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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to composite structural members and particularly to
electric transmission or distribution poles, and to a method of
constructing such members. Electric transmission and distribution poles
currently being used are, for the most part, either solid wood, hollow
steel tubes or concrete, the latter being either hollow or solid. Concrete
poles are further classified as being either conventionally reinforced,
pre-stressed, post stressed, or partially pre-stressed. The cross-sections
of the poles are either rectangular or circular. For the most part the
poles are continuously tapered or, in the case of hollow metal poles,
lengths of telescoping tubes of successively smaller diameter achieve
cross-sectional reduction in discrete steps.
All of the conventional used poles have inherent drawbacks. Wood poles are
subjected to attack by rot-producing fungi, wood-boring insects,
woodpeckers, fires, and the like. Steel poles, likewise, have a number of
distinct limitations. The steel poles used are generally thick-walled,
heavy, and expensive since the use of weight-saving thin walled tubes of
high strength steel cannot be fully realized because of the thickness
limitations imposed by local buckling considerations. In addition, the
interior of steel poles is inaccessible resulting in a danger of
undetected corrosion. Galvanizing of the poles is often used, but it has
been difficult to achieve good quality control of the interior of the
poles and as a result the danger of corrosion persists. Also, steel poles
are often formed from sections which are welded together and it is
necessary for the welded section to develop the full tensile strength of
the adjacent metal. However, quality welds are difficult to achieve and
brittle fractures originating at flaws in the welds are also an ever
present danger. The thickness limitation imposed by local buckling
considerations also aggravates the welding difficulty by requiring welding
of thick sections. Because steel poles require special manufacturing
facilities, they must be fully assembled at relatively few central
facilities and thus require shipping over long distances, adding
significantly to the cost of the pole.
Concrete poles are also fraught with disadvantages. Conventional steel
reinforced concrete poles must be designed for a bending moment
substantially less than the ultimate bending moment of the pole
cross-section because the concrete on the tension side will crack at
relatively low tensile stresses. These cracks will admit water which,
particularly in a marine environment, could cause corrosion of the steel
reinforcement members. In addition, conventional steel reinforced concrete
poles, because of their low design strength, are usually heavy and
uneconomical to use.
Pre-stressed or post-stressed concrete poles may avoid the cracking
problems of conventional steel reinforced concrete poles because the cable
tensioning will close any cracks after the overload on the pole is
removed. However, if overloading and crack opening is repeated too often,
particles of matter may become embedded in the cracks to keep them open.
Should that happen, the corrosion of the steel members may result from
water entering the opened cracks.
Pre-stressed and post-stressed concrete poles are usually tapered to a
smaller upper cross-section. Because the number of cables or tendons is
constant along the full length of the pole, the pre- or post-stress at the
lower section is less than at the top. Therefore, if the lower section of
the pole is stressed the desired amount, the upper section would be
overstressed. On the other hand, if the upper section is stressed the
desired amount, the lower section would be under-stressed. The additional
strength required at the lower section could be provided by adding
conventional reinforcing steel. However, that practice could add
considerably to the cost of the pole. Thus, because of the pole taper, the
stressing is unbalanced between the upper and lower sections resulting in
a loss of economy of the poles.
Although concrete poles require less specialized equipment for their
manufacture than metal poles, the forms used are cumbersome and costly,
especially where the poles are formed by centrifugal casting. Thus, a
large capital expenditure is required in the forms used for producing
concrete poles in order to provide a profitable production rate. In
addition to the costly forms, special steam curing equipment is often
required to reduce the time the forms are in use for each pole made.
There are various other disadvantages of conventional steel, wood, steel
reinforced, concrete, pre-stressed concrete and post-stressed concrete
poles as those skilled in this art are well aware of. The present
invention overcomes most of the disadvantages enumerated above as well as
those not enumerated. This invention in its various embodiments includes
thin-walled metal shells and cores of non-metallic settable rigid material
such as concrete embedding pre-stressed steel cables or tendons. The
ability to use thin-walled casings results in cost savings in material,
fabrication and shipping costs. In addition the thin-walled casing
eliminate the need for special forms and rapid curing techniques. Also,
the shell serves to protect the concrete core by covering the cracks which
would result during overstressing of the pole while at the same time the
concrete core serves to protect the interior of the steel casing from
corrosion, thereby eliminating the need for costly galvanizing of the
casing. The structural members of this invention also reduce the danger of
weld failure as well as allowing simple inclusion of additional lower
section tendons to provide balanced pre-stressed structural members. Also,
the structural members of this invention may be formed in a relatively
simple fabrication facility set up near the use area thereby reducing
capital requirements and shipping costs as compared with the same costs of
a complex factory of the nature required for forming conventionally used
poles, for example. Various other advantages of this invention will be
expressed at a later place in this specification.
SUMMARY OF THE INVENTION
The present invention provides a novel composite structural member, such as
an electrical transmission or distribution pole, and a novel method for
constructing the structural member. The composite structural member of
this invention in its preferred form includes an elongated outer metal
casing; a body of non-metallic settable material such as concrete, encased
by and extending between the ends of the casing; a plurality of elongated
reinforcing members embedded within the body of settable material and
extending between the ends of the casing and communicating with the
casing, each reinforcing member being under a predetermined magnitude of
applied stress and being arranged to transmit the applied pre-stress only
to the casing; securing means supported by the casing for securing the
reinforcing members to the casing such that the applied pre-stress remains
permanently on the casing only.
The preferred method of constructing a composite structural member of the
present invention includes the steps of securing one of the ends of each
reinforcing member to the securing means and disposing the other ends of
the reinforcing members loosely within the other securing means; applying
a predetermined stress to the reinforcing members by pulling them in a
direction opposite to the originally secured ends, whereby the stress
applied to the reinforcing members is transmitted to the casing; securing
the other ends of the reinforcing members in the other securing means;
pouring a fluid non-metallic settable material such as concrete into the
casing to embed the reinforcing members; and allowing the fluid material
to set whereby the stress transmitted by the reinforcing members to the
casing remains permanently on the casing only and is not transmitted to
the body of set material.
Various other advantages, details and modifications of the present
invention will become apparent as the following descriptions of a present
preferred embodiment and present preferred method of constructing the
embodiment proceed.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings we show a composite structural member which
illustrates a present preferred embodiment of this invention in which:
FIG. 1 is an elevation view in cross-section of an electric transmission or
distribution pole embodying the composite structural member of the present
invention;
FIG. 2 is a cross-sectional view taken along the line II--II of FIG. 1;
FIG. 3 is a cross-sectional view taken along the line III--III of FIG. 1;
FIG. 4 is a plan view of the end cap;
FIG. 5 is a cross-sectional view taken along the line V--V of FIG. 1; and
FIG. 6 is a diagrammatic cross-sectional view of a concrete filled casing
which does not represent this invention, but is merely to aid in an
explanation of the reason of the advantages of the embodiment of this
invention.
Referring now to the drawings there is shown a composite structural member
of the present invention in the form of an elongated electric transmission
or distribution pole 10, which includes casing 11 formed of a lower casing
12 of circular cross-section and an upper casing 14 also of a circular
cross-section but having an outer diameter of a smaller size than that of
the lower casing. Although only two sections of casing 12 and 14 are
shown, more may be used. The respective lower and upper casings 12 and 14
are formed from relatively thin steel, which could be on the order of 1/8
to 1/4 inch thick or less but could be thicker with larger diameter
casings, as those skilled in this art would readily understand. The lower
and upper casings 12 and 14 are joined by a transition section 16 which is
suitably welded respectively to the upper end of lower casing 12 and the
lower end of upper casing 14. To insure proper alignment of the lower
casing 12 to the transition piece 16, a plurality of alignment clips 18
formed of flat sections of steel bent at intermediate points to an angle
generally conforming to that of the transition piece 16 are secured as by
welding to the lower end region of the transition piece. Similarly,
alignment clips 20 of the same general configuration of clips 18 are
secured to the upper end portion of transition piece 16. Other alignment
means of various designs could be substituted for the clips 18 and 20 as
those skilled in this art would readily appreciate.
The pole 10 also includes a core 22 of rigid non-metallic settable
material, such as a non-shrinking conventional or steel-fiber reinforced
concrete, plastic foam, or the like. The type of material used to form
core 22 would generally have the strength and stiffness characteristics
comparable to concrete. For sake of ease of description, the material of
core 22 will be referred to as being concrete with the understanding that
the invention is not limited to concrete. The core of concrete 22 extends
between the ends of the entire pole and embeds a plurality of reinforcing
members in the form of steel cables 24 arcuately spaced from each other in
a circular pattern all being radially equidistant from the respective
lower and upper casing 12 and 14. The upper and lower ends of the cables
24 are secured by clamps 28 of well-known design to upper end cap 29 and
lower end cap 30 which respectively fit snugly into and are secured to the
upper end of upper casing 14 and lower end of lower casing 12. Upper cap
29 has a recessed center section 31 which is provided with a series of
openings sized for receiving the ends of the cables 24, and a larger
opening 32 which serves to permit fluid concrete to be poured into the
pole 10. Lower end cap 30 also has a recessed center section 33, a large
center opening 34 for receiving a mandrel which allows a hollow core
concrete body to be formed, and two series of concentrically arranged
openings for receiving the lower ends of cables 24 and the lower ends of
other cables to be described later.
As will be more fully described hereinafter, the cables 24, as shown, are
pre-stressed prior to pouring the concrete. It is advantageous from a
stress standpoint to have the cables 24 close to the casings. Since lower
casing 12 has a greater diameter than upper casing 14 the cables 24 must
change direction between the casings in order to maintain an equidistant
spacing to the respective upper and lower casings. Otherwise, the spacing
would be greater between the cables 24 and lower casing 12 than between
cables 24 and upper casing 14. If the spacing disparity were allowed to
exist, the desired increase in strength of the pole 10 at the lower
section would not be achieved. In order to provide the equidistant spacing
between the cables 24 and lower casing 12, transition piece 16, and upper
casing 14, guides 40 and 42, as clearly shown in FIGS. 2 and 3, are
provided. Guide 40 includes an annular central section 44 arranged to be
coaxial with the axis of upper casing 14, and an annular guide section 46
coaxial with the central section and secured thereto with four ribs 48
spaced 90 degrees apart and welded to the respective sections 44 and 46.
Four equally arcuately spaced support ribs 50 are secured by welding to
the outer surface of the guide section 46 and extend radially to fit onto
the upper surface of four equally spaced alignment clips 20 to which they
are secured as by welding. The inner diameter of the guide section 46 will
be selected as desired to provide the desired spacing between the cables
24 and upper section 14. Guide 42, as shown clearly in FIG. 3, includes a
flat plate 52 having a generally disc shape with an outer diameter
slightly less than the inner diameter of lower casing 12. Equally
arcuately spaced scallop sections 54 are cut out of the plate 52 and are
sized and shaped to receive the cables 24 and support them at the same
distance from the lower casing 12 as they would be from the upper casing
14. The guide 42 is secured to the lower ends of alignment clips 18 by
welding thereto. Guide 42 is also provided with a central opening 54 and a
series of smaller openings 56 arranged in a circular pattern as shown.
Opening 54 is coaxial with the axis of lower casing 12 and thus also with
upper casing 14 and annular section 44 of guide 40, and it serves with the
annular section 44 to receive a mandrel when a hollow core concrete body
is cast within the casings. The openings 56 serve to receive the ends of
cables 60 which are embedded in the lower portion of the concrete body 22
to provide additional strength to the lower section of the pole 10, as
will be more fully elaborated upon later on in this description. The upper
ends of the cables 60 are secured by clamps 62 to the guide 42 while the
lower ends of the cables are secured by clamps 64 to the lower end cap 30.
In forming pole 10, the upper end cap 29 is placed in position to be mated
to the upper end of upper casing 14. The guide 40 is welded into fixed
orientation with the lower section of upper casing 14. The cables 24 are
fixed at their upper ends to the upper end cap 29. The transition piece 16
is fitted over the alignment clips 20, and then the lower guide 42 is
secured to the alignment clips 18 of the lower casing 12. The cables 24
are moved through the upper casing 14 and within the confines of the guide
section 46 of the guide 40, and then through the transition piece 16. At
this point the cables 60 are placed within the lower casing 12 and their
upper ends clamped in place on the guide 42. The cables 24 are now
directed through the scallop sections 54 of guide 42 and out the lower end
of the lower casing 12. The lower ends of the cables 24 and 60 are guided
through the openings provided for them in the lower end cap 30. The upper
and lower end caps 29 and 30 are fitted into place and a few of the cables
24 lightly tensioned to draw the upper end cap 29, upper casing 14,
transition piece 16, lower casing 12, and lower end cap 30 together, and
all the components are then secured in place by welding. The next step
would be to pre-stress the cables 24 and 60 to a predetermined magnitude
and then the cables are secured in place in the lower end cap 30 such that
the applied pre-stress remains permanently on the casing 12. A metal
mandrel is then positioned within the casing 12 and 14 by inserting it
through opening 34 of the lower end cap 30. The mandrel used would be
generally cylindrically shaped but could be tapered with larger diameter
poles. The mandrel would be centrally positioned within the upper and
lower casings since it would be inserted into the openings of central
section 44 of guide 40 and the opening 54 of guide 42. Concrete would now
be poured into the casings by pumping it through the opening 32 of the
upper end cap 29. Suitably arranged openings through the wall of the
casing could also be provided through which concrete could be poured. it
is to be noted that the mandrel may be eliminated and a hollow body of
concrete formed by pouring concrete into the casings and rotating the
entire pole about its longitudinal axis. Regardless of how the concrete is
handled, when it sets it is not under any applied stress and remains that
way for the life of the pole. The mandrel, if used, is removed and the
recessed portions of the respective end caps 29 and 30 filled with a
suitable sealing material.
By pre-stressing the cables 24 and 60 prior to pouring of the concrete a
composite structural member of superior bending stress and stiffness is
realized than has been heretofore found. This will be best understood by
those skilled in this art after reading the following mathematical
explanation of the interaction of the elements.
FIG. 6 is a diagrammatic sketch of the cross-section of a concrete filled
steel casing 100 with diameter D and thickness t. The shell is filled with
concrete and the section subjected to a bending moment such that side A of
the shell is in compression and side B in tension. The line a--a is the
diametric center of the casing. The portion of the concrete in the tension
region, however, is considered cracked and, therefore, the neutral axis of
the composite section shifts toward the compression side to line b--b by
an amount d. The result of this shift is that the distance from the
neutral axis b--b to the farthermost compression fiber is D/2-d and the
distance to the farthermost tension fiber is D/2+d. Thus, the concrete
will reach its yield stress in tension at a steel casing bending moment
less than that required to cause yield of a hollow steel casing of the
same dimension without a concrete core wherein the neutral axis is along
line a--a.
To better illustrate this situation, and to further illustrate the
advantages of this invention, a sample calculation is provided. Assume a
steel casing 100 with an outside diameter, D, of 36 inches, a wall
thickness, t, of 0.25 inches, and a yield strength of 65,000 pounds per
square inch (psi). Conventional calculations of well-known engineering
mechanics will show the moment of inertia about the axis a--a to be
I.sub.a--a =4485.9 in..sup.4
From the well-known beam bending equation, the moment resisted by the
cross-section at the steel yield stress of 65000 psi is
M.sub.1 =(65000 (4485.9)/18=16,199,083 inch-pounds
where, in the above equation, the number 18 in the denominator is the
distance from the neutral axis (a--a) to the extreme fiber (tension and
compression being the same in this case). However, if, for example, the
neutral axis shifted toward the compression side to line b--b by an amount
d=4 inches due to the presence of concrete that is partially cracked, the
moment of inertia of the steel casing will increase (from the well-known
expression I.sub.b--b =I.sub.a--a +Ad.sup.2 where A is the cross-sectional
area of the steel) to
I.sub.b--b =4485.9+(28.1) (4).sup.2 =4935.5 in..sup.4
However, the distance to the extreme fiber in tension from the new neutral
axis b--b is now 18+4=22 inches, and the bending moment required to cause
the tension side of the beam to reach its yield strength of 65000 psi is
now
M.sub.2 =(65,000) (4935.5)/22=14,582,159 inch-pounds
It is noted that the moments M.sub.1 and M.sub.2 above refer only to the
bending moment resistance of the steel casing. In comparing M.sub.1 and
M.sub.2 it is further noted that M.sub.2, the bending moment resistance of
the steel casing in the presence of a concrete core, is less than that of
the same casing without concrete. The embodiment of the invention just
described above all obviate this detrimental effect.
The moment M.sub.2 described above is that which causes the tension side B
of the casing in FIG. 6 to reach 65,000 psi, the yield stress. At the same
time, the stress in the compression side A is
S=(14,582,159) (-14)/4935.5=-41,364 psi
However, if the steel casing is first (before the addition of concrete)
uniformly prestressed in compression by an amount of 14,000 psi by means
of pre-stressing cables 24 and 60 as described placed within the casing
and secured to the casing at the distal ends, the stress in the tension
side B after the curing of the encased concrete and the application of the
bending moment M.sub.2 will be
S.sub.3 =65,000-14,000 = 51,000 psi
and the stress in the compression side A will be
S.sub.2 = -41,364-14,000= -55,364 psi
By the introduction of pre-stressing of the casing, the tension side B and
compression side A of the casing are not yet stressed to the yield stress
of 65,000 psi when the steel casing resists the bending moment M.sub.2. If
the attainment of yield is the criterion for bending capacity, additional
moment, M.sub.3, can be added to cause the tension side B to increase
14,000 psi to a total of 51,000+14,000=65,000 psi. Moment M.sub.3 is
calculated from the beam bending equation to yield
M.sub.3 =(14,000) (4935.5)/22=3,140,773 inch-pounds
The total moment resisted by the steel casing, with the beneficial effects
of pre-stressing, is M.sub.2 +M.sub.3, or 17,722,932 inch-pounds--a value
greater than that resisted by the shell in the absence of concrete when
the neutral axis is symmetrically located along line a--a in FIG. 6. It is
understood that the moment capacity of the composite steel casing,
pre-stressed cables and concrete core is greater than M.sub.3 because the
resistance contributed by the steel area of the pre-stressing cables and
the concrete area must be added. However, the addition of the
pre-stressing of the steel casing has overcome the problem of yielding of
the steel at reduced moment capacity due to the eccentric location of the
neutral axis. This problem has not previously been overcome in
transmission or distribution pole design and manufacturers and designers
have generally avoided filling poles with concrete because of the
knowledge that the cracked concrete section will cause a shift in the
neutral axis as portrayed in FIG. 6.
It was earlier noted that the diminishing diameter of the structure causes
difficulty stressing if the cables are continuous from one end of the
casing to the other. The difficulty arises from the fact that, because the
total force is constant but the cross-sectional area of the casing varies,
the stress in the structure will be different at one end than at the
other. The use of guides 42, as also noted, serves as an intermediate
anchoring device for additional cables 60 that are incorporated when the
pole diameter has been increased. It is necessary to insure that the
thickness of the guide 42 is sufficient to transfer the additional
pre-stressed cable forces to the wall of the lower casing 12.
Regardless of whether conventional concrete, light-weight concrete or
steel-fiber reinforced concrete, plastic foam, or whatever type of
non-metallic settable material is used to form core 22, it is to be
clearly understood that one of the functions of the enclosed core, in
addition to preventing local buckling of the steel casing and adding
bending strength and stiffness, is the obviation of corrosion of the
inside surface of the steel casing. Thus, it is a requirement that all or
any non-metallic settable material used have additives or compositions
such that it is either non-shrinking or slightly expansive. If this is not
adhered to, the slight shrinking will leave a thin gap between the inside
of the steel casing and the core and invite corrosion due to entrapped
moisture.
The number of cables 24 and 60 required depends on the desired stress to be
placed on the structure. The total stressing force equals the stress in
the steel casing times its cross-sectional area. The desired stress of the
casing would be known as would the respective moduli of elasticity of the
casing. Thus, the total force required by the cables may be simply
calculated. The total number of cables required would be determined by
dividing the total force by the allowable force for each cable which would
also be known.
Some of the advantages of the composite structural member of the present
invention were referred to earlier. Some of those advantages will now be
elaborated upon and others not previously mentioned will be set forth. As
stated earlier the structure of the present invention overcomes the
disadvantages stated with respect to wood, metal, and concrete poles.
The wall thicknesses of the metal casings that form the structure's casings
are much smaller than those normally associated with conventional hollow
metal poles. The wall thicknesses can be on the order of 1/8 to 1/4 inch,
or less, but there is no limitation posed by buckling. This attribute
arises because the metal casing is fixed in radial position by the
presence of the concrete core. Although local buckling is not totally
obviated, the sensitivity to buckling phenomena is greatly decreased. The
cylindrical casing thickness selected will be dictated by the local
strength required for attachments such as cross-arms, and strength during
handling of the empty casings. The ability to utilize thin-wall casings
leads to savings in material, fabrication and shipping costs. The
thin-wall pole casing previously described, in addition to providing
structural strength to a composite concrete-steel pole, serves as the
casting form and thus eliminates the need for special forms and rapid
curing techniques. The casings may be assembled at any convenient location
that will provide access to a concrete pump and concrete mixing equipment.
In addition, the interior of the metal casing is protected against
corrosion by the material that is cast against it. Thus, the interior of
the metal casing need not be galvanized. Also, the metal casing protects
the core by providing a watertight protective covering. Thus, the
occurrence of cracking at bending moments close to the ultimate blending
moment of the core is not a matter of serious concern. Together with the
above advantages, the ability to add pre-tensioning cables at intermediate
locations where there is a change in cross-sectional dimensions of the
pole is a novel method of pre-tensioning tapered poles in a balanced
manner.
In addition to the above advantages of the structure of the present
invention other additional advantages are obtained. The separate
components can be standardized to permit the assembling of a pole of given
geometry and strength from a number of standard cylinders, transition
pieces, and pre-tensioning cables. Thus, the fabrication center need not
be a complex factory, but could be readily set up near the area where the
poles are to be installed, reducing shipping costs and capital
requirements. The cylindrical metal casing acts to confine the core. This
is an example of synergism inherent in this unique design. Thus, the core
protects the inside of the metal casing against corrosion and increases
its resistance to buckling, while the metal casing, in turn, seals cracks
in the core. Also, the welds that join the sections of the cylindrical
casing together are stressed in compression by the pre-tensioning. When
the stresses due to bending of the pole are superimposed on these initial
compression stresses, the welds remain in compression until the initial
compressive stress is totally cancelled. Thus, for a significant portion
of the ultimate bending resistance of the pole, no welds are in tension
and weld failure is minimized.
It should also be noted that the reinforcing members (i.e. cables 24) need
not have a uniform arcuate spacing as described and illustrated. As those
skilled in the art would recognize, non-uniform spacing of the cables
would be arranged where it is anticipated that the pole would bend in one
direction only. In that event the cables would be so arranged that the
compressive stress in the casing due to pre-stressing would be higher on
the side that would be subjected to live load or dead load tension
stresses. In other words the spacing of the cables in the tension side of
the pole would be closer than the spacing on the compression side.
While we have shown and described a present preferred embodiment of this
invention and have also described a present preferred method of
constructing the embodiment, it is to be distinctly understood that the
invention is not limited thereto, but may be otherwise variously embodied
and constructed within the scope of the following claims.
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