Civil Engineering Reference
In-Depth Information
4
Design Issues
4.1 OVERVIEW
The design equations and procedures presented in the next three chapters are
based on the knowledge and principles put forth by ACI 318-11 (2011) and ACI
440.2R-08 (2008). Strain compatibility as well as force and moment equilibrium
must be enforced. Strain at the concrete substrate, at the time of strengthening,
also needs to be considered. FRP is not allowed to carry compressive stresses,
according to ACI 440.2R-08. Therefore, FRP used in tension is ignored during
moment-reversal calculations. Environmental factors need to be incorporated
into the design, as shown in Chapter 3. Limiting strains in shear and column
confinement are based on effective strains that are obtained experimentally.
Strengthening levels are limited by the capacity of the section to carry reasonable
upgraded loads in case of FRP loss during fire. This applies to shear and column
confinement as it applies to flexure.
4.2 DESIGN PHILOSOPHY OF ACI 440.2R-08
A limit-state design philosophy is adopted by ACI 440.2R-08. This process defines
load and resistance factors to various limit states considered, including
1. Ultimate limit state in flexure, shear, and fatigue.
2. Serviceability limit state in short- and long-term deflections and cracking.
For each limit state, different damage or failure modes and related design parameters
are examined. It is worth mentioning that the load factors and load combinations
follow the requirements of ACI 318-11 except for strengthening limits in case of fire.
On the other hand, reduction factors for FRP were calibrated by ACI 440.2R-08
to produce a reliability index of 3.5 and above. However, the reliability index may
be reduced to 3-3.5 in the case of high FRP ratio used with low steel ratio. This
case is almost nonexistent due to the strengthening limits during fire. The values
of the reliability index of FRP strengthened members were decided based on the
work of Szerszen and Nowak (2003), which is mainly applicable to unstrengthened
reinforced-concrete (RC) structures.
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