Here is an Oak tree in the New Forest exhibiting its amazing strength.
The tonnage of wood on this horizontal branch is supported by the special cells which produce the interlocked grain at the junction with the main trunk. Also the branch will have tension wood growing above its centre to act like a bicep in an arm.
This junction is one of the strongest places on a tree and branches rarely fail at this point. The spiral grain on this particular branch also gives it greater bending strength.