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Diagram

In this case, L, theta and P are given, there will be 4 reactions at the wall? (2 horizontal and 2 vertical).

$$\begin{align} \sum F_v &= 0 \therefore F_{v,B} + F_{v,A} = P \\ \sum F_h &= 0 \therefore F_{h,B} = F_{h,A} \end{align}$$

What will be the tensile stresses along AB and AC?

However, I do not know how to obtain the other 2 equations to solve all the reaction forces at the wall and the tensile stresses? Can anyone please help? One of those equations are from the moment?

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As member AB is orthogonal to Point C it cannot impart any vertical reaction, hence Point B provides the only vertical support, and hence has to have a vertical reaction equal to P.

to find point B horizontal reaction, we set the sum of moments about point C equal to zero.

$$\begin{align} \sum M_C &=0 \\ -PL +F_{h_{B}}H &=0 \\ F_{h_{B}} &= PL/H\text{ pointing left} \\ \therefore F_{h_{C}}&=PL/H\text{ pointing right} \end{align}$$

tensile stress along AB is: $$\begin{align} T&=\dfrac{\sqrt{F_{h_{B}}^2+ F_{v_{b}}^2}}{A} \\ &= \dfrac{\sqrt{(PL/H)^2+P^2}}{A} \end{align}$$ And I let you do the stress on AC.

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  • $\begingroup$ @AndyT, please! sometimes we just miss the other guys' point. As you may know there is many guys who may not have clue and once I saw first you mistook the annotation for supports, I thought you where one of them, my apologies. $\endgroup$ – kamran May 30 '19 at 2:52
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@AndyT and @kamran explained how to technically solve this question. I would like to add a short comment about the thing seemed to confuse you the most.

Indeed, there are 4 reactions in this problem. If the triangle was made of a solid body, you would deal with a statically indeterminate problem and find it quite hard to resolve those reactions. However, since we are dealing with a truss, you should examine carefully whether its statically determinate or not:

  • The number of unknowns is 6 - the above mentioned 4 reactions + axial forces along the truss members
  • The number of equation is also 6 - 3 pin joints, each has 2 equilibrium equations (in X and Y directions)
  • Since the number of unknown equals the number of equations - you deal with statically determinate problem and can resolve all the reactions without dealing with the truss stiffness
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Resolve at point A, remembering that in a truss the members only carry axial loads:

  • Vertically P = FAB * sin theta
  • Horizontally FAB * cos theta + FAC = 0

This gives you the member forces without even needing to worry about the reactions (though they're pretty trivial once you've worked out the member forces).

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