1. Mohammed Zaid “ post tension slabs Vs Flat slabs “
POST‐TENSION FLAT SLABS NORMALLY REINFORCED FLAT SLABS
Gravity Loadings Gravity Loadings Gravity Loadings
Slab shall be designed to withstand dead , live load Slab shall be designed to withstand dead , live load
Seismic & wind Loads Seismic & wind Loads Seismic & wind
Slab to column connection capacity shall be checked Slab to column connection capacity shall be checked for Loads
for the additional shear due to seismic action the additional shear due to seismic action
a. bonded tendons a. normally reinforced Reinforcement
a.1.Slabs which shall have post‐tension ducts in two
way and average pre‐compression above 0.70 , no
reinforcement shall be required at bottom or top of
slabs , except additional reinforcement where
required at top of columns .
as per code , adequately designed slab can be
constructed with no normal reinforcement .
a.2. practically most of designers use a typical a. Normally reinforced slabs into one layer at bottom
bottom mesh for safety can be as min. as T08C300 and additional reinforcement at top of columns , or
mm or T12C600 to 1200 both way at bottom of the some designers choose to add top basic layer to reduce
slab. the additional reinforcement at top of column for ease
a.3. slab where PT ducts run in one direction only of construction .
shall have min. reinforcement required by code into b. reinforcement shall be as per code min. reinforcement
the other direction where there is no PT ducts c. reinforcement ratio in Flat slabs normally around 100
a.4. Reinforcement ratio including bottom mesh and to 120 Kilograms/cubic meters
additional rebar , trimmer bars around 40 kilograms
per cubic meters.
Cracks Formation Cracks Formation Cracks
Where slab is to be classified as un‐cracked the gross Slabs shall be designed based on Cracked section
section properties shall be used in design of slabs properties
Slab deflection limitation shall be as follow Slab deflection limitation shall be as follow Deflection
1‐ Def. due to live load only = L/360 1‐ Def. due to live load only = L/360
2‐ Long term deflection = L/250 2‐ Long term deflection = L/250
Slab deflection much less than normally reinforced slabs with thickness of L/36
Flat slab.
Slab thickness of L/48 to L/55 according to system
used and according to PTI recommendation , these
values are for guidance only
Durability Durability
a. Tendons are protected into ducts by grout Cracks which might be formed into slabs due to flexural Durability
b. Cracks are minimal so PT slabs are the best may cause reinforcement to be exposed to exposure
solution where severe exposures exist which may lead to corrosion of the reinforcement
Removal of form works Removal of form works Removal of
Slabs shall be initially stressed after 24 hours of Refer to Manual of concrete practice ( MCP ) for shoring form works
concrete cast to eliminate cracks due to shrinkage of slabs
Slabs can be stressed for final stage when concrete
grade min. of 28 Mpa for multi‐strand systems as Per
ACI‐318 requirements and then slab formwork shall
be removed.
M_as@hotmail.com
M_as3@yahoo.com

2. Mohammed Zaid “ post tension slabs Vs Flat slabs “
POST‐TENSION FLAT SLABS NORMALLY REINFORCED FLAT SLABS
Reinforcement congestion Reinforcement congestion Reinforcement
Reinforcement Placement is easy since the Reinforcement shall be heavy where the slab meets the congestion
reinforcement is reduced due to usage of Post‐ column which is costly effective due to required labors
tensioning work on the floor which is desired to be constructed
Punching shear capacity Punching shear capacity Punching
Punching shear capacity is higher than normally Punching shear capacity are limited to concrete strength shear capacity
reinforced concrete due to contribution of Post‐ of the connection .
tension forces.
Forming an opening at alter stage after Forming an opening at alter stage after Forming an
completion of concrete slab stressing completion of concrete slab stressing opening at
When forming large openings in structures after Forming an opening in normally reinforced flat plate alter stage
initial construction, regard must be given to the same procedure of bonded PT slabs after
original design concept. Post‐tensioned slabs are no
different from other forms of construction in this Forming an opening after construction of slabs refers
completion of
respect. that some of the rebar shall be cut concrete slab
Providing a large hole in a slab with bonded tendons stressing
is nota problem, since the operation is. similar to When rebar to be cut , slab shall be checked and
that for a reinforced section. Therefore attention is reinforcement shall be anchored
given here only to forming a large hole through a
slab with unbounded tendons. Additional reinforcement shall be added around opening
as per code required.
In forming larger openings it is likely that more than
one group of tendons will be affected and will need
to be removed. As mentioned above, techniques
already exist for de‐tensioning; similarly, there are
techniques available for re‐anchoring tendons and
re‐tensioning. It is therefore possible to remove
tendons which cross an opening without affecting
the remaining areas of the slab which those same
tendons pass through and support.
Span/Depth
Floor System
Ratio
One-way slabs 48
Two-way slabs 45
Two-way slab with drop
panel (minimum drop panel 50
at least L/6 each way)
Two-way slab with tow way
55
beams
Two-way waffle slab (5ftx5ft
35
grid)
Beams, b ≈ h/3 20
Beams, b ≈ 3h 30
One-way Joists 40
Span to depth ratio of PT slabs Span to depth ratio of Flat slabs
M_as@hotmail.com
M_as3@yahoo.com