Page 45 - Building Regulation and Design Guidelines - Structural (Grey Code)

SECTION: 3 GUIDELINES FOR PRE STRESSED CONCRETE WORKS (SLABS) SECTION: 3
3. Requirement for the other pre stressed tendons
shall be as per Section 6 - Technical Report TR43,
second edition.
c. Tendons
1. The maximum tendon spacing shall not exceed 8
times the slab thickness or 1.5 m whichever is lesser.
For banded distributed system, tendon spacing
should not exceed 10 times the slab thickness or 1.5
m whichever is lesser in the banded direction. 2 ducts
(with minimum 3 strands/duct) passing through the
column strip should be provided to maintain the
banded direction requirements. Column strip shall
include the area bounded within distance equal to
0.5 slab thickness all-around the column perimeter;
otherwise distributed system shall be considered in
the respected direction, Fig. (3.9) .
evently spaced tendons in span evently spaced tendons for short span
banded tendons over columns for long span
banded tendons over columns
Tendons banded in two directions Tendons banded in one direction
Fig. (3.9) Banded-distributed systems
2. Wherever there are certain difficulties to comply with
the strands distribution basis, partial pre stressed
slab shall be used provided that the PT strands to
be compensated with designed conventional steel
in the proper locations and directions.
3. The minimum horizontal spacing between the ducts
is the greater of 75mm or duct width.
4. Curved tendons are to be avoided, but in case of
difficulty to furnish the straight tendons, hair pins
should be used in additional to bottom and top steel
mesh not less than T10-200 mm, the curving shall
not exceed 1:12.
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GUIDELINES FOR PRE STRESSED CONCRETE WORKS (SLABS)
5. Strands shall not be extended from level to another.
6. Tendons shall be avoided to be stopped inside the
slab without support at ends. Support can be drop or
hidden beams, walls or columns.
d. Loads
1. Design loads shall comply with project design
criteria, BS 6399 and ASCE-7-05 requirements. In no
cases, the live load should be less than 2.5 KN/m2.
2. Jacking force should be taken as per design code and
should be not more than 80% of breaking loads.
e. Deflection control
1. Factors related to short-term elastic deflection
estimation are as per Fig.(3.10):
2. Pre cambering in the PT slab is subject to designer
justification prior to implementation.
3. Long Term Deflection shall be checked using cracked
sections criteria and shall be within the allowable
limits as per the applied code.
Loading Factor related to shor-term
elastic de ection value
Dead 3.0
Post-tensioning (after losses) 3.0
Live 1.5
Fig. (3.10) Factor taking account of long term effects
f. Conventional reinforcement in the pre stressed slab.
1. The minimum bottom shrinkage mesh shall be
applied by using T10 each 300mm.
2. All support areas shall have the applicable code
specified minimum reinforcement in the top for
purpose of distributing the cracks and strength
design requirements.
3. Conventional reinforcement should be placed along
edges of all slabs; this should include U-bars laced
with at least tow longitudinal base top and bottom,
Fig. (3.11).
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