Errores según hogares sin capacidad de pago

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Handbook toBS8IIO:1985

characteristic strengthalso require the manufacturer to provide load-extension curves for the estimation of the

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and that none should be less than 0.95

fm,.

Both Standards

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extension of tendons in stressing operations.

Since the characteristic strength of a tendon is specified in terms of breaking load.

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dimensional accuracy does not directly affect the ultimate strength of prestressed concrete members.

Further information on the manufacture and properties of steel for prestressing tendons may be obtained from reference 8.1.

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8.2 Handling and storage C

Since nearly all the types of tendon ingeneral use have a high tensile strength imparted

I-by cold-working, it is important that they should not be subjected to temperatures which would impair their properties. In handling and storage. therefore, the tendons should

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not be near cutting or welding operations without proper safeguards.Experience does not suggest that corrosion causes serious problems when reasonable care is taken to provide good conditions of storage for tendons. It must be recognized.

however, that the steel used in tendons is susceptible to severe corrosion in circumstances

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where ordinarv reinforcement would suffer little damage. Protection from ground damp L is essential, because severe corrosion has resulted when sulphates or other salts in the

soil have come into contact with the steel. Corrosion may also be caused by stray welding p currents or even the presence of bacteria near the steel. In coastal construction, protection

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from airborne spray and salt is needed.

Where storage is prolonged, provision should be made for regular inspection of tendons

for pitting. Visual examination for surface pitting is requiredand metallurgical inspection ^K should be made in cases of doubt. Reductions in tensile strength resulting from severe

and unacceptable levels of corrosion may be quite small and changes in mechanical properties may be assessed better from the changes in ductility revealed by bend tests or the extension at fracture.

8.3 Surface condition

To avoid superficial rusting, the manufacturer usually gives the steel a protective coating which needs to be removed by the method suggested to obtain good bond: if light surface rusting has developed, however, removal of the coating is not necessary.

It has been established experimentally that light surface rusting of hard-drawn wire has little or no effect on the static or fatigue strength of members in which it is incorporated.

8.4 Straightness

Except for cold-drawn wire supplied in mill coils, wire and strand complying with British Standards should pay out reasonably straight. As straightening modifies the properties

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of steel substantially and in some respects adversely, it should be done only under the manufacture(s control.

8.4.1 Wire

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8.4.2 Strand

8.4.3 Bars 160

_ K ^{-- - - - - .-} -..-.---....-~---.---K---,KK-K -‘-K-.-.K...

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Rart 1: Section 8

~J 8.5 cutting

Pre-tensioned tendons of wire or strand may be cut flush with the ends of units. No special measures for protecting the ends of the tendons against corrosion are then 1required.

The method of cutting should not impart shock to the tendon. as this might impair 1bond or cause slip in the anchorage if the tendons have not been grouted.

8.6 Positioning of tendons and sheaths

The recommendations on accuracy of placing apply to both pre-tensioned and post-tensioned tendons. Pro”ided that the tolerance of ±5mmis maintained, there should 71 be no difficulty in satisfying the requirement that the actual cover should be not lessthan the nominal co’-er less 5mm.

These tolerances are so small that they are unlikely to affect compliance with requirements for serviceability and ultimate limit states except for very shallow members.

~1 In practice, it will often be necessary to agree larger tolerances on positioning.In short members with pre-tensioned steel, it is usually sufficient to position the tendons at their ends only. but for long members some intermediate supports, which may be withdrawn before the completion of casting, may be required to prevent the tendons being displaced by vibration or other cause during the filling of the moulds. For post-tensioning, the positioning of the tendons is usually governed by the positioning of the sheaths or duct-formers which should therefore be fixed firmly during concreting. If sheaths are used. it-may be desirable to place the tendon in the sheath before concreting.

thereby stiffening the sheath, and to support the sheaths either temporarily or permanently at centres of at least 0.75m. Maintenance of the true cross-sectional form of sheaths and ducts and avoidance of leakage is needed to minimize frictional effects during the stressing operations.

8.7 Tensioning the tendons

:ij 8.7.1 General

If prestressed concrete construction is to resist cracking and comply with the requirements for construction, the prestressing forces imposed musi be as required in the design;

success therefore depends on the skill and accuracy with which the prestressing operations ] are carried out in the field and in the factory. All tensioning should be done under the direct control of a supervisor with thorough experience of the various stressing operations involved.

] The choice of system of prestressing to be used in particular circumstances does not usually present difficulty. Pre-tensioning is normally used for the mass production of similar units. such as floor beams. If they can be readily transported. they are precast in the concrete products factory, but if they are too large to be handled easily then they may be made on a prestressing bed at the site. Post-tensioning is most frequently employed in large structures and carried out in situ as construction proceeds. However, where sites are very constricred. it may be more convenient to precast the members in short sections 11 in the factory and to assemble them on site: one advantage of this is that it gives better

control of the concrete production.

Each of the methods of post-tensioning available has particular advantages which may make it more suitable in certain circumstances. For short members. bars with threaded ends are most suitable because losses of prestress due to draw-in. which could be excessive with wire or strand. are completely avoided. Ears can carry the largest prestressing forces K in individual tendons but strand has the advantage if they have to be curved. Large tendons can be built up from groups of strands which may be anchored together or in individual anchorages. or from individual wires anchored in groups by wedge-anchorages or by button-heading in a special anchorage assembly. For particularly long tendons, I both wire and strand have the merit of being available in long lengths and so do not need connectors. The longer the tendon, the less the significance of the loss of prestress

due to draw—in of the grips. _lbI

Handbook to B58110:198S 8.7.2 Safety precautions

During the life of a prestressed concrete structure. the concrete and the steel are usually most severely stressed during the operations associated with tensioning and transfer, at a time when the strength of the concrete is not fully developed and the anchoring of the

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steel may be only temporary. It is then, therefore. that the risk of failure and of accident is greatest. Although it is not possible to safeguard personnel completely from the risks of such an accident, reasonable precautions should always be taken when working with or near tendons which have been tensioned or are in the process of being tensioned.

Personnel should not stand in line with the tendons, anchorage or jacking equipment.

Simple protective measures such as stout timber shields should be placed in line wi th~

the tendons and behind the jacks to protect those passing in the course of their duties.

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Each factory or site will call for separate consideration of the most reasonable form’-’

of protection. It must be emphasized, however, that the most effective safety precaution is the proper supervision and training of personnel in prestressing techniques.

Manufacturers instructions for the use of stressing equipment should always be followed~

closely.

Notes for guidance with regard to safety precautions for prestressing operations are provided in references 8.2 and 8.3.

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8.7.3 Tensioning apparatus

Item (c) requires that the elongation of the tendon be measured. This measurement I

should be checked against the elongation calculated from the load—elongation % L relationship supplied by the manufacturers of the tendons for the batch of material being

used.

8.7.4 Pre-tensioning 8.7.4.1 General

8.7.4.2 Straight tendons 8.7.4.3 Deflected tendons

8.7.5 Post-tensioning

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8.7.5.1 Arrangement of tendons

8.7.5.2 Anchorages

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8.7.5.3 Deflected tendons

The requirement refers to deflectors for external tendons, as the curvature of internal

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tendons will be determined by that of the ducts which will be usually less onerous than the limit given here.

The use of deflectors of smaller radius of curvature or with a larger angle of deflectio is permitted as long as test data on the loss of strength are obtained. Some experimentaIL results (see reference 8.4) for strand of 12mm diameter show that the loss of strength is less than 10% for a ratio of deflector radius to tendon diameter of 2. There is therefore

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considerable scope for testing, but it should be noted that the secondary stresses that develop at sharp changes in curvature would have an adverse effect on fatigue strength under cyclic loading and could aggravate an otherwise passive situation should mildly

corrosive conditions develop in the region of the deflector.

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8.7.5.4 Tensioning procedure

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8.8 Protection and bond of prestressing tendons U

The recommendations apply only to post-tensioned steel; pre-tensioned steel is 162 adequately protected by the concretecover. provided that the requirements for thickness

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.-

-

^{— — -- —}

.

^{— 4— - :~<-.—.—~.K.-K....• . K — ..~K~.-— .~K. — —}

K.KK KK. K~KK K - — - K K

Parr 1~- Section 8 of nominal cover and for quality of concrete in 4.12.3.1.2 are satisfied. The primary reason for pouting internal tendons and for encasing external tendons in concrete is to protect the steel against corrosion and fire.

A secondary but important consideration is the effect of grouting and encasement on stiffness and strength. If other materials based on bitumen. epoxy resins or rubber are used, there may be a reduction in stiffness and strength which will need to be allowed for and the fire resistance may be affected. Great care is needed in selecting materials which are not bound by cement, to ensure not only that they are not harmful to the steel but that they cannot become so under conditions that could develop in the ducts over a prolonged period.

8.8.1 General

8.8.2 Protection and bond of internal tendons 8.8.3 Protection and bond of external tendons

In document límites alternativas estratificación Los de la en busca de Carlos Eduardo Sepúlveda Rico Denis López Camacho Juan Miguel Gallego Acevedo (página 158-169)