Lec 7 - Design of Wall footing-Introduction.PPT

Lecture

1. Reinforced Concrete Design-II
Lecture 07: Footings
Semester –Fall 2020
Dr.TahirMehmood
Reference
Design of Concrete Structures 14
th
Ed. by Nilson, Darwin and Dolan.

2. FOOTING

4. Footingarestructuralelementsthattransmittothesoil
columnloads,wallloadsorlateralloads.Iftheseloadsareto
beproperlytransmitted,footingmustbedesignedtoprevent
excessivesettlementorrotationandtominimizedifferential
settlement.
Introduction
Tolimitsettlementwithintolerablysmallamount,itis
necessary.
i.Totransmittheloadofthestructuretoasoilstratumof
sufficientstrengthand
ii.Tospreadtheloadoverasufficientlylargeareaofthat
stratumtominimizebearingpressure.

5. Ifsatisfactorysoildirectlyunderliesthestructure,itismerely
necessarytospreadloadbyfootings.Suchsubstructureare
knownas“spreadfoundation.”
Introduction
Ifadequatesoilisnotfoundimmediatelybelowthe
structure,ifbecomesnecessarytousedeepfoundationssuch
aspilestotransmittheloadtodeeper,firmerlayers.

6. TypeofSpreadFooting
i.WallFooting
Awallfootingissimply
astripofreinforcedor
plainconcrete,widerthan
thewallthatdistributesits
pressure.
ii.Isolatedspreadfootingunder
singlecolumn
Thesemaybeusuallysquare,
sometimesrectangularoroccasionally
circularinplan.

7. TypeofSpreadFootingiii.CombinedFooting
Supportingtwo
ormorecolumn
loads.Thesemay
be continuous
witharectangular
or trapezoidal
planorisolated
column footing
joinedbyabeam
whichisreferred
toas strap
footing.

8. iii.MatorRaftFoundation
Whichisonelargecontinuousfootingsupportingall
columnsofthestructure.Thisisusedwhenthesoilispoor.
TypeofSpreadFooting
Load,bearingPressures&FootingSize
Whenthesoilis
coarsegrained
Whenthesoilis
clayeyorfine
graineda
req
q
LD
A

 35.2
q
q
ult
a



9. Load,bearingPressures&FootingSize
DandL=loads(service)atthelevelofbaseofbaseoffooting
Eccentricload butedoesnotexceedthekerndistance.I
MC
A
P
q
min
max  P
M
e

10. Load,bearingPressures&FootingSize
DandL=loads(service)atthelevelofbaseofbaseoffooting
Ifeccentricityfallsoutsidethekerndistancemb3
P2
q
bm3
P
2
qo
max
max



 amaxqq

11. Thankyou