Prestressed piling

RESULTANT STRESS AT BB STRESSES DUE TO DUE TO DEAD LOAD OF BEAM IMPOSED LOADS AND PRESTRESSING FORCES

RESULTANT STRESSES AT BB

DUE TO DEAD LOAD

OF BEAM. PRESTRESSING

FORCES AND IMPOSED

LOADS

COMPRESSION

Civil engineering and building works

Where tensioning of wires is carried out after the concrete has set, as in the case of prestressed concrete reactor pressure vessels, the method is termed post­ tensioning and where the wires are stressed before the concrete is placed it is called pre-tensioning.

Post-tensioned systems are referred to as unbonded, when the wires or strands are protected against corro­ sion by specially formulated oil or grease applied prior to installation in the ducts, or by injection after installation. Bonded systems are those which are grouted-up by injection of cement grout following stressing. The grout also acts as corrosion protection provided measures are taken to ensure the absence of voids.

14.2 Prestressed piling

As described in Section 4.5 of this chapter, prestressed piles have been used in large numbers for the foun­ dations of power stations. These piles arc usually prestressed and cast in long line stressing beds, which allow several piles to be cast in one bed. Steel end plates are inserted between the piles with holes drilled to allow the uninterrupted course of the strands from the fixed achorage at one end of the bed to the stressing ’ system at the other. Good control needs to be exerted over the increase in concrete strength so as to allow the piles to be separated and lifted from the beds as quickly as possible, and to ensure that the piles are not driven before achieving adequate strength.

The layout of strands needs to jmsure that the prestress within the pile is as evenly disposed as possible. The amount of prestress is dictated not by the working load on the pile but rather by stresses imposed by lifting from the beds, storage, pitching and driving. To prevent damage to the pile head or toe during driving, the links are provided at close centres after the top and bottom length of each pile (typically 2 m to 3 m at earth end). In some cases this end reinforcement is enhanced by the addition of normal longitudinal rein­ forcing bars.

14.3 Prestressed concrete pressure vessels and containments

Prestressed concrete pressure vessels (PCPVs) and prestressed concrete containments (PCCs) play central roles in nuclear steam supply systems, although their design duties are somewhat different.

The PCPV is relatively thick-walled (4 m to 5 m) compared to the PCC (1 m to 1.5 m), since its primary function is to retain high gas pressure (20 bar to 40 bar) for the majority of its operational life. Typical PCPV dimensions are illustrated in Fig 3.45. In contrast, the PCC has a relatively passive function to fulfil during its

Chapter 3

operational life since it is only called upon to contain leakage at low pressure under normal reactor operating conditions. It is principally designed to retain higher pressures (typically 5 bar) which could result from low probability short duration events such as a rupture of the primary coolant circuit pipework. Typical PCCs are illustrated in Fig 3.46. Both the PCPV and the PCC have additional functions which are to provide biologi­ cal shielding for the station operators and to support internal and external structures and plant with small allowable deformations under sustained temperature gradients. PCCs may also function as a missile barrier, for example, against tornado-generated missiles, tur­ bine missiles or aircraft w'here applicable.

Apart from the double-barrier 1300 MW French PCCs and Canadian CAN DU plants, all PCPVs and PCCs are lined with a mild s^eel membrane, typically 13 mm thick for the PCPV and 5 mm thick for the PCC. In both types of structure the prestressing system, ip common wilh all prcslrcsscd concrete, is designed Io resist the tensile stresses induced in the concrete by the applied loadings whether these are from mechanical loads such as internal pressure or from strain-controlled loads such as temperature cross falls.

Operating conditions require that PCPVs are equip­ ped with thermal insulation and liner cooling water pipe systems to ensure that the liner and concrete are maintained at acceptable temperatures. These provi­ sions are unnecessary for PCCs where internal oper­ ating temperatures are not damaging to either steel or concrete.

The design and analysis of PCPVs and PCCs has been established over the last 25 years. The applicable Standard for PCPVs is BS4975: 1973. [26], The prin­ ciples which hail already been established in UEGB specifications and had been incorporated into the practical design and construction of PCPVs are reflected in this standard which is under revision. The equivalent standard used in the United States for PCPVs and PCCs is the ASME HI Division 2 [27].

The service load analysis approach adopted by the CEGB for the PCPV is a working stress approach, based on an analysis of the vessel for a series of idealised loadings which represent the most severe combinations of load which could be applied to the vessel. The gas pressure used for design purposes is set at 10% above the normal working pressure to allow for operating transients and tolerances. The principal loading cases are as follows:

•Prestress alone at transfer force.

•Prestress plus proof test pressure at ambient temperature; proof pressure is set at 15% above the design pressure.

•Early life operating condition including prestress, plus design pressure, plus design operating tempera­

ture distribution.

•Late life operating condition including prestress, plus design pressure, plus design temperature distribution.

•Start-up and shutdown conditions at early and late life including consideration of transient tempera­ tures arising from the start-up and shutdown modes.

•Fault studies in which a number of low probability fault conditions are considered in the design. The designer may be required to show that the vessel can continue to fulfil its safety function in the event that the postulated fault occurs.

Similar conditions apply to the design of PCCs in accordance with the ASME III Division 2 specification approach although, due to the difference in duties, the emphasis is upon factored internal and external loading combinations which represent the most severe loads which the PCC may have to sustain.

In accordance with BS4975 it is an additional require­ ment that PCPVs have an ultimate load capacity which is generally 2.5 or 3 times the design pressure. In calculating the ultimate load factor the designer has to consider all possible modes of failure of the vessel due to internal gas pressure. This is a hypothetical loading case since the safety relief valves limit pressures under the worst credible conditions to about 30% above the design pressure. However, the ultimate load analysis is an important feature of the design requirements, since it enables the designer to check that the design will have a ductile response to pressure and that non-linear behaviour will only commence at pressures well beyond the design pressure.

In order to ensure that the mode of failure of the vessel will be close to that predicted in the ultimate load calculations, model tests are performed to determine the mode of failure and the ultimate capacity of the vessel. These model tests are normally carried out on a one tenth scale replica of the vessel. The results have shown that analytical predictions of ultimate load factor and mode of failure provide conservative estimates of the pressure that the vessel can withstand. These tests have also demonstrated the large reserves of ductility built into the vessels.

Similar tests are proposed for PCCs to be constructed by CEGB and these are required to demonstrate an ultimate load factor of at least twice design pressure.

The civil and structural construction programme for PCi’Vs has to be integrated into the programme for plant and reactor installation.

The milestones in vessel construction are:

• The installation and grouting of the liner base plate, sometimes carried out as a one-piece operation with the liner walls.

• 1’he allocation and timing of bays for concrete pours to avoid heat of hydration problems in end caps and walls.

Prestressed concrete

•The assembly, temporary support and concreting of the standpipe zone which carries the fuel and control rod penetrations in a closely pitched array with tight dimensional tolerances.

•The stressing of the prestressing system.

In general, the civil engineering techniques used for PCPVs and PCCs are no more complex than those in conventional structures, apart from the large scale and the need to carry out mock-up trials in advance of construction to validate the proposed methods for certain critical or complex areas.

Following construction and prestressing, PCPVs are subjected to a proof pressure test at 15% above design pressure, and unfuelled and fuelled engineering trials prior to raise power and synchronisation with the electricity grid system.

PCCs are required to pass a structural overpressure test (SOT) at 15% above design pressure and an integrated leak rate test (ILRT) at 10% above design pressure. The latter test may be repeated at intervals throughout the service life of the PCC.

The Nil’s nuclear site licensing conditions outlined in Section 24.5 of this chapter require that-PCPVs are inspected on a regular basis and any necessary main­ tenance carried out. Each reactor is shut down regu­ larly at two year intervals for maintenance. At this time the external features of the vessel are inspected by the CEGB’s Appointed Examiner. The minimum pro­ gramme consists of the following items:

•Prestressing system load checks to determine the' residual force in the tendons.

•The condition of prestressing anchorages.

•The condition of prestressing strands or wires with­ drawn from a number of tendons.

•The condition of the concrete surface.

Other items which are included in the inspections include:

•PCPV foundation settlement and tilt.

•A summary of embedded vibrating wire strain gauge readings and their correlation with theoretical predictions. •

•A summary of vessel temperatures and their con­ formity to the operating rules for the vessel.

As a direct result of the Appointed Examiner’s respon­ sibility for regular inspection and maintenance of PCPVs, a considerable amount of information has been amassed on the performance of prestressed concrete structures.

The main conclusion that can be drawn from in­ service examinations of prestressed concrete pressure vessels is that they are remarkably robust structures and that the predictions made at the design stage have been fully borne out in practice. " .