Portal frame vs A-type frame in pre-engineered buildings

Clearing up the terms

Spend a week reading PEB tenders and you will see “portal frame”, “A-type frame”, “gable frame” and “clear span frame” used almost interchangeably. They overlap, but they are not the same thing, and it helps to be precise when you are pricing or reviewing a design.

• Portal frame is the general structural term: columns and rafters joined by moment-resisting connections at the knees, so the frame resists both gravity and lateral loads by bending. Almost every pre-engineered building main frame is a portal frame of some form.
• A-type frame is a portal frame with a symmetrical pitched roof and the ridge at mid-span. Draw it and it looks like the letter A standing on two legs. In Indian PEB practice it is the default single-span gable frame.

So every A-type frame is a portal frame, but not every portal frame is A-type. One more clarification, because it comes up: an A-type PEB frame is not a timber or residential “A-frame”, where the rafters run down to the ground. The PEB version has vertical tapered columns and pitched tapered rafters.

The common PEB frame forms

Clear-span A-type

No interior columns, a ridge at mid-span, tapered columns and rafters. The obvious choice for warehouses, factories, hangars and halls where the floor must be free. As the span grows, rafter moments grow quickly, the haunch gets deep, and the frame gets heavy per square metre.

Multi-span

One or more interior columns, often slender pinned members carrying mostly axial load, with continuous rafters over them. For a wide building where columns inside are acceptable, multi-span frames usually weigh noticeably less than a single clear span of the same width, because each rafter span is shorter.

Mono-slope and lean-to

A single roof slope, either as a standalone building or as a lean-to attached to the side of a main frame. Common for canopies, small sheds and building extensions. The high and low columns have very different heights, which affects drift and the column sections.

The layout decisions that set the weight

Here is something that does not get said often enough: the biggest savings on a PEB project are decided before any section is sized. Section optimisation then squeezes the rest out. These are the decisions we look at first.

Span and interior columns

If the client can live with interior columns, discuss it early. Going from a clear span to a two-span frame changes the rafter design completely.

Bay spacing

Wider bays mean fewer, heavier frames and longer, heavier purlins and girts. Narrower bays mean more frames and lighter secondary members. Bays in the 6–9 m range are common, with 7.5 m a popular figure, but the best spacing depends on the loads, the span and the purlin system. It is worth comparing two or three spacings for a large building.

Roof slope

Slopes around 1:10 are typical for PEB roofs, flatter for wide or multi-span buildings. Slope changes the wind pressure coefficients, the axial force in the rafters, the ridge height and the drainage. A steeper slope is not automatically heavier or lighter; it is a trade-off worth checking.

Base fixity

Pinned bases keep foundations small and are usual for frames without cranes. Fixed bases reduce knee moments and drift, which helps crane buildings and tall eaves, but they put moment into the foundations. The cheaper option overall depends on the soil as much as on the steel.

Bracing and flange braces

Flange braces from purlins and girts restrain the inner flange against lateral-torsional buckling. Where you put them changes the unbraced length the code check sees, and therefore the section. Model them as you intend to build them.

Crane buildings, mezzanines and other special cases

The basic frame forms cover most warehouses and factories, but a few features change the design enough to deserve their own mention.

Crane buildings

Overhead cranes add vertical wheel loads at a bracket on the column, lateral surge, longitudinal braking forces and a moment from the bracket eccentricity. The column below the bracket now carries much more than above it, so its taper and plates are often driven by the crane combinations rather than by wind or gravity on the roof. Drift limits at crane rail level are usually tight, and fixed bases become more attractive. Model the crane load positions that give the worst effect, not just one convenient position.

Mezzanines

A mezzanine floor framed into the main columns introduces floor loads at an intermediate level and can also brace the column, shortening its effective length. Whether the mezzanine braces the frame or simply hangs loads on it depends on how it is connected, so decide that clearly and model it the same way.

Tall eaves and large openings

Tall eave heights increase column moments under wind and make drift the governing criterion more often. Large openings such as hangar doors can change internal pressure coefficients significantly, which feeds straight back into the rafter and column design through the wind cases.

In each of these cases the principle is the same: get the loads and the restraint conditions right first, then let the section optimisation work from honest forces.

What makes an optimised frame

Once the layout is fixed, an optimised frame is one where:

• each column and rafter segment is a tapered built-up I-section whose depth follows the moment envelope across all load combinations;
• flange and web plates are as thin and as narrow as the code’s strength, stability and slenderness checks allow, using plate sizes the shop actually stocks;
• most members run at a utilisation close to 1.0 under their governing combination, with none above it;
• serviceability (deflection, drift) has been checked after the sections were finalised, and the few members governed by stiffness are sized for it;
• the connection depths at the knee and ridge are still practical for the bolt groups you will detail.

Hand design gets close to this on simple frames and drifts away from it on complex ones, because the number of combinations of flange, web and taper is too large to explore by trial and error. That gap is what PEB optimisation closes: on real projects, optimising hand-iterated frames has saved 3–10% or more of the frame steel.

Putting it into practice

Our process on a new building is simple to describe:

1. Agree the frame form with the client (clear-span A-type, multi-span or mono-slope) and the column-free zones.
2. Compare two or three bay spacings and roof slopes at a preliminary level.
3. Model the chosen frame in STAAD.Pro with loads, combinations and bracing as built (see PEB frame design in STAAD.Pro).
4. Optimise the tapered columns and rafters against IS 800:2007 or AISC 360 (how the two compare).
5. Re-analyse, check serviceability, then detail connections, base plates and foundations.

Step 4 is what the PEB Optimization Tool does: it reads the frame from STAAD.Pro, searches flange width, web depth and taper for every column and rafter, keeps the lightest section that passes, and writes it back. You can watch a portal frame get lighter on the optimizer bench, or follow a full project in the tutorials.