falsework supported by steel girder

falsework for cantilever portions of a bridge deck are typically supported by the exterior girder (steel or concrete).  North Carolina DOT allows bolt holes in a steel girder web, if done during initial fabrication.  This allows connecting a strut to the web that results in a force perpendicular to the web. The web is checked for deflection (out-of-plane) similar to a simple beam using an effective web width (along the centerline of the girder) to calculate Moment of Inertia (effective).  NCDOT uses an effective width of We=0.4*depth of web + 0.2*spacing of bolt, so I=We*(depth of web)^3/12.  I cannot find any references to confirm this effective width of the web.  Any ideas are welcome - thanks.

P.S. The bolt is left in place after the falsework is struck.

Is this bolt near the top or the bottom of the web plate? I am used to seeing the top strut element attatched to the top flange and the bottom strut element resting (in compression) against the web near the bottom.

I haven't seen your effective width formula before, but I have a few comments:

1. The effective width shouldn't be greater than the cantilever support (bolt?) spacing.
2.  The effective width will vary somewhat with the web thickness.  That is, a thicker web will distribute point loads better than a very thin one.
3.  Have you checked Roarke's formulas for this case??(infinite width thin plate with point load)

Additionally, wouldn't the I=We*(web thickness)^3/12??

Curvbridger,
Thanks for your response.

We is based upon "depth" of web. I=We*(web "thickness")^3/12 is correct.

There are two main members in the support.  The top, horizontal member of the falsework is bolted to the web approx. 15" below the top flange.  The diagonal bears against the web and bottom flange.

Your second point is also valid.

I'll check Roarke's later when I can spend some time, and perhaps I'll revisit this thread.

Thanks again.