Definition and Structural Role
A steel box column is a closed-section member with a square or rectangular hollow profile, normally fabricated by welding four steel plates along their edges. The closed, doubly symmetric shape gives the column nearly equal bending stiffness about both principal axes and a high radius of gyration for its weight, making it an efficient member for columns, bridge piers and heavy industrial supports.
The member transfers axial compression, bending and torsion through the structure to the foundations. Because the section is closed, internal diaphragms can be fitted at load-introduction points, and the cavity can serve as a concealed chase for services or be filled with concrete to form a concrete-filled steel tube (CFST) member.
Common Types of Steel Box Columns
Box columns are classified mainly by cross-section geometry. The three common families are rectangular, square and tapered box columns.
| Type | Shape and Behaviour | Typical Use |
|---|---|---|
| Rectangular box column | Two unequal side dimensions; higher stiffness in the direction of the longer side | Building frames, bridge piers and plant structures where stiffness is required mainly in one direction |
| Square box column | Equal side dimensions; identical stiffness in all horizontal directions | Exposed architectural supports, cores and members in restricted plan space |
| Tapered box column | Cross-section reduces with height, following the bending moment envelope | Canopies, masts and portal frames where load decreases towards the top |
Structural Advantages of the Closed Section
Balanced biaxial stiffness
Because the moments of inertia about the two principal axes are nearly equal, the column has no weak axis. Loads arriving from any horizontal direction produce similar bending and buckling behaviour, which simplifies connection design in moment frames and improves response under multi-directional seismic loading.
Torsional rigidity of the closed tube
The four plates form a closed tube that resists torsion much better than an open I- or H-section of the same weight. This is important wherever eccentric loads, crane actions or seismic torsional moments act on the member.
Efficient material use and low self-weight
Material is placed at the perimeter of the section, far from the centroid, which maximises the radius of gyration for a given amount of steel. The result is a lighter column for the same capacity, reducing foundation loads and simplifying handling and erection.
Space for services and concrete filling
The internal cavity can be used as a concealed route for cables, piping and ducting, or filled with structural concrete to form a CFST column with higher axial capacity, greater stiffness and improved fire resistance, as provided for in GB 50017, EN 1994-1-1 and AISC 360.
Fabrication Methods
Welded built-up fabrication
The standard route for structural box columns is to cut four plates, prepare the edges, assemble the box on a jig and weld the four longitudinal seams by automatic submerged arc welding (SAW). Internal diaphragms and stiffeners are installed at panel zones, splices and load-introduction points, often using electroslag welding (ESW) where access is restricted. Execution follows AWS D1.1 or EN 1090-2 with qualified welders and approved procedures.
Cold forming and hot forming
Cold forming by press braking is limited to small, thin-walled sections used for lightweight architectural members. Hot forming, in which plates are heated before bending, suits large or thick sections where cold forming is not feasible. Most load-bearing box columns for buildings and bridges are welded built-up members rather than formed sections.
Quality control during fabrication
Quality control includes verification of plate thickness and material certificates, dimensional checks of cross-section and straightness, and non-destructive testing of welds, typically 100% ultrasonic testing (UT) on critical full-penetration welds with magnetic particle testing (MT) where required. Surface preparation follows ISO 8501-1, usually to Sa 2.5, before coating per ISO 12944 or hot-dip galvanizing per ISO 1461.
Selection and Specification Checklist
Define the governing load combinations and column height under GB 50017, AISC 360 or EN 1993-1-1
Select section size and plate thickness to satisfy strength, stability and deflection limits
Choose the steel grade to match the governing code: ASTM A572 Grade 50 or 60, EN 10025-2 S355JR/J0/J2, or GB/T 1591 Q355B/C/D/E
Specify diaphragm and stiffener layout at panel zones, splices and load-introduction points
State the welding standard (AWS D1.1 or EN 1090-2) and NDT requirements, for example 100% UT on critical welds
Define surface preparation to ISO 8501-1 Sa 2.5 and coating per ISO 12944, or hot-dip galvanizing per ISO 1461
Confirm the splice type, erection method, and whether the cavity will be filled with concrete
Frequently Asked Questions
What is the difference between a box column and a solid column? box column is hollow, with material concentrated at the perimeter. For the same weight it provides better bending and torsional resistance and a higher radius of gyration than a solid section, while using less material.
Are box columns suitable for seismic zones? Yes. Balanced biaxial stiffness and uniform stress distribution make box columns well suited to seismic-resistant structural systems, designed under the applicable provisions of codes such as GB 50017, AISC 341 and EN 1998-1.
Can box columns be customised? Yes. Section size, plate thickness, steel grade, diaphragm spacing and surface treatment can all be adjusted to the project requirements, which is one reason welded built-up box columns are common in bespoke structures.
Are box columns corrosion resistant? Steel itself is not corrosion-proof, but box columns provide long service life when manufactured from appropriate grades and protected with suitable coating systems per ISO 12944 or hot-dip galvanizing per ISO 1461.
Can a box column be filled with concrete? Yes. Concrete-filled steel tube (CFST) columns combine the steel shell with a concrete core for higher axial capacity, greater stiffness and improved fire performance, and are designed under GB 50017, EN 1994-1-1 or AISC 360 Chapter I.
When is a tapered box column preferable? Tapered sections follow the bending moment envelope and save material where loads decrease with height, such as masts, canopies and portal frames. They add fabrication complexity, so the saving should be evaluated against the extra cost.