Alternative Support Systems for Cantilever - National Transportation ...

torsional strength of the lever arm pipe was calculated to be only 359 kip-ft, which exceeds the
concrete breakout strength and does not exceed the bearing strength. Since the estimated strength
will likely lie between those values, the lever arm pipe does not provide enough strength. For the
first test the concrete shaft diameter was reduced to 26” to decrease the concrete breakout and
bearing strength to 212 kip-ft and 333 kip-ft, respectively. However, for the second test a 30”
concrete shaft diameter was chosen because it was thought that the interaction of the flexural and
torsional failure modes would reduce the overall strength of each failure mode and the increased
value of ca1 would be compensated for.
4.3.2 Torsion Design
The basic threshold torsional strength of the concrete shaft was calculated using ACI 318-
08 11.6.1(a) to be 18 kip-ft (5). The threshold torsional strength does not take into account the
reinforcement present in the concrete shaft and therefore will likely be exceeded. Therefore, the
nominal torsional strength, which does take into account reinforcement, was used as the design
torsional strength. The cracking torsional strength was determined from ACI 318-08 R11.6.1 as
73 kip-ft (5). Since the concrete breakout and side-face blowout torsional strengths exceeded this
value, it indicated that there would be torsional cracks in the concrete shaft before it fails.
In order to calculate the nominal torsional strength of the concrete shaft, the reinforcement
needed to be specified. The starting point was derived from the previous design, with the
transverse hoop steel being comprised of #3 bars spaced at 2.5”. However, it became clear that
the torsional strength with this reinforcement scheme, 191 kip-ft, was insufficient to exceed the
concrete breakout or bearing strength of the section, 212 kip-ft or 333 kip-ft, respectively.
Therefore, the hoop steel size was increased to #4 bars and the spacing decreased to 2” to yield a
nominal torsional strength of 426 kip-ft, which exceeded the concrete breakout strength of the
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