Over the last decades,cable stayed structures,particularly cable stayed bridges have gained increasing importance in the construction activities.Consequently,the cable techniques have been improved steadily as well as the cable erection methods.

VT has joined that development from the early stages.As a result,we are able to present a stay cable system,which is state-of-the-art. VT‘s system has been proving its excellent abilities in various remarkable bridge projects.

Anchorages
VT-Stay Cable anchorages are designed to meet the highest requirements for fatigue resistance.Recommendations for fatigue testing according to PTI(Post-Tensioning-Institute)are as follows:

• Number of load cycles= 2,000.000 cycles
• Maximum stress= 0,45 x GUTS
• Stress variation= 200 N/mm2

Not more than 2 % of the number of individual wires may fail during fatigue testing.No failure shall occur in the anchorage material,or in any component of the anchorage.

The test specimen shall be reloaded after fatigue loading and shall develop a minimum tensile force equal to 95,0 % of GUTS(= Guaranteed Ultimate Tensile Strength)of the cable.

VT-stay cable specimens having various numbers of strands,with sizes up to a maximum of 91 strands,have passed these tests with perfect results.

Outer Seathing
Among the various materials in question,HDPE(High Density Polyethylene)has prevailed against the others.The advantageous properties of HDPE – tubes for stay cables are mainly:

• High mechanical strength and durability
• Good resistance against environmental effects
• Good workability(it can be cut,machined, welded)
• Not difficult to procure being a standard product

VT usually recommends and uses HDPE-tubes manufactured according to designation PE 3406(ASTM D 3350,Appendix)or classification PE 80(ISO 4427),unless a different quality is specified.

As PE-technology progresses,it can be expected that production processing of tubes made from PE 100 will be satisfactory,resulting in products with even higher technical data.

Generally,the thickness of the HDPE-tubes should not be minimized but kept at a certain level to ensure proper butt-welding and stability during erection;e.g.a thickness of at least 10 to 15 mm,depending on the tube diameter,is recommended.

In order to give the cables a more distinctive architectural appearance,VT invented the HDPE – tubes furnished with a coloured outside layer,which is co-extruded directly during tube production.Light colours will,of course,reduce sun-radiation effects to some extent.

Cables
VT-stay cables are regularly made up from VT-CMM bands,where 4 strands dia 15,7 mm(= 4x 150 mm2)are combined in one band.VT-CMM(Compact Multi Mono)is a patented product:4 strands are sheathed with PE in an extrusion process to form a band.The strands are greased or wax-covered at the same time.

Two-fold corrosion – protection is provided in this way.The band-shaped configuration of 4 strands is advantageous with regard to workability:The labour demand for production of stay cables is reduced and the proper order of strands in the cable can be achieved more easily with bands than with monostrands.Bands with only 2 strands and even single strands(also PE-sheathed and greased or waxed)can also be provided if required. Depending on the corrosion protection philosophy outlined for a project,the strands can additionally be galvanized,thus giving extra corrosion protection. As an alternative to the VT-CMM-bands,the more commonly offered monostrands could also be used for VT-stay cables.

Filling of Cables
(= filling the space between the strand sheathing and the outer sheathing)with appropriate material is certainly recommended.Amongst others,PU-foam materials and wax have been used satisfactorily.Cement grouting,in particular has displayed its merits in very many cases and is therefore most widely applied.

The most important advantages of cement grouting are:

• Grouting provides additional protection to the cables.It is common knowledge,that filled cavities generate less corrosion problems than unfilled ones.
• The natural frequencies of grouted cables are lower compared with ungrouted cables.This makes them less vulnerable to excitation from different parameters,which is the starting point of many vibration phenomena.
• The technology of grouting is commonly known and can be applied safely under nearly any conditions.
• Exchange of single strands or of the entire cable can still be performed,should it become necessary.
• There is an extended record of performance available for grouted cables.
• It should be mentioned,that cement grouting could be modified to be flexible.VT has successfully adopted this method in several instances.

Disadvantages of grouted cables might be regarded as follows:

• The cable weight is higher than that of ungrouted cables.This has to be paid attention to in the design.The cable weight,for cables shorter than 300 m,can be deemed to be no problem in either the design or the cable erection.
• Theoretically,grouted cables are expected to have larger thickness and diameter due to the grouting pressure,etc. In practice,however,cable diameters are determined much more by the general design criteria and by the needs of workability,e.g.welding of tubes,effort during erection,etc.,than by the grouting pressure.

The advantages and disadvantages of cement-grouted cables listed above will, to a certain extent, be applicable to all other filling materials.Some of the more remarkable features of the other grouting materials are:

Wax
has a lower specific weight(approx.0,9 kg/dm3)than cement grout.Therefore it is applicable when the cable weight is decisive.The equipment required is considerably larger.Sealing of the cable anchorage portions need special considerations.

PU-material
specific weight(approx.1,0 kg/dm3)which is also less than that of cement grout.Equipment and handling appear to be less easy.Sealing of the anchorage portions is not as important as for wax.PU-material is more costly than the other filling materials described,there are hardly any performance records available for this material.

Stay Cables Assembly and Erection
Out of a large variety of methods,3 methods can be focused on:

a) Assembly of cables in the works,and delivering them ready – made to site,followed by their erection.As there are always some variations in the cable lengths designed,compared with the actual lengths,it is obvious that this method has narrow limits and is therefore rarely employed.

b) Assembly of the cables on site while carefully considering the lengths measured on site at the bridge,also followed by the erection.

c) Assembly and erection at the same time.The cables are assembled at an inclination more or less in their final position.

VT prefers method(b)and has developed it to such an extent that both work progress and safety are satisfactory.
The method allows:

• Measurement of the required cable lengths on site,immediately before cable assembly.Construction tolerances can then be easily compensated for.
• Pre-assembly of the number of cables needed in the next stage(e.g.in the case of cantilever construction of the bridge deck).Erection of the ready assembled cables is very quick and thus,the construction schedule is not delayed.Cable erection is, in any case, on the critical path of the construction schedule.

Description of the VT-method
Cable assembly usually takes place directly on the bridge deck or next it on an appropriate platform.HDPE-tubes are delivered to site in straight pieces,about 12 m long.They are butt-welded to the required cable length.

Rollers are placed in a straight line,to accommodate the strand bundle.The strand bundle is then pulled into the HDPE sheathing.

All anchorage components are mounted on both anchorages,except for the anchor nuts.The ring wedges are put into their cones.They are pre-blocked at their passive end,and secured by a fastening plate,this is already the final stage.The wedges at the active end are only temporarily secured.The cable is then filled(grouted).

Special steel sleeves are fastened at several points along the cable,to allow for the lifting.The cable is now ready for erection.

Erection starts with lifting and inserting of the upper cable end.The anchor nut is screwed on.The rest of the cable hangs down to the deck and lies on the rollers.

The cable is then taken at the lower anchorage and inserted into position.The cable is lifted at one or two places in the middle in order to reduce the large sag and at the same time the lower anchorage is pulled towards its final position and the anchor nut is mounted.

There are some different methods provided for stressing of the cables:

Stressing with monostrand jack (“even stress method”):

The strands are stressed one after another to the required load level.This method is not as quick as stressing with a full-sized jack,but if space is limited,it may be the only applicable method.

Stressing with a full-sized jack:This is the most common method,and is applied in most cases.All the strands in one anchorage are stressed at one time by the same amount.

A combination of monostrand stressing and “stressing-in-one” is also possible:A group of strands from an anchorage is stressed at one time,followed by the next group etc.This method may be appropriate when the monostrand stressing is unacceptable but the handling of a full-sized jack appears to be too difficult.