By extending the concepts established in ACI 216R to FRP-strengthened reinforced concrete, limits on strengthening can be used to ensure a strengthened structure will not collapse in a fire event. A member’s resistance to load effects, with reduced steel and concrete strengths and without the strength of the FRP reinforcement, can be computed. This resistance can then be compared with the load demand on the member to ensure the structure will not collapse under service loads and elevated temperatures.
The nominal strength of a structural member with a fire resistance rating should satisfy the conditions of Eq. (9-2) if it is to be strengthened with an FRP system. The load effects, SDL and SLL, should be determined using the current load requirements for the structure. If the FRP system is meant to allow greater load-carrying strength, such as an increase in live load, the load effects should be computed using these greater loads. The nominal strength at high temperature should be greater than the strengthened service load on the member (ACI 216R should be used for ASTM E119 fire scenarios)
Rnθ ≥ SDL + SLL (9-2)
The nominal resistance of the member at an elevated temperature Rnθ may be determined using the guidelines outlined in ACI 216R or through testing. The nominal resistance Rnθ should be calculated based on the reduced properties of the existing member. The resistance should be computed for the time period required by the structure’s fireresistance rating—for example, a 2-hour fire rating—and should not account for the contribution of the FRP system, unless the FRP temperature can be demonstrated to remain below a critical temperature for FRP. The critical temperature for the FRP may be defined as the temperature at which significant deterioration of FRP properties has occurred. More research is needed to accurately identify critical temperatures for different types of FRP. Until better information on the properties of FRP at high temperature is available, the critical temperature of an FRP strengthening system can be taken as the lowest Tg of the components of the system.
Furthermore, if the FRP system is meant to address a loss in strength, such as deterioration, the resistance should reflect this loss. The fire endurance of FRP materials and FRP strengthening systems can be improved through the use of polymers having high Tg or
using fire protection (Bisby et al. 2005a).