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The following changes have been made to lateral support and the lateral stability factor KL
Setting for Points of Zero Moment in Calculation of Unsupported Length (v9 - Bug 2695)
A setting has been added to allow you to choose whether the points of zero moment (counterflexure) are to be used to delineate the unsupported length for the KL factor calculations in 6.5.6.4, which applies to both glulam and sawn lumber.
Background
The change we introduced with version 8.4 to no longer allow points of zero moment to be used to delineate the unsupported length for KL factor caused large changes in strength of certain multi-span applications, and was questioned by several users. We believe that the decision to make this change was based on sound research, and this is reflected in the change to the calculation example 7.5 in the Canadian Wood Council’s Introduction to Wood Design for the 2011 edition. However, at the end of this example, a note says, In some cases engineering may choose an alternate approach… based on .. the distance between support and zero moment or the distance between zero moments. It then refers to Example 13.3 which uses this approach.
For this reason we added an option to allow you to use zero moments.
Setting
In the Design Settings, in the existing data group Lateral Stability factor KL, a design setting has been added called. Unsupported length Lu ends at points of zero moment (counterflexure)
This setting defaults to unchecked. It is saved with the project file. If a file from a previous version is opened, the setting takes on whatever value is in the Design Settings when the file is opened.
Design
If checked, for each span the program determines the lowest of
Point of zero moment to support
Distance between two points of zero moment
User input lateral support spacing
Span length
and uses this as the unsupported length. Note that this differs from the implementation before version 8.4, which determined this distance only in the vicinity of the critical design moment value. Refer to the discussion in Bug 2708 under version 8.4 below.
Output
The line that has been added for this version to show the lateral support parameters (v9 - Feature 172, below), includes an indication of whether zero moment points or fuil span was used, if there are negative moments.
Built-up Member Width for KL Factor (v9 - Feature 209)
The program now offers a choice of whether using the full member width or the width of a single ply for the lateral stability calculations for built-up beams. .
Background
Research has recently shown that nailed and bolted beams have at most 30% composite action effect in terms of resisting torsional buckling, and for this reason it is extremely non-conservative to use the full member width as b in the expression for the slenderness ratio CB in O86 6.5.6.4.3 which is used to compute the lateral stability factor KL in 6.5.6.4.4.
Many designers have used O86 Clause 5.5.4.2.2 to indicate the use of full member width, but this clause says only that the 2.5:1 ratio in 6.5.6.3.1 (which indicates that KL must be calculated if not laterally supported according to 5.5.4.2.1) can use full member width if fastened correctly. It doesn’t say that the calculation of the slenderness ratio in 6.5.6.4.3 can be treated the same way,
Input
In the Design Settings, in the existing data group Lateral Stability factor KL, a set of selection buttons has been added to allow you to choose whether the full member width or single ply width is used for the KL factor.
The default value for this setting is single ply for new project files and full member width for files from previous versions which did not have the setting. In that case, the program issues a warning when the file is loaded.
Design
When single ply is chosen, the value b used in the slenderness ratio CB in 86 6.5.6.4.3 is the width of a single ply, that is, assuming no composite action effect. This affects both the limit of 50 for the slenderness ratio itself, and also the use of value CB in calculating the lateral stability factor KL in 6.5.6.4.4.
Output
For built-up beams, the program appends the choice of single ply or full beam width to the existing line in the CALCULATIONS section of the Additional Data giving the parameters for the lateral stability calculations (v9 - Feature 172, below).
Lateral Stability Parameters in Output (v9 - Feature 172)
In the CALCULATIONS section of the Additional Data in the Design Check output, the program now shows the unsupported length Lu, the effective length Le, and the slenderness ratio CB for the calculations of lateral stability factor KL in O86 6.5.6.4, and if applicable, the built-up member width option (v9 - Feature 209, above) and/or the zero moment point option (v9 - Bug 2695, above) .
These data are only output if the KL calculations are done; it is possible to specify in the design settings that KL = 1 because the member satisfies the conditions in 5.5.4.2.1.
Saving of KL = 1 Design Setting (v9 - Bug 2783)
Starting with version 8, the program was not saving the design option that allows you to choose between Satisfies lateral support and d/b conditions for KL =1 and Calculate KL … to the project file, instead using the default value for all existing files when they were opened. This has been corrected and this design setting is now saved to project files and appears when they are re-opened.
Unit Label for Lateral Support (v9 - Bug 2726)
The label indicating whether the input for column lateral support spacing was metric or imperial always showed “mm”, meaning millimetres, even if imperial units were selected. This has been corrected.
Lateral Support Output for Columns (v9 - Bug 2835)
For columns, lines appeared in the Materials specification of the Design Check output saying full support at "top" and "bottom", regardless of the actual support conditions and the fact that top and bottom do not apply to columns. 'They have been removed.