Tapered Frame: Why PEB Members Change Depth
A tapered frame is the primary portal frame of a pre-engineered building, built from welded plate sections whose depth varies along their length — deep where bending moment is highest, such as at the knee and ridge, and shallow where it is low — so steel is placed only where the load needs it.
By Dhruv Agarwal · · 1 min read
Why it matters
The frame is most of a PEB's steel tonnage, and tonnage drives cost. Tapering is what makes PEB economical: by varying the web depth along each member, the designer removes steel that a uniform section would carry for no structural reason.
How it works in practice
| Feature | What it means |
|---|---|
| Built-up section | Flanges and web cut from plate and welded |
| Variable depth | Deeper at the knee and ridge, shallower at mid-rafter |
| Bolted connections | Frames bolted together on site |
| Design basis | Member forces from the building's actual loads |
Common mistakes
- Comparing PEB and conventional steel on rate per tonne. PEB often uses less steel, so tonnage comparisons mislead.
- Adding crane or roof loads after design. Tapered members are optimised for the original loads and cannot absorb large additions.
- Ignoring connection quality. Bolted knees and splices must be tightened and inspected properly.
Standards referenced
Steel design under IS 800:2007.
Standards referenced
- IS 800:2007 — General construction in steel — code of practice (Bureau of Indian Standards)
Frequently asked
A rolled section has the same depth along its whole length, so it has to be sized for the worst point and carries surplus steel everywhere else. A tapered built-up member follows the bending moment, which usually makes the frame lighter and cheaper for the same span — the main reason PEB dominates large-span industrial buildings.
They are ideal for single-storey, large-span buildings such as warehouses and factories. Multi-storey buildings with heavy floor loads, complex geometry or many openings often suit conventional steel or RCC better.
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