A deck truss carries its roadway or railway above the main truss members. The deck transfers load through its supporting system to panel joints, and the truss carries those loads to its bearings. The illustration here uses one eight-panel Pratt truss in side elevation.
Deck Bridge Trusses Interactive Calculator
Change span, chord-centroid depth and the uniform load allocated to one truss. See a supported eight-panel deck truss, joint-load transfer, tension/compression colors and the bending-moment diagram. The animation varies the applied load while the truss retains its undeformed shape.
Equation Used
- Straight, rigid, pin-connected two-force members; one pin support and one vertical roller reaction.
- Gravity loads act at top panel joints; end joints take half a panel tributary load.
- No joint eccentricity, member bending, deflection, buckling, fatigue or connection capacity is calculated.
- Lateral load distribution to this truss has already been done before entering w.
This is an ideal planar statics model, not a bridge capacity or member-sizing check. Enter load already allocated to one truss; include any self-weight allowance in that load. The drawing uses separate horizontal and vertical scales.
A continuous load path below the deck
The deck sits above the top chord. Vertical posts and diagonals connect the upper and lower chords, and the end joints are supported by a pin and a roller. Every member in the drawing terminates at a connected joint; the support symbols are attached to actual end joints.
Blue-green members carry tension, brown members carry compression and gray members have zero axial force for this particular symmetric load. Those signs are calculated from joint equilibrium. The structure remains straight while the animation increases and decreases the displayed loading. Deflection is not calculated.
Explore span, depth and allocated load
The input w is the uniform load assigned to this one truss, not automatically the total load on an entire multi-truss bridge. Loads from the deck are distributed to nine top joints using tributary lengths. Each internal joint receives wL/8 and each end joint wL/16.
The drawing enlarges or compresses its vertical scale when needed to keep the members distinguishable. Calculations always use the entered dimensions.
Equilibrium of the illustrated truss
The symmetric vertical reactions are wL/2 each. Maximum bending moment is wL²/8 at the middle panel joint. For this eight-panel Pratt arrangement, the largest top-chord compression equals that moment divided by the chord-centroid depth h.
A separate joint-equilibrium solution calculates the axial force in all 33 members. With 18 joints and three reactions, there are 36 unknowns and 36 horizontal/vertical equilibrium equations. The displayed panel-load moment diagram joins the moments at panel joints with straight segments.
M/h is not a universal formula for every individual member of every truss. Geometry, support positions and loading must match the analysis.
72-metre span, 9-metre depth
With 180 kN/m assigned to the illustrated truss, total applied load is 12,960 kN. Each support reaction is 6,480 kN. The centre moment is 116,640 kN·m, giving a peak top-chord compression of 12,960 kN, or 12.96 MN.
Doubling depth at the same span and load halves that chord force. Doubling load doubles all member forces and reactions. These trends do not establish a suitable steel section or a safe bridge load.
What remains outside this calculation
The model does not select member sizes or connection details. A complete bridge analysis includes lateral load distribution, local deck and floor-system behavior, member and connection strength, buckling, fatigue, deflection and applicable load combinations. No universal economical span or prescribed plate thickness is inferred from this simple force calculation.
Deck-truss questions
Why does the bridge not visibly bend?
Deflection requires member stiffness and section properties, which are not inputs here. The moving load arrows and moment diagram show the changing applied load.
Does gray mean a member can be removed?
No. It only means zero axial force in the stated loading model; another load pattern can engage it.
Is this the load on both side trusses?
No. Enter the load already assigned to the single truss being analyzed.
Does the chord-force result size the bridge?
No. It is an ideal force result, without strength or stability checks.
References
Baker and Haynes, Engineering Statics: Trusses, pin-jointed truss assumptions, joint equilibrium and zero-force members. FHWA, Bridge Geometry Manual (2022), truss elevations and panel geometry. Hiscox, Mechanical Movements (1901), page 357, historical deck-truss illustrations. The calculator shows an illustrative Pratt arrangement rather than reproducing one historical bridge.
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