Gurudev

Design of rolled section for longitudinal shear?

11 posts in this topic

Hello forum,

Lets consider a beam with simply support and point load.

Now if we have to design this beam (not with reference to any code) by the basics of strength of machine elements;

we check it for bending stresses and transverse shear force (am i right?)

Why dont we consider longitudinal shear? OR It is not governing or pressumed as compared to bending stresses and transverse shear stresses.

If lets say it is built up I section (web is not slender), and web is made of 2 plates butt welded longitudinally.

If the beam is designed for BM and trans. SF stresses, and it is safe, still we have to design longitudinal weld for long shear stresses?

I know this may be very basic que, but i got stuck.

 

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Hello.

 

Can you please provide a sketch of the situation you're referring and also explain what do you mean by longitudinal shear?

 

Cheers

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Actually I went too fast without noticing the use of right words.

In above post transverse shear stress means average shear stress = (SF / total c/s area).

And longitudinal shear stress (mistakenly I thought as they are parallel axis of beam hence long. shear strs, but they are also perpendicular to cross sect hence they are called as transverse shear stresses) means transverse shear stress (which is max at NA and 0 at extreme fibres of beam c/s).

I have attached cross section diagram of hypothetical case I was thing about.

I-Beam.png

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First of all, you always have to design that weld for the shear flow that needs to be transferred through that joint. 

 

For all sections which are made of one material, the maximum shear stress will be at NA. For simply supported steel beam loaded with uniformly distributed load (UDL) the critical check will be either bending in the middle of the span (taking account of buckling) or shear at the supports. 

 

For multi-span beams with hogging regions the critical check might become the von-Mises stress check (in the linear elastic analysis) in the wall just above or below the flange, as the normal stresses are still quite high there, but also shear stresses are quite high.

 

Cheers

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In EN1993-1-1, 6.2.6 shear, what do we actually check,

transverse shear stress or average shear stress (Ved/shear area)?

Now I think we check transverse shear stresses, am I right?

If the web is not slender and we have qualified (checked) that it passes 6.2.6 shear criteria,

then what is the need of designing full penetration butt weld at neutral axis. (off course made of higher strength weld filler).

Since butt welds with full penetration may be assumed to have same capacity as parent metal (EN 1993-1-8)

What do you think?

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If web doesn't pass the buckling criterion in eq. 6.22 then you need to design it according to EN 1993-1-5. 

 

If web fulfills the buckling criterion then you either use eq. 6.18 for average shear stress check (plastic check) or eq. 6.19 & 6.20 for elastic shear stress check. You can use either of those checks because the choice is independent on the section class (1, 2, 3 or 4) as this section class considers only longitudinal plate buckling due to normal stresses.

 

There is no such thing as transverse shear stress as shear stress is equal and opposite, if you will, on two perpendicular planes at the same point, so the shear stress you calculate by eq. 6.20 is actually present not only longitudinally on the section considered (working in the longitudinal direction) but also transversely in the section considered (at the level considered) in vertical direction. If you want to know more read on elastic beam theory. 

 

P.S. What do you mean by full penetration butt weld at neutral axis?

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8 hours ago, ReinisGailitis said:

If web doesn't pass the buckling criterion in eq. 6.22 then you need to design it according to EN 1993-1-5. 

 

If web fulfills the buckling criterion then you either use eq. 6.18 for average shear stress check (plastic check) or eq. 6.19 & 6.20 for elastic shear stress check. You can use either of those checks because the choice is independent on the section class (1, 2, 3 or 4) as this section class considers only longitudinal plate buckling due to normal stresses.

 

No doubt about this.

But my que is regarding need of designing the butt weld joining P2 to P3 (see c/s diagram) along the length of beam (its location coincides with neutral axis), when web satisfies one of the criteria (6.18 or 6.19) and it is not slender according to 6.22  ?

Since butt welds with full penetration may be assumed to have same capacity as parent metal (EN 1993-1-8).

 

9 hours ago, ReinisGailitis said:

There is no such thing as transverse shear stress as shear stress is equal and opposite, if you will, on two perpendicular planes at the same point, so the shear stress you calculate by eq. 6.20 is actually present not only longitudinally on the section considered (working in the longitudinal direction) but also transversely in the section considered (at the level considered) in vertical direction. If you want to know more read on elastic beam theory. 

Yes, need to revise basics.

 

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Oh, I'm sorry, I missed the fact that the web consists of two plates welded together at neutral axis. Yes, if full penetration butt weld is made then no additional check for the weld is necessary as it is presumed that the strength of the weld is at least as much as that of the parent material if the right filler material has been used.

 

Cheers 

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Thanks for the confirmation.

Have a good day.

 

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Hello Reinis,

may be your PMI is full, limit is only 50.

Need some assistance.

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6 hours ago, Gurudev said:

Hello Reinis,

may be your PMI is full, limit is only 50.

Need some assistance.

 

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