Most portal frames do not use uniform beams for their rafters - they use tapered rafters. This is one of the steel industry's quietest and most effective cost-saving tricks. Understanding it helps buyers see why one design is lighter and cheaper than another.
1. What a tapered rafter is
A tapered rafter is deepest at the haunch (the junction with the column) and shallower toward the ridge, matching the bending moment. At mid-span, where bending is low, the rafter is slim; at the haunch, where forces peak, it is deep. The steel follows the stress - nothing is wasted.
2. Why it saves money
A uniform rafter must be sized for its worst point, wasting steel everywhere else. Tapered rafters reduce the steel weight of a typical portal frame by roughly 10–20% compared with uniform sections. Less steel means less material cost, less freight, lighter foundations and faster erection.
3. How they are made
Tapered rafters are welded up from plates in the factory, or cut from universal beams with a tapered profile (a tapered flange). The fabrication shop shapes each member to the engineer's drawing - a process your supplier should be fully equipped for.
4. What buyers should ask
Is the frame designed with tapered or uniform rafters?
Do the shop drawings show the depth variation clearly?
Will the saving in steel reduce my tonnage-based quote?
5. The one caution
Tapered members need correct handling and orientation during erection - installing one upside down defeats the design. Choose an erector with portal-frame experience.
In short: tapered rafters put steel exactly where the loads demand, cutting 10–20% of frame weight and reducing cost at every stage. They are a mark of a well-engineered portal frame - ask your supplier whether your design uses them.
Contact us and send us your requirements - we will get back to you as soon as possible with a quotation.