Portal frame analysis

This worked example considers the analysis of a typical 25m span haunched portal frame. Linear (1st order), non-linear (2nd order), dynamic (frequency and response) and buckling analyses have been performed and the results are presented in the computer printout at the end of this appendix.

 

This appendix considers only the analysis of the portal frame. The portal frame member and connection design is covered in Portal frame member design and Portal frame connection design.

 

This example is loosely based on the design example used in the AISC publication by Woolcock, Kitipornchai and Bradford (9). There are, however a number of significant differences between this example and the AISC example which can be summarized as follows.

  • Because SPACE GASS has facilities for projected length member loads, the live load has been input over the plan rafter length rather than its inclined length. This was a situation that the software used in the AISC example could not model.
  • Because SPACE GASS has facilities for automatically calculating haunch section properties based on the rafter size and the size of the member from which the haunch was cut, the haunch section properties are different. The AISC example simply approximates the haunch to a 530UB82 for half of its length and a 410UB60 for the other half.
  • SPACE GASS uses a value for gravitational acceleration of 9.8066, the AISC example uses 9.82.
  • SPACE GASS uses grade 300 steel, whereas the AISC example uses grade 250 steel.
  • The purlins used in the AISC design example are assumed to be spaced at a maximum of 1500mm, while the structural drawings elsewhere in the publication show them to be spaced at 1200mm maximum. This SPACE GASS example uses purlin spacings of 1200mm as they are shown in the drawings.

 

Because the members in the AISC example have been designed by hand, they have not been able to take full advantage of some of the more calculation intensive and slightly more efficient higher tiers offered by the SPACE GASS steel member design module.

 

image\ebd_24.gif The differences between this example and the AISC example prohibit the direct comparison of results. However, if you wish to do so, you should first modify the SPACE GASS example in accordance with the differences listed above. If you do the modifications, you will find that the results of the two examples agree almost exactly.