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Investigations on A220 kV Cold-formed Latticed TransmissionLine TowerDr N Subramanian, Fellow

An attempt was made to analyse and design a 220 kV transmission line tower using cold-formed lipped anglesections. Since 60 angles are not presently rolled in India, the tower was designed as one with square base.This introduced some difficulties infabrication and detailing. To verify the assumptions made in the designand to study the behaviour of the joints, a full scale testing of this 36m tower, up tofailure, was undertaken.From this investigation, it was found that cold-formed towers can be considered as a viable alternate to hotrolled towers even for square based towers.

INTRODUCTIONKeywords: Transmission line tower, Cold-formed section, Full scale testing, Economy, Connections

Transmission line towers represent about 35-40 % of the totalcost of the transmission line project. Hence, greater emphasisis now given to develop designs that are more efficient andcost less and give a better aesthetic look. Also, even a smallpercentage of the saving in the weight of one tower willresult in substantial overall savings of costly matcrial suchas steel. Transmission line towers were traditionally designedusing hot-rolled steel sections. Tile main difficulty of usinghot-rolled steel angles is the non-availability of thinner sec-tions and also the code restriction on the allowable thickness.This has resulted in the use of some minimum angle whichleads to more weight. Hence, several countries have tried tosubstitute these hot-rolled angles with cold-formed shapes.Cold-formed shapes arc readily available. in thinner sectionsand provide a viable alternative for more economical struc-tures due to savings ill weight' .The added advantage is that any section can be rolled accord-ing to specification even in small quantities. (Most of themajor cold rolling companies in India accept a minimum of5 t for any section to be rolled). Hence, long unsupportedlengths may be used for the leg members, which will in turnreduce the number of bracing members and the number ofconnecting bolts. Cold-formed members with thickness of3 nun - 6 mm are used for transmission towers.However, in India, the transmission line tower code does notinclude the use of cold-formed steel sections in these towers.Hence, an attempt was made to analyse and design a 220 kVdouble circuit tangent tower using cold-formed lipped anglesections. Since 60 angles are not presently rolled in India,the tower was designed as a square based tower. This intro-duced some difficulties in fabrication and detailing which arediscussed in this paper. In order to verify the assumptionsmade in the design and to study the behaviour of the jointsof the tower, a full scale test of this 36 m high tower wasconducted. The tower was tested up to failure. From the testresults, it was found that the analytical model predicts thebehaviour within reasonable limits, and the cold-rolled see-Dr N Subramanian is with Computer Design Consultants, 19J, NorthU sm ao R oa d, TNagar, Madras 6 00 0 17 .This paper was received on November 25, 1994. Written discussion on thepaper will be received until September 30, 1996.

Vol 77 , August 1996

tions offer a viable alternative even for square based towers,resulting in 10% savings ill cost.EXAMPLE TOWERIn order to demonstrate the economy that can be achieved byusing cold-formed sections, a 220 kV double circuit tangenttower having the configuration shown in Fig 1 was analysed.The loads acting on this tower based on sag-tension calcula-tions are shown in Fig 2.

FIGURE 1CONFIGURATION OF 220 kV DOUBLE CIRCUITTANGENT TOWER

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FIGURE 4 (b)TOWER AT TESTING STATION - AFTER TESTING

meeting at olle point. Eccentricity should be minimised.However, in cold-formed sections, other than angles, boltedconnections on one leg or one plane may' create eccentricitieshigher than those occurring in hot-rolled angles. In thesecases, the flexural component of the stress caused by theeccentricity must be considered in the design. Moreover,effect of local distortion in cold-formed steel connectionshould also be studied.Further difficulties of connecting cold-formed lipped chan-nels to the bracings are explained in Fig 3. In normal hot-rolled towers, the bracings and leg members have angle sec-tions, and hence, the bracings can be directly connected tothe leg members on both faces of the kg angie, thus eliminat-ing the use of extra gusset plates. It also, to some extent,reduces the bolt weight. As seen in Fig 3, such a methodcannot be adopted for cold-formed sections.

FIGURE 5CONTROL SYSTEM

MODEL TOWER AND DETAILS OF TK"TINGPROCEDUREIn order to check the validity of the assumptions made in thedesign and to study the behaviour of thc tower connections,a prototype tower with the configuration and member sizesas shown ill Fig 1 was tested in the Tower Testing andResearch Station of the Structural Engineering ResearchCentre, Madras. It is to be noted that this is the first time aprototype of cold-formed steel tower is tested in India. Thctower at the test rig is shown in Fig 4(a). Since it was apreliminary test to ascertain the validity of using cold-formedsections in lowers, the tower was tested only for the normalcondition, though facilities are available to test it for brokenwire conditions also.

TABLE 1TEST SCHEDULE TABLE FOR NORMAL CONDITION FS - 2.0

Direction and Horizontal Vertical Rigg Load, N Resultant Resultant Load Cell, Control Test Load asType of Load Load, kN Load, kN Load, kN Angle kN Channel No percent of

and (Range) Range1 Res!. 8.76 6.96 250 10.82 37"27" 50 1 (3) 86.562 Resl. 22.90 23.02 500 31.50 44 31' 300 2 (4) 84.003 Res!. 22.90 23.02 500 31.50 44 31' 500 3 (4) 50.404 Resl. 2530 23.02 500 33.22 4tO 40' 300 4 (4) 88.595 Res!. 25.30 23.02 500 33.22 41' 40' 500 5 (4) 53.156 Res!. 42.00 23.02 500 46.74 28' 12' 300 6 (3) 62.327 Resl, 42.00 23.02 500 46.74 28' 12' 500 7 (4) 74.78

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TABLE 2STRUCTURAL DEFLECTIONS, mm

Directions andLocation

Percentage o r Ultimate Test LoadDatum 25 50 7S at

FailureTransverse AtTop Conductor

250 (63)'evel 0 115 360 390Bottom Con-ductor Level 0 55 120 (33)' 180

Computed valuesIn the test setup, the transverse and vertical load componentsare combined into a resultant force, the longitudinal com-ponent being applied in the horizontal plane. Fig 5 schemati-cally shows the wire rope arrangement for the application ofthe resultant load at the required angle at a typical load pointon the tower, In the rope applying the resultant load, a loadcell to measure the load and an angle sensor to monitor theangle of load application are attached. The resultant load iscontrolled by the hydraulic ram at the top and the angle ofapplication is controlled by the vertical ram at the lower level.The double acting hydraulic rams are remotely controlledfrom the control room loading channels. This control is auto-matic by means of a closed loop servo system explained inthe figure. The closed loop system adjusts the resultanttransverse and longitudinal components simultaneously at allload points and ensures that the loads are proportional fromzero to maximum. Limit switches help to see that the loadapplied never crosses the ultimate value. The magnitude ofload and the deviation in the angle of application arc auto-matically recorded on charts. To identify the transducers'range, each is given its own channel number, eg, 7-(1), 7-(2),7-{3), and 7-{4). Accuracy of the measuring system duringa test is determined by calibrating the transducer/recordercombination before and after testing. A calibration rig com-pares the transducer output against a NFL calibrated proving ring.Depending on the minimum values of the ultimate test loadsrequired to be applied at any particular load point during aseries of tests, a load cell for that load point is chosen, andthe load cell-recorder combination is calibrated for therequired ranges of the load cell, allotting one control channelfor each load cell. The details of the tabulation for the testedtower is given in Table 1.After completing the rigging of the test tower, a trial test wasconducted with around 10% of loading in all load points, tocheck the rigging pattern, hydraulic loading and control sys-tem functioning. During the actual test, a pause in loadingwas made after every 5% or 10% of ultimate test load hasbeen applied. This is done to allow deflection readings to betaken and to assess the ability of the tower to accept a furtherincrease in load. In addition to visual inspection of the towerthroughout the test for probable distress in any member, atselected stages of loading, theodolite readings of the move-ment of graduated scales fitted on to certain parts of thetower were also taken for assessing the tower deflection.Table 2 gives the deflection readings taken during the test.After reaching 84% of ultimate test load, the tower failed.66

Subsequent inspection of the tower showed that both thecompression bracings in transverse fac~ immediately belowbottom cross ann level had buckled. Also the compressionlegs below bottom cross ann had buckled Fig 4(b).DISCUSSIONS AND CONCLUSIONSMost of the cold-rolled section manufacturers in India arerolling only a range of standard sections. Hence, it was notpossible to adopt the actual designed sections in the tower.The sections shown in Fig 1 are the sections used in thetesting and not the sections designed by using the computerprogram. Hence, in order to compare the computed resultswith those of the test results, the sections and loads asadopted in the test were substituted in the computer model.Comparison of the computed and tested results showed thatthe tower failed at a lower load which is 16 % less than thecalculated load. This may be attributed to the large deforma-tions (which are four times more than the calculated values)noticed during the test. The test also showed that the initialimperfections in tbe tower members will have greaterinfluence in the case of cold-rolled sections. It was alsofound that there was no appreciable deformation at the gus-setted joints, A series of tests on parts of the towers are beingplanned in order to determine the effect of eccentricity of thegussetted joints.However, this preliminary test ensured that cold-formed sec-tions could be successfully adopted in four-legged transmis-sion line towers, which will result in economy of material. Itwas also seen that the gusset plates, though marginally in-creasing the cost of the tower, docs not change the structuralperformance of the tower. More tests, including those ontriangular-base towers will throw more light on thebehaviour of such towers.ACKNOWLEDGEMENTSThe tower was fabricated and tested on behalf of M Is T IMetal Sections. The tower was tested at the Tower Testingand Research Station of the Structural Engineering ResearchCentre, Madras. The author was a consultant to M/s T I MetalSections in this projectREf'ERENCESL E H Gaylord and G M Wilhoite. 'Transmission Towers - Design of ColdFormed Angles.' Journal of Structural Engineering, Proceedings ASCE,vol Ill, no 8,August 1987, P 1810-1825.2.IS 802 (part I)-Irn7. 'Code of Practice for use of Structural Steel in Over-head Transmission Line Towers.' Bureau of Indian Standards, New Delhi,1977.3.N Subramanian and C RAnnapurani. 'Design of Transmission Line Towersusing a Micro Computer. ' The Bridge and Structural Engineer, vol 16, no 1,March 1986, p 21-31.4. IS 801 - 1975. 'Indian Standard Code of Practice for use of Cold FormedLight Gauge Steel Structural Members in General Building Constructions(Firs! Revision).' Bureau 'ofIndian Standards, New Delhi, January 1rn6.5. N Subramanian. 'Design of Transmission Towers using Cold FormedMembers.' Civil Engineering and Construction Review, vol 3, no 5, May19'K1,p 63-67.6. M AZaveland and P Faggiano. 'Design of Cold Formed Latticed Transmis-sion Towers.' Journal of Structural Division, Proceedings ASCE, vol Ill,November 1985. p 2427 - 2445.7.Test Report on 220 kV Double Circuit Tangent Tower, Report no 18, TowerTesting and Research Station, SERC, Madra." December 1991.

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