Design Considerations of Steel Portal Frame

Portal frames are generally low-rise structures, comprising columns and horizontal or pitched rafters, connected by moment-resisting connections. Resistance to lateral and vertical actions is provided by the rigidity of the connections and the bending stiffness of the members, which is increased by a suitable haunch or deepening of the rafter sections.  They are very efficient for enclosing large volumes, therefore they are often used for industrial, storage, retail and commercial applications as well as for agricultural purposes.

1.0  DESIGN CONSIDERATIONS

In the design and construction of any structure, a large number of inter-related design requirements should be considered at each stage in the design process. The following discussion of the design process and its constituent parts is intended to give the designer an understanding of the inter-relationship of the various elements of the structure with its final construction, so that the decisions required at each stage can be made with an understanding of their implications.

1.1 CHOICE OF MATERIAL AND SECTION

Steel sections used in portal frame structures are usually specified in grade S275 or S355 steel.

In plastically designed portal frames, Class 1 plastic sections must be used at hinge positions that rotate, Class 2 compact sections can be used elsewhere.

1.2 FRAME DIMENSIONS

A critical decision at the conceptual design stage is the overall height and width of the frame, to give adequate clear internal dimensions and adequate clearance for the internal functions of the building.

1.2.1 Clear Span and Height

The clear span and height required by the client are key to determining the dimensions to be used in the design, and should be established early in the design process. The client requirement is likely to be the clear distance between the flanges of the two columns – the span will therefore be larger, by the section depth. Any requirement for brickwork or blockwork around the columns should be established as this may affect the design span.

Where a clear internal height is specified, this will usually be measured from the finished floor level to the underside of the haunch or suspended ceiling if present.

1.2.2 Main Frame

The main (portal) frames are generally fabricated from UB sections with a substantial eaves haunch section, which may be cut from a rolled section or fabricated from plate. A typical frame is characterised by:

A span between 15 and 50 m

An clear height (from the top of the floor to the underside of the haunch) between 5 and 12 m

A roof pitch between 5° and 10° (6° is commonly adopted)

A frame spacing between 6 and 8 m

Haunches in the rafters at the eaves and apex

A stiffness ratio between the column and rafter section of approximately 1.5

Light gauge purlins and side rails

Light gauge diagonal ties from some purlins and side rails to restrain the inside flange of the frame at certain locations.

1.2.3 Haunch Dimensions

The use of a haunch at the eaves reduces the required depth of rafter by increasing the moment resistance of the member where the applied moments are highest. The haunch also adds stiffness to the frame, reducing deflections, and facilitates an efficient bolted moment connection.

The eaves haunch is typically cut from the same size rolled section as the rafter, or one slightly larger, and is welded to the underside of the rafter. The length of the eaves haunch is generally 10% of the frame span. The haunch length generally means that the hogging moment at the end of the haunch is approximately equal to the largest sagging moment close to the apex. The depth from the rafter axis to the underside of the haunch is approximately 2% of the span.

The apex haunch may be cut from a rolled section – often from the same size as the rafter, or fabricated from plate. The apex haunch is not usually modelled in the frame analysis and is only used to facilitate a bolted connection.

 1.2.4 Positions of Restraints

During initial design the rafter members are normally selected according to their cross sectional resistance to bending moment and axial force. In later design stages stability against buckling needs to be verified and restraints positioned judiciously.

The buckling resistance is likely to be more significant in the selection of a column size, as there is usually less freedom to position rails to suit the design requirements; rail position may be dictated by doors or windows in the elevation.

If introducing intermediate lateral restraints to the column is not possible, the buckling resistance will determine the initial section size selection. It is therefore essential to recognise at this early stage if the side rails may be used to provide restraint to the columns. Only continuous side rails are effective in providing restraint. Side rails interrupted by (for example) roller shutter doors, cannot be relied on as providing adequate restraint.

Where the compression flange of the rafter or column is not restrained by purlins and side rails, restraint can be provided at specified locations by column and rafter stays.