contribution of existing shear reinforcement
contribution of existing shear reinforcement
(OP)
Im talking about a case, when you add another layer of concrete on top of existing beam (composite concrete beam).
There are anchors at interface so the beam acts as one element (composite section).
Can I consider contribution of existing stirups when calculating shear resistance of composite beam?
What I mean is: TOTAL SHEAR RESISTANCE OF A COMPOSITE BEAM = CONTRIBUTION OF EXISTING STIRRUPS + CONTRIBUTION OF NEW STIRRUPS






RE: contribution of existing shear reinforcement
DaveAtkins
RE: contribution of existing shear reinforcement
Looking at it from a failure perspective:
- I would move to eliminate this as a solution for torsional or reversing load situations. The potential stress reversals will elongate the epoxied bars and remove composite action.
- For a simple beam support, I would investigate the debonding of the epoxied-in anchors due to a significant crack developing along its plane that exceeds a "normal" shrinkage crack that epoxy systems are tested for.
A couple ways around the debonding problem:
- angle the epoxy anchors at a 45 degree against the compression struts - the compression struts will keep the epoxy plane closed
- model the beam as two non-composite beams and only use composite action for service conditions - the epoxy anchors aren't considered effective under failure.
RE: contribution of existing shear reinforcement
I vote yes-ish for the following reasons:
1) Gut feel. A diagonal shear crack is going to cross the stirrups in both the upper and lower beams and those stirrups are all well anchored, if not necessarily anchored directly to the new compression block.
2) In most codes, shear reinforcement is conceived of using truss models. As shown below, one can come up with what I believe to be a valid truss model for this situation.
A few additional points to keep in mind:
1) Because your load is coming into the bottom of the beam, your dowels must do two important jobs. The first is transfer of horizontal shear via shear friction. The second is dragging the portion of the beam load assumed to "live" in the upper beam up to the upper beam. And these two demands will be additive.
2) I believe that the proof below is satisfactory for conditions "in the field". The B/Bernoulli regions. I haven't yet fully explored how this would work at critical D/Disturbed regions such as your support points. There may be implications for detailing.
3) In these situations, it's rather difficult to provide conventional, code compliant shear reinforcing. You'd either have to drill and epoxy rebar all the way through and turn a blind eye for the hook requirements or install something like vertical through bolts that get anchored externally beyond the reinforcement and compression block. Just providing enough shear reinforcement in one of the beams would be insufficient. So this is a question of some practical importance.
Note that, were you to follow my appproach, equilibrium concerns would prevent you from simply adding the stirrup shear contributions of both beams. Rather, the correct algorithm would be:
1) Determine least shear reinforcing density of the upper and lower beams.
2) Use the shear reinforcing density from #1 and the combined beam depth to work out the shear capacity associated with the stirrup reinforcing.
In general, this may give you a higher capacity than either the upper or lower beams considered alone but would not give you the shear capacity of both stirrup sets added together (unless the densities were equal).
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: contribution of existing shear reinforcement