Tuesday, January 12, 2016

TLAR

TLAR: Short for 'That looks about right".  It is a school of thought in the informed layman's culture of design and structural engineering - an education based on a lifetime of building and breaking things and learning from the results.

It more often tends to result in structures that are overbuilt rather than under built - easier to get away with in a sundeck or cabin, and even in a boat if it isn't meant to go really fast.  I have also seen it work with home-built aircraft design.  No engineering, just common sense with a firm respect for what has worked in the past.

However,there is a risk to making something too strong.  When a stress riser is reinforced  in one place, it just moves on.  Sometimes it is good to know where something may break instead of having a very nasty and consequential surprise elsewhere.  Flexibility and flexing keep things from breaking.  But too much flexing and you get fatigue.  Too much stiffness in a light structure and the result is highly stressed joints.  No margins for error in the design and construction of these joints.

The shape of a structure has a great bearing on its strength.  Curves are strong.  Compound curves are stronger - consider the egg.  The gain is strength combined with lightness.  The downside is complexity of construction.  A square box like I am building does not have any of those curves - save for the bottom curve-ups at either end, and the camber in the roof.  I compensate with heavier scantlings.

The greatest contributor to strength in my boat however, and in any square boat, is the fact that it is an enclosed tube.  This has been well described by Dave Zeiger in his blogs.  I can give you an idea of how well a box can be reinforced by enclosing it is by holding a simple square Tupperware container that is made of very flexible plastic.  With the cover off, you can grasp it in your hands and twist it many degrees measured from one end to the other.  Snap on the cover however, and the whole thing stiffens right up and you can't twist it at all.

Any opening in said box weakens the structure.  A load path must be created around the opening.  I'll give you an example of such load paths when used in aircraft.  A Boeing 737 (of which I have some intimate structural knowledge) is skinned with aluminum sheet that is typically .071" in thickness.  That is just over 70 one thousandths of an inch.  This skin carries most of the structural load of the fuselage.  There are frames and stringers to keep the shape of the fuselage but the skin is the primary load carrier.  Because people would be freaked out in an aircraft that they could not see out of, there must be windows.  These holes in the skin structure must be dealt with so that it is not weakened.  That is done with window frames made of cast and forged aluminum, each riveted to heavy stringers.  The loads that would normally be carried through ounces of .071" skin must now be directed around an opening with structure that weighs a few pounds.  Airplane designers would LOVE to eliminate windows altogether and save all that weight.  We may see that in the aircraft of the future (barring societal collapse) and people will look at the outside electronically.

Sometimes a window can be part of the structure as well.  Windshields and rear windows of most modern automobiles are structural and contribute to the stiffness and strength - especially in uni-body construction - pretty much as all cars are now.

The weakest openings in our boat are going to be the cabin hatches at either end.  Extra consideration for the forward one is in order, since that bulkhead and roof structure will be integral to the main mast support.  I will be beefing this area up accordingly, and no, there will not be any finite element analysis used in the design.  I just don't have the math.  We will be using good old TLAR.

Here is a stack of bulkheads:  Soon be time to set them up.







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