Shear Flow 15

[Stillerz]

I am sure most here are familiar with the concept of shear flow as it related to horizontal shear stresses in beams.
When designing a I-Shaped plate girder, most references, if not all, will design the weld between the flange and web using the shear formula VQ/I to determine the force on the welds.
My question is, isn't there bending stress on the weld as well in the form of MC/I?
If one had a simply support girder with a uniformly distributed load, the shear at the center of the beam would be = zero and the moment at a maximum. This would imply that no weld would be needed at the center, yet this is the section where having the entire cross-section engaged in bending is most critical.
What am I missing here?

 

[Stillerz]

I think what Miecz is saying is logical....it was the same thing i was thinking when i posed this question in the post.
I guess, since there is no differential strain or relative movement between the weld and the web/flange there is no force imparted by one to the other.
It would be similar to cover plates on a column; surely there is axial load in the cover plate, but that axial load is not exerting a shear force at the interface b/t column and plate. I think the same can be said here.

This is one of those things that is hard to explain, yet seems to be intuitively understood.
...Like why the Steelers beat the Browns everytime they play.

 

[Lion06]

I believe that a cover plated column does use shear at the interface to get axial load into the cover plates. How else is the load getting there?

 

[Stillerz]

yes....shear only

 

[miecz]

since there is no differential strain or relative movement between the weld and the web/flange there is no force imparted by one to the other.

OK. I guess I didn't write exactly what I meant either. When I wrote that the weld will have the same strain as the flange, I should have wrote that the weld will have nearly the same strain as the flange.

Except for the special case of a beam with moment and no shear, I believe there will be force transfer from the web to the weld, and from the weld to the flange. It will transfer in shear through the weld.

 

[Stillerz]

disregard my last couple posts....

 

[jike]

The formula that we use for the end development force that needs to be transferred into the cover plate (at its cutoff point)is MQ/I which in essence is the average stress (axial stress) in the cover plate times its area (Mc/I x area). Some engineers will conservatively use 0.6 Fy x area of cover plate.

The VQ/I is then the change that happens in that axial stress along the length of the cover plate.

 

[Stillerz]

Jike-
Isnt MQ/I really just MC/I x Area?


I just found an old 1988 Engineering Journal Article that i think lays this out nicely.
Fourth Qtr/1988.

 

[jike]

Yes. I believe that is what I said...

MQ/I which in essence is the average stress (axial stress) in the cover plate times its area (Mc/I x area).

 

[Stillerz]

I guess you'd want to use the yeild moment in he MQ/I equation (something like 0.66FY for the old ASD).

 

[Stillerz]

...in order to closely approximate and average stress.

 

[JAE]

DaveAtkins


I think you are confusing two different things.

The Mc/I stress we are all discussing is an axial stress on the weld cross section. So if you had a 10 ft. long simple span beam, the weld between the flange and web at the top would be compressed axially...like a long slender column.

This is no different than the small fillets on rolled wide flanges where the flange and web meet. These definitely have axial stress just like the flange and weld adjacent to them.

This axial stress is totally different than shear stress though the weld due to the VQ/I.

The axial stress is simply shortening the weld length due to the beam bending. The shear stress is trying to rip the weld into two long half-welds by failure through the throat thickness.

Your analogy using 0.3 x 70 is an AISC maxium allowable SHEAR stress. It is not the maximum allowable axial stress on the weld.

Check out AISC table J2.5 - For fillet welds the shear stress is listed as you indicate....but right after that is the line for [red]"Tension or Compression parallel to weld axis"[/red]. Here it states [blue]"Tension or compression in parts parallel to a weld need not be considered in design of welds joining the parts"[/blue]. So the 0.3 x 70 doesn't apply to the Mc/I stresses.

What others have said above is accurate - we normally don't check the axial forces on a weld such as this, only check the shear through the weld which gets larger towards the ends.

 

[DaveAtkins]

I still have a hard time believing there is a tensile stress trying to pull the weld apart (perpendicular to its length) at midspan, where the web is connected to the bottom flange. I think there is tension in the flange and in the lower half of the web, but not in the weld. I think the weld is just transferring the tension from the web to the flange via shear along its length.

DaveAtkins

 

[JAE]

Dave - not perpendicular to its length...parallel to its length.

I would agree with you that it is hard to visualize perpendicular stresses on the weld. The bottom flange is going to try to force itself upward - toward the web - and you'd have compression there. The top flange is the loaded surface usually and would also force itself down on the web.

 

[DaveAtkins]

What I meant is that an axial force in the weld would create a failure plane perpendicular to the longitudinal axis of the weld, and I just don't believe that kind of force exists in the weld. I think there is just a shearing force along the length of the weld.

DaveAtkins

 

[JAE]

Well, if the beam around it is compressing in strain, the weld must go with it and compress as well. Hooke's Law means that where you have strain, you must have stress.

 

[BAretired]

If top and bottom flanges are welded to the web with a fillet weld each side of the web, the fillet becomes part of the beam. Its area contributes to the beam area and the moment of inertia.

When the beam bends, the bending stress in the fillet weld varies slightly over its height because the term 'c' varies. The welds carry a shear force of VQ/I but that is not the principal shear. The maximum shear can be found by using the Mohr's circle.

At the midspan of a beam uniformly loaded by gravity loads, the upper welds are in pure axial compression and the lower welds are in pure axial tension. They have zero shear stress at that point.

BA

 

[apsix]

I think we all agree that "an axial force in the weld (that)would create a failure plane perpendicular to the longitudinal axis of the weld" doesn't exist.

 

[paddingtongreen]

To understand, read up on complementary shear stresses.

Graybeach had a good model, Suppose the flange and web are not welded but put under beam loading and deflected. The middle of the flange will be in contact with middle of the web, but the ends of the flange will have moved over the ends of the web.

Think of it like putting a cover plate on a beam. AISC makes you develop the force in the cover, close to the ends of the plate.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[paddingtongreen]

A not bad piece on complementary shear stress

http://www.codecogs.com/reference/engineering/materials/beams/shear_stress_in_beams.php

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Stillerz]

Where's Timoshenko when you need him?
I bet he could explained this in less than 30 words.