1. Assumptions in Limit state of collapse in flexure: (a) Plane sections normal to the axis remain plane after bending (b) Maximum strain in concrete at the outermost compression fibre is taken as 0.0035 in bending. (c) The relationship between the compressive stress distribution in concrete and the strain in concrete may be assumed to be rectangle, trapezoid. parabola or any other shape which results in the prediction of strength in substantial agreement with the results of tests. For design purposes. the compressive strength of concrete in structure shall he assumed to be 0.67 times the characteristic strength of concrete in cube. The partial safely factor (1.5times) shall be applied in addition to this. (d) The tensile strength of the concrete is ignored. (e)The stresses in the reinforcement are derived from representative stress strain curve for the type of steel used. For the design purpose partial safety factor (1.15 times) may be applied. (f) The maximum strain in the tension reinforcement in the section at failure shall be not less than (fy/1.15Es)+0.002 Where fy = characteristic strength of steel, Es= modulus of elasticity of steel Q13. What is modular ratio? Why is it an non-reliable quantity? What is the difference in value, assumed for this quantity between IS and BS? Ans: Modular ratio is the ratio of Modulus of elsticity of steel to that of concrete. It is considered to ba an unreliable quantity because of it is an imaginary quantity which may be either a constant in value for all strengths of concrete or one which varies with strength of concrete. CP 114 , the code used in U.K till 19473, recommended the use of a constant modular ratio of 15, independent of the strength of the concrete where as the other codes like IS 456 recommend a modulus of elasticity of concrete which varies with the strength of the concrete. 17.Define the term clear cover for steel in R.C constructions. The term clear cover for steel in R.C constructions is defined as the distance between the inner face of stirrups to the outer face of the concrete. The cover is provided to guard the reinforcement from corrosion. Explain straight line theory. This is one of the fundamental theories used to analyze reinforced concrete structures. It is based on the working stresses acting on concrete and the reinforcements. In the straight line theory or the elastic theory the ultimate compressive strength of concrete and the yield point stress of steel are divided by the same or different appropriate safety factors to determine the allowable stresses that may be permitted in the two materials under working conditions. The straight line theory assumes that the stress distribution in the concrete at working stresses of approximately half the ultimate compressive strength of concrete (σcm) is an inclined straight line as the slight curvature in the compression diagram is neglected. Hence through this method a straight line is obtained in the stress distribution diagram of the compressive zone of concrete. Explain ultimate load theory Ultimate load theory is also called as load factor method. in this method the strength of the rc section at working load is estimated from the ultimate strength of the section.this method uses a Load factor =(ultimate load /working load) which is generally taken as2. an additional fos is taken to ensure that failure occurs only by tension failure of steel and not by compression failure of concrete. Q-7:- Name the code of practices used for design structures for general building purposes: a) India b) USA c)Australia and d) UK? Answer: The Different code of practices used for design structures for general building purposes are: a) India- IS 456:2000 b) USA- ACI 318:1999 c) Australia - AS:3600 and d)UK - British code CP:110 Differentiate Factor of safety and partial safety factor Factor of safety Partial safety factor · It is the ratio of ultimate stress and · It is the ratio of characteristic strength allowable stress. and design strength. · Fos=ultimate stress/allowable stress · Partial safety factor=characteristic strength/design strength. · It is the measure of reliability of · Design loads and design strengths can be particular design. obtained from this. Explain the term nominal cover as given in IS 456: 2000. ANS: Nominal covar is the design depth of concrete cover to all steel reinforcement,including links.It is the dimension used in design and indicated in the drawings. It shall be not less than the diameter of the bar. What is lever arm? It is a distance between the line of action of the resultant compression to the resultant tension. It is expressed as ‘a’ or ‘j’. Enumerate atleast three situations in which a doubly reinforced beam becomes necessary. The situations in which doubly reinforced beam becomes necessary is, *Head room consideration. *Span is greater than 15ft. *To improve the performance of singly reinforced beam.
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