PPT DESIGN AND COMPARISON OF FLAT SLAB USING IS 456 – 2000

PRESENTATION ON DESIGN AND COMPARISON OF FLAT SLAB USING IS 456 – 2000 AND ACI 318 – 08

PRESENTED BY TUSHAR MANOJ MADAMWAR (11001063)

GUIDED BY DR. K. N. KADAM

CONTENTS 1. INTRODUCTION 2. ADVANTAGES OF FLAT SLAB 3. LITERATURE REVIEW

4. DESIGNOF FLAT SLAB USING IS 456-2000 5. DESIGN OF FLAT SLAB USING ACI 318-08

6. RESULT AND COMPARISION 7. REFERENCES

INTRODUCTION 

DEFINATION-Slab and column without beams



COMPONENTS - Drops-increase shear strength against punching. - Column Head-resists negative moment

Advantages of Flat Slab 1.

Flexibility in room layout

2.

Saving in building height

3.

Shorter construction time

4.

Ease of installation

5.

Pre-fabricated welded mesh

6.

Buildable score

PROBLEM STATEMENT Design a flat slab of size 6.6m×5.6m using direct design method by IS 456-2000 and ACI 318-08 .It is subjected to live load of 7.75 kN/m2 .The grade of steel used is Fe 415. The exposure condition is mild. Height of floor is 4m.size of column is 400mm × 400mm.

DESIGN OF FLAT SLAB USING IS 456-2000 

Column Strip A design strip having a width of 0.25l2 not greater than 0.25l1 on each side of column center-

line. For longer span=1.4m (not greater than 1.65)

For shorter span=1.65m(not greater than 1.4) 

Middle strip A design strip bounded on each of its opposite sides by the column strip.

For longer span=2.8m For shorter span=3.8m 

Panel

The part of a slab bounded on-each of its four sides by the center-line of a Column or centerlines of adjacent-spans.



Drop

The drops when provided shall be rectangular in plan and have a length in each direction not less than one- third of the panel length in that direction For longer span =2.2m For shorter span=1.866m Provide drop of 2.2m×2.2m 

Column Head

For longer span=1.65m For shorter span=1.4 Adopting 1.3m 

Depth of Flat Slab

The thickness of the flat slab up to spans of 10 m shall be generally controlled by considerations of span ( L ) to effective depth ( d ) ratios given as below: Cantilever 7; simply supported 20; Continuous 26 For longer span=260mm For shorter span= 220mm Taking effective depth of 25mm; overall depth=285mm > 125mm

Loads acting on slab Dead load(𝑊𝑑1 )=0.285× 25=6.25kN/m2 Floor finish(𝑊𝑑2 )=1.45kN/m2 Live load(𝑊𝑙 )=7.75kN/m2 Total design load=15.45kN/m2

Total design moment on slab 𝑾𝒍𝒏 𝟖

Mo =

Mo=total moment W = design load on an area l1 l2

ln = clear span extending from face to face of columns, capitals, brackets or walls, but not less than 0.65 l1 l1 = length of span in the direction of Mo. l2 = length of span transverse to l1. 𝝅 𝟒

𝜋 4

A= 𝒅𝟐 =

1.32 = 1.152𝑚 1 2

1 2

1 2

1 2

Clear span along long span =ln =6.6- (1.152)- (1.152)=5.448 >4.29 (Should not be less than 0.65 l1 ) Clear span along long span =ln =5.6- (1.152)- (1.152)=4.44 >3.64 (Should not be less than 0.65 l1 )

Total Design Load For longer span(l n=5.448 m , l2 =5.6 m)= W = w l 2 l n =471.36kN For shorter span(l n=4.44 m , l2 =6.6 m)=W = w l2 ln= 452.74k N

Absolute Sum of Negative and Positive Moments 𝑤𝑙𝑛 8

=

For longer span=Mo=

𝑊𝑙𝑛 8

For shorter span=Mo=

471.36×5.448 8

=

452.74×4.44 8

= 320.99 𝑘𝑁 = 251.2 𝑘𝑁

Stiffness Calculations 𝑘𝑐 𝑘𝑠

stiffness coefficient=ac=

=1.39

For longer span=𝑎𝑐𝑚𝑖𝑛 =0.7 1 𝑇𝑣 < 𝑇𝑐

safe design OK

Reinforcement Longer span Negative exterior reinforcement:

𝑀𝑢 =0.87 𝑓𝑦 Ast 𝑑 − 0.42𝑥𝑢 1.5× 121.34 × 106 = 0.87 × 415 × 𝐴𝑠𝑡 𝑑 − 0.42 × 0.48 × 150

𝑨𝒔𝒕 =4290mm2

DESIGN OF FLAT SLAB USING ACI 318-08 

Drop of flat slabs

Drop panel shall extend in each direction from centerline of support a distance not less than one-sixth the span length measured from center -to center of supports in that direction. Span of panel in longer direction = 16.76 ft 1 6

length of drop panel= × 16.76 × 2=5.58ft With half width on either side of the center line of support = 0.85 m 1 4

Thickness of drop = × 6 = 1.5 𝑖𝑛 = 38.1𝑚𝑚 

Thickness of the slab

the minimum thickness shall be in accordance with (a) Slabs without drop panels as ......................... 5 in. (b) Slabs with drop panels as defined.................. 4 in.

𝑙𝑛 36

Depth of the slab from deflection criteria = Minimum depth of slab=max of (

(the yield stress 𝑓𝑦𝑖 =60,000psi,≅ 415𝑁/𝑚𝑚2

16.76×12 14.22×12 , )=max 36 36

of (5.58in,4.74in)=5.58in

Providing a slab of thickness 6 in or 152.4 mm.



Design strips

Column strip is a design strip with a width on each side of a column centerline equal to 0.25 l2 or 0.25 l1, whichever is less. Middle strip is a design strip bounded by two column strips. A panel is bounded by column, beam, or wall centerlines on all sides.

Designed Load on Slab Dead load on the slab =

6 12

× 150 = 75𝑝𝑠𝑓 = 3.6𝑘𝑁/𝑚3

Live load on the slab = 161.80 psf = 7.75 KN /m2 Design load on the slab = (1.2 x 7.5 + 1.6 x 161.80) = 348.88 » 350 psf= 16. 765 KN /m2

Total factored static moment for a span 𝑾𝒖 𝒍𝟐 𝒍𝒏 𝟐 Mo = 𝟖

Wu=load per unit area acting on the slab panel ln=Clear span ln shall extend from face to face of columns, capitals ,brackets, or walls. Value of ln shall not be less than 0.65 l1 . l2=When the span adjacent and parallel to an edge is being considered, the distance from edge to panel centerline shall be substituted for l2 . In an interior span, total static moment Mo shall be distributed as follows: Negative factored moment .................................0.65 Positive factored moment ...................................0.35 350 × 170762 8×1000

Mo=

× 14.22 = 174.75𝑓𝑡 − 𝑘𝑖𝑝𝑠 = 237𝑘𝑁𝑚

Negative design moment = 237 x 0. 65 = 154 ft-kips = 208.89 kNm Positive design moment = 237 x 0.35 = 83.00 ft -kips = 113.22 kNm



Bending moment for column strip

Negative moment for column strip = 75 % of total negative moment in the pannel = 0.75 x 154.00 = 115.50 ft -kips = 157.66 KNm Positive moment for column strip = 60 % of total positive moment in the panel. = 0.60 x 83.00 = 49.8 ft -kips = 67.977 KNm Max moment (+ve or –ve ) along shorter span = 72.18 ft -kips Max moment (+ve or –ve) along longer span = 115.50 ft –kips

Shear Provision 

For flat slabs Vc =Nominal shear strength of concrete, Vc Shall be smallest of the following: [Where βc is the ratio of long side to short side of the column, concentrated load or reaction area and where as is 40 for interior columns, 30 for edge columns,20 for corner columns]

Vc== 2 +

4 𝛽𝑐

Vc== 2 +

𝑎𝑠 𝑑 𝑏0

Vc=4 𝑓𝑐 ′ 𝑏𝑜 𝑑

𝑓𝑐 ′ 𝑏𝑜 𝑑 𝑓𝑐 ′ 𝑏𝑜 𝑑

Vu= factored shear, acting at distance d/2 from face of the support. (assuming column of size 400 mm by 400 mm) Vu=350 16.76 × 14.22 − 1.31 + 0.5 1.31 + 0.5 =350 238.32 − 1. . 812 =82265.365 lb=365.91kN 𝑓𝑐 ′ 𝑏𝑜 𝑑= 4000 × 4 × 21.72 × 6 = 32968.64𝑙𝑏

(𝛽𝑐 = 1.17)

The nominal stress of concrete will be smallest of the following Vc== 2 +

4 1.17

Vc== 2 +

40×6 4×21.72

Vc=

× 32968.64 = 178650.57 𝑙𝑏 × 32968.64 = 157010.87 𝑙𝑏

𝑓𝑐 ′ 𝑏𝑜 𝑑=4× 32968.64 = 131874.56 𝑙𝑏

Vc > Vu section safe in punching shear \safe.

Reinforcement For negative moment in column strip 𝑀 R= 𝑢 𝑏(324)

=

115.5×103 =219.77 16.22×32.4

Reinforcement ratio = 0.00375 Area of reinforcement = 0.00375 x 16.22 x 6 x 12 = 4.38 in2/ft

Results and Comparisons CODE

IS 456 -2000

ACI 318-08

Shape of test specimen for concrete

Cube

Cylinder

strength (mm)

150x150x150

152.4x304.8

Grade of concrete(N/mm²)

20

20

Grade of steel (N/mm²)

415

413.7

Negative moment

188.5

208.89

90

113.22

Area of reinforcement(mm²)

4290

2829

Thickness of slab for

170

150

Safe

Safe

(KN-m) Positive moments

(KN-m)

Serviceability criteria(mm) Punching shear

Conclusions 

By comparing with different codes we concluded that ACI 318 code is more effective in designing of flat slabs.



As per Indian code we are using cube strength but in international standards cylinder strength is used which gives higher strength than cube.

 

Drops are important in increasing the shear strength of the slab. Flat slabs enhance resistance to punching failure at the junction of concrete slab & column.

 

By incorporating heads in slab, we are increasing rigidity of slab. In the interior span, the total design moments (Mo) are same for IS, ACI.

Conclusions 

The negative moment’s section shall be designed to resist the larger of the two interior

negative design moments for the span framing into common supports. 

According to Indian standard (IS 456) for RCC code has recommended characteristic strength of concrete as 20, 25, and 30 and above 30 for high strength concrete. For design

purpose strength of concrete is taken as 2/3 of actual strength this is to compensate the difference between cube strength and actual strength of concrete in structure. After that we apply factor of safety of 1.5. So in practice Indian standard actually uses 46% of total

concrete characteristic strength. While in International practice is to take 85% of total strength achieved by test and then apply factor of safety which is same as Indian standard so in actual they use 57% of total strength. 

Pre fabricated sections to be integrated into the design for ease of construction.

References 

Bureau of Indian Standards, New Delhi, “IS 456:2000, Plain and Reinforced Concrete - Code of Practice”, Fourth Revision, July (2000).



American Concrete Institute, “ACI 318-08, Building Code Requirements for Structural Concrete and Commentary”, January (2008).



Dr. V. L. Shaha & Dr. S. R. Karve “Limit State Theory and Design of Reinforced Concrete” Sixth edition



Amit A. Sathwane , R. S. Deoalate (IJERA)“Analysis and Design of Flat Slab and Grid Slab and Their cost omparision”