2pcs 25% lower beams instead of 1pcs?

[nauleem]

I read somewhere on the forum that if a beam's height is to be decreased by 25%, then the width must be increased by 100%.

Ex

1x 145x45 can be replaced with 2x 120x45 and maintain the same strength properties.

Is this correct?

 

[kashieda]

The moment of inertia, which can be used to calculate deflection, for a rectangular beam is b*h^3/12.

A reduction of 25% is thus 0.75^3, around 42% of the original. Doubling does not quite give the same as the original's resistance to bending.

 

[Testarn]

120/145 = 0.8275.... so it is not 75% but rather 82% and (120/145)^3 = 0.5668...

I believe that two 120 mm planks will correspond to a 145 mm plank quite well.

Edit: Now I read a bit more carefully, and the original question was 75% of the origin... while the example was something else...