Design a tied-arch footbridge

An arch turns load into compression. A parabolic arch under an even load carries it in pure compression with almost no bending — which is why arches have spanned rivers in stone for two thousand years. The Arch footbridge template in Struxt3D is a tied arch (also called a bowstring): two parabolic steel ribs above the deck, the deck hung from them by vertical hangers, and the deck girders acting as the tie that holds the arch ends together.

#How a tied arch carries load

Deck load goes from the deck slab into the cross beams and deck girders, then up the hangers into the arch ribs. The ribs push outwards at their ends with a horizontal force called the thrust. In a free-standing arch the ground has to resist the thrust; in a tied arch the deck girders do, in tension. The bearings then carry only vertical load, so the bridge can sit on a pinned bearing and a roller like a simple beam.

Under load on half the span the arch is no longer the ideal shape for the load, and the ribs bend. That half-span live load usually governs the rib design. Sideways, the struts between the ribs (placed only where there is at least 3 m of headroom over the walkway) and the deck frame stop the ribs buckling outwards.

#Build the model

1. Pick the template: Bridges › Arch footbridge.
2. Set the size with the parameters below. The deck height is behind More options.
3. Choose the materials. Steel by default, with groups Arch ribs, Deck girders, Cross beams & struts and Hangers. Hangers are steel rods. Set the deck thickness under Slabs and walls.
4. Set supports and loads. The bearings come with the template: pinned at the start, rollers at the end. Set finishes, wind and earthquake.
5. Run the analysis (F5).

#The template's parameters

Parameter	Default	Range	What it does
Span	40 m	20–80 m	Distance between the arch springings.
Arch rise	8 m	4–16 m	Height of the arch crown above the deck.
Deck width	3.5 m	2–6 m	Distance between the two arch planes.
Hanger panels	10	6–20, even	Number of panels along the span; a hanger at each inner panel point.
Deck height	3 m	1–15 m	Deck level above the ground (More options).

#Check the thrust by hand

For an even load w (kN/m) on the whole span, the horizontal thrust of a parabolic arch is:

With L = 40 m, f = 8 m and w = 20 kN/m on the bridge, H = 20 × 40² / (8 × 8) = 500 kN, or 250 kN per rib. The tie force in each deck girder should be close to this under the full-span load. If the app shows something far off, look for a missing hanger or a wrong support.

#What to check in the results

1. Axial. Show Axial with the Envelope: ribs in compression (largest near the springings), deck girders in tension, hangers in tension.
2. Rib bending. Show Moment on the ribs. Under the full load it should be small; under the strength combinations with live load it grows. A large rib moment under dead load alone means the arch shape and the load don't match.
3. Deck girders. They carry tension and bend between hangers. Look at Moment near the ends, where the first panel is longest from a hanger.
4. Deflection under the service combination: a smooth sag of the whole bridge.
5. Modes (with earthquake on): the first lateral mode of the ribs is a useful sign of how well the struts brace them.

#Design the bridge

Click Run design (F6). The arch ribs are designed as steel members in compression and bending with buckling; the deck girders for tension and bending; the cross beams and struts as beams; the hangers in tension; the deck as a slab. With auto-sizing on, each group gets its lightest passing section.

Look at Utilization per segment: an arch rib is often critical only near the springing or at the quarter points. Then use the Calculation report, connection details at the hanger and strut joints, and Shop drawings.

#Common problems and fixes

What you see	Why	Fix
Ribs fail in buckling	Low rise means a large thrust; long unbraced rib out of plane.	A higher rise (span/5 to span/7) or a heavier rib.
No struts appear	The rib is never 3 m above the deck.	Increase the rise; without struts the ribs rely on the deck frame alone for lateral stability.
Deck girders fail near the ends	Tension plus bending in the end panels.	More hanger panels or a heavier girder.

#Rules of thumb