Flexure design resistance

The method of obtaining creep data and their presentation have been described however, their application is limited to the exact same material, temperature use, stress level, atmospheric conditions, and type of test (tensile, compression, flexure) with a tolerance of 10%. Only rarely do product requirement conditions coincide with those of the test or, for that matter, are creep data available for all grades of material that may be selected by a designer. In those cases a creep test of relatively short duration such as 1000 h can be instigated, and the information can be extrapolated to the long-term needs. It should be noted that reinforced thermoplastics and thermosets display much higher resistance to creep (Chapter 2). 

Flexural member design requires the determination of (1) the design blast loads, (2) the initial design cross-section, (3) an idealized resistance deflection function, (4) the calculated response (maximum deflection) and, (5) allowable ultimate deflection and (6) design for shear. 

Tensile membrane behavior requires continuous reinforcement steel to support in-plane stesses. Two-way slabs and flat slabs, with fixed or simple supports, can usually satisfy the requirements for tensile membrane resistance. Design with tensile membrane resistance is the same as for flexural resistance since the moment capacity of the section is used to determine ultimate resistance. Tensile membrane resistance at 8 degree rotation must be at least... 

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. 

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". 

Structural dements resist blast loads by developing an internal resistance based on material stress and section properties. To design or analyze the response of an element it is necessary to determine the relationship between resistance and deflection. In flexural response, stress rises in direct proportion to strain in the member. Because resistance is also a function of material stress, it also rises in proportion to strain. After the stress in the outer fibers reaches the yield limit, (lie relationship between stress and strain, and thus resistance, becomes nonlinear. As the outer fibers of the member continue to yield, stress in the interior of the section also begins to yield and a plastic hinge is formed at the locations of maximum moment in the member. If premature buckling is prevented, deformation continues as llic member absorbs load until rupture strains arc achieved. 

The stability and permeability of a mix are not sufficient criteria to reflect how a material will perform under the repeated loads generated by traffic. Therefore, any mix design selection process should include an examination of the material s fatigue resistance. Figure 5 shows the relationship between sulfur content and fatigue life for some SAS mixtures at two strain levels [15]. These tests were run at constant stress in third point flexure. Both curves go through a maximum at a sulfur content of 14 percent. 

There are other proprietary systems such as polyacrylamate. It is Ashland Chemical s Airmax that is designed for use with preforms or glass mats. These reinforced plastics possess high flexural modulus, good impact resistance, and high temperature stability. Systems with similar performance from isocyanate-based polymers are also used. 

Silicon nitride ceramics should have high flexural strength and fracture toughness at room temperature and good creep resistance at elevated temperatures. Because these properties are determined by the microstructure of these ceramics, further improvement of these ceramics is possible through alloy design. The following sections will be devoted to micro-... 

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