What Are Steel Box Girders and Plate Girders?
A steel box girder is a hollow, closed-section beam fabricated by welding steel plates into a rectangular, square or trapezoidal cross-section. Because the section is closed, it develops a continuous shear flow under torsion and offers very high torsional rigidity. Box girders are widely used in curved bridges, long-span cable-stayed and suspension bridges, urban elevated highways and heavy-duty crane girders.
A steel plate girder is a built-up I-shaped (or H-shaped) beam assembled by welding or bolting individual plates - a web plate and top/bottom flange plates - together with stiffeners. It is an open section that is highly efficient in resisting vertical bending along its strong axis, and it is commonly used in simply supported and continuous highway bridges, industrial crane runways and transfer girders.
Both types are custom-fabricated structural members rather than rolled standard sections. The choice between them is governed by span length, load path, curvature, construction access and life-cycle cost. This guide explains the technical differences and gives a practical selection route for bridge and building projects.
Key Differences at a Glance
| Comparison Item | Steel Box Girder | Steel Plate Girder |
|---|---|---|
| Cross-section | Closed rectangle, square or trapezoid | Open I or H shape |
| Torsional stiffness | High (closed shear flow) | Low (open section, needs bracing) |
| Flexural behavior | Excellent in all directions | Excellent about strong axis only |
| Shear resistance | Shared by two or more webs | Carried by a single web |
| Global stability | High resistance to lateral-torsional buckling | Requires lateral bracing and stiffeners |
| Fabrication complexity | High (enclosed cells, many stiffeners, full-penetration welds) | Moderate (planar assembly, automated lines possible) |
| Site assembly | Mostly welded segments, high-altitude welding | Often high-strength bolted splices |
| Typical application | Curved ramps, long spans, wide decks | Straight medium spans, crane girders, transfer beams |
| Maintenance | Enclosed void, dehumidification needed, small exposed area | Larger exposed surface, easier inspection |
Structural Performance Compared
Torsional Stiffness
The closed box cross-section develops a closed shear flow when twisted, which restrains warping and gives far greater torsional resistance than an open I-section of similar weight. This makes box girders the standard choice for horizontally curved bridges and for decks loaded eccentrically by heavy vehicles.
Flexural Capacity
In a box girder, the top and bottom flange plates are wide and contribute a large moment of inertia, giving high global flexural capacity in both directions. A plate girder concentrates material in two flanges separated by a single deep web; it is very efficient for gravity loads acting vertically, but its lateral (weak-axis) stiffness is limited.
Shear Resistance and Stability
Shear in a box girder is shared by two or more webs, and the internal diaphragms restrain the webs against shear buckling, so thin webs can be used economically. A plate girder relies on one web; deep slender webs require transverse and longitudinal stiffeners to prevent diagonal shear buckling. For the same reason, plate girders are more susceptible to lateral-torsional buckling and need a bracing system for long unbraced lengths.
Fabrication, Erection and Maintenance
| Aspect | Steel Box Girder | Steel Plate Girder |
|---|---|---|
| Welding demand | Full-penetration welds, restricted interior multi-cell welding, demanding NDT | Mostly fillet or partial-penetration welds; distortion easier to control |
| Shop prefabrication | Segments (single-cell or multi-cell) built in shop, transported to site | Whole or segmented girders; lighter pieces, lower transport demand |
| Site assembly | Segments joined by welding; alignment control is critical | Often high-strength bolted splices; faster installation |
| Lifting | Heavy self-weight, large-tonnage cranes required | Moderate weights, less demanding on crane capacity |
| Corrosion protection | Enclosed cavity needs internal coating or dehumidification; external area is small | All surfaces exposed; shot blasting, coating and inspection are simple |
For box girders, quality control relies on qualified welding procedure specifications (WPS), non-destructive testing of critical joints and shop trial assembly of matching segments. For plate girders, automated welding lines and standard dimensional checks are usually sufficient. Both types are supplied with material mill certificates and surface treatment such as shot blasting to Sa 2.5 with zinc-rich primer and finishing coats, in line with ISO 12944 for the project corrosivity category.
How to Choose Between Box and Plate Girders
Select a steel plate girder when the alignment is straight, the load path is mainly vertical and the budget is tight: it can be mass-produced on automated lines, is lighter to transport and lift, and connects quickly with high-strength bolts. It is a proven solution for simply supported and continuous spans in the medium range, crane girders and building transfer beams.
Select a steel box girder when the bridge is curved or skewed, when live loads are eccentric, when long spans or wide decks require fewer piers, or when a smooth, modern appearance and low maintenance surface area are priorities. Box girders are also preferred for bridges in high wind regions because the closed profile can be shaped aerodynamically, and for incremental launching schemes.
Both types should be designed to the governing code of the project region - for example AASHTO LRFD in North America, Eurocode 3 (EN 1993) in Europe, or GB 50017 / GB 50010 series in China - using structural steel grades such as ASTM A709 Grade 50, EN 10025 S355, or GB/T 1591 Q355/Q420 with corresponding weldability and toughness requirements.
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Frequently Asked Questions
What are the main types of box girders?
Box girders are classified by cross-section shape as rectangular, trapezoidal or circular, and by cell arrangement as single-cell, multi-cell or multi-box girders. The trapezoidal (triangular-bottomed) steel box is common in long-span cable-stayed and suspension bridges because of its aerodynamic efficiency.
Why are box girders preferred for curved bridges?
A curved bridge deck is loaded both vertically and torsionally. The closed box section resists twisting with a closed shear flow and needs no external bracing, which keeps the deck compact and allows tight horizontal radii without excessive distortion.
Are plate girders cheaper than box girders?
Generally yes for simple, straight spans. Plate girder fabrication is planar and more easily automated, material is used close to the neutral axis only where needed, and erection is faster with bolted splices. The comparison depends on span, deck width, steel grade and site conditions, so an engineering estimate is needed for each project.
What welding and inspection requirements apply to box girders?
Main seams are typically full-penetration welds executed under qualified welding procedure specifications (WPS) and inspected by ultrasonic (UT), radiographic (RT) or magnetic particle (MT) testing. Reference documents include AWS D1.5 for bridge welding and EN 1090 for execution of steel structures.
Can a plate girder be used for long spans?
Yes, with a deeper web and a bracing system, plate girders can span considerable distances, but beyond a certain span the extra steel for stiffeners and bracing makes the box girder more economical, especially where torsion or curvature is involved.
What steel grades are typically used for bridge girders?
Common grades include ASTM A709 Grade 36/50/50W for US projects, EN 10025 S355J2 and S355NL in Europe, and GB/T 714 or GB/T 1591 grades such as Q355 and Q420 in China. The final grade depends on yield strength, toughness at the design temperature and weldability requirements of the project.