Why You Can Drill a Big Hole in an I-Beam Web
It seems counterintuitive: you can cut a large hole in the middle of a steel beam’s web and barely reduce its load-carrying capacity. But once you understand where stress lives in a beam, it makes perfect sense.
Where Bending Stress Lives
When a beam bends, the top and bottom fibers carry almost all the bending stress. The formula:
σ = My / I
Where y = distance from the neutral axis (center). At the top and bottom flanges, y is maximum → maximum stress. At the center (neutral axis), y = 0 → zero bending stress.
This means the material right at the center of the beam contributes almost nothing to bending resistance. The flanges do the heavy lifting.
What About Shear?
The web does carry shear stress. Shear stress in a beam follows a parabolic distribution — maximum at the neutral axis, zero at the flanges. So the web matters for shear.
However, in most beam applications (especially floor joists and headers), bending controls the design, not shear. Shear is only critical near supports or under concentrated loads.
The Rules of Thumb
For steel I-beams and open-web joists, standard practice allows web penetrations with these guidelines:
- Hole diameter ≤ 2/3 of web depth (conservative)
- Hole centered on the neutral axis (vertically centered in the web)
- Located in low-shear zones (near mid-span, NOT near supports)
- Minimum spacing between holes ≥ 3× hole diameter
- Minimum edge distance from hole to flange — typically the hole radius
For wood I-joists (TJI / engineered joists), the manufacturer publishes specific hole charts. These are typically even more generous because the flanges carry bending and the web is just oriented strand board (OSB).
Why This Matters Practically
Every building has HVAC ducts, plumbing, and electrical that need to pass through floor and ceiling framing. Without web penetrations, you’d need deeper cavities, dropped ceilings, or soffits everywhere. Understanding that the web center is a stress-free zone means you can run a 4″ pipe right through the middle of a 12″ joist — and the engineer who signed off on it sleeps fine at night.
When You CAN’T Cut the Web
- Near supports — shear is highest here; web penetrations drastically reduce capacity
- Off-center vertically — cutting near a flange removes high-stress material
- Too close together — adjacent holes create a “perforated” web that buckles
- Under concentrated loads — point loads create local shear peaks
The I-Beam Is Brilliantly Designed
Think about it: an I-beam puts material where stress is highest (flanges, far from center) and removes material where stress is lowest (web center). It’s the structural equivalent of “work smarter, not harder.” The hollow web isn’t a weakness — it’s an optimization.
Key Takeaways
- Bending stress is zero at the neutral axis — the web center is the least stressed part of a beam
- Flanges carry bending, web carries shear
- Centered holes in low-shear zones barely affect beam capacity
- Always check manufacturer specs for engineered lumber — they publish exact allowable hole sizes and locations