steel crane box girder

[AKRAMCRANE]

I am wondering in a steel crane, box girder, which required a diaphram every 4 ft, I saw the some example the diaphrams welded
a)all around
b)the diaphram not all the way to the bottom?
any idea why the cut the diaphram short and not welded to the bottom flange

 

[jike]

I would guess that it may have to do with the practicality of fabrication. Welding the stiffeners on 3 sides of a "open" box and then the bottom flange plate doesn't get welded to the stiffeners.

This is just my guess.

 

[graybeach]

It could be that the diaphragm braces the compression zone and is not needed in the tension zone, and/or maybe the stiffener was not welded to the tension flange so as not to decrease the allowable stress range. This assumes that the bottom of the box girder is in tension.

 

[AKRAMCRANE]

What stress range in the tension flage and how can it increase and due to what. any paper or book i can refer to?
thanks foks

 

[jike]

I believe he is refering to fatigue. See AISC Manual Appendix.

 

[Robbiee]

I think Tension Field Theory explains well the behavior of this system. I would be surprised to see stiffeners not continued near the supports of the girder. The stiffeners are to prevent finite deformations or wrinkling of the webs. Some times these stiffeners are welded inclined at an angle close to 45 degrees, perpendicular to the compression struts that develop in the webs.
Read this paper on the Tension Field Theory. Oh.. One more thing, the concept of this theory was employed in developing The Compression Field Theory to understand the behavior of concrete beams in resisting shear.

http://diablo221.altervista.org/Tension-field Theory.pdf

 

[FLCraneBuilder]

Most bridge crane box girders do not have the diaphrams welded to the bottom flange - except, as Alimar noted, at the support points. It is common to stitch weld the diaphrams.
There may be cases where the bottom flange is welded, such as very heavy capacities, but in these cases, the box would have to be large enough for weldor to crawl thru the box after assembly (with manholes in the diaphrams) to perform the welds.