General design criteria
2.4 STRUCTURAL ANALYSIS
2.4.4 Load and strength criteria
2.4.4.3 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,
General design criteria 57
Table 2.15c NESC Strength Factors (Grade B New Construction).
Structure1or Component with Rule 250B Strength Factor (φ)
Metal and Prestressed Concrete Structures, Crossarms and Braces6 1.00 Wood and Reinforced Concrete Structures, Crossarms and Braces2,4 0.65 Fiber-reinforced Polymer Structures, Crossarms and Braces6 1.00
Support Hardware 1.00
Guy Wire5,6 0.90
Guy Anchors and Foundation6 1.00
Structure1or Component with Rule 250C and 250D Strength Factor (φ) Metal and Prestressed Concrete Structures, Crossarms and Braces6 1.00
Wood and Reinforced Concrete Structures, Crossarms and Braces3,4 0.75 Fiber-reinforced Polymer Structures, Crossarms and Braces6 1.00
Support Hardware 1.00
Guy Wire5,6 0.90
Guy Anchors and Foundation6 1.00
Note: Rule 250B refers to NESC District Loading (Light, Medium, Heavy or Warm Islands) Rule 250C refers to NESC Extreme Wind Loading
Rule 250D refers to NESC Extreme Ice and Concurrent Wind Loading
1Includes Pole
2Wood and reinforced structures shall be replaced or rehabilitated when deterioration reduces structure strength to 2/3 of that required when installed. When new or changed facilities modify loads on existing structures, the required strength shall be based on revised loadings. If a structure of component is replaced, it shall meet the strength required by this table. If a structure or component is rehabilitated, the rehabilitated portions of the struc-ture shall have strength greater than 2/3 of that required when installed.
3Wood and reinforced structures shall be replaced or rehabilitated when deterioration reduces structure strength to 3/4 of that required when installed. When new or changed facilities modify loads on existing structures, the required strength shall be based on revised loadings. If a structure of component is replaced, it shall meet the strength required by this table. If a structure or component is rehabilitated, the rehabilitated portions of the structure shall have strength greater than 3/4 of that required when installed.
4Where a wood or reinforced concrete structure is built for temporary service, the structure strength may be reduced to values as low as those permitted by footnotes 2 and 3 provided the structure strength does not decrease below the minimum required during the planned life of the structure.
5For guy insulator requirements, see Rule 279 of NESC.
6Deterioration during service shall not reduce strength capability below required strength.
(Courtesy: IEEE NESC®C2-2012) National Electrical Safety Code Reprinted With Permission from IEEE, Copyright IEEE 2012 All Rights Reserved.
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
58 Design of electrical transmission lines
Table 2.16 Load Conditions Considered in Design of a Transmission Line.
No. Type of Loads Load Case Description
1 Weather Extreme Wind in Any Direction
2 Extreme Ice Combined with Reduced Wind
3 Unbalanced Ice without Wind
4 Reduced Ice Combined with Substantial Wind
5 Failure Containment Broken Conductors or Ground Wires
6 Construction and Maintenance Stringing of Wires
7 Structure Erection
8 Legislated NESC-mandated Cases
(with permission from ASCE)
Table 2.17a Weather Cases for a Typical PLS Criteria File.
Ice
Wire Wind Pressure Thickness Constant
No. Description Temp. (◦F) (psf ) (in) k (lb/ft) Remarks
15 4 ¼ 0.20 Structure
Design
6 Galloping (Swing) 32 2 ½ 0 Weather Cases
7 Galloping (Sag) 32 0 ½ 0 for Galloping
Checks Note: 1 psf= 47.88 Pa, 1 inch = 25.4 mm, 1 mph = 1.609 kmph, 1 lb/ft = 14.594 N/m, deg C = (5/9)(deg F-32)
1All structures, including those below 18 m (60 ft) in height, shall be checked for Extreme Wind condition without any conductors with wind acting in any direction.
2Wind pressure must be calculated using Equation 2.1a and including all applicable parameters.
General design criteria 59
Table 2.17b Weather Cases for a Typical PLS Criteria File (cont’d).
Wire Wind Ice Constant
Temp. Pressure Thickness k
No. Description (◦F) (psf) (in) (lb/ft) Remarks
8 Low Temp –10 0 0 0 Uplift Cases
9 Low Temp –20 0 0 0
10 T-0 0 0 0 0
11 T-32 32 0 ½ 0 Conductor Separation;
Differential Ice Loading
12 T-50 50 0 0 0 Various Wire
13 T-60 60 0 0 0 Installation
14 T-70 70 0 0 0 Temperatures
15 T-80 80 0 0 0
16 T-90 90 0 0 0
17 T-100 100 0 0 0
18 T-120 120 0 0 0
19 T-167 167 0 0 0 High Temp.
20 T-212 212 0 0 0 Transmission Sag
21 T-392 392 0 0 0 (Clearance Purposes)
Note: 1 psf= 47.88 Pa, 1 in = 25.4 mm, 1 mph = 1.609 kmph, 1 lb/ft = 14.594 N/m, deg C = (5/9)(deg F-32)
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.
60 Design of electrical transmission lines
Table 2.18 Typical Load Cases for Analysis and Design.
NESC
Wind Wind Constant Ice
Speed, mph Pressure, psf ‘k’ Temperature Thickness,
Load Case (kmph) (Pa) (lb/ft) (◦F) Inches (mm)
NESC Heavy 40 (64) 4 (190) 0.30 0 ½ (12.5)
NESC Medium 40 (64) 4 (190) 0.20 15 ¼ (6.5)
NESC Light 60 (96) 9 (430) 0.05 30 0
NESC Warm Islands 60 (96) 9 (430) 0.05 50 0
40 (64) 4 (190) 0.20 15 ¼ (6.5)
NESC Extreme Wind 90 (144)1 20.7 (991)1 0 60 0
Extreme Ice 0 0 0 32 1.0 (25)
NESC Extreme ice with 40 (64) 4 (190) 0 15 1.0(25)
Concurrent Wind
Construction 28 (44.8) 2 (96) 0 60 0
Broken Wires 40 (64) 4 (190) 0 0 ½ (12.5)
Failure Containment2 40 (64) 4 (190) 0 0 ½ (12.5)
Uplift 0 0 0 –10 to –203 0
Deflection 28 (44.8) 2 (96) 0 60 0
1Values shown are typical. For other wind speeds, use appropriate values.
2All wires are cut on one side of the structure.
3Cold curve.
Definition of NESC Constant ‘k’.
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
General design criteria 61 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 domino-like 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.
62 Design of electrical transmission lines
Cold curve
Cold curve
Uplift exists at center structure
No uplift at center structure, check for allowable insulator swing
Figure 2.10 Uplift Condition (Source: RUS/USDA).
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 conduc-tors 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.
General design criteria 63 2.5 FOUNDATION DESIGN CRITERIA
Each structure must be securely embedded or anchored into the ground and facil-itate safe transfer of structure loads to the ground strata below. To determine foundation requirements, the engineer must first evaluate the nature and condi-tion of the soil in the vicinity of the structure. The choice of eventual foundacondi-tion type will further depend on geotechnical characteristics of strata underneath, struc-ture material, configuration, loads, constructability and economy. A majority of tangent poles are directly-embedded into ground; but systems defined by large lat-eral forces and moments require concrete drilled shafts whose design is a bit more labor-intensive.
Design criteria for foundations depend on type of soil and loads imposed. Design loads are usually factored reactions obtained from structural analysis from com-puter programs. In lattice towers, the loads transmitted are primarily compression and uplift loads. For single pole structures, the loads transmitted are overturning moment, shear (lateral) and axial loads. Where moment foundations such as drilled shafts are required, it is important to specify allowable deflection or rotation criteria, both elastic and non-recoverable. Chapter 4 will contain more information on this issue.
As with structures, foundations are also designed for given Strength Factors shown in Tables 2.15a and c. However, unlike structures, guidance in the area of foundation design is not well laid out by the codes and standards. Therefore it is common practice to use internal design criteria for foundations. These criteria vary from utility to utility.
The structural design of all reinforced concrete systems is in general governed by the ACI 318 (2011). For drilled shafts with a diameter greater than 30 in (0.76 m), ACI-336-3R (1998) is recommended.
Computer programs such as CAISSONTM (2011), LPILETM (2015) or MFADTM (2015) are used to quickly size a drilled shaft or directly-embedded pole under moment loads. Drilled shafts under moment loads are primarily designed as laterally-loaded piles. Guyed systems require anchors to transmit guy wire loads to the ground. Anchor types range from classical (log) to grouted (rock) to helical (screw) with widely-varying holding strengths depending on site-specific soils. If soil data is available, helical anchors are evaluated using HeliCAPTM(2007).
Geotechnical properties of ground strata are an integral part of the design crite-ria. In general, properties such as allowable bearing capacity, unit weight (dry, moist and submerged), friction angle, cohesion, subgrade modulus and others are required.
Chapter 4 will discuss these issues in greater detail.
2.6 CONSTRUCTABILITY
Constructability refers to the “readiness to be built’’ and to the process of review-ing and ensurreview-ing that a particular transmission line project can be actually built as designed, given the numerous technical and non-technical parameters that control the construction phase. This “review’’ looks at the construction constraints, environmental barriers, political issues, risks and acceptance to local community.
64 Design of electrical transmission lines