Pre-stressing

The most challenging of these applications has been the location and characterisation of anomalies in thick concrete structures using seismic methods and the detection of reinforcing steel and pre-stressing cables in congested structures using radar. 

It has been found that the contrast in film density caused by very small local variations in mass density of the concrete is considerable, e g. S D = 0.12 for a 6 mm diameter hole in a 250 mm thick concrete beam. The image quality provided by fine-grained films (Agfa Gaevert D7) was sufficient to distinguish the thin walls of a pre-stressing duct in a 750 mm thick concrete bridge slab. 

Radiography provides the only means of reliably detecting voids in pre-stressed cable ducts or of detecting loss of section or fracture of eables inside the duets. The maximum thiekness of eonerete whieh ean be radiographed for confident loeation of voids inside ducts is of course dependant on a number of variables, e g. amount of reinforcing bars, size of void in duet etc... 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

Cherry, B., Cathodic Protection of Buried Pre-Stressed Concrete Pipes . In Cathodic Protection Theory and Practice, 2nd International Conference, Stratford upon Avon, June... 

For a given grade of PA 46, Figure 4.64 displays an example of an SN or Wohler s curve concerning flexural tests with maximum stress of S and pre-stress of 20 MPa. 

Figure 4.64. SN curve of neat PA 46, examples of maximum stress S (MPa) versus number of cycles at rupture (N) with a 20 MPa pre-stress...

The stress distribution within the thick-walled structure can be turned to better eveness by introducing appropriate benefitial pre-stresses with multilayer shrink fitting or autofrettage. 

In the multilayer design, half-shells, formed from sheets 6 up to 22 mm-thick sheets, are applied to the core tube, which is seamless or welded by a longitudinal seam. The shells are joined one to the other, and to the underlying layer, by two longitudinal seams (three-sheet seams). The half-shells are tangentially pre-stressed by the welding contraction. Uniform dis... 

The most commonly used types of closures are shown in Fig. 4.3-18 [14], The cover seals (Fig. 4.3-18 A - E) are attached by means of threaded bolts and are used for all diameters and pressure ranges. The bolts are generally tightened hydraulically or pneumatically. It is, however, also possible to pre-stress the bolts longitudinally and then to tighten the nuts. 

This effect of pre-stressing is due to physical breakdown of some structure of the filler/rubber composite, its exact nature being unimportant as regards testing procedure. It is self-evident, however, that if a rubber during service is subjected to repeated deformations testing, should be carried out after prestressing rather than in the initial state where the result may be different due to the effect of the unstable structure. 

Most fatigue tests apply a fixed pre-stress or strain, partly because without bonding of the test piece it is necessary to hold the rubber in place. The amount of pre-stress or strain will affect the fatigue life in particular the fatigue life is appreciably shortened if the cyclic deformation passes near to or through zero strain. 

It is also possible to use cycles of constant energy which is quite commonly the situation met by such products as dampers and shock absorbers, but this is more difficult from the apparatus point of view. The pre-stress can in fact also be a constant stress or a constant strain. 

Attractive and simple as the technique is in theory, in practice there are a number of difficulties which severely limit its value. Only areas of disbond, not a weak bond, can be detected although very weak areas can be made to part by pre-stressing, which is in any case necessary to separate the debonded areas. Notwithstanding these remarks, there have been considerable developments in ultrasonic flaw detection over the years although there has not been any widespread adoption of the technique in the rubber industry generally. 

Pre-stressed concrete outer container with steel liner... 

The functioning of a spring valve is based on a pre-stressed spring. When the pre-stress force is reached, the disc starts to move and the valve opens. With lifting equipment attached to the valve spindle, the valve is opened and the pre-stress force measured, taking into account the above factors (seat area and operating pressure). 

When limiting our attention to purely spherical pre-stress we find analytical forms for the solutions of Bessel or Modified Bessel equations in dependence on the coupling coefficient Ksf. The obtained density profiles may show an oscillating behavior we prove the conjecture that oscillating profiles are unstable as well as the non-oscillating ones which correspond to sufficiently high absolute values of Ksf. 

Long filaments Reinforced columns and panels Pre-stressed concrete... 

National Research Council of Canada, Corrosion Behaviour of Pre-stressed Stainless Steel Embedded in Chloride Contaminated Concrete, Report A-264 1. 1, December 4, 1997. 

Fig. 8. Peak leakage current of pre-stressed samples as a iiinction of the receding angle, with advancing angle as parameter.

Soil nailing is similar to ground anchors or tiebacks in that a steel rod is grouted into a pre-drilled hole. There are, however, several important differences. Nails are considerably smaller and shorter than anchors, and while anchors are pre-stressed after placement, nails are not (with few exceptions in which a very small pre-stress is applied), and do not pick up load until the soil mass deforms. Nails, like anchors, add shear resistance to the soil mass. 

Phillips Master trainer, 150, 230 Piccadilly tube tunnel, 232, 234 Pickwick Quarry, 45, 207 Admiralty lease of, 151 Pickwick Road, Corsham, 196 Piaor, Alan, 16, 17, 18, 154 Pillboxes, 121-123 Pioneer Corps, 121,131 PlesseyLtd, 148 Pockeredge House, 68 Poison canon, 206 Portal, Lord, 173 Post-war rearmament, 135 Power-egg, 171-172 Power-jets Ltd, 225 Pre-stressed concrete, 86 Priory Street, Corsham, 196 Property Services Agency (PSA), 141,144... 

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