A bridge truss uses connected triangular panels to carry loads to its bearings. This interactive example shows a simply supported Pratt-style side truss, with deck loads applied at the bottom joints. Change span, depth, load and panel count to compare a beam estimate with the axial forces calculated for the illustrated members.
Bridge Truss Interactive Calculator
Explore one side truss under uniform deck load delivered at panel points. Autoplay highlights each member and its force while the structure remains stationary. The beam estimates and the illustrated joint-equilibrium calculation are labeled separately.
Equation Used
- Ideal 2D pin joints, straight members and concentric loads at panel points.
- Left pin and right vertical roller reaction; no imposed displacement.
- Entered line load applies to one side truss; no automatic two-truss split or load factors.
- Member self-weight is omitted unless represented in the entered deck load.
- Vertical drawing scale is labeled; calculations use the actual span and depth.
- No section capacity, buckling, connection, fatigue, deflection or lateral-stability calculation.
Deck loads enter at the bottom joints and pass through the members to the pin and roller supports. Member colors show tension or compression from joint equilibrium. The end-post force excludes the half-panel load delivered directly to the bearing; M/h gives a separate beam estimate.
How the illustrated bridge truss carries load
The deck delivers vertical loads to the bottom-chord panel points. The connected members transfer those loads to a pin bearing on the left and a roller bearing on the right. Each ideal member carries axial tension or compression. Blue and orange identify the calculated sign; gray marks a zero-force member under the selected symmetric load.
The sloping end posts close the end triangles. Interior diagonals descend toward the center in this Pratt-style layout. With an odd number of panels, the middle panel has one diagonal; under the symmetric load it can be a zero-force member. Autoplay highlights members in sequence; the structure stays stationary because displacement is not calculated.
What the drawing represents
This is one side truss, not a complete three-dimensional bridge. The deck strip and bearing blocks provide visual context. Cross-bracing, floor beams, member sections and connection details would need separate design. The display expands or compresses the vertical scale to keep the truss readable; dimensions and force calculations retain their entered values.
Where truss bridges are used
Truss arrangements are used for transport crossings and covered bridges. Their panel geometry, load placement and joint construction matter. Pratt, Howe and lattice arrangements differ, so a force result from this one illustrated layout should not be applied unchanged to another bridge.
The calculation behind the picture
For span L, depth h, panel count n and line load w on one side truss, panel length a=L/n. Each support reaction is R=wL/2. The deck places wa at every interior bottom joint and wa/2 at each end joint.
The displayed end-post compression is (R−wa/2)/sin(atan(h/a)). The load already delivered directly to the bearing joint does not pass through the end post.
The beam comparison uses M=wL²/8 and F=M/h. Actual colored member forces are found independently by solving horizontal and vertical equilibrium at every joint. This distinction explains why a discrete truss chord need not equal the continuous-beam estimate exactly.
Worked example using the default inputs
With L=60 m, h=3 m, n=5 and w=1 kN/m, a=12 m. Interior bottom joints receive 12 kN and end joints receive 6 kN. Each bearing reaction is 30 kN.
The continuous-beam moment estimate is 450 kN·m and the M/h chord estimate is 150 kN. The left end post instead carries (30−6)/sin(atan(3/12))=98.95 kN in compression. These are force calculations for the idealized geometry, not a choice of steel section or a load rating.
Compare geometry without confusing force and capacity
Increasing depth lowers the M/h chord estimate at fixed span and load. Increasing span raises that estimate quadratically when depth and load stay fixed. Panel count changes nodal load, diagonal slope and individual member forces while leaving the continuous-beam moment estimate unchanged.
Those trends do not establish an optimum bridge. Member length, buckling, connections, floor-system weight, clearances and construction all affect the finished design and are outside this calculator.
Questions about this model
Does the load describe the whole bridge?
No. Enter the load carried by the one side truss shown. Distribution from a complete deck to multiple trusses is not calculated here.
Why does the bridge not flex during autoplay?
The model calculates forces, not displacements. Autoplay highlights real members and their calculated axial force while keeping the geometry stationary.
Why can a gray diagonal have no force?
A member can have zero axial force for this particular symmetric load case. That does not mean it is unnecessary under other load positions.
Can these outputs select a member size?
No. The model does not check section properties, buckling, fatigue, connections or bridge loading rules.
References and model basis
- FHWA Covered Bridge Manual, Chapter 4: truss types — arrangement and historical distinction of Pratt and Howe systems.
- FHWA Covered Bridge Manual, Chapter 12: force and stress analysis — ideal axial-force assumptions and the effects of real joint details.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
