Running the analysis

Run analysis solves the model: it builds the loads, meshes the members and slabs, and finds the displacements, member forces, plate forces and reactions for every load case and combination. It does not check or size anything — that is Run design. Keeping them apart lets you look at how the structure behaves before you ask whether it is strong enough.

#How to run it

• Press F5 (or Ctrl+Enter) anywhere in the app.
• Click Run analysis at the end of the Supports & loads step.
• Click Run in the top bar or in the hint at the bottom of the Simple workspace.
• In the Full interface, choose Analyze › Run analysis.
• Click Results when there are no results yet.

When it finishes, the workspace switches to Results mode and shows the displacement under the service combination, and a message says how long the solve took, for example Analysis done in 240 ms. Run Design (F6) to check and size the members, slabs and foundations.

#Progress

The solver runs in a background worker, so the page stays responsive. A small card in the middle of the screen names the stage it is on:

1. Building loads — the load cases from your setup and any added loads.
2. Meshing — members are split at every node along them, and slabs and walls are cut into four-node shell elements.
3. Solving — the stiffness matrix is assembled and solved for all static load cases at once.
4. Finding the modes — only with Earthquake on.

Most models solve in under a second. Large models with many slabs, and earthquake runs, take a few seconds.

#What is solved

#Static load cases

Each load case is solved on its own as a linear elastic static analysis:

Case	Contents
Dead	Self-weight of members, slabs and walls, plus finishes and services
Live	The floor use's imposed load, roof or footbridge load, and any point and line loads you added (they go in Live unless you change their case)
Wind +X, −X, +Y, −Y	With Wind on: pressure on cladding or drag on open members
Earth, Wave	Template loads: earth pressure on a retaining wall, wave and current on a jacket

#Combinations

The code's strength combinations (ULS) are formed from the cases, plus one service combination, D + L (service), used for deflections and soil bearing. The envelope is the worst of all strength combinations at each point. Read Loads and load combinations.

#Earthquake

With Earthquake on, the analysis finds the first 12 vibration modes using the mass of the dead load plus the code's share of the live load, then runs a response-spectrum analysis in X and in Y. The modal responses are combined by CQC into two more cases, Seismic X and Seismic Y, which enter the seismic combinations. See Earthquake design with response spectrum analysis.

#The model the solver sees

• Members are 3D beam elements (six degrees of freedom per node), or axial-only bars for pin-jointed groups and cables.
• Slabs, walls and plates are four-node shell elements with bending and membrane action.
• Supports are fixed, pinned or roller restraints, or springs (soil under a raft).
• Member end releases are applied at the released ends.

The status bar of the Full interface shows the number of degrees of freedom and the solve time.

#When the analysis cannot run

Instead of results, a red message explains what is wrong. The common ones:

Message	What to do
The model has no members yet.	Pick a template or draw some members.
The model has no supports.	Add supports in the Supports & loads step or with the Support tool.
The structure is unstable: node … can move freely …	The model is a mechanism: some part can move without resistance. The message names the node, its coordinates and the direction. Add a support or bracing there, or connect the member to the rest.

#Typical causes of instability

• A member drawn to a point that is not joined to anything else.
• A frame with pinned bases and pinned beam ends, and no bracing, so it can sway.
• A member with both moments released at both ends but not torsion, free to spin about its axis.
• A whole structure on rollers only, free to slide.

The supports and releases tutorial explains how to make a structure stable.

#Messages

The Full interface's Messages tab lists warnings about the model before you run — no supports, a node joined to nothing, a member with no group — and notes from the analysis, such as:

• The 12 modes found reach 84 % of the mass in X … — codes ask for 90 % of the mass in each direction. The response is likely to be underestimated.
• The Sturm check counts … modes below … but … were found — some modes may be missing.
• No vibration modes were found (the model has no mass).

Click a message that names a member or node to select it.

#After the analysis

• Read the results on the model — see Viewing results.
• Run the design with F6 or the Run design button.
• Export the analysed model to OpenSees — see Saving, opening and exporting.

Any change to the model, its sections or its loads clears the results. Run again to see the new ones.