Abstract:
|
In reinforced concrete beams subjected to concentrated loads, the shear resisted increases
considerably as the loads approach to the support. There is not a
consensus in the current codes of
practice about any simple design formulation which adequately considers this phenomenon.
A mechanical model has been developed at the Polytechnic University of Barcelona, which predicts
the shear strength of beams. The mo
del is originally applicable to slender beams (a/d >2.0
-
2.5), and
is under consideration for adoption into the Eurocodes. The model deals with shear
-
flexure failure,
in which flexural cracks develop inclined through the web and stabilized near the neutral
axis. Failure
is assumed to happen when this flexural crack develops inside the compression chord, subjected to
normal and shear stresses, which takes place when the stresses reach the biaxial failure envelope of
Kupfer.
The model has been extended to ca
ses where the loads are applied near the supports. In this case,
the
Bernoulli
assumption (Plane sections remain plane) is no longer valid, and the problem must be
faced as a typical discontinuity “D” region, because the dimensions (a/d<2) and because the
vertical
stresses introduced by the vertical loads by means of the bearing plates interact with the state of
stresses of the compression chord.
The extension of the model proposed at UPC consists on incrementing the neutral axis depth as the
load approach
es the supports and including the vertical stresses due to the applied concentrated
loads, as confinement stresses in the concrete, thus changing the state of principal stress in the
failure envelope from tension
-
compression to compression
-
compression, thu
s enhancing the shear
capacity
In this thesis, a data base of beams with loads near the supports, with and without stirrups, has been
collected. A
comparison between the predictions of the proposed model and the predictions of the
Eurocodes has been made
, using the collected data base of 121 short
-
span beams which failed in
shear. 43 of the beams had stirrups, while the last 78 were not transversally reinforced, all the
beams have a/d
-
ratios of less than 2.5.
The results obtained in this thesis are appro
ximate to the results achieved in other literature. The
proposed model compares favourably to the Eurocode for beams with and without shear
reinforcement. In addition, some of the assumptions of the new model have been investigated using
a 2D finite elemen
t model. The results obtained with the 2D FEM confirm the assumptions used in
the model. |