This report investigates the application of the
Strut-and-Tie Method (STM) for the design and evaluation of reinforced concrete
bridge bent caps in North Carolina. The study addresses practical challenges in
implementing STM, seeks to improve design and assessment consistency, and
examines structural performance under both ultimate and service load
conditions.
A literature review was conducted to establish the foundation for
STM design. Multiple bent cap typologies common in North Carolina were
identified, and templates were developed to guide engineers through STM design
in alignment with AASHTO LRFD provisions.
An experimental program involving six
large-scale deep beam specimens was undertaken to validate the STM templates
and assess the influence of different reinforcement levels and geometries.
Specimens were constructed with transverse and horizontal reinforcement ratios
of 0%, 0.13%, and 0.30%.
Crack Pattern of beam at failure load
Results demonstrated that transverse reinforcement
significantly improves shear capacity and serviceability. Members with 0.13%
reinforcement achieved higher shear capacities than unreinforced specimens,
while those with 0.30% reinforcement offered improved crack control with only
modest strength gains. Reinforcement also influenced crack width distribution,
with higher ratios resulting in smaller crack widths and more crack control. Geometric
parameters, particularly shear span-to-depth ratio, affected crack behavior,
with shorter spans exhibiting smaller crack widths.
Based on these findings, a
minimum distributed reinforcement ratio of 0.13% to 0.30% is recommended to
ensure adequate service-level performance. The results also support the use of
full nodal efficiency factors in evaluating existing members with distributed
reinforcement that may be less than the minimum. This work provides a description
for integrating STM into the NCDOT’s design workflow and offers guidance for
both new and existing bent cap evaluation.
