Reinforced Concrete Slab

#ReinforcedConcreteSlab

Q1. RC slab rebar တြင္၊ torsion bar ထည့္သင့္ပါသလား။
Q2. rebar % မည္မၽွရွိသင့္သနည္း။
Q3. ၾကမ္းခင္အထူကို၊ မည္သို႔၊ မွန္းဆမည္နည္း။
Q4. Support ေပၚမူတည္ၿပီး၊ slab ဘယ္ႏွစ္မ်ိဳး၊ ရွိပါသနည္း။
Q5. မည္သည့္ rebar size ကို၊ အသံုးျပဳမည္နည္း။
Q6. Cover မည္မၽွထားမည္နည္း။
Q7. Rebar မ်ားကို၊ end support အတြင္း၊ မည္သို႔ ျမဳ တ္သြင္းအဆံုးသတ္မည္နည္း။

1. Supported slab - ၃-မ်ိဳး###

Slab ကို ၄င္းအား(ေထာက္မ) ထားေသာ၊ support ေပၚမူတည္ၿပီး၊ ၃-မ်ိဳးခြဲျခားထားပါသည္။
1.1 beam မပါဘဲ၊ column ျဖင့္သာ (ေထာက္မ)ထားေသာ slab ကို Flat slab(Post Tension) ဟု ေခၚပါသည္။

1.2 wall ၂-ဖက္ (သို႔) beam ၂-ဖက္ျဖင့္ (ေထာက္မ) ထားေသာ slab ကို One Way Slab ဟုေခၚပါသည္။

1.3 Slab အနား၄- ဘက္လံုးအား (ေထာက္မ) ထားေသာ slab ကို Two Way Slab ဟု ေခၚပါသည္။

2.  slab ၂- မ်ိဳးႏွင့္၊ rebar direction###

2.1 slab ၏ႀကီးေသာအနားကို ငယ္ေသာအနားႏွင့္စားေသာ္ ၂ထက္ႀကီးပါက one way slab ဟုေခၚၿပီး၊ ၂-ထက္ငယ္ပါက two-way slab ဟုေခၚသည္။

2.2 one way slab တြင္ main Steel bar သည္ shorter direction ျဖစ္ၿပီး၊ distribution steel bar သည္ longer direction ျဖစ္သည္။

2.3 two way slab တြင္steel direction ၂-ခုစလံုးသည္ main bar မ်ားျဖစ္ၾကသည္။

3. Support - ေဒါက္ (ေထာက္မ)###

3.1 slab ကိုေထာက္မ ထားေသာ (ထမ္းထားေသာ) တိုင္မ်ား၊ နံရံမ်ားအေပၚမူတည္ၿပီး
- simply supported slab
- continuous slab ႏွင့္
- cantilever slab ဆိုၿပီး ခြဲျခားရပါေသးသည္။

3.2 Two way slab

Two way slab မ်ားတြင္ ေထာင့္ေနရာမ်ား၌ ခံုးႂကြတတ္သည့္ သေဘာရွိပါသည္။

3.3 ထို႔ေၾကာင့္ အဆိုပါ ေထာင့္ေနရာမ်ားတြင္၊ torsion bar ကို အပိုေဆာင္းၿပီးထည့္ေပးရပါသည္။ ပံု(၁) ကို၊ ၾကည့္ပါ။

4. Slab design###

Slab တခုကို structure designer မရွိေသာအခါ ကြင္းအင္ဂ်င္နီယာမ်ားအေနျဖင့္ ေအာက္ပါပုံေသနည္းကို အသံုးျပဳ ၿပီးတည္ေဆာက္ႏိုင္ပါသည္။
slab thickness = span to depth ratio of IS456-2000

1.  Types of Slab by Support
Reinforcement detailing of a slab is done based on its support conditions. Slab may be supported on walls or beams or columns.
1.1 flat slab
Slab supported directly by columns are called flat slab.
1.2  One Way Slab
Slab supported on two sides and bending takes place predominantly in one direction only is called One Way Slab.
1.3 Two Way Slab
On the other hand, when slab is supported on all four sides and bending take place in two directions are said to be Two Way Slab.

2. Types of Slab by rebar direction
2.1 The slabs having ratio of longer length to its shorter length (Ly/Lx) greater than 2 is called one way slab otherwise as two way slab.
2.2 In one way slab main reinforcement is parallel to shorter direction and the reinforcement parallel to longer direction is called distribution steel.
2.3 In two way slab main reinforcement is provided along both direction.

3. Support
3.1 Slabs could be simply supported, continuous or cantilever.
3.2 In two way slab the corners may be held down by restraints or may be allowed to lift up.
3.3 Additional torsion reinforcement is required at corners when it is restrained against uplifting as shown in Fig.1.

4.  Slab design
4.1 Thickness of the slab is decided based on span to depth ratio specified in IS456-2000.
4.2 Min reinforcement is 0.12% for HYSD bars and 0.15% for mild steel bars.
4.3 The diameter of bar generally used in slabs are: 6 mm, 8 mm, 10 mm, 12 mm and 16 mm. 4.4 The maximum diameter of bar used in slab should not exceed 1/8 of the total thickness of slab.
4.5 Maximum spacing of main bar is restricted to 3 times effective depth or 300 mm whichever is less.
4.6 For distribution bars the maximum spacing is specified as 5 times the effective depth or 450 mm whichever is less.
4.7 Minimum clear cover to reinforcements in slab depends on the durability criteria and this is specified in IS 456-200. Generally 15 mm to 20 mm cover is provided for the main reinforcements.
4.8 Alternate main bars can be cranked near support or could be bent at 1800 at the edge and then extended at the top inside the slab as shown in Fig.1. Curtailment and cranking of bars and is shown in Fig. 2.

5. Torsion reinforcement
5.1 Torsion reinforcement shall be provided at any corner where the slab is simply supported on both edges meeting at that corner and is prevented from lifting unless the consequences of cracking are negligible.
5.2 It shall consist of top and bottom reinforcement, each with layer of bars placed parallel to the sides of the slab and extending from the edges a minimum distance of one fifth of the shorter span.
5.3 The area of reinforcement per unit width in each of these four layers shall be three quarters of the area required for the maximum mid-span moment per unit width in the slab.
5.4 Torsion reinforcement equal to half that described above shall be provided at a corner contained by edges over only one of which the slab is continuous. Torsion reinforcement to be provided is shown in Fig. 3 below.
5.5 Torsional Reinforcement in Slabs
The drawing showing the detailing of reinforcement has a plan showing typical reinforcement in both direction and sectional elevations. Typical detailing of slab is shown in Fig.4 and 5.

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