Unbraced length of Top Flange in Steel Tub Girders

[KM2]

Hi,

I'm working on design of a Curved Steel Twin Tub Girder System; running grid analysis in MDX. The Top Flange Lateral bracing system is warren truss type and has an internal cross frame spaced at every other strut location. Following are the 2 questions I have:

1. Unbraced length of the Top Flange: AASHTO LRFD 8th ed. 6.11.3.2 recommends the unbraced length should be taken as the distance between interior cross frames. However, the commentary on the section leaves it to the discretion of the engineer if the unbraced length is considered as the distance between the struts. This reduces the unbraced length in half for the design. Wondering what have some of you folks considered the unbraced length in a similar situation.

2. Unbraced length in MDX: From help file, the INB-i command in MDX is used to give a list of spacings between the struts of the internal horizontal truss bracing of the top flanges for box girder "i" (feet or meters). The location of internal vertical cross frames with respect to the struts is defined with the SCRATIO command.

I have input SCRATIO as 2 which would place an internal cross frame at every other INB spacing provided.

The software then auto generates the ‘BR’ command for each girder based on the INB command defined under Girder System Layout. The BR command is defined as a list of brace spacings for top flange for box girder. This auto generated BR locations coincide with the spacings defined under INB command which would imply that the software is using the distance between the struts as the unbraced length of the top flange. Unfortunately, I cannot find the unbraced length used in the analysis in the output file.

Does anyone know the command to change the unbrcaced length in MDX or knows where I could find this information in the output file?

I have contacted the software help center with my question but haven't heard back from them yet.

Any help will be appreciated! Thanks!

 

[BridgeEI]

Unbraced length is found in the “Case Data” section after the analysis is completed. You have to scroll for a ways to find it. There’s also another spot but can’t remember where.

Unless your horizontal brace sections, I’m assuming tee sections, are rigid enough to laterally brace the top flange then I would use the distance between your interior cross frames.

 

[ukbridge]

Word of warning, from UK perspective it is very common to use plan bracing for Open Tub box girder sections even if you can justify it from a ULS perspective the second-order deformations and rotations can be significant.

http://www.newsteelconstruction.com/wp/ad-330-open-top-box-girders-for-bridges/