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Transcript of Chapter 6 Results and Discussion 11 NEW
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Chapter 6 Results andDiscussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software
1
GENERAL
The analysis of flat slab is done with the use of Microsoft excel spread sheets. Two-way
equal spanning flat slab geometry is used for the analysis. Results of flat slab analysis are
compared with post-tensioned flat slab with same spanning geometry. For the comparison,
analysis of flat slab in reinforced concrete flat slab and post-tensioned flat slab both are
started from three span of 3 m x 3 m two-way slab and performed up to three span of 6 m x 6
m two-way flat slab. Analysis and design of post-tensioned flat slab is done with the analysis
tool ADAPT 6.15- Structural Concrete Software System. The software gives required post-
tensioned tendons and extra non-prestressed reinforcement over support or mid span. For the
labor cost of post-tensioning tendons layout, rates are taken from the website of PTSI.
Comparison is done in three parts, named flat slab, flat slab with drop cap and waffle slab.
Fig. 6.1 Typical geometry of flat slab
L L L
L
L
L
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software3
Table 6.2 Compar ison between Span Vs Span Moment
Span Moment in RC slab Span Moment in PT slab Span in meter
37.42 27.32 3.0097.20 66.72 4.00
213.50 116.51 5.00
407.17 261.13 6.00
It can be seen from the graph that as span increases, span moment in Reinforced concrete flat
slab increases much higher as compared to Post-Tensioned flat slab. At 6.0 m span, span
moments is around double in Reinforced concrete flat slab compared to Post-Tensioned flat
slab.
Graph of span moment Vs span for RC and PT flat slab
37.42
97.20
213.50
407.17
27.3266.72
116.51
261.13
0
50
100
150
200
250
300
350
400
450
3.00 4.00 5.00 6.00
SpanMome
ntin(kNmt.)
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software4
Table 6.3 Comparison between Span Vs Bottom f iber stress
Stress in RC flat slab Stress in PT flat slab Span in meter
9.58 3.5 3.00
11.40 6.41 4.0012.80 6.21 5.00
15.40 6.53 6.00
In Reinforced concrete slab stresses are increasing continuously as span increases but stresses
are more over constant in the case of Post-Tensioned flat slab. Here thickness of Post-
Tensioned flat slab is less then also stresses are lesser which is due to load balancing by
applying prestressing force through post-tensioning tendons.
Graph of bottom fiber stress Vs span for RC and PT flat slab
9.58
11.40
12.80
15.40
3.5
6.41 6.21 6.53
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
3.00 4.00 5.00 6.00
Bottomf
iberstressinN/mm2
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software5
Table 6.4 Comparison between Span Vs Quanti ty of steel requi red
Steel required in RC flat slab Steel required in PT flat slab Span in meter
585.807 451 3.00
1148.96 834 4.00
2086.19 1576 5.003751.24 2841 6.00
Observing the graph, it shows that as span increases, lesser steel quantity is require in Post-
Tensioned flat slab compared to Reinforced flat slab. Graph shows that around 25% of
required steel quantity can be reduced by using Post-Tensioned flat slab.
Graph of required total steel Vs span for RC and PT flat slab
585.807
1148.96
2086.19
3751.24
451
834
1576
2841
0
500
1000
1500
2000
2500
3000
3500
4000
3 4 5 6
Quantityofs
teelrequiredinkg.
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software6
Table 6.5 Comparison between Span Vs Volume of concrete
Concrete volume of RC slab Concrete volume of PT slab Span in meter
10.13 10.13 3.0023.04 18.00 4.0045.00 33.75 5.00
74.52 64.80 6.00
Observing the graph below, it shows that as span increases, lesser quantity of concrete is
require in Post-Tensioned flat slab compared to Reinforced flat slab. Graph shows that
compared to Reinforced concrete flat slab 20-25% of concrete quantity can minimized by
using Post-Tensioned flat slab.
Graph of concrete volume consumed Vs span for RC and PT flat slab
10.13
23.04
45.00
74.52
10.13
18.00
33.75
64.80
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
3.00 4.00 5.00 6.00
Volumeofc
oncreteinm3
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software7
Table 6.6 Compar ison between Span Vs Cost of steel
Cost of steel in RC slab (Rs.) Cost of steel in PT slab (Rs.) Span in meter
19039 23043 3.0037341 40091 4.0067801 75107 5.00
121915 136355 6.00
Required total steel quantity in Post-Tensioned flat slab is less but cost of Post-Tensioning
tendons is much higher because of this reason cost of steel is higher in Post-tensioned flat
slab as compared to Reinforced flat slab. Thus the most significant factor affecting the cost of
slab system of post-tensioning is the tendon length. It is always advisable to obtain budget
prices from a post-tensioning supplier.
Bar chart of cost of steel Vs span for RC and PT flat slab
19039
37341
67801
121915
23043
40091
75107
136355
0
20000
40000
60000
80000
100000
120000
140000
160000
3.00 4.00 5.00 6.00
Costof
steel
Span in meter
RC flat slabPT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software8
Table 6.7 Comparison between Span Vs Total cost of slab
Total cost of RC slab (Rs.) Total cost of PT slab (Rs.) Span in meter
19039 23043 3.0037341 40091 4.0067801 75107 5.00
121915 136355 6.00
Bar chart of total cost Vs span for RC and PT flat slab
In the above chart it is observed that considering design parameters, values of Post-
Tensioned flat slab is lower than Reinforced concrete flat slab. But considering the cost of
the flat slab, because of the higher cost of Post-Tensioning tendons and the higher cost of
layout of Post-Tensioning tendons, the over all cost of slab goes higher. Above graph shows
that Reinforced concrete flat slab is cheaper than the Post-Tensioned flat slab. But as
reaching to the span 6 m, it is observed that cost of Reinforced concrete and Post-Tensioned
flab slab is nearby equal, so it reflects that as increasing the span more than 6m, post-
Tensioned flat slab will become cheaper than the Reinforced concrete flat slab.
55221
119124
227060
401441
79758
143965
251275
424705
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
3.00 4.00 5.00 6.00
Costofsteel+concre
te+labor
Span in mt.
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software9
6.2 FLAT SLAB WITH DROP CAP
Fig. 6.2 Typical geometry of flat slab with drop cap
In above discussion, it shows that Reinforced concrete flat slab without drop cap is cheaper
than the Post-Tensioned flat slab. In the case of flat slab with drop cap, it gives better
resistance to the punching shear than simple flat slab without drop cap and due to this benefit
it allows to go for a span length up to 8 m.
Here analysis and design of flat slab with drop cap is started from span length 5.5 m and it is
extended up to 7.5 m. Increment of span is 0.5 m. results are compared as follows.
L L L
L
L
L
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software10
Table 6.8 Comparison between Span Vs Thickness of slab
Thickness Of RC slab Thickness Of PT slab Span in meter
slab with drop slab with drop
150 225 125 175 5.50175 225 125 175 6.00200 250 125 225 6.50
230 280 150 200 7.00230 300 150 250 7.50
Looking on the above graph it shows that as span increases, thickness of Reinforced concrete
flat slab is also increases whereas in Post-Tensioned flat slab comparatively less slab
thickness is required. Based on bar chart it can be said that thickness of Post-Tensioned flat
slab with drop cap can be reduced by 30% by providing drop cap around column which takes
care of punching shear. compared to Reinforced concrete flat slab with drop cap.
Bar chart of thickness of slab Vs span for RC and PT flat slab with drop cap
150
175
200
230 230
125 125 125
150 150
0
50
100
150
200
250
5.50 6.00 6.50 7.00 7.50
Thicknessofslabinmm
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software11
Table 6.9 Compar ison between Span Vs Span Moment
Span Moment in RC slab Span Moment in PT slab Span in meter
217.16 126.82 5.50309.34 150.85 6.00428.54 183.8 6.50
618.65 276.9 7.00734.78 330.7 7.50
It can be seen from the graph that as span increases, span moment in Reinforced concrete flat
slab increases much higher as compared to Post-Tensioned flat slab with drop cap. Graph
shows that at higher moments are around 55% more in Reinforced concrete flat slab
compared to Post-Tensioned flat slab with drop cap.
Graph of span moment Vs span for RC and PT flat slab with drop cap
Table 6.10 Compar ison between Span Vs Bottom f iber stress
217.16
309.34
428.54
618.65
734.78
126.82 150.85183.8
276.9
330.7
0
100
200
300
400
500
600
700
800
5.50 6.00 6.50 7.00 7.50
SpanMoment(kNmt.)
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software12
Stress in RC slab Stress in PT slab Span in meter
21.20 6.15 5.50
20.20 6.7 6.0019.80 10.9 6.5021.00 10.5 7.00
22.20 11.8 7.50
In Reinforced concrete slab, stresses are more but in Post-Tensioned flat slab with drop cad,
stresses are comparatively too lesser. Stresses in Reinforced concrete slab are almost double
than the stress develops in same spanning of Post-Tensioned slab.
Graph of bottom fiber stress Vs span for RC and PT flat slab with drop cap
21.20
20.2019.80
21.0022.20
6.15
6.7
10.9 10.5 11.8
0.00
5.00
10.00
15.00
20.00
25.00
5.50 6.00 6.50 7.00 7.50
Bottom
fiberstress(N/mm2)
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software13
Table 6.11 Comparison between Span Vs Quanti ty of steel requi red
Steel required in RC slab Steel required in PT slab Span in meter
3197 1675 5.503622 2130 6.004962 3001 6.50
6080 3443 7.008204 4118 7.50
Observing the graph, it shows that as span increases, lesser steel quantity is require in Post-
Tensioned flat slab with drop cap compared to Reinforced flat slab with drop cap. Here it is
observed from graph that use of Post-Tensioned floor can reduce the required steel up to
around 35-45% compared to Reinforced concrete flat slab with drop cap.
Graph of required total steel Vs span for RC and PT flat slab with drop cap
31973622
4962 6080
8204
16752130
3001 3443
4118
0
1000
2000
3000
40005000
6000
7000
8000
9000
5.50 6.00 6.50 7.00 7.50
Quantityofsteelrequiredinkg.
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software14
Table 6.12 Compari son between Span Vs Volume of concrete
Concrete volume of RC slab Concrete volume of PT slab Span in meter
63.57 35.57 5.5074.70 42.30 6.0097.44 51.77 6.50
126.29 68.61 7.00155.81 81.56 7.50
Observing the graph, it appears that as span increases, lesser quantity of concrete is require in
Post-Tensioned flat slab with drop cap compared to Reinforced flat slab with drop cap. It can
be seen from graph that around 30-40% of concrete volume can minimize by using Post-
Tensioned flat slab with drop cap compared to Reinforced concrete flat slab with drop cap.
Graph of concrete volume consumed Vs span for RC and PT flat slab with drop cap
63.5774.70
97.44
126.29
155.81
35.57 42.30
51.7768.61
81.56
0.00
20.00
40.00
60.00
80.00100.00
120.00
140.00
160.00
180.00
5.50 6.00 6.50 7.00 7.50
Volumeofcon
creteinm3
span in mt.
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software15
Tabl e 6.13 Compar ison between Span Vs Cost of steel
Cost of steel in RC slab (Rs.) Cost of steel in PT slab (Rs.) Span in meter
103889 92495 5.50117730 115483 6.00161277 160423 6.50
197594 196796 7.00266617 240204 7.50
Here it can understand that cost of required steel in Reinforced concrete flat slab with drop
cap is nearer to the cost of required total steel in Post-Tensioned flat slab with drop cap. In
some cases cost of steel of Reinforced slab is higher than the cost of total steel of Post-
Tensioned slab. For the span of 7.5 m in post-tensioned flat slab with drop cap, cost of
required steel is reduced around 10% less of the steel required in the Reinf. Conc. Slab.
Bar chart of cost of steel Vs span for RC and PT flat slab with drop cap
103889117730
161277
197594
266617
92495
115483
160423
196796
240204
0
50000
100000
150000
200000
250000
300000
5.50 6.00 6.50 7.00 7.50
CostofsteelinRs.
Span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software16
Table 6.14 Comparison between Span Vs Total cost of slab
Total cost of RC slab (Rs.) Total cost of PT slab (Rs.) Span in meter
304171 334063 5.50390047 402853 6.00517215 505123 6.50
657857 626575 7.00836572 743360 7.50
Graph of total cost Vs span for RC and PT flat slab with drop cap
Above graph shows clearly that increasing span of flat slab beyond 6 m, cost of Reinforced
concrete flat slab with drop cap will be comparatively higher than the cost of Post-Tensioned
flat slab with drop cap. In the other words it can be said that Post-Tensioned flat slab is
proven to be a cheaper and economic option for the long span geometry of slab with out an
obstruction of intermediate column. For greater span, cost of Post-Tensioned flat slab can bereduced by 10-12% as compared with Reinforced concrete flat slab with drop cap.
304171
390047
517215
657857
836572
334063402853 505123
626575
743360
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
5.50 6.00 6.50 7.00 7.50
Costofsteel+concrete+la
bor
span in meter
RC flat slab
PT flat slab
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software17
6.3WAFFLE SLAB
Design of Reinforced concrete waffle slab is started from the 8 m span. Comparison between
reinforced concrete waffle slab and Post-Tensioned flat slab with drop cap is presented
graphically.
Table 6.15 Comparison between Span Vs Thickness of slab
Thickness Of RC waffle slab Thickness Of PT slab Span in meter
Slab Topping Slab with drop
300 75 175 250 8.00300 75 200 250 9.00
350 75 225 300 10.00
400 75 225 325 11.00
Thickness of Reinforced concrete waffle slab is also increases whereas in Post-Tensioned flat
slab comparatively less slab thickness (around 40-50% less) is required.
Bar chart of thickness of slab Vs span for RC waffle and PT flat slab with drop cap
300 300
350400
175
200 225 225
0
50
100
150
200
250
300
350
400
450
8.00 9.00 10.00 11.00
Thicknessofslabinmm
Span in meter
RC waffle slab
PT flat slab with
drop cap
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software18
Table 6.16 Compar ison between Span Vs Bottom f iber stress
Stress in RC waffle slab Stress in PT slab Span in meter
7.60 10.5 8.00
9.62 10.6 9.00
9.02 11.4 10.00
8.63 12.4 11.00
In Reinforced concrete waffle slab, stresses are lower than Post-Tensioned flat slab with drop
cap. Because of more thickness of Reinforced concrete waffle slab, stresses are lesser than
the stresses develops in same spanning of Post-Tensioned slab.
Graph of bottom fiber stress Vs span for RC waffle slab and PT flat slab with drop cap
7.60
9.62 9.028.63
10.5 10.611.4
12.4
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
8.00 9.00 10.00 11.00
Bottomf
iberstressinN/mm2
Span in meter
RC waffle slab
PT flat slab with
drop cap
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software19
Table 6.17 Comparison between Span Vs Quanti ty of steel requi red
Steel required in RC waffle slab Steel required in PT slab Span in meter
12915 5092 8.00
17041 7202 9.00
20830 10247 10.00
25650 13096 11.00
Observing the graph, it shows that as span increases, lesser steel quantity is require in Post-
Tensioned flat slab with drop cap compared to Reinforced concrete waffle slab. According to
graph required steel quantity of steel in waffle slab is around 50% more than quantity
required in Post-Tensioned flat slab with drop cap.
Graph of required total steel Vs span for RC waffle slab and PT flat slab with drop cap
12915
17041
20830
25650
5092
7202
10247
13096
0
5000
10000
15000
20000
25000
30000
8.00 9.00 10.00 11.00
Quantityofsteelre
quiredinkg.
Span in meter
RC waffle
slab
PT flat slab
with drop
cap
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software20
Table 6.18 Compari son between Span Vs Volume of concrete
Concrete volume of RC waffle slab Concrete volume of PT slab Span in meter
204.00 105.61 8.00
246.70 151.88 9.00
321.20 210.30 10.00
408.00 257.15 11.00
Observing the graph, it appears that as span increases, quantity of concrete decreases in Post-
Tensioned flat slab with drop cap compared to Reinforced concrete waffle slab. Around 35-
40% of concrete volume can be reduces in Post-Tensioned flat slab with drop cap compared
to Reinforced concrete waffle slab.
Graph of concrete volume consumed Vs span for RC waffle slab and PT flat slab with drop
cap
204.00
246.70 321.20
408.00
105.61
151.88
210.30
257.15
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
450.00
8.00 9.00 10.00 11.00
Volumeofcon
creteinm3
Span in meter
RC waffle slab
PT flat slab
with drop cap
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software21
Tabl e 6.19 Compar ison between Span Vs Cost of steel
Cost of steel in RC waffle slab
(Rs.)
Cost of steel in PT slab
(Rs.)
Span in meter
419724 280307 8.00
553834 394489 9.00
676973 570710 10.00
833617 752343 11.00
Cost of required reinforcement in Reinforced concrete waffle slab is higher is more than the
cost of required steel in Post-Tensioned flat slab with drop cap. Around 10-25% cost of steel
can be minimize in Post-Tensioned flat slab with drop cap as compared to Reinforced
concrete waffle slab.
Bar chart of cost of steel Vs span for RC waffle slab and PT flat slab with drop cap
419724
553834
676973
833617
280307
394489
570710
752343
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
8.00 9.00 10.00 11.00
Costofsteel(Rs.)
Span in meter
RC waffle slab
PT flat slab
with drop cap
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Chapter 6 Results and Discussion
Design of Reinforced Concrete & Post-Tensioned flat slab using software
Table 6.20 Comparison between Span Vs Total cost of slab
Total cost of RC waffle slab
(Rs.)
Total cost of PT slab
(Rs.)
Span in meter
1431773 1070429 8.00
1796264 1558221 9.00
2268619 1962815 10.00
2827330 2447328 11.00
Bar chart of total cost Vs span for RC waffle slab and PT flat slab with drop cap
Based on the results taken form analysis and design, it can be concluded that Post-Tensioned
flat slabs are proven to be a cheaper option of such kind of long span floors. Here above
graph shows that Reinforced concrete waffle slabs are around 12-14% costlier than the same
span of Post-Tensioned flat slab with drop cap. Reinforced concrete waffle slab is having
other disadvantages also like self weight is more than the Post-Tensioned flat slab, cost of
form work is more and skilled labor is required and is a time consuming activity also.
1431773
1796264
2268619
2827330
1070429
1558221
1962815
2447328
0
500000
1000000
1500000
2000000
2500000
3000000
8.00 9.00 10.00 11.00
Costofsteel+concrete+labor
Span in meter
RC waffle slab
PT flat slab
drop cap
