Either of those two beams should support the load, the question is: how
much safety factor do you want to build into it? Is the 750 the max load
it will ever see or the standard load everyday? Is this something that
is subject to impact or impuslse loads when the load is dumped on the
beam? Is this something that is human safety related?
The 2"X4" gives a max stress of approx 11340PSI, about the max I would
want for 6061 (yeild 37-40KSI, ultimate about 42-45KSI... depends on
the referance. These are from Machinery's 25th ed) as the safety factor
is only a tad above 3. Not knowing the loading type, I would
personnally want a higher safety factor. It is likely ok IF the
following conditions are met: Not a life-critical application; the load
is suspended such that no stress concentrations are produc at the load
point (so don't hang the load by running a 1/4" bolt vertically through
the tube); NO impact loading or cyclic loading that will lead to
fatigue issues (impact loading derates, typically, by at least a factor
of two. may be a factor much higher than two. Fatigue issues get rather
involved... generally a concern when there is cyclic load variations of
more than 50% of the maximum load, for thousands of cycles. Note that
this can occur from external vibrations, as well as from the ayatem
The 2"X6" give a max stress of 6120PSI, for a safety factor of aout 5.
If the loading was steady, not shocks, not applied suddenly, no
vibration, I might be happy with this. Any shock, impact, or vibration,
especially not knowing how the load is applied, I would still not be
real happy with this.
My target would be about 4 to 5KSI for the max stress, including
consideration of how the load is applied, if there is to be any shock
loading or vibration, or if there is any life risk. Without more detail
on how the load is applied, and what the actual cirumstances are, this
is the best I can tell you.
References: Machinery's 25th, P 234 (stress in beams) P224 (moments of
inertia) P569 (mechanical properties of aluinum alloys);
_The_design_Of_Welded_Strutures_ by Blodget;
Thanks, I should have given the specifics. this is for a race car trailer
where the load will be one end of a 1,000 pound car being lifted three feet
on a pair of ramps. the car is placed on the ramps, one end is lifted and
locked in place then the other end is lifted and locked in place. Another
way of doing this is to have the car winched up on the ramps with the front
end already locked up in place, so the car goes up at an angle, then the
back end is winched up and locked. In that case the light end of the car is
lifted, maybe a load of 500 pounds. The load is steady, and the ramps are
locked on the uprights as soons as they are in place.
Follow the link below to a program called Engineering Power Tools.
You can dowload a free trial version & then buy either the plus version
(I strongly endorse it) for $50 or the standard version for $25. This
FANTASTIC program has information in it for ALL of the building trades.
In your case, it will give information about stress & torsional
deflection in beams. Check it out...you'll be glad you did:
Hope this helps. -Wayne-
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