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Basic Principles of Design of RCC Structures

By

Sri. N. Krishnam Raju, Adv. A.P.H.B

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LIMIT STATES DESIGN OF R.C. STRUCTURESINTRODUCTION

• Purpose Of Structural Design: The purpose of structural design is providing a safe structure complying with the user’s requirements. The design should evolve a structural solution for safety and serviceability throughout the design life, which gives the greatest overall economy for the first cost end for maintenance costs.

• Limit states: Limit states are concerned with structural safety and serviceability and cover all forms of failure. A structure could be rendered unfit for use in many ways and these factors are conveniently grouped into three major categories.– Ultimate limit states: collapse of the structure due to normal or

exceptional loadings or the occurrence of exceptional events like earthquake etc.

– Serviceability limit states: Deflection, cracking and vibration.– Other limit states: Fatigue, Durability, Fire resistance, Lightning etc.

It is often possible that a given structure is required to satisfy one or more limit states simultaneously.

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• Deflection limit: The designer must therefore ensure that though the structural element is safe and strong, the deflection is not excessive. This limit state usually controls the depth of the section.

These span/effective depth ratios are to be modified depending on the amount tension steel and compression steel used in the section. If more tensions steel is used than a certain amount, the neutral axis depth increase and more concrete comes under compression causing more shrinkage and creep deflection. Further providing more tension steel would require more effective depth. The provision of the compression steel reduces the neutral axis depth and hence reduces the effective depth of the Beam. The effect of percentage of tension reinforcement and compression reinforcement are shown in table3 and table 4.

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WORKING STREE METHODS

Where the limit state method cannot be adopted, working stress method may be used.

Assumption for design of Members:

Based on elastic theory, the following assumption shall be made. At any cross section, plan section before bending remain plane after

bending. All tensile stresses are taken up by reinforcement and none by concrete

except as otherwise permitted. The stress strain relationship of steel and concrete, under working loads is a

straight line. The modular ratio m has the value 280/36cbc where 60bc is permissible

compressive stress due to bending in concrete in N/mm2

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LIMIT STATES METHODS

• The acceptable limit for the safety and serviceability requirements before failure occurs.

• The aim of design is to achieve acceptable, probabilities that the structure will not become unfit for the use for it is intended.

• Ensure an adequate degree of safety and serviceability

• Design should be based on characteristic values for material strengths and applied loads.

Term ‘characteristic loads’ means that value of load which has a 95% probability of not being exceeding during the life of structure.

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Design Values

Materials – fd=f/rm

fd=Design strength of materials

f=characteristic strength of material

rm=Partial safety factor appropriate to material and the limit state being considered.

Loads

Design Load Fd=F rf

F=Characteristic load

rf=partial safety factors to nature of loads.

Partial safety factors

rm - 1.5 for concrete

rm - 1.15 for steel

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LIMIT STATE OF COLLAPSE

Assumptions: Design of the limit state of collapse in flexure shall be based on Plane section normal to the axis remain plane after bending. The maximum strain in concrete at the outer most compression fibre is

taken as 0.00035 in bending. For design purpose, the compression strength of concrete in structure shall

be assumed as 0.67 times the characterisistc strength. The partial safety factor rm=1.5 shall be applied in addition to this.

The tensile strength of the concrete is ignored. Stresses in the reinforcement are derived from respective stress strain

curve for the type of steel used.

For design purpose partial safety factors rm= 1.15 shall be applied.

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BUILDING MAINTENANCE, COMMON DEFECTS AND REMEDIAL METHODS

Maintenance plays a vital role in the execution of buildings. Very often difficult problems are encountered in the maintenance of building than in original work.

• Every aspect of maintenance has to be carefully thought out in its entirety aiming at over all sound ness of structure in all the seasons of the year. Most buildings may develop cracks usually soon after construction and sometimes later. Much of the early cracking is superficial, can be easily repaired.

• Several factors contribute in producing defects. Before repairs or remedies are sought, one needs to know the causes of cracking and its effects on the performance of the buildings.

• Timely action in mitigating the distress phenomena through repair and rehabilitation is essential for sustaining performance of such structures. Concrete is basically meant to last for ever without any major repairs and maintenance. However deleterious agents in the environment itself often leads to premature deterioration of concrete structures.

• Cracks in buildings are common occurrence. A building component develop cracks whenever stress in the component exceeds its strength.

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5. Durability can be achieved by proper maintenance. Therefore maintenance is equally important as design and construction stages. But, maintenance is always given a least importance. The importance given to planning and execution of project is missing in maintenance activities. The more efficient maintenance results in increase in life of structure and creates good image of the society. The various problems in maintenance are occurring due to inefficient design/planning and bad quality of construction. The designer shall use the best quality of materials by which reduce maintenance problems. Most of the problems in maintenance are repetitive type and directly affect the durability of structure. Some of the problem are seepage/leakage, spalling of concrete and corrosions of steel.

6. Principal causes of occurrence of cracks1. Forces like Dead, Live, Wind, Seismic etc.2. Foundation settlement3. Moisture changes4. Thermal variation5. Chemical reaction etc6. Poor workman ship

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Main Common Defects: 1. Foundations2. Walls3. Concrete/RCC Frame

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1. Foundation: a) Engineers need to know the character and magnitude of forces in order to design and

construct structures.b) One has to study the system of soil below the earth surface at various levels under ground

depending upon the past experience.c) Repairs to foundations are expensive. Structures should be founded as stable soils.d) Certain soil deposits wherein wetting of the soil beyond a stress level causes steep

reduction in stiffness resulting from disruption of soil structure.e) Subject to rate of loading, disruption in soil structures takes place at a faster pace than the

development of new structural bonds which leads to vertical deformation at locations of higher stress due to disturbance of soil structures.

f) Problems associated with foundation in clay soil are well known. Swelling clays create large uplift forces on the peripheral wall during rainy season. A reverse situation may arise at region of moderate rainfall when the central region of a building founded an clay soil is prone to swelling during dry spells.

a) Differential settlement due to unconsolidated fill.b) Differential settlement due to uplift of shrinkage soil, shrink and expand with changes in

moisture content. Vertical and diagonal cracks are noticed in external walls.

g) The problems of dampness in building requires a systematic approach to determine the causes of leakage, the source from which are likely to prove effective.

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2. WallsWalls are constructed using a variety of materials such as mud, stone, clay bricks, concrete blocks, Fal-G Bricks etc. Common burnt clay bricks as per IS 1075-1951, Bricks shall be hand or machine molded classifying Class1, Class2 Bricks maintaining characteristics like water absorption to 20% and Efflorescence slight.

1. Although the walls are built of reasonably non- porous bricks, the mortar itself is relatively porous and so rain water penetrate into to the mortar and will be finally sucked up on the inside surface causing discolouration and dampness. The moisture which was absorbed by the wall tries to escape by break through plaster, which otherwise reduces the strength of materials in the wall. Porous mortar than water tight mortar for plaster is advisable.

2. Faulty joints are common cause of entry by rain so that if bricks are adequate for their purposes, pointing needs to be examined and mortar replaced.

3. Number of causes of failures of brick wall have been reported. High intensity wind causes masonry walls to collapse due to their in adequate lateral restraint. Quality of bricks workman ship. Spacing of pilasters, size of wall panels etc. Influence the lateral resistance of the walls structure.

4. Generally walls constructed with RC columns with in filled brick walls have performed better during cyclones.

5. Failures of brick masonry walls can be avoided by suitable choice of panel size which in term would depend on the tensile strength of brick and quality/workman ship. It is advisable for provide a continuous RC bond beam on top.

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6. Brick work may become cracked especially at door and window opening as a result of excessive drying shrinkage. Rich cement mortar rendering, fail because they shrink and crack. The familiar map pattern cracking is typical of drying shrinkage in renderings.

7. Cement based mortars may be attached by sulphates derived from clay bricks themselves. Some times from external sources such as sulphates bearing soils or flue gases. The attack is gradual and occurs when the brick work remain wet for long periods, which produces various forms cracking and deformation of bricks.

8. Junction of the concrete lintels and masonry walls and junction of RCC. Sun shades and walls are vulnerable places for the penetration of moisture, as these two different materials always give rise to their cracks at the junctions, water dripping on the wall surface also causes dampness.

9. Finished surface of roof should have a slope of 1 in 80.10. Special attention should be paid to junction of roofs and parapets, outlets to drain out to rain

water to be properly executed. Every 200 sft of roof areas should be provided with one outlet.

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3 Concrete and RCC itemsThe common problems are

1. Seepage/leakage in buildings and their controlling methods: Excessive dampness in buildings is one of the major problems in recent years. If such seepage/leakage is allowed to continue unchecked, unhygienic conditions will prevail and also the building may deteriorate to the extent that ultimately it becomes uninhabitable. The source of seepage/leakage can be rain water, leakage in pipe lines condensation or ground water.Causes of seepage in building:Seepage mainly occurs from walls and roof ceiling in buildings.a) The causes of seepage/leakage through the roof are:

1. Lack of proper slope thereby causing stagnation of water.2. Lack of proper drainage system3. Lack of goals, coping etc.4. Poor maintenance of pipe connection and joints.5. Poor quality of construction.

b) Causes of seepage/leakage through the wall are1. non provision of damp proof course.2. lack of plinth protection3. lack of chajja, facia over openings4. poor orientation and wind direction5. lack of stone cladding/water proof plastering and painting.

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Seepage controlling methods:Water proffing treatment is necessary especially for areas like, water tanks, sunken slabs, roofs, terrace gardens, foundations, planters, service floors, etc., As a preventive measure in recent years a number of water proofing treatment methods are being used by making use of different water proofing materials.1. mud phuska with proofing materials.2. multi layer asphalt treatment.3. brick coba treatment.4. chemical injection treatment.5. polymer modified bitumen based treatment6. glass fibre tissue based treatment (7 course)7. lime based treatment

There are different water proofing methods available for pre and post construction stages of buildings. By good design/planning constructions and maintenance, the problem of seepage in buildings can be minimized.

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Spalling of concrete:This is a common problem being faced by the maintenance engineer. Spalling of concrete causes in convenience, shabby look and more affects the durability of structure.

Some of the reasons for spalling of concrete are as follows:1. Defective design.2. Improper diameter of reinforcement bars.3. Use of substandard materials.4. Poor quality of construction.5. High water cement ratio.6. Seepage/leakage.7. Inadequate cover to reinforcement bars.8. Corrosion of steel.9. Lack of water proofing treatment in areas like terrace, sunken slab, basement.10.Lack of external treatement fro exposed concrete sufraces.11.Environmental conditions12.Neglected maintenance.

Large number of destructive and non destructive tests are available to assess its state of concrete and techniques are also available to combat various deteriorating causes.

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For repairing such affected areas different materials like cement, polymer, epoxy materials, polymer modified bitumen are being used. Steps to be taken for repairing the affected areas as:1. Remove all loose materials.2. Clean the areas with compressed air.3. Remove rust from reinforcement4. Apply anticorrosive paint.5. Apply cement/resin/polymer based mortar

Corrosion of Steel:Corrosion of steel reinforcement in concrete structure is a common phenomenon which require utmost attention. This occurs because of inefficient design/ drafting and poor quality construction.

To avoid corrosion of reinforcement, special care has to be taken regarding the following.1. Design mix2. Water cement ratio3. Garding of concrete4. Cement content.5. Quality cement, aggregate, water6. Covert to reinforcement.7. Compaction, admixtures.8. Treatment to exposed surfaces.9. Environmental conditions.

Therefore, it is suggested that the dampness which is the main cause for corrosion should be avoided by good design and quality construction to achieve dense concrete.

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Scope of Investigations/ assessment of structural damage decision of Restoration.

1. To assess the extent of structural damage to RCC elements of the building2. To arrive at the residual strength of concrete and reinforcing steel.3. Report covering the above aspects.

1. Debris insepction2. Visual inspection of affected members.3. Institution field testing.4. Lab test5. Damage classification of structural member.

Visual: 1. Surface appearance. a. Condition of plaster/finish

b. Colourc. Crazing

2. Structural condition.a. Spalling.b. Exposure and condition of main reinforcement.c. Cracksd. Distortione. Construction joint, honey combing, delimitation

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A) Condition of plaster and finish:RC Members rendered with cement mortar which in general (1:3) may be cladded with other materials (wood/marble etc.) condition of finishs are categorized as 1) unaffected 2) peeling 3) substantial loss 4) total loss.

B) Colour of concrete may change as a result of heat due to fire.C) Crazing: Development of fine cracks on surface of concrete due to sudden cooling of surface

with water is termed as crazing.D) Spalling of concrete:E) CracksF) Distortion in the form of deformation (deflection, twisting)G) Honey combing/construction joints: due to original construction defects.

Delamination of concrete means that a layer of some part of concrete has separated out from the parent body but still not fallen out, Hallow surroundings etc.

Remedial Measures

Hammer test, Core test compressive strength estimation. Based on the severity of the damage of the structural members, different types of repairs methods are to be adopted to restore their structural integrity.

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Class-I Superficial For repair, use cement mortar trowelling using cement slurry bonding.

Class-II General Minor structural repairs like restoring cover to reinforcement using cement based polymer,modified mortar polymer slurry as bondinglayer and nominal light. Fabric mesh or usingepoxy mortar over primary coat of epoxy primer.

Class-III Principal Repair Where concrete strength is significally reduced strengthing to be carried out with shot creting. In case of slabs and beam, andJacketing incase of columns. Bonding material shall be epoxy formulation, additional reinforcement shall be providedin accordance with load carrying requirementof member.

Class-IV Major repair Demolition and recasitng.

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BUILDING MAINTENANCE, COMMON DEFECTS AND REMEDIAL METHODS

1. Generally buildings are constructed in two categories. Framed structure usually built with column and beam and with one brick thick wall and half brick walls for above two or more floor structures.

2. Especially in cyclone prone areas RCC frame with evaluation of a geometric layout consistent with functional utility and the site dimension is designed with high wind speed to mitigate any eventualities in future.

3. It has been the constant endeavor of structural Engineer to improve the concepts of analysis and design so that an economical structure is obtained with safety and serviceability. Introduction of high strength steel has helped in achieving considerable economy and reducing the cost of construction.

The design of a structure presents two fold problema) It has to be so constructed that it serves the need efficiently for

which it was intended (Functional Design).b) It has to be strong enough to resist the loads and forces to which it

is subjected during its service (Structural Design)

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The structural design consists of planning the frame work of the structure to meet the above needs and to carry the loads economically with a design life suited to the services in view.

The important aspects in the structural design are

a. to determine the loads forces which the frame work will be required to support.

b. Selection of a suitable structural arrangements and materials of construction.

c. Analyzing the internal stresses in the frame work.d. Proportioning the members of the frame work to resist safely and

economically the internal stresses produced.

A structure may be subjected to (1) Dead Loads (2) Live Loads (3) Wind Loads (4) Seismic forces. For the sake of standardization and legal binding on all question of properties and working stresses for various materials are covered by standard specifications. For the design of building in concrete, steel, masonry, basic considerabations are followed referred to :

(1) I.S.Code 456-2002 – Code of practice for plain & RCC structures.(2) I.S.Code 800-1984 – Structural steel in building construction.(3) I.S.Code 875-1984 – Code of practice fro Live loads and Wind Loads.(4) I.S.Code 1893-1984- Criteria for earthquake resistant design of structures.(5) I.S.Code 4326-1976- Code of practice for Earthquack resistant design &

construction of building

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(6) I.S.Code 1904-1986 – Code of practice for design and construction of foundation in slab.

(7) I.S.Code 1905-1980 – Code of practice for masonary walls.(8) I.S.Code 1786 – High strength deformed bars and Fe 415 grade.(9) I.S. Code 269/8112/12269 – Code of Cement Grades(10) I.S. 9103 – Code of Practice for Super Plasticizers(11) I.S. 14687 – Formwork (12) I.S. 2502 - Assembly of Reinforcement(13) I. S. 10262 – Design of Mix(14) I.S.383 – Coarse and Fine Aggregates(15) I.S. 13920 – Ductility Detailing

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Engineers have been designing the structures primarily on strength and behavior considerations. Durability and life expiatory of a structure depends upon quality of basic materials used in the construction, such as Water, Cement aggregate and admixtures and methods of construction. The designer and builder should ensure that right type of materials are used which can withstand loads and environmental forces and other exposure conditions. There is no substitute for good concrete. BIS has also recommended availability successful use of super plasticisers in improving the workability without increasing the w/c ratio in strength of concrete.

Mix Proporation - Shall be selected to ensure the workability of the fresh concrete and when concrete is hardened, it shall have the required strength, durability and surface finish. (1) Design Mix(2) Nominal Mix. Design Mix concrete is preferred to nominal Mix

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GENERAL DESIGN CONSIDERATION

1. Aim of Design - Aim of design is to provide a safe and economic structure complying to the users requirement.

2. Methods of Design- Structure and structural elements shall normally be design by Limit state method. Calculations alone do not produce safe, serviceable and durable structures. Suitable materials, quality control, adequate detailing and good supervision are equally important.

3. Durability, workmanship and materials- It is assumed that the quality of concrete, steel and other materials and of the workmanship, as verified by inspections is adequate for safety, serviceability and durability.

4. Design process- Design including design for durability, construction and use in service should be considered as a whole. The realization of design objectives requires compliance with clearly defined standards for materials, production, workmanship and also maintenance and use of structure in service.

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LOADS AND FORCESIn structural design, account shall be taken of the dead, imposed and wind loads and forces such as these caused by earthquake, and effects due to shrinkage, creep temperature etc., where applicable.

Dead loads shall be calculated on the basis of unit weights specified as per IS code 1911.

Imposed load, wind loads and snow loads shall be assumed in accordance with IS 875 (2), (3), (4) respectively.

Earthquake forces shall be calculated in accordance with IS 1893.Shrinkage, creep and temperature effects shall be considered as per IS code 875

part (5).Analysis – All structures may be analysed by the linear elastic theory to calculate

internal actions produced by design loads. In liew of rigorous elastic analysis simplified analysis as given in 22.4 & 22.5 of IS 456 may be adopted. With the aid of computers using STAAD PRO evaluation of analysis and design of members has become simple.

Structural Frames- Simplyfying assumption may be used in the analysis of frames.a. Consideration may be limited to combinations of 1. Design dead load on all spans with full design imposed load on two adjacent

spans and2. Design dead load on all spans with full design imposed load on alternate span

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b. When design imposed load does not exceed three fourth of the design load, the load arrangement may be design dead load and design imposed load on all the spans.

Substitute Frame- For determining the moments and shears at any floor or roof level due to gravity loads, the beams at that level together with columns above and below with their far ends fixed may be considered to constitute the frame.Where side sway consideration become critical due to unsymmetrical in geometry or loading, rigorous analysis may be required.For lateral loads, simplified methods may be used to obtain the moments and shears for structures that are symmetrical. For unsymmetrical or very tall structures, more rigorous method shall be used.Behavior of concrete structures Earth quakes cause not only large lateral forces on structures but also large lateral In addition to structure is also subjected to load due to violent ground shaking . The basic principle of earthquake resistant design is to ensure ductility of the structure so that it can absorb large deformations by an earthquake without significant damage the ductility or concrete structures can be ensured by proper ductility the reinforcement as per the codes of practice IS 13920

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Quantitatively the base shear force on a single storyed structure is given by

F=a/g x w

a=Ground accelerationg=acceleration due to gravityw=weight of structure.

Multistoried- structures with cellard may service earthquakes better than those on shallow isolated footings

Foundation- Apart from structural system, the various types of foundations to be adopted based on the soil characteristics are discussed. Code of practice IS 1904 -1986 shall be followed for design of size of foundations.

1. Strip foundation.2. Isolated footing with constant thickness3. Isolated footing with variable depth.4. Raft foundation.

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The Depth of FoundationThe depth of foundation is measured from the ground level to the bottom surface of the lean cement. The depth of the foundation should be taken so as to avoid any damage to the foundation concrete and to protect the soil below the foundation and also depends on to nature of soil.

Design of deep foundation- A deep foundation is one which derives its main strength and stability from the properly of the depth of foundation and it is classified into

1. Pile foundation- IS 2911. Cast in site/pre cast piles2. Well foundation

Strip foundations:- Where the width of foundation required exceeds to width of spreak of load at to level of the foundation transverse reinforcement will be necessary and ship foundation of suitable design shall be adopted.

3. Combined footings- Sometimes columns are closely spaced because of high loading, constraints and considerations in building. At times even if the columns are reasonably well spaced the bearing capacity of the soil may be lower and will not allow separate footing to each of the columns. Practical considerations and economic consideration may force a combined footing for two or more columns even though that the design of combined footing is normally discussed for two columns, it is applicable to multiple columns. When a footing is designed for a row of columns, it can be considered as a combined strip footing and designed as a

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as a continuous beam. Similarly a footing designed for a set of columns is usually called a raft or foundation.The section given under present design of footing for two columns. Typical footing are shown hire.

4. Design of Raft FoundationA raft foundation is basically a shallow foundation in which the load on the foundation is function of orthogonal directions. It is a plot type of structure, spread over a large area and supporting a number of column or the entire superstructure a single unit. The bending moments on the footing and to soil pressure distribution are functions of the two directions. A raft foundation is also called as make or spread foundation. Such foundations are used when the columns of a structure are closely spaced, or the load on the columns are large and they are usually provided for multistoried buildings, over head tanks etc. A raft foundation might become unavoidable in submerged structures is some multistoried structures where basements is to be provided and in retaining walls the mat or raft foundation is designed flat slab.

Example – Design of rectangular raft foundation.Columns spread @ 6 m a part in two perpendicular direction.Load from each column on the foundation = 2880 KNSize of column=500 mm/500 mmHeight of column above foundation = 5 met

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Soil in silty clay with S.B.C = 90 KN/m2

M25 grade concrete, HYSD bars

Net bearing capacity of soil Pa= 90 KN/m2

Design of foundation1. Assume the average thickness of the raft approx. 0.60 mr for the purpose of

calending the self using slabs.2. The difference in the weight of concrete slab and the soil can be assumed as 10

KN/m3

3. The gross load on the foundation per panel size consists of the load from the one column + weight of slab + weight of soil over burden.

4. Since the net bearing capacity is given, only the net load on the soil need to be computed for the purpose of bearing pressure.

Bearing area available per panel 6(6) = 36 m2

Load from each column = 2880 KNDifference in to weight of slabs and soil is asumed as = 6(6)(0.6)(10)= 218 KN

Total net load on pannel =2880 + 218 =3098 KN.

Net bearing pressure on soil, P = 3098/36 = 86.06 KN/m2 < 96KN/m2

Hence safe.

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Footings:- Footings shall be designed to sustain the applied loads moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as nearly uniform as possible, and the safe bearing capacity of the soil is not exceeded. (See IS code 1904)Is slopes or stepped footing the effective cross section in compression shall be limited by the area above the neutral plane, and the angle of slope or depth and location or steps shall be such that the design requirements are satisfied at every section. Sloped and stepped footings that are designed as a unit shall be constructed to assure action as a unit. In reinforced and plain concrete footing thickness at edge shall be not less than 150 mm for footing on soils nor less than 300 mm above the tops of piles for footings on piles.

Moments and forces- In the case of footing on piles, computation for moments and shears may be based on the assumption that the reaction from any pile is concentrated at the centre of pile.For the purpose of computing stresses in footings which support a round or octagonal concrete column or pedestal, the face of the column or pedestal shall be taken as the side of a square inscribed within the perimeter of the round, octagonal column or pedestal. Bending manent at any section shall be determined by passing through the section a vertical plane which extends completely across the footing, and computing and moments of the forces acting over the entire area of the footing on one side of the said plane.

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Shear and Bond- Shear strength of footing is governed by the a. The footing acting essentially as a wide beam, with a potential diagonal crack

extending in a plane across the entire width, the critical section for this condition shall be assumed as a vertical section located from the face of column, pedestal at a distance equal to the effective depth of footing for footings on piles.

b. Two-way action of the footing, with potential diagonal cracking along the surface of truncated cone or pyramid around the concentrated load. In this case, the footing shall be designed for shear in accordance with the critical section for shear at a distance d/2 from the periphary of the column.

Example:SBC of soil = 25 T/M2

Max load = 200 Ton=PSize of footing 200/25 = √8 = 2.82X2.82 metersSize of column pedastal 60cm x 60 cmP=200/2,82x2.82 =25.15 T/M2

Mt=25.12 x 2.82 x 1.112/2x100 = 4369 tonesMu=4369 tonnes= 0.87 fy Ast d(1-(Astxfy/ bdfck)Fy = Characteristic strength of reinforcementd = eff. DepthAst = area of tension reinforcementFck = Characteristic strength of concreteb = width of compression faceMu = Moment of resistance of section

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Compression members - Column is a compression member, the effective length of which exceeds three times least lateral dimention.A compression member may be considered as short when the slenderness ratio lex/d and ley/b are less than 12.lex = effective length in respect of major axis.D = depth in respect of major axisley = effective length in respect of minor axisb = width of member

Minimum eccentricity- All columns shall be designed for minimum eccentricity = unsupported length of column/500 + lateral dimention/30

subject to minimum of 20mm.

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Short axially load members in compression:- The member shall be designed by considering the

assumption when the minimum eccentricity does not exceed 0.05 times the lateral dimension, the

members may be designed by the following equation.

P = 0.4 fck AC + 0.67 fy AscP – Axial load on the member.fck = Characteristic strength of compressive strength of concrete.Ac = Area of concrete.fy = Characteristic strength of compression reinforcement.

Asc = Area of longitudinal steel for columns.

For design purposes, the compressive strengths of concrete in the structure shall be assumed as 0.67 times the characteristic strength

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Members subjected to combined Axial load and unaxial bending using sp16 Design axials for reinforced concrete to IS456.

Members subjected to combined axial load and Biaxial Bending.

The resistance of a member subjected to axial force and Biaxial bending shall be obtained on the basis of equilibrium and minimum eccentricity with the neutral axis so chosen as to satisfy the equilibrium of load and moments about two axes.

As suggested by ‘Bresler’ such members may be designed by the following equation.

( Mux) αn + (Muy) αn < 1.0 Mux1 Muy1

Mux, Muy = Manent about X and Y axes due to design loads.

Mux1, Muy1= Max Uniaxial manent capacity for axial load of Pu, bending about x and y axes respectively.

αn = related to Pu/Pu2Puz = 0.45 fck Ac + 0.75 fy Ascαn = = Pu = 0.4fck Ac + 0.67 fy Asc Pu2 0.45 fck Ac + 075 fy Asc

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Minimum requirements in column:-The cross sectional area of longitudinal reinforcement shall be not less than 0.8% not

more than 6% of gross sectional area of column.

Max percentage of steel may be limited to 4% to avoid problems.

Minimum percentage of steel shall be based upon the area of concrete required to resist the direct stress and not upon the actual area.

Minimum number of longitudinal bars in column shall be four in rectangular and six in circular columns.

Bar dia shall not be less than 12mm.

RCc column having helical reinforcement shall have at least six bars of longitudinal reinforcement.

Spacing of longitudinal bars measured along the periphery of the column shall not be exceed 300mm.

In case of pedastals in which longitudinal reinforcement is not taken in account in strength calculation, nominal longitudinal reinforcement not less than 0.15% of the cross sectional area shall be provided. Pedastal is a compression member, the effective length of which does not exceed three times the least lateral dimension.

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Transverse reinforcement:-A reinforced concrete compression member shall have transverse or helical

reinforcement so disposed that every longitudinal bar nearest to compression face has effective lateral support against buckling.

Beams:- Rectangular, T beam & L Beam.

Effective depth of a beam is the distance between the centroid of the area of tension reinforcement and the max. compression Fibre.

T-Beams and L-Beams:- A slab which is assumed to act as a compression flange of a T beam or L beam shall satisfy the.

(a) The slab shall be cast integrally with the web or the web and the slab shall be effectively bonded together in any other manner and

(b) of the main steel of the slab is parallel to the beam, transverse steel shall be provided which shall not be less than 60% of the main reinforcement at mid span of the slab.

Effective width of flange:- The effective width of flange shall be (a) For T-Beams = bf = 1o + bw + 6 dt

6(b) For L-Beams = bf = 1o + bw + 3dt.

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Bf = Effective width of flange1o = distance between points of zero moments in Beam.bw = breadth of webDt = Thickness of flange.b = actual width of flange.

Note:- for continuous beams & Frames ’1o’ may be assumed as 0.7 times the effective span.

Deflection of structure to be limited to span / 250.

The vertical deflection limits may generally be assumed(a) Span to effective depth ratios for span upto 10 meters.

Cantilever -- 7Simply supported -- 20Continuous -- 26

Slenderness limits for beams to ensure lateral stability:-A simply supported or continuous beam shall be so proportioned that the clear distance between the lateral restrictions does not exceed 60b or 250b2

d

whichever is less, d is effective depth of beam and b is breadth of compression face.

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For cantilever, the clear distance from the area free end of the cantilever to the lateral restaurant shall not exceed 25b or 100b2 whichever is less.

d

Beams – Tension Reinforcement:-

(a) Minimum area of tension reinforcement shall not be less than that.As 0.85bd fy

As = Minimum area of tension reinforcement.b = breadth of beam or breadth of web of T-Beam.d = effective depth.fy = characteristic strength of reinforcementin N/mm2.

(b) Max. reinforcement:- Max are of tension reinforcement shall not exceed 0.04bd.

Compression reinforcement:-

The Max. area of compression reinforcement shall not exceed 0.04 bd. Compression reinforcement in beam shall be enclosed by stirrup for effective lateral restraint.

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Side face reinforcement:-Where the depth of web in a beam exceeds 750mm side face reinforcement shall be provided along the two faces. The total area of such reinforcement shall not be less than 0/1% of the web area and shall be distributed equally on two faces at a spacing not exceeding 300mm or web thickness whichever is less.

Transverse reinforcement:- The transverse reinforcement in beams shall be taken around the outer most tension

and compression bars. In T-beam & L-T Beam, such reinforcement shall pass around longitudinal bars located close to the outer face of the flange.

Max. spacing of shear reinforcement:-

The max. spacing of shear reinforcement measured along the axis of the member shall not exceed 0.75d for vertical stirrup and d for inclined stirrup at 450, where d is effective depth of the section. In no case shall the spacing exceed 300mm.

Minimum shear reinforcement in the form of stirrup shall be provided such that.

Asv > 0.4 bsv 0.87fy

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Where Asv = Total cross sectional area of stirrup legs effective in shear.

Sv = Stirrup spacing along the length of member

bs = breedth of beam or breedth of web of flanged beam.

fy = characterstic strength of stirrup reinforcement in N/mm2 which shall not greater than 415N/mm2.

When a member is designed for torsion, torsion reinforcement shall be provided. Reinforcement in flanges of T&L beams shall satisfy the requirements where flanges are in a tension, a part of the main tension reinforcement shall be distributed over the effective flange width or a width equal to one tenth of the span whichever is smaller. If the effective flange width exceeds one tenth of span, nominal longitudinal reinforcement shall be provided in the outer portions of the flange.

Slab:- For design of slabs Annex-D of IS code 456 may be adopted.

Development of stress in Reinforcement:-The calculated tension or compression in any bar at any section shall be developed on each side of the section by an appropriate development length or end anchorage or by a combination there of.

Development length Ld = φσs/ 4τbd

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R.C Slabs Solid Slabs:-1. When the ratio of length to width of slab > 2, most of the load is carried by shorter span, called as one way slab.2. When the ratio of ly/lx < 2, slab is called as two way slab. Here the load is carried in two directions, however more load is carried by shorter to longer span.

Effective Span of Slab:For simply supported = clear span + effective depth For fixed slab = Clear span.

As per IS code 456-2000:-1. For slab span in two directions the shorter of the two span should be used for calculating span to effective depth ratios.2.For two way slabs of shorter span (upto 3.5 mtrs), the span to over all depth ratiod given below may be assumed to satisfy vertical deflection limits for loading class up to 3 KN/m2.

Simply supported slabs = 28 Continues slab = 32 ( for HYSD bars of Fe 415 grade)

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Slab spanning in two directions at Right Angles:-

Slabs spanning in two directions at right angles and carrying U.D.L may be designed by using coefficients.

The maximum BM per unit width is a slab by Mx = α x X w X Lx2

My = α y X W X Lx2

Where α x and α y are coeffeclient based on edge conditions. W = Total design load per unit area. Mx , My = Moment on strips on unit width spanning Lx, Lly respectively. Lx, Ly shorter and longer span lengths.

Minimum Steel:- To minimize the shrinkage and temperature effects and consequent cracking , minimum reinforcement in the slabs should be 0.12 % of gross area of the section for HySD bars.

Maximum Steel:- Limited to 4% of the cross section.

Diametere of the bar not more than 1/8 of thickness of slab

Spacing of main reinforcement:-Should not be more than two times thickness of slab.

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Minimum cover to Steel:-15mm or dia of bar.

Design of sheer :- Sheer stress is not normally critical in slabs, however to ensure that nominal sheer stress is not less than the allowable sheer stress.

Allowable sheer stress in slabs τ cs = Ks X τ c

Ks = Modified sheer stress.

Normal sheer stress = τ v = V/bd

V = Sheer force per unit width.

B = unit width.

Effective slab depth d = V/b X τ c

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RCC BUILDING ELEMENTS

BY

N.KRISHNAM RAJUADVISOR TO APHB

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Structural Planning:- In case of framed structures,1.The most important aspect of structural planning is the arrangement of columns and

beam. The size of column, beams and slabs depend upon the spacing and arrangement of the frame.

2. For taller building cross bracing either with RCC wall or bracing girder is essential.

Preliminary design of RCC framea. For fixing up tentative sizes of the member of frame.

Detailed design of RCC frame1. Fix sizes of slabs, beams and columns on the above basis2. Calculate column loads etc various floor levels3. Analyse the RCC frame to arrive the sizes of members

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RCC Elements

Foundation:

Footing:- Footing shall be designed to sustain the applied loads, moments

and forces and to ensure that the safe bearing capacity of soil is not

exceeded.

Column:- Column is a compression member usually subjected to combined

axial compression and bending

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3. Beams:- A horizontal bracing member connecting the columns to take care of load

and moments

4. Slabs:- RCC slabs are most commonly used in floor and roofs of building.

Thickness is small compared will the other dimensions. Steel is compared will the

other dimensions. Steel is provided to minimize shrinkage, temperature effects

and cracking.

5. Stair case:- To provide access between various floors.

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6.Shear walls: RC walls designed to take care of lateral forces and stability.

7 Choice of Mix:- Based on the number of floors and flexural stresses for

beam, slabs, and footing, predominant stresses in compression for column

members.

8.Assembly of reinforcement a) Reinforcement shall be bent and fixed in

accordance once IS 2502. b) Barbending schedule for reinforcement wall

9.Expansion joints:- To allow variation is temperature, expansion joints in

frames are essential normal @ 45 meters length and shape of building.

10.Construction Joints:- To comply with IS 11817. To provide at accessible

locations.

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Concepts Introduction:- The important characteristics of soil one should know in the design of

RCC foundation1. Type of Soil2. Bearing capacity3. Settlement at different pressures4. Water Table5. Friction angle.

a. Soils:- conforming ( to IS 1498)Clay: A plastic stage moderate to wide , range of water content.Silt: a fine grained soil will little or plasticity.

Sand& gravel: cohesionless aggregates of rounded, angular, flaky..

b. Bearing capacity of the soil is governed by its shearing resistance. If stress due to shear exceeds what the soil can bear, failure occurs.

c. SBC of soil to be ensured based on the soils in the location duly conducting soil exploration and necessary lab tests.

d. Foundation:- That part of the structure which is in direct contact and transmitting loads to the ground.

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Raft Foundation:- A foundation continues in two direction. Covering an area

equal to or greater than the base area of a building.

Strip Foundation:- A foundation providing a continues longitudinal bearing.

Wide strip foundation:- A continues foundation providing a continues bearing of

such width that transverse reinforcement is necessary.

Foundation Beam:- A beam in a foundation transmitting a load to pile/slab or

other foundation

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General considerations for design conforming to IS 1904

1. Loads on Foundation: a Dead load + Live Load

b Dead Load + Live Load + W L + E Q F

2. Depth of foundation: The depth to which foundation should be

carried depends upon the principal features.

a. Adequate bearing capacity

b. In case of clayey soils penetration where shrinkage and swelling

due to seasonal weather changes are likely to cause appreciable movement.

c. In fine sand and silts, penetration below the zone in which trouble

may be expected from frost.

All foundation shall extend to a depth of at least 80 cm below natural

ground level.

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Type of Foundation

a. Spread foundations

b. Strip Foundation

c. Steel grillage foundation

d. Raft Foundation

e. Pier foundation

f. Pile Foundation

Selection of type of foundation:- As per site conditions and soil met with and safe

bearing capacity of soil.

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a.Spread Foundation: The area of the footing which has the largest percentage of

live load to total load should be determined. By total load/allowable soil pressure

b.Strip Foundation: Where the width of foundation required exceeds the width of

spread of load at the level of foundation transverse reinforcement is necessary and

ship foundations of suitable design shall be adopted.

c.Steel crollage foundation: In designing grillage a method that assures of flexibility

in both the base plate of the column and the reload steel beams may be used.

d.Raft Foundations: Are used where the bearing power of the soil is so low. A raft

shall be so shaped and proportioned that the centre of area of the ground bearing

shall be vertically under the centre of gravity of the imposed load. The soil usually a

raft shall be protected from alternate shrinking and swelling due to moisture changes

e.Pile foundation: The principal uses of piles is to transit loads taro soft or unstable

surface soils to harder soils.

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General design consideration

No pile shall be loss then 30 cm in diameter

Piles shall be spaced sufficiently far apart to ensure that zones of sois surrounding them, do not over lap, spacing of piles shall be not less than 100 cm.

The edge of caps shall extend at least 15cm beyond edge of pile. The caps will not be less the 60 cm thick

Piles and pile caps shall be designed for all column loads.

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5.Reinforcement of to pile shall be carried into cap and anchored into it just as the

reinforcement of column is anchored to develop full tension value .top of all piles

shall be embedded in caps not less than 7.5 cm.

Multi storied buildings and important aspects.

1.Types of construction

1. Load bearing construction (upto 2 to 3 floors)

2. Composite construction (upto 5 to 6 floors)

3. Reinforcement concrete framed construction (any floor)

4. Steel framed construction (for economy of space and quicker program of

construction)

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BUILDING CONSTRUCTION IN STAGES

A Building is a structure having various component like foundation, walls, columns, Floors, roof, doors, windows, ventilators, stairs, lifts, surfaces, finishes etc.

In general, every structure consists of 1 Foundation2 Super structure

Specifications1.Earth Work:

1. Excavation of foundations2. Filling in foundation3. Filling in Basement4. Pile foundation

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2 Concrete3 Steel Reinforcement4 Brick Masonry5 Stone Masonry6 Flooring7 Roofing and ceiling8 Plastering, painting etc9 Wood work10 Painting & varnishing

Construction Stages1.BC1 a Site examination

b Soil explorationc Marking & set outd Earth work excavatione Antitermite treatmentf Mortars and Masonryg Brick work and stone masonry

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2 BC2 a Damp proof and work proofing b Timber and plywoodc Word workd Steel worke Roof and Roof coveringsf Stairs, Lifts & Elevators

3 BC3 a Assembly of Reinforcement (As per IS 2502)b Cutting, Tying and placing on reinforcementc Plastering & External renderingd Flooringe Painting & Polishing

4 BC4 Structural Concretea Materialsb Grade of concretec Proportioning of concreted Admixturese Equipmentf Mixing of concrete

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g Transport and placing of concreteh Compaction of concreteI Construction Jointsj Finishing

BC5 Building Servicesa Formation of roadsb Plumbing servicesc Electrical Servicesd HVAC Servicese Acousticsf Installation of Lifts & Escalavationsg Fire Safety Measures (IS 1641 to IS 1646)

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CONSTRUCTION STAGES1 EXCAVATION OF FOUDNATION2 FILLING IN FOUNDATIONS3 FILLING IN BASEMENT4 PLAIN CEMENT CONCRETE PCC FOR FOUNDATION5 FORM WORK (CONFORMING TO IS 14687)6 REINFORCEMENT7 WATER8 PLACING OF CONCRETE

COMPACTIONSLUMP TEST

9 MASONARY (CRS)10 BRICK WORK11 PLASTERING12 SUMMARY

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CONSTRUCTION PRACTICES• Placing of concrete (As per clause No. 13.2 of IS 456/2000)

1. Design mix to be obtained.2. The concrete to be deposited as nearly as practicable in its final position.3. Avoid lengthy handling and segregation of mix.4. The concrete shall be placed and compacted before initial setting of

concrete.5. Avoid segregation or displacement of reinforcement form work.

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CONSTRUCTION PRACTICES• Compaction (As per clause No. 13.2 of IS.456/2000)

1. Concrete to be compacted with pan vibrators for slabs and pin vibrators for beams/columns

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CONSTRUCTION PRACTICES• Slump Test (As per clause No. 13.2 of IS 456/2000)

1. For concreting of lightly reinforced sections, mass concreting with very low and low degree of workability, the slump is to be between 25 to 75 mm.

2. For concreting with heavily reinforced sections with medium degree of workability the slump is to be between 50 to 100 or 75 to 100 as directed by Engineer-in-charge.

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CONSTRUCTION PRACTICES• Stone masonary

1. Coursed rubble stone masonry1. The face stones shall be squared on all joints with beds horizontal.2. They shall be set in regular courses of uniform thickness fom bottom to

top throughout.3. No face stone shall be less width in plan than 150 mm for walls of 400

mm thick 200 mm for walls of 450 mm thick and 250 mm for walls of 600 mm thick and above.

4. The face stones shall be laid headers and stretchers alternatively so as to break joints.

5. The stones shall be solidly bedded, set in full mortar with joints not exceeding 12mm and extend back into the hearting.

6. The height of the stone shall not exceed breadth at face nor the length inwards.

2. Through stones and Headers1. In all the works upto a width of 600mm, bond stones running though the

wall to be provided at an intervals of 2 m in each course.2. For walls thicker than 600mm, a line of headers each headers each

header overlapping by 150mm minimum shall be provided from front to back at 2 m intervals in each course.

3. The position of the stones shall be marked on both the faces.

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CONSTRUCTION PRACTICES• Brick work

1. The thickness of joints in case of masonry with first class brigcks shall not be more than 10mm.

2. In case of masonry with second class bricks joints shall not be more than 12 mm.

3. The bricks shall be thoroughly soaked in clean water.4. The cessation of bubbles when the bricks are immersed in water is an

indication of thorough soaking of bricks.5. The bricks shall be laid with joints full of mortar.6. The face joints shall be racked by jacking tool when the mortar is green.7. The wall construction shall be taken up truly plumb.8. All courses shall be laid truly horizontal.9. All vertical joints shall be truly vertical.10.The thickness of brick course shall be kept uniform and with their frogs kept

upward.

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CONSTRUCTION PRACTICES• Plastering

1. Water the brick wall before start of plastering.2. Chicken mesh at joints of brick wall and R.C.C member to be provided. 3. Dry mixing of cement and sand is to be done on impervious platform.4. Holes provided for scaffolding are to be closed along with plastering.5. Level marking must be done in advance form time to time.6. Chip off concrete surface before starting plastering.7. Gaps around door window frames to be filled.8. Base coat of plaster to be checked before application of finishing coat.

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SUMMARY OF QUALITY CHECKS TO BE DONE ON BULLDINGS WORKS.

• Bearing capacity of soil to be checked in advance.• Material to be approved in advance.• Quality of materials to be checked periodically.• Steel to be obtained from main manufacturers only.• Size of footings, pedestals, columns, beams are to be checked.• Design mixes to be obtained in advance.• Cover to the reinforcement as per structural requirement to be checked. • Thickness of plastering to wall be checked.• Proportion, workability and vibration of CC mix and cement mortar proportion be

checked.• Cube samples be collected for testing in lab.