![]() ![]() Given that, the node now has an upwards force which must be countered by AB, which must point down. Since that resultant will point left-and-up, it's pointing towards the node and is therefore compression. That being so, BC's vertical component must point up, since BC's resultant force must be purely axial and therefore parallel to BC's inclination. That happens by making BC's horizontal component point towards the left. ![]() Only BC has a horizontal component, so it has to be solely responsible for countering that force. ![]() In this case, you have a rightwards external force at B. Therefore you just need to figure out the direction for each of the arrows in the node diagram. Likewise, a beam under tension feels outward-facing external forces, so its reaction points towards the column, and away from the node. Therefore, the column's reaction points outwards from the column. If a beam is under compression, the external force it feels is pointing inwards (think of a column with a downwards force above and an upwards reaction force below). The way to see this is to think in terms of Newton's Third Law: the node is applying a force on the beam, and the beam is applying an equal and opposite one on the node. The equilibrium diagram in your question (just the arrows, ignoring the values) is a cheat-sheet: all arrows pointing towards the node (such as the one representing BC) are compression, those pointing away (such as AB) are tension. ![]() The last way is to look at the individual nodes. That's the basic gist of this method: look at the expected deformed configuration and see which elements get stretched (tension) and which get squeezed (compression). Which tells us that AB is under tension and BC, under compression. No matter what the real displacement of B is, this will always be the case: AB gets slightly longer, BC gets shorter. All that matters is that AB got longer and BC got shorter. The numbers themselves are irrelevant (after all, they're based on a made-up displacement of B).
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