A Structural Engineer’s Guide to Dome Framing

by | May 16, 2024 | News, Structural

Dome structures have been used for centuries, from ancient Roman architecture to modern-day sports arenas. These structures are not only aesthetically pleasing, but they also offer unique structural advantages. As a structural engineer, understanding the principles and techniques of dome framing is essential for designing and constructing these impressive structures.

In this guide, we will explore the basics of dome framing, including its types, materials, and techniques, to help you better understand the behavior of domes and how to successfully incorporate them into your projects.

What is Dome Framing?

Workers install the dome at the Blessed Stanley Rother Shrine on Thursday, Aug. 12, 2021 in Oklahoma City, Ok

Photo Courtesy: Archdiocese of Oklahoma City by Steve Sisney

Dome framing is a structural technique used to create a curved or spherical roof or ceiling. It involves constructing a series of triangular or polygonal panels that form a dome shape. These panels are then connected to a central point, known as the compression ring, which supports the weight of the dome.

Types of Dome Framing

Domes can effectively be broken into two types of framing:

1.   Structural steel infill: Radiused stud framing between radiused structral steel members-typically rectangular HSS. The spacing between the steel framing would have the largest impact on the size of these CFS members. This framing style uses lighter studs, but requires expensive structural steel and more coordination between different trades. Domes of this type will not need to meet any of the requirements of this document.

2.   Full CFS domes: Dome framing with all ribs made of Radius Track studs, with structural steel compression and tension rings. This dome design includes everything needed for easy installation and allows for offsetting supporting steel beams. This would be the traditional dome framing approach prefered and recommended by MEC/RTC.

Behavior of Domes

Domes are complex structural shapes that require 3D finite element models to analyze. While the models can not be solved by handwritten calculations, their behavior is predictable. Figure 1 shows a typical engineering model that would be generated. This model will have two types of loading applied: vertical and horizontal forces.

Typical Finite Element Model using RISA software

Figure 1: Typical Finite Element Model using RISA software

Vertical loads: Dead (self-weight), Live (maintenance personnel), and uniform snow loading cause the dome to compress downward. Unbalanced snow creates a horizontal effect. The combination of the vertical forces creates the primary forces the tension/compression ring must resist. Figure 2 shows the movement from vertical loads.

Dome Behavior – Vertical Loading

Figure 2: Dome Behavior – Vertical Loading

Horizontal loads: Wind and seismic cause the dome to tilt sideways. Additionally, unbalanced snow loads cause the same behavior. These forces impart the largest effects on the studs and can require larger connections to resist the movement of the dome, since the loads are not uniform. These loads will often try to roll the compression ring, which may require heavier connections at the top to resist. The sheathing design is controlled by these horizontal forces as well.

Figure 3 shows the exaggerated movement of a compression ring that does not have a stiff enough top connection. To prevent this rolling, the connections at the top can be increased – primarily by adding top and bottom plates to the connection.

Dome Behavior – Horizontal and unbalanced Loading

Figure 3: Dome Behavior – Horizontal and unbalanced Loading

Loads domes are highly directional, and unsymmetric. To get the most efficient results dome loads are calculated into small regions and applied precisely to each component. Figure 4 shows an unbalanced snow load applied to a dome. Each of these loads is calculated as a unique parameter.

Unbalanced loading – Snow Loading

Figure 4: Unbalanced loading – Snow Loading

Components of Dome Framing

Stud Framing

The radiused stud framing has reduced strength and stiffness based upon Radius Track testing data. These proprietary crimped studs from Radius Track have different capacities than traditional studs and requires precise modeling of their modified section properties. Adding additional blocking and strapping in rings around the dome can increase strength. Stiffness can be improved with stiffer (thicker) sheathing or heavier studs.

Stud sizes are impacted by loading, span, and bracing.

Sheathing

Domes rely on their sheathing for bracing of studs and load transfer. This sheathing is traditionally plywood screwed to the studs.

The plywood and fasteners are part of an engineered system. The thickness of the plywood is important for evaluating the stiffness of the dome system. Non-plywood sheathed systems should be evaluated on a case-by-case basis.

Tension Ring

The tension ring prevents the bottom of the studs from kicking outward under vertical loading. The primary effect on the size of the tension ring is the number of connections to the supporting structure below and the spacing between these connections. On very small domes these rings can be cold-formed steel tracks. Typical medium and large domes require structural steel, typically a plate, or on very large domes HSS; however, a track (or angle if preferred) is still required as an attachment point for the sheathing.

Domes with discrete (e.g., not every stud) attachment points to the underlying structure, such as a hexagonal steel layout, with the dome only attaching at the intersecting points require structural steel, with the span and loading dictating the type of steel ring.

If domes are directly fastened to a structural system that provides continuous support, (example: radiused HSS beam directly under CFS framing) the tension ring is not required as the structural itself becomes a tension ring system. Each rib would be clipped directly to structure in this configuration.

On hoisted systems, the tension ring acts as beam between the pick points that supports the framing while being hoisted. Even if you are sitting on a structural system once installed, the tension ring would need to be a structural steel plate on smaller domes, or a radiused HSS on larger domes.

Compression Ring

The compression ring prevents the top of the studs from collapsing inward under vertical loading. The ring itself can be made of CFS, steel plate, or HSS depending on loading requirements. Inside the ring are spokes unless the project requires an open center. Each materially is generally used as follows:

  • CFS track compression ring: limited to interior and low loading/small dome applications.
  • Rolled steel plate compression ring: will work for most domes. Typically requires spokes on the interior of the ring.
  • Rolled HSS, or built up plate assembly: Required for larger domes. Generally needed for domes that have an open center. Also needed when large loads are present directly above the compression ring (such as a coupola above).

Compression ring spokes are either CFS studs clipped to the ring, or steel plates welded to the assembly.

  • CFS spokes: used for typical applications, and finials/religious symbols with lower loading, generally less than 2ft tall.
  • Steel plate spokes: large concetrated loads at dome tops, generally from tall finials/religious symbols. Recommended for finials 2ft or larger.

The center of spokes should attach to a pinwheel assembly. For finials, McClure recommends steel pipes to bolt through or that be attached to assemblies separately welded to the center pipe.

Hoisting

Dome Build on Ground

Photo Courtesy: Radius Track

Domes are designed to be stable in their final, in-place location. Hoisting as a means and methods item for constructing the dome on the ground and lifting into place is possible on small and medium sized domes.

If requested, the dome engineering model is analyzed with the pick points as the supports and with construction loads only. Modifications and reinforcements are then detailed so both the hoisting and typical loading conditions can be accommodated. These pick points usually concentrate the loading on the dome, resulting in two general effects:

  • The studs at the pick points need reinforcement. Either heavier typical studs, multiple typical studs, or temporary framing installed that would need to be removed after the hoisting is completed.
  • The tension ring acts more like a ‘beam’ between the pick points, resulting in significantly more force on the weakest direction of the tension ring. This requires a heavier tension ring to work, typically an HSS.

If the dome is fastened to a continuous structural steel support, and that assembly is hoisted with the dome installed, the dome itself experiences no hoisting forces and requires no additional engineering.

Hoisting without an engineered approach is not recommended and would be performed on the contractor’s own liability.

Interested in constructing a dome?

Radiustrack is our industry partner that specializes in dome fabrication, as well as all of your curved cold-formed steel solutions. Reach out to Radius Track at radiustrack.com.

Josh Garton, PE, SE

Josh Garton, PE, SE

Josh brings 10 years of structural engineering experience. His design experience includes calculations and production of construction documents for multi-story load-bearing cold-formed steel structures, load-bearing wood buildings, and wood pole barns, as well as design of industrial structural steel components, non-bearing cold formed steel of both panelized and stick framed construction, and building cladding elements.
Josh’s analytical experience includes finite element modeling, Excel, and various design programs including cold-formed steel, the RISA suite, Woodworks, Winbeam, and Hilti Profis. Josh has also led the development of custom cold-formed steel connection design software used by design engineers at McClure.
Josh has developed a background in radiused and unique structures framed with cold formed steel. He has utilized the training from his master’s degree to create out-of-the-box solutions for problems that move in 3D spaces, where traditional details are not effective.

Josh Garton, PE, SE

Josh Garton, PE, SE

Josh brings 10 years of structural engineering experience. His design experience includes calculations and production of construction documents for multi-story load-bearing cold-formed steel structures, load-bearing wood buildings, and wood pole barns, as well as design of industrial structural steel components, non-bearing cold formed steel of both panelized and stick framed construction, and building cladding elements.
Josh’s analytical experience includes finite element modeling, Excel, and various design programs including cold-formed steel, the RISA suite, Woodworks, Winbeam, and Hilti Profis. Josh has also led the development of custom cold-formed steel connection design software used by design engineers at McClure.
Josh has developed a background in radiused and unique structures framed with cold formed steel. He has utilized the training from his master’s degree to create out-of-the-box solutions for problems that move in 3D spaces, where traditional details are not effective.

Learn More

Learn more about McClure’s Structural services or contact us at info@mcclurevision.com.