GZ1 Box Column: 200×200, 4Φ12 Inside, 11.5 m Frame

English Version

Title: 200×200 Box Column, 4Φ12 Inside: Holding Up an 11.5 m Cold-Store Frame

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Overview

A box column is a square tube: four plates welded into a closed section. It looks simple, and it is — until you have to weld the inside, paint the inside, and prove the inside is dry.

This cold-store frame uses GZ1 at 200×200, with a small reinforcement cage inside — 4Φ12 vertical bars and Φ6 stirrups at 200 mm — running from the foundation beam to the underside of the roof slab.

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Column Data (from Drawing)

 

Item	Value
Mark	GZ1
Section	200×200 box (square tube)
Vertical bars	4Φ12
Stirrups	Φ6@200
Height range	Foundation beam to underside of roof slab
Detailing reference	12G614-1, page 15

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Frame Data (from Drawing)

 

Item	Value
Grid lines	2A, 2B, 2C, 2D, 2E, 2F, 2G — 7 lines, 6 bays
Bay spacing	8,000 / 8,500 mm
Total length	49,100 mm
Eave level	+11.500
Floor level	±0.000
Beam levels	Two intermediate levels shown between floor and eave

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How a Box Column Works

• Closed section, equal stiffness both ways. A 200×200 tube bends the same in either direction — useful when wind and crane loads come from more than one side.
• High torsional stiffness. The closed cell resists twisting far better than an open H-section of the same weight.
• The inside is the weak point. Four plates mean four internal corners that cannot be painted or inspected after the column is closed. Vent holes and sealed ends are not optional.
• Small bars are detailing, not structure. 4Φ12 with Φ6 stirrups at 200 is a construction cage — it keeps the fill material together and provides fire cover, it does not carry the frame.
• One column, two jobs. The tube carries the load; the fill (concrete or blockwork tie) handles fire and connection detailing per 12G614-1.

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Advantages

1. Same size, same capacity in both directions — no need to orient the column on site; fewer erection errors.
2. Torsion-resistant — a closed tube is the right section for frames with bracing forces and eccentric loads.
3. Clean appearance — four flat faces, no flanges to conceal; good for exposed columns in cold stores and clean rooms.
4. Easy to fire-protect — a closed box takes board or spray protection well, and an internal fill adds cover from the inside.
5. Simple connections — end plates and bolted splices work on a flat face; no flange copes needed.

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Disadvantages / Limitations

1. The interior cannot be inspected — once closed, corrosion inside is invisible; you rely on venting, sealing, and coating discipline.
2. Thin plates buckle locally — 200×200 boxes have thin walls; local buckling governs before the section yields.
3. Welding distortion — four corner welds pull the tube out of square; straightness must be checked and corrected.
4. More fabrication than an H-section — cutting four plates, fitting corners, welding inside and out; cost per tonne is higher.
5. Connection tolerance is tight — a box has no flange to absorb misalignment; end plates must be drilled as matched pairs.

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Typical Use Cases

1. Cold stores and clean rooms — exposed columns with flat faces and easy cleaning, as in this frame.
2. Frames with bracing on both axes — where torsion and two-way bending occur.
3. Architectural exposed columns — atria, canopies, and entrance frames.
4. Columns with internal fire fill — situations where fire rating is met by filling the tube.
5. Machine and equipment support columns — small footprint, high torsional stiffness.

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Fabrication and Erection Notes

• Section: 200×200 box; check wall thickness and squareness before assembly.
• Weld: corner seams full length; balanced sequence to limit distortion; check diagonal dimensions after welding.
• Interior: coat before closing; provide vent holes at the top and drain holes at the bottom.
• Reinforcement: 4Φ12 verticals, Φ6@200 stirrups, set per 12G614-1 page 15.
• Base: set the base plate on the foundation beam; check plumb before the first bay is braced.
• Erection: bolt splices with matched hole pairs; do not site-weld the shaft unless the drawing requires it.

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Bottom Line

A 200×200 box column is a small, square, torsion-resistant member that carries an 11.5 m frame and looks tidy doing it. The 4Φ12 cage inside is detailing, not muscle — the tube does the work. Close it dry, vent it, and check it square, and the column will outlast the cladding around it.

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**Email**: eddie@zdsteels.com, wells@zdsteels.com