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Prestressed Concrete Flat Slabs - Introduction

CDIO 496

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, and due to the much cheaper shuttering

required, they became popular for residential and office buildings.

A number of catastrophic shear failures of flat plates occurred, including some where several floors of a building suffered

progressive collapse. As a result a large amount of research into shear in flat slabs has taken place, and various methods

of reinforcing slabs against shear failure have been developed. Because of the brittle nature of shear failures, conservative

design is necessary.

1.3 Prestressed Flat Slabs and Plates

Prestressed flat slabs and plates have been developed mainly in Australia and the USA over the last 30 years. They have

a number of outstanding advantages. Among these are a shallower depth (for the same deflection), quicker stripping of

shuttering, and greater shear strengths than plain reinforced slabs of the same depth. Prestressing is also applied to waffle

type slabs to achieve even greater spans.

Tendons in post-tensioned concrete are considered to be either bonded or unbonded. Unbonded tendons are usually single

strands covered with grease and an outer plastic sheath.Bonded tendons usually consist of a number of strands in a metal

sheath, which is grouted after the tendons are stressed.. Unbonded tendons have the advantage of low friction values,

maximum lever arm and drape due to the smaller diameter, fast placing and avoidance of grouting operations. In the USA

and UK, and in South Africa prestressed flat slabs have been almost entirely unbonded, whereas in Australia bonded

tendons are the rule.

The disadvantage of unbonded tendons is that the prestress depends on the anchorage remaining intact throughout the life

of the structure. Corrosion or accidental damage could cause tendon failures at any time, and detailing must take account

of this, to try and prevent or reduce the possible effects. The USA Uniform Building Code requires that for one-way slabs

unprestressed reinforcement sufficient to carry the Dead load + 1/4 live load at ultimate, is provided to prevent

catastrophic failures in the case of loss of prestress. The committee considers that prestressed flat slabs do not have a better

record than one-way slabs, and its recommendation is given below.