shear flow with y = 0

huangyhg

shear flow with y' = 0?
i am having a bit of a problem with a shear flow problem.  i have a wf with strong axis bending (and some torsion) that is getting a vertical plate welded to (1) side of it.
this plate is to take out the torsion.  when i am calculating the shear flow, the a' is the area of the plate, but y' is the distance from the centroid of the composite section to the centroid of the connected piece (plate).  in this case the centroidal axes are coincident, and y' = 0.  this is obviously problematic because i can not calc shear flow with this.  i have considered calcing the shear flow at the juncture of the web/flange and distributing it based on relative thickness between the flange and plate; this is not at the exact juncture that i am seeking, but i feel it is close enough since the weld will most likely be overdesigned by a significant factor.
does anyone have any suggestions.
not sure i understood yr problem correctly - are you welding a plate to side of web, thickening the web?
no, i am welding the plate to the edges of the flanges in an effort to simulate an hss section.  
see this thread  
willis- will that work?  it appears to me that the w you are calculating is a vertical line load that will produce the required deflection.  i agree with that.  how does that vertical line load of w get transferred into a horizontal shear flow resisted by the bolts (in the case of thread you referenced)?  if, for example, the w turned out to be 9.3 kips/ft - would you say (1) - 3/4" a325n bolt/12" is ok?  
yes.
myerges
i think your approach is correct.
if you think of this as welding the flange to the plate and the web, instead of the plate to the flange, then your weld will take a portion of the total shear flow. the question is, what portion?  distributing it based on the relative thickness of the plate and the flange makes sense to me.  after all, if the web and plate deflect the same, then the shear in each will be proportional to the relative thickness, as opposed to the relative size of the welds.
jmiec..i'm not so sure on that - if you are looking at deformation compatibility the load in each will be proportional to the moment of inertia of the plate vs. that beam - not the areas of plate to flange.  
willisv
oops, i meant to say the distribution should be based on the relative thickness of the web and the additional plate.  that would make the shear flow proportional to the thickness of the plate vs. that of the web of the beam.
here's how i got to that conclusion.  imagine that the section is made up of four plates, two flanges and two webs. now weld the flanges to the web plate, and to the other plate.  all the welds will have shear flow, proportional to the thickess,(and moment of inertia) of the vertical plates.  
willisv
i don't believe the thinking that went into your response to
myerges:  i assume your i-beam y axis is vertical, with bending about the x axis, so your statement y' = 0 doesn't seem to make sense. the section cut for shear flow is generally on (or close to) only one plane, not two different planes. so there's no way to completely section off a free body of your attached plate and, at the same time, have a meaningful y' value.
instead, your section cut would be, e.g., near the inner face of your i-beam flange, cutting the web and your attached plate (or weld) simultaneously. so a' would basically be the cross-sectional area of one flange, not of the attached plate. then, the shear stress can be roughly assumed to be evenly distributed across this section cut, meaning it is distributed approximately in proportion to the web and attached plate thicknesses.