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11
Where flexural shear, torsion, and moment. It is based on the provi-
sions of Article 5.8.6 and, therefore, applicable to a segmental
Tn = (2AoAtfycot )/s (Equation C5.8.3.6-2) post-tensioned concrete box-girder bridge.
At /s = Tn/(2Aofycot )
For the exterior web of a box section, the combined area of Step 1 Determine the Controlling
both stirrup legs in the web, Astirrups, contributes to Av and At, Load Cases
therefore the maximum spacing of the stirrups, Smax, is The design for flexural shear and torsion in segmental
given by: bridges shall be performed at the strength limit state per Ar-
ticle 5.8.6.2. The shear component of the primary effective
Smax = Astirrups/[(Av/s)flexural shear + (Av/s)torsion]
longitudinal prestress force, Vp, shall be added as a load effect
with a load factor of 1.0. The component of inclined flexural
Step 7 Check the Longitudinal compression or tension, in the direction of the applied shear,
Reinforcement in variable depth members shall be considered when deter-
mining the design factored shear force.
The required tensile capacity of the longitudinal reinforce-
In accordance with Article 5.8.5, principal stresses at the
ment on the flexural tension side of the member is found
neutral axis of segmental bridges shall not exceed the tensile
from Equation 5.8.3.5-1.
stress limits of Table 5.9.4.2.2-1 at the Service III limit state
(Comment Clarify that the flexural shear, not the equivalent and Table 5.14.2.3.3-1 during construction
factored shear force, is used for Vu, as the additional longitudinal Determine the controlling load cases for each of the three
reinforcement for torsion is determined separately.) applicable limit states separately (Strength, Service III, and
The longitudinal reinforcement required for torsion, in during construction). Consider the concurrent actions on the
addition to that required for flexure, is found from section. As a minimum, consider the following two cases for
each of the limit states:
Al = Tn ph/(2Aofy) (Equation 5.8.3.6.3-2)
1. Maximum flexural shear and concurrent actions
Comments: 2. Maximum torsion and concurrent actions
Article 5.8.3.6.3 would benefit from a clarification that Al is
also in addition to the required tensile capacity from Equa- Perform Steps 3 through 6 separately for each of the above
tion 5.8.3.5-1 when Equation 5.8.3.5-1 exceeds the longitudinal Strength cases and any additional Strength cases that may
reinforcement required for flexure. govern the design.
The distribution of Al within the cross section needs to be clar- In Step 4, check the principal stresses separately for each of
ified. Article 5.3, "Notation," and Article 5.8.6.4, "Torsional the above Service III and construction cases and any additional
Reinforcement," define it as the area of longitudinal torsion re- Service III and construction cases that may govern the design.
inforcement in the exterior web of the box-girder, which appears
to be incorrect. LRFD Equations 5.8.3.6.3-2 and 5.8.6.4-3 are
essentially identical to the equation in Article 12.3.8 of the Seg- Step 2 Determine the Cross-Section
mental Guide Specification, which specifies that Al shall be dis- Parameters
tributed around the perimeter of the closed stirrups. Ao area enclosed by the shear flow path (in.2) (Arti-
Prestressing steel should also be permitted to satisfy Equa- cle 5.8.6.3)
tion 5.8.3.6.3-2 similar to Article 12.3.8 of the Segmental Guide be effective width of shear flow path, but not exceeding
Specification and LRFD Commentary Article C5.8.6.4 for seg- the minimum thickness of the webs or flanges compris-
mental bridges. The area of longitudinal torsion reinforcement ing the closed box section.(in.) be shall be adjusted to
in the flexural compression zone should be permitted to be re- account for the presence of ducts as specified in Arti-
duced similar to Segmental Guide Specification Article 12.3.9 cle 5.8.6.1. (Article 5.8.6.3)
and LRFD Equation 5.8.6.4-4 for segmental bridges. be may be taken as Acp/pc (Article 5.8.6.3)
Acp area enclosed by outside perimeter of concrete cross
Design Steps (Segmental section (in.2) (Article 5.8.6.3)
Box-Girder) pc the outside perimeter of concrete cross section (in.)
(Article 5.8.6.3)
The following outlines the basic steps of designing the ph perimeter of the centerline of the closed transverse tor-
exterior web of a box section for the combined actions of sion reinforcement (in.) (Article 5.8.6.4)