FAQ #23
What is an appropriate choice for bracing connection offset (GCONNDST) for box girders?
-----------------------------
This issue demonstrates the inherent difficulty in modeling bracing between box girders in grid analysis. First of all, the effect of the external bracing stiffness on the load effects of the girders is typically but not necessarily small. There are two extremes:
1. GCONNDST 0. leads to the least stiff bracing between box girders, which would influence the dead load effects in different ways depending on the framing plan characteristics and would tend to lead to higher live load effects as live loading wouldn't distribute as much, so this choice generally would lead to slightly conservative load effects on the girders. But, this choice also would generally lead to lower forces in the bracing.
2. GCONNDST given to set the bracing member lengths using the connection at the top flanges leads to the most stiff bracing between box girders, which would influence the dead load effects in different ways depending on the framing plan characteristics but would tend to lead to lower live load effects than would GCONNDST 0, so this choice generally would lead to less conservative but perhaps more rational load effects on the girders. This choice also would generally lead to higher forces in the bracing.
By running the analysis both of these ways you can generally bound the solution for the load effects on the girders as far as the influence of GCONNDST is concerned. Again, these differences would depend on the framing plan characteristics and may or may not be significant.
The effect of the GCONNDST on kL/r for bracing members has 2 extremes:
1. GCONNDST 0. conservatively leads to the largest unbraced length.
2. GCONNDST given to set the connection at the top flanges, which would only be appropriate for the bracing members that connect through there.
So, given all these factors, how is GCONNDST chosen?
1. The trivial case is if neither the bracing forces nor kL/r are constraining the bracing designs the influence of GCONNDST only has meaning in relation to the load effects for the girders, which can be investigated by running the analysis at the two extremes described above.
2. Otherwise, GCONNDST is to be selected rationally taking all of these factors into account.
So there is no one answer. You probably need to make multiple runs with these factors in mind to see how the choice of GCONNDST affects your results and then make your best choice based on what factors are important in your case.