What is a castellated beam?
A castellated beam is a steel beam with hexagonal openings in its web, formed by cutting a rolled I or H section along a zigzag line, sliding the two halves half a pitch apart and welding them back together at the contact points. The result is a deeper beam made from the same amount of steel: identical weight per metre, a considerably larger section modulus.
How the geometry works
The proportions follow from the cutting pattern. In the classic Peiner cut:
| Quantity | Value |
|---|---|
| New depth H | about 1.5 times the depth h of the parent section |
| Pitch of the openings | equal to H |
| Cutting angle | 63.5 degrees, so tan alpha = 2 |
The earliest known castellated beams date from the early 1930s and already spanned 12 m clear.
Advantages and limits
The gain is depth. A deeper web gives a larger section modulus and greater stiffness while the steel weight per metre stays the same as the parent section. That makes long spans economical where the load is relatively light and evenly distributed. The openings are also directly usable as service routes for pipework, ducts and cabling, which saves clear headroom in a building.
The limit shows up under concentrated loads and high shear. At an opening the web is reduced and cannot carry shear over the full depth. Where a point load lands, for example at a column position or a hanging point, individual openings are filled in with welded infill plates. The web is normally left solid at the supports as well.
Where they are used
Castellated beams are common as roof beams in industrial buildings, sports halls and warehouses, and they also appear in bridgework. When the openings are round rather than hexagonal, the member is called a cellular beam.
Related terms
- Plate girder
- Truss
- Rolled section
- Section modulus
- Span
- Steel frame
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.