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as per IS 456-2000

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- 1. by S.Praveenkumar Assistant Professor Department of Civil Engineering PSG College of Technology Coimbatore Design of Footings
- 2. IntroductionIntroduction ► The ultimate support for any structure is provided by the underlyingThe ultimate support for any structure is provided by the underlying earth or soil material and, therefore, the stability of the structureearth or soil material and, therefore, the stability of the structure depends on it.depends on it. ► Since soil is usually much weaker than other common materials ofSince soil is usually much weaker than other common materials of construction, such as steel and concrete, a greater area or volume ofconstruction, such as steel and concrete, a greater area or volume of soil is necessarily involved in order to satisfactorily carry a givensoil is necessarily involved in order to satisfactorily carry a given loading.loading. ► Thus, in order to impart the loads carried by structural members ofThus, in order to impart the loads carried by structural members of steel or concrete to soil, a load transfer device is necessary.steel or concrete to soil, a load transfer device is necessary. ► TheThe structural foundationstructural foundation serves the purpose of such a device.serves the purpose of such a device. ► A foundation is supposed to transmit the structural loading to theA foundation is supposed to transmit the structural loading to the supporting soil in such a way that the soil is not overstressed and thatsupporting soil in such a way that the soil is not overstressed and that serious settlements of the structure are not caused.serious settlements of the structure are not caused. 2
- 3. ► The type of foundation utilized is closely related to the properties of theThe type of foundation utilized is closely related to the properties of the supporting soil, since the performance of the foundation is based onsupporting soil, since the performance of the foundation is based on that of the soil, in addition to its own.that of the soil, in addition to its own. ► Thus, it is important to recognize that it is the soil-foundation systemThus, it is important to recognize that it is the soil-foundation system that provides support for the structure; the components of this systemthat provides support for the structure; the components of this system should not be viewed separately.should not be viewed separately. ► The foundation is an element that is built and installed, while the soil isThe foundation is an element that is built and installed, while the soil is the natural earth material which exists at the site.the natural earth material which exists at the site. 3
- 4. General types of foundationsGeneral types of foundations The various types of structural foundations may be grouped into twoThe various types of structural foundations may be grouped into two broad categoriesbroad categories 1) Shallow foundations1) Shallow foundations 2) Deep foundations.2) Deep foundations. 4
- 5. Further classification of shallow foundations and deep foundations is asFurther classification of shallow foundations and deep foundations is as follows:follows: 5
- 6. Spread footingsSpread footings 6 Spread footing foundation is basically a pad used to ‘‘spread out’’ loads from walls or columns over a sufficiently large area of foundation soil
- 7. Strap FootingsStrap Footings 7 A ‘strap footing’ comprises two or more footings connected by a beam called ‘strap’. This is also called a ‘cantilever footing’ or ‘pump-handle foundation’
- 8. Combined footingsCombined footings 8 A combined footing supports two or more columns in a row when the areas required for individual footings are such that they come very near each other.
- 9. Raft FoundationsRaft Foundations 9 A raft or mat foundation is a large footing, usually supporting walls as well as several columns in two or more rows. This is adopted when individual column footings would tend to be too close or tend to overlap; further, this is considered suitable when differential settlements arising out of footings on weak soils are to be minimized.
- 11. IS 456 Provisions for Design of FootingsIS 456 Provisions for Design of Footings Design Loads for Foundation DesignDesign Loads for Foundation Design 1.Loads for determination of size of foundation1.Loads for determination of size of foundation 2.Loads for limit state design of foundation2.Loads for limit state design of foundation 11
- 12. In multi-storeyed buildings, one should also consider the allowableIn multi-storeyed buildings, one should also consider the allowable reduction in live load for residential and office buildings.reduction in live load for residential and office buildings. 12
- 13. Design Centrally loaded Isolated footingsDesign Centrally loaded Isolated footings The major steps in the design of column footing square or rectangular canThe major steps in the design of column footing square or rectangular can be summarized as follows.be summarized as follows. Step-1-Step-1- Determine the plan area from the allowable nearing capacity andDetermine the plan area from the allowable nearing capacity and service loads from the column, assuming a reasonable (10% of loading)service loads from the column, assuming a reasonable (10% of loading) as weight for the footing.as weight for the footing. Step-2-Step-2- Taking the factored dead and live loads, determine the ultimateTaking the factored dead and live loads, determine the ultimate soil reaction for the factored design load.soil reaction for the factored design load. Step-3-Step-3- Determine the depth for one-way shear, assuming a design shearDetermine the depth for one-way shear, assuming a design shear strength valuestrength value ττcc .Theoretically, this value depends on the percentage.Theoretically, this value depends on the percentage of steel in slab. However, for preliminary design, a value ofof steel in slab. However, for preliminary design, a value of ττcc =0.35=0.35 N/mmN/mm22 corresponding to 0.25% steel may be assumed.corresponding to 0.25% steel may be assumed. Step-4-Step-4- Determine the Depth from bending Considerations.Determine the Depth from bending Considerations. 13
- 14. Step-5-Step-5- Check the depth adopted for safety against punching shear. If it isCheck the depth adopted for safety against punching shear. If it is not sufficient increase the depth so that the footing is safe in punchingnot sufficient increase the depth so that the footing is safe in punching shear.shear. Step-6-Step-6- Choose the largest depth required considering steps 3, 4, and 5Choose the largest depth required considering steps 3, 4, and 5 and provide the necessary cover.and provide the necessary cover. Step-7-Step-7- Calculate the reinforcement required in the X and Y directionsCalculate the reinforcement required in the X and Y directions from the bending moment considerations. The steel provided at thefrom the bending moment considerations. The steel provided at the section for maximum moment should not be less than the minimumsection for maximum moment should not be less than the minimum specified for slabs.specified for slabs. Step-8-Step-8- Check the Development length required and choose the properCheck the Development length required and choose the proper diameter of bars.diameter of bars. Step-9-Step-9- Detail the steel as specified in IS.Detail the steel as specified in IS. Step-10-Step-10-Provide the necessary cover to reinforcement and find the totalProvide the necessary cover to reinforcement and find the total depth of footing required.depth of footing required. Step-11-Step-11- Verify the design by chartsVerify the design by charts 14
- 15. Wall FootingsWall Footings ► Wall footings are continuous strip footings under masonry walls.Wall footings are continuous strip footings under masonry walls. ► In ordinary low rise buildings, we use masonry walls to carry theIn ordinary low rise buildings, we use masonry walls to carry the superstructure loads to the ground.superstructure loads to the ground. ► The types of foundations generally used in them are the following:The types of foundations generally used in them are the following: Simple plain concrete continuous strip wall footingSimple plain concrete continuous strip wall footing Reinforced concrete continuous strip wall footingReinforced concrete continuous strip wall footing Reinforced concrete continuous T or U beam wall foundations( whenReinforced concrete continuous T or U beam wall foundations( when the beam is upstanding, it is called a T-beam and when it is downthe beam is upstanding, it is called a T-beam and when it is down standing, it is called a U-Beam)standing, it is called a U-Beam) 15
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- 17. Design of Isolated Footings with Vertical LoadsDesign of Isolated Footings with Vertical Loads and Momentsand Moments 17
- 18. 18 Case 1
- 19. 19 Case 2
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- 22. 22 Case 3
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- 24. Combined Footings for Two ColumnsCombined Footings for Two Columns ►The term combined footings is usually used for one footingThe term combined footings is usually used for one footing which is designed to carry the loads of two separate columns.which is designed to carry the loads of two separate columns. ►When three or more column loads in one line are supported onWhen three or more column loads in one line are supported on one long single footing, it is called a column strip footing.one long single footing, it is called a column strip footing. ►Rafts and mats support many columns located both in x and yRafts and mats support many columns located both in x and y –axes.–axes. 24
- 25. Types of combined footingsTypes of combined footings ►In general, we can have the following three types of combinedIn general, we can have the following three types of combined footings.footings. Combined slab footing with a continuous slab bond.Combined slab footing with a continuous slab bond. Combined longitudinal beam and slab footing with continuous slab baseCombined longitudinal beam and slab footing with continuous slab base and as longitudinal strap or spine beam to the ends of the slaband as longitudinal strap or spine beam to the ends of the slab connecting the two columns.connecting the two columns. Combined transverse beam and slab footing with continuous slab baseCombined transverse beam and slab footing with continuous slab base and separate transverse beam under columns.and separate transverse beam under columns. 25
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- 28. 28 Thank You !!Thank You !!