Box Beam Internal Diaphragm

Torsional stiffeners.

Mike McCann, PE, SE (WA)

I don't believe that internal stiffeners (diaphragms) are particularly effecting in helping with either torsional strength or stiffness. My guesses:

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.

KootK said:

I don't believe that internal stiffeners (diaphragms) are particularly effecting in helping with either torsional strength or stiffness.

Click to expand...

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.

I also agree with stiffeners for bending for appearance sake as opposed to stiffeners on a built-up I beam that can be not as architecturally pleasing as a plain box type girder with hidden internal stiffeners.

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)

For a truss that has a built up box section, the internal diaphragm is more a fabrication aid to keep the section square. I don't know of any hard and fast rules for spacing other than for convenience or access. I believe AREMA (railroad code) has some requirements for spacing. There is usually a hand hole (an oval shaped cutout) to provide access in order to place/replace bolts or rivets. It's just large enough to fit your hand and arm into.

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.

Distortion will cause havoc with a steel box without regularly spaced diaphragms (although usually these are K-braces for the typical composite steel-concrete boxes.) And then plate diaphragms at bearing locations as BridgeEI mentioned.

For anyone interested, Hambly's got a good section explaining the distortion behaviour of box girders

as others have mentioned:
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.

Torsion presents itself as shear in the individual plates. As such, the stiffeners could also serve as "torsion" stiffeners in that they would reduce b/t for torsion induced plate shear.

Sail said:

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.

Click to expand...

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.

depending on h/t, b/t, etc...using typical pl theory....not that familiar with cold-formed member design....