How StiffStruct works

1. Open the calculator. It reopens the model you were working on (autosaved in this browser), or starts with a small example beam.
2. Choose File → New model… or use the New model… button in Quick actions. You can also open app.html?new=beam directly.
3. Pick the structure type. Beam: members in a line, loaded across their length (shear, moment, deflection). Truss: pin-jointed bars that carry axial force only. Frame: rigid joints that carry axial force, shear and bending.
4. Pick a template. Start from a Blank model or from a parametric template (continuous beam, Pratt or Howe truss, portal frame, multi-storey frame, 3D tower…). The preview updates as you change the dimensions.
5. Set units and a name, then press Create. You land in the Model step.

1. Geometry. Type the total Beam length. To make a continuous beam, list the span lengths from left to right (for example 4, 6, 4) under Spans. Each span end becomes a joint.
2. Supports. Choose a type: Pin, Roller, Fixed, Guided, Spring or Hinge (an internal hinge for Gerber beams).
3. Position. Type a position or several, such as 0, 6, 12, or use the L M R buttons for the left end, the middle and the right end. L stands for the beam length, so L/2 and 0.25*L also work.
4. Press Add support. A Settlement can be added to a support if you need one.

The status line under the tabs says whether the beam is stable and statically determinate, and how many redundants it has.

Do you need a section and a material? Only for deflections and slopes.

• Reactions, shear force and bending moment do not depend on E, I or A when the beam has the same stiffness along its length. A statically determinate beam never depends on them; an indeterminate beam depends only on the relative stiffness of its spans.
• Deflection and slope need the real section and material.

1. Open the Sections tab and choose Forces only — no section needed if you only want reactions and the shear and moment diagrams. The material and section editors disappear, and the deflection numbers show “needs section”.
2. Or choose Real section & material, pick a Material and a Section (search the library of IPE, HEA, HEB, AISC W, HSS and pipe sections, or enter your own properties), and assign it to the whole beam or to part of the beam.

Forces-only mode switches itself off if you add something that needs stiffness, such as a spring support or the beam’s self-weight.

Press Member detail… under the charts for a closer look at one member.

• Four large diagrams: shear V, moment M, deflection δ and slope θ.
• Probe: drag the slider or type a position x to read every quantity at that point.
• The equations of the diagrams along the member and its points of interest.

Tables and pasting from Excel

1. The left panel lists Nodes, Members, Supports, Loads and Combinations as spreadsheet-like tables.
2. Click a cell to select it; click again or press F2 to edit. Cells accept expressions and units, such as 3*1.5 or 12 kN.
3. Paste rows from Excel with Ctrl+V. Copy a block of cells in your spreadsheet, select the first cell here and paste. Add appends a row; the copy and paste buttons under each table move whole tables.

Supports and member loads

• Supports: pin, roller, fixed, or custom restraints per degree of freedom (tick ux, uy, rz…). Springs, inclined rollers and prescribed settlements are available.
• Member loads: point and distributed loads, in global or local axes, plus temperature and fabrication-error loads.
• Releases: mark a member end as pinned (for example a truss bar inside a frame) with the release columns, or tick Truss for an axial-only bar.

1. Click the unit button in the header (for example SI (kN, m)) and choose a system, or choose Custom… to set a unit for each quantity separately.
2. You can change units at any time. All values are stored in SI internally, so changing the display never changes a result.
3. You can also type units in any field (12 kN, 3 ft, 250 mm). A number without a unit is read in the current display unit.

1. Add a case. In the beam wizard use the + next to the load case selector; in the other modes use the load case table. Give it a name and a category (dead, live, wind, snow, temperature…).
2. Add loads to the active case. New loads go into the case that is selected. Tick Include self-weight to add the weight of the members to a case (this needs a section and a material).
3. Create combinations. Open the Combinations table. Add a row and type a factor for each case, or press Generate… to create the combinations of a design code.
4. Read the results. The Case menu at the top right switches between a single case, a combination and the Envelope of everything, which shows the largest and smallest value at every position.

1. Press Code in the step bar and choose a tab: Python, MATLAB / Octave or JSON model.
2. Include library puts the whole solver into the file so it runs on its own. Plots adds the diagrams.
3. Run in browser runs the Python script on the page, with nothing to install, and shows its printed report and plots. The first run downloads the Python runtime (Pyodide) once from a public CDN, so it needs an internet connection.
4. Copy or Download the file, then run it on your computer: pip install numpy matplotlib, then python the_file.py. The script prints the step-by-step report and writes CSV files and PNG plots next to it.

1. Before you sign in you can draw the structure, add loads and press Analyze. You see the shape of the shear force, bending moment and deflection diagrams.
2. To see the values (reactions, extremes, tables, the step-by-step report and the code export), choose Sign in in the header or the card in the results panel, then Create free account.
3. Sign up with an e-mail address and a password, or continue with Google. The account is free and needs no payment details; it shows the diagram shapes. To see the values (reactions, diagram values, tables), start the 3-day free trial of Student or Pro.
4. After signing in, the screen updates at once. You stay signed in on this device until you choose Sign out from the account menu.

• Autosave: the model is saved in this browser after every change (the status bar shows Saved). It is never uploaded. Clearing your browser data removes it.
• Save JSON / Open JSON: download the model as a small file and open it later or on another computer. Ctrl+S saves.
• Copy share link: the model travels inside the link, so anyone who opens it gets a copy. Nothing is stored on a server.
• Project info: add a title, description, author and project number; they appear in the report.
• Undo and redo: every edit can be undone with Ctrl+Z.
• Settings: the gear icon sets the number of decimals, the theme (auto, light or dark) and the text contrast. The same choices apply on every page of this site.

Open a model from a link

These addresses start the calculator in a given state, which is handy for teaching and for bookmarks:

• app.html?new=beam starts a new beam (also truss2d, frame2d, truss3d, frame3d).
• app.html?example=frame-portal-lateral opens a worked example.
• Add &mode=results, &mode=report or &mode=code to jump to that step.