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Design of prestressed Concrete flat slabs.pdf
The South African Institution of Civil Engineering
Postnet- Suite 81
Private bag X65
Halfway House 1685
South Africa
This Report is intended to serve as a manual of good practice for the design of prestressed concrete flat slabs..
In addition to the recommended procedures, other methods are described for the sake of completeness and to compare
different methods of design.
The Report was produced by a sub-committee of the Joint Structural Division of the South African Institution of Civil
Engineers, and the Institution of Structural Engineers.
PRESTRESSED CONCRETE FLAT SLABS
1.0 Introduction
In 1989 the Structural Division of the South African Institution of Civil Engineers created a sub-committee to examine
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committee decided to produce a booklet of recommendations for good practice.
The matter was considered especially important because the South African Loading Code was changed with effect from
1990, and the required factor on D.L. is now 1.2, whereas it was previously 1.4. This has the effect of reducing
reinforcement areas, and cracking and deflection require more attention. To make allowance for this, SABS 0100 was
revised, and among other changes, the allowable concrete shear stress was reduced by 10 percent, to lessen the probability
of brittle shear failures.
1.1 Flat Slabs
Flat slabs were originally invented in the USA at the beginning of this century, and there were a number of patented
systems.
The early reinforced concrete flat slabs all had drops, and columns with capitals, and were considered to be the structure
of choice for warehouse construction and heavy loads. Because of the columns capitals and drops, shear was not really a
problem.
Design was based on tests on stresses in reinforcement at working loads, and the early codes required a total moment in
a span of WL2/11.
It was realized that statically a total moment of about WL2/8 was required for equilibrium, (If the column diameter is D,
the statically required moment is (very closely) W(L-2D/3)2/8 where L-2D/3 is the effective span. The difference between
WL2/11 and WL2/8 was attributed to a mystical '2 way action'. In fact it was due partly to tensile stresses in the concrete
and partly to arching effects reducing the measured stress in the reinforcement.
The philosophy, and the empirical coefficients, persisted until the 1950's when the allowable stresses in reinforcement
were increased, limit state design was introduced, and the statically required moment of WL2/8 was introduced into the
codes. This was because it was felt that it was not safe to rely on arching or tensile strength of the concrete. In addition
to the changed moment coefficients, the frame method of analysis was required in certain cases.
1.2 Flat Plates
Flat plates were subsequently developed, with no drops and no column capitals/
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Ch. 14 the ocean floor
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Design of prestressed Concrete flat slabs.pdf
Design of prestressed Concrete flat slabs.pdf
The South African Institution of Civil Engineering
Postnet- Suite 81
Private bag X65
Halfway House 1685
South Africa
This Report is intended to serve as a manual of good practice for the design of prestressed concrete flat slabs..
In addition to the recommended procedures, other methods are described for the sake of completeness and to compare
different methods of design.
The Report was produced by a sub-committee of the Joint Structural Division of the South African Institution of Civil
Engineers, and the Institution of Structural Engineers.
PRESTRESSED CONCRETE FLAT SLABS
1.0 Introduction
In 1989 the Structural Division of the South African Institution of Civil Engineers created a sub-committee to examine
the design of prestressed concrete flat slabs. It was found that a certain amount of poor design was prevalent, and the
committee decided to produce a booklet of recommendations for good practice.
The matter was considered especially important because the South African Loading Code was changed with effect from
1990, and the required factor on D.L. is now 1.2, whereas it was previously 1.4. This has the effect of reducing
reinforcement areas, and cracking and deflection require more attention. To make allowance for this, SABS 0100 was
revised, and among other changes, the allowable concrete shear stress was reduced by 10 percent, to lessen the probability
of brittle shear failures.
1.1 Flat Slabs
Flat slabs were originally invented in the USA at the beginning of this century, and there were a number of patented
systems.
The early reinforced concrete flat slabs all had drops, and columns with capitals, and were considered to be the structure
of choice for warehouse construction and heavy loads. Because of the columns capitals and drops, shear was not really a
problem.
Design was based on tests on stresses in reinforcement at working loads, and the early codes required a total moment in
a span of WL2/11.
It was realized that statically a total moment of about WL2/8 was required for equilibrium, (If the column diameter is D,
the statically required moment is (very closely) W(L-2D/3)2/8 where L-2D/3 is the effective span. The difference between
WL2/11 and WL2/8 was attributed to a mystical '2 way action'. In fact it was due partly to tensile stresses in the concrete
and partly to arching effects reducing the measured stress in the reinforcement.
The philosophy, and the empirical coefficients, persisted until the 1950's when the allowable stresses in reinforcement
were increased, limit state design was introduced, and the statically required moment of WL2/8 was introduced into the
codes. This was because it was felt that it was not safe to rely on arching or tensile strength of the concrete. In addition
to the changed moment coefficients, the frame method of analysis was required in certain cases.
1.2 Flat Plates
Flat plates were subsequently developed, with no drops and no column capitals/
Sinkhole
Sinkholes are depressions in the ground formed by the dissolution of carbonate rock underground, such as limestone, by groundwater. There are two main types of sinkholes - cover-collapsed sinkholes which form suddenly and cause damage, and cover-subsidence sinkholes which form slowly over time. Sinkholes are more likely to occur in karst terrain where carbonate rock lies close to the surface. Sinkholes can damage infrastructure and property, and mitigation efforts include monitoring areas prone to sinkholes and stabilizing loose soil.
Ch. 14 the ocean floor
Nearly 71% of Earth's surface is covered by the global ocean. The ocean floor can be divided into four main regions: continental margins, ocean basin floors, and mid-ocean ridges. Continental margins are the zones between continents and ocean basins, and include continental shelves and slopes. Ocean basin floors contain features like abyssal plains, seamounts, and deep-ocean trenches. Mid-ocean ridges are underwater mountain ranges found near the centers of ocean basins where new seafloor is formed through seafloor spreading.