# 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.

## 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**.

## Frequently asked questions

### Why not just use standard rolled steel sections?

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.

### Are tapered frames suitable for every building?

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.

## Sources

- [IS 800:2007](https://standards.bis.gov.in/website/published-standards/department-wise) — General construction in steel — code of practice

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Source: https://hagerstone.com/glossary/tapered-frame-peb