Shear Analysis of Weld of Built up Section

[Buzzbromp]

Hi,

I have a beam that is considerably corroded. Our repair calls for a welded channel along the length of the web of this beam for reinforcement. Analyzing the beam now as a built up section with included detioration, my sectional modulus is acceptable. My question is how do I analyze the stress in the weld? I have a fillet weld running continuously along the bottom of the channel's bottom flange, and have to slot the web just beneath the top flange for welding access. These slots are fillet welded also. I took the maximum moment in the beam, calculated the new neutral axis, and what equal and opposite forces from the top (compressive) and bottom (tensile) welds would create that moment about the neutral axis. Then i applied this compressive load as a shear on the full length of slotted welds to see if my weld was acceptable. I feel like this approach is conservatie, what do you think?

 

[desertfox]

Hi bradleydawson

I am no expert in this field but was trying to find something to help you and will keep looking in the meantime I do have a couple of questions:-

1 I thought the best way to reinforce a beam was welding
plates to the flanges of the beam, I don't quite
understand why your just welding a channel to the web or
is it that your worried about web buckling?

2 Assuming that the web takes mostly the shear forces would
it not seem reasonable to create a shear force diagram,
determine your maximum shear force and use that to stress
the fillet weld ie:- fillet weld with a transverse load?

regards

desertfox

 

[Buzzbromp]

Desertfox,

Yes, the best way would be to reinforce the beam by welding plates to the flanges of the beam. However, the top flange of this beam is embedded in concrete, and access to both sides of the bottom flange is extremely difficult.

I have looked at the transverse shear, but i was thinking there would also be a longitudinal shear along the length of the weld due to the bending along the beam. Although the web of the I beam takes mostly the shear force, I was thinking this built up section would transfer a portion of the moment to the channel. If so, wouldn't the welds resistance to the moment transfer act as a longitudinal shear force? When you weld plates to the bottom flange of a beam, do you calculate the average stress in the plate at the location of highest moment, multiply this stress by the plates cross-sectional area and use that as your maximum shear force acting against the entire length of weld? Or do you integrate these shears along the length of the beam to get a total shear loading?

 

[desertfox]

Hi bradleydawson22

You have shear in both directions ie complementry shear is needed to maintain equilibrium.
Doing some digging I believe there is a specification for
welded plate girders published by the AISC you might like to check it out.
Secondly I came across a reference to built up girders in an old "Strength of Materials" book by R.C. Stephens which states:- " welds that connect flange plates to a web transfer the horizontal shear force between web and flange.
Thus if the shear stress at the top and bottom of the web is
[τ], the horizontal force in a welded joint, per unit length of beam is:-

[τ] * t * l

where [τ]= shear stress

t= web thickness

l= length.

So I would calculate my shear stress at a point in the beam web which coincides with my fillet weld on the channel and thus find the force on the weld from the above and hence the stress.


hope this helps

regards

desertfox

 

[desertfox]

Hi

I found this also about welded girders from another thread:-

http://lists.topica.com/lists/Structuraldzine/read/message.html?mid=1605005596&sort=d&start=0

regards

desertfox