Portal frames and the use of haunches
Portal frames are widely used for single-storey buildings because they provide an economical solution for large spans. They are typically used for industrial halls and warehouses where large, unobstructed internal spaces are required. A common portal frame consists of columns, rafters, and rigid frame corners that transfer significant bending moments.
The frame corners experience the highest bending moments and are therefore often strengthened. Haunches are introduced to increase bending resistance, improve stiffness, and optimise rafter depth.
Portal frames may be single-span or multi-span. In multi-span systems, both frames with internal columns and frames without internal columns are used in practice.
Purpose of strengthening frame corners
Strengthening portal frame corners provides several structural benefits:
- Increased bending resistance of the rafter in the region of maximum moment
- Improved stiffness of the frame corner, contributing to overall frame stability
- Increased lever arm for bolt design
- Potential reduction in required rafter depth
These effects are particularly relevant for structures with long spans or high roof loads.
Types of haunch solutions
Portal frame corners are commonly strengthened in three ways.
Beam width mini haunch
This is a small haunch, often neglected in global frame analysis. It is mainly used to increase the bolt's lever arm rather than to improve the overall member's resistance.
Haunched beam
A haunched beam includes a triangular extension at the end of the rafter. This increases both the bending resistance of the member and the lever arm for the connection. This type of haunch has a structural effect and requires careful consideration during modelling.
Tapered beam
Tapered beams are fabricated as welded plate girders with variable depth. They are structurally efficient but more complex to fabricate and less suitable for verification using prismatic member models.
The discussion here focuses on mini haunches and haunched beams.
Modelling haunched connections
A typical connection model includes a column, two inclined rafters, end plates, wideners representing the haunches, and, where necessary, stiffeners. Loads are imported from a global analysis model and checked for equilibrium before calculation.
Member alignment, end plate offsets, bolt layout, and weld definition are critical to obtaining realistic results. Wideners are commonly used to represent haunch geometry, with their behaviour depending on their length relative to the connected member.
Member length and load application
Members in the connection model are loaded through rigid links over a defined effective length. This length affects how internal forces are distributed along the member.
For long haunches, extending this effective length can lead to unrealistic stress distributions. To reduce this risk, wideners are often shortened so that the effective length is applied behind the last operation rather than along the full haunch.
Internal forces are recalculated within the model, leading to differences in force distributions between the global and connection models.
Loading of haunched beams
For mini haunches that are neglected in the global analysis, internal forces can be transferred directly to the connection model.
For haunched beams that influence global stiffness, this approach is not always valid. Including haunches in the global analysis changes moment distribution, normal forces, and deflections, especially in statically indeterminate frames.
When internal forces are derived from models that include haunch geometry, eccentric normal forces can introduce additional moments. These effects must be excluded when transferring forces to a prismatic connection model.
Rotational stiffness of haunched connections
Haunched frame corners are often assumed to be rigid, but their actual stiffness can vary. Connection stiffness influences the distribution of internal forces in the frame and affects both corner moments and mid-span moments.
Stiffness can be evaluated using a moment rotation curve. Based on defined criteria, connections are classified as rigid, semi-rigid, or pinned. The evaluation considers initial rotational stiffness, moment resistance, rotational capacity, frame type, and member length.
The stiffness of the connection itself is obtained by separating member deformation from the overall response.
Buckling considerations
In addition to stress-strain analysis, buckling analysis can be performed. Results are presented as buckling modes and their associated critical load factors.
For global buckling involving entire members, higher safety margins are required. For local buckling of plates or stiffeners within the connection, lower factors may be acceptable, subject to engineering judgement. Buckling shapes can be reviewed directly in the model to identify critical areas.
Summary
Haunched portal frame connections require careful modelling and interpretation. Reliable design depends on understanding how haunch geometry affects stiffness, load transfer, and member behavior.