FOUNDATIONEVERY STRUCTURE CONSIST OF THE FOLLOWING

TWO PARTS –SUPER STRUCTUREFOUNDATION (SUB STRUCTURE) THE FOUNDATION IS A PART OF SUB

STRUCTURE .FOUNDATION IS THE LOWEST ARTIFICIALLY PREPARED PARTS OF THE STRUCTURES WHICH ARE DIRECT CONTACT WITH THE GROUND AND WHICH TRASMIT THE LOADS OF THE STRUCTURES TO THE GROUND .

“FOUNDATION SHOULD BE DESIGNED SO THAT SETTELEMANT INTO THE GROUND IS LIMITED AND UNIFORM UNDER THE WHOLE OF THE BUILDING.

THE SOLID GROUND ON WHICH THE FONDATIONS REST IS CALLED FOUNDATION BED .

PURPOSE OF FOUNDATION :-

TO DISTRIBUTE THE LOAD OF THE STUCTURE,ON LARGE AREA ,SO THAT THE INTENSITY OF LOAD DOSE NOT EXCEED THE SAFE BEARING CAPICITY OF THE UNDERLYING SOIL.

  TO PROVIED A LEVELLED AND HARD SURFACE FOR THE SUPER

STRUCTURE TO BE BUILT OVER IT.  TO INCREASE THE STABILITY OF THE STUCTURE AS A

WHOLE ,AGAINST SLIDING , OVERTURNING OR OTHER DISTRUBING FORCES LIKE WIND RAIN,ETC.

  TO PREVENT THE LATERAL MOVEMENT OF THE SUPORTING

MATERAIL SO THAT SAFETY OF THE STRUCTURE IS NOT ENDANGERED.

 

LOADS ON FOUNDATION: DEAD LOAD – FORCE DUE TO STATIC WEIGHT OF

ALL WALLS, PARTITIONS, FLOORS, ROOFS & FINISHES & ALL OTHER PARAMENT CONSTRUCTION.

IMPOSED LOAD – LOAD ASSUMED TO BE PRODUCED BY THE INTEATED USE OF BUILDING, INCLUDING DISTRIBUTED, CONCENTRATED, IMPACT, INERTIA AND SNOW.

WINDLOAD – EFFECTS OF WIND PRESSURE AND SUCTION.

SIESMIC FORCES - DURING EARTHQUAKES.

TYPES OF FOUNDATIONa) Shallow Foundation System

i) Spread Foundation ii) Mat / Raft Foundation

b) Deep Foundation Systemi) Pile iii) Diaphragham wallii) Pile walls iv) Caissons

SPREAD FOUNDATION

Spread footings may be built in different shapes & sizes to accommodate individual needs such as the following: a) Square Spread Footings / Square Footings b) Rectangular Spread Footings c) Circular Spread Footings d) Continuous Spread Footings e) Combined Footings f) Ring Spread Footings

a) Square Spread Footings / Pad Foundation - support a single centrally located

column - use concrete mix 1:2:4 and

reinforcement - the reinforcement in both axes are to resist/carry tension loads.

PAD FOUNDATION

b) Rectangular Spread Footings - Useful when obstructions prevent construction of a square footing with a sufficiently large base area and when large moment loads are present

c) Circular Spread Footings - are round in plan view - most frequently used as foundation for light standards, flagpoles and power transmission lines.

d)Continuous Spread Footings / Strip Foundation

- Used to support bearing walls

e) Combined Footings - support more than one column - useful when columns are located too close together for each to have its own footing

f) Ring Spread Footings - continuous footings that have been wrapped into

a circle - commonly used to support the walls above-

ground circular storage tanks. - The contents of these tanks are spread evenly across the total base area and this weight is

probably greater that the tank itself - Therefore the geotechnical analyses of tanks

usually treat them as circular foundations with diameters equal to the diameter of the tank.

Ring Spread Footings

RAFT FOUNDATION A foundation system in which essentially the entire building is placed on a large

continuous footing. It is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls.Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure.

MAT/RAFT FOUNDATION

It is normally consists of a concrete slab which extends over the entire loaded

area. It may be stiffened by ribs or beams incorporated into the foundation. Raft foundations have the advantage of

reducing differential settlements as the concrete slab resists differential movements between loading positions.

They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

Mat Foundation often considered to be

used when dealing with the following

conditions:a) The structural loads are so high or the soil

condition so poor that spread footings would be exceptionally large. As a general rule of thumb, if spread footings would cover more than 50% of the building footprint area, a mat or some type of deep foundation will usually be more economical.

b) The soil is very erratic & prone to excessive differential settlements. The structure continuity and flexural strength of a mat will bridge over these irregularities.

The same is true of mats on highly expansive soils prone to differential heaves.

c) The structural loads are erratic and thus increase the likelihood of excessive differential settlements. Again, the structural continuity and flexural strength of the mat will absorb these irregularities.

d) The lateral loads are not uniformly distributed through the structure and thus may cause differential horizontal movements in spread footings and pile caps.

The continuity of a mat will resist such movement.

e)The uplift loads are larger than spread footings can accommodate. The greater weight and continuity of a mat may provide sufficient resistance.

f) The bottom of the structure is located below the groundwater table, so waterproofing is an important concern. Because mats are monolithic, they are much easier to waterproof. The weight of the mat also helps resist hydrostatic uplift forces from the groundwater.

Differing ground conditions, proximity of trees, backfilled land, soil types, proximity of drains, wind speeds,  all dictate the form our foundation work will take.

FOUNDATION SYSTEMS CAN BE CATOGORIZED IN BASIC 3 CATEGORIES ACCORDING TO THE GEOMETRIC ANALYSIS OF POINT, LINE AND PLANE.

POINT SYSTEM:- ISOLATED FOUNDATION

LINE SYSTEM:- STRIP FOUNDATION

PLANE SYSTEM:- RAFT FOUNDATION THIS CHOICE ALSO EFFECT THE POTENTIAL FORM OF THE

SUPERSTRUCTURE. THIS IS TO UNDERSTAND THE INNOVATIONS IN

FOUNDATION SYSTEMS.

CLASSIFICATION OF FOUNDATION SYSTEMCLASSIFICATION OF FOUNDATION SYSTEM

ISOLATED FOOTINGCOMBINED FOOTINGECCENTRIC FOOTINGCANTILEVER FOOTING

ISOLATED FOOTINGALSO KNOWN AS COLUMN FOOTING. IN CASE OF HEAVY LOADED COLUMNS, STEEL REINFORCEMENT

IS PROVIDED IN BOTH THE DIRECTIONS IN CONCRETE BED.THE FOOTING IS REINFORCED BOTH WAYS BY MEANS OF MILD

STEEL RIBBED BARS PLACED AT RIGHT ANGLES TO ONE ANOTHER AT EQUAL DISTANCES APART.

GENERALLY, 15cm OFFSET IS PROVIDED ON ALL SIDES OF CONCRETE BED.

8 CM THICK LAYER OF PLAIN CEMENT CONCRETE IS 8 CM THICK LAYER OF PLAIN CEMENT CONCRETE IS PROVIDED BEFORE REINFORCED CONCRETE FOOTING IS PROVIDED BEFORE REINFORCED CONCRETE FOOTING IS CONSTRUCTED OVER IT.CONSTRUCTED OVER IT.

AREAS OF USAGEAREAS OF USAGE IF A MASONARY WALL IS SUBJECTED TO A VERY HEAVY IF A MASONARY WALL IS SUBJECTED TO A VERY HEAVY

LOAD & BEARING CAPACITY OF THE SOIL ON WHICH FOOTING LOAD & BEARING CAPACITY OF THE SOIL ON WHICH FOOTING IS TO REST IS VERY LOW, RCC FOOTING MAY BE PROVIDED.IS TO REST IS VERY LOW, RCC FOOTING MAY BE PROVIDED.

SECTION

ISOLATED FOOTING

COMBINED FOOTING

A COMBINED FOOTING SUPPORTS TWO OR MORE COLUMNS IN A ROW.

TYPES OF COMBINED FOOTING RECTANGULAR- USED WHERE BOTH COLUMNS CARRY

EQUAL LOADS. TRAPEZOIDAL- USED WHERE BOTH COLUMNS CARRY

UNEQUAL LOADS AS WELL AS THERE ARE SPACE LIMITATIONS.

AREA OF USE: WHEN THE BASE OF THE COLUMN IS VERY NEAR TO THE SITE

BOUNDARY AND CANNOT SPREAD UNIFORMLY AROUND. WHEN FOUNDATIONS OF ADJACENT COLUMNS ARE LINKED TO RESIST UPLIFT,OVERTURNING OR OPPOSING FORCES. AS A SUBSTITUTE TO ECCENTRIC FOOTING.

IN THE DESIGN OF FOOTINGS, THE FOOTING IS ASSUMED TO BE RIGID AND RESTING ON A HOMOGENEOUS SOIL.

AIMTHE LOCATION OF CENTRE OF GRAVITY OF COLUMN LOADS AND

THE CENTROID OF FOOTING SHOULD COINCIDE SO THAT UNIFORM PRESSURE DISTRIBUTION TOOK PLACE.

• THE MAXIMUM BENDING MOMENT IS TAKEN AS THE DESIGN VALUE FOR THE RCC FOOTING & SHOULD BE CHECKED FOR MAXIMUM SHEAR AND BOND ETC.

COMBINED FOOTING

ECCENTRIC FOOTING

(1) THE FOUNDATION MAY BE SUBJECTED TO ECCENTRIC LOAD UNDER THE FOLOWING CIRCUMSTANCES;

(A) WHEN A WALL OR COLUMN IS SITUATED AT THE PROPERTY LINE, ITS FOUNDATION CANNOT BE EXTANDED BEYOND THE PROPERETY LINE THUS RESULTING ECCENTRIC LOAD ON THE FOUNDATION

(B) CANTILIVER BALCONIES, BRACKETS, ETC ATTACHED TO THE WALLS OR COLUMNS BENDING MOMENT IN THEM AND THUS CAUSE ECCENTRIC LOADING ON THE FOUNDATION

ECCENTRIC LOADS CAUSE UNEVENDISTRIBUTION OF PRESSURE ON THE SOIL BELOW THE FOUNDATION. IF ECCENTRICITY IS SMALL,ALTHOUGH DITRIBUTION OF THE SOIL WILL BE VARYIN, BUT IT WILL REMAIN COMPRESSIVE. IN THIS CASE, IT SHOULD BE ENSURED THAT THE

MAXIMUM INTENSITY OF PRESSURE SHOULD NOT BE MORE THEN THE SAFE BEARING CAPACITY OF SOIL. IF ECCENTRICITY OF THE LOAD IS QUITE LARGE, IT IS JUST POSSIBLE THAT WALL OR COLUMN MAY EVEN OVERTURN. FOLLOWING ARE SOME OF THE METHODS WHICH CAN BE USED TO ENSURE THE STABLITY OF THE WALL OR

COLUMN WITHOUT ENCROACHING THE AREA BEYOND THE LIMIT OF PROPERTY LINE.

(2) OFFSETTING THE FOOTING- IN THE CASE OF THE BOUNDARY WALL, FOOTING IS PROVIDED WITH OFFSET ON THE INSIDE OF THE WALL ONLY, WHEREAS THE OUTER FACE IS MAINTAINED FLUSH WITH THE BOUNDARY LINE, RIGHT UPTO THE BOTTOM OF THE FOUNDATION. THE LINE OF ACTION OF LOAD WILL BE AWAY FROM THE CENTRE OF THE FOOTING.

(3) THE PRESSURE DISTRIBUTION IN THIS CASE WILL BE SUCH THAT MAXIMUM PRESSURE INTENSITY WILL BE AT THE OUTER FACE A OF THE FOOTING AND MINIMUM AT THE INNER FACE B OF THE FOOTING.

(4) MASONRY STRUCTURE ARE NEVER ALLOWED TO DEVELOP TENTION AND AS SUCH, THE WIDTH OF FOOTING IS SO

PROPORTIONED THAT THE C.G. OF THE LOAD REMAINS WITHIN MIDDLE OF THE BASE OF THE WALL FOOTING SO THAT NO TENTION IS DEVELOPED ANYWHERE IN THE FOUNDATION. IT IS ALSO SEEN BY SIDE THAT INTENSITY OF PRESSURE AT POINT A DOES NOT EXCEED THE SAFE PERMISSIBLE VALUE OF THE SOIL.

(5) BY PROVIDING STRAP FOOTINGS. THE OFFSETTING METHOD DESCRIBED ABOVE IS APPLICABLE ONLY IN THE CASE OF HARD SOILS. THIS METHOD IS NOT SUTIABLE WHEN FOUNDATION SOIL IS SOFT AND LOAD TO BE TRANSMITTED IS HEAVY AS INTENSITY OF MAXIMUM PRESSURE DEVELOPED BELOW THE FOUNDATION WILL BE EXCEEDED, THE SAFE BEARING CAPACITY OF THE SOFT UNDERLYING SOIL. UNDER SUCH CIRCUMSTANCES STRAP BEAM FOOTING MAY BE ADOPTED.

IN THIS CASE,LOAD ON THE OUTERCOLUMN SITUATED AT BOUNDARY LINE IS BALANCED BY THE LOAD ON THEINNER COLUMN THROUGH A CANTILEVER BEAM ACTING ABOUT A FULCRUM.

THIS TYPE OF FOOTING CAN BE CONTRUCTION IN R.C.C. OR STEEL GRILLAGE. IF INTERIOR COLUMN CANNOT BE PROVIDED, A HEAVY CONCRERE BLOCK MAY BE CONSTRUCTED TO NECESSARY LEVERAGE TO SUPPORT THE LOAD ON THE OUTER WALL.

CANTILEVER FOOTINGUSED WHERE THE DISTANCE BETWEEN THE COLUMNS IS

SO GREAT THAT A COMBINED TRAPEZOIDAL FOOTING BECOMES QUITE NORROW WITH HIGH BENDING MOMENT

THE STRAP BEAM DOES NOT REMAIN IN CONTACT WITH THE SOIL;SO A STRAP DOES NOT TRANSFER ANY PRESSURE TO SOIL HOWEVER,BECAUSE THE STRAP IS INFINITELY STIFF,HENCE IT SERVES TO TRANSFER COLUMN LOADS ON TO THE SOIL WITH EQUAL AND UNIFORM SOIL PRESSURE UNDER BOTH FOOTING

CANTILEVER\STRAP FOOTINGCANTILEVER\STRAP FOOTING

TYPES OF FOUNDATION FAILURES : OVERSTRESSING OF SOIL: IF THE SOIL IS OVERSTRESSED, IT

MAY LEAD TO A SHEAR FAILURE RESULTING IN THE SLIDING OF THE SOIL ALONG A PLANE OF RUPTURE AND THUS RESULT IN THE COLLAPSE OF THE STRUCTURE.

TILTING OF STRUCTURE: IF THE FOUNDATION AREA OF A STRUCTURE IS SUCH THAT THE CENTRE OF GRAVITY OF THE LOADS DOES NOT COINCIDE (IN PLAN) WITH THE CENTRE OF GRAVITY OF THE FOUNDATION AREA THE CONSEQUENT BEARING ACTION WILL BE NON UNIFORM. AT THE EDGE CLOSER TO THE CENTRE OF THE GRAVITY OF THE LOADS THE PRESSURE INTENSITY WILL BE HIGHER RESULTING IN A GREATER SETTLEMENT OF THE SOIL AT THIS EDGE.

SETTLEMENT: TO ENSURE UNIFORM SETTLEMENT,IT IS NECESSARY THAT THE FOUNDATION AREA IS SO PROVIDED, THE INTENSITY OF SOIL REACTION IS THE SAME UNDER ALL THE FOOTINGS OF A STRUCTURE.

OVERTURNING OF SUPERSTRUCTURE

SLIDING OF SUPERSTRUCTURE :