Truss hand calculations via the method of joints are available in the results section.

- All members are connected by pins. This implies that the member fixities are “FFFFRR” or “FFFFFR”
- All members are two force members and thus only support axial loads. This implies that only point loads that are applied to the nodes are allowed.

**If either of the above conditions are not met, then the “Hand Calculation” button will not show**

In addition, it should be noted that:

- Statically indeterminate members cannot be fully solved by the “Method of Joints” however, the hand calculations will still attempt as many solutions as possible for reactions and axial forces.
- All axial forces will be assumed to be in tension. This means that all positive results will confirm tension, negative results will determine compression, and zero results will determine 0 force members.
- In some cases, the “Method of Joints” is unable to provide a solution, and this is a limitation on the method. In these instances, the “Method of Sections” may be more appropriate.

**Example**

1) Model your truss in S3D.

## Truss Instability Formula

The software can only solve structures that are stable. A helpful check for stability, are the following formulas:

- The condition m >= 2j – 3 must be satisfied for internal stability
- The condition m + r >= 2j must be satisfied for external stability

Where;

*m = number of members
j = number of joints
r = number of reactions*

Note: These calculations are made in the SkyCiv Truss software in the above Hand Calculation Module

So for example, in the following structure:

We have 9 members, 7 joints and 3 reactions. This fails both the internal and external stability check criterion:

9 is not greater than 2(7) – 3 = 11 for internal stability

(9 + 3) is less than 2(7) for external stability