# Is there a distinctive difference between girders and beams?

Discussion in 'Class Societies' started by ldigas, Jan 2, 2012.

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### ldigasSenior Member

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.

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### Perm StressSenior Member

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.

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### ldigasSenior Member

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).

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### Perm StressSenior Member

Strength engineering is exciting, is it ?

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### ldigasSenior Member

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.

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### rxcompositeSenior Member

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.

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### poweraboutSenior Member

Beam me up Scotty
Girder me up Scotty - doesnt work

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### conceptiaNaval Architect

I too felt it that way..

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### aldy syahriJunior Member

Amazing! After read and read several times. I am clearly understand the concept. 2012 threads still helpful!
I hope there is no big difference in rule about the definition.

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### Will GilmoreSenior Member

"From its very earliest appearance in English in the 17th century, “girder” has meant “the main beam supporting a floor” or a supporting element that plays the equivalent role in a bridge. ... By the way, “gird” has another relative in English, “girdle,” which springs from the same root and has a similar history."
https://www.google.com/url?sa=t&sou...Vaw0cSecakAn7t_SEV_vJAd5A&cshid=1606395397160
In construction, a ridge pole would be a girder and the beams are the ribs that form the pitch of the roof. A beam is always supported, as in post and beam construction. To girdle something is to cross its entire span.
Beams are generally horizontal. Language is an interesting phenomenon. It grows and moves like an amoeba.

-Will (Dragonfly)

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### BarrySenior Member

I would SUGGEST that the difference could be defined by the primary forces within in the particular components.

So a Beam could be defined as an element when loaded experience Beam stresses, say a horizontal beam with vertically down forces creating vertical shear, tension, compression and shear flow stresses. ( not having to be down but for simplicities sake) And these stresses governed by the BEAM EQUATIONS

While a girder would be like a truss with the primary stresses being either compression or tension.

Primary being the stresses resulting from loading as tension or compression member of a truss would experience secondary bending stresses due to its own weight and if a floor is attached with additional loading.

An aside
Most people who do simple stress calculations use the standard beam equations are familiar with S=Mc/I to find the max compression or tension stresses in a beam. Unless doing advanced design, most stop there because this tension stress
will normally cause failure. (in a homogeneous constant area moment of inertia beam) ( with the assumption that there is not any torsion

But an in depth stress analysis of a beam would include (in addition to determining the max yield stress to calculate yield)
1) stresses induced due to the weight of the beam
2) Vertical shear analysis
3) Shear flow analysis
4) stresses induced in a material resulting from changes in size/length due to expansion and contraction, with constrained ends
5) stresses induced by dynamic loading, ie wind load or momentum caused by changing loads

(and torsion if it is present)

Last edited: Nov 26, 2020
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### TANSLSenior Member

In my opinion :
• "beam" is a transverse reinforcement of a deck. The transverse elements are often called "beam" when they belong to a deck and "frame" when they belong to the hull. It can be a primary or secondary element of the structure. When it is a primary element, it is also called "transverse web frame" (deck, side or botton)
• "girder" is a longitudinal primary reinforcement. It can be a "deck girder", "side girder" or "botton girder".
• Both elements work with loads perpendicular to their length and are usually accompanied by a piece of plate, called attached plate, which helps to support the loads on them.
• When an element works in compression it is usually called "pillar".
• A "strut" is a piece that, placed obliquely, secures frames or structures.
More concepts :
The structure is frequently arranged such that the plating panels are supported at their boundaries by a set of grid of “secondary” stiffeners directly supporting plating (e.g. stringers), which themselves are supported by another set of “primary” striffeners (e.g. frames, bulkheads, etc.)

Last edited: Nov 27, 2020

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### Ad HocNaval Architect

Tansl is correct.
A beam = transverse member,
A girder = longitudinal member.

Also noted HERE.

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