Single fillet weld in build up hollow section - subjected to lateral bending

[KubusKubus]

Dear All,
As per bs5950-1-1990-structural-steel-hot-rolled.pdf - single fillet weld should not be subjected to bending.
But I can see in that rectangular build-up hollow section is allowed to be used - webs are welded as single fillet weld.
How can I understand possibility of using this sections in terms of torsion in a beam - which creates bending moments in a fillet weld. I want to asses safety of this solution.

 

[RPMG]

Torsion causes shear stress. The one-sided fillet weld issue is like replacing a door hinge with fillet weld. When you open the door, moment is applied to the fillet weld, and it breaks.

 

[KootK]

How can I understand possibility of using this sections in terms of torsion in a beam - which creates bending moments in a fillet weld.

I'm not sure that I agree that torsion would create significant bending moments in the fillet welds. I can see a nominal amount of bending developing owing two section warping but I'd not think that enough to be concerned about.

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.

 

[dik]

On a single fillet weld, torsion could generate significant shear stresses...

Dik

 

[dik]

What is the program and what does it develop... section properties, strength, weld sizes required?

Dik

 

[KubusKubus]

I did quick study of internal forces in walls of the section under eccentric to the axis load - as you can see if the walls are connected to the flanges by fix connection ( single fillet weld) - there is much bending in a fillet weld - you can see based on deformation.
If I try to provide hinge connection between walls and flanges - this system is unstable (if you imagine long beam).
So how can I understand this case - if we heave significant torsion - only full penetration butt weld can be used?
Software is RFEM - this section is only for characteristics - it does not provide weld calculations.

 

[handofthelion]

there is much bending in a fillet weld - you can see based on deformation.

The above deflected shape is the result of restrained warping (as a result of your boundary conditions) rather than pure torsion, which would not deform the web and flanges in this way. I agree with KootK - if you had pure torsion (no warping) and the weld throat was big enough with respect to the connecting thicknesses, the shear stresses would still be able to 'flow' through the weld.

In your case, the fillet welds will have to resist a certain degree of warping, the magnitude of which depends on the level of warping.

 

[WARose]

Normally warping does not cause significant stresses in the weld.....but pure torsion can cause shear stresses. I've run into this with I-Beams with stiffeners. (I.e. the stiffener weld to the flange.)

Does your software not give forces at the plate corners? If not, if you tell me the parameters, I should be able to run it on mine.

 

[dik]

WARose... what software are you using?

Dik

 

[WARose]

WARose... what software are you using?

STAAD Pro

It doesn't give them in the post-processing/graphical part. But if you input this in the edit file:

PERFORM ANALYSIS PRINT ALL
PRINT ELEMENT FORCE LIST [List of plates you want to see the forces for]

You will get those forces (in the global directions) in the output file.

 

[dik]

WARose: Thanks..., Dik

 

[dhengr]

KubusKubus:
The bending moment and the bending stress issue with single fillet welds primarily has to do with the fact that you do not want a situation which causes a tensile stress field which runs perpendicular to the weld axis and across/around the root of the weld. The member parallelograming in end view would cause a tension stress field at the root of two corners and compression at the other two corners. The root tends to be the weak area in the weld cross section, prone to defects; with some potential for poor fit-up, slag deposits, lack of good fusion and penetration, possible root cracking, etc. And, all of these potential defects tend to run parallel to the root and perpendicular to that tensile stress field, they are stress raisers w.r.t that stress field. Furthermore, when you have that stress field and root condition, you have essentially stress at a point (a line, the root) and this leads to very high local stresses under the best of defect conditions. The single fillet weld is less sensitive to stress fields running parallel to the root, or these types of defects, and torsional shear stresses run primarily in that parallel direction also. The bending normal stresses also run parallel to that weld or at the web to flange radius on all WF beams, etc, and we deal with that on every beam. But now, you will also see that at two corners on the beam the bending normal stress and the torsional shear stress are additive and at the other two they will reduce the stress a little, and this should be looked at for the max. stress level.

As for your box beam section, by working your way out from the inside of the section, you can get double sided fillet welds on one flg. of the beam. And, there are situations where you want the double sided weld, and the other flg. is less critical in this respect. On the second flg. you are pretty much stuck with welding from the outside only (a single side fillet), unless the box beam is large enough so a welder can get inside it. Some internal diaphragms and the end details will normally limit the most detrimental warping. As long as you control the web/flg. distortion represented by RPMG’s ‘hinge motion’ (@ 12NOV17, 16:22) you shouldn’t have a significant bending tension stress at the root of the single fillet weld

 

[WARose]

I ran a sample model of a tube shaped section and generated some plate end forces at the location where the webs meet the flange. Even with some tremendous torques applied to the section (generated by 2 joint loads applied to the flanges), the moments generated that would (for a lack of a better way to put it) put a twist on the weld were very minimal (less than 0.1 kip-in/in at most; in most cases, less than 0.05 kip-in/in). The rest of the forces were all pretty manageable. (Working out to a resultant of about 3-4 kips per inch.)

Needless to say though, when you check that single weld with that twist on it (i.e. about it's own longitudinal axis)......you get an explosive amount of force (even with the very minimal moments I was applying).....because it's just a single weld. If you had a weld on the other side you'd be set. With such minimal forces maybe a little bit of weldment at the ends (on the other side) is the answer. (Unless there is some legit way to break up a single weld into more than one.)