Method of Joints Query: Shedding Light On Truss Calculations

[tina-duncan]

Hello all,

I have recently been attempting to refresh my memory in regards to method of joints and have been making some reasonable progress, however I have ran into a stumbling block that has left me quite puzzled.

I have been using the Skyciv truss calculator to verify my work and for the 1st analysis shown below I am fine and have no problems working out all the forces. However, in the 2nd analysis when I add further members, I cannot understand how the force values in each member are the values stated. Supports are at A & B, with an external force applied to D at 45 degrees.

Could someone possibly shed some light on how this works please?

 

[PhanthomJay]

Looks like you've introduced internal static indeterminacy by adding in those members, such that you cannot solve the truss using the joint equilibrium equations alone. Often these extra members carry no load when added outside the original load/support 'envelope', but apparently in this case because of the member arrangement there are some small forces introduced based on joint and member displacements ( if FC was not there, the other additional members would not carry load). Difficult to solve indeterminate trusses without computer aid.