Clear-span buildings

What a clear-span frame is

A clear-span (CS) frame is the classic pre-engineered building: two rigid frame columns and two rafters joined by moment connections at the knees and ridge, spanning the full building width with no interior supports. The columns and rafters are usually tapered built-up members, deepest at the knee where bending moment is greatest and shallower at the base and towards the ridge.

Most clear-span frames have a symmetrical double-slope (gable) roof. Column bases are normally pinned, which keeps foundations simple; fixed bases are used when drift limits, cranes or tall eaves require extra stiffness. Endwalls are either light post-and-beam framing or, where future extension is planned, a full rigid frame identical to the interior frames.

Typical dimensions

Dimension conventions – steel line, eave height and clear height – are defined in building dimensions and terminology.

When to choose a clear span

• Interior columns would obstruct operations: aircraft hangars, sports halls, exhibition spaces, manufacturing lines with moving equipment.
• Flexible layouts are needed for racking, forklift routes or future changes of use.
• The building is narrow enough that a single span is economical – roughly up to 40–50 m for typical loads.
• Overhead cranes must travel the full width; see crane buildings.

Typical applications include warehouses, aircraft hangars, factories, showrooms and indoor sports buildings.

Clear-span variants and options

The basic two-column frame can be adapted in several ways without losing its column-free interior:

Two clearances matter to users more than the nominal width: the horizontal clearance between the inside flanges of the columns, and the vertical clearance under the haunch at the column. Both are reduced by deep tapered members, so if racking, doors or vehicles need a specific envelope, state it as a clearance requirement rather than as a building width or eave height.

Design limits and considerations

As span grows, rafter moments increase roughly with the square of the span, so frame weight per square metre rises. Very wide clear spans also bring:

• Deflection and drift. Vertical rafter deflection and lateral sway at the eave govern many designs, especially with cranes, brittle wall finishes or sprinkler systems; see serviceability and deflection limits.
• Base thrust. Wide pinned-base frames produce large horizontal reactions at the foundations.
• Transport. Deep haunch sections and long rafter segments must fit 40 ft containers or flat racks, which limits segment length and depth for export projects.
• Erection. Long rafters need larger cranes and careful temporary stability.
• Snow and ponding. In snow regions, drift at parapets and adjacent buildings can govern rafter design.

If a span much above 50 m is being considered, ask the designer to compare it with a multi-span frame. One or two interior columns can significantly reduce frame weight if the layout allows them.

Clear-span buildings from World Steel

World Steel designs clear-span frames to the project's code – for example ASCE 7 and AISC 360, NBC and CSA S16, AS/NZS 1170 and AS 4100, or EN 1991 and EN 1993 – and fabricates the tapered members on automatic welding lines at its Tay Ninh plant. Frame segments are sized for container shipping where required. See primary frame supply and long-span roofs for larger structures.