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Cbip manual for transmission line pdf free download |
Cbip manual for transmission line pdf free download
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Practice in the tower industry is also to specify negative body extension, i. For lines traversing in hilly terrain, negative body extensions can be used in tension towers from the consideration of economy. Tower Geometry Leg extensions are generally used in hilly regions to reduce benching or cutting. The alignment of leg extension is done with the first section of a tower.
Installation of leg extension calls for high degree of expertise in tower erection. Leg Extensions are also used with negative and positive body extension in suspension and tension towers from the consideration of economy. Cleats are provided on the stubs to offer resistance against uprooting of the stubs. A sub set consists of four members whereas the number of anchor bolts depends upon uplift and shear on the bolts. A cleat is also provided between the last leg and stub to strength the unsupported portion of stub above concrete chimney and portion of leg upto to the last bracing to take care for bending due to shear load and direct load.
Tower outline is defined in terms of the following parameters:. The minimum ground clearance is determined in accordance with the stipulations of Chapter 4 Electrical clearances of this manual. The maximum sag of a conductor occurs under maximum temperature and still wind condition. The maximum; sag is considered in fixing the height of a line support. Appropriate provision for sagging error generally mm is also to be considered. It is a continuous process and takes place throughout its life.
The rate of creep is higher initially but decreases with time. Creep compensation is provided by one of the following methods The present practice is not to make any provision for creep compensation for 66 kV, kV, and kV lines.
For transmission lines of kV and above, the creep is compensated by adopting negative temperature compensation. The length of an insulator string is a function of insulation level LIWL and SIWL , power frequency voltage service voltage dynamic over voltage and service conditions Pollution, altitude, humidity.
The depth of the jumper is affected by phase to grounded metal clearance which also is determined on the basis LIWL, SIWL service voltage, short circuit level, altitude, humidity, etc. The length of V string for the purpose of determining the height of tower is the vertical distance between the lower main member of cross arm and center of lowest conductor. For preparing clearance diagram the nearest live part from the grounded metal has to be considered.
The number and size of discs, length of single and double suspension and tension string for various system voltages are given in Table 3. The minimum phase to phase and phase to grounded metal clearances are generally determined on the basis of lightning impulse levels for lines of voltages up to kV. Minimum phase to phase clearances for different voltage level are given in Chapter — 4 Electrical Clearances of the Manual. Calculations for estimating the spacing between two adjacent cross-arms or two power conductors of suspension and tension towers are given in Figures 17 a and 17 b.
The Tension Insulator Drop is the vertical displacement of the jumper lug point with respect to attachment point of tension string at strain plate. The drop is maximum under the maximum sag condition and minimum under the minimum sag condition. While drawing clearance diagram it is necessary to check the clearance of jumper for the minimum as well as maximum drop conditions of insulator string. For such cases, the jumper may be modified to obtain the appropriate clearance. The width depends upon the magnitude of the physical loads imposed upon the towers by conductors, wind loads and the height of application of the loads from ground level.
Towers with larger base width result in low footing cost and lighter main leg members at the expense of longer and heavy bracing members. There is a particular base width which gives the best compromise and for which total cost of the tower and foundations is the minimum. Based on experience extending over a number of years, certain empirical relations have been developed which are good guide in determining the base width.
The base width of the tower is determined from the formula given below. Under the values of constant K for suspension and angle towers are chosen correctly, the wide range suggested for K can lead to conflicting results.
With a view to arriving at a simpler relationship, figures relating to total weight of tower and their base widths are tabulated in Table 3. Where the way leave is a problem, the design is optimized with the maximum permissible base width.
Typical slopes of bottom-most leg members with the vertical for towers various voltage ratings are given in Table 3. Table 3. This width is mainly decided by torsion loading.
The torsional stresses are distributed in the four faces of the tower. Larger width reduces torsional forces transmitted to the bracings below that level and thus helps in reducing the forces in bracings of the tower body.
These parameters are covered in Chapter 4 of the Manual. The two swing angles correspond to maximum transverse load and vertical load, and average transverse load and vertical load. In case where weights are used for restricting the swing of the insulator string, the width of cross arm shall be determined taking cognizance of the same.
The electrical clearance diagrams considering length and configuration of string and electrical air clearances Ref. The analytical calculations for electrical clearances are given in Annexure-I where reference is to be made to Figure The electrical clearance diagram of a tension tower is given in Figure The analytical calculations for electrical clearances on tension towers also are given in Annexure-I where reference is to be made to Figure Note : i Size of discs for insulator strings for voltage up to and including kV is xmm.
E Rules and its latest amendment. New regulations on construction of Electric lines have also come into force. Similarly some new regulations like electrical clearances for navigable rivers, railways etc. Experience in the fields of Design, Construction and Operation of various transmission lines of kV have also been obtained which was lacking at the time of earlier CBIP publication. Keeping the above in view, the chapter on electrical clearances has been revised.
The electrical and mechanical performance of transmission line influence the reliability and security of an electrical power system. The contributing factors in mechanical performance viz. The contributing factors in electrical performance are electrical stresses, air gap clearances, insulation coordination etc.
Besides restricting line outage rates, the electrical clearances provided should also satisfy the safety of public and maintenance crew during operation and maintenance of transmission lines. Optimizing of electrical clearances and tower configuration also offer overall economics of the transmission line. As such selection of electrical clearances is important from the perspective of reliability, security, safety and economy of the transmission lines.
The electrical clearance design of transmission line involves selection of suitable air gap clearances, insulation length and insulator strings to:. A transmission line is exposed to various kinds of electrical stresses which include steady state operating voltage of the system as well as various over voltages that generally occur in the system.
These are discussed below:. The steady state operating voltage is expressed in terms of service voltage, i. Its value is generally 1. In a transmission line, switching over voltage is represented by a probability distribution. Its value is generally 2. These are fast front over voltages 1. The location of the earth wire s and their distance and angle w. For a transmission line, the magnitude of lightning over voltages depends on the intensity and probability of thunder storms in the regions through which the transmission lines are constructed.
For systems upto and including kV voltage rating, the line insulation is determined from the power frequency voltage and lightning impulse voltage requirement whereas for system above kV rating, the power frequency voltage and switching impulse voltage are the governing criteria.
The other factors which affect the electrical insulation are climatic conditions i. In probabilistic approach of line insulation design the probability distributions of over voltages are coordinated with distributions of withstand strengths of air gaps and the electrical clearances are accordingly selected.
The Over Voltages experienced due to various events in electrical system is given in Table-A1 at Annexure The minimum clearance to be provided above ground as per regulation 58 4 of CEA regulations on Safety and Electricity Supply - for A.
C Transmission lines is stipulated as 5. Accordingly the minimum electrical clearances above ground to be provided for A. C transmission lines is as under:. Table 3 DC Voltage kV Minimum clearance above ground mm Accordingly the minimum ground clearances being kept by the utilities in India are as follows:. Note: The minimum clearances given above are Normative as these are dependent upon conductor configuration, phase to phase distances, other mitigation measures etc.
The maximum over voltage occurs very rarely and likewise insulation strength of an air gap very rarely decreases to its lowest value. The likelihood of both events occurring simultaneously is very limited. Therefore considerable economy may be achieved by recognizing the probabilistic nature of both voltage stress and insulation strength and by accepting a certain risk of failure. The decrease in line cost must be weighed against the increased risk of failure i. This philosophy of insulation coordination is considered while deciding Live- Metal electrical clearances corresponding to various swing angles of conductor.
Maximum swing angle is generally considered with reference to 25 - 50 years return period of wind. Similarly the switching or lightning over voltages are less probable and are hence generally combined with the maximum probable air gap length and corresponding insulation strength. For this, generally, swing angle corresponds to stationary wind conditions zero swing angle or moderate wind conditions swing angle generally corresponding to 5 days to 2 years return period.
The aspect of probabilistic design becomes more pertinent with increase in voltage levels. For transmission lines up to kV class, the air gap clearance at stationary and moderate wind speeds corresponds to lightning impulse voltage requirement and for transmission line above kV class, air gap clearance at stationary and moderate wind speeds corresponds to switching impulse voltage.
The other factors which influence the selection of air gap clearances are climatic conditions, i. The swing angles of insulator string shall dependent upon various factors like wind velocity, wind span, weight span, span factor, conductor type, insulator weight, ice thickness and weight, etc. Similarly the minimum electrical clearances to be kept across air gaps between live and metal parts depend upon the magnitude of switching overvoltage, air gap factor, altitude corrections etc.
Hence the specific swing angles and corresponding clearances are unique to each transmission line. For the typical self supported lattice structure configurations adopted in India, the phase to phase clearance is generally dictated by the live metal clearances.
For AC lines the minimum clearance generally adopted between conductors of different phases under stationary condition is based upon various empirical formulae as given below:. Table 5 A. For HVDC lines the pole to pole clearance adopted is based upon the interference levels to be maintained and are generally as follows:.
Table 6 Voltage level Conductor type Min. The height and location of ground wires shall be such that line joining the groundwire to the outermost conductor shall make an angle with the vertical equal to desired shield angle.
The angle of shield adopted for various voltage transmission lines in India is given as under:. The location of ground wire is related to the position of power conductor which is fixed depending upon length and configuration of insulator string, swing of insulator string, electrical clearances etc.
On transmission lines having large horizontal spacing between phases, two ground wires are provided to achieve required angle of shield. The protective zone between two ground wires forms a semicircle with the line connecting two ground wires forming the base diameter in case of horizontal configuration tower. The middle phase conductor shall not fall within this semi-circle. The mid span clearance between the conductor and groundwires is kept more than the clearance at tower to avoid flash over from ground wire to conductor when hit by the lightning stroke.
This arrangement also improves angle of shield in the middle of span. For UHV lines kV this clearance is dictated by the corona performance of the line. The minimum mid-span clearances generally followed for different voltages rating lines are given as under:. The clearances being generally followed by utilities in respect of navigable rivers for A.
The minimum electrical clearances between the lowest power conductor of crossing line over the crossed line as per regulation 69 of CEA regulations on Safety and Electricity Supply is given as under:. However, based on the clearance study, reduced clearances as under may be adopted:. No blasting within meter from electric supply line of voltage exceeding V or tower structure shall be permitted without written permission of owner. No cutting of soil within 10 meter of tower of kV or above voltage shall be permitted without written permission of owner.
No construction of brick kiln or pollution unit shall be permitted near the transmission line of kV and above voltage within a distance of meter. Energizing 2. Table-A2 Swing Angle of suspension string in degrees. Note: No guest is required to be considered for conductor due to extremely low probability of guest acting perpendicular to full span causing maximum displacement.
Electrical Clearances Chapter 5. It is essential to collect all the necessary design parameters in consultation with the power utility before starting the design work.
The design parameters required for developing a transmission line tower design are described hereunder in this chapter. These design parameters should be correct and authentic to ensure reliability of transmission line under given conditions. The transmission capacity of a transmission line is a function of voltage rating of transmission line and as such is vital parameter. All the electrical parameters such as air gap clearance from conductor to steel structure, phase to phase clearance, ground clearance above ground etc.
The power is transmitted either through AC System alternating current or through DC System Direct Current depending upon the requirement of power system in terms of power to be transmitted, distance of transmission, system frequency, etc. In India, the following transmission voltages have been standardised for transmitting the power:. A high-voltage, direct current HVDC electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems.
For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For shorter distances, ever though the higher cost of DC conversion equipment compared to an AC system, DC system may still be warranted, due to other benefits of direct current links. HVDC allows power transmission between unsynchronized AC systems and can increase system stability by preventing cascading failures from propagating from one part of a wider power transmission grid to another.
The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. In bipolar transmission, a pair of bundle conductors is used, each at a high potential with respect to ground, in opposite polarity.
Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor.
However, there are a number of advantages to bipolar transmission which can make it attractive option. This reduces earth return loss and environmental effects. A bipolar scheme can be implemented so that the polarity of one or both poles can be changed. This allows the operation as two parallel monopoles. If one conductor fails, transmission can still continue at reduced capacity. A back-to-back station B-t-B is a plant in which both static inverters and rectifiers are in the same area, usually in the same building.
The length of the direct current line is kept as short as possible. HVDC back-to-back stations are used for. The DC voltage in the intermediate ac circuits can be selected freely at HVDC back-to-back stations because of the short conductor length.
The DC voltage is as low as possible, in order to build a small valve hall and to avoid series connections of valves. For this reason at HVDC back-to-back stations, valves with the highest available current rating are used. The transmission line AC System can be classified into three categories depending on the number of circuits.
Each circuit consists of three phases. However, each phase may further consist of single, twin or multiple bundle of conductors. The three classifications based on the number of circuits are :.
Single circuit and double circuit transmission lines are popular throughout the world. Some of the utilities of the world have constructed multi-voltage lines which have more than two circuits of different voltage levels.
Wherever Right of Way constraints are foreseen, multi-circuit and multi-voltage lines are preferable specifically near substations and in forest stretches. The reliability of a transmission system is largely dependent on the accuracy of the parameters related to climatic conditions considered for design. The design of tower will vary with variation in climatic conditions.
The following are the main climatic parameters which play vital role in developing design of transmission line towers:. Wind 2. Temperature 3. Isokeraunic level 4. Seismic Intensity 5. Ice formation. Transmission lines shall be designed for the reliability levels as given in Table below.
These levels are expressed in terms of return periods in years of climatic wind loads. Description Reliability Levels 1. Return period of design loads, in years, T 50 Yearly reliability, Ps Reliability level 1 shall be adopted for EHV transmission lines up to kV class Twin bundle conductor Reliability level 2 shall be adopted for EHV transmission lines above kV class. Triple and quadruple circuit towers up to kV lines shall be designed corresponding to the reliability level 2.
Reliability level 3 shall be adopted for Tall River crossing towers and special towers, although these towers are not covered in this chapter.
The basic wind speed data have been published by Bureau of Indian Standards in IS: in active cooperation with Indian Meteorological Department as shown in Figure 1. This map represents basic wind speed based on peak gust velocity averaged over a short time interval of about 3 seconds and corresponds to 10 m height above mean ground level in terrain Category-2 for yr return period.
Based on the wind speed map, the entire country has been divided into six wind zones with max. Responsibility for the correctness of internal details shown on the map rests with the publisher. In case the line traverses across the border of wind zones, the higher wind speed may be considered. However, it depends on the length of line in each section and its financial impact involving designs, spares 2.
Reference may be made to IS Part-3 for basic wind zone maps. Where Ko is a factor to convert 3-second peak gust speed into average speed of wind during 10 minutes period at a level of 10 meters above ground. Ko is to be taken as 1. Reference wind speed obtained in 5. Table 2 gives the values of Risk Coefficient K1 for different wind zones for three Reliability Levels.
Design Parameters The design wind pressure on towers, conductors and insulators shall be obtained by the following relationship:. Design wind pressure Pd for all the three Reliability levels and pertaining to six wind zones and the three terrain categories have been worked out and given in Table 4.
These panels should normally be taken between the intersections of the legs and bracings. Values of Cdl for the different solidity ratios are given in Table 5.
The projections of the bracing elements of the adjacent faces and of the plan-and-hip bracing bars may be neglected while determining the projected surface of a face. Values of GT for the three terrain categories are given in Table 6. Note: Intermediate values may be interpolated. For height above 80m, refer manual for river crossing tower. The load due to wind on each conductor and groundwire, Fwc in Newtons applied at supporting point normal to the line shall be determined by the following expression :.
Values of Gc are given in Table 7 for the three terrain categories and the average height of the conductor above the ground. The total effect of wind on bundle conductors shall be taken equal to the sum of the wind load on sub- conductors without accounting for a possible masking effect of one of the sub-conductors on another.
Up to 10 1. Up to 10 2. Note: Length of Insulator shall be considered as follows:. Values of Gi for the three terrain categories are given in Table 6. To evolve design of tower, three temperatures i.
The temperature range varies for different parts of India under different seasonal conditions. The absolute maximum and minimum temperatures which may be expected in different localities in country are indicated on the maps of India in Fig 2 and Fig 3 respectively. The temperatures indicated in these maps are the air temperatures in shade.
The maximum conductor temperatures may be obtained after allowing increase in temperature due to solar radiation and heating effect due to current etc. After giving due thought to several aspects such as flow of excess power in emergency during summer time etc.
For region with colder climates -5 deg C or below , everyday temperature to be considered as 15 deg. As the overhead transmission lines pass through open country, these are subjected to the effects of lightning. The faults initiated by lightning can be of the following three types:. The territorial waters of India extend into the sea to a distance of twelve nautical miles measured from the appropriate base line. Government of India Copyright 1W6. The above types of faults can be minimized by suitably choosing the shielding angle and keeping the tower footing resistance to the minimum.
Lightning is a very unpredictable phenomenon. Moreover, not enough data is available, at present, to treat them by statistical technique. The only data available are the isokeraunic levels, i. In the view of the above fact, the following shield angles are provided in EHV line towers as per present practice in the country:. Horizontal Configuration Outer Ph. Delta Configuration Outer Ph. The transmission line tower is a pin-jointed light structure comparatively flexible and free to vibrate and max.
Wind pressure is the chief criterion for the design. Concurrence of earthquake and maximum wind condition is unlikely to take place and further seismic stresses are considerably diminished by the flexibility and freedom for vibration of the structure. This assumption is also in line with the recommendation given in cl.
Seismic considerations, therefore, for tower design are ignored and have not been discussed here. However in regions where earthquakes are experienced, the earthquake forces may be considered in tower foundation design in accordance with IS: The dip from the center point on a line joining the two supports called Sag. The Sag of a conductor at Null point is inversely proportional to the tension in the conductor. For all practical purposes, the catenary is simplified as a parabola without much error.
A sag-tension calculation predicts the behavior of the conductors based on recommended tension limits under varying loading conditions. Sag and tension at all other weather conditions depends on the initial weather conditions considered for the calculation.
For spans normally adopted for transmission line, the catenary is very nearly a parabola and hence the sag is calculated by the formula for the supports at same levels:. The shape of the wire strung between the supports will form a part of catenary and therefore, the lowest point of catenary will not lie in the middle of the span.
Let H be at a horizontal distance of a units from A and b units from B. Sag corresponding to taller support i. A : Null Point within the Span Fig. B : Null Point outside the Span. Similarly, weight span for the other side of towers, can be calculated and total weight span obtained. It is also evident that the maximum weight span are obtained by worst condition of wind loading when T is maximum, which means the vertical component of worst load sag should be taken for cold curve in order to assess uplift on tower.
Generally the transmission line corridor requirement for different voltage lines are as follows:. Horizontal Configuration 52 kV Double Ckt. Horizontal Configuration 85 kV Single Ckt. While deciding tower and conductor configuration of Transmission Lines at kV and above, the interference level should be maintained within the following limits:.
PTCC 1. Maximum value of induced electromagnetic voltage for Volts fault duration equal to or less than ms 2. C Min. UTS The earthwire to be used for transmission line has been standardised. Continuously run galvanised steel earthwires are to be used for lines and earthed at every tower point. Co-efficient of linear The following type of insulator strings are generally used on transmission lines, depending on actual insulation requirement and mechanical strength, other suitable insulators can also be used.
Type of Tower Type Size of No. Quadruple All types of Angle 4 x 51 tension Towers. Single Tension For Transposition - x 1 x 35 Towers. Quadruple All types of Angle - 4 x 35 tension Towers. Quadruple Horz. Quadruple All types of Angle x 4 x 23 tension Towers. Single For Transposition 1 x 24 Tension Towers. Double All types of Angle 2 x 23 Tension Towers. Single All types of Angle x 1 x 15 Tension Towers.
Single All types of Angle x 1 x 10 90 90 Tension Towers. Single All types of Angle x 1x6 90 90 Tension Towers. Triple All types of Angle x 3 x 64 Tension Towers. Quadruple All types of 4x 41 Tension Angle Towers. Normal design spans for various voltage transmission lines considered are as follows.
The wind span is the sum of the two half spans adjacent to the support under consideration. For plain terrains, this equals the normal ruling span. The weight span is the horizontal distance between the lowest points of the conductors on the two adjacent spans. For design of towers the following weight spans are generally considered:. Normal Span m Wind Pressure on Cond. Conductor temperature. Case no. Tower loading is most vital input for tower design.
Various types of loads are to be calculated accurately depending upon the design parameters. In the load calculations, the wind plays a vital role. The correct assessment of wind load will lead to proper load assessment and reliable design of tower structures. Overhead transmission lines are subjected to various loads during their life span which are classified into three distinct categories:.
Some guideline has been furnished in clause 6. Failure of items such as insulators, hardware joints etc. In order to prevent the cascading failures, angle towers shall be checked for anti-cascading loads for all conductors and earthwire broken in the same span under Nil Wind condition.
Only Suspension Towers to be designed under this condition. As an important and essential requirement, Construction and Maintenance Practices should be regulated to eliminate unnecessary and temporary loads which would otherwise demand expensive permanent strengthening of Towers.
Loads imposed on tower due to action of wind are calculated under the following climatic criteria:. Note: 1 Criterion ii above is to be adopted for Suspension towers under security condition. The load due to wind on each conductor and ground-wire normal to the line applied at supporting point shall be determined by the following expression:.
Values of Gi for the three Terrain Categories are given in Table 2. These panels should normally be taken between connecting points of the legs and bracings. To calculate wind loads separately in transverse and longitudinal directions, above formula can be further simplified in two components as follows. Cdt L and CdtT for different solidity ratio are given in Table 3. Loadings Values of GT for the three terrain categories are given Table 2.
Note: i Solidity ratio is equal to the effective area Projected area of all the individual elements of a frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction.
However, in case the bracing on the leeward face is not shielded from the windward face, then the projected area of the leeward face of the bracing should also be taken into consideration. In case of Normal tower with extension of any voltage rating, one more level at the top of extension panel shall be considered. This wind speed is to be considered as Reference wind speed VR. Wind load shall be calculated as prescribed in 6. Loads due to weight of each conductor and groundwire based on appropriate weight span, weight of Insulator strings and accessories.
For intact wire the vertical load shall be considered as given in clause No. Tension Tower with Vertical Lifting point distance in. For intact wires these loads shall be considered as nil. Longitudinal loads due to unequal spans may be neglected.
Longitudinal loads during stringing on Suspension Tower should be nominally imposed only by the passing restriction imposed during pushing of the running block through the Sheave. It will apply only on one complete phase of sub-conductors or One Earthwire. It will be taken as 10, N per Sub- conductor or 5, N per Earthwire. However, the structure will be strengthened by installing temporary guys to neutralize the unbalanced longitudinal tension. These guys shall be anchored as far away as possible to minimize vertical load.
In such a case the transverse and vertical loads shall be transferred to outer limb attachment point. Icing tends to occur when temperatures have been below freezing, making conductors cold. If the air temperature above the ground rises, then any precipitation falls through the warm air as rain and freezes on contact with the cold conductor. If the air above the conductor is too cold, the precipitation freezes in the air and does not stick to the conductor.
Wind velocities tend to be low when ice forms. Generally temperature drops and wind velocity increases after ice forms. To consider the loading cases for iced condition various practices being prevailed in different countries.
Ambient Conditions Temp. Final decision to be taken by the utility. To take care of Galloping due to ICE shedding. Annexure - A. The role of cross section classification is to identify the extent to which the resistance and rotation capacity of cross sections is limited by its local buckling resistance.
The classification of a cross-section depends on the width to thickness ratio of the parts subject to compression. The Limiting width-to-thickness ratios for compression parts should be obtained from Table A. Table A. Elements which exceed semi-compact limits are to be taken as of slender cross-section Class 4. A-2 Axial Resistance in Tension. Table C.
The methods covered in this Chapter are Graphical diagram method, Analytical method, Computer aided Analysis Plane truss method or 2 dimensional analysis,Space truss method or 3 dimensional analysis. The exact stress analysis of transmission tower requires calculation of the total forces in each member of the tower under action of combination of loads externally applied plus the dead weight of structure.
The design of structure must be practical so that it is done as a production assignment. Basically the stress analysis of any tower requires application of the laws of statics. As, tower is a space frame, the solution becomes complex, if all external loads are applied simultaneously.
Different categories of loads are taken separately for calculation of stress in each member. Stresses so calculated, for different types of loads are superimposed to arrive at overall stress in the member.
Similarly, the longitudinal loads are shared equally by the two longitudinal faces. Stress-Analysis by graphical method, i. Even the line thickness makes some difference in stress value. Further, for each load on each face, separate stress diagram is required. Sometimes, due to space limitation in a drawing sheet, each stress diagram bears different Scale and overall computation of the stresses become difficult. There is likelihood of some human error creeping in, while computing the stresses.
Thus, the graphical method of drawing stress diagram has now become obsolete. However, a typical stress diagram for a Tower is shown at Annexure 4 2 Sheets. Basically, all the assumptions which are made in stress analysis of Tower by Graphical Method, are also made while using Analytical Method. However, the calculation of stress in leg-members with staggered bracings on transverse and longitudinal faces are slightly more intricate.
Annexure 5 8 sheets shows the formats for calculating stresses by Analytical Method for the following tower members Leg Member Bracings-Transverse and longitudinal faces. Cross-Arm: Various Members. In the previously described methods of stress analysis, viz. With the advent of Digital Computer, now available as an aid to a Designer, his capability is enhanced to try out number of iterations with several permutations and combinations, so as to achieve the optimum design and accurate stress analysis.
Two different methods of stress analysis with the aid of computers are being practiced. This is exact replica of analytical method, covering all the steps as before but with unlimited scope of trials for variations in tower geometry of bracing systems. Various organizations have developed several computer programs suitable to use with particular computer system available with them.
Some computer programs are so elaborate that even optimum Tower Geometry is selected automatically by a Computer. But most practical one is that Computer Software working on Interactive mode, let amalgamates the experience of a designer to try a particular geometry along with capability of a computer to try numerous permutations and combinations.
The main objective of such an elaborate aid from a computer is to achieve optimum design of a tower, which will withstand simultaneous application of worst loadings and achieve reliability as well as optimum strength of all tower members. The tower structure is basically a statically indeterminate structure.
Stiffness matrix analysis with the help of appropriate powerful computer is essential. Annexure 6 3 sheets. Connectivity of members between the Nodes and the sectional areas of the members. The loads on each Node for all three directions. These inputs can also be created through computer programs.
The first stage gives the 3-D analysis of the tower for each member for each load case. The second stage uses the out-put of the first stage as input and then gives the summary of critical stresses for members of each group Ref.
Annexure 8, 3 sheets. The 2nd stage also requires the Group file as an input. This summary output is then utilized by designers for final design. Comparison of stress analysis by graphical, analytical and computer method reveals, though it does not affect the practical stress design of tower much, the 3-D analysis by computer gives more insight into stress distribution in various members due to the various external loads.
Whereas, in the case of graphical and analytical methods it is assumed that the transverse faces take care of transverse loads and members of longitudinal faces carry stresses due to longitudinal loads only, the 3-D stress analysis by computer shows the stress distribution in the members of all the four faces of the tower due to any type of external load applied to the structure.
Similarly, while doing analysis by graphical and analytical method, stresses are only calculated in the members at the level of the externally applied load and below it, the 3-D analysis gives the magnitudes of stresses even in the members above the level of the externally applied load.
Again in the Cross-arm analysis we assume that the main members carry the transverse and longitudinal loads and a portion of vertical load, and the top inclined members carry the vertical loads, but the 3-D analysis indicates the top members share even the transverse and longitudinal loads. As per IS Part I , the concept of limit load theory has to be followed and the tower loadings, covered in Chapter 6 are based on this concept.
Design of Tower Members Since Towers are manufactured in factory environment and have to be assembled at site, the ease of transport and assembly during tower erection are equally important points for consideration. So far, the practice is over whelming in favour of the use of Hot Rolled Angle Steel Sections in the design of Towers but in some countries formed angles are also used. As per IS: , the following minimum thicknesses for members are specified: Sl.
Generally, two grades of steel i. The salient properties of these grades of steel are tabulated in Annexure 11, Annexure 12 and Annexure 13 2 sheets. Properties of angle sections which are normally used in Towers, are furnished.
For achieving desired strength of tower members and optimum weight of full Tower, a Designer adopts several Geometrical patterns for bracings, with and without the use of secondary members. This code suggests for use 6 different curves for calculation of the permissible compressive stresses in different tower members.
Refer Annexure 13 5 sheets. This Design should follow stipulations of Curve-1 to Curve-6, described above Ref. Annexure The net effective areas of angle sections in tension to work out the permissible tensile load in a member shall be determined as under The net effective sectional area in this case is given by,.
The back to back angles are to be connected or stitched together throughout their length in accordance with the requirements of IS : Code of Practice for use of Structural Steel in General Building Construction 7. They are also required to carry 2. Connection will be designed for the relevant shear and bearing stresses and the class of bolts used.
There will be no restriction on the number of bolts. ANNexure-4 Sheet No. Face Long. Bolt - 6 Nos. Bolts 6 No. Note: All loads in kg and all lever arms in Metres. Bolt 2 Nos. ST x2. Annexure-7 Sheet No. The compressive stresses in various members multiplied by the appropriate factor of safety shall not exceed the value given by following formulae As per IS Part-I With With Rxx or With Kg. Ultimate With Kg. Members to be checked for 2.
Notes : 1. Intermediate stress values can be obtained by interpolation. Redundants considered with one bolt connection at either end. Lengths wt. Lengths Lengths kg kg mm kg mm kg mm mm A 35 0. Bolt Dia. Transmission line towers are highly indeterminate structures. In the analysis and design of these structures and their detailing, a number of theoretical assumptions are made. The structures are mass produced and the quality of materials, fabrication and the assembly require checking.
It is desirable that the Designers and Users both are convinced that the tower can withstand most critical loads for which it is designed and are therefore subjected to a full scale prototype test.
For a Prototype test, the material used shall be of same quality standards as those that will apply to all towers during mass production.
The full scale testing of tower is generally termed as Prototype Test and for conducting such Prototype tests, a tower testing station is required, where it is possible to measure the applied loads and deflections and observe the behavior of the tower on application of the external design loads. Longitudinal Mast P is a structure of adequate dimension and height, constructed at a sufficient distance from the tower bed and equipped with all Rigging arrangements for applying longitudinal loads.
The Transverse loads are applied through pulleys positioned on the Transverse Mast B. Control room shall have the facility to have the complete view of transverse and longitudinal testing arrangements of the test tower.
All the electrically operated machines and instruments shall be connected to and controlled from the Control Room. In order to ensure the correctness and reliability of all measuring instruments and in turn the validity of the tests, the calibration of all instruments before the test is conducted.
Calibration of the load cells is done with Universal Testing Machine prior to rigging of test tower. A typical calibration chart is shown in Appendix- I. The Prototype tower, fabricated as per structural drawings approved by the Purchaser shall be assembled and erected on a fixed base ensuring unbraced portion of stub above chimney top to the point of connection of bracing with leg.
Fitment of any member shall be easy, natural and shall not be a forced one. The bolts should be tightened with suitable torque wrench simultaneously on all four faces. To enable application of the external loads in the most representative manner and to simulate tower design conditions, the tower structure is rigged suitably.
Impact of any variance in inclination of rigging wires with respect to the directions accounted for in designs is considered while preparing Rigging Chart. Loads are applied as per these approved rigging charts. The load cells shall be attached to the tower through the rigging wires.
The Prototype Tower is erected on the test bed and all the rigging arrangements are completed. Before rigging arrangement, the tower shall be carefully examined to ensure that all the bolts and nuts are properly tightened and tower is made truly plum within tolerance limit of 1 in and square.
All its members shall be checked for any visible defect. Two graduated metallic scales are fixed at Peak and Top Cross arm level on the transverse face. Readings on these scales with reference to the plumb line are taken by Theodolite or Total station. In order to eliminate as far as possible, the play between the bolts and the holes throughout the structure, Bolt slip test is done in the beginning.
The loads on the tower are held for 1 minute. The loads on the tower are then reduced to zero or to as low a value as possible. The deflection reading is once again taken for this Zero loading. The differences between the two zero loadings are the permanent deflections on tower.
For subsequent test purposes, the readings with zero loads taken after the Bolt Slip Test taken are considered as the Initial readings. Testing of Towers Sequence of test loading cases shall be pre-determined.
The choice of the test sequence shall largely depend upon simplification of the operations necessary for carrying out the test program. If the purchaser so desires, the tower shall be tested to destruction. Destruction test shall be carried out under normal condition or broken wire condition as agreed between the Purchaser and the Contractor. The destruction test, however, can be discontinued beyond a certain limit on mutual agreement between the Purchaser Designer and Testing station authority.
Galvanized tower shall be preferred which is similar to tower used on the transmission Line. The reinforced tower will be put to test again and subjected to balance tests, unless the failure is of major nature, which will require all the tests to be repeated, or as mutually agreed between the Purchaser and the Supplier.
For each load level, the applied load measurements shall be considered acceptable if they are within the limits as shown in below Table Ref: IEC No further adjustment shall be made for the loads on the tower during the waiting period and the waiting period starts thereafter.
In the event of failure during testing of any load case, the structure shall be modified and retested. Material of the prototype tower shall be checked for mechanical and chemical characteristics. Sample selected by the Purchaser from Test Tower shall be subjected to such tests. The type of tested tower. The name and address of the tower manufacturer and of the tower designer.
The name and address of the client. The dates and location of testing. The names of persons presented during the tests. A list of various assembly and detail drawings related to the tower tested with updated modifications of the drawings referred to. A schematic line diagram of the tower showing the various load points and directions of loading to be applied and a table with the specified loads.
Diagram showing the rigging arrangement used to apply the test loads. One table per test showing the loads required at the various points on the structure and for the various loading steps. One table per test showing the various deflection values measured. Results of Mechanical and Chemical Test carried out on samples taken from the tower. Photographs showing the whole of the structure and details of the failure. Video of testing tower can be recorded if required by Client. The Test Report shall be prepared in quadruple.
Entity Max. This chapter covers the provisions relating to the materials, fabrication, galvanizing, inspection and storage requirements of Towers. Various grades of steel used in towers-details of sections, bolts and nuts and other accessories, need a detailed scrutiny and quality control procedure before being processed for fabrication, assembly etc. Annexures I and II give chemical composition and mechanical properties of mild steel and high tensile steel used in towers.
Annexure III a to c gives sectional details and properties of hot- rolled angle and channel sections. Annexure IV gives unit weights of plates of all sizes. Annexures V and VI give dimensions of hexagon nuts and bolts and their mechanical properties respectively. Annexure VII gives the properties of tower bolts metric screw threads.
A well plan, implementable, result oriented and executable quality assurance plan is necessary to ensure delivery of acceptable material in an agreeable schedule. Reliability of a transmission structure depends not only upon its design, but also on the development of structural arrangement, detailing of connections, uniformity of quality of structural sections, and accurate fabrication.
Proper fabrication while maintaining permissible tolerances and galvanizing of towers are, therefore, very essential. The design of structure must be practicable so that it is done as a Fabrication assignment. Maximum efficiency in fabrication of structural steel by Modern shops is entirely dependent upon close co-operation between design office, drafting room and shop. After design is complete, the structural assembly drawings should be prepared according to IS: The drawings shall show the complete design dimensions, member length, slope factors or triangles, section sizes, bend lines, gauge lines, diameter, length and number of bolts, spacers, plain and spring washers, detailing of joints, sizes of gusset plates, position of holes, etc.
Sufficient number of elevation, cross-section and plan views should be presented to clearly indicate the details of joints and arrangement of members.
All members should be clearly shown and respective identification mark allotted to each member. The drawings should be drawn to a scale large enough to convey the information adequately.
All connections should be detailed to minimize eccentricity of connections. Due consideration should be given to the additional stresses introduced in the members on account of eccentricity of connections. Dimensions of all members and the distances such as hole-to-hole, length, gauge distance etc.
These drawings should clearly show the member sizes, length and mark the hole positions, gauge lines, bend lines, edge distances, amount of chipping, notching etc. For Gusset fabrication, separate individual item wise templates can be made to facilitate gusset fabrication as well as inspection. In case of members to be bent, shop drawings should indicate the provisions for variation in length.
Items requiring steep bends can be cut and welded as per approved welding procedure. Quality welding should be ensured to attain desired strength. Each fabricator or detailer has his own method of preparing details.
It is not recommended that specifications be established in so far as actual bending details are concerned. This should indicate grade of steel, mark numbers, section sizes, member lengths, their calculated weights, number of bolts, nuts and washers and their sizes, total quantities required and structural drawing numbers.
No reduction in weight due to drilling, punching of bolts holes, skew cuts, chipping, notching, chamfering etc, should be made while computing calculated weights of the members. Steel sections used should be as per IS: and all angle sections should have dimensions as per IS : In case more than one grade of steel is used in the structural members, proper identification marks of various grades of steel being used should be made on the material to ensure their ultimate use in proper location in the tower before taking up fabrication.
On the shop sketch HT steel marking is added for identifying high tensile steel items. This way, it is ensured that no mix-up of MS and HT steel materials can take place. For each project, several types of towers in different quantities have to be fabricated.
For each type of tower, number of sections may vary as per design and in length. Ingenuity in planning with the help of computer for preparing cutting plan leads to optimizing wastage of raw material as well as achieving completion of tower fabrication as per commitment.
Straightening should not damage the material. Adjacent surfaces of the parts when assembled should be in close contact throughout keeping in view the tolerances specified. The surfaces so cut should be clean, smooth and reasonably square and free from any distortion. Difficulty Beginner Intermediate Advanced. Explore Documents. Cbip Manual PDF. Uploaded by Girish R Rao.
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