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A Bird's Home

Structural Concerns Of




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 stress-strain 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(D2rd-d2rd)      or      0.7854(D2rd-d2rd)  Where D is the outside diameter and d is the inside diameter.



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.



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