Why sign boards joints are made by an angle of 45 degrees?

Hello Afroz ..

45 angles equally distribute loads on structures. The resulting moment is way less than in the case of90 degrees angles.

I am illustrating this below:

On pivot B , the negative sign means that the resulting moment of air drag is in the opposite direction of the moment reulted from the weight of the sign itself.This means that the net moment will decrease.

In case A, the net moment is the very moment resulted from the mass of the sign.. No other component is there to oppose it. 

Now, since the moment is less in case of inclined angles, this means that welding will live longer because it's subjected to less loads.

Good luck

PS in case the illustration is too small to show clearly, then please check for it on my google plus m the link is here:

https://lh4.googleusercontent.com/GXM-Hn3b2m8b8vOWbx6DW5ThxxQmcXBflUgcBunBokU=w1249-h900-no

Its mainly related to facilating the fabrication process by cutting all the elements the same equal angle that can form the90 degrees rectangular/ square shape. Having other shapes you need to figure out the required angle that sute the fabrication process.

first to decrese force in inclined member

second that chamfer in steel is easy to cut45 degree or60 degree