Designing and Constructing Crossbeams

[fburton]

Hello Ahmed

Okay I’ll play teacher for a change.

Create your beam as 3m long; with a cantilever support at one end.
Assume the weight of a single hull is 300kg which is placed at the far end of the beam
See Loading Screen below:-

Now state the Beam-Properties
Simply this is the I (the moment of Inertia) (which comes from your beam sectional shape)
And E the Modulus of Elasticity (which is the material the beam is made from)
See Beam-Properties below:-
But of course unless you’re an engineer you probably aren’t familiar with these terms.

So if you don’t know the Moment of Inertia touch Calculate and fill in the Beam section dimensions.
See Calculate Moment of Inertia below:-
Enter cross section (probably solid rectangle for you)
Width 100
Height 300
Touch Calculate button to process the Moment of Inertia
Then touch Use These Values to slot them into the previous screen.

You can then touch Caluculate-Calculate Stress,Deflection to see
The following results
See Calculations Screen below:-


In my example I used Douglas Fir with a Modulus of Elasticity of 13.

I don’t know what the prescribed solution for plywood would be but my thoughts would be:-
i. only half the layers are acting as a beam structure (so you could just double the width).
ii. Perpendicular fibres still offer 12.5% strength
iii. I’ve seen people state that a laminated frame is 25% stronger than a bent one.

All in all, at a guess, I would suggest, that a plywood made from Douglas would have an E value of:-
((13 / 2) + (13 / 2 * 12.5%)) * 1.25% = 9

Just mess with Beamboy; you’ll soon find that a 25% increase in the height is the same as doubling the width.
It gets even more playful with hollow sections. Who’s beaming now.

Best Fred

[catsketcher]

Ahmed,

We had a discussion on this a while ago. try this thread

cheers

Phil Thompson

http://boatdesign.net/forums/showthr...atamaran+beams

[sigurd]

Hello, I also don't understand much about beams. In order to get going, I have started reading here: http://www.brantacan.co.uk/
It is a nice page that starts out pretty basic.

I am wondering the difference in torsion stiffness between an I beam and a rectangular box beam - And while I'm at it a tube and a triangle too. A plywood I beam looks like the simplest to make but I want good torsion stiffness for the height. It would be cantilevered from an axle through holes in each locally reinforced flange.

Plywood beams like the one in post #2, can they be made with only epoxy glue, no screws or glass, except at attachments? Or will it delam before breaking?

[BWD]

Yes only epoxy is good.
The wood fibers fail before either the plywod glue or the epoxy if done right (primary chemical bond, etc.) with good plywood. Wood-epoxy doesn't need screws for a true composite structure.
The loads are different from the floor-joist though; a box or tube is better I think. For an I beam you'd need bigger flanges....

[sigurd]

thanks BWD; thinking about it some more a box shouldn't be any harder to make and only a web heavier. I bet the second web does more for torsion than would its weight in the flanges?

[catsketcher]

Hello Sigurd

I am not an engineer but I have built a few multis and designed two. Most designers are not engineers but do a careful mix of baselining from what has gone before and very cautious modification.

I would be looking at something similar to the boat you are drawing and using the beams of this similar boat as a base. Starting from first principles is a bold step as you can so easily leave out a load condition or over-estimate your build quality.

My advice is to read JE Gordon's two books - The science of strong materials and Structures. Get them at Amazon and from there get into some aeronautical books on structures. Reverse engineer boats around you and find out the safety factors from them. I would not recommend you try to work your way to a design without carefully basing any calculations on real world testing.

Most cats use box or round aluminium beams for crossbeams. The box beam is great in torsion as well as simple bending and cantilever. A box is easy to build. For the design of box beams you can't go past "Stress without tears" by Bernard Rhodes. It goes through the design of several different types including a few boxes, a flanged beam from wood, aluminium built up beams and alloy tubes.

I am about to start more testing of the structure of my folding catamaran so that I can get some real data to back up my calculations. I have already had the prototype sailing for a while and want more data still.

cheers

Phil Thompson

[Ahmed Wahab]

Thank you fburton for your clear instructions,Now I can use the program!!

another issue
Should be another support to the other side of the hull which is the inside gunwale?to have more precise calculation.I think the stress on this point would be the greatest.
the calculation shows the deflection in minus and plus mm.
Should I add them together to have the total deflection of the beam !!
Are these calculations within the yield strengh of the beam

How to add a safety factor to the beam calculation,like doubling the load !!!
Would it be 100% a good safety factor

cheers
Ahmed

[sigurd]

Hi cat; it ain't a cat, and it is not a folding beam; and there ain't anything very similar to compare to! thanks for the help.

[Spiv]

I have just posted a new thread in the design forum regarding the design of "I Beams".
If anybody who followed this forum is interested, please go to http://www.boatdesign.net/forums/sho...428#post205428