How the Pantheon Dome Survived for Nearly 2,000 Years: A Blender Explainer Video

Status: Draft (2026-10-31). Building statistics are values confirmed from the sources listed in "References" below; ranges are provided where sources differ. The 3D models in the video and illustrations are simplified, and values calculated from the models are explicitly marked as "Model calculation" (they are not measured values of the actual building). Series: "Architectural Structures in 3D", Part 1.

Table of Contents

• Video
• 1. What Kind of Building Is It?
• 2. Concrete is Strong in Compression and Weak in Tension
• 3. Four Ways Weight Was Reduced
• 4. Flow of Forces: How a Cracked Dome Stands
• 5. Summary
• How This Video Was Made
• References

The Pantheon in Rome is an ancient building whose roof remains intact to this day. The interior diameter of the dome is about 43 meters, and it is made of unreinforced concrete. It is widely considered the world's largest unreinforced concrete dome to date (Masi et al., 2018). This post is the result of creating a "3D graphics + brief commentary" video explaining why the dome does not collapse.

Video

In accordance with the site's video rules (no sound, under 30 seconds per segment), four sections were created, one segment each. The dialogue corresponds to the subtitles at the bottom of the screen.

1. What Kind of Building Is It?

The shape is not a precise sphere. Precise surveys show that the dome and cylinder deviate slightly from a perfect hemisphere and cylinder (Plunkett 2016). The video and illustrations are simplified based on a 43.3 m diameter sphere.

2. Concrete is Strong in Compression and Weak in Tension

Stone and concrete withstand compressive forces well, but crack quickly under tensile (pulling) forces. Based on the values used by Masi et al. (2018) for their dome analysis, the compressive strength of the concrete was set to about 5 MPa, while its tensile strength was set to 0.08 MPa—a difference of over 60 times (these are assumed values for analysischosen from experimental literature).

Therefore, the design of the Pantheon can be interpreted in two ways: one is reducing the weight itself, and the other is channeling the remaining forces into compressive forces.

3. Four Ways Weight Was Reduced

1. Thickness. The dome is thick at the bottom and grows thinner toward the top, ranging from about 5.9 m at the base to about 1.5 m at the top (Masi et al., 2018).
2. Aggregate. Roman concrete consists of mortar mixed with volcanic ash (pozzolana) and stone pieces (aggregate) (Masi et al., 2018). Heavy travertine was used in the foundations, while lighter tuff and pumice were used at the highest points of the dome (Moore 2002). The regional densities used in analytical papers are 2,000 for the foundation, 1,750 for the drum wall, 1,600 for the lower dome, 1,500 for the middle, and 1,350 kg/m³ for the top (Masi et al., 2018, Table 1, citing values from Mark & Hutchinson 1986). However, at what height the density changes and how many materials were used remain undetermined (Plunkett 2016).
3. Coffering. The interior of the dome is excavated into 5 tiers of 28 panels each. This serves as ornamentation while also functioning as a weight-reduction structure (Moore 2002, Masi et al., 2018).
4. Oculus. An opening with a diameter of about 9 m is left open at the top center. It acts as a window for light while removing weight from the highest point of the dome.

The reason the structure must be lighter toward the top is that weight higher up increases the outward pushing force (thrust) at the base of the dome. Lighter concrete at the top reduces the thrust exerted on the supporting walls, making the dome more stable (Plunkett 2016).

Model Calculation. Using the simplified model in the video, we reversed each design feature one by one to compare the mass "above the height where the dome begins" (the calculation script is in "How This Video Was Made" below).

This table varies greatly depending on how the model's zone boundaries and coffer depths are set, so please use it only as a sense of scale. It is not a mass comparison of the actual building. While "thickness" contributes the most to mass and the oculus contributes the least, the oculus and upper aggregates are located at the highest point, so their effect in reducing thrust is greater than their mass proportion would suggest.

4. Flow of Forces: How a Cracked Dome Stands

Vertical Cracks. A hemispherical dome of uniform thickness tends to pull apart along its lower circumference due to its weight (circumferential tension). Theoretically, the boundary is at a height of about 52° (Masi et al., 2018, citing Flügge). The interior of the Pantheon dome features vertical cracks extending from bottom to top, mapped as 14 lines in surveys from the 1930s. The cracks terminate between a height of about 54° (summarized by Mark & Hutchinson 1986, Moore 2002) and about 57° (Masi et al., 2018). The upper portion is a zone under circumferential compression, preventing the cracks from propagating further.

Multiple Arches. The cracked dome does not function as a single shell, but rather like multiple wedge-shaped archesleaning upward from the drum wall toward the apex. Arches stand through compressive forces. Mark & Hutchinson (1986) proposed this view, and the 3D analysis by Masi et al. (2018) confirmed that a cracked dome behaves in this manner. Masi et al. identified initial concrete shrinkage and self-weight as the primary causes of the cracks.

Oculus Compression Ring. The point where the arches meet is the perimeter of the oculus. Because this ring receives the inward pushing forces of the arches from all directions, it is under compression. The perimeter of the Pantheon's oculus contains a vertically laid brick ring and a bronze ring (Moore 2002).

Stepped Rings. Interpretations vary regarding the seven stepped tiers on the exterior at the base of the dome.

• Mark & Hutchinson (1986): If the dome were an uncracked monolithic shell, the steps would offer little help; however, in a dome behaving like arches due to cracking, they act as weight pressing down on the haunches of the arches, functioning like buttresses (summarized by Moore 2002, Masi et al. 2018).
• Brune (2010, summarized in Masi et al. 2018): The steps reduce tension along the lower circumference, estimating a reduction of about 58.5%.
• Masi et al. (2018): Both interpretations complement each other. Considering self-weight alone, maximum circumferential tension was about 27.1% lower when the steps were present, and after cracking occurs, they add weight to the arch mechanism.
• Whether the true reason for the steps was construction convenience (stacking lower walls instead of a steep inclined surface) or structural necessity remains an open question (Plunkett 2016).

5. Summary

The Pantheon dome avoided the material's weakness (tensile force) through its geometry. By making it thinner and lighter toward the top, it reduced thrust, and even if it cracked, the remaining structure was able to stand purely through compression like an arch. The ring around the oculus counters those arches, and the thick drum wall transfers that force down to the foundations.

How This Video Was Made

Tools. Blender 5.2.3 LTS (official release), Blender's Python API, the Eevee renderer, ffmpeg for video editing, and Python (Pillow) for drawing subtitle text. The font used is Noto Sans CJK included in Blender (SIL Open Font License). Not a single 3D asset was downloaded; everything was created via scripts.

Procedural Modeling. The half cross-section of the building is defined as a polygon of (radius, height) coordinates, which is then revolved around the vertical axis to create a solid. Because each density zone's cross-section was cut by height and revolved separately, removing the front half reveals cut surfaces colored by zone.

def spin_profile(name, poly, a0, a1, seg=96, mat=None):
    """(r, z) 다각형을 a0→a1 (라디안) 만큼 돌린 닫힌 회전체."""
    bm = bmesh.new()
    verts = [bm.verts.new((r * cos(a0), r * sin(a0), z)) for r, z in poly]
    face = bm.faces.new(verts)
    bmesh.ops.spin(bm, geom=[face] + verts + list(face.edges), cent=(0, 0, 0),
                   axis=(0, 0, 1), angle=a1 - a0, steps=seg, use_duplicate=False)
    bmesh.ops.recalc_face_normals(bm, faces=bm.faces)
    ...

# 앞쪽 반(π..2π)과 뒤쪽 반(0..π)을 따로 만든다. 단면 보기는 앞쪽 반을 들어 올려 치우는 것.
for half, (a0, a1) in {"back": (0, pi), "front": (pi, 2 * pi)}.items():
    for name, poly, mat in pieces:            # 기초·원통 벽·돔 아래·가운데·위·오큘러스 고리·계단
        ob = spin_profile(f"{name}_{half}", poly, a0, a1, mat=mat)

For the 140 coffered panels, a hexahedron was created for each cell and carved out using booleans (difference), with separate materials assigned to the cut faces so they glow blue in Section 2. Cracks, arches, and arrows were grown by keyframing the curve's "end mapping factor" from 0 to 1. Camera angles, visibility toggles, and background colors were all keyframed along a single timeline (in seconds).

Rendering. 1920×1080, 30 fps, 3,000 frames (100 seconds) were rendered using Eevee (64 samples, ray tracing enabled) on a single laptop (Apple Silicon GPU). The median time per frame was 0.86 seconds (slowest frame 1.43 seconds), and the total process including saving took 45 minutes and 33 seconds. The script to generate the scene itself finishes in under 4 seconds (including booleans).

Subtitles and Editing. We had planned to include narration using text-to-speech (TTS), but this version was made with subtitles only. The site's video rules also do not accept audio tracks. Subtitles, name tags, and legends were overlaid as transparent planes frame by frame, then combined into H.264 using ffmpeg. For the site, the video was cut by chapter, downscaled to 1280x720, and metadata such as encoder information was stripped. The legend colors were sampled directly from the rendered cross-sections rather than the material colors (because color management makes the on-screen colors differ from the material values).

Sources

• Masi, F., Stefanou, I., Vannucci, P. (2018). On the origin of the cracks in the dome of the Pantheon in Rome. Engineering Failure Analysis 92, 587–596. doi:10.1016/j.engfailanal.2018.06.013. Author manuscript preprint: https://hal.science/hal-01719997
• Plunkett, J. W. (2016). The Roman Pantheon: scale-model collapse analyses. S.M. thesis, MIT Department of Architecture (Advisor: J. Ochsendorf). http://hdl.handle.net/1721.1/107867
• Moore, D. (2002). The Pantheon: Crown Jewel of Roman Concrete. CONSTRUCTOR, September 2002 issue, pp. 22–26. Article based on the author's book The Roman Pantheon: The Triumph of Concrete .
• Mark, R., Hutchinson, P. (1986). On the Structure of the Roman Pantheon. The Art Bulletin 68(1), 24–34. doi:10.2307/3050861. The original text was not accessed; the content related to this in the article was verified through the summaries by Masi et al. (2018) and Moore (2002).
• PBS, Building Big: Pantheon. https://www.pbs.org/wgbh/buildingbig/wonder/structure/pantheon.html (Oculus 27 ft)