Strengthenning of an H-shape against torsional moment 1

[Gourile]

Hi every body,
I faced a rather rare problem of an H-Beam under torsional moment togehter with bending and shearing force. The designer seemingly designed it to resist against flexure and shear, as maximum nominal shear stress in the web is 0.38Fy, very close to the maximum alloeable 0.4Fy. A relatively large amount of torque is applied to the section which is indifferntly ignored during the design process. The end conditions are fixed to column by means of end plate bolt connections, friction type. Analysis show that shear stress in the web due to torsion may exceed twice the allowable, so does the bending in the flanges due to warping torsion. I made some kind of strengthening, but have no theoretical base. I'l be glad to know about your view points. If anyone is expert in this, I can send him detailed information and drawings. Thanks a lot.

 

[ishvaaag]

The best way to deal with this kind of torsion problems is to convert a primary torsion case in a secondary torsion case. To do such yo need to take the loads causing the primary torsion by flexural members loading in your present beam, and beyond to an anchorpoint -in another beam, joist or column- able to sustain the induced uplifting force safely. Then any remanent torsion becomes of no structural interest and you have the beam quite coles to the situation assumed by the designer, contrary to that you have discovered actually to be.

 

[Gourile]

Sorry ishvag, but I don't get your point. The situation is like this: a long and stiff plate girder carrying a heavy bunker of row material is supported on a wide flange beam. The support is an end plate bolt connection type. The end plate is wlded ro the flanges of the H-section, so that the girder is fixed to it on its end plate, forming a T-connection. The H-section is fixed with end plates to the columns. The torsion will be there, whether we take it into account or not. There are only two ways for the H-section:
1- To fail due to excessive shearing stress, a catastrophie.
2- To form a torsional plastic hinge as a result of yielding in some sections. The structure is under vibration, as it is a screening tower for row material. This vibration may cause plastic fatigue in the yielded section after certain number of cycles and may cause fracture. As you see the risk is so high and I can't leave anything to chance. To have a figure, the shearing force is 60tones and is coupled with 7.1 ton-m torsion and about 45 ton-m of flexure. The section is HE-A600, 7m long.

 

[ishvaaag]

THen what happens is that you have strong torsion because yo can't -we are assuming- extend your girder beyond the supporting (if I unserstand well) HEA 600 and then anchor beyond as said. For most building situations torsion is from facade loads and there's always the possibility to anchor teh bending substituting of torsion part within the building. Then your torsion remains big because you seemingly can't use such prolongation, the stiffness of which -girder- could be made as big compared to the torsional rigidity of the supporting beam as to reduce very much the torsion present there.

In any case, if you are to support this load in primary torsion, of necessity you will need to conform to some code, or at least good practice. It is obvious that if the torsional strength of the supporting beam is what is in question, you may convert your supporting H in a box (with the original H within) by adding vertical plates parallel to the web, between tips of flanges. This at the proper size should solve any bending, shear torsion problem at the supporting beam level. Of course the new forces may affect to the girder supporting the bunker and the columns wehre the then new box would be fixed, themselves needing further redesign. In reality, I in this see more than anything a resizing problem, finding more difficult how to ascertain the effect of the heavy vibration you announce, that to be dealt with no doubt requires either quite precise code-speified procedures or at least empirical knowledge of when the built items start to show structural distress. This would be for me the more difficult part, and in absence of such reliable information, I would say a mere modest impact factor wouldn't satisfy to me, since the fatigue-stress concentration aspects may be very relevant. This would convert your subassemblage in the target of analysis by the specialized fatigue analysis software packages out there.

 

[Gourile]

Thanks for the comments. In fact the problem rises from a strange off-centeric fixed connection of a very stiff girder to a rather more flexible H-beam as support. Unfortunately, the box method you mentioned is not possible, as it chocks the space for fastenning the bolts! The best I could do was to use a combination of vertical and diagonal web stiffenners. I reasoned that in any vertical section we should have a strengthenning section, so mere vertical stiffenners would not be satisfactory. Diagonals, on the other hand, sew the top and bottom flanges and provide web with extra cross sectional area. I also written to the designer to check back the design and give comments, as the responsibility is theirs. The only thing which bothers me, as you mentioned, is the amount of vibration when combined with excessive use of welding.