Copying and network storage prohibited. Amendment No. Amdt 2, March NOTE 1 In terms of the Standards Act, Act 29 of , no person shall claim or declare that he or any other person complied with an SABS standard unless a such claim or declaration is true and accurate in all material respects, and b the identity of the person on whose authority such claim or declaration is made, is clear. Construction works. Selection of pipes for buried pipelines.

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Preliminary design of Design optimization of Influence of key facto Cathodic Protection of Superstructure desi Design procedure for p Reliability-based desi Design and Site Applic Load-Deflection Respon Prestressed Concrete. Michel Virlogeux--New The Design and Analysi Report No. In addition to the recommended procedures, other methods are described for the sake of completeness and to compare different methods of design.

The Committee consisted of: Mr. Attwell Dr. Cross Mr. Glover Mr. Gohnert Mr. Goldstein Dr. Liebenberg Dr. Lunt Mr. Moore Dr. Pretorius Prof. Scholz Mr. Spitz Mr. Wium Chair Editor Corresponding A. Goldstein wrote most of the text, drew the diagrams, and wrote the computer programs.

It should be noted that a decimal point has been used in the text. Computer output generally uses a point and not a comma, and it was felt that mixing two systems would be confusing. Diagram 9. 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 , and the required factor on D.

This has the effect of reducing reinforcement areas, and cracking and deflection require more attention. To make allowance for this, SABS was revised, and among other changes, the allowable concrete shear stress was reduced by 10 percent, to lessen the probability of brittle shear failures.

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. In fact it was due partly to tensile stresses in the concrete and partly to arching effects reducing the measured stress in the reinforcement. 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. 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.

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. 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 committee considers that prestressed flat slabs do not have a better record than one-way slabs, and its recommendation is given below.

Prestressed Concrete Slabs 2. Technical Reports 17 and 25 are based on the load balancing method for preliminary design, and on a frame method for analysis. The ACI code also uses the frame method but takes account of the torsional flexibility of the slab at the columns See 5.

Recently a certain amount of analysis has been done by the finite element method, and by grillage programs. These are discussed later. If premature failure due to shear is prevented, the interior panels of flat slabs have considerable reserves of strength due to two way arching action, and membrane stress. There is no recognised design method which takes advantage of this at present, so it is not feasible to take the extra strength into account.

It does, however, emphasize the fact that exterior panels are more vulnerable, and greater care must be taken with them. Because of the reserve of strength due to arching, the Committee recommends that the exterior and corner spans be designed with additional non-prestressed reinforcement, similar to the USA Uniform Building Code for one-way slabs , but it is not considered necessary for internal spans. A reinforcement of 0. In a lecture given some years ago it was suggested that less than 1.

Many slabs are now designed with stresses less than 0. It is the Committee's philosophy that there is a continuous spectrum of concrete from plain reinforced to fully prestressed concrete, and that there should be no arbitrary limits on prestress levels. It is considered that prestressed flat slabs are essentially ordinary flat slabs, subjected to additional lateral loads from tendons, which reduce the effective load on the slab, and to longitudinal prestress which helps to reduce cracking.

However if prestress levels appreciably lower than the ones recommended in Reports 17 and 25 are used, there is a greater onus on the designer to satisfy himself that the requirements for deflection and cracking are met. The main advantage of prestressing slabs is that the prestress acts as an upward load resisting the effects of dead load, reducing the long term dead load creep which is the cause of most of the deflection.

For this reason it is considered that if less than half the dead load is balanced, a good deal of the advantage of prestressing is lost. See section 3. However, the actual amount of prestress is an economic decision. Preliminary Manual Design The method recommended depends on the load-balancing method made popular by Lin.

Prestressed Concrete Slabs Page 3 Prestressing is treated analytically by removing the prestressing tendon and replacing it by the equivalent forces that it applies to concrete.

By taking reverse curvature of the tendon into account, the complete prestress loading diagram on the slab is derived Diag. The system may be described as follows Decide on column centres, concrete grade usually 30 or more and preliminary slab thickness, as well as drops, capitals, etc.

Choose a preliminary thickness from paragraph 3. The shear will often govern, unless capitals or drops are used, and should be checked first. Decide on the amount of load to be balanced. This is an economic decision 3. Using the maximum drape possible, decide on the preliminary geometry of the tendon.

From 2 and 3 calculate the prestress force required. Analyse the slab for Dead Load, Live Load for pattern loading see 4. Calculate the working stresses at various points and check for allowable tensile stress and deflections. It should be noted that the limitation of tensile stress is not a good way to limit cracking, but is acceptable for preliminary design.

Final design should be based on limiting crackwidth. Calculate the reinforcement required and check the maximum compressive stress in the concrete for the ultimate limit state. Check the shear stress, and calculate the reinforcement required, if any. Depending on the results of 5 and 6 it may be necessary to adjust the thickness and prestress, and perhaps to supply additional non-prestressed reinforcement to control cracking or shear.

Non-prestressed reinforcement is always required over columns, and in external spans. Check the prestress losses due to shrinkage, creep, friction etc.

It should be noted that although the sample calculations take account of the variation in prestress along the length of the structure, and of the effect of curvature of cables over the supports, it is sufficient for preliminary design to assume a uniform prestress along the length of the structure except where the prestress is varied by changing the number of tendons, and to assume that the cables hang from the supports i.

See Diag 3. Some desi gn ers ma ke the above assumptions even for final design. However it should be realised that one is designing the non-prestressed reinforcement for the difference between the downward loads due to permanent load and live load, and the upward load due to prestress. The difference between these quantities may be sensitive to small variations in either one.

Several programs are available which enable changes in the slab thickness, concrete grade, number of tendons etc. The programs show stress, deflections, and additional reinforcement required, as well as highlighting areas of excessive shear stress.

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Partial factors for selected reinforced concrete members: background to a revisionof SANS Contact details. The application of Eurocode EN in revising the South African standard for structural concrete design SABS will require the determination of partial factors in accordance with the reliability requirements of the revised South African loading code SANS Further research is required on the model uncertainty for different structural members flexural members, shear, columns, walls and the theoretical models of basic resistance variables related to quality control.


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