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OCR for page 43
43
(a) Specimen P615-3B (b) Specimen P1035-3B
Figure 27. Components of shear friction behavior.
2.6.2.3 Components of Shear Friction equation. Different assumptions of µ will shift the curves in
a linear manner.
The behavior described in the previous section is illus-
These findings demonstrate that Equation 11 does not rep-
trated in Figure 27, which shows the load-crack width (V-w) resent the shear friction mechanism since it implies that the
plots decomposed into their steel and concrete components maximum concrete and steel components of the shear fric-
for Specimens P615-3B and P1035-3B. Similar plots were tion occur simultaneously. In fact, as seen in Figure 27, the
developed for all specimens (Zeno 2009). In Figure 27, the concrete component contributes to the majority of the shear
steel clamping component of shear friction was calculated friction capacity before the ultimate shear load is reached
using the measured interface reinforcement strain (s) to and then falls to a residual value while the steel component
calculate the actual steel stress and assumes a friction factor, increases. However, the steel component never reaches its
, equal to 1.0 (consistent with AASHTO provisions); thus, peak value, Avf fy, before the ultimate shear load is reached.
the steel clamping force component of the total shear fric- Nonetheless, empirically limiting the yield strength, fy, to 60 ksi
tion is Avf sEs. The concrete component was calculated by in Equation 11 does provide safe design values.
subtracting the steel component from the applied shear Values of the nominal design shear friction capacity (Vni)
load: V - Avf sEs. Figure 27 also shows the calculated capac- calculated from Equation 11 are given in Table 21. Provided
ity for the specimens obtained using Equation 11 and mea- the limitation fy < 60 ksi is imposed, Equation 11 gives conser-
sured values of fy. From Figure 27 it can be seen that at its peak, vative estimates of capacity. However, if the measured values
the apparent concrete component greatly exceeds the nomi- of fy are used, Equation 11 becomes significantly unconserva-
nal concrete component (cAcv) and contributes to the major- tive when the higher strength A1035 bars are used (shaded
ity of the shear friction capacity of the specimens. The entries).
corollary of this observation is that the steel component is
significantly lower than the assumed design value (Avf fy)
2.6.3 Conclusions with Regard
and reaches its peak value well after the shear friction capac-
to Shear Friction
ity of the specimens is exceeded. As can be seen in Figure 27,
steel yielding was observed in P615-3B but only after a crack The present AASHTO requirement for shear friction capac-
opening of 0.09 in. while the steel in P1035-3B did not yield. ity (Equation 11) may be safely adopted for use with high-
The behavior of P1035-3B appears to achieve a "steady strength steel reinforcement and other steel not experiencing
state" (i.e., balance between the steel and concrete compo- a well-defined yield plateau provided the value of fy used in the
nents after a crack opening of about 0.08 in.). Similar behav- formulation is not taken greater than 60 ksi. This recommen-
ior was exhibited by all specimens having A1035 reinforcing dation is the present requirement and no change to AASHTO
(Zeno 2009). In all cases shown, the prescribed limits on LRFD §5.8.4 is required to accommodate high-strength steel.
shear friction capacity (K1 f cAcv and K2Acv) are greater than
the values of cAcv + Avf fy shown. Furthermore, it is acknowl-
2.7 Compression Members
edged that the use of the empirical value = 1 is arbitrary
although supported by current codes and much previous Analytical parametric studies were performed to examine
research. The assumption of a value is necessary to resolve behavior of columns reinforced with A1035 longitudinal and
the concrete component from an otherwise indeterminate transverse reinforcement. The current AASHTO requirements
