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(TECHNICAL)(TECHNICAL)

Structural Design of RCC Bldg Structural Design of RCC Bldg ComponentsComponents(Session (Session –– 2) 2)

WELCOME

• This is a presentation for the E4-E5 Civil Technical

Module for the Topic: Structural Design of RCC Bldg

Components (Session 2)

• Eligibility: Those officers of civil wing who have got the

Up-gradation from E4 to E5.

• This presentation is last updated on 21-4-2011.

• You can also visit the Digital library of BSNL to see this

topic.



AGENDA

Design of Various Structural Components

Column Design

Design of Slab

Beam Design

Isolated Footing Design – Imp. Considerations.

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.


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.


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.


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.


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.


Design of Columns

Design Of Columns – Reinforcement Provisions as per

IS:456-

A. 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


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


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.).


BIS 456 EXTRACT


• 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


BIS 456 EXTRACT


• 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.


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

�The horizontal distance between parallel

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 & spacing of

reinforcing bars


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 one-

fifth of the shorter span.

• Area of reinf. in each of these four layers is three-

quarters of the area required for maximum mid-span

moment in slab


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.


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 >450mmFor 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


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


FOUNDATION DESIGN


• 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.


FOUNDATION DESIGN


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


For one way action

For one way shear action, the nominal shear stress is

calculated as follows:-

Vuτv = -------

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)


FOUNDATION DESIGN


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 calculatedin accordance with clause 31.6.2 of the code as follows:-

Vuτv = ----------

b0.d

Where b0 = Periphery of the critical section


FOUNDATION DESIGN


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.


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

