Koi Koi Grace
by on March 1, 2019
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The utility industry now uses tubular steel poles as well as steel lattice towers, spun prestressed concrete poles, wood poles and composite poles. Simple suspension (tangent) and small-angle single pole structures can be quickly analyzed using spreadsheets. The advent of powerful digital computers and software such as PLS-POLETM, TOWERTM and PLS-CADDTM, now enables accurate modeling and analysis of not only individual structures of a transmission line but also the entire transmission line in a single modeling session. POLE and TOWER also have provisions for input of foundation capacities to facilitate fuller representation of the structure. Plan and Profile drawings can now be digitally processed, printed and saved in various formats and sizes.

 

1-) PLS-POLE

PLS-POLETM is a powerful structural analysis and design program for transmission structures. The program is capable of handling wood, laminated wood, steel, concrete and composite structures and performs design checks of structures under specified loads. It can also calculate maximum allowable wind and weight spans to aid in structure spotting. Virtually any transmission, substation or communications structure can be modeled, including single poles, H-Frames and A-Frames. These models can be rapidly built from components such as pole shafts, davit arms, cross arms, guy wires, X- and V-braces and all types of insulator configurations.

Pole shaft databases for standard classes of steel poles from various suppliers are built into the program and cover both galvanized and weathering steel poles. The component databases that are used with PLS-POLE include various steel shapes (round and multi-sided pole shafts), wood, laminated wood, concrete and composite poles of various classes, guy wires and insulators etc. Custom elements can be added by the user as needed.

PLS-POLE is capable of performing both linear and nonlinear analyses. Nonlinear analysis allows 2nd order or P-Delta effects, helps to detect instabilities and to perform accurate buckling checks. The program can perform steel pole design checks based on ASCE 48-11 or other specified codes. The current version of the program has built-in checks for various international codes. For poles used as communication structures carrying antennae, the program also has a design check option using ANSI TIA-222 (2006) standards.

 

Structures assembled in PLS-POLE can be exported to PLS-CADD and installed in the line’s 3-dimensional model. Figures 2.6a, 6b and 6c show renderings of typical structure models developed with PLS-POLE.

 2-) TOWER

TOWER is a powerful structural analysis and design program from Power Line Systems (PLS) for lattice steel transmission towers based on a 3-dimensional finite element analysis scheme for axially-loaded trusses. Both self-supported and guyed towers can be modeled. The program performs design checks of structures under user specified loads. For a given lattice tower structure, it can also calculate maximum allowable wind and weight spans and interaction diagrams between different ratios of allowable wind and weight spans.

The program is capable of performing both linear and non-linear analysis and facilitates detailed input (angle members, bolts, bolt configurations, member slenderness ratios, connection eccentricity and restraint etc.) It calculates the forces in the members and components and compares them against calculated capacities for the selected code or standard. Overstressed members easily identified graphically and in the TOWER output reports.

TOWER is capable of performing design checks based on various world standards: ASCE Standard 10-15, ANSI/TIA-222, Canadian Standard CSA S37, ECCS, CENELEC, AS 3995, BS 8100 and others. The component databases that are used with TOWER include various steel angles (equal and unequal legs, single and double angles), tower bolts of various specifications and insulators etc. Unique shapes (flat bars, channels, T-sections for example) can be created where necessary. Structures assembled in TOWERTM can be exported to PLS-CADDTM and installed in the line’s 3-dimensional model. Figure 2.7 shows rendering of a double-circuit deadend tower developed with TOWERTM.

 

 

3-) PLS CADD

PLS-CADD is a line modeling, analysis and design program for transmission lines and structures. This program in integrated with its sister modules PLS-POLE and TOWER. A typical 3-dimensional view of a line segment showing single poles with braced post insulators is shown in Figure 2.8. A Plan and Profile drawing generated using the software is shown in Figure 2.9.

3-a) Load and strength criteria

The fundamental design philosophy behind current transmission structure design is that all structures shall be designed and detailed in such a way so as to sustain imposed factored design loads without excessive deformations and stresses. The objective is to design a structure with resistance exceeding the maximum anticipated load during its lifetime and to produce a structure with an acceptable level of safety and reliability. Mathematically,

The strength factor ‘∅’ limits the resistance ‘R’ and accounts for the variability of the resistance property. The load factor LF accounts for the uncertainty of the given load and/or simplifying assumptions made in the analysis.

This concept is similar to the LRFD (Load and Resistance Factor Design) used in general structural design where the erstwhile deterministic approach is replaced with a more robust probabilistic approach, matching strengths with loads of various types and incorporating statistical variations associated with different load categories and strength of components.

Strength and Load factors suggested by RUS and NESC are shown in Tables 2.15a, 2.15b and 2.15c. All values refer to new, Grade B Construction which is the highest grade associated with safety and reliability per NESC. Both NESC and RUS also provide guidelines for strength factors for replaced or rehabilitated structures. The strength factors of RUS are somewhat conservative and are often adopted by utilities in rural areas and whose lines and structures are designed and built according to RUS/USDA guidelines.

 

Grades of construction

NESC defines three grades of construction, namely, Grades B, C and N. Grade B is the highest or the type of construction with stringent strength standards while N is the lowest grade. Grade B standards are typically used by US utilities for transmission lines.

 

3-b) Structural Design Criteria

Overhead transmission lines are continuous structural systems with supporting elements (poles, towers, frames) and cable elements (conductors and shield wires). To ensure safe and reliable service, these are typically designed by considering the various load conditions listed in Table 2.16.

3-c) Weather Cases

A comprehensive analysis and design of a transmission line involves various weather cases related to both the performance of the line itself (conductors and insulators) as well as the supporting structures (poles, frames and towers). A collection of these weather cases is called “Criteria File’’ which is a critical input to line design software PLS-CADD. Tables 2.17a and 2.17b show the full list of weather cases that are typically considered in transmission design in the United States. Several parameters such as wind pressures and ice thickness magnitudes vary from utility to utility and the numbers provided in the table are for illustration purpose only. The case labeled Warm Islands is applicable only for islands located from 0 to 25 degrees latitude, north or south. This zone covers Hawaii, Guam, Puerto Rico, Virgin Islands and American Samoa. The ‘k’ factor is explained below.

 3-d) Load Cases

Table 2.18 gives a typical list of load cases needed for transmission structure analysis and design. These loads are applied on the structures, supported wires, hardware, insulators and other equipment. The philosophy behind each of the load cases is explained below. The reader must note the additional NESC Factor ‘k’ applied for the NESC district load cases only. This is an arbitrary factor added to the resultant of the vertical and horizontal loads as shown in the figure at the bottom of the Table 2.18.

 

 

NESC district loadings

These 4 cases – also called District Loadings – are included to meet the requirements of Rule 250B of NESC and include the mandated climactic and load factors. The ‘k’ factor is an additional load item applied to the resultant of the conductor vertical and wind load components while determining the sag and tension of the wire. Only the structural design of angle and deadend structures and tangent structures located on

 

the line angle are impacted by the ‘k’ factor because the tension of the wire is affected by the factor.

The designations of Heavy, Medium, Light and Warm Islands are based on ice and wind expected in those areas. The user may refer to NESC for geographic boundaries of these 4 zones. For a given transmission line, only one of the four zones is applicable unless the line crosses more than one zone.

Extreme wind

The purpose of this NESC load is to ensure that the structure is capable of withstanding high winds that may occur within the geographic territory. As mentioned earlier, the wind force used is based on the fastest 3-second gust at 33 ft (10 m) above ground. Per NESC, this load is currently applied to structures over 60 ft (18 m) in height. However, RUS Bulletin 200 recommends all structures be checked for Extreme Wind regardless of height. Maps showing design wind speeds are provided by NESC as well as RUS Bulletin 200. NESC also requires all structures (irrespective of height) to be checked for extreme wind applied in any direction on the structure without conductors.

 Extreme ice

This load case considers the possibility of extreme ice storm or a storm that develops icing conditions. Usually this case is defined by accumulation of radial ice of 1 in (25.4 mm) thickness on conductors; but often utilities located in icing regions adopt a higher value of 1.5 in (38.1 mm) or more.

 

Extreme ice with concurrent wind

The intent of this NESC load case is to design a structure for extreme ice accompanied by wind. Per NESC, this load is currently applied to structures over 60 feet (18 m) in height. Since ice can stay on conductors for 4 to 5 days and may see subsequent wind, a 40 mph (64 kmph) wind at 4 psf (190 Pa) wind is shown in Table 2.18; this is used to satisfy ASCE Manual 74 requirement that wind supplementing ice must be equal to about 40% of extreme wind case. Maps showing uniform ice thickness with concurrent wind speeds are provided in NESC as well as RUS Bulletin 200. These maps show uniform ice thickness typically ranging from ¼ inch (6.35 mm) to 1 inch (25.4 mm) and concurrent wind speeds ranging from 30 mph (48.3 kmph) to 60 mph (96.5 kmph), equivalent to a wind pressure of 2.3 psf to 9.2 psf (110 Pa to 440 Pa). In all NESC load cases in which wind is included, the horizontal wind pressure is applied at right angles to the direction of the line, except where wind is applied in all directions without wires. Also, NESC does not consider ice on structures and wind-exposed surface areas.

 

Construction

This load case is to ensure the structural integrity of not only the main structure but also the arm or steel vang supporting the insulator/stringing block and arm strength during wire tensioning. This is because one of the worst loadings that a given arm or conductor attachment point will endure is during the stringing of the conductors. A small wind is also considered in this case. This load is applied as additional vertical and horizontal loads to the phase that induces the highest structural stresses with all conductors installed and stringing in the last conductor. A typical example will be tensioning at the structure tensioner down slope (1:1).

 

Broken wires

The idea behind including this case is to ensure that in the event any phase wire or overhead ground wire fails, the failure does not cause any additional damage to the structure or lead to a cascading type of line failure. Another motivation is related to the cost and availability of replacement structures and the long lead times for fabrication. Designing a structure for potential broken wire cases and the slight cost associated with it is well worth considering that removal of a line from service, even temporarily, is avoided. This is critical for lattice transmission towers carrying HV and EHV circuits.

Broken wire loads are generally applied at selected wire location (conductor or shield wire) that induces the greatest stress in the structure. More than one load case may be needed if the highest stress location is not readily apparent. ASCE Manual 74 provides more information on design longitudinal loading on structures, historically called everyday wire tension or broken wire load.

 

Failure containment

This load case is designed to reduce potential for catastrophic failures. Severe climactic conditions often produce a cascading type of failure where one structure fails and collapses completely. This subjects adjoining structures with severe unbalanced loads – much more than what they were designed for – upon which they collapse in a dominolike sequence. Deadends and heavy angles, and in some cases tangent structures too, are subjected to such a load criteria. Containment loading is characterized by the absence of ALL wires on one side of the structure. The conditions that define this load case are usually set by the utility based on their judgment.

ASCE Manual 74 also provides guidelines for longitudinal loading on structures including a procedure based on Residual Static Load (RSL), which is a final effective static tension in a wire after all the dynamic effects of a wire breakage have subsided.

 

 

 

Uplift

Uplift is defined as negative vertical span. Figure 2.10 depicts the situation where uplift occurs in a transmission line. On steeply inclined spans in hilly terrains, when the cold sag curve shows the low point to be above the lower support structure, the conductors in the uphill span exert upward forces on the lower structure. The magnitude of this force at each attachment point is related to the weight of the loaded conductor from the lower support to the low point of sag. This uplift force is more pronounced at colder sub-zero temperatures. Uplift must be avoided for suspension, pin-type and post insulators. A quick check for uplift can be made using a sag template (see Chapter 5).

 

Deflection

In addition to the above loading cases, designers also often check situations such as Deflection Loading where tangent poles and frames are designed to limit the pole top deflection to a specified level, usually to 1% to 2% pole height above ground for a given loading. This ensures adequate stiffness of the structure to limit flexural deformations and thereby help keep insulators plumb and clearances intact. The climactic conditions for this load case usually include the average annual ambient temperature for the structure geographical location.

 

 

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