Box Beam Internal Diaphragm
Box Beam Internal Diaphragm
(OP)
All --
As some of you know, I end up working on lots of projects that involve reviewing as-builts and shop drawings for old bridges.
I got to wondering the other day. Many bridges with built up box section beams or truss members have internal diaphragms -- a thin plate (maybe 1/2") transverse to the section at regular intervals down the member. It often has a manhole, whether that was for fabrication or maintenance access, I'm not sure. As I was thinking, I realized, I don't know what the structural purpose of this diaphragm is.
Any insights?
Thanks!
As some of you know, I end up working on lots of projects that involve reviewing as-builts and shop drawings for old bridges.
I got to wondering the other day. Many bridges with built up box section beams or truss members have internal diaphragms -- a thin plate (maybe 1/2") transverse to the section at regular intervals down the member. It often has a manhole, whether that was for fabrication or maintenance access, I'm not sure. As I was thinking, I realized, I don't know what the structural purpose of this diaphragm is.
Any insights?
Thanks!






RE: Box Beam Internal Diaphragm
Mike McCann, PE, SE (WA)
RE: Box Beam Internal Diaphragm
1) Aid to fabrication.
2) Stiffening of plate elements for compression or shear (b/t ratio stuff).
3) Shear transfer from pier to box girders at supports.
I've also heard of them used as deviators for post-tensioning but, if that were the case, I would think that it would be pretty apparent.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Box Beam Internal Diaphragm
Might have to back peddle on that. Perhaps the diaphragms could be considered torsional stiffeners in the sense that they limit torsional distortion?
Looking forward to hearing from some bridge guys on this.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Box Beam Internal Diaphragm
As for the torsional aspect, it really depends how the beam is loaded as to how these stiffeners will perform in fact. Whatever the loading pattern, they will have the ultimate effect of limiting any distortion of the box shape, or any shape for that matter, and I believe, increase torsional stiffness therein.
Mike McCann, PE, SE (WA)
RE: Box Beam Internal Diaphragm
In a section like KootK posted, we will have a solid plate diaphragm usually at bearing locations but they are tied to all of the girders. They provide torsional rigidity, more so on curved bridges. They also have a manhole opening that's the size of a person to fit through for maintenance access. The insides are lighted so workers can inspect the structure when it's in service.
RE: Box Beam Internal Diaphragm
For anyone interested, Hambly's got a good section explaining the distortion behaviour of box girders
RE: Box Beam Internal Diaphragm
a)to keep the section square or enable it to maintain it's shape..for fabrication, for shipping and for
overall stability under load.
b)depending on the size of the box bm, the sides may act as girders and exceed the h/t ratio and requre
intermediate stiffeners for tension-field action.
c)again, depending on the size and especially if constructed out of steel, the corners of the box bm may
be the only elements resisting compression and one ends up essentially with a box truss.
d)and ,ofcourse, for torsional rigidity.
RE: Box Beam Internal Diaphragm
Are steel box girders sometimes designed to be partially effective in the same way that cold formed members are?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Box Beam Internal Diaphragm