US20090031646A1

US20090031646A1 - Utility pole

Info

Publication number: US20090031646A1
Application number: US11/826,405
Authority: US
Inventors: Peter Badgley; Stephen Dunstall; Christian Deveau; Michael Thorpe
Original assignee: Stelco Inc
Current assignee: US Steel Canada Inc
Priority date: 2006-07-14
Filing date: 2007-07-16
Publication date: 2009-02-05

Abstract

A tubular utility pole for supporting a load, adapted for direct burial of a preselected end thereof in a soil material. The pole includes a body having a substantially constant cross-section substantially along a length thereof. The body includes a steel tube with holes therein to permit attachment of the load to the body.

Description

FIELD OF THE INVENTION

This invention is related to a utility pole adapted for direct burial of an end thereof having a body with a substantially constant cross-section.

BACKGROUND OF THE INVENTION

Wooden and steel support poles for supporting electrical utility transmission lines, telephone and cable wires, transformers, and the like are known. Wooden support poles have a number of disadvantages, however. For instance, wooden support poles tend to rot. To minimize and/or retard rot, wooden poles are often treated with chemicals. Because of this treatment, however, the treated poles are generally considered hazardous waste, so that their disposal is relatively costly. Also, because the wooden poles originate as natural products, their characteristics vary from one wooden pole to the next, and in particular, the strength of the wooden poles can vary from one pole to another. Wooden poles are generally tapered, due to the natural taper of the tree trunks from which the poles are formed.
Various steel tubular utility poles are known in the art. However, the prior art steel utility poles typically have tapered tubular bodies (see, e.g., U.S. Pat. No. 3,942,296; Korean Patent No. 20040020447; and Japanese Patents Nos. 2004211292 and 2005188279), which have relatively high manufacturing costs. U.S. Pat. No. 6,705,058 discloses a pole that can be directly embedded into the ground, however, the pole is comprised of telescoping tubular sections above-ground and an in-ground concrete base.
For instance, a tapered metal support pole of the prior art is indicated generally by the reference numeral 10 in FIG. 1. As can be seen in FIG. 1, the pole 10 has a bottom end 12 with a preselected diameter and a top end 14 with a much smaller diameter. The pole 10 is tapered along its length, from the bottom end 12 to the top end 14.
A straight (i.e., non-tapered) steel pole which is mounted on a concrete foundation is also known. For instance, Japanese Patent No. 08-326356 discloses a straight steel pole positioned on a concrete foundation for erection. However, this type of pole is not designed for supporting electrical utility transmission lines, telephone and cable wires, transformers, and the like. This type of prior art pole is not suitable for such applications because of the relatively high costs thereof, and also because such poles are unable to withstand the loads.
There is therefore a need for an improved utility pole which overcomes or mitigates one or more of the disadvantages of the prior art.

SUMMARY OF THE INVENTION

In its broad aspect, the invention provides a tubular utility pole for supporting a load, adapted for direct burial of a preselected end thereof in a soil material. The pole includes a body having a substantially constant cross-section substantially along a length thereof. Also, the body is a steel tube with holes therein to permit attachment of the load to the body.
In one of its aspects, the invention additionally includes a bearing plate positioned on the body at the preselected end, to retard the settlement of the pole into the soil material.
In another of its aspects, the invention provides a method of producing the tubular utility pole. The method includes, first, the step of roll-forming a coil of strip steel to a predetermined configuration so that the strip has at least two edges adapted to mate with each other. Next, the two edges of the strip are seam welded together to form the body. Finally, the method includes the step of cutting the body to a predetermined length.
In another aspect, the method additionally includes the step of attaching a bearing plate to the body at the preselected end, to retard settlement of the pole in the soil material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the attached drawings, in which:

FIG. 1 (previously discussed) is a side view of a tapered utility pole of the prior art;

FIG. 2A is a side view of an embodiment of a utility pole of the invention;

FIG. 2B is a bottom view of an embodiment of a bearing plate of the invention;

FIG. 3A shows design constraints and loading used in design calculations for an embodiment of the pole of the invention;

FIG. 3B is a cross-section of the pole of FIG. 3A;

FIG. 4A is a diagram schematically illustrating the results of finite element modeling conducted to estimate stresses to which an embodiment of the pole of the invention may be subjected; and

FIG. 4B is a contour plot legend for the stresses of the finite element analysis of FIG. 4A.

DETAILED DESCRIPTION

Reference is first made to FIGS. 2A and 2B to describe an embodiment of a tubular utility pole in accordance with the invention indicated generally by the numeral 20. Preferably, the utility pole 20 is provided with a bearing plate 21 positioned on a preselected end 24 of the pole 20, and the end 24 is to be positioned in soil material 25 (i.e., direct burial), as will be described. The utility pole 20 includes a tubular body 22 having a substantially constant cross-section along a length thereof. The body 22 preferably comprises a steel material, as will be described. The tubular body 22 preferably includes holes 26 positioned in it, as shown in FIG. 2A, to permit insertion and/or attachment of one or more members (not shown) for supporting a load (not shown).
As noted above, the pole 20 preferably includes the bearing plate 21. It is preferred that the bearing plate 21 is mounted on the body 22 at the end 24. The bearing plate 21 is for retarding settlement of the pole 20 into the soil material 25.
It is preferred that the steel material of which the pole 20 is made is coated. For the purposes hereof, “coated” refers to any coatings (such as those resulting from galvanizing or galvannealing) added after the steel has been made, to protect the steel from corrosion. Preferably, the coating is selected from a group consisting of zinc, aluminum, and an alloy of zinc and aluminum. It is also preferred that the steel is pre-coated, i.e., the steel is coated shortly after the steel has been made, and before the steel has been formed into the body 22. However, the steel may also be post-coated. “Post-coated” refers to any coating (e.g., such as those resulting from galvanizing) added after the body 22 has been formed, i.e., manufactured.
In addition, it is preferred that the preselected end 24 of the body (i.e., the end to be inserted into the soil material 25) is provided with a material (not shown) adapted for retarding corrosion of the body. Such material preferably is a layer of an organic material such as epoxy, or any suitable polymer, as would be known to one skilled in the art. It will be understood that, for the purposes hereof, “soil material” refers to any suitable soil or aggregate material in which the end 24 of the body 22 is positioned.
Preferably, the holes 26 are provided in the body 22 to facilitate the attachment of members on which the load is supported (or otherwise to permit attachment of the load to the pole) generally towards an upper end 28 of the body 22. Preferably, the holes 26 are spaced apart from the end 24 by predetermined distances. As indicated above, the load may be, for example, electrical utility transmission lines, telephone and cable wires, transformers, and the like, and also may include related gear, such as members inserted in the holes 26 (FIG. 2A).
To form the steel utility pole 20, the following steps are taken. First, a coil of strip steel is roll-formed in a continuous rolling mill to a predetermined configuration, so that the strip has two or more edges which are adapted to mate with each other. Next, the two edges are seam welded together to form the body 22. Next, the body 22 is cut to a predetermined length, as required for the application. The bearing plate 21 is also attached by any suitable means.
As noted above, the pole is adapted for direct burial of the end 24 thereof. As is known, a hole is dug in the soil, in which the end 24 is receivable, to the depth required. Before burial of the end 24 in soil, the preselected end 24 of the utility pole 20 preferably is provided with a layer of material (i.e., whether a polymer or epoxy) which is adapted for retarding corrosion of the body 22. The end 24 is subsequently inserted and/or buried directly in the soil material, being such suitable material as may be available. After the end 24 is positioned in the hole, soil material is backfilled, i.e., positioned around the pole in the space remaining in the hole, and such material is tamped down or compressed, to a suitable extent, to hold the end 24 so that the pole remains substantially vertical.

Engineering and Design Principles

There are three major factors to be considered in designing the steel pole of the invention. The factors are: expected strength, expected life, and expected performance.
1) Expected Strength: The primary structural component, the body 22, is subjected to transverse wind and ice loading. Its resistance is characterized by a bending-strength at groundline (GL) (FIGS. 2A, 3A). However, the pole bending capacity could be limited by local buckling, depending on pole diameter (D) and thickness (t) ratio, D/t. Steel pole manufacturers would typically provide a nominal strength P_n(or a minimum guaranteed strength f_ymin) to determine the moment capacity, and an ultimate vertical load P_u(or the buckling critical load based on Euler's formula).
The many sources of uncertainty in observed pole strength include inherent material property variability, manufacturing effects, and variation in testing methods. Thus, the current practice requires that the strength of the pole must be characterized by a probability density function (normal distribution type) with a mean value and a coefficient of variation (COV). For steel towers the standards assume that nominal strength P_nhas an exclusion limit in the range of 5% to 10% and COV is in the range of 10% to 20%.
2) Expected Life: A steel pole of the invention has a service life of up to approximately 80 years (i.e., more than twice the typical service life of a wooden utility pole). Steel utility poles of the invention can be coated with zinc or made from weathering steel. (Weathering steel poles of the invention can provide the same benefits as galvanized steel poles.) Accordingly, the steel pole of the invention is an economically viable alternative to traditional wood poles. Preferably, and as noted above, an organic, epoxy material (not shown) is applied at the end 24 of the pole, to further protect the directly embedded end of the pole. Various epoxy materials may be suitable. All serve the same purpose of protecting the steel from aggressive corrosion conditions that sometimes exist in the environment.
3) Expected Performance: The following three factors are to be taken into consideration when assessing expected performance:

- a. Groundline Deflection;
- b. Handling; and
- c. Field Use.

Preferably, the utility pole of the invention is designed to various diameters that vary in steel grades, strength, and thicknesses (gauge), depending on the application, i.e., depending on the dimensions and the load. The steel utility poles of the invention are manufactured and tested to stringent quality and performance requirements. In general, the utility poles of the invention do not fail catastrophically. They exhibit forms of material yielding when damaged, often allowing the utility line to remain in service and providing enough time for utility workers to replace or repair the pole without causing any major service interruption. Additionally, the steel utility poles of the invention can be designed for applications where wood poles are required to be guyed, i.e., the pole of the invention is usable in such circumstances without a guy. The steel utility poles of the invention provide a solution for tight spacing requirements and need not be guyed or otherwise exteriorly supported.

Installation

In the present invention, utility poles are installed in a very similar fashion to wood poles and conventional tapered steel utility poles with direct burial in the soil material (FIGS. 2A, 3A). Utility poles are also relatively lightweight and easier to install and manoeuvre than wooden poles. Due to the increased strength and engineered dimensional stability, utility poles can be smaller in diameter than prior art wooden poles and can also utilize smaller-sized auger equipment. (This is especially advantageous because only one auger size needs to be selected whereas wood poles can differ in diameter sizes and require more auger sizes to be required). Changing augers can increase installation times when a series of poles is being installed. With more control over pole diameters, backfill practices are minimized.
Among other advantages, the utility poles of the invention require less maintenance, resulting in fewer risks for utility linemen.

Storing and Handling

Utility poles preferably are shipped as single pieces due to their length (less than approximately 60 feet). Since the utility poles of the invention are lighter than wood poles, the number of poles per truckload is governed by volume, not weight. Utility poles are also easier to nest tightly together than the conventional tapered steel utility poles. When traditional wood poles are transported to the holding site, cribbing practices must be in place in order to avoid storage stains. Unlike wood poles, steel poles do not require periodic rotation while in storage to avoid potential moisture problems.

Testing

Steel utility poles of the invention preferably are tested to verify the maximum load carrying capacity by bending tests, whereas wood poles are tested to determine the mean rupture strength.

HSS Utility Pole Examples

EXAMPLE 1

In the present invention, the utility pole is dipped in zinc for corrosion prevention. Preferably, and as described above, the buried portion of the pole (i.e., the end 24) is further coated with an organic material prior to burial.
According to American National Standards Institute (ANSI) standards and, in general, the typical standards of hydro utilities in North America, utility poles are defined by height above ground and class, where the class identifies the load-bearing capacity (see Table 1). Body 22 behaves as a cantilever beam, with end 24 fixed at groundline (GL) and a transverse load P, depending on the service class. In accordance with such standards, the prescribed load is applied 2 feet below the pole top and is intended to represent the applied force from service equipment, wires, and the like mounted to the pole. The groundline is assumed to be at a distance from the pole bottom which is approximately ten percent of the pole length, plus 2 feet (FIG. 3A).
Table 2 shows three design cases according to the current invention. The parameters are illustrated in FIGS. 3A and 3B. These designs were arrived at following the design steps as per ANSI 05.1 Utility Distribution Pole Class Design Requirements.
1) Calculate pole nominal bending moment M_nomat GL:
M _nom @GL=P*[L−(0.1L+2)−2]*12, [in-Ibf]
2) Calculate pole section modulus, S_nom:
S _nom =D _n ⁴−(D−2t _n)⁴)/32D _n, [in³]
3) Calculate nominal bending stress f_bnom:
f _bnom =M _nom /S _nom, [psi]
4) Select material minimum yield, f_y:
f_ymin=f_bnom
5) Check for buckling limitations, as per ASCE manual 72, with a strength factor, φ=1:
D/t≦6,000φ/f _ythen allowable bending stress f _ba =f _ymin , [ksi]
6000/f _y <D/t<12000φ/f _ythen allowable bending stress f _ba=0.7f _y+800/(D/t)
6) Calculate Euler's critical buckling load P_u(for a column of length L_B, fixed at the base and free at the top):
P _u=π² EI/4L _B ² , [Ibf]

- I=section moment of inertia, [in⁴]
- L_B=distance from GL to 2 ft from the top of pole, [in]

The three design solutions described above and in Table 2 were verified through finite element modeling (FIGS. 4A, 4B).

TABLE 1

Utility Distribution Poles ANSI 05.1 Class Requirements**

	Working	Wood Horizontal	Equivalent Steel
Class	Load (lbs)	Load (lbs)* 4×	Load (lbs)* 2.5×
(“X”)	(“Y”)	Safety Factor	Safety Factor

2	925	3,700	2313
3	750	3,000	1875
4	600	2,400	1500
5	475	1,900	1188
6	375	1,500	938

*Point load located 2-ft below top of pole based on requirements for new and replaced Grade C Structures (NESC)
**Table from American Iron and Steel Institute (AISI)

TABLE 2

Examples in Table Format

Design Calculations

Bending

Design

Section

Moment

Stress,

Stress²,

Steel

Ultimate

O.D.

Thickness

Length

Weight

Pole

Load

Modulus

@GL

Calculate

FEA

Grade

Vertical Load

D, [in]

t, [in]

L, [ft]

[lbf]

Class¹

P, [lbf]

S_nom, [in₃]

D/t

M_nom, [in-lbf]

f_bnom, [ksi]

f_bFEA, [ksi]

[ksi]

P_u, [kips]

9.625	0.188	40	757.1	3	1875.0	12.90	51.2	720,000.0	55.8	58.0	80	301.2
9.625	0.188	40	757.1	4	1,500.0	12.90	51.2	576,000.0	44.7	47.0	60	301.2
9.625	0.188	40	757.1	5	1,187.5	12.90	51.2	456,000.0	35.4	37.0	50	301.2

¹per ANCI05.1-NESC Type C Construction
²Predicted results from a 1st order linear finite element model.

Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. §112, paragraph 6.
It will be appreciated by those skilled in the art that the invention can take many forms, and that such forms are within the scope of the invention as claimed. Therefore, the spirit and scope of the appended claims should not be limited to the descriptions of the preferred versions contained herein.

Claims

1. A tubular utility pole for supporting a load, the pole being adapted for direct burial of a preselected end thereof in a soil material, the pole comprising:

a body having a substantially constant cross-section substantially along a length thereof, and

the body comprising a steel tube with holes therein to permit attachment of the load to the body.

2. A tubular utility pole according to claim 1 additionally comprising a bearing plate positioned on the body at the preselected end, to retard the settlement of the pole into the soil material.

3. A tubular utility pole according to claim 1 in which said steel is coated.

4. A tubular utility pole according to claim 3 in which said steel is coated with a coating selected from the group consisting of zinc, aluminum, and an alloy of zinc and aluminum.

5. A tubular utility pole according to claim 1 in which the preselected end of the body is provided with a layer of material adapted for retarding corrosion of the body.

6. A method of producing the tubular utility pole of claim 1, comprising:

(a) roll-forming a coil of strip steel to a predetermined configuration such that the strip has at least two edges adapted to mate with each other;

(b) seam welding said at least two edges together to form the body;

and

(c) cutting the body to a predetermined length.

7. A method according to claim 6 additionally comprising:

(d) attaching a bearing plate to the body at the preselected end, to retard settlement of the pole in the soil material.

8. A method according to claim 7 additionally comprising:

(e) providing the preselected end of the support pole with a layer of material adapted for retarding corrosion of the body.