Design a concrete girder bridge in 3D

A girder bridge is the workhorse of short and medium spans: a row of parallel girders carries the deck from support to support, cross-girders tie them together, and piers take the load down to the ground. This tutorial walks through the Girder bridge template in Struxt3D — how the structure works, how to set it up, what to look for in the results and how to design every part.

#How a girder bridge carries load

Load on the deck — its own weight, surfacing, people — first bends the deck slab across the girders. The girders then carry it along the bridge as beams. Because the template's girders run continuously over the piers, they hog (tension on top) over each pier and sag (tension at the bottom) in the spans, which makes them lighter than a chain of simple spans.

The cross-girders share load between girders, so a heavy load over one girder is spread to its neighbours. At the piers each girder sits on a short post on a cap beam, and the cap beam spans between two pier columns down to the foundations. At the abutments the girders rest on bearings.

Sideways, wind on the girders and piers is resisted by the deck acting as a deep horizontal beam, by the pier frames (two columns and a cap beam) and by the pinned bearings at the first abutment.

#Build the model

1. Pick the template. Open the app, choose Bridges in the gallery and click Girder bridge. Or use Open the template above.
2. Set the size. The How big is it? step lists the parameters in the table below. The 3D model rebuilds as you type. The number of cross-girder bays is behind More options.
3. Choose the materials. The template starts in concrete: girders, cross-girders and piers are reinforced concrete sections. Pick your design code first — it sets the concrete grades, the load combinations and the default wind speed. Then pick a section for Girders, Cross-girders and Piers, and set the Deck thickness under Slabs and walls (250 mm by default). You can switch the material to steel to get steel I-girders on steel piers.
4. Set supports and loads. The abutment bearings are fixed by the template (pinned at the start, rollers at the end). Under Bases choose whether the pier columns are Fixed or Pinned at the ground, and switch on Pad footings if you want footings designed under them. Set Finishes and services (surfacing, parapets spread over the deck), wind, and earthquake if needed.
5. Run the analysis. Click Run analysis or press F5.

#The template's parameters

Parameter	Default	Range	What it does
Spans	3	1–6	Number of equal spans; a pier between each pair.
Span length	30 m	15–50 m	Length of each span.
Girders	4	2–8	Parallel girders across the deck.
Girder spacing	2.5 m	1.5–4 m	Centre-to-centre distance between girders.
Deck height	8 m	4–25 m	Height of the deck above the pier bases.
Cross-girder bays per span	4	2–10	Cross-girders divide each span into this many bays (More options).

Each pier has two columns at a quarter of the deck width from each edge, a cap beam 1.2 m below the deck and a short post under every girder.

#Loads on the bridge

• Dead — self-weight of girders, cross-girders, piers and the deck slab, plus the finishes you set.
• Live — the deck carries the code's footbridge crowd load over its whole area (for example 5.0 kN/m² in Eurocode, 4.31 kN/m² in ASCE/AISC, 4.3 kN/m² in NSCP 2015).
• Wind +X, −X, +Y, −Y — the bridge has no cladding, so wind acts as drag on every member, with a force coefficient of 2.0 on the section width and the pressure growing with height.
• Seismic X and Y — when you switch on Earthquake, a response-spectrum analysis in both directions.

#What to check in the results

Once the analysis finishes, the workspace switches to Results. Work through these in the Show panel:

1. Deflected shape. Pick Displacement with the service combination (D + L). The spans should sag smoothly with the end spans moving most. Use Animate to see it move.
2. Girder moments. Pick Moment and the Envelope. Look for the hogging peaks over the piers and the sagging peaks near 40% of the end spans. The diagram is drawn on the tension side of each member.
3. Shear. Pick Shear: the largest values are in the girders right next to the piers.
4. Axial in the piers. Pick Axial: the pier columns are in compression; wind across the bridge adds compression to one column and takes it from the other.
5. Deck. Slab moment shows how the deck bends between the girders.
6. Reactions. The Load on the supports card and the Reactions table show what goes to the abutments and piers. The total vertical reaction should equal the bridge's weight plus the live load — a quick check that nothing is lost.

For more on reading these, see Reading analysis results.

#Design the bridge

Click Run design (F6). Struxt3D checks every member to the chosen code: the girders and cross-girders as reinforced concrete beams (bending, shear, deflection under D + L within span/250 by default), the pier columns as RC columns, the deck as a slab, and pad footings under the piers if they are on. With Size members for me on, it picks the lightest section in each group that passes.

• Switch the result to Utilization: members are coloured from blue (lightly used) to red (failing). Click a member to see its governing check, clause and combination.
• Reinforcement bars shows the designed bars in 3D for girders, piers, deck and footings.
• Calculation report writes the whole design with step-by-step calculations; select a girder first and use the inspector's report button to get the report for that one element.
• Shop drawings produces reinforcement drawings from the design.

The member checks are explained in RC beam design, RC column design and Slab design.

#Common problems and fixes

What you see	Why	Fix
Girders fail on deflection	Span too long for the section depth.	Use a deeper girder, more spans, or a shorter span length.
Pier columns fail in bending	Wind and the frame action of a tall pier.	Larger columns, fixed bases, or a lower deck height.
Deck shear ratio is high	The deck sits on the girder axes, so it shares the girders' bending and attracts in-plane force.	Read it as conservative; check the deck locally with a thicker slab if needed.
Large sideways movement	Only one abutment holds the deck in plan.	Expected for a long deck; check that the pinned bearings and piers carry the wind reactions.

#Rules of thumb

#What this model leaves out

• Moving traffic loads, braking, temperature, creep, shrinkage and prestress.
• Construction stages: the analysis assumes the whole bridge is built at once.
• The vertical offset between the deck and the girder centroids: deck and girders sit on one plane.
• Bearing stiffness and soil–structure interaction at the abutments.

Read Accuracy and limitations for what Struxt3D does and does not analyse.