Reinforced Concrete Design for Thermal Effects on Nuclear Power Plant Structures
|Publication Date:||1 June 2007|
ACI 349, Appendix A, provides general considerations in designing reinforced concrete structures for nuclear power plants. The Commentary to Appendix A, Section A.3.3, addresses three approaches that consider thermal loads in conjunction with all other nonthermal loads on the structure, termed "mechanical loads." One approach is to consider the structure uncracked under the mechanical loads and cracked under the thermal loads. The results of two such analyses are combined.
The Commentary to Appendix A also contains a method of treating temperature distributions across a cracked section. In this method an equivalent linear temperature distribution is obtained from the temperature distribution, which can generally be nonlinear. Then the linear temperature distribution is separated into a pure gradient ΔT and into the difference between the mean and base (stress-free) temperatures Tm - Tb.
This report offers a specific approach for considering thermal load effects which is consistent with the above provisions. The aim herein is to present a designer- oriented approach for determining the reduced thermal moments which result from cracking of the concrete structure. Chapter 2 addresses frame structures, and Chapter 3 deals with axisymmetric structures. For frame structures, the general criteria are given in Sections 2.2 (Section Cracking) and 2.3 (Member Cracking). The criteria are then formulated for the moment distribution method of structural analysis in Section 2.4. Cracked member fixed-end moments, stiffness coefficients, and carry-over factors are derived and presented in graphical form. For axisymmetric structures an approach is described for regions away from discontinuities, and graphs of cracked section thermal moments are presented.
This report is not intended to represent a state-of-the-art discussion of the methods available to analyze structures for thermal loads. Rather, the report is intended to propose simplifications that can be made which will permit a cracking reduction of thermal moments to be readily achieved for a large class of thermal loads, without resorting to sophisticated and complex solutions. Also, as a result of the report discussion, the design examples, and graphical presentation of cracked section thermal moments, it is hoped that a designer will better understand how thermal moments are affected by the presence of other loads and the resulting concrete cracking.