Free-cantilever-method PSC box girder, three-span continuous. Drag the main span and the side-span ratio — the elevation, depths and proportioning checks update live against typical built-bridge bands. A first-sizing aid for layout, not a code check.
Free-cantilever-method PSC box girder, three-span continuous. Drag the main span and the side-span ratio — the elevation, depths and proportioning checks update live against typical built-bridge bands. A first-sizing aid for layout, not a code check.
| # | Check | Formula | Value | Typical band | Status |
|---|
The pier-table depth-to-span and midspan depth-to-span ratios here are the ones owners' engineers check first in review — land inside them and the conversation is easier. Remember the governing case for the pier is often during construction, one segment out of balance, not the finished bridge.
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A free-cantilever bridge is a three-span (or multi-span) continuous prestressed box girder whose superstructure is built from the pier tables outwards. During construction each pier carries two symmetric cantilevers; the structure is therefore statically determinate for its own weight until the closure segments make it continuous, and the design must follow both states.
| Item | Typical range | Why |
|---|---|---|
| Main span L | 70–250 m | economic band for PSC boxes; longer spans move to steel or hybrid sections |
| Depth at pier | L/16 – L/20 | cantilever moment ≈ w L²/8 over the pier during construction |
| Depth at midspan | L/35 – L/50 | continuity moment after closure; slenderness for appearance |
| Side span | 0.60 – 0.70 L | positive end reaction, balanced cantilevers |
| Segment length | 3–5 m (cast in place), 2.5–4 m (precast) | traveller capacity 60–120 t; one segment per 5–7 days per traveller |
| Web thickness | 350–600 mm | shear at the pier and tendon anchorages |
| Bottom slab | 250 mm midspan → 600–1,000 mm at the pier | compression from the cantilever moment |
Two tendon families do different jobs: cantilever tendons in the top slab, anchored in each new segment, carry the hogging moment of the growing arms; continuity tendons in the bottom slab, stressed after closure, carry the sagging moment from superimposed dead load and live load. External tendons inside the box are common for the continuity group. After closure, creep redistributes part of the locked-in cantilever moments towards the midspan — typically 10–20 % — and the design covers both the day-of-closure and the long-term distributions.
Pier table on falsework or brackets (8–12 m) → travellers installed → segment cycle: move traveller, place reinforcement and ducts, cast, cure, stress cantilever tendons → repeat to the closure gap → side-span closure on falsework or with a temporary pier → midspan closure → continuity tendons → remove temporary fixity at the piers. Unbalanced cases (one segment ahead, traveller drop, wind) fix the temporary pier–deck connection, which is the governing load case for many piers.
FCM, also called balanced cantilever construction, builds a box girder outwards from each pier in segments of 3–5 m, cast in form travellers or lifted as precast segments, keeping the two arms balanced until they meet at midspan with a closure pour. It needs no falsework in the span, which is why it is used over rivers, valleys, railways and deep water.
What spans suit balanced cantilever bridges?Typically 70–250 m for prestressed concrete box girders, with the longest around 300 m (Shibanpo 330 m uses a steel midspan section). Below about 60 m other methods (MSS, precast girders, incremental launching) are usually cheaper.
How deep is an FCM box girder?Haunched: about L/16–L/20 at the pier and L/35–L/50 at midspan, where L is the main span. A 150 m span therefore has roughly 8 m at the pier and 3.5 m at midspan. Constant-depth boxes are used up to about 90 m at L/18–L/22.
What is the side span ratio?Side spans are 0.6–0.7 of the main span so that the end reaction stays positive and the cantilever moments balance; shorter side spans need counterweights or tie-downs at the abutments.