Simple Beam Moments

Structural Layout Vignette and Multiple Choice

Simple Beam Moments

Postby focusteck » Wed Oct 30, 2013 3:42 am

Can someone help me understand and explain why a simple beam will have a greater maximum moment and more deflection than either a continuous beam or a fixed end beam supporting the same load on the same span.

Thanks in advance.
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Re: Simple Beam Moments

Postby ssarch25 » Wed Oct 30, 2013 1:58 pm

focusteck wrote:Can someone help me understand and explain why a simple beam will have a greater maximum moment and more deflection than either a continuous beam or a fixed end beam supporting the same load on the same span.

Thanks in advance.


I will probably just end up rambling and making no sense but hopefully this will help.

Think about about it in a small scale with model pieces of wood or pencils, whatever.
1. A simple beam has two posts with a beam in the middle, push down in the middle and it deflects.

2. Same beam, now with 3 reactions instead of 2. Push down on the middle you are pushing down in the same axis as the middle support so it obviously deflects less. Push down on the middle between supports and the portion of the beam over the middle support will push upward on both sides giving you more leverage.

3. Fixed end beam. Image holding a pencil on both ends with a firm grip and having someone push down on the middle of it. It will be much stronger than the simply supported beam with post supports.

Just my thought process, I hope that helps. In reality if you did a sample problem and put math to each of those examples it will explain it in a calculated manner.
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Re: Simple Beam Moments

Postby patjohns » Fri Nov 01, 2013 12:26 pm

Simply supported beams have zero moments at the supports. Continuous beams have a negative moment over the supports. Because of this negative moment, the positive moment between the supports is less in continuous beams. Therefore, Deflection is less.
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