Design a steel truss bridge

A truss bridge carries load by triangles. Instead of one deep girder resisting bending, a top chord in compression and a bottom chord in tension form a couple, and the web members carry the shear between them. This makes trusses light and stiff for their weight. The Truss bridge template in Struxt3D is a through bridge: two Pratt trusses on either side of the deck, with the walkway or roadway running between them at bottom-chord level.

#How a Pratt truss works

In a Pratt truss the verticals are in compression and the diagonals slope down towards the middle of the span, so under gravity they are in tension. That suits steel well: the long diagonals are the members that would buckle if they were compressed, and in a Pratt they are pulled straight instead.

The deck load reaches the trusses through the floor system: the deck slab spans onto stringers running along the bridge, the stringers onto floor beams at every panel point, and the floor beams into the bottom-chord joints. Loading the trusses only at their joints keeps the chords and webs mainly in axial force.

Sideways, wind is carried by two horizontal trusses: wind bracing between the top chords (struts plus X-braces) and X-bracing in the floor plane. The top-chord bracing also holds the compressed top chord against buckling sideways.

#Build the model

1. Pick the template. In the gallery choose Bridges › Truss bridge.
2. Set the size with the parameters below. Stringers are behind More options.
3. Choose the materials. The template is steel. Choose the code and the steel grade, then a section for each group: Chords, Web members, Floor beams & stringers and Wind bracing. Set the Deck thickness under Slabs and walls.
4. Set supports and loads. The bearings are part of the template: pinned at both trusses at the start, rollers at the end. Set the finishes (surfacing on the deck), the wind speed and, if needed, the earthquake inputs.
5. Run the analysis with Run analysis or F5.

#The template's parameters

Parameter	Default	Range	What it does
Span	40 m	20–80 m	Distance between the bearings.
Panels	8	4–16, even	Number of truss panels along the span (kept even so the diagonals meet at midspan).
Width between trusses	5 m	3–10 m	Clear width of the deck between the two trusses.
Truss depth	5 m	3–10 m	Height from bottom chord to top chord.
Stringers	2	0–4	Longitudinal beams between the trusses (More options).

Web members and wind bracing are analysed as pin-ended truss members. Chords, floor beams and stringers are continuous frame members, so they also pick up the bending that a real riveted or welded truss has.

#Loads

• Dead: self-weight of all steel and the deck slab, plus finishes.
• Live: the code's footbridge crowd load on the whole deck.
• Wind ±X, ±Y: drag on every member (force coefficient 2.0 on the section width). Wind across the bridge (±Y) is the case that loads the wind bracing.
• Seismic: response spectrum in X and Y when switched on.

#What to check in the results

1. Axial forces. Show Axial with the Envelope. The top chord should be in compression, largest at midspan; the bottom chord in tension, also largest at midspan; diagonals largest near the supports, where the shear is.
2. Deflection. Show Displacement with the service combination. The midspan should sag smoothly. A kink or one joint dropping alone points to a missing member or a wrong release.
3. Floor beams. Show Moment: the floor beams bend like simple beams between the trusses.
4. Wind bracing. Pick the Wind +Y load case and show Axial: the X-braces in the top and floor planes take the wind back to the bearings.
5. Vibration modes. With earthquake on, the Vibration modes list shows the periods. For a footbridge the first vertical and lateral frequencies are worth noting: pedestrian comfort guides commonly ask for vertical frequencies above about 5 Hz and lateral above about 2.5 Hz, or a dynamic check.

A quick hand check: midspan chord force ≈ M / d, where M = wL²/8 for the whole bridge's share per truss and d is the truss depth. If the app's chord force is in the same range, the model is behaving.

#Design the bridge

Click Run design (F6). The chords are checked for compression with flexural buckling and for combined axial force and bending; the webs and braces for tension and compression buckling; floor beams and stringers as beams with lateral-torsional buckling; the deck as a slab. With Size members for me on, each group gets the lightest section that passes.

With Steel connections on in Design settings…, Struxt3D also designs the joints: gusset plates for the truss members, end plates and fin plates for the floor beams. Show Steel connections to see them in 3D, and click a joint to open its checks and dimensioned sketch. See Steel connection design.

Use Calculation report for the full design and Shop drawings for the steel drawings.

#Common problems and fixes

What you see	Why	Fix
Top chord fails in buckling	Long unbraced length sideways.	More panels (shorter chord lengths) or a heavier chord section.
End diagonals fail	The end posts and first diagonals carry the full support shear.	A deeper truss reduces chord forces; for the ends, a heavier web section.
Deflection too large	Truss too shallow for the span.	Increase the truss depth to about span/8–span/10.
Analysis reports an unstable model	A truss member edited by hand left a joint free to rotate or move.	Check the edited joint; see Supports and releases.

#Rules of thumb

#What this model leaves out

• Moving loads, fatigue and impact.
• Second-order effects and the eccentricities of real gusset joints.
• Pedestrian-induced vibration analysis (the modes are shown; the comfort check is yours).

See also Accuracy and limitations.