Beam me up Scottie - I or H?

Anonymous · Post by **Anonymous** » Tue Nov 23, 04 2:35 pm

I always wondered this, but never asked anyone. I get that the names come from the sectional shape but what are the actual differences between an I beam and a H beam rod? Why would you want one over the other?

Dave-R · Post by **Dave-R** » Tue Nov 23, 04 3:17 pm

I am sure I have posted this before so you must have missed it.

The H-beam is stronger and usually lighter.

Anonymous · Post by **Anonymous** » Tue Nov 23, 04 3:20 pm

so why do people still sell (& buy) I beams? Is there something inherantly more expensive about H beams? (ie for the given of the same material being used)

Dave-R · Post by **Dave-R** » Tue Nov 23, 04 3:45 pm

Dunno. But typical performance I-beam rods are about $200-$300 a set and H-beam are around $400-$500.

Anonymous · Post by **Anonymous** » Tue Nov 23, 04 4:05 pm

It's to do with maximum bending stress. To get the same strength at a lighter weight you have to satisfy a certain equation (I'll dig it out later). The key to the whole lot is the maximum distance from the neutral axis (I'll explain later) known as the I or Y value (depending on circumstance).

It all adds up that material can be removed from certain areas as long as you maintain the same second moment of area (Ixx, or radius of gyration, J, in a circluar type thing).

It's the same thing for why a hollow pipe used for a propshaft is as strong a solid bar at a smaller diameter and lighter too. In this case we take torsional stress and the radius of gyration, but it's the same basic equation.... :-s

No, that doesn't make much sense does it?

Well you did ask, now I have realised I forgot all that stuff and I'm going to have to go away and study it again so I can do all the calcs for the suspenion force path analysis

Anonymous · Post by **Anonymous** » Tue Nov 23, 04 4:07 pm

bottom line is we guess they are more expensive to make then.

Or just that the precieved benefits mean that they can charge more for the product even if it costs the same to make?

Dave-R · Post by **Dave-R** » Tue Nov 23, 04 4:08 pm

Don't ever let go of Teabag James. You will never get another woman with patter like that at parties.

Anonymous · Post by **Anonymous** » Tue Nov 23, 04 5:40 pm

LOL! Well, im sure some girls would interested in the part about shaft and Gyration

MrNorm · Post by **MrNorm** » Tue Nov 23, 04 6:18 pm

The extra expense might be to do with the manufacturing process? I beams can have a pretty simple die, but H-beams couldn't be extracted from a simple die due to their shape. They probably need either a multi-piece die with sliders etc, or maybe they do a lot of machining instead - either way it's more cost.
dunno, just a thought

I got H beams!

Alex · Post by **Alex** » Tue Nov 23, 04 6:27 pm

steady....

Anonymous · Post by **Anonymous** » Tue Nov 23, 04 8:23 pm

On second thoughts I won't be busting out the max bending stress equation....

I just got another bit of coursework which involves some truly filthy maths! Anyone know how to apply a second order differential equation to an indeterminant bending force?

Why couldn't I have done sport science?!? It's them that get this famous student lifestyle, not us poor engineering types.

Kev · Post by **Kev** » Tue Nov 23, 04 9:54 pm

Firstly; indeterminant is spelt indeterminate.
If I'm not mistaken I'd say it's a fairly simple case of involving the unknown function y, its derivatives y' and y'', and the variable x. Then use the "guessing" method, which will, of course, only work under the following conditions;
Condition 1: the associated homogeneous equations has constant coefficients;
Condition 2: the nonhomogeneous term g(x) is a special form.
Sorted, Down the pub!

TrevD · Post by **TrevD** » Tue Nov 23, 04 10:09 pm

well i must agree with kev , too many YYYYYYYYYs , its all abbout loading on a given area displaced over another known area , ( done all this years ago ,didnt understand it

then) but iff you can afford it H beam seems to be the way to go.

Guy · Post by **Guy** » Wed Nov 24, 04 6:55 am

I've got I beams but my sidelights still dont work

Dave-R · Post by **Dave-R** » Wed Nov 24, 04 10:02 am

There was no-one making H-beams with Chrysler sized ends/lengths when I put my 440 together. That is why it has those stupid heavy 6-pack rods that it originally came with.