Quantcast Figure 33. Arrangement of Reinforcement for Combined Flexure and Torsion -Cont.

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whichever is greater.  When using Equation (54b), the value of 2At shall
satisfy the following;
EQUATION:
2At >/= 50bs/fdy
(55)
where,
fdy = dynamic yield stress, psi
(11) It should be noted that Equation (54a) requires the volume of
longitudinal reinforcement to be equal to the volume of the web
reinforcement required by Equation (51) or (52) unless a greater amount of
longitudinal reinforcement is required to satisfy the minimum requirements
of Equation (54b).
(12) Longitudinal bars should be uniformly distributed around the
perimeter of the cross section with a spacing not exceeding 12 inches.  At
least one longitudinal bar should be placed in each corner of the closed
stirrups.  A typical arrangement of longitudinal bars is shown in Figure
33b, where torsional longitudinal bars that are located in the flexural
tension zone and flexural compression zone may be combined with the
flexural steel.
(13) The addition of torsional and flexural longitudinal
reinforcement in the flexural compression zone is not reasonable.  It is
illogical to add torsional steel that is in tension to the flexural steel
that is in compression.  This method of adding torsional steel to flexural
steel regardless of whether the latter is in tension or in compression is
adopted purely for simplicity.  For blast resistant design, flexural
reinforcement added but not included in the calculation of the ultimate
resistance could cause a shear failure.  The actual ultimate resistance
could be significantly greater than the calculated ultimate resistance for
which the shear reinforcement is provided.  Therefore, torsional
longitudinal reinforcement cannot be indiscriminately placed but must be
placed only where required.
(14) In the design of a beam subjected to both flexure and torsion,
torsional longitudinal reinforcement is first assumed to be uniformly
distributed around the perimeter of the beam.  The reinforcement required
along the vertical face of the beam will always be provided.  However, in
the flexural compression zone, the reinforcement that should be used is the
greater of the flexural compression steel (rebound reinforcement) or the
torsional steel.  In terms of the typical arrangement of reinforcement in
Figure 33b, either A's or A*l is used, whichever is greater, as the design
steel area in the flexural compression zone.  For the tension zone at the
mid span of a uniformly loaded beam the torsional stress is zero and
torsional longitudinal reinforcement is not added.  Conversely, the tension
zone at the supports is the location of peak torsional stresses and
longitudinal torsional reinforcement must be added to the flexural steel.
2.08-80





 


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