Diaphragms of box girders

[HighPanda]

Hello,

I just read a book about box girder design and one of sections talks about how to work out the bending moment in a diaphragm if the bearings are not right underneath the webs.

I am used to design diaphragms as a "simple beam" or use strut tie method. The method shown in the attachment is not new to me, but a bit different from what I think.

Please take a look at Sec 4.13.2, it outlines the steps of calculating BM in a diaphragm:
1) Suspend all shear stress carried by web under point A, where a 45 deg line starting at the bearing edge intersects the web centre line. --> My question is why the shear force in the hatched area is suspended?

2) Balance the moment (R.d) induced by the beaing offset.
--> Does the "R" refer to the bearing reaction or the total shear force in the hatched area or the bearing reaction minus shear the force in the hatched area?

 

[ishvaaag]

In the case where the bearing is directly aligned under the web the presstress compress the section over the bearing and so the web there is under vertical compression.

If, by a displacement towards the center of the bearing, the bottom support disappears, the downwards compression from atop of the prestress is still present. Hence it could cause ejection in the shape of a segment of a circle at the bottom of the longitudinal web, and to ensure integrity there it must be that the recommendation to hang the lower part of the shear in the web appears; it seems to think that from there up the total force is transmitted efficiently by a strut at 45º with the edge of the bearing.

Then, the total shear in the inclined web, bottom and upper part, is causing cantilever moment in the transverse diaphragm, and hence must be used for design.

 

[ishvaaag]

In the last paragraph, the total REACTION exerted by shears from both spans.