Analysis Types
AutoCalcs offers different analysis methods to suit various structural problems. Choosing the right analysis type is crucial for obtaining accurate results.
Linear Analysis
Best for: Most standard structures, beams, frames, and trusses where deformations are small.
This is the default analysis type. It assumes:
- Small Displacements: The structure's deformed shape does not significantly affect the equilibrium equations.
- Linear Material Behavior: Materials follow Hooke's Law (stress is proportional to strain).
Tension/Compression Only Support
Although termed "Linear", this solver is capable of handling Tension-Only or Compression-Only members and supports. It performs an iterative process to deactivate members or supports that are acting in their forbidden direction (e.g., a cable in compression).
P-Delta Analysis (Second Order)
Best for: Tall buildings, slender columns, and flexible structures where sway is significant.
P-Delta analysis accounts for geometric nonlinearity. It considers the additional forces and moments created when vertical loads (P) act on the laterally displaced (Delta) position of the structure.
Key effects captured:
- P-δ (small delta): Effect of axial loads on the bending stiffness of individual members (e.g., beam-column effect).
- P-Δ (large Delta): Effect of vertical resultants acting on the sway of the entire frame.
Note: This is an iterative non-linear analysis and may take longer to compute than Linear analysis.
Comparison Guide
| Feature | Linear | P-Delta |
|---|---|---|
| Equilibrium Formulation | On undeformed shape | On deformed shape |
| Stiffness Matrix | Constant | Updated iteratively |
| Superposition | Valid | Not Valid (Loads must be combined before analysis) |
| Use Case | Standard steel/concrete frames | Slender structures, drift-sensitive designs |
Which one should I use?
Start with Linear Analysis. It is faster and sufficient for the vast majority of low-rise structures.
Consider P-Delta if:
- You are designing a Moment Resisting Frame (MRF)
- Your structure has slender columns
- Lateral drift is a governing design criteria
- Axial loads are a significant fraction of the buckling load