Is there a distinctive difference between girders and beams?

[ldigas]

Originally Posted by PAR

By initial post was from a structural engineering stand point and is how it's described in text books. You can buy into the hoopla of "we called it this or that" but in strict engineering terms a beam carries a bending load, typically along a single axis and a girder bending and torsional loads often along multiple axises. Anything else is discussion fodder, but not "footnote" qualified or quantified.

That is a good definition, but unfortunatelly even after all this, I still cannot tell which is which in a particular case, which makes the whole point of the name moot.

[Perm Stress]

Originally Posted by ldigas

The dashed line actually represents the boundaries of the plate which forms the main deck, starting from the bow and then going along the side.

Along the middle of the plate, at the side of the boat, goes the longitudinal (below the plate, of course).
Below the deckhouse walls (where you assumed was the longitudinal) is a, I don't know, FB40x5 profile, also, along the whole length of the deckhouse wall.

The stiffeners are also below that plate at every other frame, connecting the deckhouse walls (the FB profile), and the longitudinal to the side frames.

Now I see your situation.
Longitudinal in the middle of long narrow plate.
So from the start:
*your deck plate is well supported from all four sides: ship side/bulhkead/deckhouse wall/bulkhead.
*your transversals are well supported at both ends: at side and at deckhouse wall
*your longitudinal connect the transversals at about mid-span, running from bulkhead to bulkhead
*transversals and longitudinal are of the same profile

What do we have from all of this?

*span of transversals is much shorter as that of longitudinal.
*this mean, that for the same amount of load (weight) transversals will deflect much less than longitudinal, if we disregard their interaction
*further, from two points just mentioned we have a doubtless conclusion: transversals act as supports for longitudinal
So:
*free span for transversals is from ship side do house side
*free span for longitudinal would be from transversal to transversal
HOWEVER
The property of structure just described (a grid of members of the same size) is very good at distribution of point loads to several nearby members. If you calculate necessary scantlings for longitudinal on basis just described, you will find much smaller section necessary; but then stiffness of such section will be not enough to push down several beams, when heavy person push it down at one point. In this kind of arrangement, it is normal practice to make longitudinal and transversal profiles the same. As longitudinal lay on "spring mattress" of transversals, analysing it as beam, supported at the ends is meaningless.
In this particular situation, the longitudinal serve the purpose to tie together all the beams for concerted reaction to point loads (it is a deck after all, and some persons weigh well over 100kg ).
If carefully engineered, scantlings of beams could be reduced considerably. If careful engineering and saving the last 0.52kg is not your goal, than simply enjoy the added strength as an extra safety factor.

[ldigas]

Originally Posted by Perm Stress

Now I see your situation.
Longitudinal in the middle of long narrow plate.
So from the start:
*your deck plate is well supported from all four sides: ship side/bulhkead/deckhouse wall/bulkhead.
*your transversals are well supported at both ends: at side and at deckhouse wall
*your longitudinal connect the transversals at about mid-span, running from bulkhead to bulkhead
*transversals and longitudinal are of the same profile

What do we have from all of this?

*span of transversals is much shorter as that of longitudinal.
*this mean, that for the same amount of load (weight) transversals will deflect much less than longitudinal, if we disregard their interaction
*further, from two points just mentioned we have a doubtless conclusion: transversals act as supports for longitudinal
So:
*free span for transversals is from ship side do house side
*free span for longitudinal would be from transversal to transversal
HOWEVER
The property of structure just described (a grid of members of the same size) is very good at distribution of point loads to several nearby members. If you calculate necessary scantlings for longitudinal on basis just described, you will find much smaller section necessary; but then stiffness of such section will be not enough to push down several beams, when heavy person push it down at one point. In this kind of arrangement, it is normal practice to make longitudinal and transversal profiles the same. As longitudinal lay on "spring mattress" of transversals, analysing it as beam, supported at the ends is meaningless.
In this particular situation, the longitudinal serve the purpose to tie together all the beams for concerted reaction to point loads (it is a deck after all, and some persons weigh well over 100kg ).
If carefully engineered, scantlings of beams could be reduced considerably. If careful engineering and saving the last 0.52kg is not your goal, than simply enjoy the added strength as an extra safety factor.

This is the answer I've been looking for! Thanks ... uhmm, Perm it explains a lot (more over, now I understand a bit better how this is approached).

[Perm Stress]

Originally Posted by ldigas

This is the answer I've been looking for! Thanks ... uhmm, Perm it explains a lot (more over, now I understand a bit better how this is approached).

Strength engineering is exciting, is it ?

[ldigas]

Originally Posted by Perm Stress

Strength engineering is exciting, is it ?

More and more, expecially as you start to understand the concepts (one by one) ...


btw, thanks to, well ... everyone ... for taking the time to explain some of them, and the principle in general.

[rxcomposite]

Kind of a latecomer in this thread, This is a "grillage" structure where members are of the same size accepted by LR. The cross connection, however, needs special treatment in composites.

[powerabout]

Beam me up Scotty
Girder me up Scotty - doesnt work

[conceptia]

Originally Posted by rxcomposite

Kind of a latecomer in this thread, This is a "grillage" structure where members are of the same size accepted by LR. The cross connection, however, needs special treatment in composites.

I too felt it that way..