Aluminum/Stainless Steel Tubing Strength 1

[gunship]

Sir/Madam,

I am not an engineer. I am a retired banker and I was hoping that you might point me in the direction that would help me solve a vexing problem.

I live on my 46' ketch and have devised a way of securing my dinghy that requires the use of either aluminum tubing or stainless steel tubing. My problem is that I cannot find a reference the formula for determining the strength of these items.

For example: assume you have an aluminum tube that is 8' feet long, one and one-half inches in diameter, with a wall thickness of one eighth of an inch. If you attach a band of material (webbing, for example) that can support 1000l pounds, what dimensions are required for the aluminum tube that will support a load of 300 pounds?

If you have read this far, I thank you. I am not looking for an answer to that specific set of measurements, but, for standard quality aluminum/stainless steel, what is the formula for determining the proper tubing to use? Is there a set of reference tables that one can use to come up with the answer to any length of tubing, weight, etc.

I have searched the internet trying to find formuli or reference materials, to no avail. Any help that you can render would be much appreciated.

Thanks and Regards,

Dutch Huffman
gunshipdutch@aol.com

 

http://euler9.tripod.com,

"Metals Handbook, MIL-HDBK-5H," Sect. 3.6.2, Table 3.6.2.0(c1), says Aluminum 6061-T6, Pipe, yield strength is Fty = 35 ksi. At same URL, "MIL-HDBK-5H," Sect. 2.7, gives austenitic stainless steel 304 ("301 and other&quot strength properties, but it's only for sheet, plus I don't know the typical hardness condition for stainless steel (SS) tubing, so we'll have to reply on another expert for type, condition, and Fty of SS commonly used in typical SS tubing, and the URL for its yield strength Fty, sometimes called Sy. E.g., same URL, above, under "MatWeb," "type of material," "Stainless Steel," "300 Series," list item number "258," "AISI Type 304 SS" (

http://www.matweb.com/SpecificMaterial.asp?bassnum=Q304A)

says Fty = 31.18 ksi, but I don't know if this is the correct SS, perhaps not.

Assuming fabric strap is located at tube midspan, compute M = PL/4, where M = bending moment at tube midspan, P = applied transverse load (300 lbf in your example), and L = tube span length (96 inch in your example). Compute tube moment of inertia I = pi[D^4 - (D-2t)^4]/64, where D = tube outer diameter (OD) and t = wall thickness. Compute tube bending stress sigma = 0.5(M)(D)/I. Compute tube margin of safety MS = {Fty/[FS(sigma)]} - 1, where FS = factor of safety. A typical value for FS is FS = 1.67. If MS is zero or positive, the selected tube and material are adequate to resist the bending stress. If MS is negative, select a larger or thicker tube cross section and/or a stronger material and repeat the process until MS is zero or positive. Good luck.

 

[LPPE]

try

http://www.aluminum.org

or

http://www.matweb.com

 

Correction to my previous answer, above. If your applied load P is in units of lbf and Fty is in units of ksi, before performing the calculations I prescribed, either convert your P to kip by dividing P by 1000 or convert your Fty to psi by multiplying Fty by 1000. It doesn't matter which system of units you use, provided you are consistent throughout. Examples of consistent units are (1) N, mm, MPa; (2) kip, inch, ksi; (3) lbf, inch, psi. Good luck.

 

[ishvaaag]

On aluminum yo can look the book

Aluminum Structures, a guide to their specifications and design.
J. Randolph Kissell, Robert L. Ferry
Wiley Interscience

a very good book. To do anything well takes no less than what there said.

Out of memory of what there read (I am not aluminum structures designer), in any case typical aluminum structural alloys seem to be a bit less strong than 36 ksi steel, the shear modulus is quite lower, and safety factors for strength were in the vicinity of 2, what would give for pipes in only bending about 16 ksi, but depending on the alloy.

http://www.euro-inox.org/e/frames/main/fp.html

gives a pdf on design of stainless steel structures.

Note that sizing structural members is subject to many things, and so is always better a professional looks at the thing.

 

[ishvaaag]

The download of the quoted link failed for me.

This other has typical properties of some stainless steel

http://www.aksteel.com/markets/pdf/12_SR_data_sheet.pdf

 

Dutch:  The majority of structural stainless steel (SS), including structural SS tubing, is indeed SS 304, 304L, 316, and 316L; usually 304, because 316 is slightly more expensive. So the SS 304 as listed in my original post appears to be right on target. Good luck.

 

Dutch: Now that you have gotten started on your analysis, let's go back and address the M = PL/4 equation assumed in my first post. Please answer true or false, or yes or no, to each of the following questions about your given problem, or elaborate as necessary where a misunderstanding is identified. Please cut and paste the following and insert your answer after each sentence for sake of brevity and clarity. Thanks.

(1) You have a tube supported at each end of, say, a 96-inch span length with no other supports in between these two span end supports? (2) You have one or more webbing straps attached transversely (perpendicularly) to this span, and each strap has a tensile capacity of 10001 lbf? (3) How many webbing straps are attached within the given span length? (4) If there is more than one strap per tube span, do the straps have any characteristics such as uniform spacing, symmetry of attachment locations relative to midspan, etc? (5) If there is more than one strap per tube span, does each strap carry the same applied load P (simultaneously)? Or do the applied loads vary from strap to strap? (6) Can the straps significantly slide longitudinally along the tube? (7) Will each strap load be applied slowly, gradually, very gently, and monotonically? Or could a load be applied suddenly? (8) Are the tube supports completely rigid connections? Or can the tube span ends rotate even a small amount in or at the supports (and I am not referring to the tube spinning about its own axis here but instead to any minute rotation about the other two axes)? (9) If the tube ends can rotate even a small amount in the span end supports, is the tube cut at the 96-inch span ends? Or is it continuous into an adjoining span at each span end? (10) What is the width of each strap?

 

[arvi20]

Please let me know the tensile strengh of the Aluminium material as early as possible.

 

[ishvaaag]

Table 3.5 of Ferry reference as above gives for different alloys of aluminum used for structural design work

Minimum Rupture Tensile Strength .... from 20 to 45 ksi
(more typical 30 to 37 ksi)

Minimum Yield Strength 7 to 36 ksi
(more typical form 20 to 29 ksi)