Design Philosophy

The design of portal frames has two distinct stages:-

Stage 1 - Selecting the main sections assuming adequate restraint.

Stage 2 - Checking the adequacy of the restraints.

The design of portal frames are commonly based on the ultimate limit state for dead plus live loading and only check the adequacy of the resulting frame for other loading combinations. For three or more bays the snap through must be checked separately.

Note that to reduce deflection and increase the sway resistance of the frame the following actions can be taken:-

1.Use UBs instead of UCs as internal columns to increase the frame resistance to sway.

2.Use the 0/10/20% base fixity option to reduce the deflections in the service load cases.

3.Design the internal bases as fully fixed. The fixed internal columns will usually absorb small moments but the fixity will substantially reduce the frame deflection.

Stage 1: Selecting the Main Sections

When selecting the main steel sections the following points are relevant:

Wide flange sections are better suited for use in portal frames because of their higher lateral stability thus requiring fewer lateral and/or torsional restraints.

In general the rafter section is made smaller than the column section by one, two or even three serial sizes, (e.g. for a 533x210 UB as a column section the rafter section will be 457x191 UB, 457x152 UB or 406x178 UB).

1. In selecting the sections, consideration should be given to the sway stability and the deflection of the frame.

2. Plastic hinges should only be allowed to form at higher loads, usually not less than 85% of the ultimate load. This guarantees a good balance between the column and rafter sizes and ensures that no plastic hinges are formed at service or close to service loading.

3. Only one plastic hinge can form in any area limited by two points of contraflexure (a pinned support is considered as such a point). This means that plastic hinges cannot occur simultaneously under the haunch (in the column) and at the end of the haunch (in the rafter).

Stage 2: Checking the adequacy of the restraints.

These checks include:

a) Check purlins and side rails spacing using the Axial with Moments check between adjacent purlins/side rails.

b) Check the elastic and/or plastic stability between points of torsional restraints using Appendix-G check.

Important Note: The automatically selected section sizes are dependent on the bending moments, which are influenced by the stiffness of the original section sizes. Where the difference in stiffness of the altered and original section sizes is considerable, the altered section sizes may not be the most suitable for continuing with a plastic analysis. This situation is mitigated by conducting the design steps twice before performing plastic analysis.