Design a cable-stayed footbridge

A cable-stayed bridge hangs its deck from straight cables running to a tall pylon. Each stay acts as an elastic support for the deck, so a long span behaves like a continuous beam on many short spans. The Cable-stayed footbridge template builds two equal spans on either side of a central H-pylon, with stays in a semi-fan arrangement anchoring to edge girders.

#How a cable-stayed bridge carries load

Load on the deck goes into the edge girders and cross beams, then up the stays to the pylon. Each stay pulls the deck up and towards the pylon, so the deck is compressed along its length, most strongly near the pylon where all the horizontal stay components add up. The pylon is compressed by the vertical components of all the stays and carries the deck's weight to the ground.

Because the two spans are equal, the stays on each side balance each other at the pylon top under symmetric load. Load on one span only — a crowd on one side — unbalances them and bends the pylon.

#Build the model

1. Pick the template: Bridges › Cable-stayed footbridge.
2. Set the size with the parameters below; the deck height is behind More options.
3. Choose the materials. Steel by default. The groups are Girders (edge girders), Cross beams, Pylon (tubes) and Cables. Cables are always steel rods, whatever material you choose for the rest. Set the Deck thickness under Slabs and walls.
4. Set supports and loads. The deck bearings are pinned at the start and rollers at the end. Choose Fixed or Pinned for the pylon bases; fixed is usual. Set finishes, wind and earthquake.
5. Run the analysis (F5).

#The template's parameters

Parameter	Default	Range	What it does
Span each side	30 m	15–60 m	Length of each of the two spans; the whole bridge is twice this.
Deck width	3 m	2–6 m	Distance between the edge girders.
Pylon height above deck	20 m	8–45 m	Height of the pylon top above the deck.
Stays per side	5	2–12	Stays on each side of the pylon, in each of the two cable planes.
Deck height	5 m	2–20 m	Deck level above the pylon base (More options).

The pylon's two legs stand 0.6 m outside the deck edges, joined by a cross beam at the top and, when there is room, a strut at the lowest stay anchor. The stays anchor 0.3 m below the top and up to 1.2 m apart down the legs; the deck anchors are evenly spaced along each span.

#What to check in the results

1. Stay forces. Show Axial. Every stay should be in tension in every combination. The longest stays usually carry the most.
2. Deck compression. Still on Axial, look at the edge girders: compression grows towards the pylon.
3. Girder bending. Show Moment. Between stay anchors the girders bend like a continuous beam. The longest unsupported pieces — at the abutments and at the pylon — bend most.
4. Deflection. Show Displacement with the service combination. The deck dips between the anchors and the pylon top moves towards the more loaded span.
5. Pylon. Show Moment and Axial on the legs, and check the cross beams that tie them.

Struxt3D also does not model cable sag, the stiffening of cables under tension, or the pre-tension that real stays are set to so the deck sits level under dead load. The deck therefore sags under its own weight in the model where a real bridge would be adjusted to its design profile.

#Design the bridge

Run Run design (F6):

• Cables are checked in tension only: N / (φ A f_y), with the clause of your code (for example EN 1993-1-1 6.2.3 or AISC 360 D2(a)).
• Edge girders and cross beams as steel beams with axial force, bending and lateral-torsional buckling.
• Pylon legs as steel tubes in compression and bending.
• Deck as a slab, when slab design is on.

Auto-sizing picks the lightest section in each group that passes. All stays share one section, so the most loaded stay sets the size. Then use Calculation report, the element report of the most loaded stay, and Shop drawings.

#Vibration

Footbridges are light, and people walking can make them sway or bounce. Switch on Earthquake to get the Vibration modes, and note the first vertical and lateral frequencies. Typical guidance asks for a dynamic comfort check when the vertical frequency is below about 5 Hz or the lateral one below about 2.5 Hz. Struxt3D shows the frequencies; the comfort check itself is not part of the design.

#Common problems and fixes

What you see	Why	Fix
Girders fail in bending near the abutments	The last stay is far from the bearing.	More stays per side.
Pylon fails in bending	Unbalanced live load, or wind on a tall pylon.	A heavier pylon tube, fixed bases, or a lower pylon.
Stays very highly used	Stays are flat (low pylon), so they need a large force for their vertical share.	A taller pylon: stays at 25° or more to the deck work well.
Large deck deflection	No pre-tension in the model, and slender girders.	Deeper girders; read the dead-load sag as an upper bound.

#Rules of thumb

Compare with the arch footbridge, which carries the same deck by arching instead of hanging it from a pylon.