Flexural design, reinforced concrete

The advantages of using reinforced concrete for the design of blast-hardened structures and the important recent changes to the design criteria of flexural elements have been summarized. Detailed design of hardened structures should be in accordance with the criteria in the tri-service design manual, TM 5-1300/NAVFAC P-397/AFM 88-22, "Structures to Resist the Effects of Accidental Explosions". 

Reinforced concrete is a complex material to model due to the brittle nature of concrete and non-homogenous properties. Although sophisticated methods are available to model crack propagation and other responses, simplified methods are normally used in blast design of facilities. These methods are based on a flexural response and rely on elimination of brittle modes of failure. To achieve a ductile response for concrete, proper proportioning and detailing of the reinforcing is necessary. 

The primary failure mechanisms encountered in reinforced concrete buildings arc flexure, diagonal tension, and direct shear. Of these three mechanisms,. flexure is preferred under blast loading because an extended plastic response is provider prior to failure. To assure a ductile response, sections are designed so that the flexural capacity is less than the capacity of non-ductile mechanisms. 

Maalej, M. and Li, V.C. (1995). Introduction of strain-hardening engineered cementitious composites in design of reinforced concrete flexural members for improved durability. ACI Structural Journal, 92(2) 167-176. 

Similar to the design of flexural strengthening, the following states for the shear design of reinforced concrete (RC) members can be distinguished as the stmctural loads increase ... 

The design of FRP-reinforced concrete members in flexure is analogous to the design of steel-reinforced concrete members. Flexural capacity can be calculated based on assumptions similar to those made for members reinforced with steel bars. The design of members reinforced with FRP bars should take into account the mechanical behaviour of the FRP materials (ACI-440.1R-03 2003). 

The required minimum characteristic design concrete flexural strength for concrete pavements with a design traffic of 10 HVAG or more is 4.5 MPa at 28 days, except for steel fibre-reinforced concrete, where the requirement is 5.5 MPa. 

Chaallal, O., Nollet, M. J. and Perraton, D. (1998), Strengthening of reinforced concrete beams with externally bonded fiber-reinforced-plastic plates Design guidelines for shear and flexure , Canadian Journal of Civil Engineering, Vol. 25, Issue 4, pp. 692-704. 

PeSic, N. and Pilakoutas, K. (2005), Flexural analysis and design of reinforced concrete beams with externally bonded FRP reinforcement . Materials and Structures, Vol. 38, Issue 2. pp. 183-192. 

Dr. Duan s research interests cover areas including inelastic behavior of reinforced concrete and steel structures, structural stability, seismic bridge analysis, and design. With more than 70 authored and coauthored papers, chapters, and reports, his research focuses on the development of unified interaction equations for steel beam-columns, flexural stiffness of reinforced concrete members, effective length factors of compression members, and design of bridge structures. 

Direct Shear. For type I cross-sections (0 < 2°) the concrete between the flexural reinforcement Is capable of resisting direct shear. However, because cracking at the support yield line reduces the shear capacity, diagonal bars must be provided to at least resist the shear capacity of the concrete, v. For type II and III cross-sections (0 > 2 ), with little or no concrete shear resistance, The diagonal reinforcing bars must be designed to resist the entire shear load at the support. 

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