The Bending Moment is the sum of all the moments and reaction loads passing through the centre axis of the truss considered at any point,in one word, it is the force which is needed to deflect a truss.
How would Bending Moment occurs? Below are some situations:
1. Self-weight caused by the force of gravity.
2. Live loading such as lighting fixture or sound equipment.
The Bending Moment acts as a compression force in the top main tube and as a tension in the bottom main tube. The braces are used to maintain the distance between the top and bottom main tube. .
And the maximum allowable Bending Moment can be increasing by enlarging the distance of the top and bottom main tubes. The other way to increase its Bending Moment is to increasing the diameter or the thickness of the main tubes.