Tension or compression?No guessing — the sign decides

Why assume tension? (30 seconds)

• One assumption for all the members → one sign that is easy to read.
• Arrow out = the member pulls. Got plus? The assumption is right — tension.
• Got minus? The member actually pushes — compression. Write the magnitude without the minus + 'compression' (for example 4.243 t compression), and correct the arrow in the sketch.

A compressed member moves on to the next joint

In the truss of the method of joints, joint A gave: N1 = −4.243. What do we do with it at joint D?

• Joint A
  N1 = −4.243 → minus = compression ✓
• Joint D
  N1 enters with an inward arrow and a positive magnitude: 4.243 · 0.707 = +3
• The equation
  ΣFy: +3 − 4 − N5·0.707 = 0 → N5 = −1.414 ✓

Inward arrow or minus — never both.

Tension and compression in the whole truss (for checking)

Reactions: AY = 3 t, BY = 2 t

Member	Between joints	Force	State
N1	A–D	4.243 t	Compression
N2	A–C	3 t	Tension
N3	C–D	0	Zero-force member
N4	D–F	2 t	Compression
N5	D–E	1.414 t	Compression
N6	C–E	3 t	Tension
N7	E–F	1 t	Tension
N8	F–B	2.828 t	Compression
N9	E–B	2 t	Tension

3 sign traps

• Arrow in + minus together. The sign is counted twice → at D you get N5 = −9.9 instead of −1.414.
• Arrow in because it "looks compressed". Then plus = compression — the opposite of the convention, and all the following joints get mixed up.
• Not fixing the arrow after a minus. At the next joint it is no longer clear what to substitute.

Checking tip: the chord = the row of horizontal members. In a simple truss with a downward load, the top chord is usually in compression and the bottom one in tension. Not a law — but if you got the opposite, it is worth checking.