The format for steel unbraced lengths (Lb pos, Lb neg, L33, L22, and LTorsion) changed in V11. The help context in the member design parameters worksheet discusses the format. This can be accessed by clicking on a cell in the Members – Steel Design Parameters worksheet and hitting F1 on your keyboard.
Prior to V11, unbraced lengths were entered as a single value in the steel design parameters. If no value was entered, the unbraced length was assumed to be equal to the physical length of the member (j node to k node). If a non-zero value was entered, that value was used as the unbraced length for every station considered during the design of the beam. However, parameters such as Cb were always calculated using the physical length of the member. This limitation could be unconservative.
In V11, unbraced lengths need to be entered such that the sum of the unbraced lengths is equal the physical length of the member. For example, if a beam is 20’ long and the unbraced length is 5’, the unbraced length should be entered as 5;5;5;5. This allows the program to calculate parameters such as Cb for the actual unbraced segment rather than the physical length of the member. A tool button was introduced to rapidly generate the unbraced lengths.
However, this new method introduced some limitations into the program. For example, it is not possible to enter an unbraced length that is longer than the physical length of the member. This might be a necessity if you are modeling a bent or simulating a curved member. In addition, it is not possible to assign multiple Cb values to correspond to the unbraced lengths that were assigned. In other words, only a single Cb value can be entered and it will be used for all segments. If Cb is left blank, it will be calculated for all segments.
V12 and Later Implementation
The V12 implementation functions as described below.
i. The engineer doesn’t type anything in the spreadsheet cell (either directly or with the tool) and thus the value remains the default “0”: RE uses the physical length of the member as Lb and Cb will be automatically calculated based on the physical length of the member.
ii. Just one value is entered in the cell a. If the value is less than the member’s physical length, RE uses that value for all pertinent code checks (just like before v11.0) and Cb = 1. Besides printing Cb (as 1) in the design report, RE adds a note at the bottom of the report saying “Cb not calculated for the Lb specified. It is conservatively prescribed as 1”. However if there’s a value in the Cb cell other than zero “0”, then that value is used in all segments and the note is not printed.
b. If the value is equal to the physical length of the member, then it would be exactly the same as i.a above. c. If the value is larger than the member’s physical length, RE uses that value for all pertinent code checks and Cb = 1. Besides printing Cb (as 1) in the design report, RE adds a note at the bottom of the report saying “Cb not calculated for the Lb specified. It is conservatively prescribed as 1”. However if there’s a value in the Cb cell other than zero “0”, then that value is used in all segments and the note is not printed.
iii. Multiple values are entered in the cell (either directly and separated with semicolon or using the tool):
a. If the sum of the values is less or equal to the physical length of the member, RE calculates the difference for the last segment and Cb is automatically calculated for each segment. The sum is validated for unintended, non-summing entries. b. If the sum of the values is larger than the physical length of the member, RE does NOT allow it. In other words, one single unbraced length for the member, could be larger or smaller than the distance from node to node, but multiple unbraced lengths, must match the member’s physical length.
Note: Old models (prior to v11.0) opened in v12.0 will retain the original unbraced length assignments. These will fall under option ii above. The engineer must keep in mind that the final results may still vary because before, Cb was calculated with the physical length (which now only applies to option i above) and now with Cb = 1.
Similar behavior is applicable to L33, L22 and Ltorsion.Adjusting the unbraced length of Built-Up members (double angles) to accommodate Intermediate Connectors locations.
In the Steel design criteria for AISC 360 there are two user variables that control this:
See AISC 360 Section E6 for further details.
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