Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/articles/pipemast/pipemast

Text File | 1996-06-30 | 17KB | 377 lines

DESIGN DATA FOR PIPE MASTS

R. P. Haviland, W4MB


 One of the best materials available for building self-
supporting antenna masts is steel pipe. It is widely
available, uniform in quality, and reasonable in
price. A well-designed mast is adequately strong, neat and
attractive, and relatively light weight. And, using steel pipe,
it's not too difficult to design a fold-over mast which allows
all antenna work to be done at ground level. Even main-
tenance on the mast itself does not require work at any great
height.

 However, attaining all of these advantages does require
some design work. This is particularly important for safety.
The purpose of this article is to present a set of design
curves which will give a safe and satisfactory design, while
using the minimum of material.

Construction

 The general construction of a typical fold-over pipe mast
is shown in Figure 1. At the top are the antenna and rota-
tor, carried by the smallest size pipe. This is inserted into
the upper end of the next size pipe for a short distance,
and fastened by through-bolts or welding. The second sec-
tion is inserted into the next larger, and so on. The bottom
section is hinged to a fixed upright pipe, which gives the
fold-over feature. It, in turn, nests into a larger section of
pipe set into the ground. A yoke is provided to fasten the
mast to the upright after erection. Figure 1 shows a block
and tackle for pulling the mast to the vertical position, but
a winch fastened to the upright may be used instead.

 Most mast designs use the widely available standard
weight pipe, each size of which nests neatly into the next
larger size, over the range from 1-1/2 to 4 inches. Larger
sizes still nest, but there is a gap between the walls. Very
high masts, or those with unusually heavy top loads, can
be built with extra-strong or double extra-strong pipe, but
such designs are not considered here as the data are cal-
culated for standard weight pipe. (See note at end)

Design Criteria

 Because of the change in diameter, beam formulas can-
not be applied to a stepped diameter mast as a whole.
Instead, each individual pipe section must be analyzed by
itself, as a free body, starting at the top. The section load
must then be transferred to the next lower section. This is
done by converting the lateral load to a couple, acting
across the diameter of the section, then multiplying the cou-
ple magnitude by the ratio of pipe diameters to get the top
load of the next section. Intermediate antennas can be
assumed to be concentrated at the junction of sections. The
next section is then considered.

 The critical or design load on a section may be caused
by wind load when the mast is vertical, or by erection load
as the mast is being raised. Both loads should be calcu-
lated and the design chosen for the worst of the two.

 For wind load, two design winds are commonly used.
For most of the country, it is assumed that the worst wind
to be encountered is 85 mph, a value to be expected once
in 50 years or so. For Florida, the Gulf Coast, and locations
like Cape Hatteras, a maximum wind of 125 mph is also
used. Your county engineer can provide the recommende
value for your location (see reference 1).

 During erection there is some deflection, or bending, of
the mast. The greatest load occurs when each section
horizontal; this is the loading which must be designed for.

 The wind and erection impose two different types of load
on the section. One is the concentrated load at the topmost:
end of a section due to the forces on the section above
The second is the distributed load acting along the length
of the section. As the concentrated load becomes large
there is less strength left for the distributed load, so the sec-
tion length must become smaller. Accordingly, the problem
of design is to determine the allowable section length.

 The concentrated load during erection is the weight of
the antenna, rotator, and sections above the one being con-
sidered. The concentrated wind load includes the sum of
all wind loads above the section being considered. The
usual load is calculated on the basis of projected area. This
is the area covered by the shadow of the object. If the object
is not symmetrical, like a Yagi beam, the largest projected
area is used. The loading depends on whether the object
is flat or round, as follows:
 Wind loading in pounds per square foot
                85 mph wind       125 mph wind
Flat objects         30.3            65.9
Round objects        18.1            39.0

The projected area is often given in the instructions for con
mercially made antennas and rotators. It is easily calculated
from the dimensions of the element.

 Given this concentrated load on the topmost section,
design of the mast itself involves solving section load equa-
tions for allowable section length. To simplify this process
the equations have been reduced to a series of graphs -
Figures 2 and 3 for load during erection, and Figures 4
A and B and 5 A and B for wind loads. Use of these curves
will be explained through an example.

Example

 Assume that the design is for an all-tubing 6-meter
antenna, having 2 square feet projected area and weigh-
ing 15 pounds. A small TV rotator is available, having 1/2
square foot of mostly flat plate area, and weighing 8
pounds. This area is not subjected to unusual winds. Mast
height is 40 feet.

 The concentrated load on the top section is 15 + 8, or
23 pounds. Entering Figure 2 at the bottom with this weight
and moving upwards, it is seen that the top section could
consist of 12 feet of 1-1/2 inch pipe, 16 feet of 2-inch pipe,
or 20 feet of 2-1/2 inch pipe. In keeping with the scale of
the antenna, suppose the 1-1/2 inch diameter pipe is used.

 The concentrated wind loading is due to 2 square feet
of antenna and 1/2 square foot of rotator. From the table
above, the loading is (2 x 18.1) + (0.5 x 30.3), or 51
pounds per square foot. Reading upward from this load
on Figure 4, it is seen that the maximum allowable length
for 1-1/2 inch pipe is 8 feet. Since this is the critical value,
it becomes the length of the topmost section.

 Assume that the sections are to be fastened by welding,
with 6-inch insertion into the next section. From Figure 3,
the weight of the 8-1/2 foot total of the top section is 23
pounds. The wind loading on the exposed 8 feet from Fig-
ure 5 is 25 pounds per square foot. Thus, the weight load
at the top of the second section is 23 + 23, or 46 pounds
and the wind loading is 51 + 25, or 76 pounds per square
foot.

 Using Figure 2 again, the maximum allowable length of
the next section with the nesting 2-inch pipe is 11-1/2 feet
for erection loads. From Figure 4, the allowable length for
wind loads is 9 feet, which becomes the section length.
Proceeding as before, the loads on the next section are 46
+ 35, or 81 pounds during erection, and 76 + 35, or 111
pounds per square foot for wind.

 Again, using Figures 2 and 4, the allowable length of 2-
1/2 inch pipe is 13 feet for erection load, and 12-1/2 feet
for wind load. The 12-1/2 feet is the length La, in Figure 1.
The load on the section Lb in Figure 1 is the same in mag-
nitude, so this part could also be 12-1/2 feet long. However,
a stock length for pipe is 21 feet. Assume that this is all that's
available. Then the third section will need to end 1 foot
above ground to reach the desired 40-foot total height. This
is not unreasonable.

 If a counterweight is added to the lower part of the third
section to just balance the top weight, the erection loads
on the fixed upright pipe are essentially zero. Even if no
counterweight is used, the balancing effect of the part Lb
of Figure 1 reduces the load on the upright to less than
the load on section R, of Figure 1. Thus, if the upright is
no smaller than the lowest mast section, it will have ade-
quate strength for erection.

 The wind load on the upright is that of the upper sec-
tions plus that on the top 10-1/2 feet of the lower section,
plus some amount on the upright. Assume that the upright
is fully exposed (a safe assumption). The wind load to the
top of the upright is 111 + 55, or 166 pounds per square
foot maximum, the exact value depending on the final
choice of upright length. From Figure 4, the upright can
be only 6 feet long if it is 2-1/2 inches in diameter, or 13
feet long if it is 3 inches in diameter. Since 12-1/2 feet is
needed as a minimum, this is just about right (half of the
21-foot length of the 2-1/2 inch section, plus I-foot ground
clearance).

 Even with the curves, the process is somewhat tedious
and it's easy to make mistakes. Most of the tedium and mis-
takes can be avoided by transferring the relations to a com-
puter program. (Found in the W4MB program section)

 While this design is intended to be used without guys,
they can be added for greater safety or increasing the allow-
able wind load. Usually the wall thickness is sufficient to
withstand the compressive forces caused by guy tension,
but this should be checked if a guyed design is attempted.

 Factors affecting the length of pipe buried in the ground
are discussed below. For this example, assume that this is
10 percent of mast height, or 4 feet. Total upright length
is thus 13-1/2 + 4, or 17-1/2 feet. The jacket section buried
in the ground needs to have i-inch clearance, so it must
be a 4-foot length of 9inch diameter pipe.

 The results of this design example are:

Top section: 1-1/2 inch diameter top section, total length
8-1/2 feet, exposed 8 feet.

Second section: 2-inch diameter second section, total
length 9-1/2 feet, exposed 9 feet.

  Lower section: 2-1/2 inch diameter lower section, total
;  length 21 feet, hinge at 12-1/2 feet, i-foot ground clearance
  at bottom.

  Upright: 3-inch diameter upright, total length 17-1/2 feet,
  exposed 13-1/2 feet, buried 4 feet.

  Jacket: 5-inch diameter, total length 4 feet, all buried.
  If necessary, this design could be carried higher, using
  larger pipe sizes.

   It is often necessary to try several initial assumptions as
  to length and diameter of the top section. With a little prac-
  tice, this can be done in a few minutes.

  Construction Details

   The 6-inch overlap assumed in the example is sufficient
for either welding or bolt fastening. Bolts are suggested as
they are simpler and allow disassembly.

 Two bolts at right angles passing completely through both
pipe sections are recommended. The thread root diameter
should be no less than the thickness of the larger section.
As a refinement, drill and tap the outer pipe for alignment
screws to be placed just above the top bolts and just below
the bottom ones. These are a necessity if the pipe sections
differ much in size (for example, if a 4-inch pipe is to be
nested into a 5-inch one). The space between pipes can
be filled with silicon rubber in the final assembly.

 The "U" strap hinge shown in Figure 1 should have a
thickness at least as great as the wall thickness of the pipe
it supports. For strength in bending, its width can be about
12 times the thickness. The pin hinge diameter should be
at least twice the wall thickness for bending strength. (These
bending forces are likely to occur in handling and erection,
and are difficult to estimate).

 A second "U" and pin can be placed at the very bottom
of the movable mast part to anchor it to the ginpole sec-
tion. The pin can be drilled for insertion of a padlock, to
prevent sabotage or tampering. A bicycle chain does nearly
as well. Another refinement is to wrap both the ginpole and
lower pipe section with several turns of barbed wire, about
8 feet above ground level. This helps prevent anyone from
climbing the mast.

 The suggested assembly routine is to mark each sec-
tion with the bolt locations and the nesting length. Then
lay the pipe on the ground, with blocks or pegs to hold it
in place. Use a cord to get the correct alignment. Drill one
of the bolt holes, insert the bolt, and then drill for the other
one. Without shop facilities, it's nearly impossible to pre-drill
these holes and have them line up.

 Weight and area aloft can be reduced by turning the
entire mast. This complicates the attachment to the ginpole
section. However, the bearings needed can be simple
sleeve bearings -- essentially "U" straps with filler blocks,
plus bearing rings attached to the pipe. The vertical load
on these bearings can be removed by mounting a heavy-
duty rotator under the very bottom of the mast and using
a scissors jack to raise the rotator and mast just enough
to take the load off the straps. Look at one of the commer-
cial designs for ideas.

 Since guys are not needed, the rotating mast type is
excellent for stacked beams.

Foundations

 Because of the great variability of soils, it isn't possible
to provide a set of all-purpose design curves for founda-
tions. The best way of proceeding is to work with your
county engineer, and use the practices developed for your
particular area. The local power or telephone company
should also be able to supply the necessary data.

 For reasonably good soils, like firm loams or clays, a good
starting point is to assume that the foundation depth is equal
to 10 percent of the height, with the jacket set in concrete
of sufficient size to keep the soil load to a safe value. A max-
imum load of 4000 pounds per square foot is often used,
with the design adjusted to give a 100-percent safety fac-
tor above the design load. If you haven't done this work
before, the county engineer can show you the steps.

 The ginpole pipe section going into the ground must be
protected from rust and corrosion on the inside and out-
side. This is especially important to prevent rusting at the
waterline, if free water is present.

 Usually, adequate protection can be assured by paint-
ing the pipe with a grout of cement and water. Even better
protection can be obtained by wrapping the outside with
several layers of builder's felt, painted with cold application
roofing tar as the felt is wound on.

 Pipe sections can be sealed with wooden plugs and a
layer of silicon putty. The entire mast and all hardware
should be painted as a last step before installation of the
antennas. Aluminum Rustoleum TM is suggested, as it is
compounded to remain flexible, and is nearly as good for
rust prevention as a zinc coating.

Safety

 More and more communities are requiring permits for
structures of this type. There may be height restrictions.
Know your local laws!

 In many areas, one requirement for obtaining a permit
is certification by a professional engineer. You can usually
save time and cost by doing the preliminary design and
analysis yourself; use standard formulas or the curves here.
Do the work neatly, in an easy-to-follow form. The engineer
will want to at least check the method and critical loads.
If he wants to do a complete analysis, you'll be able to use
it to argue about the cost of insurance coverage (a gener-
ous policy is recommended).

 Any antenna mast can become a hazard if good safety
practices are not followed. Remember that a quarter- or half-
ton of steel 30 to 70 feet in the air is no toy. If you lack exper-
ience or don't have the proper facilities, get qualified help.
Always remember, safety is no accident.

REFERENCE

1 John J Nagei. KIKJ.  How to Calculate Wind load ng on Towers
 and Antenna Structures
Ham Radio, August 1974, page 16

(Note)

 Standard and extra strong (ASTM nomenclature) are the two pipe weights com-
monly encountered. The American Petroleum Institute has a separate designation
for well casing, but this is called tubing rather than pipe -- although some sizes
are identical to pipe sizes. The critical dimensions for standard weight pipe are:
     Size       Outer diameter Wall thickness
4 inch           4.5 inch         0237 inch
3-112 inch       4.0 inch         0226 inch
3 inch           35 inch          0216 inch
2-112 inch       2.875 inch       0203 inch
2 inch           2.375 inch       0.154 inch
The ASTM recommended fiber stress values for standard weight pipe is 20,000
psi (bending). The design procedure presented here uses a 10-percent reduction
from this stress figure, based on good used pipe.

 Note that the extra-strength and double extra-strength sections do not nest
because of thicker walls. Such heavier pipe can be used for the topmost section
and for the standing or ginpole section. However, the curves apply only to stan-
dard weight pipe or tubing of the sizes given in the table. Editor


This article first appeared in the September 1974 issue of Ham Radio Editor