Flexural members - Reinforced Concrete with FRP Bars: Mechanics and Design - page 111

occurred. However, no moment redistribution occurred when either the
midspan or middle support section reached its respective moment capac-
ity due to the brittle nature of FRP reinforcement rupture or concrete
crushing [16].
4.8.3 Compression FRP in flexural members
Differently from steel reinforced flexural members, the contribution of FRP
bars in compression does not increase the strength nor reduce the effects
of concrete creep of FRP reinforced flexural members due to the limited
compressive strength and modulus of the FRP bars. For this reason, ACI
440.1R-06 does not recommend relying upon compression FRP reinforce-
ment in flexural members, but allows its use for fabrication purposes.
The lack of effectiveness of FRP reinforcement in the compression zone
of flexural members does not necessarily preclude its use in columns. In
fact, Chapter 5 presents a methodology for the design of FRP RC columns
subjected to combined axial load and flexure. For analysis purposes, this
methodology is based on the assumption that the area of FRP bars in com-
pression can be replaced with an equivalent area of concrete—as if the FRP
bars were not present in the cross section.
COMMENTARY
Studies from Washa and Fluck [17] conducted on steel RC simply supported
beams demonstrated that the effect of compression reinforcement is benefi-
cial in reducing the long-term deflections under sustained loads. Such behav-
ior is attributed to the creep of concrete that transfers a portion of the
load from the concrete itself to the compression steel, thus reducing the
overall stress in the concrete and the resulting deflection caused by the sus-
tained load. Similar studies are not available for FRP RC members; however,
because of the relatively low elastic modulus of some FRP products (particu-
larly GFRP reinforcement) compared to steel and creep characteristics of
FRP (which are resin dominated), it is believed that such beneficial effects on
long-term deflection due to sustained loads are negligible.
In the technical literature, there is no direct measurement of time-dependent
creep of FRP bar coupons subjected to compression. Experimental evidence
obtained from GFRP pultruded shapes indicates that the ratio creep strain to
initial elastic strain is low for stress levels up to 45% of the proportionality
limit [18]. Accordingly, creep should not cause geometrical integrity problems
at the stress levels typical of FRP bars in compression.