I ran across this text browsing through a book I picked up recently - ASME Engineer's Data Book by Clfford Matthews (Second Edition , ASME Press):

"6.1 Engineering Structures: Where Are All the Pin Joints?

Much of engineering mechanics is based on the assumption that parts of structures are connected by pin joints. Similarly, members are continually assumed to be “simply-supported” and structural members pretend to be infinitely long, compared with their section thickness. The question is: Do such members really exist?

They are certainly not immediately apparent – look at a bridge or steel tower and you will struggle to find a single joint containing a pin. The structural members will be channels, I-beams, or box sections surrounded by a clutter of plates, gussets, and flanges, not simple beams of nice prismatic section. So where is the relevance of all those clean theories of statics and vector mechanics?

Fortunately, the answer exists already, hidden in two hundred years of engineering experience. Calculations based on simple bending theory, for example, have been validated against actual maximum stresses and deformations experienced in real structures and proved sufficiently accurate (say ±10%) to represent reality. Once a factor of safety is introduced, then the simplified calculations are as accurate as need be. They are, to all intents and purpose, correct.

Simply-supported assumptions work the same way. The complicated-looking supports of a bridge deck do act like simple supports when you consider the length of the beam-like members they are supporting. Equally, the members themselves dissipate stresses induced by constraint from the “real” supports within a short distance from the support, so they act like long thin members, even though they may not be.

The design of engineering structures is built around findings like this. They have been proven quantitatively, by using strain-gauges and measuring deflections, and by advanced techniques such as FE analysis and photo-elastic models. Complete structures, airplanes, ships, and buildings have been investigated to demonstrate the validity of taught theories of statics and mechanics. The result is that all these types of structures in the world are designed using equations that are unerringly similar – proof enough to the validity of the theories behind them. Try to improve theoretical techniques, by all means, but don’t ignore what has been found already, including those assumptions about pin joints and simply supported beams."

Sir,

Thanks for this excellent quote.

Simple assumptions are easier to learn and comprehend and if in closed form, useful in troubleshooting in locations and timing where even a computer and software is not available.

Advanced analyses are often costly and can be selectively used in high stake areas.

regards,

sam