Ch1b-E4-E5 Civil-Structural Design of RCC Bldg Components Session -2 [Compatibility Mode]

E4--E5 E4 -E5 CIVIL (TECHNICAL) Structural Design of RCC Bldg Components (Session – – 2) For internal circulation circulation of BSNL only

WELCOME • This This is is a prese present ntat ation ion for for the the E4-E E4-E5 5 Civil Civil Tec Techn hnic ical al Module for the Topic: Structural Design Desi gn of RCC Bldg Components (Session 2) • Eligibil Eligibility: ity: Those Those officer officerss of civil civil wing who have have got the Up-gradation from E4 to E5. • This This pr pres esen enta tati tion on is la last st upda update ted d on 21-4-2011. • You You can can als also o visi visitt the the Digi Digita tall libr librar ary y of BSNL BSNL to see see thi thiss topic.

For internal circulation circulation of BSNL only

WELCOME • This This is is a prese present ntat ation ion for for the the E4-E E4-E5 5 Civil Civil Tec Techn hnic ical al Module for the Topic: Structural Design Desi gn of RCC Bldg Components (Session 2) • Eligibil Eligibility: ity: Those Those officer officerss of civil civil wing who have have got the Up-gradation from E4 to E5. • This This pr pres esen enta tati tion on is la last st upda update ted d on 21-4-2011. • You You can can als also o visi visitt the the Digi Digita tall libr librar ary y of BSNL BSNL to see see thi thiss topic.


AGENDA Design of Various Structural Components Column Design Design of Slab Beam Design Isolate Isolated d Footin Footing g Design – Imp Imp.. Consider Consideratio ations. ns. Development Length


DESIGN OF RCC STRUCTURE Design of Various Structural Components – • After load calculation & analysis for vertical & horizontal loads, design & of various structural components e.g. – – –

Columns, Foundations,

–

Beams,

– Slabs & staircase etc are carried out as per various clauses of IS codes with help from charts & tables given in BIS handbooks.

For internal circulation of BSNL only

Design of Columns Design of Columns • After obtaining (i) Vertical load, (ii) Moments due to horizontal loads on either axis & (iii) Moments due to vertical loads on either axis, acting on each column, at all floor levels of the building, • Columns are designed by charts of SP-16(Design Aids). • Design of each column is carried out from the top of foundation to the roof, varying the amount of steel reinforcement for suitable groups for ease in design. Slenderness effects in each storey are also considered for each column group. For internal circulation of BSNL only

Design of Columns Column A compression member, the effective length > three times the least lateral dimension. Short and Slender Compression Members When both slenderness ratios lex /D and ley /b are <12 • Column is a short column • If more than 12, then it is long or slender column. • Slender Columns are designed for Additional Moments as per Clause 39.7 of IS456 Effective height of column:• For effective column height refer table 28 (Annexure E) of IS:456-2000. For internal circulation of BSNL only

Design of Columns Design Of Columns – Important Considerations (ii) Unsupported Length – In beam-slab construction, it is the clear distance between the floor & under side of shallower beam framing into columns in each direction at next higher floor level.

(iii) Slenderness limits for columns – The unsupported length between end restraints shall not exceed 60 times the least lateral dimension of a column.

(iv) Minimum Eccentricity – All columns shall be designed for emin ≥ l/500+ D/30 ≥ 20 mm Where l= Unsupported length of column in mm. D= Lateral dimension of column in the direction under consideration in mm. For internal circulation of BSNL only

Design of Columns Design Of Columns – Design Approach • The design of column is complex as it is subjected to axial loads & moments which may very independently. Column design requires – – Determination of the cross sectional dimension. – The area of longitudinal steel & its distribution. – Transverse steel. • The maximum axial load & moments acting along the length of column are considered for design of the column section. • The transverse reinforcement is provided to impart effective lateral support against buckling to every longitudinal bar. For internal circulation of BSNL only

Design of Columns Design Of Columns – Reinforcement Provisions as per IS:456A. Longitudinal reinforcement • Area of longitudinal reinforcement shall be not less than 0.8% nor more than 6% of cross sectional area of the column. • However maximum area of steel should not exceed 4% to avoid practical difficulties in placing & compacting concrete. • In pedestals, in which the longitudinal reinf. is not taken into account in strength calculations, nominal reinforcement should be not be less than 0.15% of cross sectional area. • Minimum dia of longitudinal bar should be 12 mm For internal circulation of BSNL only

Design of Columns Design Of Columns – Reinforcement Provisions as per IS:456 A. Longitudinal reinforcement • Spacing between bars < 300mm along periphery of column • The minimum number of bars shall be four in rectangular columns & six in circular columns. B. Transverse reinforcement (STIRRUPS) • Diameter of lateral ties should not be less than 1/4th of dia of the largest longitudinal bar & in no case should be less than 6 mm. • Spacing of lateral ties should not > least of the following: –Least lateral dimension of the column. –16 times the smallest diameter of longitudinal bars to be tied. –300 mm. For internal circulation of BSNL only

SLAB DESIGN TYPES OF SLABS Based on Ratio of long span to short span – • One way slab – Long span (ly)/Short span (lx ) > 2 • Two way slab – Long span (ly)/Short span (lx ) < 2 Based on Edge Conditions • Simply supported • Restrained – Edge Conditions of supporting edge • Cantilever For internal circulation of BSNL only

SLAB DESIGN • The design of floor slab is carried out as per –  Clause

24.4 &  Clause 37.1.2 & Annexure D of IS:456-2000 . 

The Bending moment coefficients are taken from  Table- 26 or  Table

– 27 of BIS code • depending on support conditions •

Bending moment is calculated & reinforcement steel is obtained from charts given in SP-16.


BIS 456 EXTRACT Clause 22.2 Effective Span – • Effective Span of slab or beam to be considered in design is based on support condition- simply supported, continuous, cantilever etc. & width of support. • For RCC frame construction, generally centre to centre distance is considered.


BIS 456 EXTRACT EFFECTIVE DEPTH Clause 23.0 • Effective depth of beam or slab = distance between centroid of area of tension reinf. & maximum comp. fiber, • Excluding thickness of finishing material not placed monolithically with member and the thickness of any concrete provided to allow for wear.


BIS 456 EXTRACT Clause 23.2 CONTROL OF DEFLECTION The deflection shall generally be limited to following: • Final deflection < span/250 (Due to all loads & measured from as-cast level of supports of floors, roofs and all other horizontal members.) • Final deflection < span/350 or 20mm whichever is less (Including effects of temperature, creep & shrinkage occurring after erection of partitions & application of finishes.). For internal circulation of BSNL only

BIS 456 EXTRACT Clause 23.2 CONTROL OF DEFLECTION • For beams, vertical deflection limits may generally be assumed to be satisfied provided that span/depth ratio are not greater than the value obtained as below – (a) Basic values of span/effective depth ratios for spans up to 10m: Cantilever 7 Simply supported 20 Continuous 26 For internal circulation of BSNL only

BIS 456 EXTRACT Clause 23.2 CONTROL OF DEFLECTION • For spans >10m, values in (a) may be multiplied by 10/span in meters, Modification Factors are applied – • Based on area & type of

steel

for

tension

reinforcement (As per Fig. 4 of IS456) • Based on area of compression reinforcement (As per Fig. 5 of IS456) • For flanged beams (As per Fig. 6 of IS456)


BIS 456 EXTRACT Clause 24.1 SLABS –Control of Deflection • The provisions of 23.2 for beams apply to slabs also. • For slabs spanning in two directions shorter of the two spans to be used for span/effective depth ratios. • For two-way slabs of shorter spans (≤3.5 m) with mild steel reinf., span/depth ratios given below may generally be assumed to satisfy vertical deflection limits for loading class up to 3 kN/m2. Simply supported slabs 35 Continuous slabs 40 For HYSD bars grade Fe 415 & Fe500, values given above to be multiplied by 0.8. For internal circulation of BSNL only

BIS 456 EXTRACT 26.5.2 Requirement of Reinforcement – SLABS 26.5.2.1 Minimum reinforcement • Mild steel reinf. in either direction in slabs ≥ 0.15 % of total cross sectional area. • For high strength deformed bars ≥ 0.12 % of total (Fe415/Fe500 bars) cross sectional area. 26.5.2.2 Maximum diameter • The dia of reinforcing bars < 1/8th of total thickness of slab


BIS 456 EXTRACT Requirement of Reinforcement – SLABS 26.3.3 Maximum distance between bars  The horizontal distance between parallel main reinforcement bars ≤ 3d or 300 mm horizontal distance between parallel  The reinforcement bars provided against shrinkage and temperature ≤ 5d or 300 mm whichever is smaller.


SLAB DESIGN Steps for Design of Slabs – • Step 1: Selection of preliminary depth of slab • Step 2: Calculate design loads, bending moments • Step 3: Determination/checking of the effective and total depths of slabs • Step 4: Determination of areas of steel • Step 5: Selection of diameter & reinforcing bars


spacing of

BIS 456 EXTRACT • Torsion reinforcement is provided at any corner where the slab is simply supported on both edges meeting at that corner. • It consist of top and bottom reinforcement, each with layers of bars placed parallel to sides of slab & extending from edges a minimum distance of onefifth of the shorter span. • Area of reinf. in each of these four layers is threequarters of the area required for maximum mid-span moment in slab For internal circulation of BSNL only

BEAM DESIGN 26.5.1.1 Tension reinforcement a) Minimum reinforcement As = 0.85 bd fy where AS = minimum area of tension reinforcement b = breadth of beam or the breadth of the web d = effective depth of T-beam fy = characteristic strength of reinforcement in N/mm2 & b) Maximum reinforcement - The maximum area of tension reinforcement not to exceed 0.04 bD. For internal circulation of BSNL only

BEAM DESIGN • Compression reinforcement • The maximum area of compression reinforcement not to exceed 0.04 bD

• Side face reinforcement •Where depth of web in a beam >750 mm, side face reinf is to be provided along the two faces. •The total area of such reinf. should not < 0.1 percent of web area and •It shall be distributed equally on two faces at a spacing not > 300 mm or web thickness whichever is less. •Also to be provided in beams having torsion & with width or depth >450mm For internal circulation of BSNL only

BEAM DESIGN Minimum shear reinforcement (Clause 26.5.1.6) • Minimum shear reinforcement in the form of stirrups shall be provided such that: Asv = 0.4 bsv 0.87fy Maximum spacing of shear reinforcement (Clause 26.5.1.5) • The maximum spacing of shear reinforcement measured along axis of member shall be < 0.75 d for vertical stirrups and d for inclined stirrups at 45 degrees. • In no case shall the spacing to be >300 mm.


BEAM DESIGN Steps for Design of beams– • Step 1: Selection of preliminary cross sectional dimension of beam • Step 2: Calculate design loads, bending moments & shear force • Step 3: Determination/checking of the effective and total depths of beam/ Revise if necessary. • Step 4: Determination of areas of steel for flexure • Step 5: Determination of shear reinforcement • Step 6: Detailing as per IS 456 & IS13920 provisions For internal circulation of BSNL only

FOUNDATION DESIGN Design of Foundations – Important Considerations • Foundations transfer loads from the building or individual columns to earth. Foundations must be designed to prevent – • • • •

Structural Failure Shear failure of soil Excessive settlement & To minimize differential settlement

• Depth of footing is determined from the consideration of – (a) Bending Moment (b) One way shear (c)Two way shear For internal circulation of BSNL only

FOUNDATION DESIGN Design of Foundations – Important Considerations • To determine area required for proper transfer of total load on the soil, the total load (the combination of dead, live and any other load without multiplying it with any load factor) need to be considered. Total Load including Self Weight of footing Plan Area of footing

=

----------------------------------------------Allowable bearing capacity of soil

Thickness of the edge of footing – The thickness at the edge shall not be less than 15 cm for footing on soils. For internal circulation of BSNL only

FOUNDATION DESIGN Design of Foundations – Important Considerations Bending Moment (Reference Clauses- 34.2.3.1 & 34.2.3.2) • The critical section for bending Moment is considered at the face of column, Pedestal or wall. Shear (Reference Clause 33.2.4.1) • The critical section for one way shear is at the vertical section located at a distance equal to the effective depth (d) from the face of the column, pedestal or wall of the footing in case of footings on soils.


FOUNDATION DESIGN Design of Foundations – Important Considerations For one way action For one way shear action, the nominal shear stress is calculated as follows:Vu τv = ------b.d Where τv = Shear stress, Vu = Factored vertical shear force b = Breadth of critical section, d = Effective depth τv < τc ( τc = Design Shear Strength of concrete based on % of longitudinal tensile reinforcement refer Table 61 of SP-16) For internal circulation of BSNL only

FOUNDATION DESIGN Design of Foundations – Important Considerations For Two Way Action (Punching shear ) Critical section for punching shear is at d/2 from the face of column or pedastal For two way shear action, the nominal shear stress is calculated in accordance with clause 31.6.2 of the code as follows:Vu τv = ---------b0.d Where b0 = Periphery of the critical section


FOUNDATION DESIGN Design of Foundations – Important Considerations Development Length (Reference Clause 34.2.4.3) • The critical section for checking the development length in a footing shall be assumed at the same planes as those described for bending moment in clause 34.2.3 of code and also at all other vertical planes where abrupt changes of section occur. Reinforcement –  Minimum % of steel in footing slab should be 0.12% &  Maximum spacing should not be more than 3 times effective depth or 300mm which ever is less. For internal circulation of BSNL only

DETAILING • Reinforcing steel of same type and grade shall be used as main reinforcement in a structural member. • Simultaneous use of two different types or grades of steel for main and secondary reinforcement is permissible. • 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 thereof.


Development Length Development Length of Bars Ld = φσst /4τbd, φ = nominal diameter of bar, τbd = design bond stress σst = stress in bar at the section considered at design load • Design bond stress in limit state method for plain bars in tension is given in clause 26.2.1.1 • For deformed bars conforming to IS 1786 these values are to be increased by 60 %. • For bars in compression, the values of bond stress for bars in tension is to be increased by 25 percent


Development Length


Ch1b-E4-E5 Civil-Structural Design of RCC Bldg Components Session -2 [Compatibility Mode]

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