Multi-span buildings

How a multi-span frame works

A multi-span (MS) frame is a rigid frame whose rafter is supported at intermediate points by interior columns. The exterior columns and rafter are joined by moment connections as in a clear-span building, while the interior columns are usually slender members – pipe, square hollow section or small built-up I-sections – pinned at top and bottom so they carry mainly axial load. The rafter becomes a continuous beam over several supports, which dramatically reduces its bending moments and depth. Because the frame is lighter and the base thrust smaller, the foundations of the exterior columns are also reduced, although more individual footings are needed for the interior columns.

The roof is normally a single gable over the whole width, so there are no internal valley gutters: water drains to the two sidewalls. This is the main difference from a multi-gable building.

Typical dimensions

When to choose multi-span

• Very wide buildings – large warehouses, distribution centres and manufacturing halls – where interior columns can be placed between racking or production zones.
• Projects where steel weight and foundation cost are critical: shorter rafter spans mean lighter frames and smaller base thrust.
• Buildings where a single roof plane without valley gutters is preferred for watertightness.

The interior column grid should be coordinated with racking layouts, aisles, dock doors and fire-sprinkler mains at concept stage – moving a column line later changes the whole frame design. Column positions also influence fire-sprinkler layouts, lighting and the location of internal partitions, so a coordinated grid saves rework across several trades.

Variants: one, two and three interior columns

Multi-span frames are usually described by the number of interior columns per frame. The most common configurations are summarised below; the width ranges are typical, not limits, and depend on loads and the module chosen.

• Unequal modules are common: a wider central module for a production line with narrower side modules for storage.
• Omitting interior columns in selected frames is possible with jack beams that carry the rafter over the missing column.
• Rigid interior columns – built-up I-sections moment-connected to the rafter – are used when drift, cranes or mezzanines require more stiffness than the exterior columns can give alone.
• For very wide buildings with extremely low slopes, the space-saver variant keeps the peak height down.

Design considerations and limits

• Lateral stability. With pinned interior columns, the exterior moment-connected columns and rafter provide all transverse stiffness. On very wide buildings, some interior columns may be made rigid to control drift.
• Drainage. All roof water runs to the sidewalls; on wide roofs the long run increases water depth on the sheeting and gutter sizes. Standing seam roofs with continuous sheets are often specified.
• Thermal movement. Wide frames expand across the building; slotted connections or transverse expansion joints may be required.
• Cranes. Cranes can run in individual modules, with runway beams supported on brackets on interior and exterior columns – see crane buildings.
• Foundations. Interior columns introduce more footings but each carries a smaller load.

Interior columns in forklift areas need impact protection. Specify bollards or concrete pedestals early so base plate elevations are set correctly.

Multi-span frames from World Steel

World Steel optimises multi-span frames by testing module widths and interior column positions during concept design, then fabricates continuous rafters, exterior tapered columns and interior pipe or built-up columns as one primary frame package. The anatomy of the frame is shown in anatomy of a PEB.