Chapter -2 STRUCTURAL DESIGN OF RCC BUILDING COMPONENTS PDF

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Chapter - 2 STRUCTURAL DESIGN OF RCC BUILDING COMPONENTS Rajendra Mathur Dy. Dir(BS-C) 09412739 232(M) e-mail – [email protected] ©BSNL India For Internal Circulation Only Page: 1 Structural Design of RCC Building Components 1.0 Introduction The procedure for anal ysis and design of a g...

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Chapter - 2

STRUCTURAL DESIGN OF RCC BUILDING COMPONENTS

Rajendra Mathur Dy. Dir(BS-C) 09412739 232(M) e-mail – [email protected]

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Structural Design of RCC Building Components 1.0 Introduction The procedure for anal ysis and design of a given building will depend on the t ype of building, its complexit y, the number of stories etc. First the architectural drawings of the building are studied, structural sys tem is finalized sizes of structural members are decided and brought to the knowledge of the concerned architect. The procedure for structural design will involve some steps which will depend on the t ype of building and also its complexity and the time ava ilable for structural design. Often, the work is required to start soon, so the steps in design are to be arranged in such a way the foundation drawings can be taken up in hand within a reasonable period of time. Further, before starting the structural design, the following information of data are required: (i) A set of architectural drawings;(ii) Soil Investigation report (SIR) of soil data in lieu thereof; (iii) Location of the place or cit y in order to decide on wind and seismic loadings;(iv) Data for lifts, water tank capacities on top, special roof features or loadings, etc. Choice of an appropriate structural system for a given building is vital for its economy and safet y. There are two t ype of building s ystems:(a) Load Bearing Masonry Buildings. (b) Framed Buildings. (a) Load Bearing Masonry Buildings: Small buildings like houses with small spans of beams, slabs generall y constructed as load bearing brick walls with reinforced concrete slab beams. This system is suitable for building up to four or less stories.(as shown in fig. below). In such buildings crushing strength of bricks shall be 100 kg/cm 2 minimum for four stories. This s ystem is adequate for vertical lo ads it also serves to resists horizontal loads like wind & earthquake by box action . Further, to ensure its action against earthquake , it is necessary to provide RCC Bands in horizontal & vertical reinforcement in brick wall as per IS: 43261967( Indian Standards Code of Practice for Earthquake Resistant Construction of Buildings.) . In some Buildings, 115mm thick brick walls are provided since these walls are incapable of supporting vertical loads, beams have to be provide along their lengths to support adjoining slab & the weight of 115mm thick brick wall of upper storey. These beams are to rest on 230 mm thick brick walls or reinforced concrete columns if required. The design of Load Bearing ©BSNL India

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Masonry Buildings are done as per IS:1905-1980 (Indian Standards Code of Practice for Structural Safety of Buildings: Masonry Walls(Second Revision).

Load bearing brick wall Structural system (b) Framed Buildings:In these t ypes of buildings reinforced concrete frames are provided in both principal directions to resist vertical loads and the vertical loads are transmitted to vertical framing system i.e columns and Foundations. This t ype of system is effective in resisting both vertical & horizontal loads. The brick walls are to be regarded as non load bearing filler walls onl y. This system is suitable for multi -storied building which is also effective in resisting horizontal loads due to earthquake. In this system the floor slabs, generall y 10 0-150 mm thick with spans ranging from 3.0 m to 7.0 m. In certain earthquake prone areas, even single or double storey buildings are made framed structures for safet y reasons. Also the single storey buildings of large storey heights (5.0m or more ) ,like electric substation etc. are made

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framed structure as brick walls of large heights are slender and load carrying capacit y of such walls reduces due to slenderness.

Framed Structural system

2.0 Basic Codes for Design . The design should be carried so as to conform to the following Indian code for reinforced concrete design, published by the Bureau of Indian Standards, New Delhi: Purpose of Codes National building codes have been formulated in different countries to lay down guidelines for the design and construction of structure. The codes have evolved from the collective wisdom of expert structural engineers, gained over the years. These codes are periodicall y revised to bring them in line with current rese arch, and often, current trends. The codes serve at least four distinct functions .

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Firstl y, they ensure adequate structural safet y, by specifying certain essential minimum requirement for design. Secondl y, they render the task of the designer relativel y simple; often, the result of sophisticate anal yses is made available in the form of a simple formula or chart. Thirdl y, the codes ensure a measure of consistency among different designers. Finall y, they have some legal validit y in that they protect the structural designer from any liabilit y due to structural failures that are caused by inadequate supervision and/or fault y material and construction. (i)IS 456 : 2000 – Plain and reinforced concret e – code of practice (fourth revision) (ii) Loading Standards These loads to be considered for structural design are specified in the following loading standards: IS 875 (Part 1-5) : 1987 – Code of practice for design loads (other than earthquake) for buildings and structures (second revision) Part 1 : Dead loads Part 2 : Imposed (live) loads Part 3 : Wind loads Part 4 : Snow loads Part 5 : Special loads and load combinations IS 1893 : 2002 – Criteria for earthquake resistant design of structure (fourth revision). IS 13920 : 1993 – Ductile detailing of reinforced concrete structure subject to seismic forces. Design Handbooks The Bureau of Indian standards has also published the following handbooks, which serve as useful supplement to the 1978 version of the codes. Although the handbooks need to be updated to bring them in line with the recently revised (2000 version) of the Code, many of the provisions continue to be valid (especiall y with regard to structural design provisions). SP 16 : 1980 – Design Aids (for Reinforced Concrete) to IS 456 : 1978 SP 24 : 1983 – Explanatory handbook on IS 456 : 1978 SP 34 : 1987 – Handbooks on Concrete Reinforced and Detailing.

General Design Consideration of IS: 456-2000.

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The general design and construction of reinforced concrete buildings shall be governed by the provisions of IS 456 –2000 AIM OF DESIGN The aim of design is achievement of an acceptable probability that structures being designed shall, with an appropriate degree of safety –  Perform satisfactorily during their intended life.  Sustain all loads and deformations of normal construction & use  Have adequate durability  Have adequate resistance to the effects of misuse and fire. METHOD OF DESIGN –  Structure and structural elements shall normally be designed by Limit State Method.  Where the Limit State Method cannot be conveniently adopted, Working Stress Method may be used MINIMUM GRADE OF CONCRETE The minimum grade of concrete for plain & reinforced concrete shall be as per table below –

26.4 ©BSNL India

Nominal Cover to Reinforcement For Internal Circulation Only

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26.4.1 Nominal Cover Nominal cover is the design depth of concrete cover to all steel reinforcements, including links. It is the dimension used in design and indicated in the drawings. It shall be not less than the diameter of the bar. 26.4.2 Nominal Covers to Meet Durability Requirement Minimum values for the nominal cover of normal weight aggregate concrete which should be provided to all reinforcement, including links depending on the condition of exposure described in 8.2.3 shall be as given in Table 16. Table 16 Nominal Cover to Meet Durability Requirements (Clause 26.4.2) Exposure Nominal Concrete Cover in mm not Less Than Mild

20

Moderate

30

Severe

45

Very Severe

50

Extreme

75

NOTES 1. 2. 3.

For main reinforcement up to 12 mm diameter bar for mild exposure the nominal cover may be reduced by 5 mm. Unless specified otherwise, actual concrete cover should not deviate from the required nominal cover by + 10 mm For exposure condition ‘severe’ and ‘very severe’, reduction of 5 mm may be made, where concrete grade is M35 and above.

26.4.2.1 However for a longitudinal reinforcing bar in a column nominal cover shall in any case not be less than 40 mm, or less than the diameter of such bar. In the case of columns of minimum dimension of 200 mm or under, whose reinforcing bars do not exceed 12 mm, a nominal cover of 25 mm may be used.

26.4.2.2 For footing minimum cover shall be 50 mm.

26.4.3 Nominal Cover to Meet Specified Period of Fire Resistance

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Minimum values of nominal cover of normal-weight aggregate concrete to be provided to all reinforcement including links to meet specified period of fire resistance shall be as given in Table 16A.

21.4 Minimum Dimensions of RC members for specified Period of Fire Resistance

DESIGN LOAD ©BSNL India

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Design load is the load to be taken for use in appropriate method of design. It is –  Characteristic load in case of working stress method &  Characteristic load with appropriate partial safety factors for limit state design.

LOAD COMBINATIONS As per IS 1893 (Part 1): 2002 Clause no. 6.3.1.2, the following load cases have to be considered for analysis:  1.5 (DL + IL)  1.2 (DL + IL ± EL)  1.5 (DL ± EL)  0.9 DL ± 1.5 EL  Earthquake load must be considered for +X, -X, +Z and –Z directions.  Moreover, accidental eccentricity during earthquake can be such that it causes clockwise or anticlockwise moments. So both clockwise & anticlockwise torsion is to be considered.  Thus, ±EL above implies 8 cases, and in all, 25 cases must be considered. It is possible to reduce the load combinations to 13 instead of 25 by not using negative torsion considering the symmetry of the building.

STIFFNESS

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22.3.1

Relative Stiffness: The relative stiffness of the members may be based on the moment of inertia of the section determined on the basis of any one of the following definitions: a) b)

c)

Gross Section Transformed Section Cracked Section

The cross-section of the member ignoring reinforcement The concrete cross-section plus the area of reinforcement transformed on the basis of modular ratio The area of concrete in compression plus the area of reinforcement transformed on the basis of modular ratio

The assumptions made shall be consistent for all the numbers of the structure throughout any analysis. 22.3.2

For deflection calculations, appropriate values of moment of inertia as specified in Annexure of IS 456-2000 should be used.

STRUCTURAL FRAMES 22.4

The simplifying assumptions as given in 22.4.1 to 22.4.3 may be used in the analysis of frames.

ARRANGEMENT OF LIVE LOAD 22.4.1 a) Consideration may be limited to combinations of: 1) Design dead load on all spans with full design live load on two adjacent spans; and 2) Design dead load on all spans with dull design live load on alternate spans. 22.4.1 b) When design live load does not exceed three-fourths of the design dead load, the load arrangement may be design dead load and design live load on all the spans. Note: For beams continuous over support 22.4.1 (a) may be assumed. 22.4.2

22.4.3

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. 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 methods should be used.

MOMENT AND SHEAR COFFICIENTS FOR CONTINUOUS BEAMS

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22.5.1

Unless more exact estimates are made, for beams of uniform cross-section which support substantially uniformly distributed load over three or more spans which do not differ by more than 15 percent of the longest, the bending moments and shear forces used in design may be obtained using the coefficients given in Tables below. For moments at supports where two unequal spans meet or in case where the spans are not equally loaded, the average of the two values for the negative moment at the support may be taken for design. Where coefficients given in Table below are used for calculation of bending moments, redistribution referred to in 22.7 shall not be permitted.

22.5.2

Beams Over Free End Supports Where a member is built into a masonry wall which develops only partial restraint, the member shall be designed to resist a negative moment at the face of the support of W1/24 where W is the total design load and 1 is the effective span, or such other restraining moment as may be shown to be applicable. For such a condition shear coefficient given in Table below at the end support may be increased by 0.05.

------------------------------------------------------------------------------------------------------BENDING MOMENT COFFICIENTS ------------------------------------------------------------------------------------------------------Span Moments Support Moments ------------------------------------------------------------------------------------Types of Load Near Middle At Middle At Support At Other of End Span of interior next to the Interior span end support Supports ------------------------------------------------------------------------------------------------------Dead load and 1 1 1 1 imposed load +-+-(- )-(- )-(fixed) 12 16 10 12 Imposed load (not fixed)

1 +-10

1 +-12

1 (- )-9

1 (- )-9

Note:

For obtaining the bending moment, the coefficient shall be multiplied by the total design load and effective span. -------------------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------------------SHEAR FORCE COFFICIENTS -------------------------------------------------------------------------------------------------------

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Type of Load

At End Support

At Support Next At All Other to the end Support Interior Support Outer side Inner Side ------------------------------------------------------------------------------------------------------Dead load and imposed load 0.40 0.60 0.55 0.50 (fixed) Imposed load (not fixed)

0.45

0.60

0.60

0.60

Note:

For obtaining the shear force, the coefficient shall be multiplied by the total design load. ------------------------------------------------------------------------------------------------------CRITICAL SECTIONS FOR MOMENT AND SHEAR 22.6.1

For monolithic construction, the moments computed at the face of the supports shall be used in the design of the members at those sections. For non-monolithic construction the design of the member shall be done keeping in view 22.2.

22.6.2

Critical Section for Shear

22.6.2.1

The shears computed at the face of the Support shall be used in the design of the member at that section except as in 22.6.2.1 When the reaction in the direction of the applied shear introduces compression into the end region of the member, sections located at a distance less than d from the face of the support may be designed for the same shear as that computed at distance d.

REDISTRIBUTION OF MOMENTS 22.7

Redistribution of moments may be done in accordance with 37.1.1 for limit state method and in accordance with B-1.2 for working stress method. However, where simplified analysis using coefficients is adopted, redistribution of moments shall not be done.

EFFECTIVE DEPTH 23.0

Effective depth of a beam is the distance between the centroid of the area of tension reinforcement and the maximum compression fibre, excluding the thickness of finishing material not placed monolithically with the member and the thickness of any concrete provided to allow for wear. This will not apply to deep beams.

CONTROL OF DEFLECTION

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23.2

The deflection of a structure or part thereof shall not adversely affect the appearance or efficiency of the structure or finishes or partitions. The deflection shall generally be limited to the following: a) The final deflection due to all loads including the effects of temperature, creep and shrinkage and measured from the as-cast level of the supports of floors, roofs and all other horizontal members, should not normally exceed span/250. b) The deflection including the effects of temperature, creep and shrinkage occurring after erection of partitions and the application of finishes should not normally exceed span/350 or 20mm whichever is less.

23.2.1 For beams, the vertical deflection limits may generally be assumed to be satisfied provided that the span to depth ratio are not greater than the value obtained as below: a) Basic values of span to effective depth ratios for spans up to 10m: Cantilever Simply supported Continuous

7 20 26

b) For spans above 10m, the values in (a) may be multiplied by 10/span in metres, except for cantilever in which case deflection calculations should be made. c) Depending on the area and the type of steel for tension reinforcement, the value in (a) or (b) shall be modified as per Fig. 4 d) Depending on the area of compression reinforcement, the value of span to depth ratio be further modified as per Fig. 5 e) For flanged beams, the value of (a) or (b) be modified as per Fig. 6 and the reinforcement percentage for use in fig. 4 and 5 should be based on area of section equal to bf d.

Note: When deflections are required to be calculated, the method given Annexure ‘C’ of IS 456-2000 may be used.

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CONTROL OF DEFLECTION – SOLID SLABS 24.1 General The provisions of 32.2 for beams apply to slabs also. NOTES

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1. For slabs spanning in two directions, the shorter of the two spans should be used for calculating the span to effective depth rations. 2. For two-way slabs of shorter spans (up to 3.5 m) with mild steel reinforcement, the span to overall depth rations given below may generally be assumed to satisfy vertical deflection limits for loading class up to 3 kN/m2. Simply supported slab 35 Continuous slabs 40 For high strength deformed bars of grade Fe 415,the values given above should be multiplied by 0.8. Simply supported slab 28 Continuous slabs 32 23.3

Slabs Continuous Over Supports Slabs spanning in one direct...