Structural
Concerns Of
Poles
for Purple Martin
Housing
OK! You've probably either heard or read all kinds
of claims about which different materials and shapes are best suited
to be used as a pole to hold up purple martin housing, but no one
ever goes into the actual numbers proving it. Well, that's
just what we plan to do here. Once and for all, let's look in
detail at what type of material and shape of pole is best.
Then, you can read the numbers and from that, make your own informed
decisions.
Some background:
I'm a Mechanical Design Engineer by profession and deal
with the strengths of different materials on a daily basis. My
entire intent of this page is to inform folks that do not work with
this stuff on a regular basis (or at all) which material is best and
what geometrical shape will give you the best structural integrity
and why.
First, let's say that we're not going into all the
formulas and calculations in full detail here because that would
probably be too boring for most folks, and (meaning no disrespect to
anyone) a good number probably wouldn't understand them
anyway. What we'll do instead is give enough detail and
explanations (simplified) so that all can understand what is being
said. We'll give some engineering terms, give a general
explanation what they mean, and how they apply to what we're doing
and then the numbers to back them up. We'll won't carry
the calculations out to the Nth degree... Instead, we'll round
close and give the answers in terms that EVERYONE can
understand. So, with that said...
Let's do some engineering...
First, for those that don't want to go through all the
numbers and are only interested in the final results, then you can
skip down to the end of the page to the "Final Results" and
find the resulting numbers from all the calculations.
For those that are a little more interested, let's first
understand there are a number of different factors that come into
play in the strength of ANY product or material. Factors such
as, the material itself, the material shape, what it's being used
for, where it's being used, the time it's to be used and the
different elements and factors that will have input on it.
Safety is always a big issue in the design of any product and is a
must consideration here, too. For our purposes, we're
going to use only the pertinent factors that will apply to holding
up purple martin housing. In our case, this will mean a pole
of some sort, structurally mounted in the ground and sticking
straight up in the air for a height of 10'. To properly
compare our numbers, we'll use a second arbitrary number of 15'
because that will show how the longer a pole gets, the weaker it
gets.
For those that do not know, ( " ) is the symbol for
'inches' and ( ' ) is the symbol for 'feet'.
Some terms:
(W) Load... The amount of pressure or force
applied to a body to make it move or bend.
(d) Deflection... The distance a part will
move or bend when (W) is applied to it.
(I) Moment of Inertia... Standard formulas
for calculating the actions of material particles of area or mass
using their locations from reference axis.
(E) Modulus of Elasticity... The ratio of
stress to strain within proportional limits of a material, usually
in tension or compression.
Elastic Limit... The maximum point at which
a material can be bent and then return to normal shape.
Yield Point... The point on a stressstrain
curve at which a material will no longer return to normal and
instead, will stay deformed.
(L) Length of beam The length
of the beam being used for the calculation. (In our case, a
pole).
Geometric shape... Round... Square...
Triangular...
Size... Large... Small...
Material... Steel... Aluminum... PVC...
Wood...
Our Scenario:
We have a pole that is mounted solidly in the ground and
extends to a height of 10' (and 15') up in the air. How
much will it bend if a force (W) of 10 lbs, (pounds) is
applied horizontally to the very top of it. For our purposes,
10 lbs, simple to understand, easy to calculate and it's a number
that is commonly applied to the side of a martin house because of
wind on the house.
The pole is to be considered hollow, will have an outside
dimension of 1.9" and a wall thickness of .145", the same as
standard schedule 40 steel tubing. These numbers will be used
across all geometric shapes to give a comparison of the shapes in
relation to each other.
The 3 materials we'll do the numbers on are; Cold
Rolled Steel, Aluminum and PVC (Poly
Vinyl Chloride).
The 3 geometric shapes we'll do our numbers on are
round, square and triangular.
Some things we need to know:
'Modulus of Elasticity' (E) is usually a good
indicator of how strong a material is... The higher a material's
modulus number, the stronger it is. This number is usually
constant for a given material.
(E) for Cold Rolled Steel is 30x10 to the 6th
power (30,000,000psi)
(E) for Aluminum is 10.3x10 to the 6th power
(10,300,000psi)
(E) for PVC is .4 x 10 to the 6th power
(400,000psi)
NOTE: Not being metallic, type 1 PVC doesn't
have a direct Modulus of Elasticity, but instead has what is known
as a 'flexible modulus' of 400 x 10 to the 3rd power or
400,000psi.
To calculate the strengths of different geometric shapes,
we need to know the 'Moment of Inertia' (I). This
number uses the cross sectional shapes of beams and changes with
size variations and wall thicknesses. The higher the number,
the better. Tubing in general comes in different shapes, sizes
and wall thicknesses. As stated, for our calculations, we are
using tubing with the outside dimensions of 1.9" and a wall
thickness of .145".
For comparative purposes, the Moment of Inertia for the
three basic shapes being considered are:
(I) for a round 1.9" Outside Dimension tube, .145
wall, = .608
(I) for a square 1.9" Outside Dimension tube, .145
wall, = .416
(I) for a triangular 1.9" Outside Dimension tube,
.145 wall, = .191
For those that would like to calculate their own
(I) for a different size and wall thickness round pipe, the
formula is:
Pi(D2rdd2rd)
or 0.7854(D2rdd2rd)
Where D is the outside diameter and d is the inside
diameter.
4
The generic formula for the calculation of deflection
(d) at the end of a beam, rigidly mounted at one end is:
d = WL3rd
3 E I
Final Results
After going through the calculations; (The smaller
the number, the better (stronger) the material and geometric
shape). In other words, if you were to push against the top of
the pole with 10 pounds, it would move per the following
numbers.
10'
Pole
15' Pole
(d) for round
steel
=
.240"
.811"
(d) for square steel
=
.365"
1.232"
(d) for triangular steel
= .533"
1.800"
(d) for round alum
=
.700"
2.361"
(d) for square alum
=
1.069"
3.588"
(d) for triangular alum
=
1.553"
5.243"
(d) for round PVC
=
18.000"
60.750"
(d) for square PVC
= 27.380"
*
92.390" *
(d) for triangular PVC =
40.000"
*
135.000" *
*My plastics source says square and triangular PVC
are not available 'off the shelf', meaning a person would have to
pay for special tooling and minimum runs of the desired
shapes. However, we did the numbers anyway just to show the
comparative results.
As we can see by comparing the above numbers, steel is by
far the strongest of materials, moving less than a 1/4 of an inch
for a round, 10' long pole. Aluminum is second moving a little
less than 3/4 inch and PVC is a far and distant third moving a full
foot and a half. In fact, the 18' PVC pole would probably
snap long before it reached the number calculated, (over 5 feet)
because it would have passed through its elastic limit and
beyond its yield point. When you bend a coat hanger,
you take the material passed its yield point and it stays
bent. In PVC, this yield point is probably where it
would fail, (snap).
As for which shapes are best, round was proven to be # 1,
square #2 and triangular #3.
(Since wood can only be safely used in a 4x4 mode, it was
not used in this scenario. Wood should not be considered a
good material to make a martin pole out of unless it is at least 4
inches thick).
Some extras:
All tubing products of different shapes and materials do
not come with the same outside dimensions and wall thicknesses and
instead, vary greatly. However, in some instances the three
that we used, do, and we used the one wall thickness across the
board to compare different geometric designs and find the strongest
one. This gave us numbers that we could compare between shapes
and products. For our purposes, we assumed 'off the shelf'
tubing that the average person can go and purchase.
Since tubular products can, and do, come with different
wall thicknesses, a 'thicker wall' in the alum could equate to the
same strength of schedule 40 wall of steel.
A 'larger diameter' with a 'thinner wall' could also
accomplish the same feat.
For general talking purposes, for the same diameters and
wall thickness, steel is 3 times stronger than aluminum and 75 times
stronger than PVC.
The longer a pole gets, the weaker it gets.
Steel is not UV sensitive, nor is aluminum. PVC
however, is UV sensitive and in time will deteriorate
badly.
Steel and aluminum may bend, but PVC will snap.
Adding another material such as cement to the inside of a
PVC pipe doesn't really help that much and in fact makes things
worse. It gives any bending motions a fulcrumatic point and
that will make the snapping of PVC even worse.
Final Suggestions:
Spend a little extra and get a good pole that will
give you peace of mind and protect your birds. A good pole may
cost you between $20 and $40, but in the long run, it will turn out
to be the cheaper product since you'll never have to replace
it. Galvanized may cost more, but will never require
painting.
Back to Chuck's Purple Martin
Page
This page created
and maintained
by Chuck Abare
Woodside Gardens
The Registry of Nature Habitats
Copyright 1999 
All Rights Reserved
Last Updated:

