chapter 10 - cables and terminals.pdf

10. Cables and terminals

Wire strand ropeWire ropes are normally composed of a number of steel wires spuntogether to form six strands, which in turn are spun together around afibre core to form a rope. This is the simplest common rope formationand there are many variations of it, each designed to suit a particular setof operating conditions. For instance, a rope which is required to operateon a winch with a relatively small-diameter drum must be flexible. Such arope would have a strand formation involving a larger number of smallwires. On the other hand, a rope for use, say, as a mast stay does notrequire flexibility and would be made from a smaller number of largewires. Between these two extremes there is a large range of permutationsof strength and flexibility, each being particularly suited to a specificrequirement.

Ropes for structural use generally do not require flexibility. Indeed,rigidity is desirable. For these applications it is usual to use single-strandropes which are composed of layers of wires built up around a centralking wire until the required strength is achieved. These ropes may becomposed entirely of round wires, in which case they are known as spiralstrands. Alternatively, they may be of the locked coil type which employsone or more layers of interlocking wires on the outside of the rope toproduce a smooth external finish. Both these types of rope are capable ofoperating satisfactorily in structures, and the choice between the two islargely a matter of the personal preference of the designer. Tables 10.1and 10.2 give typical properties for spiral-strand and locked coil cablesproduced by British Ropes Ltd.

When considering structural applications it has become common torefer to ropes of all types under the collective term 'cables'. In thischapter this term is used to refer to all types of ropes, spiral and lockedcoil strands. In terms of cost there is some advantage to the spiral strandbut, particularly in the larger sizes of strand (say 90 mm and above), thebetter fill factor (ratio of steel area to strand area) gives the locked coilconstruction some advantage in terms of breaking load to diameter ofstrand.

Table 10.3 compares typical properties of the two constructions. Thetable is based on cables produced from standard tensile strength wiresspun together using standard constructions. In practice, the requirementsfor individual projects are so varied that special designs are usually drawnup to suit the particular requirements of the designer.

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Cables and terminals

Table 10.1. Spiral-strand bridge cables

Diameter(mm)

25-030-533-142-045-966075-086-4

10201160127-0137-0

Table 10.2.

Diameter(mm)

24-035-838-848-051-563-273-687-2990

101-51160

Table 10.3.

Diameter(mm)

507090

100110

Minimum breakingload (kN)

557-2795-69100

1500-9180503747-4484616131-38524-9

10486-913371-015 068-2

Elastic weight(kN/100 m)

2-994-545-488-48

101521-1127-3535-8949-97660178-338910

Locked coil bridge cables

Minimum breakingload (kN)

50901106-81314-52001-32452-53492-44708-86553-18721190350

12 3900

Elastic weight(kN/100 m)

3066-587-92

12-5715-0821-4229-6941-1451-8956-2075-96

Steel area(mm2)

374546678

10331240257033304330602078629450

11015

Steel area(mm2)

373802966

15301840261036175012632069009251

Comparison of locked coil and spiral-strand cables

Locked coil

Minimumbreaking load

(kN)

2188439572108907

10644

Spiral

Weight Minimum(kN/m) breaking load

0-13240-26450-43210-54510-6599

(kN)

21394238683884769928

Modulus(kN/mm2)

169-7169-7169-7169-7169-7157-9157-9147-2147-2147-2147-2147-2

modulus(kN/mm2)

158-4158-4158-4158-4158-4158-4158-4158-4158-4158-4158-4

Weight(kN/m)

0-11840-23660-38910-48270-5833

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Introduction to cable roof structures

SteelGenerally the wires used in structural cables have large diameters sinceflexibility is not required and, indeed, where corrosion is a problem, thelarge-diameter wires give more body of steel to withstand its effects. Thewires, which are cold drawn from steel rods, have carbon contents usuallyin the range 0-5%-0-8% and are normally galvanized to give addedprotection. To the wiremaker the basic raw material is in the form of hot-rolled rods with diameters up to approximately 12 mm. This materialmay then be subjected to a patenting process which involves heating thewire to a temperature of approximately 950°C and quenching in a bath ofmolten lead at around 550C to achieve a grain structure suitable fordrawing down to the final diameter or shape. The action of drawingthrough a tungsten carbide die reduces the cross-sectional area of the wireat the same time as it increases the length and raises the tensile strength.

Modern wire-drawing machines produce the required reduction as acontinuous process through a series of dies. In this way the tensilestrength of the material is raised and a careful assessment is made ofthe effect of the ductility of the wire which is measured in terms ofpossible bend, torsion or wrap tests. At this stage a typical round wire forstructural use will have a tensile strength in the range 1-6-1-8 kN/mm2,with ductility measured by its ability to withstand a number of turnsaround a mandrel, equivalent to three times the diameter. As mentionedabove, for structural usage it is normal to galvanise the wires to giveprotection against corrosion.

Manufacture of cablesTypical cables (Fig. 10.1) for use in cable structures will normally notemploy a fibre core. Such cables have a tendency to stretch as the fibrecore reduces in diameter and this can lead to problems, particularly in ahumid environment. This is due to denaturing of the fibre core, allowingpartial collapse of the rope which, in turn, produces increase in length.Thus, the most suitable cables for long-term use in structures are thosewhich are composed solely of steel. For some small lightweight nets six-stranded cables with an independent wire rope core are suitable, but forthe heavier applications the more suitable constructions are spiral-strandand locked coil.

In the case of the traditional six-stranded cable, a typical constructionmight be a cable composed of six strands spun around a small wire ropecentre (independent wire rope core). Depending on the flexibility re-quired, the strand construction might range from a simple six wires spunaround one to a more complex 19-wire strand made up of nine wires overnine wires over one. These six strands are then spun together to form thecable centre. This type of cable would normally be employed in what isknown as ordinary lay, i.e. strands laid up in one direction, usually left-hand, with the cable spun up in the opposite direction, usually right-hand.

For the larger spirals and locked coil constructions the method ofmanufacture is similar to that for one strand of the conventional cable.

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(a)

(b)

(c)

W)

(e)

Fig. 10.1. (a) 6 x 7 fibre core; (b) 6 x 7 independent wire rope core; (c) 6 x 19(9/9/1) independent wire rope core; (d) spiral-bridge cable; (e) locked coil bridgecable

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The cables are built up layer after layer by passing through the ropemak-ing machine using round or shaped wires in combinations to produce thefinal diameter, breaking load and modulus values. During this process,carefully controlled lay lengths are employed which basically govern thebreaking load and extension characteristics of the finished cable. In thecase of a spiral-strand cable one tensile strength of wire is used through-out the construction, whereas in a locked coil cable it is normal for theshaped wires to have a lower tensile strength than the round wires; thus, alocked coil cable may have two tensile ranges in its construction; one forthe round wires and one for the full lock wires. During the manufacture,corrosion-resistant dressings are normally applied to each layer of wiresto eliminate corrosion as far as possible when the cable is in service. Forstructural cables the lubricant must be of a type which gives long-termprotection and does not readily flow from the cable under the influence ofhigh temperatures. At the same time, if the structure has to withstandvery low temperatures, the protective lubricant must not have anytendency to go brittle and flake from the cable surface.

British Ropes, for example, normally uses a resin-based protectivecompound containing aluminium platelets which form a flexible barrierbetween the surface of the steel and the atmosphere. This compound isknown as Metalcoat and has proved satisfactory in practice. Laboratorytests have shown that coatings of 200 um give protection against corro-sion for a very long period. The compound is applied at every stageduring the manufacture of the cables, with a final coat or coatingsapplied after the erection is complete.

Another method of protection now available is to encase the cable inan extruded sheathing of plastic material such as high-density polyethy-lene. Typically, the sheathing is 5-7 mm thick and very tough anddurable. This sheathing, when combined with watertight end fittings,provides the best possible protection for the cable provided the sheathingis not seriously damaged.

Environmental factors affecting steel cablesSteel cables, as machines, frequently work with at least some of theirload-bearing members (the wires or strands) exposed to the prevailingenvironmental or climatic forces. In many cases these forces can alsoaffect the cable internally.

Assuming the ubiquity of oxygen, the climatic and environmentalforces of importance in the deterioration of materials can be taken as:

(a) moisture (usually with dissolved solids)(b) heat and cold(c) solar radiation(d) solid particles(e) biological agents.

These forces can, of course, act independently or in concert. In thefollowing sections they will, for simplicity's sake, be discussed indepen-dently.

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MoistureThe most serious of the environmental deterioration agencies from thecable usage aspect is undoubtedly moisture, and all components in a steelcable may be susceptible to a greater or lesser extent. The factors to beconsidered in respect of moisture are its form (liquid or vapour), its state(static or dynamic), and the nature of the substances dissolved in it. In theliquid state moisture exists either in mass (sea, river, lake or pond water,etc.) or as discrete droplets (rain or driven spray), and can cause deteriora-tion by chemical action, physical action, or some combination of these.

Water in mass, (sea, river, lake or pond water)Water in mass can cause deterioration by chemical and/or physicalmeans. Under conditions of total immersion, and ignoring chemicalreactions for a moment, the deteriorative potentialities of water in massare probably some function of its velocity past the cable. The higher thisvelocity, the greater is the deterioration. Under normal conditions ofcable usage the velocity varies from negligible for a cable immersed in atank or lake to about 18 knots or more for such cables as trawl warps. Atthe lower velocities the deteriorative action of water is mainly due tochemical action; at the higher velocities both chemical action and erosionmay be important.

Water as discrete droplets (rain or driven spray)Again a possibility exists of materials' deterioration by chemical and/orphysical action, and of importance in the latter are the kinetic forcesinvolved when water droplets hit rope. These will vary with droplet sizeand impact velocity; droplet size varies from 0-001 to 0002 cm for mist to0-04-0-5 cm for heavy rain; impact velocity from almost zero for a cableenveloped in mist to over 100 miles/h for a cable exposed to the fury of agale. Under conditions of mist, light drizzle, etc., the predominantdeteriorative mechanisms are undoubtedly chemical, although evendrizzle can cause the physical removal of some cable dressings. In heavyrain both physical and chemical factors may play a part.

Water vapourWater in the vapour phase is, to all intents and purposes, always presentin the atmosphere; the amount being dependent on temperature and theavailability of liquid moisture. The measure of air saturation with watervapour is relative humidity (RH), which varies from below 10% intropical desert climates to 100% + in tropical rain forests. Under thelatter conditions deterioration by water vapour is probably at its worst,but even in England and other countries with similar climates it is a factorthat must be considered, especially in respect of metal corrosion. It hasbeen shown that carbon steel will corrode if the RH is over 70%, and thefollowing figures have been quoted for Kew, London, in June:

• 2200 h-0600 h over 80% RH• 0600 h-0900 h 70-80% RH• 0900 h-1900 h below 70% RH• 1900 h-2200 h 70-80% RH.

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In this location, June had longer periods of relatively low humidity thanany other month, and in December the relative humidity was over 80%all the time.

As the dynamic potentialities of water vapour are negligible for presentpurposes, its deteriorative potentiality can be considered as mainlychemical.

Heat and coldAs with moisture, heat and cold can cause deterioration by chemical and/or physical means. For instance, as the temperature rises, the rates ofmany chemical reactions increase and new reactions become possible.Some materials change state (solid to liquid; liquid to gas). Conversely, asthe temperature falls reaction rates may decrease, some liquids solidify,and some plastic solids become brittle. Under natural conditions theextremes of temperature likely to be encountered are -40° to 90°C. Thereis evidence to show that the temperature of cables standing exposed tothe sun for long periods under tropical desert conditions can reach 90°C,with a shade temperature of 45°C. (In England, cable temperatures inexcess of 50°C have been encountered with a shade temperature of28-30°C.)

The change in cable length as a result of temperature is a factor thathas to be taken into consideration, and a value for coefficient of linearexpansion of 0-0000125 per °C is generally used.

Solar radiationThe deteriorative factors of solar radiation would seem to be caused bytwo distinct groups of radiation: the infra-red (wavelengths above about10 000 A) and the ultraviolet or near-ultraviolet (wavelengths belowabout 4000 A). Of these, the former is mainly important from thetemperature aspect; absorption of infra-red is the cause of the hightemperatures when a cable is exposed to the sun. Its effects can, forpractical purposes, be considered along with the effects of temperature ingeneral.

Ultraviolet, on the other hand, causes physical and chemical changes inmaterials by activating photochemical reactions. It is, however, difficultto consider it on a quantitative basis, or to define limits. All cablesexposed outdoors may be considered subject to it, but it is probably at itsworst under tropical desert conditions, and absent for cables indoors.Similarly, only the outside layers of a cable are subjected to it, for thewires form an effective barrier against its penetration to the internalsurfaces. When cables are sheathed to protect against corrosion, stepsmust be taken to minimize the ultraviolet degradation and the indicationsare that the incorporation of carbon black into the sheath provides thenecessary resistance.

Solid particlesThe term 'solid particles' includes dirt, sand, grit, etc., and again these areagents which can act both chemically and physically. Chemical reactionbetween roping materials and solid particles is of importance only in the

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presence of moisture and, perhaps, can be considered in this context.From the physical aspect, a survey of the pertinent factors indicates thatthese are perhaps important wherever substantial quantities of solidparticles come into contact with the cable. In general, these may be takento be:

(a) cables in contact with the ground(b) cables in excessively dusty and dirty environments(c) cables immersed in water with a high suspended solids content, i.e.

near the sea bottom.

Protective coatingsIn practice, cables working in corrosive environments are usually madefrom carbon steel wires, which are inherently corrodible materials, withsome form of protection against corrosion. While a degree of protectioncan be provided by suitable lubricants and rope dressings, experience hasshown that these may not be sufficient to provide the very long-termprotection demanded by structural usage. Therefore, it is normal practiceto provide additional protection by having galvanized or aluminizedwire-wires with protective metal sheaths which are not inherently corro-sion-resistant but are less susceptible to corrosion than plain carbon steel.However, even with wires with metallic sheathing, it must be acceptedthat the main corrosion protection is provided by lubricant or dressing.With the exception of permanent sheathing with, say, high-densitypolyethylene, it is therefore necessary to inspect cable surfaces regularlyto ensure that the lubricants or dressings have not been removed byabrasion or chemical reaction and to ensure that additional coats areapplied when necessary. The time interval between applications willdepend on the particular environment in which the cable is employed.Of the two metallic protective coatings, galvanizing is by far the morecommon and may be deposited on the wire surface by either electroplat-ing or hot-dip methods. Highly refined zinc with a purity of 99-95% isemployed, and coating thicknesses up to 300 g/m2 are normal.

Cable propertiesThe designer of most structures employing steel wire cables will generallyrequire the highest possible modulus of elasticity E in order to limitextension under load and to keep the weight of the unit to a minimum.Young's modulus for the steel wire itself will always be in the order of190 kN/mm2 and this value will remain constant within the tensile rangesnormally employed in structural ropemaking.

When the wire is spun into a cable the modulus of the whole is less thanthat of the individual wires, dependent on the lay length or helix pitch atwhich the wires are spun. The longer the lay length, the nearer themodulus value comes to the straight-wire value and vice versa. Typicalmodulus values for structural cables are:

• spiral strand, 145-170 kN/mm2, depending on size• locked coil, 158 kN/mm2.

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Similarly, the breaking strength of the complete cable is dependent on thelay length at which it is spun, being relatively low for short lay lengthsand greater for longer lay lengths. Thus, it is apparent that after thetensile range of the wire the major factor governing both modulus ofelasticity and breaking load is the lay length of the cable. The lay lengththat can be employed is largely dependent on the application for whichthe cable is intended and the construction chosen. Spiral-strand con-structions will normally have lay lengths in the range 9-12 times the cablediameter, depending on the size of the finished strand and the number oflayers of wires employed. The smaller sizes of strand may have the longerlay lengths and, hence, greater E values and relative breaking loads. Thelarger and more complex the spiral becomes, the more the lay has to beshortened in order to produce a good, tight strand. This, in turn, leads toa lower value for E and breaking load.

Locked coil construction, however, because of the interlocking of theouter wires, may have correspondingly longer lays which are more or lessconstant throughout the size range and in the larger sizes, say 90 mm andabove. It offers better modulus values, although breaking load is notautomatically better because there are limitations on the tensile strengthof the shaped wires.

Another important factor, particularly for structural applications, isthe fill factor of the cable. This is the relationship between the total cross-sectional area of the wires in the rope and the area of a circle correspond-ing to the diameter of the rope. Typical values of fill factor are:

• six-stranded ropes with fibre core, 50%• six-stranded ropes with wire core, 60%• spiral-strand, 75%• locked coil, 85%.

Prestressing strands developed for use in concrete structures normallyconsist of seven wires spun six-around-one. After spinning, the completedstrand is given a low-temperature heat treatment which results inmodified load/extension characteristics. Typical modulus of elasticitywill be around 198 kN/mm2, and limit of proportionality 80%. Normalgalvanized roping wire will have a modulus of elasticity approximately186 kN/mm2 with no definite limit of proportionality. Prestressed strandscan therefore be operated at higher stress levels than conventional cableswithout permanent extension occurring. However, more complex cablesconsisting of more than one layer of wire over a king wire cannot besuccessfully heat-treated, and this introduces a practical limit to the useof prestressing strands. Table 10.4 gives the types, dimensions and someof the properties of prestressing strands produced by British Ropes Ltd.

Cable terminationsCables throughout the size range can be terminated with sockets employ-ing a hot metal fill material, usually high-grade zinc or a cold-pouredpolyester resin. The cable end is opened up for the required length by

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Table 10.4. Types, dimensions and properties of prestressing strands

Type

BS 5896Standard

BS 5896Super

BS 5896Drawn

Diameter(mm)

9-311012-515-2809-6

11-312-915-712-715-2180

Prestressing

Breaking load(kN)

92012501640232-0

70010201390I860265-0209030003800

strand

Weight(kN/100 m)

0-4000-5460-71610690-2920-4240-5790-77011580-8731-2701-717

Steel area(mm2)

527193

139385575

100150112165223

Note: Typical values for the elastic modulus range from 195 to 205 kN/mm2.

unlaying the individual wires, which are then cleaned using suitabledegreasing fluids. When the wires are completely cleaned and dried thesocket is positioned on the 'brushed' section and the socked 'basket' iscompletely filled with the socketing material. When correctly designedand fitted, these sockets will always achieve the full breaking load of thecable on test.

The external shape of the socket can be varied to suit the individualdesign requirement, but for each shape variation the basic dimensions ofthe socket cone or basket will remain the same for a given size cable.Sockets are generally steel castings but can be produced machined fromsolid steel, and in either case are nearly always hot-dip galvanized toBS 729, Part 1 (Figs 10.2(a) and (b)). All bridge sockets are designed andstressed to utilise either medium- or high-tensile steel, the overall basketlength and diameter depending upon the cable diameter and breakingload. Generally, when a single conical basket is used the length of thebasket is approximately 5'A-6 times the cable diameter and the majordiameter of the cone is approximately 2-3 times the cable diameter(Fig. 10.3).

In addition to metal- or resin-filled sockets, some smaller cables, ofdiameter up to approximately 38 mm, may be terminated by means ofhydraulically compacted fittings. These fittings can incorporate a male-threaded end or fork end, etc., for coupling to the adjacent structure.Such fittings can achieve a guaranteed 95% of the cable's minimumbreaking load (Figs 10.4(a), (b) and (c)), and are referred to as swayedterminals.

All terminations of whatever type should be subjected to rigoroustesting during manufacture by non-destructive methods, which may take

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(a) (b)

Fig. 10.2. (a) Socket terminal with pin connector; (b) socket terminal with screwthread

the form of X-rays, ultrasonics, surface dye penetrants or magneticparticle checks. Testing of terminals after fitting can take place in theform of a proof load test but this is limited to a maximum of 50% of thecable's breaking load to ensure the cable does not yield. Usually thedestruction test of the cable is carried out through identical sockets tothose of the finished product and this, together with rigorous control orfitting procedures, is acceptable proof of the assemblies' efficiency.

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/O.i. Bridge socket

Linearization of cables—prestressingAll structural cables are prestressed and then measured to length whileheld under their calculated load. The modulus figures quoted above referto cables in the prestressed condition. To appreciate the description ofprestressing which follows, it is necessary that certain statements must beaccepted:

(a) that a wire cable is a machine composed of many working parts(b) that no complicated machine has the same characteristics when

new as those it develops when fully 'run in'

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II1

I

II

i

ni ii ii ii ii ii ii i

bd

(a) (b) (c)

Fig. 10.4. (a) Swaged terminal with pin connector; (b) swaged terminal with screwthread; (c) swaged terminal with eye bolt head

(c) that wire cable, consisting as it does of many moving parts, in its'running-in' period must stretch when subjected to load

(d) that the modulus of elasticity E, frequently needed in calculationsis a variable factor, being low when the cable is new andunstressed and greater during the useful life of the rope.

Where a reasonably accurate apparent modulus is needed for calculation,or where it is important to maintain an exact cable length (i.e. whenprovision for length adjustment is small or non-existent), it is of primeimportance that the cable be prestressed. The major benefits of prestres-sing are found when cables are used as stationary load members onsuspension bridges, suspended roofs, television mast guys, boom sup-

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ports on large-face shovels, etc. In these cases, no cable adjustment isusually allowed for, bearing in mind that the facility to adjust for lengthalso brings with it the ability to create uneven stresses in the structure.

Considering the new cable as a machine, all the individual componentsare relatively loose, the lays being those at which they are manufactured,yet the application of load creates an elongation of the lay helix, atightening of the individual wires and a compressive force in the centre orcore of the cable. As these individual components adjust themselves tothe effect of the load, the cable elongates. This settling-in or 'running-in'period is known as the period of initial or constructional stretch.

If a length of straight unstressed new cable is taken and subjected toload then, on the release of that load, it will be found that the cable hasextended. This amount of stretch is frequently taken as %% of the lengthfor lightly loaded cables up to a figure of approaching 1 % for heavilyloaded cables. Initial stretch cannot be accurately determined by anytheoretical means. Thus, if a prescribed length of cable is required at agiven load and this new cable is given one application of load only, cutand socketed, it will be found in installation to measure the prescribedlength. However, new and unused wire cable has greater elongation thanused cable so it will continue to stretch until the initial or constructionstretch is completely removed.

Because the greatest proportion of initial stretch occurs during theearly period of the cable life, it follows that the modulus of elasticity isalso smallest during this time. In addition, it will be appreciated that aftersome time under the load the modulus will increase. The amount ofincrease is affected by the length of service of the cable, the intensity ofworking loads, whether loading is constant or variable, and the vibrationto which the cable is subjected. During this period, the load in the cablewill be reducing due to the increase in the length of the cable as theremainder of the initial stretch is removed. Therefore the cable will haveto be retensioned and if tests were carried out it would be found that ateach retensioning the apparent modulus of the cable had increased.

After the initial or constructional stretch in the cable has beeneliminated, the cable will behave as a truly elastic medium, whereelongation is truly proportional to the applied load. During prestressingthe cable becomes truly elastic and, at the same time, develops the fullyapparent modulus of the cable. The load is then reduced to the calculatedworking or marking load, the prescribed length being marked and cut.

As an example of cable behaviour, consider the type of graph formedby an autographic testing machine when testing a cable to destruction,the load being applied at a uniform speed. Initially, the graph will show asteep rise or elongation for small progression along the load scale, thisbeing the initial or constructional stretch as the wires bed down. Soon thegraph will show considerable reduction in the rate of elongation for equalincrements along the load scale. This line is straight and this is the zonewithin which elongation is proportional to the applied load. If loading iscontinued further, it will be seen that the graph breaks away from thestraight line and the rate of elongation increases. The rapid lengthening

159


of the cable in this portion is known as permanent stretch which will notrecover. Further application of load will cause the graph to rise evenmore steeply although still progressing along the load scale, until a pointis reached where the extension increases rapidly to result in final fractureof the cable without any further application of load.

Referring again to methods of prestressing, the cable is loadedrepetitively to loads that are well within the elastic limit of the materialuntil the autographic recorder shows that the hysteresis loops formed bythis loading and unloading are coincident. The coincidence of these loopsis definite proof that the sample is behaving as a truly elastic materialand, therefore, developing the maximum apparent modulus betweenthese loads. When marked in this state and with the end attachmentsaccurately positioned, the cable can be erected with the minimum ofdifficulty, and with great accuracy, in the certain knowledge that if themarking load is then applied, the cable will return to its measured lengthor, conversely, if the cable is returned to its measured length, then thetension in the cable will be the same as the marking load. In short, thepurpose of prestressing is the removal of constructional or initial stretch.It must be borne in mind that prestressing should only be applied tocables with steel centres. Prestressing is not recommended for fibre-coredcables.

After the cable has been prestressed and removed from the plant andsubsequently coiled for despatch, wires forming the cable are disturbed intheir relationship one to another and, when the first small application ofload is applied on installation, the cable will behave in a similar mannerto a non-prestressed cable. However, the wires rapidly return to theposition that they previously occupied at the working load. After two orthree days, a check is made to ensure that the cables are still retaining theload applied on installation.

After prestressing, when the cable is being brought to the markingload, the cable temperature must be accurately determined for themarking length will be listed as a length at a given load at a standardtemperature. Correction for temperature variation must be made.

As an example of a modern prestressing plant, British Ropes Ltd offersa length accuracy of ±6 mm at a stipulated marking load and tempera-ture. Cables can be prestressed under loads of up to 7500 kN and inlengths of up to 731m. Longer lengths can be accommodated byprestressing in sections.

CreepAll materials creep under the influence of constant load, and steel wire isno exception. However, the extent to which wire will creep is the subjectof a great deal of research at the present moment and it is difficult to giveactual figures. If the cable into which the wire is spun is of normallyaccepted construction and lay length and it has been prestressed in anacceptable manner, then the creep likely to occur during its working life isso small as to be, in most cases, unimportant.

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FatigueAll steel cables have a definitive fatigue life beyond which the wires in thecable will begin to fracture. This fatigue limit is governed by the level ofintensity of the fluctuating stress at which the cable is working and theconditions under which the stress is applied.101' 10 2

In the case of running ropes, such as crane and lift applications, fatigueis caused by a combination of tensile and bending stresses, together withthe effects of abrasion and fretting. Typical fatigue curves for these typesof rope are shown in Fig. 10.5. The curves give a good general basis onwhich to assess the likely fatigue performance of running ropes, but are ofmuch less value when estimating the fatigue life of structural cables. Suchcables are mainly subjected to loading in tension only, of which a largeproportion is due to gravitational loading such as pretension, self-weightand snow loading. Fluctuating stresses in cables, which may cause fatiguefailure, are, in the case of cable roofs, due to the non-steady-statecomponents of the wind velocity.

Surprisingly little information is available with respect to the fatiguelife of structural cables as compared to running cables. Some informationis available from fatigue tests carried out in Germany during the past 20years.103 This indicates that the stress range should not exceed 200-250 N/mm2 in order to ensure a safe fatigue life corresponding to twomillion cycles or more. This stress range refers to stresses due to tensiononly, and care should be taken not to add fluctuating bending stresses,particularly by designing too rigid cable attachments. Thus terminals

to

40

35

30

25

?n

\

\

\

\

\

\

\

\

\

\

\

s

\Langs lay

Ordinary lay>———•—=

001 0-1 0-2

Stress reversals x 106

10 20

Fig. 10.5. Fatigue curves for flexible ropes taken from BS302

161


such as those shown in Figs 10.2(a), 10.3 and 10.4(a) and (c), whichpermit rotation of the cable, are preferable to those shown in Figs 10.2(b)and 10.4(b).

A stress range of 200-250 N/mm2 for the cables in Tables 10.1 and10.2 corresponds to 131% to 16-8% of the breaking load for spiralstrand and 15-4% to 20-3% for locked coil, assuming fill factors of 0-75and 0-85 respectively as previously quoted. From this it would appearthat locked coil cables, size for size, have better fatigue properties thanspiral-strand, but that both will perform satisfactorily provided that thepoint made with respect to the attachment of the cables' support points isadhered to.

Where prestressing strands are attached by means of wedge grips,particular care should be taken since such constructions may sufferfatigue failures with a live tensile stress range as low as 10% of thebreaking load of the strand. To overcome this problem the strand shouldbe grouted in as shown in Fig. 12.17.

Flexibility of cablesWith spun single-strand cables such as the spiral-strand or locked coil,the minimum radius of curvature around which cable can be bent isgoverned by the physical limit point at which the outer wires are socrowded on the inside radius of the bend that they are displaced outwardsfrom their correct position. The minimum diameter will vary with theconstruction of the cable, but for the recognized constructions it will be inthe region of 30D (where D is cable diameter). The minimum value ofapproximately 30Z) is often used for coiling the cable for transporting tosite, but for permanent bending it is normal to employ ratios greater than30.D. When a cable is bent a stress is generated due to bending whichmust be added to the static and dynamic tensions to obtain the totalstress imposed. There are various methods for evaluating this stress, butthat which is most commonly used and has been found in practice to givesatisfactory results is based on the stress in the outer wires (these beingthe most affected by the bend). This tensile stress is governed by theformula

EdAstress — ——-

where E is the modulus of elasticity of cable, d is the diameter of outerwire, A is the area of cable, and D is the diameter of the drum or saddle.

Another factor which must be considered is the pressure between cableand saddle. For steel cables on steel saddles this is normally not aproblem. The formula

ITF

is used to determine pressure, where T is the tension in cable, D is thediameter of saddle, and d is the diameter of cable.

While there is not normally a problem with steel on steel, with theincreasing application of sheathed cables for structural purposes this

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factor does become important as the permissible pressure levels are muchlower. Typically acceptable bearing pressures with sheathed cables arelikely to be in the region of 1.0 N/mm2 maximum, depending on thesheathing material.

References10.1 Grover, H.J., Gordon, S.A. and Jackson, L. R. Fatigue of metals

and structures. Thames and Hudson, London, 1956.10.2 Gabriel, K. & Dillmawn, U. Hochfester Stahldraht fur Seile und

Biindel in der Bautechnik. Weitgespannte Flachentragwerke, Son-derforschungsbereich 61 Universitat Stuttgart, Mittelungen 21/1982, Werner-Verlag, Diisseldorf.

10.3 IABSE. Fatigue of steel and concrete structures. Colloquium,Theme 7, Lausanne, 1982, 595-680.

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