civil engineering by sandeep jyani - wifistudy.comsuch a slabs are also designed as one way slabs....

Civil Engineering by Sandeep Jyani

Design of Slabs

• Slabs are plate elements forming floors and roofs of the buildings and carrying distributed loads primarily by flexure

• Inclined slabs may be used as ramps for multi-storey car parking

• Stair case may be considered as an inclined slab

• A slab may be supported by beams or walls or may be used as a flange of a T –Section beam

• Slab may be simply supported, continuous over one or more supports and is classified according to manner of support


IS code Recommendations1. Minimum area of reinforcement

• 𝑭𝒐𝒓 𝑭𝒆 𝟒𝟏𝟓 = 𝟎. 𝟏𝟐% 𝒐𝒇 𝒕𝒐𝒕𝒂𝒍 𝒂𝒓𝒆𝒂 = 𝟎. 𝟏𝟐% × 𝑩×𝑫

• 𝑭𝒐𝒓 𝑭𝒆 𝟐𝟓𝟎 = 𝟎. 𝟏𝟓% 𝒐𝒇 𝒕𝒐𝒕𝒂𝒍 𝒂𝒓𝒆𝒂 = 𝟎. 𝟏𝟓% × 𝑩× 𝑫

2. Maximum diameter of steel bar in slab

• ∅𝒎𝒂𝒙 =𝑻𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 𝒐𝒇 𝒔𝒍𝒂𝒃

𝟖

3. Maximum spacing of the reinforcement

a) Main bar = 𝐥𝐞𝐬𝐬𝐞𝐫 𝒐𝒇ቊ𝟑𝒅

𝟑𝟎𝟎𝒎𝒎

b) Distribution bar = 𝐥𝐞𝐬𝐬𝐞𝐫 𝒐𝒇ቊ𝟓𝒅

𝟒𝟓𝟎𝒎𝒎‘d’ is the effective depth of slab.


Design of One Way Slab• One way slab is

a) IF supported on two opposite sides, it is always one way slab

b) If the lab is supported over two sides and 𝒍𝒚𝒍𝒙

ratio is

greater than 2𝒍𝒚𝒍𝒙> 𝟐

𝒍𝒚 = 𝒍𝒐𝒏𝒈𝒆𝒓 𝒅𝒊𝒎𝒆𝒏𝒔𝒊𝒐𝒏 𝒔𝒑𝒂𝒏 𝒐𝒇 𝒔𝒍𝒂𝒃𝒍𝒙 = 𝒔𝒉𝒐𝒓𝒕𝒆𝒓 𝒅𝒊𝒎𝒆𝒏𝒔𝒊𝒐𝒏 𝒔𝒑𝒂𝒏 𝒐𝒇 𝒔𝒍𝒂𝒃

Note: Slab is always designed for shorter span


a) Also, if the slab is supported on all four edges and the ratio of longer span (ly) to shorter

span (lx) i.e, 𝑙𝑦𝑙𝑥> 2, practically the slab spans across the shorter span. Such a slabs are also

designed as one way slabs. b) In this case, the main reinforcement is provided along the spanning direction to resist one

way bending.

Design of One Way Slab

• Temperature and shrinkage reinforcement are invariably provided at right angles to the main longitudinal reinforcement because it occupies large area and so load and temperature has to be distributed evenly

• Distribution bars are always placed above main bar in case of one way slab to ensure higher effective depth

• In two way slab, both reinforcement are main reinforcement however higher load transfer will be on the shorter direction and hence we keep shorter direction reinforcement closer to tensile surface to get higher bending moment (higher effective depth)


Important Points

• Minimum dia of main bar is 10mm for Fe 250 and 8mm for HYSD (practical recommendation, not as per IS code)

• Minimum dia of distribution steel is 6mm in both cases i.e. Mild steel and HYSD

• No maximum reinforcement recommendation is given in IS code so we consider the maximum reinforcement criteria same as in case of beam i.e. 4% of bD


Effective Span of Beam/Slab 1) Simply Supported beam or Slab: The effective span of a simply

supported member is taken lesser of the following:I. 𝒍 = 𝑳𝒄 + 𝒅II. 𝒍 (centre to centre distance between supports)Where 𝒍 = centre to centre distance between supports

𝑳𝒄 = clear span𝒅 = 𝒆𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 𝒅𝒆𝒑𝒕𝒉 𝒐𝒇 𝒃𝒆𝒂𝒎 𝒐𝒓 𝒔𝒍𝒂𝒃

𝐿𝑐

𝒍


Effective Span of Beam/Slab 2) Continuous Beam or Slab: The effective span of a continuous member

is taken as:i. If width of support 𝒕𝒔 ≤

𝑳𝒄𝟏𝟐

, then effective span is taken as lesser of …

a) 𝒍 = 𝑳𝒄 + 𝒅b) 𝒍 (centre to centre distance between supports)

Where 𝒍 = centre to centre distance between supports𝑳𝒄 = clear span𝒅 = 𝒆𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 𝒅𝒆𝒑𝒕𝒉 𝒐𝒇 𝒃𝒆𝒂𝒎 𝒐𝒓 𝒔𝒍𝒂𝒃

ii. If width of support 𝒕𝒔 ≥𝑳𝒄𝟏𝟐


a) 𝒍 = 𝑳𝒄 + 𝟎. 𝟓 𝒅b) 𝒍 = 𝑳𝒄 + 𝟎. 𝟓 𝒕𝒔

𝐿𝑐

𝒍 𝒍

𝐿𝑐𝒕𝒔 Civil Engineering by Sandeep Jyani

Deflection


Depth of Slab• The depth of slab depends on bending moment and deflection criterion. the trail

depth can be obtained using:• The effective depth d of two way slabs can also

be assumed using cl. 24.1,IS 456 provided short span is ≤ 3.5m and loading class is < 3.5KN/m2


Design of One Way Slab• Step 1: Find load over 1m of span

a) 𝐋𝐢𝐯𝐞 𝐥𝐨𝐚𝐝

b) 𝑺𝒆𝒍𝒇 𝒘𝒆𝒊𝒈𝒉𝒕 = 𝑻𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 × 𝟏𝒎 × 𝟐𝟓

c) 𝑭𝒍𝒐𝒐𝒓 𝒇𝒊𝒏𝒊𝒔𝒉𝒊𝒏𝒈 = 𝑻𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 𝒐𝒇 𝒇𝒍𝒐𝒐𝒓 × 𝟏𝒎 × 𝟏𝒎× 𝟐𝟒𝑙𝑥

𝑙𝑦

1𝑚

• Step 2: Bending Moment

a) 𝑩𝑴𝒎𝒂𝒙 =𝒘𝑳𝟐

𝟖( for simply supported beam)

• Step 3: Area of Steel

• 𝑨𝒔𝒕 =𝟎.𝟓𝒇𝒄𝒌

𝒇𝒚𝟏 − 𝟏 −

𝟒.𝟔 𝑩𝑴

𝒇𝒄𝒌𝑩𝒅𝟐 ×𝑩 × 𝑫

• Step 4: Check for Shear

• 𝝉𝒗 =𝑽

𝑩𝒅≯ 𝝉𝒄 𝒎𝒂𝒙

• 𝑽 =𝑾𝑳

𝟐


Note: Slab is not designed for shear, shear reinforcement shall not be provided as it is safe in shear

Two way Slab

• A rectangular slab supported on four edge supports, which bends in two orthogonal directions and deflects in the form of dish or a saucer is called two way slabs.

• If a slab is considered to be two way slab, a) if the slab is supported on 4 edges /sides

b) The span ratio 𝒍𝒚𝒍𝒙≤ 𝟐

• Since, the slab rest freely on all sides, due to transverse load the corners tend to curl up and lift up. The slab looses the contact over some region. This is known as lifting of corner. These slabs are called two way simply supported slabs

• If the slabs are cast monolithic with the beams, the corners of the slab are restrained from lifting. These slabs are called restrained slabs


Que 1. Minimum area of reinforcement in

RCC slab shall be

(a)𝐹𝑒 415 = 0.12% 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎

(b)𝐹𝑒 250 = 0.15% 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎

(c) Both a and b(d) None


2. Maximum diameter of steel bar in RCC

slab

(a)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

3

(b)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

8

(c)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

4

(d)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

6


2. Maximum diameter of steel bar in RCC

slab

(a)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

3

(b)𝑻𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 𝒐𝒇 𝒔𝒍𝒂𝒃

𝟖

(c)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

4

(d)𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑠𝑙𝑎𝑏

6


3. Spacing of main bars in an RCC slab

shall not exceed

a) 3 times the effective depth

b) 3 times then overall depth

c) 30 times the dia of main bar

d) 30 cm


4. In two way slabs, the depth of slab is based on

a) Long span

b) Mostly long span but sometimes short span

c) Short span

d) Independent of span


5. A reinforced concrete beam, supported on columns at ends, has a clear span m and m effective depth. It carries a total uniformly distributed load kN/m. The design shear force for the beam is…?

a) 275 kN

b) 200 kN

c) 175 kN

d) 150 kN



𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑙𝑒𝑛𝑔𝑡ℎ = 5 + 0.5 = 5.5𝑚

𝑆ℎ𝑒𝑎𝑟 𝑓𝑜𝑟𝑐𝑒 =𝑤𝑙𝑒𝑓𝑓

2

⇒ 𝑆ℎ𝑒𝑎𝑟 𝑓𝑜𝑟𝑐𝑒 =100 × 5.5

2

⇒ 𝑆ℎ𝑒𝑎𝑟 𝑓𝑜𝑟𝑐𝑒 = 275 𝑘𝑁

5. A reinforced concrete beam, supported on columns at ends, has a clear span m and m effective depth. It carries a total uniformly distributed load kN/m. The design shear force for the beam is…?

a) 275 kN

b) 200 kN

c) 175 kN

d) 150 kN


6. Deflection can be controlled by using the appropriate

a) Aspect Ratio

b) Modular Ratio

c) Span/Depth Ratio

d) Water/cement ratio


7.Maximum permitted spacing of main bars in a slab is

a) 100 mm

b) 3d

c) 5d

d) 450mm


7 .Maximum permitted spacing of main bars in a slab is

a) 100 mm

b) 3d

c) 5d

d) 450mm


8. The effective span of a simply supported slab is

a) Distance between the centre of the bearing

b) Clear distance between the inner face of the wall plus twice the thickness of the wall

c) Clear span plus effective depth of slab

d) None of these


Effective Span of Beam/Slab 1) Simply Supported beam or Slab: The effective span of a simply

supported member is taken lesser of the following:I. 𝒍 = 𝑳𝒄 + 𝒅II. 𝒍 (centre to centre distance between supports)Where 𝒍 = centre to centre distance between supports

𝑳𝒄 = clear span𝒅 = 𝒆𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 𝒅𝒆𝒑𝒕𝒉 𝒐𝒇 𝒃𝒆𝒂𝒎 𝒐𝒓 𝒔𝒍𝒂𝒃

𝐿𝑐

𝒍


Effective Span of Beam/Slab 2) Continuous Beam or Slab: The effective span of a continuous member

is taken as:i. If width of support 𝒕𝒔 ≤

𝑳𝒄𝟏𝟐


a) 𝒍 = 𝑳𝒄 + 𝒅b) 𝒍 (centre to centre distance between supports)

Where 𝒍 = centre to centre distance between supports𝑳𝒄 = clear span𝒅 = 𝒆𝒇𝒇𝒆𝒄𝒕𝒊𝒗𝒆 𝒅𝒆𝒑𝒕𝒉 𝒐𝒇 𝒃𝒆𝒂𝒎 𝒐𝒓 𝒔𝒍𝒂𝒃

ii. If width of support 𝒕𝒔 ≥𝑳𝒄𝟏𝟐


a) 𝒍 = 𝑳𝒄 + 𝟎. 𝟓 𝒅b) 𝒍 = 𝑳𝒄 + 𝟎. 𝟓 𝒕𝒔

𝐿𝑐

𝒍 𝒍

𝐿𝑐𝒕𝒔 Civil Engineering by Sandeep Jyani

8. The effective span of a simply supported slab is

a) Distance between the centre of the bearing

b) Clear distance between the inner face of the wall plus twice the thickness of the wall

c) Clear span plus effective depth of slab

d) None of these


9. If the maximum Bending moment of a simply supported slab is M-kg-cm, the effective depth of the slab is

a)𝑀

100 𝑄

b)𝑀

10 𝑄

c)𝑀

𝑄

d)𝑀

100𝑄


9. If the maximum Bending moment of a simply supported slab is M-kg-cm, the effective depth of the slab is

a)𝑀

100 𝑄

b)𝑀

10 𝑄

c)𝑀

𝑄

d)𝑴

𝟏𝟎𝟎𝑸


10. The minimum cover in the slab should neither be less than the diameter of the bar nor less than

a) 10 mm

b) 13 mm

c) 20 mm

d) 25 mm


11. A slab is designed as a two way slab if the ratio of long span to short span is less than

a) 1.0

b) 1.2

c) 1.5

d) 2.0


12. The horizontal distance between parallel main reinforcements in RC slab shall not be more than

a) 4 times of effective depth of slab

b) 5 times of effective depth of slab

c) 3 times of effective depth of slab

d) 2 times of effective depth of slab


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civil engineering by sandeep jyani - wifistudy.comsuch a slabs are also designed as one way slabs....

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