.png)
The Elastic Critical Load Factor brief presents the calculation of the αcr (alpha crit) factor by which it can be determined whether or not the structure is sensitive to second order effects. The calculation of αcr is determined by either the approximate method given in BS EN 1993-1-1 Clause 5.2.1 (4), where
αcr = (HEd / VEd) * ( h / δH,Ed)
Where:
HEd is the total design horizontal load, including equivalent horizontal forces, transferred by the storey (storey shear force)
VEd is the total design vertical load on the frame, transferred by the storey
δH,Ed is the horizontal displacement as the top of the storey, relative to the bottom of the storey.
h is the height of the storey under consideration
Alternatively, the Elastic Critical Buckling Load will use the Buckling Analysis method from Masterframe for any load case where this 2nd order analysis option has been activated. The Elastic Critical Load Factor will indicate the method used to determine the αcr value in the results area of the brief.
When the brief has been applied to a model, the initial screen is as shown below.
The section AutoDesign tab allows the user to control which elements of any model are considered as part of the Elastic Critical Load Factor calculation. Members can be select to be excluded from the calculation by using the mouse pointer and selecting the member(s) to exclude graphically. The member number(s) will appear in the rectangle below the Section Auto Design heading. A typical use of this feature would be to exclude parapet members, which may be relatively more flexible than the main structural members and so could potentially lead to the return of a lower αcr value.
While nodal displacements can be viewed in the results area of MasterFrame, the nodal displacements for all serviceability cases can also be viewed under the Nodal Displacements tab of the Elastic Critical Load Factor brief. Within this area, it is possible to select specific nodes for part or all of the model and review the vertical or horizontal displacements, as well as specifying a limiting value for each grouping of nodes.
To view nodal displacements:
1. Click inside a Nodes cell on a free row to place the cursor there.
2. Select the node(s) from the frame geometry area to include for the displacement check.
3. Enter a description of the displacement check in the next cell, eg, eaves, apex. This will be used during the export or printing.
4. Clicking in the Case cell displays a drop down list of the serviceability loading cases. Select the required loading case from which to display the displacements.
5. From the Direction cell, select the desired displacement directions.
6. The Value cell will now display the maximum displacement for the selected nodes for the chosen loadcase and direction.
7. To set a displacement limit, input the required limiting value in the Limit cell for the chosen row. The software will check the maximum displacement value against the input limit. If the displacement is less than the limit, the Verdict cell will display the message "OK". If the maximum deflection value exceeds the input limit, the Verdict cell will display the message "Warning".