Built-up (boxed) sections
Why sections are built up
Section titled “Why sections are built up”More capacity under concentrated load
Section titled “More capacity under concentrated load”Jambs beside openings, heavily loaded wall sections, joist ends, column ends — a single section would not be enough at these points.
Better torsional behaviour
Section titled “Better torsional behaviour”Two C sections back to back give a nearly symmetrical cross-section; nested, they give a more closed one. Both improve resistance to flexural-torsional buckling considerably. Flexural-torsional buckling.
Simpler manufacturing
Section titled “Simpler manufacturing”A stronger member is built from the same profile instead of introducing a new cross-section.
The typical arrangements
Section titled “The typical arrangements”Back to back
Section titled “Back to back”The two webs lie against each other. The result is a doubly symmetrical, I-like cross-section with good bending and torsional behaviour.
Nested
Section titled “Nested”One section fits inside the other, giving a more closed shape. Space-efficient, and it fits the build-up better.
Boxed with added plates
Section titled “Boxed with added plates”Where extra capacity is needed, a closed section can be formed with plates.
The critical question: acting together
Section titled “The critical question: acting together”Two sections sitting next to each other do not act together. That takes interconnection, at the right spacing.
If the interconnection is sparse:
- the two sections buckle separately and the system does not reach the expected capacity,
- shear force develops in the connections, which can overload the screws.
So the spacing of the interconnection is a designed value, not an installer’s decision.
What goes wrong on site
Section titled “What goes wrong on site”- Wider screw spacing than the drawings show.
- Interconnection only at the ends, nothing in the middle.
- Building up two sections of different thickness, contrary to the design.
All of these quietly degrade the structure: to the eye everything looks fine.
Introducing the load
Section titled “Introducing the load”Where a built-up member receives load, that load has to be passed to both sections. A connection that attaches to only one web does not use the build-up.
The point
Section titled “The point”Building up is not about quantity of material but about acting together. The spacing of the connections is what turns two profiles into one stronger member.