A word from the engineer
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A preliminary bridge design in your browser: length (up to 1 km), deck width or lanes, spans and girder type in, and the tool draws the bridge in an ink-wash landscape and returns quantities, an approximate cost, carbon and a construction programme. Open the detailed check for an AASHTO LRFD check of the girder, the pier and the foundation, or the Detail programme to watch the bridge being built. A sketch for design intuition, not for construction.
Preliminary bridge design is the stage where the big decisions are made (how many spans, what girder type, how deep the deck, what kind of pier and foundation) before anyone draws a reinforcing bar. Most of the cost and most of the carbon are locked in here. This free online bridge design tool runs that stage in a few seconds so you can compare options instead of guessing: change the length, the deck width, the soil class or the girder type and watch the drawing, the quantities, the cost and the carbon badge move.
Deck width. Type the deck width directly, or let the lane count set it. Footways on either side, a shared path, light-vehicle lanes, one or two LRT or metro tracks with their overhead line, and a median take their share; the tool recommends the number of traffic lanes from what is left (3.65 m lanes, shoulders of at least 0.6 m) and you can lower it by hand. Footways carry the AASHTO pedestrian load of 3.6 kPa and add their build-up and parapets to the dead load; light-vehicle lanes and LRT tracks sit inside the vehicular zone and are loaded as HL-93 design lanes, which is conservative for both. Heavy freight rail is not covered.
Up to 1 km. The total length goes up to 1,000 m and 60 spans. A long continuous deck is split into continuity units with an expansion joint between them (about 400 m for precast girders, 600 m for boxes), and the joints become modular where a unit is long. The detailed check runs on the first unit, which holds the end span; a long run of simple spans is checked on its first three spans.
Detail programme. The Detail programme button opens the construction schedule on a page of its own. A few decisions come first, as in the bridge game: the start month, the contract (construction only or design & build), the climate, the working pattern, piling rigs, pile load tests, substructure crews, a second erection front, form-traveller pairs, an own precast yard or a supplier, local or overseas steel and road possessions. Each option shows what it does to the finish date and to the project budget. The programme then shows the whole project and its phases (foundations, substructure, superstructure, paving and finishing) as bars that fill as the time cursor moves, an animation of the bridge being built in the order of the programme, and a bar chart with one bar per support or span and the critical path in red. It feeds back into the design: the build duration, the time-related preliminaries, plant and crews beyond the standard set, and the escalation to the works midpoint all come from it.
How the programme is planned. As on most sites, the substructure is got well ahead and the superstructure then runs without stopping: the launching gantry, MSS, floating crane or travellers arrive only when every pier will be finished a few weeks before they reach it, and the precast yard or the steel fabricator is timed to keep just ahead of the erection. Starting the superstructure as early as possible is offered as a decision; it finishes a little sooner but the plant stands and waits, and that standby is priced. Yard and plant set-up count as preparatory work; the superstructure starts with casting or fabrication. Productivities allow for public holidays, the first spans being slower and the curing and storage times of concrete. A major plant table lists the rigs, cranes (sized from the girder or segment weight and the lifting radius), gantries, travellers, barges, tugs and boats the programme needs, how many, when they are on site and roughly what their hire costs. Under the months the time axes also count the days from the start.
| Girder type | Span range | Checks (AASHTO LRFD, 10th ed.) |
|---|---|---|
| PSC I-girder, PSC bulb-tee / wide-flange, PSC U-girder | 20 – 55 m | strand layout, losses, transfer and service stresses, Strength I flexure and shear, camber; AASHTO Types I–VI, PCI bulb-tees and generic metric wide-flange sections |
| PSC adjacent box beams / voided slabs, PSC spread box beams | 8 – 32 m | AASHTO box beams BI–BIV and 1.2 m voided slabs; adjacent-unit (f/g) and spread-box (b/c) distribution factors, shear keys and CIP topping |
| PSC box (constant depth) | 35 – 80 m | tendons per web, secondary moments, principal web tension, shear, deflection |
| Balanced cantilever box (FCM) | 80 – 200 m | cantilever stages, top and bottom tendon groups, pier and midspan stresses |
| Steel I composite, steel twin-girder ladder deck, steel tub / box girders | 30 – 150 m | staged stresses (bare steel → 3n → n), flange and web slenderness, shear connectors, fatigue, stiffeners; ladder decks with the rigid-cross-section distribution and a cross-girder check |
| RC slab, RC T-beam | 6 – 30 m | strip widths, flexure, shear, crack control, bar fit |
| Substructure | all | column P–M interaction with slenderness (Strength I, III, IV, V, Service I), cap beam, spread footing or pile cap with pile loads, seat abutment stability and sections |
| Section details | all | reinforcement and prestressing sketches drawn from the check: sections at midspan and at the support with strands, ducts, bars and stirrups, a profile elevation (harped strands, tendon drape, cantilever tendon groups, plate changes, stud pitch, stirrup zones) and a bar & strand schedule with masses |
It is a concept-level sketch. There is no site data in it: soil is a class you pick, wind is the code map, there is no seismic, scour or vessel-collision load, and every unit rate is a placeholder. Use it to compare options and to size a first proposal; a qualified engineer redoes every line before anything is built.
Enter the total length (up to 1 km), what the bridge crosses, the deck width or the number of lanes and the soil class, then choose a girder type. The sketch updates as you move the sliders. Press Detailed check to run the AASHTO LRFD checks on the girder and the substructure; use Apply to sketch to take the checked sizes back into the drawing, and Compare to set a second girder type against the first.
The AASHTO traditional minimum depths: about L/18 – L/22 for prestressed I-girders (0.045 L simple, 0.040 L continuous), L/25 – L/35 for steel composite girders, L/15 – L/20 for RC T-beams, and a haunched box at L/19 at the pier and about 0.42 of that at midspan for balanced cantilever construction. The check then deepens the section when the rule of thumb is not enough.
It depends on the pier reaction and the pile capacity. The tool works out the factored reaction from the girder analysis (dead load plus the true live-load total, lanes × multiple presence), adds the pier's own weight, and divides by the pile resistance from skin friction and end bearing for the soil class. A typical four-lane, 40 m span pier ends up on 4 – 6 piles of Ø1.2 – 1.5 m in dense ground and 8 – 12 piles of Ø1.0 m in soft clay, laid out in two or three rows along the pier; the check also carries the longitudinal and transverse moments into the group. With a depth to rock entered, the piles end 1.5 diameters into the rock and are designed as rock sockets (AASHTO 10.8.3.5.4: overburden shaft plus socket side resistance and 2.5 qu at the tip, never more than the pile's structural working load), so the count drops at the rock line rather than slowly with length, which is how piles are really chosen where rock is within reach.
A portal frame (two columns and a cap beam) suits decks up to about 20 m wide: it uses less concrete than a wall, its foundation is compact and it lets the transverse live-load eccentricity work as push–pull in the columns. A wall pier is the choice in rivers, where the streamlined shape and the stiffness against ship or debris impact matter; it is heavier and its pile group runs the full length of the wall.
With the placeholder GCC rates in the tool (materials plus erection and temporary works, preliminaries & general and overhead & profit, i.e. the construction contract value), typical results per m² of deck are about 1,000 – 1,500 USD for prestressed concrete girder bridges, 1,300 – 2,300 USD for cast-in-place and balanced-cantilever boxes and 1,800 – 2,800 USD for steel composite; the site's Budget estimator, which uses world-average benchmark rates, sits 10 – 30 % above these for concrete types. A marine site (piers standing in the sea) roughly doubles the construction share: cased piles through the water, cofferdams or precast cap shells, floating cranes or a launching gantry, a trestle in shallow water, splash-zone protection and weather standby, and it adds 30 – 50 % to the programme. Site conditions, foundations and the market move any of these by a factor of two either way. The cost panel shows materials, construction (erection and temporary works by the chosen method: crane, launching gantry, floating crane, falsework, MSS, incremental launching, precast segmental or balanced cantilever), direct and indirect cost, the construction contract value, then fees, contingency, escalation and VAT to a total project budget (the owner's side adds about 35 – 45 % on top of the contract) and a benchmark verdict (reasonable / low / high) of the contract value per m² against world-average rates. The split and the verdict are usually more useful than the total.
It picks one from the girder type and the site and prices the erection and temporary works from quantities for that method: precast girders by crane on land, by launching gantry over a road or river and by floating crane at sea; cast-in-place slabs and T-beams on falsework, or on staging over water; boxes on falsework, a movable scaffolding system, incremental launching (with temporary piers at mid-span for 60 to 100 m spans), precast segments span by span or whole precast spans placed by an overhead gantry on long viaducts; balanced cantilever with form travellers or precast segments; steel girders by crane, by crane onto temporary bents for spans beyond one lift, by launching or by floating crane; spliced precast girders (segments on temporary towers, post-tensioned continuous) for 45 to 100 m spans; and, for one or two spans over a live road or railway, the span built beside the crossing and slid or carried in by SPMTs during one possession. The same methods, with the same cycle times, are in the Methods guide and the Bridge Game. The Layout tab lets you override the method, and the Duration tile names the one in use. The construction group is never allowed below a floor share of the material cost (30 – 45 % by type, more over water), so a cheap-looking site cannot hide the erection cost.
Yes, as sketches. Below the substructure report a "Section details" panel draws the superstructure section at midspan and at the support with the strand pattern on its 50 mm grid (harped strands raised at the ends), the post-tensioning ducts in each web, deck bars from the strip check, stirrup loops and spacing zones, and, for steel, the plates, shear studs and stiffeners; a profile elevation shows the strand or tendon drape, the cantilever tendon groups of a balanced-cantilever box, the plate changes and stud pitch of a steel girder and the stirrup zones; a bar and strand schedule gives counts, lengths and masses per girder and per square metre of deck. Cover, laps, anchorage zones and deviators are not designed: it is a preliminary drawing for a design intuition, not a construction drawing.
Yes. Type the deck width in the Site tab, open "Footways, rail & other users" and set the footways, a shared path, light-vehicle lanes, LRT tracks and the median. The tool recommends how many traffic lanes fit in what is left and warns when the deck is too narrow; the Section view and the strip under the sliders show how the width is shared out. Footways are loaded with the AASHTO pedestrian load (3.6 kPa), light-vehicle lanes and LRT tracks as design lanes.
The Detail programme works it out from the quantities and typical productivities: about 12 to 14 months for a 120 m precast girder bridge on land, 17 to 22 months for a 400 m box or balanced-cantilever viaduct and around two years for a 1 km precast viaduct. Piling and the substructure crews usually drive the first half and the erection method the second; more rigs, more crews or a double shift shorten it at a cost that the decision cards show before you choose.
Yes. Under the carbon breakdown the A1–A5 total is converted to money in two European ways: the shadow cost of carbon that the European Commission asks for in the cost-benefit appraisal of infrastructure (for example €199 per tonne for works in 2027, in 2016 prices) and the EU ETS market price (about €82 per tonne in the third quarter of 2026, the CBAM certificate price). You can type your own price as well. The result is shown next to the budget and in the carbon advisor notes, so a saving in tonnes reads as a saving in money; it is not added to the budget.
No. It is a preliminary check with class-based soil values, code-map wind and no seismic or scour loads, meant for design intuition and for comparing concepts. Every figure has to be redone by a qualified engineer with site data before construction.