Joints or sections?Pick a method in ten seconds

Why? (30 seconds)

• With joints you advance joint after joint. For a member in the middle you have to go all the way to it.
• With a section you jump straight to the member — three equations at most.
• But a section has a condition: 3 cut members at most, and the truss is split into two parts.

N5 in the middle of the truss — two ways

• Joints
  A → C → D → N5 = −1.414: three joints, 5 equations (A: 2 · C: 2 · D: 1)
• Sections
  After the reactions: a cut through N4, N5, N6 → one ΣFy → N5 = −1.414

Same answer. One equation instead of five.

Which method?

What is asked	The method
All the members	Joints
A member next to a support	Joints — one joint or two
A member in the middle, with a cut of 3 members through it	Sections
No valid cut through the member	Joints

3 traps

• Sections without a valid cut. A member where every cut through it crosses 4 members — you get stuck.
• Joints for a member in the middle. Every joint on the way = one more chance for a mistake that carries on.
• Not checking. With two methods there is a built-in check — a pity to give it up.