Regelungen nach Eurocode 3 Teil 3

(High Strength Tension Members and Guys)

 

1 Definitions

(a) Guy: a tension only member, connected at each end to terminations, forming a guy assembly which provides horizontal support to the mast column at discrete levels. The lower end of the guy is anchored to the ground and generally incorporates a means of adjusting the tension in the guy.

(b) Wire: an individual filament of steel, the smallest single tension component in ropes. Usually circular in cross section with a diameter between 3 and 8 mm but may be non-circular in locked coil strands. High strength usually obtained by cold drawing.

(c) Spiral strand: an assembly of a small number of wires laid helically around a central straight wire. Most commonly seven-wire and nineteen-wire configurations, with successive layers frequently being wound in opposite directions.

(d) Locked coil strand: with the wires of at least the two outer and (usually) serveral inner layers of wires so shaped that they interlock with each other, leaving virtually no voids in that part of the cross section. lnner layers of shaped wires are sometimes of trapezoidal cross section, but more usually all layers of Z section wires.

(e) Parallel wire strand: an assembly of wires laid side by side in parallel and either bound together or with the spaces between the wires held constant by spacer elements.

(f) Wire rope: an assembly of spiral strands laid helically around a central fibre or steel core, normally laid in the opposite direction to the wires in the outer layer of the strands (ordinary lay) or sometimes in the same direction (Lang's lay). Wire ropes with fibre cores are not normally used as permanent structural elements of masts.

(g) Cable is an assembly of one or more strands or ropes of the types described above. The strands may be bound in tight contact or kept apart by spacers.

(h) Termination: A device fixed to the ends of a cable, strand or wire to enable the loads to be transferred between it and the mast column or the steelwork attached to the guy foundation.

(i) Anchorage: a device comprising all components and materials such as sockets, pins, threaded rods, etc, required to retain the force in a rope or strand and to transmit this force to the structure of the mast, foundations or other structural elements.

(j) Socket: a permanent enclosure at the end of a rope or strand to enable stress to be transferred from the rope or strand to the rest of the anchorage. Usually a permanent fixture on the rope.

(k) Socket filler material:a material introduced while liquid into a socket to surround the individual elements of the rope and which then solidifies or sets hard to provide a structural bond or wedging action between the wires and the filler material.

(l) Damper: a device clamped to a stay to absorb vibration energy. May be a freely attached inertia type or a viscous type fixed between stay and structure.

(m) Lay Length of a strand: the length measured along the axis of a strand in which a helically wound wire makes a full 360° revolution. May vary from layer to layer thus reducing the residual torque in the strand to a minimum, and usually measured as a multiple of the diameter of the layer.

(n) Lay angle of a strand: the angel a wire of the strand makes with the axis of the strand. Note: The lay angle, , and the lay length, kD where D is the pitch circle diameter of the relevant layer of wires, are related by the expression k = / tan

 

2   Actions

2.1  Dead Weight of Strands

(1) In the absence of an appropriate European Standard, the following approximate expression for the nominal dead weight g per unit length of the strands themselves may be used for preliminary design:

where:

d is the external diameter of the strand, including sheathing for corrosion protection if used

f is the steel solidity ratio, defined as the ratio of the net steel cross sectional area to the gross area of the strand including sheathing.

r is the weight density of steel

w is the weight factor, to allow for corrosion protection

f and w may be obtained from the table which is focussed on guy properties:

 

    f w
Strands formed from locked coil strand 1 layer of shaped wires 0,81 0,83
Strands formed from locked coil strand 2 layers of shaped wires 0,84  
Strands formed from locked coil strand >2 layers of shaped wires 0,88 0,83
Strands formed from single spiral or parallel wire strands, with corrosion protection by galvanising and painting, with 3 or more layers of wire 0,76 0,93 0,83
Strands formed from single or multiple spiral or parallel wire strands, with corrosion protection by plastic sheathing and cement or grease filling, with 3or more layers of wire 0,60 1,05  

 

2.2 Wind Effects

(1) Wind effects to be considered shall include:

(a) The static effects of wind drag on the guys, including deflections and hence possible bending effects near the ends of the guys.

(b) Aerodynamic excitation leading to possible oscillation of the cables.

 

2.3 Thermal effects

(1) Thermal effects to be considered shall include the effects of different temperatures between the guys and the mast or chimney shaft.

(2) In the absence of more exact calculations or detailed information, it should be assumed that a guy can be

- 15°C warmer (light coloured)

- 30°C warmer (dark coloured)

- and 10°C cooler than the average temperature of the rest of the mast.

 

2.4 Initial guy forces

(1) The initial guy forces are definded as those forces and moments which would remain if no meteorological actions act.

(2) The partial safety factor for initial forces should be taken as gF =1.0.

(3) When adjustment of the guys is not provided, the design values of the total effects of the permanent actions plus preload shall allow for the range of error which may arrise in the preload.

 

3   General Requirements

3.1 General Requirements for Guys

(1) Materials other than steel can be used but may require special concideration to achieve an acceptable modulus of elasticity and the prevention of vibrations in higher frequencies

(2) The make-up and form of construction shall take into account:

(a) The strength of the cable and its attachments.

(b) The fatigue resistance of the cable and its attachments.

(c) The sensitivity of the cable to dynamic excitation by wind or

     otherwise

(d) The stiffness of the guy (both axial and flexural)

(e) The requirements for corrosion protection of the guy (see   

      Annex ??).

(f ) The requirement for maintenance and replaceability of the guy.

(g) The requirements for dielectric properties

 

3.2 General Requirements for Anchorages

(1) Anchorages shall be designed such that:

(a) Facilities are available for providing adequate adjustment of guy length to meet the requirement of the specification in regard to the specified initial load, geometrical tolerances, etc, both on original installation and subsequently.

(b) The strand entry to the anchorage is sealed to prevent the ingress of moisture.

(c) Bending of the strand due to transverse wind load or vibrations is minimised, for example by use of universal joints (cross pinned).

(d) Articulation is provided in the anchorage details to cater for manufacturing and erection imperfections.

(e) All weldings of the anchorages shall tested non destructive. This normally requires a special design of the weldings.

 

Note:  The enhanced requirements for guy assemblies and anchorages provides for the wider strenght variation and inspection difficulties in this area as well as the more dynamic nature of the loads.

 

(2) Anchorages for guys and their bearing elements within the structure shall be proportioned such that the ultimate load and the fatigue resistance exceeds that of the actual guy.

(3) Anchorages provided by sockets attached to the ends of the strands may be assumed to meet the requirements of (1) above provided that:

(a) The socket and filler materials comply with G.4.2.

(b) The wires of the strands are separated and splayed out within the length of the socket chamber to fill the cone in uniform manner.

(d) The ULS verification specified in (G.7.3) is carried out.

 

4   Materials

4.1 Wire for Strands

(1) The wire for strands shall be cold drawn steel wire complying with ENV 3.

(2) The wires shall be galvanized.

(3) The wire shall have adequate ductility to allow redistribution of stress within a strand.

(4) The requirements of (2) above may be deemed to be satisfied if, at fracture, the strain in the wire is not less than 2.5% over the full length and 3.5% over a gauge length of 5 times the wire diameter, which length includes the actual fracture zone.

 

4.2 Materials for Sockets

(1) Sockets shall be cast from steel to EN.., forged from steel to EN.., or machined from steel to En 10025 or EN.. The grade specified shall have an energy absorption of not less than 27J in impact tests conducted at -2O°C.

(2) The filling material for the sockets shall be selected taking into account service temperature and loadings, and basket and strand design, such that continued creeping of the loaded strand through the socket is prevented.

(3) Socketing with molten metals and resins should be carried out in accordance with EN.. The filling material should be chosen from one of the following:

 

(a) Molten metal (e.g. zinc or zinc/aluminium alloy) to..

(b) Plastic to ..

(c) Epoxy resin filied with steel balls to.

(d) Filling material for cables in antennas must be metallic..

 

4.3 Non Metallic Guys

1) Synthetic materials are generally proprietary products and reference must be made to manufacturer´s data for properties. Careful selection is necessary with these materials as the low modulus of elasticity of some products make it dangerous to substitute strenght for strenght with steel guys.

(2) They can also require higher initial tension to compensate for low stiffness which can be lead to high frequency vibrations.

(3) The ends of such ropes must be sealed as to prevent entrance of moisture and thus preventing the discharge of lightnings.

 

5   Mechanical Properties

5.1 Strength of Wires, Ropes and Strands

1) The characteristic strength of wire may be taken as the specified nominal value of the breaking strength.

(2) The characteristic value of the 0.2% proof stress of wire may be taken as the specified nominal value.

(3) The characteristic strenght of a parallel wire strand or locked coil strand may be taken as the aggregate characteristic strength of the constituent wires of the strand.

(4) The characteristic strength of a spiral strand of lay length not less than ten times the strand diameter may be taken as 95% of the aggregate characteristic strength of the constituent wires of the strand.

(5) The characteristic strength of a wire rope may be taken as 90% of the characteristic aggregate strength of the constituent wires of the rope.

 

5.2 Stiffness of Guys (table 3.4.1 in Part 3.1 is scattering to much in my opinion)

(1) The effective modulus of elasticity should preferably be determined by testing strands of the actual make up used in the design. In the Absence of such tests, it may be calculated as described in Annex XA (ENV1993-2, Annax 5.2, clause 2) or, for preliminary design, the following values under live loading may be assumed:

- Hard drawn wires either single or in parallel wire strands: 200 000 N/mm2

- Locked coil strands: 170 000 N/mm2

- Spiral strands with a lay length of not less than 10 times the strand diameter: 50 000 N/mm2

- Wire ropes: < 120 000 N/mm2

(2) The values in (1) above do not allow for any flexibility resulting from the change of geometry under load of a strand hanging in catenary.

(3) Attention is drawn to the fact that until a helically wound strand of any type has bedded down (efther by prestretching in a fabriator´s shop, or by stressing during erection) the effective modulus of elasticity is indeterminate and generally significantly less on first loading than the values given in (2) above. Furthermore, the strain caused by first loading is not generally 100% recoverable.

(6) The effective flexural stiffness of a strand should preferably be determined by testing strands of the actual make up used in the design. In the absence of such tests, it may be calculated as described in ENV 1993-2, Annex XA clause XA.3.

(7) It is recommended that guys be prestretched prior to measuring on site, in order to ensure that the rope is in truly elastic condition as it reduced the amount of retensioning necessary during the early life of the mast. Prestretching should be carried out by loading the guy cylically between 10% and 40% of ist breaking load. The numbers of cycles should not be less than ten. The loading process should not be carried out by passing the loaded guy around a sheave wheel. Parallel wire strands does not need prestretching and the process is less effective on small diameter ropes. The requirement for prestretching is most necessary when using multi-stranded ropes, ropes in excess of 20mm diameter and to a lesser extent spiral and locked coil ropes.

 

6   Structural Analysis

6.1 General

(1) In addition to any standard requirements, the structural analysis of a guyed mast shall take into account:

(a) The initial guy forces

(b) The effective modulus of elasticity of the guys (G.5.2).

(c) The reduction of effective axial stiffness of a guy hanging in catenary.

 

7   Ultimate Limit State

7.1 Partial Safety Factors

(1) Partial safety factors gM at the ULS for structural elements of a guyed system shall be taken as follows:

(a) The more severe of (i) or (ii) below

(i) for cables in tension, based on the breaking stress of the guy: gM = 1.8

(ii) For ropes in tension, based on the 0.2% proof stress of the guy: = 1.4

(b) For other stress conditions in cables, and other structural elements of guyed systems: gM =1.1

 

7.2 Guys

(1) Guys shall be designed taking into account the axial force in the guy.

(2) The characteristic resistance of a guy to tensile force should be taken as the aggregate characteristic strength of the constituent strands as defined in G.5.1 (2) to (4).

(3) Provided that the general requirements for anchorages (see 3.2) are maintained, local bending effects may be ignored.

 

7.3 Anchorages and Terminations

7.3.1 General Requirements

(1) Each guy assembly should incorporate a linkage system to enable coarse and fine adjustments to be made to the length (tension) and to facilitate installation or replacement. Due consideration should be given to lateral movements of the guy in service when sizing link plates and pins, and in the design of attachments to the mast. Where prising action can occur only a bolt or pin and locked nut should be used to make the connection.

(2) Guy link plates should be sized to provide reasonable lateral stiffness.

(3) The guy anchor strap should be designed to prevent lateral flexing and cracking of the concrete due to lateral movement induced by the guy.

(4) All components of anchorages shall be designed to be stronger than cables which they are anchoring.

 

7.3.2 Guy Sockets

(1) In the absence of detailed verification by testing, guy sockets proportioned as in G.3.2 may be verified as below.

(2) The design force applied to a guy socket (Sd) should be taken as 1.05 x the characteristic strength load of the strand to which it is attached, irrespective of the design load in the strand.

(3) The design longitudinal stressld) at any section of a guy socket should be calculated as:

 

where:

Sld may be assumed to vary linearly from Sd at the bearing or anchored end of  he guy socket to zero at the free end

A is the cross-sectional area of the guy socket at the section being considered

k1 is a factor to correct for the variation of load transfer from the strand to the guy socket, and may be taken as 1.5.

(4) The design value of the total ring force (Rd) in the guy socket should be taken as:

 

where:

Sd is as in (3) above

f is the angle of friction between the socketing material and the socket and may be taken as:

17°   for metal socketing

22°   for epoxy socketing

a is the angle of the cone (see 3.2).

(5) The total ring force Rd may be distributed along the guy socket to give the local intensity rd as shown in the Figure.

where:

rd is the local intensity of the ring force obtained from (6)

d0, di are the outer and inner diameters of the socket at the section being considered

k2 is a factor to take into account the uneven stress distribution over the wall thickness, and may be taken as 1.5.

(7) The maximum value of the principal stress in the socket material at any cross section should be determined from rld and rrd each section and should not exceed

 

7.3.3 Loops

(a) Hand Splicing - Efficiency 60%-90%).  This is the most traditional method of forming a terminal loop.

(b) Ferrule Secured Eye Termination: (Efficiency 90%-95%).  The ferrule is placed over the looped back end body of the rope and then swage pressed.

(c) Rope Clamps: (Efficiency 60%-90%). Clamps and Bulldog grips permit an relatively easy site termination. They are generally not suitable for ropes greater than 26mm in diameter and required re-tightening at regular intervalls.

 

7.3.4 Performed Rope Grips

(1) These are preformed strands having been previously coated by some

    substance (such as abrasive grit), which are wrapped around the periphery of the wire rope for a lenght of up to 2m, dependent on guy size and type. The preformed strand therefore provides a loop termination on site which can be reused a limited number of times.

 

8   Fatigue

8.1 General Requirements

(1) The fatigue endurance of a cable and ist attachment under varying axial loads shall be determined using the action specified in EC 1 Part 2.4 using the appropriate category of structural detail.

(2) Fatigue failure of guys due to bending effects near the sockets shall be excluded by use of pin joint connections.

(3) In the absence of the tests described in 9.l(2) above, the category of detail may be taken as follows subject to the qualifications listed in (2) below.

(a) Locked coil or spiral strands with metal socketing:         Category 112

(b) Parallel wire strands with epoxy socketing:                    Category 160

(4) The categories stated in (3) above are only valid provided that:

(a) Sockets comply with the general requirements of 3.2 to 3.4.

(b) Serious aerodynamic oscillations of cables are prevented ( ENV 1993-2 An.XB).

(c) Adequate protection against corrosion is provided (see ENV 1993-2 Annex XC).

 

9   Information Required for Cable Suppliers

9.1 Mechanical properties of cable and fittings

(1) Rather than detailing the make up of a guy, a designer may consider providing a performance specification in which case the requirements to be specified should include:

(a) Breaking strength of constituent wires and complete guy including terminations

(b) Ultimate mode of failure (eg in cable rather than at termination)

(c) 0.2 % proof stress of constituent wires

(d) Elongation at fracture (ductility) of constituent wires

(e) Stiffness (both axial and flexural) of complete guy

(f) Fatigue resistance of compiete cable including terminations

(g) Any special requirements for lay length / angle of cable and a minimum of residual torque.

(h) Any special requirements for socket type and provision for jacking

(i) Dimensional tolerances of cable and fittings

(j) Method of prestretching and marking to length

(m) Which items should be designed as replaceable

(n) Any possibility of aerodynamic excitation (which might require dampers)

(o) Codes and Standards to be used

(p) Any special requirements for marking for identification

(q) Any special precautions for transportation

(r) Any special requirements for quality assurance

 

9.2 Protective treatment

(1) Rather than specifying the details of protective treatment, a designer may consider providing a performance specification, in which case the items to be specified should include:

(a) The environment of the bhdge (exposure, pollution type and levels, access, etc)

(b) Particular design aspects which might affect the protection (eg stress range or liability to oscillation might prohibit the use of certain types of sheathing)

(c) Particular requirements for protective treatment (eg galvanising of wires)

(d) Required life time of treatment to "minor maintenance"

(e) Required life time of treatment to full reinstatement

(f) Codes and Standards to be used

(2) The designer should specify details of guarantees to be provided for the system.

 

10   Requirements for Tests

10.1 General

(1) A designer shall specify those tests on the cables and their fittings which are required to ensure that they perform as intended.

(2) The tests listed in G.11.2 to G.11.5 will normally be required to satisfy (1) above. When they are covered by an appropriate European Standard, specification should be by reference to it.

(3) The frequency of testing, the size of the sample, the number of the samples, and the action to be taken on failure to meet criteria, should all be specified.

 

10.2 Tests on wire

(1) Wire should be tested in an approved mechanically operated tensile testing machine, maintained to specific standards.>

(2) Tests should be carried out for tensile strength, proof stress and elongation. The accuracy, and speed of operation of the testing machine should be specified.

(3) A bend test should be specified not only for the wire but also for the galvanising (see below). This should normally be done by wrapping the wire round a mandrel of diameter three times the wire diameter without causing fracture of the wire or flaking or cracking of the zinc coating.

 

10.3 Tests on the zinc coating

(1) As well as the wrap test referred to in G.11.2 (3) above, tests to determine the uniformity and weight of the zinc coating should be specified.

 

10.4 Tests on guys

(1) Where possible, complete (socketed) samples of each size of a guy should be tested to destruction with the test load applied through the sockets in the same way as in the mast.

(2) When the behaviour of a structure is critically dependent on the stiffness of the cables, and a designer is not satisfied with the approximate methods of determination given in G.5.2, this should be determined by testing of complete guys, socketed as intended in the final structure. Several cycles of loading and unloading of the cable before it is cut to length and socketed will be required to ensure that it has bedded down and reached a stable condition. The cyclic loading should be carried out on every guy of a mast and the records of stiffness should be obtained for at least one guy of each diameter.

3) Where the information available on the fatigue performance of a particular guy is inadequate for safe estimates to be made of the fatigue life, tests should be specified to determine this.

 

10.5 Other tests

(1) The designer should specify any other tests considered necessary to justify the design assumptions.These may include:

(a) Friction tests (estimates of both maximum and minimum value)

(c) Long-term (creep) tests on socketed strands

(d) Visual, ultrasonic and radiographic tests on sockets