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The Partial Member End Fixity option allows a partial release to be applied to the ends of a member or members within a MasterFrame model. The partial fixity is defined as a percentage of the bending stiffness of the member itself. Partial fixities are defined through the Member Loading menu and are treated in a similar manner as member loads – the partial fixities are assigned to a load group and are added into particular load cases by applying a factor to the load combination.
Since partial fixities are applied to members are treated in the software in a similar manner as member loads, the partial fixities are found in the Member Loading/Attributes screen. Therefore, the member loading screen can be accessed by going to Loads>Member Loading or by going to Restraints>Partial Member End Fixity.
The initial Member Loading screen is shown below.
The partial fixities are found in the ‘Add Attributes’ area, which can be expanded by clicking on the ‘More Attributes’ icon. The expanded attributes menu is shown as: -
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To add a partial fixity to a member: -
1.In the graphics window, move the cursor over the required member
2.Left click the mouse to select the member. The selected member will highlight in red and be shown in 3D, in the main graphics window. The member will also display in the upper right-hand pane.
3.Click on the ‘Partial Fixity’ icon.
When done, the partial fixity ‘load’ will now display in the bottom right hand pane. The partial fixity will be shown as: -
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When first applied to a member, the default partial fixity is defined as: -
UT PartFix 00.00 --- --- (Mt My Mz)
These symbols have the following meaning: -
UT – Unity Load factor. This means the partial fixity will apply globally to the model
00.00 - this indicates a zero percent fixity
--- - there is no fixity applied to the Mt, My or Mz axis of the member at the member start node
--- - there is no fixity applied to the Mt, My or Mz axis of the member at the member end node
The Mt, My and Mz indicators refer to the member torsional, minor and major axes.
To add a partial fixity to a specific axis, a ‘+’ need to be added to in the relevant position in the ‘---’ indicator, at the relevant end of the member. This can be done by overtyping the + in the required position. Where a ‘-‘ occurs, then no partial fixity is considered about that axis at the specified end of the member. This means the member will be ‘fixed’ to the node it connects to.
The partial fixity is specified as a percentage of the member bending stiffness about the various axes. For example, a value of 00.00 indicates a partial fixity of zero percent, which is equivalent to a pinned support. A value of 20.00 would give a partial fixity of 20% of the member stiffness about the specified axis and ends. An input value of 99.99 is taken to be a fully fixed connection.
The first two letters refer to the load group that the partial fixity will be considered to be acting in. By treating partial fixities as load groups, it is then possible to add the partial fixities to specific load cases and so have different partial fixities being applied in different load cases. An example of this would be to have a partial fixity of 0% in Ultimate Load cases, 10% in the Sway cases and 20% partial fixity acting in the Serviceability load cases.
When the Load Group is set to UT, the partial fixity for the member is taken to act in all loadcases.
To add a partial fixity to a specific load case for a member or set of members: -
1.Select a member to have a partial fixity defined at one or both ends.
2.Click on the ‘Partial Fixity’ button to add a partial fixity ‘load’ to the member
3.Specify the Load Group the partial fixity is to be added to
4.Define the partial fixity percentage and directions per end
To include a partial fixity in a specific load case, the load group for the partial fixity needs to be included in the load combinations with a factor of 1.0 against the select load for the partial fixity. To navigate to the Load Combinations, go to Loads>Load Cases and select the Load Combinations tab from the top right-hand side of the Load Case Combinations pane. The load cases can be navigated from the Load Case drop down. With the correct load case showing, input a value of 1.0 in the required load case input box. Repeat for all required load cases.
For situations where the Partial fixities are to be added to multiple load cases, the ‘Edit Load Group’ feature can be used. This can be selected by clicking on the ‘Edit’ icon. This will open the ‘Load Case Combinations’ pane in the right-hand side of the screen.
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The load cases to have the partial fixity added can be highlighted in the load case list. At the bottom of the table, a number of options can be selected to automatically highlight loadcases. To multi-select loadcases, or add loadcase to those automatically selected, hold the CTRL-key and then left mouse click on the required loadcase.
To add a load group to the load cases, select the required load group from the ‘Load Group’ drop down. For partial fixities, where the load factor is to be 1.0, add 1.0 into the Load Factor input box. To add the partial fixities, ensure the ‘Add’ radio button is selected, then click on the ‘Apply’ icon.
Having clicked on ‘Apply’ and then confirmed the selection, the software will return to the Load Combinations tab.
Partial Fixities and column bases
The partial fixity function applies a rotational restraint to the end of a member. This gives a degree of rotational restraint in the specified direction with the degree of restraint being specified as a percentage of the stiffness of the member itself. Where the degree of restraint to the base of a column is non-zero, this means that the column will produce an end moment, but this is dependent upon the nodal support condition. If the base to the column is pinned, the base cannot resist any moment and so the base of the column will be pinned regardless of the degree of partial fixity defined. Therefore, for the base of a column, partial fixities will only be effective when the nodal support to the base of the column is defined as a fixed support.
For further details see the technical note on how to apply pinned and partially fixed bases.
Partial Fixities and beam ends
Similar to a column base, the partial fixity for the ends of a beam rely on the structure coincident to the end of the beam being able to resist bending. Thus, if the beam is defined to have a pinned end, the partial fixity will not generate any moment since the beam is free to rotate at the node. Therefore, partial fixities at the ends of beams can only be effective if the appropriate beam axis has not had an end release condition applied.
Multiple partial fixities
Each partial fixity allows for the specification of a single Load Group and a single partial fixity percentage. Therefore, to specify partial fixities in different Load Groups, or to apply different percentage fixities in different load combinations, it will be necessary to apply more than one partial fixity to a member.
The method outlined above for adding a partial fixity to a member can be repeated as often as necessary, to add the required number of partial fixities to any member. Therefore, by using multiple partial fixities on a member, it is possible to have a different percentage of fixity in different axes, or to have different percentages of fixity about the same axes in different loadcases.
One particular area where this is useful is in the design of portal frame columns, where it is common practice to design a column in the ultimate limit state as a pin-ended column, while treating the serviceability case with a degree of partial fixity to the column. In this case, the column nodal supports would be defined as fixed and two partial fixities would be added to the columns. A partial fixity would be defined in one load group and set to have 00.00 fixity, while the second partial fixity would be added to another load group and defined to have a fixity of 20.00. By adding these to the appropriate loadcases, the column would be pinned in the ultimate load cases and partial fixed in the serviceability cases.
Partial fixity load factor
Within MasterFrame, partial fixities are treated by the software in a similar manner to member loads. To include the partial fixities, it is necessary to define them to act in a load group and then include the load group in the appropriate load combinations, by including the load factor in the appropriate load group.
However, the load factor applied to the partial fixity load is only designed to act as an on/off switch to activate the partial fixity. In this case, if a factor greater than 1.0 is specified, the percentage fixity is still that as specified for the partial fixity – the percentage of partial fixity is not multiplied by the load factor.
If a partial fixity is set in the same load group as another member attribute or member load, then both will be applied in a load case based on the value of the load factor. In this case, the partial fixity cannot be applied independently of the loads or attributes. In general, it is advisable to define partial fixities in load groups with no other loads or attributes, to avoid situations where partial fixities are applied accidentally when trying to include loads, or vice versa.