can i count on shear-friction capacity of the reinforcemen

huangyhg

can i count on shear-friction capacity of the reinforcement?
i have checked an existing concrete beam, its shear capacity is not enough, and there is no shear reinforcement(i.e. no tie rebar), but its flexural capacity is over kill and it has an extra layer of longitudinal rebar,  can i take this extra layer of rebar as shear-friction rebar and caculate its shear-friction capacity to compensate the lack portion of the shear capacity?
thanks a lot.
probably not.  the problem is that the critical shear location is away from the critical moment location (for simply supported beams), and at the critical shear location that rebar is most likely not developed.  the bars need to be fully developed to count on it yielding and getting the clamping force associated with the shear friction model.
ok, i need to mention that the development length is long enough from the critical shear section (rebar extended long enough pass the beam supporting point. so the development length is not a problem.
i raise this issue here is becausei haven't seen any example of using this longitudinal reinforcement in  shear capacity caculation for beams. but base on my engineering knowledge, these longitudinal rebar should has contributions to beam shear resistance and also aci318 does talk about shear-friction in chapter 11.7.4. what i don't understand is it seems no people used this shear-friction in beam design? am i wrong?  
i believe that any reinforcing you use (to have a vs in your shear capacity) must have some incline to them...see aci 318, section 11.5.1.2 for the definition - this requires a 30 degree minimum incline.
see also 11.5.6.4 and 11.5.6.5.

i would say, no.  commentary r11.7 says that the shear friction provisions "provide design methods for conditions where shear transfer should be considered: an interface between concretes cast at different times, an interface between concrete and steel, reinforcement details for precast concrete structures, and other situations where it is considered appropriate to investigate shear transfer across a given plane."
the shear friction provisions are based on testing of the specific conditions mentioned.  it is not appropriate to apply the provisions to situations other than those for which the testing was performed.  specifically, it is not appropriate, in my opinion, to apply shear friction to slabs and beams, except at the interface with another concrete element.
my 2 cents:
i think you can use the shear friction capacity, but it is not additive to the shear capacity of the concrete beam. it is an either or situation. you can use either the straight shear capacity of the beam or you can use the shear capacity via the shear friction method, but they do not add togther.
shear frictition is really a clamping force to generate a high enough friction to keep two planes of material from sliding past each other. this implies, in my mind, that there already is a plane of interface bewteen two surrfaces, which, in your case, would mean that the concrete has already cracked due to shear failure, and you are computing whether there is enough steel to keep the now two seperate surfaces in close enough contact so as to not slide past each other.
jmho, concrete shear topics are definately not my strong suit.  
with no stirrups in the beam, how will the shear friction bars help with respect to the diagonal tension in the beam due to the shear?
the simple answer is no.  others here have touched on the reasons:
1) there is no "forced" shear plane, it is an inclined failure plane = diagonal tension.
2) flexural reinforcement cannot serve as shear-friction reinforcement in the way described.  all the flexural reinforcement will provide is "dowel action" - which is included in aci's vc.
shear friction relies on aggregate interlock, with the clamping force provided by the reinforcement.  the two faces along the failure plane essentially have to "slip" a tiny bit to really engage.  by the time your stirrup-less beams "slips", it has an inclined diagonal tension crack = failure.

miecz: i think any place where the concrete is loaded to a point where a crack may form should be checked for shear-friction regardless if poured at different times.
i've always considered shear-friction to be a secondary check to the normal shear capacity calculation.  i've never seen that it can be used as a substitute, but i don't know for sure.  
perhaps if you were able to get a strut and tie model to work, you could justify it.  
like several have stated, you're crack will be diagonal, so you could possibly only use a portion of the shear friction tension bars for this purpose (see 11.7.4.2)  
you cannot use shear friction in the case of a beam with a diagonal shear crack.
here's why:
if you look at section 11.7.4, there is equation 11-26 which provides you the shear friction capacity for a bar inclined to the shear plane.
in the case of a beam, the shear plane is about a 45 degree diagonal.  but if you look at figure r11.7.4, it indicates that the shear direction across the crack is such that it drives tension into the reinforcing bars.  for a beam, longitudinal reinforcing is oriented opposite to the bars shown in the figure.  thus, the value of αf is 135 degrees, not 45 degrees.
(see the attached drawing)
so from equation 11-26:
vn = avf(fy)(μsin(135) + cos(135))
   = avf(fy)(0)
   = 0 kips
now if the reinforcing bars are rotated 90 degrees (i.e. vertical stirrups), then equation 11-26 is:
vn = avf(fy)(μsin(45) + cos(45))
   = avf(fy)(1.41)

the case of an assumed failure plane that would result in compression in the reinforcing is discussed on pages 253-254 in reinforced concrete design by s unnikrishna pillai, et al (see attached).