Stability & Fire Boundary
Elastic Critical Load Factor

The Elastic Critical Load Factor check can be used in the portal frame checking process. It is unlikely that a portal frame will fall beneath the critical factor of 10 and so the check is usually not going to be an issue. Occasionally some of the non-portal minor members could give a factor under 10 and so should be checked.

The Elastic Critical Load Factor can be determined from either a buckling analysis or from form empirical design code sway deflections. The option to use a buckling analysis is given in the MasterPort General tab. Otherwise 'Sway Stability' loading cases are automatically generated that include equivalent horizontal notional loads only, producing the necessary deflections on which the empirical approach is based.

The sway deflection method is based on the maximum h/200*δH,HNL from all vertical member. When using this method It is possible to omit some vertical members from the check, eg, parapet members or canopies that are not going to influence the overall stability of the building. To omit members set the cursor into the large box at the bottom of the screen, then click on the individual members that you wish to omit. These particular members will be removed from the check. (The numbers of the omitted members appear in the box.)

For in depth details of the elastic critical load factor check see the relevant section in the MasterFrame manual 📄 Elastic Critical Load Factor and the MasterKey Steel Section design Manual 📄 Elastic Critical Load Factor Brief

For Eurocode design, when the elastic critical load factor is determined using the sway deflection method, SCI P399 section 7.5 is used

• α,cr,s = h/200*δH,HNL 

and 7.6.1 is used to account for significant rafter axial force adjustment.

• α,cr,s,est = 0.8 (1- Ned/Ncr)* α,cr,s