Shear flow in thin wall members

[PhanthomJay]

ok , understand it...nw , i understand that why there's no horizontal shear stress in web .

But , i still don't understand why there's no vertical shear stress in flange , can you explain about it ?

there is a small vertical shear stress in the flange, but if you do the math using the same area above cut b, you get the same Q, but the flange thickness is 200, not 20, so the vertical shear stress is only about 10 percent on average
of the vertical shear stress in the web at b, or perhaps just 5 percent of the max vert shear stress at the NA, and further,much of that vert flange shear stress is fictitious because there is no shear stress at the free edges of the flange, vert shear is internal within the flange. Vert shear in flange is only useful when you want to strengthen the beam by welding a plate to the top flange, and the welds required are quite small because the vert shear stress is so small.

 

[fonseh]

same Q,

removed

 

[fonseh]

there is a small vertical shear stress in the flange, but if you do the math using the same area above cut b, you get the same Q, but the flange thickness is 200, not 20, so the vertical shear stress is only about 10 percent on average
of the vertical shear stress in the web at b, or perhaps just 5 percent of the max vert shear stress at the NA, and further,much of that vert flange shear stress is fictitious because there is no shear stress at the free edges of the flange, vert shear is internal within the flange. Vert shear in flange is only useful when you want to strengthen the beam by welding a plate to the top flange, and the welds required are quite small because the vert shear stress is so small.

ok , i don't understand why when we cut the portion of the flange or web , there are only 2 shear stress acting ? shouldn't be 3 stress ? for example , in the vertical cut at flange , the stress only act to the 2 faces of the beam flange only ? How about the top part ? why there's no stress over there ?

 

[PhanthomJay]

ok , i don't understand why when we cut the portion of the flange or web , there are only 2 shear stress acting ? shouldn't be 3 stress ? for example , in the vertical cut at flange , the stress only act to the 2 faces of the beam flange only ? How about the top part ? why there's no stress over there ?

in the flange cube, I didn't show the small vert shear stress which exists downward on the left face, and with its accompanying longitudinal stress which lies on the green area. In the web cube, there is no horiz shear stress, so that face has no stress shown. Note that in both cases there are equal and opposite shear stresses on the far faces of the cube, not shown for clarity.

 

[fonseh]

Note that in both cases there are equal and opposite shear stresses on the far faces of the cube, not shown for clarity.

far faces of the cube means the face behind ?

 

[fonseh]

in the flange cube, I didn't show the small vert shear stress which exists downward on the left face, and with its accompanying longitudinal stress which lies on the green area.

do you mean they have the stress like this ( blue part) ? just they are not shown in your diagram ?

 

[PhanthomJay]

Vert shear stress blue down on left face and it's complimentary long shear stress blue right on top face. Get rid of the other blue ones. There are 6 faces of cube I showed three, back faces have equal and opposite shear stresses to maintain rotational equilibrium.

 

[fonseh]

Vert shear stress blue down on left face and it's complimentary long shear stress blue right on top face. Get rid of the other blue ones. There are 6 faces of cube I showed three, back faces have equal and opposite shear stresses to maintain rotational equilibrium.

do you mean the shear stress is like this ? why the shear stress is vertical down at the left face ? do you mean there's no vertical shear stress on the right face ? why ?

For the top part of the beam , is the shear stress direction correct ?

 

[PhanthomJay]

do you mean the shear stress is like this ?

yes

why the shear stress is vertical down at the left face ?

the left face is the front face on the xy plane

do you mean there's no vertical shear stress on the right face ? why ?

no vert shear on right face in the yz plane. Vert shear acts in the xy plane only,and it's complimentary long shear acts on top face in xz plane, in blue. In the plane of the cut.

For the top part of the beam , is the shear stress direction correct ?

yes

Remember that tis is a cut of the flange. Horizontal cut in flange for vert shears in blue and vert cut for horiz shears in black.

 

[fonseh]

yes the left face is the front face on the xy planeno vert shear on right face in the yz plane. Vert shear acts in the xy plane only,and it's complimentary long shear acts on top face in xz plane, in blue. In the plane of the cut.
yes

Remember that tis is a cut of the flange. Horizontal cut in flange for vert shears in blue and vert cut for horiz shears in black.

i think your explanation of x , y , z axes is based on this ?

 

[fonseh]

no vert shear on right face in the yz plane. Vert shear acts in the xy plane only,

why there's no vertical shear act on right face in the yz plane ??

 

[PhanthomJay]

i think your explanation of x , y , z axes is based on this ?

Yes

 

[fonseh]

Yes

could you explain why there's no vertical shear act on right face in the yz plane ??

 

[PhanthomJay]

could you explain why there's no vertical shear act on right face in the yz plane ??

Fonseh

These questions are going way beyond 'homework help', but I've been trying to assist since there is no forum here on Civil/Structural Engineering. In the future, you should probably post in the Mechanical Engineering forum for questions related to general understanding that are NOT homework related.

Anyway, applied loading is in the vertical y direction and bending is about the NA in the y direction about the xx axis. For equilibrium of bending and shear stresses, vertical shear in the xy plane exists on the front and back faces of a cubed element , and complimentary longitudinal stresses occur on the top and bot faces in the xz plane. There is nothing going on in the side yz plane faces since there is no bending taking place in the x direction about the yy axis, the beam supports are at either end along the z axis..

 

[fonseh]

whe

Mechanical Engineering forum

re is it ? can you provide the link ?

 

[PhanthomJay]

from the home page, just toggle down to ENGINEERING/Mechanical Engineering. Just be sure that your question is of a general nature and is NOT Homework related.

https://www.physicsforums.com/forums/mechanical-engineering.101/