I-Beam Question...

[PetePetePete]

All these aluminum I-Beam calculators have me befuddled...

Cantilever question.

If I have a 30' beam... and the middle 10' are supported on each end of said 10'...

So...

----------|----------|----------
E---------S---------S--------E

"S" is support.
"E" is cantilevered end.

If I need to place 800lbs on "E" (let's say 800lbs on both ends at the same time)... what size alumimun I-Beam do I need?? (I really only need 400lbs, but I want a safety margin of 2x)

It looks like I need A, B, and C for this...


Thanks,
Pete

[a64pilot]

Alloy and heat treat level will I believe play hugely into this too

[PetePetePete]

I was told:
6061 I-Beam... but I need the size...

Thanks,
Pete

[thinwater]

All you have is a 10' cantilever. The rest is irrelevant, just holding it down in this case. Or a 20' span. Same stress.

How much deflection (bending) is acceptable? That may be an issue in sizing.

You need a 5:1 safety factor. You really do.

[valhalla360]

What is your assumed webbing and flange thickness?

As thinwater said, how much deflection is acceptable?

Safety factor: 5-1 may be OK depending on the load. Is it a static load or dynamic?

If fatigue a factor?

What are you trying to do?

[3Days_at_a_time]

Originally Posted by thinwater

All you have is a 10' cantilever. The rest is irrelevant, just holding it down in this case. Or a 20' span. Same stress.

How much deflection (bending) is acceptable? That may be an issue in sizing.

You need a 5:1 safety factor. You really do.

If those supports are just supports and not a clamped connection than this is not a cantilever beam situation. It is a simply supported beam and the equation for it is very different. Also you may want to think about deflection because with an aluminum beam with a 10' unsupported region you will have a large amount of deflection before failure. You may select a beam that is strong enough to support the load, but there might be enough deflection that it won't work for your application.
I can walk you through this analysis if you want to pm me.

[PetePetePete]

The beam in question would be secured to the two supports.

This is for a walkway for a boat-life.

The load would be a person, and at most, holding a lower unit. But would still like it designed for 800lbs (safety factor included). The actual weight will never exceed 400lbs. Ever.

I would rather it not "flex".

[svHyLyte]

Originally Posted by PetePetePete

All these aluminum I-Beam calculators have me befuddled...

Cantilever question.

If I have a 30' beam... and the middle 10' are supported on each end of said 10'...

So...

----------|----------|----------
E---------S---------S--------E

"S" is support.
"E" is cantilevered end.

If I need to place 800lbs on "E" (let's say 800lbs on both ends at the same time)... what size alumimun I-Beam do I need?? (I really only need 400lbs, but I want a safety margin of 2x)

It looks like I need A, B, and C for this...


Thanks,
Pete

Assuming you are referring to the loading on a boom, firstly, how does one know that the end loads will always be symmetric?

Beyond the foregoing, in your example the maximum bending moment at the supports or at the mid-point between the supports would be 96 in-kip. Assuming 6061 T-6 aluminum, Young's Modulus (E) would be 10,000 KSI. In this case the determining factor in the design would be the maximum permissible vertical displacement, which will determine the moment of inertia (I) you need in the section What are your limits?.

[skipmac]

Another factor, besides alloy is whether or not this is a static load or will be subject to dynamic stress, like on a boat underway.

If static, like on a dock or or a lift then calculations based the weight of your proposed load will be sufficient. However, if this is for a structure on a boat that will be used when the boat is underway then the dynamic loads can far exceed the actual weights.

[skipmac]

OK, Ignore my previous post. It came in after you posted the application which sounds like it will be pretty much a static load unless you have people jumping up and down on the walkway.

[PetePetePete]

Guys. Sorry if it got complicated.

Take a look at my original post.

At the "E" points (the cantilevered ends)... if I want to put a 800lb iron weight on the end. What size beam do I need for it to be safe? The actual load would be well under 400lbs.

The two points that are at the 10' mark and the 20' mark would be fixed to the beam... so, an approximate 10' cantilever.

Hope that clears it up.

[PetePetePete]

Originally Posted by svHyLyte

Assuming you are referring to the loading on a boom, firstly, how does one know that the end loads will always be symmetric?

Beyond the foregoing, in your example the maximum bending moment at the supports or at the mid-point between the supports would be 96 in-kip. Assuming 6061 T-6 aluminum, Young's Modulus (E) would be 10,000 KSI. In this case the determining factor in the design would be the maximum permissible vertical displacement, which will determine the moment of inertia (I) you need in the section What are your limits?.

I would prefer for it to flex as little as possible. It there is an "acceptable" amount of flex, that's is totally fine. As long as it does not fail. In this case, "fail" would mean that the 800lb "iron weight" would break the beam and/or bend it to the point where it was not longer the shape it started as.

[Cheechako]

Aluminum flexes way more than steel by comparison. How much deflection is acceptable? that will likely be your limiting factor.

[PetePetePete]

Originally Posted by Cheechako

Aluminum flexes way more than steel by comparison. How much deflection is acceptable? that will likely be your limiting factor.

I would prefer for it to flex as little as possible. If there is an "acceptable" amount of flex, that's is totally fine. As long as it does not fail. In this case, "fail" would mean that the 800lb "iron weight" would break the beam and/or bend it to the point where it was no longer the shape it started as.

[Nord Sal]

It doesn't have to break to fail. Serviceability will govern over structural yielding for this project. You should decide on what your acceptable limits of deflection are. That might be, say 1/2" or 1" at the overhangs, then solve for the section modulus that is required for the beam. Use the section modulus that you determine to look up a suitable WF shape that has that section modulus or greater.

Don't forget to divide your load by two! (you'll have two WF beams to support your gangway.)

This is NOT a cantilevered beam but rather a simply-supported beam with 10 foot overhangs. With a load on an overhang (or both of them) the aluminum WF will deflect downward on the overhangs and upward in the middle of the spans. The overhangs will deflect upward for a load in between the supports.

Also, be wary of statements like this: "The actual weight will never exceed 400lbs. Ever." Sometimes things get used differently than they were intended. This is onme of the greatest challenges in engineering is to try to anticipate what people might do.