possibly a very silly question

huangyhg

possibly a very silly question
hello all,
i have a very silly question that i would like to pose. i have a steel beam with a simple connection, clip connection bolted to the web and welded to an embedded plate within a concrete wall. the weld connecting the angle and the embedded plate is inadequate, because the embedded plate is only within the top 5" of the 24" beam. my supervisor states that we can add an angle seat at the bottom flange of the beam, but i am not sure how that will be able to transfer the shear? (i can certainly visualize the effectiveness of the seat, but theoretically, i thought shear is only carried mostly by the web of the beam? i am a confused! ;(
thanks,
clansman
check out our whitepaper library.
the shear is carried by the web, but will transfer in bearing through the bottom flange.  this is an issue of stiffness, as in if your weld is already in place the seat needs to be welded to ensure that it does not rotate (and thus stress relieve, resulting in an overstress on the weldment).  with that caveat, i would agree with your supervisor...
given that your supervisor is your supervisor, i would presume she/he would be pleased to have a chat with you about this.  i often have asked just as silly/simple seeming questions in the past... no question is stupid, only not asking is stupid.
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
i am assuming that the seat angle would be epoxy bolted to the wall below the mis-placed (seemingly) embed plate.  is that accurate?
if so, i would suggest sizing the seat angle to handle all of the shear as aisc doesn't allow mixing of bolts and welds.  i would bet that the same goes for epoxy anchors and welds.
additionally, in an effort to ensure the epoxy anchors see pure shear (or as close as possible to pure shear), i would either provide a stiffener on the seat angle (assuming the vertical leg is down) or provide bolts from the beam's bottom flange to the seat angle.  you will have to provide some positive connection there, so include the bolts and call it a day.  just make sure to size the thickness.
i agree with your supervisor.
adding a seat angle below the beam when the embed plate is misplace (or sometimes missing completely) is a fairly common fix.
the trick, if it is beam with a large reaction, is getting enough expansion or epoxy anchors to handle the load. if this becomes a problem, i have taken angles (one on each side of the web) and run them vertically down the wall. weld the legs against the web to the web, expansion anchor the outstanding legs to the wall with as many anchors as required. it basically looks like a standard double angle shear connection, but you run it as far down the wall as required to get the required number of anchors. this may require that the bottom flange of the beam be coped away, but with a torch and a grinder, that is usally not a big issue.
i agree with structuraleit in that i would size what ever fix is decided on to take the entire beam reaction, and ignore the contribution from the 5" of angles that is connected to the embed plate.
jmho.  
p.s.
definately not a silly question!
hold on everybody:  this is no longer a simple "cleat" shear connection... as soon as you move a second fixing below the existing to any appreciable distance, you have two, separate, fixings with unique loading on each.
if you want to keep this as a simple, shear-only, connection, you need to provide a slip connection at the top.  it is not possible to maintain a "pin" assumption for the forces if you have a semi-rigid connection near the top of your beam and another near the bottom.
i do hope that is more clear.  if not, i'm happy to provide a sketch.  i would also like to hear a detailed explanation from anyone who feels i am wrong.
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
i definitely see the point about creating some fixity, but i guess i am looking at the sketch i attached and thinking that between the top angles flexing, the ability of the bolts through the beam web to slip, and the ability of the bolts at the bottom beam flange to slip that this condition would allow at least as much beam end rotation as a full depth shear connection that was (likely)originally specified.
seit,
i agree with ys.  you now have a moment connection, and it won't take much rotation to do some damage to the concrete at the top.  and you need to make the bottom seat much stiffer than your sketch implies.
well, it was just a generic seat angle, i didn't actually design it.
i certainly see the concern of the restraint near the top flange, so just provide horizontal slotted holes in either the beam or the angles.
suggestion to clansman--since i take it this beam is already in place, after installing the seat, i would just take out the bolts and weld a plate on the other side of the web so the web can slide as it rotates.
thank you all for the informative discussion and hints.
yes, the beam is already in place and the holes in the existing double clip connection are not slotted, so from what   i can gather from the above discussion, i can expect fixity in my end restraint when i add my seat angle even if i provide slotted holes in my seat angle.
hookie,
i did not quite understand your last post. are you suggesting that i get rid of the existing double clip connection once my seat is in place and weld a shear tab instead that has slotted holes allowing for sliding action?
thanks again everyone,
clansman