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Dave and Anke
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Tuesday, April 28, 2015

Girder Construction in Square Boats

Rotary Girder
A term used when a person is referring to something technical about which he has no real knowledge.
Originally used in the movie "Tommy Boy" with Chris Farley.

[CAUTION: This post is a 'rotary girder' on my part, based on my take-away from technical information that was more or less over my head in its fine detail. While our experience has apparently confirmed my understanding, common sense ajd a grain of salt are advised.] 

Girder Construction in Square Boats

Girder construction is fundamental to TriloBoat and most other box barge/scow/square boats, especially when built with sheet materials.

A girder separates parallel faces by edge-bonding one or more faces at a substantial angle to them (in square boats, this angle is typically 90deg). Relevant examples are box, I, H and T girders. Separated faces are prevented by this attachment from moving, relative to one another.

This enhances rigidity in two ways:
  • Stress loads try to bend the girder. The face away from the load lies along a longer radius than that toward the load, so acts in tension to resist flexion (bending). That face can't appreciably stretch, so until it fails (it or the edge-bond tears asunder) the girder resists the load.
  • As the stress tries to bend the girder, the connective faces at right angles to the stress resist torsion (twisting). Its region toward the stress acts in compression while that away from the stress acts in tension (try bending a playing card on edge). That face can't appreciably stretch, so until it fails (buckles or tears) the girder resists the load.
The wider the separation of a girder's faces, perpendicular to the load, the greater its rigidity. The greater the cross section of its faces in line with the load, the greater its rigidity.

Clearly, the edge connection along all adjoined faces must be very strong.

Metal girders, built from sheets may be welded. Typically, they will not require further reinforcement along their edges.

In wood construction, we seek excellent glue adhesion (a function of surface area and the glue's working PSI), and/or substantial fasteners. Notably, the strength of a timber, running along that edge is secondary. It provides surface area for adhesion, but does not, itself, come under significant stress unless the bonding mechanism (glue or fasteners) fail. It is the bonded faces which provide strength, not the timbers framing the girder.

An interesting phenomenon in materials is that stresses tend to run along outer skins. Two relevant consequences:
  • Web Frames - Generally, you can cut large, rounded edge holes in girder faces without losing significant rigidity. If the face material is substantial and stresses are low (or distributed) then simple cutouts suffice. If not, the edges of the the cutouts can be framed to resist buckling. This feature is especially useful for internal bulkheads, windows, and hatches.
  • Solid Structure vs Girder Failure - We've noticed in solid ply leeboards that, in hard going, we may crack a veneer (the outermost 'skin') to leeward (the side of stress loading). From that point, in fairly short order, it will walk through the board, veneer by veneer (actually, transverse veneers put up no resistance... in effect, we lose two at a go).

    By separating sheets of ply in a girder arrangement, however, the full thickness of ply is now the 'skin', and all its veneers work in concert at full strength. Unless the leeward sheet of ply tears asunder or its edge bonds fail, the board holds. This holds generally true as well for solid vs hollow spars.


Square Boat = Box Girder

The hull and decks of a TriloBoat comprise a modified box girder. Sides are edge-joined to bottom and decks at near right angles to one another. In addition, transverse bulkheads (among which I'll include transoms) internal to this girder form sub-girders.

Picture attempting to bend the hull up or down at the ends, like a banana. You will be strongly resisted by the vertical sides.

Try bending laterally, like a banana on its side. You will be strongly resisted by the horizontal bottom and decks.

Try twisting it, or collapsing it sideways (like a cardboard box with its endflaps open). You will be strongly resisted by the bulkheads.

These are analogous to the major forces acting on any hull as a whole.

Hull areas which are curved - the bottom end curves and crowned decks - have a great deal of inherent resistance to stresses from outboard. Their inboard skins work in compression to resist; the principle of an arch. Accordingly, they require less internal support.

Large, flat panels left unsupported - say, deadflat areas between bulkheads - are not inherently rigid. Their inboard faces work in tension, and allow considerable flex. So I often recommend girder furnishings; furnishings built as boxes bonded to bottom and sides. Like the hull entire, these resist flexion and serve to much reduce the open flats within the hull, stiffening the flats and contributing to overall rigidity.

A final technique is to double hull surfaces (decks, sides and/or bottom), making girders of them. Simple longitudinal, bulkhead spanning stiffeners (rub-rails, leeboard guards, etc.) suffice, in conjunction with girder furnishings, but doubled is hell-fer-stout.

Girders within girders within girders! The result, robustly joined, is an exceptionally rigid hull.

Perhaps you've noticed the care with which a competent crew will crane and block a Curvy Dog?

In the water, CDs use monocoque principles to distribute stresses widely, diminishing their point loads. Try to crush a raw egg on end between the palms of your hands... you can do it, but it's surprisingly hard. But use your finger tips and you can easily rupture the shell. Out of the water, wracking forces and point loads from poor support can wreak havoc on the hull and its interior joins.

Not to disparage Curvy Dogs, but out of the water, they are like fish out of water.

A girder boat, on the other hand, remains rigid on land or sea. It can be jacked from any girder interstice, side-to-side or end-for-end, or cantilevered from three, poorly placed high points. Not that these are best practices, but they happen, and afford little concern. You can practically juggle them!

It seems to me that the fiber strength of the materials in use (ply and connective timber) in PSI (Pounds per Square Inch, or equivalent) is the limiting factor for adhesive bonding. Many modern glues well exceed the failure thresholds of the woods they bond.

Two strategies: a) increase the glue surface area to sufficient (increase timber faces or tape n' glue), and/or b) through-bolt on a schedule that raises the strength of the bond.

I personally favor glue-centric approaches. We've used 1 1/2in gluing surfaces, minimum, for structural joins along the outer hull in ZOON, LUNA (32ft) and SLACKTIDE, with no signs of failure. Fasteners were light and I consider them only useful for temporary clamping pressure (structurally negligible).

I'm unqualified to recommend this much reliance on adhesives. Consider running your construction solutions by an qualified Naval Architect for approval. Consider backing up the adhesives by through-bolting along the major hull edges, from both outboard faces. Err on the side of caution.

Girder construction is found from houses to bridges to jetliners to super-tankers to skyscrapers. Without girders, much of the modern world's architecture would be impossible.

So let's gird ourselves for DIY!

Girders within girders... walls and deck are ply/foam/ply
If additional lateral rigidity were deemed necessary,
lockers could have fixed lids with hatches cut into them.

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