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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

Strength of materials question: square tube vs angle iron



 
 
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  #1  
Old April 14th 08, 03:57 PM posted to rec.crafts.metalworking
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Posts: 63
Default Strength of materials question: square tube vs angle iron

Hi all; about to start building a trailer. The trailer plans I
bought from trailerplans.com call for 2"x3"x1/4" angle iron. I
want to use steel tube instead. Can anybody give me an idea of what
size steel tube would have the same strength? Is there somewhere
this can be looked up?

--
-Ed Falk,
http://thespamdiaries.blogspot.com/
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  #4  
Old April 14th 08, 07:27 PM posted to rec.crafts.metalworking
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Posts: 1,527
Default Strength of materials question: square tube vs angle iron

Don Foreman wrote:
On Mon, 14 Apr 2008 14:57:50 +0000 (UTC), (Edward
A. Falk) wrote:

Hi all; about to start building a trailer. The trailer plans I
bought from trailerplans.com call for 2"x3"x1/4" angle iron. I
want to use steel tube instead. Can anybody give me an idea of what
size steel tube would have the same strength? Is there somewhere
this can be looked up?


2" x 3" x 1/8" rectangular tube should be as strong as 2" x 3" x 1/4"
angle and have about the same weight per foot.


Actually it'll likely be stiffer in bending and way stiffer in twist.
It'll have less strength at any one weld 'cause you're going into 1/8
inch of material instead of 1/4 inch, although that could be overcome
with gussets if necessary.

If the trailer is designed to have a stiff chassis and well-sprung
wheels it'll probably be better with the tube; if the trailer needs to
have some flex in the chassis (unlikely if it's remotely normal) then
the stiffer tubing plus weaker welds may tear it apart over time.

Chances are the thing was designed for angle iron because the folks
doing the plans felt that'd be easiest to build, and because for a lot
of things 'easy' is better than 'absolute optimal weight/strength ratio'.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
  #5  
Old April 14th 08, 07:34 PM posted to rec.crafts.metalworking
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Posts: 762
Default Strength of materials question: square tube vs angle iron

You can look up or use one of the calculators to get the section
modulous for the various angles and tubes you would consider. Typically,
a tube shape has more strength in both directions than a rectangular
tube of the same weight per foot.

But a word of warning: tube and angle fail in different modes, you
really can't just sub one for the other without a bit of care. That
said, 2x3x1/4 angle is fairly wimpy stuff if used as frame rails.

Edward A. Falk wrote:
Hi all; about to start building a trailer. The trailer plans I
bought from trailerplans.com call for 2"x3"x1/4" angle iron. I
want to use steel tube instead. Can anybody give me an idea of what
size steel tube would have the same strength? Is there somewhere
this can be looked up?

  #6  
Old April 14th 08, 10:45 PM posted to rec.crafts.metalworking
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Posts: 13
Default Strength of materials question: square tube vs angle iron

I wouldn't substitute tube for the angle in a trailer application. Reason is
that short of hot dip galvanizing, you can't rust proof the inside of the
tube. Water will get inside and eventually the frame will rust from the
inside out leading to failure. Do yourself a favor and follow the original
plans.
Just my .02 YMMV.
Paul

"Edward A. Falk" wrote in message
...
Hi all; about to start building a trailer. The trailer plans I
bought from trailerplans.com call for 2"x3"x1/4" angle iron. I
want to use steel tube instead. Can anybody give me an idea of what
size steel tube would have the same strength? Is there somewhere
this can be looked up?

--
-Ed Falk,
http://thespamdiaries.blogspot.com/



  #7  
Old April 15th 08, 04:23 AM posted to rec.crafts.metalworking
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Posts: 1,527
Default Strength of materials question: square tube vs angle iron

On Mon, 14 Apr 2008 14:45:28 -0700, 42 wrote:
(top posting compulsively fixed)

"Edward A. Falk" wrote in message
...
Hi all; about to start building a trailer. The trailer plans I bought
from trailerplans.com call for 2"x3"x1/4" angle iron. I want to use
steel tube instead. Can anybody give me an idea of what size steel
tube would have the same strength? Is there somewhere this can be
looked up?

--
-Ed Falk,
http://thespamdiaries.blogspot.com/


I wouldn't substitute tube for the angle in a trailer application.
Reason is that short of hot dip galvanizing, you can't rust proof the
inside of the tube. Water will get inside and eventually the frame will
rust from the inside out leading to failure. Do yourself a favor and
follow the original plans.
Just my .02 YMMV.
Paul


Fill it full of linseed oil & roll it around to cover thoroughly.

In general, "follow the original plans" is good advise, unless you know
the ramifications of deviating from them.

On the one hand I'm tempted to say that if you have to use plans to make
a trailer, you should follow them. On the other hand, if you never
deviate from other's plans you'll never make the mistakes you need to
make to learn how to be innovative _and_ correct.

Just check the safety-related stuff often, if you do deviate from those
plans....

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
  #8  
Old April 15th 08, 05:15 AM posted to rec.crafts.metalworking
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Posts: 63
Default Strength of materials question: square tube vs angle iron

In article ,
Tim Wescott wrote:

Chances are the thing was designed for angle iron because the folks
doing the plans felt that'd be easiest to build, and because for a lot
of things 'easy' is better than 'absolute optimal weight/strength ratio'.


That was my guess. I'm just hoping to hear something like "2x2x1/8 will
do fine"

--
-Ed Falk,
http://thespamdiaries.blogspot.com/
  #9  
Old April 15th 08, 05:16 AM posted to rec.crafts.metalworking
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Posts: 63
Default Strength of materials question: square tube vs angle iron

In article ,
42 wrote:
I wouldn't substitute tube for the angle in a trailer application. Reason is
that short of hot dip galvanizing, you can't rust proof the inside of the
tube.


I had not thought of that particular issue. Thanks.

--
-Ed Falk,
http://thespamdiaries.blogspot.com/
  #10  
Old April 15th 08, 05:30 AM posted to rec.crafts.metalworking
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Posts: 762
Default Strength of materials question: square tube vs angle iron



That was my guess. I'm just hoping to hear something like "2x2x1/8 will
do fine"


Anybody who really knows the answer is not too likely to give you the
all clear over the internet. Too many idiots out there. Not to mention
there may be some reasons why the original designer chose angle iron.
(other than hot rolled angle is the absolute cheapest per pound material)
 




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