Truss Analysis simulation
A 6-joint Pratt truss (A, B, C, D, E, F) with 9 members. Pin support at A, roller support at B. Two adjustable point loads P₁ at joint C and P₂ at joint D. Three analysis modes you can switch between.
Method of Joints (blue) Step through joint by joint. Each step highlights the current joint with a gold ring, shows the solved member forces, and explains what equilibrium equations are being applied.
The solve order is A → B → E → C → F → D, always starting at joints with supports where you have at most 2 unknowns.
Method of Sections (red) Step through a section cut. Step 1 draws a vertical cut line through members CD, ED, and EF (slicing exactly 3 members).
Step 2 highlights the cut members.
Step 3 explains how ∑M about E gives CD directly, ∑Fy gives ED, and ∑Fx gives EF. Three unknowns, three equations, done.
All Results (green) Shows every member force at once. No stepping. Instant overview of the full truss with all tension/compression values labeled.
Try this simulation yourself
The sim loads with a 6-joint Pratt truss carrying a single downward load at the bottom chord. Members are gray (unsolved). Support reactions are labeled at joint A (pin: Ax, Ay) and joint F (roller: Fy). The first thing to notice: pin gives two reactions, roller gives one. Three unknowns, three global equilibrium equations. The reactions solve before any member analysis begins.
Select Method of Joints (default tab). Tap the Step button once. Joint A highlights. The free body diagram of joint A appears: the two reaction forces (Ay upward, Ax horizontal) and the two unknown member forces along AB and AC. ΣFy = 0 gives the force in the vertical member. ΣFx = 0 gives the force in the horizontal member. Members turn blue (tension) or red (compression) as they are solved.
Tap Step again. The solver moves to the next joint that has at most two unknowns. This is the rule: you can only solve a joint where two or fewer member forces are unknown, because you only have two equations (ΣFx, ΣFy). The solver picks joints in the correct solvable order automatically. Watch the color and thickness propagate across the truss one joint at a time.
Keep stepping until all members are solved. The process table below the truss shows every member's axial force and whether it is in tension (T, blue) or compression (C, red). Top chord members are typically in compression. Bottom chord members are typically in tension. Diagonal members alternate. This pattern is universal for Pratt trusses under gravity loading.
Now switch to Method of Sections. A vertical cut line appears through the truss, slicing three members. The left portion of the truss is isolated as a free body. The three cut members have unknown forces shown as arrows along their axes. Taking moments about the point where two of the three unknowns intersect eliminates both and solves the third directly. This is the shortcut: Method of Sections can solve any single interior member without analyzing every joint first.
Move the load. Drag the load slider to shift the applied force from one bottom chord joint to another. Watch every member force change. When the load is at the center, the truss is symmetric and left/right member forces mirror each other. Move the load to one end and the far-side members carry almost nothing. This is how influence lines work: each member's force is a function of the load position.
Increase the load magnitude. Drag the force slider from 10 kN to 50 kN. All member forces scale linearly (trusses are linear systems under the assumptions of pin joints and axial loads only). The member thickness on the diagram grows proportionally. A member that was safe at 10 kN might exceed its buckling capacity at 50 kN, and compression members (red) always buckle before tension members fail. This is why compression members are designed with larger cross-sections.
Look for zero-force members. At certain load positions, some members carry exactly 0 kN and stay gray. A zero-force member occurs at a joint where two non-collinear members meet and no external load is applied. These members are not useless: they prevent buckling of long compression members and carry load under different loading conditions. But in the current configuration, they do nothing.