Branch pipe required replacement area

[rww88]

I am having difficulty understanding the factor (2–sin ?) in the area replacement calculation. By my way of thinking, the parallelogram cross section resulting from a “skewed” branch pipe has the same area as the rectangular cross section when ? = 90°. Why must the area be augmented by (2–sin ?)?

 

[bvi]

The area is greater because it is a lateral and concerns regarding the geometry, thus some additional reinforcement is required.

The other thing is you need to use the actual opening size, which is larger than the branch ID when you have a lateral. For a lateral, the opening size is the branch ID (also considering erosion, corrosion and mill tolerance) divided by sin(Beta)

 

[rww88]

Thanks bvi. Will you explain the derivation of 2-sin(beta)?