Longitudinal shear stress in steel beams

[SuG89]

Hello,
Checking the transverse shear stress in beams is a common practice. Does the AISC explicitly mention something about the longitudinal shear stress (which is actually equal to the transverse shear stress in magnitude) check? Should it be combined with the normal stresses (due to direct tension/compression or bending)?
Regards,
Su

 

[dik]

I've never considered longitudinal shear stress in all my years of practice. Transverse shear stress, other than at openings is not common unless you have highly loaded short spans. My spreadsheet checks for it, but is not normally highly stressed. One project for the cross beam for a large hydraulic ram for a local floodway was designed for transverse shear... maybe 1 in 20 years.

Dik

 

[JoshPlumSE]

The only time I've done it is when I'm checking a weld for a built up member. And, in that case you're calculating the shear flow and such because the stress is not identical at all locations from the neutral axis.

 

[dik]

The only time I've done it is when I'm checking a weld for a built up member.

Did you find it critical then? I've done a few castellated girders and never come close to weld being critical.

Dik

 

[KootK]

Does the AISC explicitly mention something about the longitudinal shear stress (which is actually equal to the transverse shear stress in magnitude) check?

Not that I know of. I think that it would be implied in rare cases where it would be a concern however. Usually, you've got miles of horizontal length to deal with for horizontal shear so it's not critical. A local over-stress can redistribute elsewhere as with shear stud in composite decks. Of the top of my head, situations that would require a horizontal shear check would be built up member welds (per JP) and openings such as occur in penetrated beams and castellated beams (per dik).

Should it be combined with the normal stresses (due to direct tension/compression or bending)?

In general, I would say no. Firstly, shear stresses are not directly additive to normal stresses. You'd need to combine using something like Von Mises interaction. Secondly, in most situations, you would assume the flanges to be carrying the lions share of the normal stresses, as with the transverse shear check.


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]

Unless in high compression... your NA is generally pretty low for flexural generated stresses <G>.

Dik

 

[JoshPlumSE]

The case I'm thinking about was adding load to an existing built up beam. The flange was stitch welded, I believe. Gosh, it could have even been bolted. Same basic calculation though. Use the shear flow to calculate the force demand on that connection. Long time ago.... I'm pretty sure it ended up working just fine.

 

[Shu Jiang PEng SE]

Yes. it is normal practice to connect channel to W section for crane beam. The shear flow along the connection between the two member is calculated by t=VQ/I. t: shear flow, force/length; V: shear force; Q section modules, length^3