[Shop-talk] weight capacity of aluminum channel

Donald H Locker dhlocker at comcast.net
Fri Jul 2 07:11:50 MDT 2021 

Some central supports would certainly help. I think my bigger concern
(which a good central support would help) would be stress distribution
and channel twisting. If either of the channels tipped (due to
unbalanced loading or uneven ground) the loads would not be squarely on
the flanges and could twist the channel => bad bongos!

A central support the prevented twisting would be a great boon to
reliability, I expect.

I am NOT a structural engineer; all I've said could be wrong, so check
carefully. (But I'd probably try it myself, FWIW.)

Donald.
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On 2021-07-02 8:42 a.m., Jeff Scarbrough wrote:
> I haven't done it on 35 years, but at least your equations look
> familiar.  I find that comforting.
> 
> If the capability is questionable, would solid wood blocks of a suitable
> height to support the channel somewhere near the center help?
> 
> On Fri, Jul 2, 2021, 06:34 Donald H Locker <dhlocker at comcast.net
> <mailto:dhlocker at comcast.net>> wrote:
> 
>     First oops - I used 10ft length, not 8ft. At 8ft, the numbers look
>     better by about 20%
> 
>     Donald.
>     --
>     *Plain Text* email -- it's an accessibility issue
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>     /\ <https://www.georgedillon.com/web/html_email_is_evil.shtml
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> 
>     On 2021-07-02 6:16 a.m., Donald H Locker wrote:
>     > BIG NOTE: please check my calculations and assumptions; I haven't
>     > actually done any of this for nearly 40 years.
>     >
>     > TLDR: They might not work.
>     >
>     > The significant supporting bits are the two flanges (3in x 0.5in); the
>     > plate across the top adds very little strength.
>     >
>     > All calculations below in pounds and inches.
>     >
>     > The maximum stress on the webs (if the weight is perfectly divided
>     > between them; more on that later) will be (equation 1-1 in
>     reference 1)
>     > with the weight at the center of the ramp. Fb (bending stress in the
>     > flange of the channel) must be less than the yield stress of the
>     channel
>     > material.
>     >
>     >   Fb = M*c/I
>     >
>     > [Ref 2] M for each web: PL/8 = P*120in/8 = P*15in [P is point
>     load, lb]
>     > [email] c for each web: 1.5in
>     > [Ref 4] I for each web: (b*(h^3))/12 = ((0.5in)*(3in)^3)/12 =
>     1.125in^4
>     >
>     >   Fb = (P*15in*1.5in)/1.125in^4 = P*20 lb/in^2
>     >
>     > For the given situation, p=3840lb, so Fb (max) is 3840*20 =
>     76800lb/in^2
>     > distributed between the two channels, each with two flanges. Since Fb
>     > must be less than the material yield stress, 10000 psi for some
>     kind of
>     > structural Al, but not high-strength material [Ref 3], this
>     doesn't look
>     > very good. Each flange would be loaded to about 19200lb/in^2 if all
>     > loads were perfectly distributed. (Note that high-strength Al (e.g.
>     > 6061) can have a yield stress of 45000lb/in^2 so looks much more
>     likely
>     > to succeed, but I don't know how much I'd trust that.)
>     >
>     > My biggest concerns would be the division of stresses between flanges
>     > (if one channel tilts a little or is not sitting perfectly flat on the
>     > trailer or ground, stress sharing will be uneven and one flange make
>     > take all stress) and the potential for flange instability (see
>     notes in
>     > 1.3.1.5 in Ref 1.) This may not be a problem because the flanges
>     are not
>     > particularly deep, but I watched a boat hung from I-beams drop when a
>     > structural I-beam twisted slightly from imperfect loading and the
>     entire
>     > structure quickly twisted into a totally unsupportive mess.
>     >
>     > References:
>     > 1 Beam stress/strain:
>     >
>     <https://engineeringlibrary.org/reference/simple-beam-bending-air-force-stress-manual
>     <https://engineeringlibrary.org/reference/simple-beam-bending-air-force-stress-manual>>
>     >
>     > 2 Beam force/reaction:
>     >
>     <https://engineeringlibrary.org/reference/beam-forces-moments-air-force-stress-manual
>     <https://engineeringlibrary.org/reference/beam-forces-moments-air-force-stress-manual>>
>     >
>     > 3 Material properties:
>     >
>     <https://www.engineeringtoolbox.com/properties-aluminum-pipe-d_1340.html
>     <https://www.engineeringtoolbox.com/properties-aluminum-pipe-d_1340.html>>
>     >
>     > 4 Area moments:
>     >
>     <https://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html
>     <https://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html>>
>     >
>     > ATB,
>     > Donald.
>     > --
>     > *Plain Text* email -- it's an accessibility issue
>     > () no proprietary attachments; no html mail
>     > /\ <https://www.georgedillon.com/web/html_email_is_evil.shtml
>     <https://www.georgedillon.com/web/html_email_is_evil.shtml>>
>     >
>     > On 2021-07-01 10:56 p.m., john niolon wrote:
>     >> I'm thinking about using aluminum channel for a set of ramps for
>     my car
>     >> hauler.  I have some 'available' that is  10" wide with .50 web
>     >> thickness and almost 3" flange... about 15# per ft.
>     >>  
>     >> ramps will be 7-8' long …..  and will go from ground to 17" high
>     trailer
>     >> deck...  truck weight is 5300 +/- … with front end weighting
>     3840.   I
>     >> can't find a table telling me weight capacity of
>     >> channel with the load traveling up the wide web...   like this   any
>     >> ideas on capacity ??
>     >>  
>     >> Image87ABA31D
>     >>  
>     >> thanks
>     >> john
>     >>  
>     >>  
>     >>
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