Bending Strength Of Aluminum Vs Wood?

[FAST FRED]

All the math is fine , but the practical setup is to simply use a band saw and chop them out of dimensional lumber a 2x10 or 2x12.

Stand on one with a friend while its upright and you can decide how many to install.

No math , just a trip to the big box store.

FF

[TollyWally]

There are joist span tables all over for building house floors and decks. It's not hard to get the numbers off the tables and reverse engineer something that is ballpark close. Sure point loads etc. have to be taken into consideration but those are details to be refined a little further down the road. Unless you're absolutley optimising for a certain criteria painting with a broad brush at first and then refining your ideas is a pragmatic way to go about it. From what I've seen more deflection is acceptable in a boat. But as has been said, jumping up and down on a simple mockup on saw horses is pretty damn easy.

[portacruise]

Originally Posted by jdworld

Anyone know? If you have a wood 2 x 4 supported each end spanning 9' alongside an aluminum tube 2x4 (1/8" walls?) spanning 9' and you stood on each one at about mid span, which one is going flex and/or fail more?

Also - if anyone can point me to a place online that has SIMPLE aluminum span charts that would be great - have had no luck with my googling.

thanks!

IF you can use an "I" beam shape instead of box in either one it should be stronger, lighter and maybe cheaper for the same desired strength.

Porta

[Submarine Tom]

Perhaps he/she is looking for a certain amount of deflection, it's a mystery.

However, no one has answered the original question so the simple test

originally proposed would seem the next logical step...

-Tom

[jdworld]

Originally Posted by Submarine Tom

jdworld,

Obviously my input was of no use to you as you didn't

even acknowledge it.

Because your post was all about the laminated spruce beams you made - doesn't really apply to my original post about a store bought wood 2x4 vs alum box beam. Thanks anyway though.

Originally Posted by messabout

The OP asked a simple question that need not have generated a controversy.

Wood fiber is not as strong as aluminum fiber.

.

i agree / good to know

Originally Posted by Submarine Tom

Perhaps he/she is looking for a certain amount of deflection, it's a mystery.

However, no one has answered the original question so the simple test

originally proposed would seem the next logical step...

-Tom

No specific deflection needed. Just wondering which material in that situation would deflect more. Yeah, I could have just gone to the store and bought a wood 2x4 and found a place to buy an alum 2x4 and done a quickie test. But I wouldn't have been able to return the alum 2x4 and don't have an extra $50 to spend on something I can't return just to see which deflects more. Instead I thought I would draw upon the vast amounts of aluminum experience here. If/When I get to the point of needing and exact spec I will go to an engineer, but in the meantime all I needed was a gut feeling based on experience. Thanks to all, I pretty much have a feel for it now......a box alum beam would likely bounce less/ feel more rigid.
And even if it doesn't for 9', I can always go with an alum I beam if needed.

[Frosty]

Well heres my 2 pence worth, wood rots, it will also sag, if you leave it long enough.

Ally does neither and looks nicer, it wont stain from dirty hands and need no painting.

[Guest625101138]

Originally Posted by jdworld

No specific deflection needed. Just wondering which material in that situation would deflect more. ...

The question is not about "strength" but rather elasticity. The elastic modulus of wood in longitudinal bending varies from about 8 to 15GPa depending on wood type. Aluminium is around 70GPa with some variation depending on the alloy. For comparison, steel is typically around 210GPa and carbon fibre as high as 800GPa.

There is a neat little program called BeamBoy (Google it) that enables you to easily analyse beams. I have used this for the two examples you posed only I have used metric units.

The deflection of the solid timber beam is about 3X that of the hollow aluminium. The example in the attached images compares deflection with a 1000N load - equivalent to 220lbf.

Rick W

[Submarine Tom]

Do the test at the store.

No need to buy the aluminum.

-Tom

[messabout]

You don't need to go to the store, Just believe Rick Willoughby. He knows what he is talking about. Earlier I gave you some simple math to do. Prediction of beam elasticity is based on those fundamental concepts. Never mind the math, just respect Ricks input. Also be advised that an I beam is not necessarily stiffer than a box beam. It depends on the dimensions of the I beam versus the dimensions of the box beam.

[jdworld]

ok thanks Rick - so it looks like the aluminum beam will deflect about 1", and the wood beam about 4" - that tells me what i was wanting to know. Rule of thumb answer, wood is way more springy than aluminum

[jdworld]

wait, what wall thickness did you use?

[Guest625101138]

Originally Posted by jdworld

ok thanks Rick - so it looks like the aluminum beam will deflect about 1", and the wood beam about 4" - that tells me what i was wanting to know. Rule of thumb answer, wood is way more springy than aluminum

You have not converted correctly. The figures show 9.84mm for the wooden beam and 2.92mm for the aluminium. The aluminium section had a 6mm (say 1/4") wall.

Wood flexes more readily than aluminium of the same section. Springy really has a different meaning to me. I consider this to mean how far something will flex before it yields. This is a function of elasticity and yield strength.

It is interesting that alloys of a particular material like iron or aluminium do not have much variation in elastic modulus but they can have an incredibly wide range in yield strength.

One of my favourite materials is spring steel. The small round bar I use has yield up around 1500MPa. So it will flex a hell of a long way and fully recover. There are also some nice alumium alloys that are almost as "springy". Although they have lower modulus and lower yield, some as good as 400MPa, they are not far behind the spring steel in terms of ability to recover from being flexed just that it takes less effort to flex.

So when you are considering springiness you are bringing in another material property.

Now I can see you will be heading down the local hardware just to confirm the figures. If you cannot find the exact sections I have used I can redo the calcs. In fact you may have a bit of aluminium tube in the workshop you can load.

It always pays to check the theory with some testing so you have verified it is close to reality. Also avoids gross errors in conversion.

Rick