Highly skewed bridges and camber

[gwynn]

Simple question for anyone who has designed highly skewed steel girder bridges. Only applicable in cases where the diaphragms are square to the girders and not parallel to the bearing line. Do you take into account the influence that diaphragms have on camber or base the camber on simple beam bending?

 

[Bridge1892]

Typically, the concern is less with camber values and more with the bridge rating/strength, however, the two are interrelated since most analysis programs give both a girder rating and deflection/camber values.

Our office has designed a number of bridges with varying skew (20 degrees to 55 degrees) where the diaphragms are placed perpendicular to the girders. Towards the low end of this range, we ignore the diaphragms and perform a line girder analysis. At the high end of this range, we typically would perform a 2d grillage analysis, which considers the cross-frames/diaphragms in all calculations (strength, deflection, etc.).

In the end, the consideration of the diaphragms in calculations for skew bridges is a judgement call. I have outlined a few considerations below:
- More of a strength concern, but: Do the diaphragms form a discontinuous or straight line i.e. are they staggered? Discontinuous diaphragms/cross-frames requires consideration of flange lateral bending (See AASHTO C6.10.1.) See attached files NYSDOT specifies diaphragms in a continuous line. CTDOT specifies discontinuous diaphragms.

- cross-frames (K or X -shaped) are stiffer than diaphragms (C-Shapes) and thus would have more of an impact on the deflection of the girders.

That being said, girder camber is not an exact science, despite the fact that many engineers report cambers to 0.001". We just consider the diaphragms for our camber in highly skewed bridges because a grillage analysis using a program like MDX is not much more difficult than a line girder analysis.

 

[bridgebuster]

Typically, simple bending was the easiest way to go, unfortunately someone invented computers. The attached file is from NSBA; it's very helpful regarding analysis of skewed bridges.

Also, think about the concrete placement sequence & whether you want the steel plumb before or after deck placement.