Behavior of spandrel beams strengthened with steel fibers under combined loadin
Loading...
Date
2015-10-01
Authors
Omer Farouk Ibraheem
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Important concrete members are subjected to significant torsion accompanied by
bending and shear. Until recent years, the design codes of reinforced concrete members
assumed that the effects of torsion could be safely neglected due to high safety factors
for shear and bending moment. Thus, members under combined loading were not treated
with serious attention. However, this assumption cannot be applied anymore as torsion
issues become common and play a significant role in structural members, such as
spandrel beams. The spandrel beam, or the L-beam, lies on the perimeter of buildings.
Any failure in spandrel beams can seriously damage slabs, beam-column connections,
and punch concrete flat-plates. By incorporating steel fibers, it can enhance torsional
behavior of spandrel beam under combined load in addition to the structural
performance such as maximum load, ductility and cracking resistance. Steel fibers may
provide resistance to combined loading as stirrups and longitudinal bars, this
investigation is still scare and limited. Moreover, a worldwide interest in utilizing fiber
reinforced concrete structures for civil infrastructure applications has increased. This
study presents the advantage of using steel fiber concrete in strengthening spandrel
beams under different reinforcement and loading cases. An experimental investigation
was conducted to assess the behavior of steel fiber reinforced concrete spandrel beams
subjected to combined torsion, bending, and shear. A total of 18 spandrel beams were
prepared and tested with two common loading combinations i.e. high torque to bending
ratio and low torque to bending ratio. All beams were divided into three groups, namely,
plain concrete beams, specimens with longitudinal reinforcing bars and specimens with
bars and stirrups. All cases were examined with 0%, 1%, and 1.5% steel fiber volume
fractions. Fibrous concrete beams exhibited improved overall torsional performance
with respect to the corresponding non-fibrous control beams. The main contribution of
steel fibers on the torsional behavior is mainly observed after concrete cracking. The
addition of steel fibers was essential to the beams without conventional steel
reinforcement since fibers were the only reinforcement and proved capable to provide
enhanced torsional moment capacities. Besides, a numerical analysis by finite element
method was suggested against the experimental data to predict the structural behavior of
spandrel beams. A new technique was presented to incorporate the effects of steel fibers
within concrete element and the softening effect of concrete in compression and tension.
The general behavior of the finite element model represented by the torque-twist plot
and load-deflection plots show good agreement with the test data from the plain and
fiber reinforced spandrel beam. However, the finite element models show higher
stiffness than the test data in the nonlinear ranges for beams without steel reinforcement.