TheBridgeEng

FCM — first sizing

FCM box-girder sizing

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.

Girder section at pier

Girder depth

—

H pier 0.055L + 0.546

—

H mid 0.022L + 0.626

Main span150m

50125200

Side-span ratio×0.60

0.500.600.70

#	Check	Formula	Value	Typical band	Status

Bands: L/H pier 18–22 (opt. 20), L/H mid 35–50 (opt. 45), L side/L main 0.60–0.70 — international practice ranges for variable-depth segmental box girders (ASBI / PCI, ASPIRE). The depth formulas give a starting section: below a band means the section is deeper than typical (stockier, conservative); above a band means more slender, worth a closer look. Note these formulas are dimensional (metres only) and are a layout starting point — final sections must pass full strength, service and construction-stage checks.

Field notes — what to watch for

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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