# how to determine the direction of support reactions in a truss?

Hi, I am not sure how to determine the direction of support reactions at point A and K. I know point A has a pin support, therefore, it can take in horizontal and vertical loads and point K has a roller support, therefore can only take in vertical load. But I am not sure in which direction the forces should be for both points. Help would be much appreciated.

• The sum of the forces in your system have to be equal to 0 or else you system is accelerating. Take your sum horizontal at A, your sum vertical at A and your rotational about A. You can initially assume a direction for your reaction force and apply the appropriate signage. If at the end your reaction force is negative, it means you have assumed it in the opposite direction to what it is. Commented Jun 23, 2022 at 4:55
• I am not saying A is the best location, but sometimes a location can be chosen which simplifies the equations you need to work with. Commented Jun 23, 2022 at 5:09
• This seems like a homework problem, have you ever given it a try? Please show your effort, then ask questions, if any.
– r13
Commented Jun 23, 2022 at 11:48
• Does this answer your question? Finding the reaction force at the hinge
– Abel
Commented Jun 27, 2022 at 20:10

I would assume the system is in equilibrium and point K is the support for the truss.

The truss is usually taken as a single unit in these calculations so creating a simple force diagram would assist immensely in reducing the apparent complexity.

Point A has an anticlockwise moment, point K doesn't.

The diagram would be like a beam of 20 length... Work it from there.

Choose a direction of horizontal and vertical reactions at A and Vertical reaction at K. If the answer is negative you put the arrow in the wrong direction. You do not need to get the direction right to start with.

The other answers tell you how to find the (instantly) obvious forces at A and K.

The next step in all probability is to find the force in each member, where you will be greatly advantaged if you number the space between each force and member (this called Bow's notation) and construct a Cremona diagram

https://www.northernarchitecture.us/allowable-stress/truss-analysis.html

the advantage of this is that it is self checking, elegant and has all the information in one plot

https://en.wikipedia.org/wiki/Cremona_diagram

The other common methods are joints (which is similar to this, but bitty) and sections, which gets messy with complex structures. Both are correct, but Cremona diagrams are harder to teach, but infinitely more satisfying. IM(H)O.