Steel U Frame Footbridge

[Mac75]

Hi

I have designed a U frame footbridge.The fabricator has proposed to split the bridge in 3 section at cross members level. The cross members are 120x120x8mm SHS.The design loading on cross members are:-
Moment 20kN.m
Shear 20kN
Tension 22kN

The fabricator proposal is to use a plate 25mm thk with 4 nos of M24bolts for connecting the cross members.He is recommending to use 6mm fillet weld on 3 sides of the SHS section and one side full penetration Butt weld.

The required length for weld is 140mm.Is it correct to assume that the tension load of 189kN will be share by sides of the weld.

Any help would be great.

 

[ishvaaag]

At 25 mm lamellar tearing shouldn't be an issue. Quality, alignment, ductility and creation of in-built cracks are. Have the fatigue issues been dealt with? It might result you need to butt weld the complete section. By table D3.1 in AISC case 1 all your sides of the tube are effective section, since you are connecting all the 4 sides (symmetrically one would expect, in load terms). I have not AASHTO code now so you'll have to check there.

Also, prying action by the bolts means also lever action at the fillet welds and transverse flexure for the butt weld (in sections perpendicular to the welds at each side), compounded with tension. In the case of the butt weld you may say well this is just more on one side, less another, plasticity will care. But in fillet welds it is simply additional tensile force. Care of this additional stresses will add to the required dimension of weld. I would be conservative as is proper when dealing with tensile members, more if not redundant.

 

[hokie66]

Mac75,

Ishvaaag seems to understand your connection query, but I don't. A sketch would help.

 

[ishvaaag]

It also comes to my mind that for the rules that I remember in Spain (former code, the current would have to look at) two thicknesses as dissimilar as 25 mm and 8 mm can't be welded by fillets following the code. I also know this is one of the least respected clauses of it, for very necessary if not good practical reasons. Anyway, compliance with such aspect of the code would place you either practicing butt welds anywhere or maybe on special heat treatment.