- Flexural
Strengthening: it is realized by
bounding one or more laminas or sheets on the tension side
of structural members, and when necessary it is allowed
also on the sides, but in this case the stress in the
FRP needs to be competed at every point and not only at
the most solicited one (i.e. further away from the neutral
axis). For negative moment regions it is also very efficient
the NSM technology.
1.Strengthening with FRP allows flexural strength increments
up to 40/50% of the nominal cross sectional strength but
on the other hand it
reduces deformations and ductility of the section;
2. The efficiency of FRP strengthening is only at ultimate
state. In service FRP do not provide significant performance
increase since they do not contribute mostly;
3.To assure that the FRP is efficient also in service it
is necessary to tension the lamina through special tensioning
devices that many times result very expensive and not easy
to install or alternatively jacketing the structure in order
to partially unload it from dead weights and once the strengthening
has been completed, released it from the jacketing making
effectively acting the retrofit also under service loads;
4. A correct design with FRP is such that at ultimate limit
state, concrete crushes first while FRP has not reaches
its ultimate stress (such mechanism is considered less fragile
that FRP rupture) and in addition that FRP does not delaminate
by providing appropriate anchoring lengths or anchoring
solutions (not mechanical such as for example using “U”
wrapping along the bean anchoring the longitudinal flexural).
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