way between the edge of the base plate

and the face of a steel column. It is ¼ of

“c” inside of a masonry column. (This iscommon for wall footings).

d b ' f V 2 cc φ φ =


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Design for one-way shear

The ultimate shearing force at section m-m can be

calculated

−−= d

c LbqV

22

uu

If no shear reinforcement is to be

used, then the minimum value of d

can be calculated.

Set Vu = φVc

b f

V d

2 c

u

φ =

ae303 footing design presentation part 1

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