Which is better for your building - single-span or multi-span rigid frames? The answer, as in many decisions is, “It depends.”
It depends on how large your building will be. It depends on what you intend to do with the building. And it depends on whether you can tolerate a space interrupted by columns. Considering these “dependencies” helps you determine whether your structure is feasible with single-span or if you will need multiple spans with the attending columns.
What Is Meant By Rigid Frame Construction?
Rigid frame construction is also called fixed or moment-resisting frame construction. These frames have rigid connections to provide stability in at least one direction, and the connections transfer bending moment forces to the foundation. There is complete structural continuity between any two adjacent members and are appropriate when you want to eliminate vertical bracing in one or both directions.
Pinned frame construction, also called simple connection, and semi-rigid frame construction, also called partial strength connection, differ from rigid frame in that the connections are made to allow transfer of axial and shear forces but not bending moments.
In a pinned joint, the simple connections may provide a slight amount of rigidity but so little as to be ignored in the structural design. Pinned connections include cleated, thin or partial depth end-plates and fin-plate connections.
Semi-rigid or partial strength connections have continuity but are not classified as full-strength since they do not have the bending resistance of the connected members. Semi-rigid framing is used in low-rise construction with smaller horizontal forces or in beams with end fixity to help control deflections.
Single-Span Rigid Frame Construction
The primary benefit of single-span construction is the ability to create open spaces uninterrupted by columns.
Single-span construction is appropriate for smaller structures with a frame width between 60 feet and 120 feet and eave heights between 10 feet and 24 feet. It is also preferred when headroom at the exterior walls is not critical since the span slopes to a maximum height at the center of the building’s width and slopes downward toward the exterior side walls.
The knee is the deepest part of the frame, where the beam and column join. A typical design is a two-hinge frame where the frame section is the shallowest midway between the knee and the ridge. Any splices are typically made at the knee, ridge and, if needed, elsewhere on the beam. Splices are made with bolted end-plate connections.
Single-span construction allows tapered columns and the frames are classified as high, medium and low profile, referring to the slope of the metal roof.
- Low profile is considered to have a slope of 1/4:12 to 1:12.
- Medium profile refers to slopes of 2:12 and 3:12.
- High profile has a slope of 4:12, suitable for roofs requiring significant roof slope or for buildings requiring large, clear heights near the mid-span.
Single-span frames can reach widths over 200 feet for extended flexibility of design and function. Single-span construction is preferred for auditoriums, gymnasiums, aircraft hangars, warehouses and recreational facilities where unobstructed space is desired.
Gable symmetrical clear span, gable unsymmetrical clear span, single slope clear span and lean-to building frames are all examples of single-span rigid frame design.
Multi-Span Rigid Frame Construction
Multi-span construction is used for the largest of structures, such as distribution centers, resource recovery facilities or any other extremely wide building without interior bearing walls.
Where single-span widths are typically limited to 120 feet due to economics, a multi-span building can reach 300 feet in width or more. Anything more than 300 feet is still feasible, but thermal movement may require the addition of expansion joints for such gigantic structures.
- Multi-span framing is also called continuous-beam, post-and-beam or modular frame.
- Multi-span frames can be assembled using tapered or straight columns. Tapered columns are placed on the exterior.
- Rafters are usually tapered members and are considered pinned, rather than full-moment connections.
- Columns are designed for axial loads only.
Multi-span buildings may have uneven building movement as different parts of the ground settle. The effect is more noticeable than in buildings with no interior supports. Additionally, once the columns are set, it is difficult and expensive to move them in the future. If renovation or remodeling may be required later in the structure’s lifetime, you may wish to rethink multi-span rigid frame.
Determining Which Rigid Frame Is Right for You
Since size is the primary difference between single-span and multi-span construction, you can begin by determining the frame width, clear span and eave height requirements of your design.
- Frame width is measured between the outside surfaces of the girts or eave struts.
- Clear span is measured between the inside faces of the columns to either side of the clear space.
- Eave height is measured between the bottom of the base plate of a wall column and the top of the eave strut.
Frame widths of 120 feet and under are appropriate for single-span construction. You can build single-span up to 200 feet wide but with diminishing financial returns. The larger a single-span building becomes, the less cost-effective the design and materials will be. Unless you truly need a 200-foot wide unobstructed expanse, you may wish to consider multi-span for buildings between 120-feet and 200-feet wide.
For extremely large buildings, multi-span rigid frame is the choice to make. Most multi-span rigid frame structures are used for warehousing and storage, where the occasional interior column doesn’t matter. For other uses, such as office space, you can always hide the columns within the walls or use the columns for architectural interest. There is no established maximum length of a rigid frame building, although the size of the building site will certainly limit that factor.
The distribution of moments in rigid frame construction is based on several variables including the size of the members. Therefore you must establish the size members you expect to use early in the design process.
Single-span and multi-span rigid frame construction joined the mainstream decades ago and continue to be the "go-to" design for structures that require open space unobstructed by columns or extremely large structures in which space is spread out at ground level instead of rising upwards.
Armed with the frame width, eave height and clear span, you have enough data to decide which style of rigid frame structure is right for your operations.