Question 1

Neutral axis properties in homogeneous beams For a homogeneous prismatic beam under bending that obeys the Euler–Bernoulli assumptions, the neutral axis of any cross-section must:

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Introduction / Context: The neutral axis (N.A.) is the line within a cross-section where the bending normal stress is zero. Correctly locating the N.A. is fundamental to computing stresses and deflections. In homogeneous, isotropic materials under pure bending, the N.A. passes through a specific geometric location regardless of section shape. Given Data / Assumptions: Homogeneous, isotropic material with linear stress-strain behaviour. Plane sections remain plane; pure bending assumptions. No axial force (only bending moment considered). Concept / Approach: From compatibility and equilibrium, strain varies linearly with distance from a neutral surface; zero strain line maps to zero stress line. For a homogeneous section with no axial force, the internal compressive and tensile resultants must be equal and opposite, so the centroidal axis is the only location where the first moment of area about the N.A. is zero. Therefore the N.A. must pass through the centroid. Step-by-Step Solution: Let y = 0 define the N.A.; integrate σ dA over the section to satisfy axial force = 0.With σ ∝ y, the condition ∫ y dA = 0 requires the axis to pass through the centroid. Verification / Alternative check: For non-homogeneous composite sections, the elastic neutral axis passes through the transformed-section centroid, not necessarily the geometric centroid—highlighting why the homogeneity assumption matters. Why Other Options Are Wrong: “Equidistant from top and bottom” is only true for symmetric sections; not general. “Axis of symmetry” may not exist (unsymmetric sections can be used). “Always coincide with mid-depth” is again only true when the section is symmetric about that depth. “None of these” is incorrect because the centroidal condition is definitive. Common Pitfalls: Confusing geometric centroid with elastic neutral axis for composite materials. Final Answer: pass through the centroid of the section.

Question 2

Eccentric loading on circular columns — rule for no-tension condition To ensure no tensile stress anywhere on the cross-section of a circular column under axial load with eccentricity, which empirical rule is typically applied to limit the eccentric…

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Introduction / Context: When a compressive load acts with eccentricity on a column section, bending stresses superimpose on direct compression. Designers often use simple “kern” rules to ensure the resultant remains within a central core so that tensile stresses do not develop (important for masonry and unreinforced concrete). Given Data / Assumptions: Circular, homogeneous, isotropic cross-section of radius R. Axial compression with eccentricity in any direction. Linear elastic stress distribution σ = P/A ± M y / I. Concept / Approach: The kern (core) of a section is the locus of load resultants for which normal stress stays compressive over the entire section. For a circle, the kern is a concentric circle of radius R/4. Equivalently, the resultant load must lie within the “middle fourth” of the diameter—hence the middle fourth rule. Step-by-Step Solution: For a circle: I = π R^4 / 4, A = π R^2, extreme fiber distance c = R.Kern radius r_k = I / (A c) = (π R^4 / 4) / (π R^2 · R) = R / 4.Therefore, any eccentricity e ≤ R/4 keeps σ ≥ 0 over the section → “middle fourth rule”. Verification / Alternative check: By setting σ_min = P/A − M c / I ≥ 0 and using M = P e, you obtain e ≤ I/(A c) = R/4, confirming the rule analytically. Why Other Options Are Wrong: Middle third rule applies to rectangles (kern limits at ±b/6, ±h/6). Middle half and middle eighth are not the correct kern sizes for circular sections. “None” is incorrect—there is a well-defined rule. Common Pitfalls: Applying the rectangular middle-third rule to circular columns by mistake. Confusing diameter quarters with area quarters; the rule concerns diameter (R/4 from center). Final Answer: Middle fourth rule.

Question 3

Rivet geometry — terminology for spacing The distance measured along a row between the centres of two adjacent rivets in the same row is called:

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Introduction / Context: Accurate terminology for fastener spacing is critical for detailing and checking riveted or bolted connections. Spacing affects strength, deformation compatibility, and the likelihood of tearing or block shear failures. Given Data / Assumptions: Row of rivets or bolts arranged along the load direction. Centre-to-centre measurements are used for layout. Standard definitions per steelwork practice. Concept / Approach: Pitch (p) is the longitudinal centre-to-centre distance between adjacent fasteners in the same row. Gauge (g) is the transverse distance between adjacent rows. Staggered pitch refers to the diagonal centre-to-centre spacing between offset fasteners in adjacent rows. Edge distance is from fastener centre to the plate edge; lap refers to plate overlap length in lap joints. Step-by-Step Solution: Identify the asked spacing (same row, adjacent fasteners, along the row).By definition, this is the pitch. Verification / Alternative check: Design provisions specify minimum pitch to avoid net-section tearing and to ensure proper bearing; these clauses explicitly use the term “pitch”. Why Other Options Are Wrong: Lap: joint overlap length, not fastener spacing. Gauge: spacing between rows, not along the row. Staggered pitch: diagonal spacing across rows. Edge distance: from fastener to plate edge, not between fasteners. Common Pitfalls: Confusing gauge with pitch, especially in multi-row patterns. Measuring from edge rather than centre when applying minimum pitch rules. Final Answer: pitch.

Question 4

Axial stress in a round steel rod under tensile load A steel rod of diameter 20 mm and length 5 m is subjected to an axial tensile force of 3000 kgf. If the measured elongation is 2.275 mm (not needed for stress), what is the average normal stress d…

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Introduction / Context: Direct (normal) stress under axial loading is one of the first checks for tension members. It is simply the applied force divided by the loaded cross-sectional area. Unit consistency is key to obtaining a correct value. Given Data / Assumptions: Diameter d = 20 mm = 2 cm. Axial tensile force P = 3000 kgf. Uniform circular cross-section, no stress concentrations considered. Average normal stress σ = P / A. Concept / Approach: Compute area in cm² because the requested stress unit is kg/cm². For a circular section A = π d² / 4. Insert values and divide the load by the area. Step-by-Step Solution: Convert diameter: d = 20 mm = 2 cm.Area: A = π d² / 4 = π (2)² / 4 = π cm² ≈ 3.1416 cm².Stress: σ = P / A = 3000 / 3.1416 ≈ 955.41 kg/cm². Verification / Alternative check: If you convert to N/mm² (MPa) for intuition: 1 kgf/cm² ≈ 0.09807 MPa, so 955.41 kgf/cm² ≈ 93.7 MPa, a reasonable stress level for mild steel in service. Why Other Options Are Wrong: 9.5541 and 95.541 kg/cm² come from misplacing decimal points (area units confused). 9554.1 kg/cm² is off by a factor of 10 (using mm² but keeping kg/cm² units). 190.0 kg/cm² corresponds to using area ≈ 15.79 cm² (incorrect for 20 mm diameter). Common Pitfalls: Mixing mm² and cm² when the requested stress unit is kg/cm². Using diameter in mm directly in a cm² area expression without conversion. Final Answer: 955.41 kg/cm².

Question 5

Stress–strain behavior near the elastic limit As a ductile material approaches its elastic limit during a tensile test, how does the tensile strain respond to further increases in applied stress?

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Introduction / Context: This question checks your understanding of the stress–strain curve for engineering materials, especially the transition from the linear elastic region to the nonlinear region near the elastic (or proportional) limit. Knowing how strain responds as you approach this limit is vital for selecting safe working stresses and avoiding permanent deformation. Given Data / Assumptions: Tensile loading of a typical ductile metal specimen (mild steel–type behavior). Quasi-static loading; temperature effects ignored. Elastic limit is close to but not necessarily identical to the proportional limit. Concept / Approach: In the elastic range, Hooke’s law holds and strain is proportional to stress. As the material approaches the elastic limit, the curve deviates from linearity. The tangent modulus reduces and a given increment of stress produces a larger increment of strain compared to the linear region. This manifests as strain increasing more rapidly with stress. Step-by-Step Solution: Identify the linear segment of the curve where epsilon = sigma / E.Locate the onset of nonlinearity (near proportional or elastic limit).Beyond that point, the slope decreases, so additional stress causes disproportionately larger strain. Verification / Alternative check: Many test curves show a clear knee near yield. The secant modulus beyond the knee is lower than E, confirming faster strain growth. Why Other Options Are Wrong: Proportional increase (option c) applies only strictly within the proportional limit, not as the limit is reached. Decrease in strain with increasing stress (options b and d) contradicts material behavior. Constant strain (option e) cannot occur while stress is still rising. Common Pitfalls: Confusing elastic limit with yield point or assuming perfect linearity up to yield for all materials; many alloys deviate slightly before yield. Final Answer: Increases more rapidly (nonlinear rise begins)

Question 6

Hooke’s law — the correct validity range Every material obeys Hooke’s law within which portion of its stress–strain response?

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Introduction / Context: Hooke’s law states that stress is proportional to strain. While widely used, it only holds over a specific range of behavior. Distinguishing between the proportional limit and the elastic limit helps avoid design errors, especially for metals that show slight nonlinearity before yield. Given Data / Assumptions: Tension test on a typical metal specimen. Small strains; room temperature. Standard engineering definitions for limits. Concept / Approach: The limit of proportionality is the largest stress at which stress is strictly proportional to strain (linear relation). The elastic limit is the maximum stress at which the material will still fully recover its original dimensions on unloading. Some materials exhibit a small nonlinear elastic region between these two limits. Therefore, the strict validity of Hooke’s law is only up to the proportional limit. Step-by-Step Solution: Define Hooke’s law: sigma ∝ epsilon.Identify the region where the stress–strain curve is a straight line.Recognize that linearity ceases at the proportional limit, even though the material may still be elastic up to the elastic limit. Verification / Alternative check: Compare unloading paths: up to the elastic limit, unloading returns to zero strain; however, strict linear proportionality ends earlier at the proportional limit. Why Other Options Are Wrong: Elastic limit (option a): elastic recovery can include slight nonlinearity; not strictly Hookean. Plastic point and upper yield (options b and e): by then, proportionality is lost. None of these (option d): incorrect because the proportional limit is the accepted answer. Common Pitfalls: Using the terms proportional limit and elastic limit interchangeably; they are close but not identical for many materials. Final Answer: Limit of proportionality

Question 7

Location of maximum bending moment using the shear-force relation In a loaded beam, at what location does the maximum (or minimum) bending moment occur with respect to the shearing force diagram?

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Introduction / Context: The relationship between shear force and bending moment is foundational in structural analysis. Designers routinely use the shear-force diagram (SFD) to locate critical points for the bending-moment diagram (BMD). Given Data / Assumptions: Beam under transverse loads with well-behaved diagrams. Sign convention is consistent (sagging positive). Concept / Approach: The differential relationships are: dM/dx = VdV/dx = wwhere M is bending moment, V is shear force, and w is the distributed load intensity. A stationary point (extreme) of M occurs where dM/dx = 0, i.e., where V = 0. In practice, this is where the SFD crosses the axis (changes sign). Step-by-Step Solution: Obtain V(x) from loading.Find points where V(x) = 0.Evaluate M at these points to determine maxima or minima in the BMD. Verification / Alternative check: Sketch SFD and BMD for common cases (point load at midspan, uniform load). In each case, the peak of M aligns with a zero crossing of V. Why Other Options Are Wrong: Maximum or minimum V values (options a and b) do not generally correspond to extrema of M. Any section with nonzero shear (option c) implies dM/dx ≠ 0, not an extremum. Midspan regardless of loading (option e) is only true for certain symmetric cases. Common Pitfalls: Confusing zeros at supports (M = 0 for simple supports) with internal maxima; always check V = 0 within the span. Final Answer: Where shear force changes sign (passes through zero)

Question 8

Effect of doubling beam width on central deflection A simply supported prismatic beam carries a single concentrated load at midspan. If the beam's width b is doubled (all other parameters unchanged), how does the central deflection change?

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Introduction / Context: Deflection control is a key serviceability requirement. For rectangular sections, the flexural rigidity E * I depends on the geometry, so changing width or depth alters deflection in predictable ways. Given Data / Assumptions: Simply supported beam, concentrated load P at midspan. Span L, Young’s modulus E, depth h remain constant. Rectangular section with width b. Concept / Approach: For a midspan point load on a simply supported beam, the maximum deflection is: delta_max = P * L^3 / (48 * E * I)For a rectangle, the second moment of area is: I = b * h^3 / 12Therefore, delta_max is inversely proportional to b when h, E, L, and P are fixed. Step-by-Step Solution: Original I_1 = b * h^3 / 12.After doubling width: I_2 = (2 b) * h^3 / 12 = 2 * I_1.Since delta ∝ 1 / I, delta_2 = delta_1 / 2. Verification / Alternative check: Check dimensional consistency and observe that increasing section size increases stiffness, reducing deflection proportionally. Why Other Options Are Wrong: Options a, b, c, e suggest larger deflections, which contradict stiffness increase when width increases. Common Pitfalls: Confusing the stronger influence of depth (h appears as h^3) with width (linear effect). Doubling depth would reduce deflection by a factor of 8; doubling width only halves it. Final Answer: It becomes 1/2 of the original deflection

Question 9

Required cross-sectional area at working stress For a prismatic tension member carrying a given load P, if the computed stress equals the specified working (allowable) stress, what can be said about the required cross-sectional area?

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Introduction / Context: Allowable stress design sizes members so that actual stress does not exceed a prescribed limit. This question asks you to connect that concept to the required area for a given load. Given Data / Assumptions: Axially loaded prismatic member. Working (allowable) stress = sigma_allow. Applied load = P (constant). Concept / Approach: Stress is defined by sigma = P / A. To ensure sigma ≤ sigma_allow, the area must satisfy A ≥ P / sigma_allow. The minimum area that just satisfies the limit (and is therefore most economical in terms of area) is A_min = P / sigma_allow. Step-by-Step Solution: Start with sigma = P / A.Impose the design constraint sigma ≤ sigma_allow.Rearrange to A ≥ P / sigma_allow; choose A = P / sigma_allow for minimum compliant area. Verification / Alternative check: Using any smaller area violates the allowable stress; using any larger area is safe but not minimal for area economy. Why Other Options Are Wrong: Zero or infinite area are physically meaningless for a finite load. Maximum area is not required; larger area is conservative but uneconomical. Poisson’s ratio is irrelevant for uniaxial allowable-stress sizing. Common Pitfalls: Confusing working stress with ultimate or yield stress; forgetting that design often seeks minimum adequate size, not maximum. Final Answer: Minimum that satisfies the allowable stress

Question 10

Pure torsion — identify the statement that is NOT an assumption Which one of the following statements is NOT an assumption of the elementary (pure) torsion theory for a uniform circular shaft?

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Introduction / Context: Elementary torsion theory underpins the design of shafts. It relies on a set of assumptions that lead to the familiar torsion formula tau = T r / J and angle-of-twist relation. Given Data / Assumptions: Uniform circular shaft subjected to pure torque. Saint-Venant torsion conditions. No warping for circular sections. Concept / Approach: The correct assumptions include material linearity and isotropy, plane circular sections remaining plane, straight radial lines, and (for constant T and constant GJ) a constant rate of twist. However, the shear stress is not uniform over the section; it varies linearly with radius: tau(r) = T r / J, being zero at the axis and maximum at the surface. Step-by-Step Solution: Recall the torsion formula: tau(r) = T r / J.Observe that tau depends on radius r ⇒ not uniform.Therefore, any statement claiming uniform shear across the section is not an assumption of pure torsion theory. Verification / Alternative check: The torque equilibrium integral T = ∫(tau * r) dA requires tau ∝ r for circular shafts to satisfy compatibility. Why Other Options Are Wrong: Options a, b, c, and e are standard assumptions or results for a prismatic circular shaft under constant torque. Common Pitfalls: Confusing uniform shear stress (false) with uniform rate of twist (true) for prismatic shafts under constant torque. Final Answer: Shear stress is uniform over the cross-section at a given torque

Question 11

Differential relationships among load, shear, and bending moment Which statements correctly express the fundamental relations for a beam (with consistent sign conventions)?

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Introduction / Context: The shear-force and bending-moment diagrams arise directly from equilibrium applied locally to a beam element. Remembering the differential relations helps locate maximum M and relate changes in V to the applied load intensity. Given Data / Assumptions: Transverse loading on a slender beam. Consistent sign convention (e.g., sagging M positive; downward w positive). Concept / Approach: The governing relationships are: dM/dx = VdV/dx = wThese are obtained by taking moments and forces on a differential element and letting the element length tend to zero. They are independent of material properties and are purely from statics (with the chosen sign convention). Step-by-Step Solution: Write equilibrium of a small beam slice of length dx.Summation of vertical forces ⇒ V(x + dx) − V(x) = w dx ⇒ dV/dx = w.Summation of moments ⇒ M(x + dx) − M(x) = V dx ⇒ dM/dx = V. Verification / Alternative check: Integrating w over a span gives the change in shear, and integrating V gives the change in bending moment — exactly how SFD and BMD are constructed. Why Other Options Are Wrong: Only one correct (options a or b) ignores the other true relation. Neither correct (option d) contradicts statics. Option e swaps the relations; that is incorrect. Common Pitfalls: Sign mistakes when drawing diagrams; always stick to a consistent convention and apply the differential relations carefully. Final Answer: Both dM/dx = V and dV/dx = w are correct

Question 12

Statically determinate beam with three reaction unknowns If a beam is supported such that there are exactly three independent reaction components, which equilibrium equations are sufficient to determine them?

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Introduction / Context: A planar, statically determinate problem can be solved using only the equations of static equilibrium. For beams in the plane, there are three independent equations available. Given Data / Assumptions: Planar system (2D), rigid body equilibrium. No additional redundants or internal releases. Supports provide exactly three independent reaction components in total. Concept / Approach: The independent equilibrium equations in a plane are: ∑H = 0∑V = 0∑M = 0These three equations can determine three unknown reaction components if the structure is statically determinate. Step-by-Step Solution: Count unknown reactions = 3.Provide three independent equations of equilibrium.Solve the linear system to obtain reactions. Verification / Alternative check: If more than three unknowns exist, the structure is statically indeterminate to that degree and requires compatibility (deformations) to solve. Why Other Options Are Wrong: Options a, b: insufficient equations. Option c: duplicates ∑H and omits the others. Option e: omits ∑V = 0; system remains underdetermined. Common Pitfalls: Counting dependent reaction components or missing the moment equation about a convenient point. Final Answer: ∑H = 0; ∑V = 0; ∑M = 0

Question 13

Name of the point where the bending moment changes sign In a continuous bending-moment diagram, the point at which the bending moment passes through zero and changes sign is called the:

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Introduction / Context: Terminology matters when interpreting diagrams. The location where bending moment switches from sagging to hogging (or vice versa) has a specific name, often used in detailing and analysis notes. Given Data / Assumptions: Beam or frame with regions of positive and negative bending. Continuous bending-moment diagram with at least one internal zero crossing. Concept / Approach: The point where the bending moment M changes sign is called the point of contraflexure (also written as counterflexure). At this point, curvature changes sign as well, although the point of inflexion is a broader mathematical term where curvature changes sign in a deflection curve; in beam theory, the two are related but the standard term tied to M = 0 sign change is contraflexure. Step-by-Step Solution: Draw the BMD and identify where M crosses zero within the span.Label that location as the point of contraflexure.Use it to plan tension/compression reinforcement transitions in RC beams or to understand flange stress reversals in steel beams. Verification / Alternative check: Check the corresponding shear diagram; a zero in M does not necessarily require zero shear. Why Other Options Are Wrong: Point of inflexion (option a) describes curvature change in the deflected shape; terminology in beam BMD context prefers contraflexure. Point of virtual hinge (option c) refers to a plastic hinge or a released moment location, not merely a sign change of M. All of the above (option d) mixes distinct concepts. Common Pitfalls: Confusing zero bending moment at supports of simple spans with internal points of contraflexure. Final Answer: Point of contraflexure

Question 14

Basic material property — recovery after unloading The property that enables a body to return to its original shape and size upon removal of the deforming force is called:

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Introduction / Context: Material property definitions are the foundation for interpreting test data and selecting materials. This question contrasts elasticity with other properties commonly discussed in mechanics of materials. Given Data / Assumptions: Small deformation, uniaxial loading for clarity. Deforming force is removed completely. Concept / Approach: Elasticity: ability to recover original shape and size when unloaded.Plasticity: tendency to retain permanent deformation after unloading.Ductility: ability to undergo significant plastic strain in tension before fracture.Malleability: ability to undergo plastic deformation in compression (e.g., rolling, hammering into sheets).Toughness: ability to absorb energy before fracture (area under stress–strain curve). Step-by-Step Solution: Identify that the object returns to original geometry after load is removed.This excludes plastic behavior and directly corresponds to elasticity.Therefore, the correct property is elasticity. Verification / Alternative check: Examine a typical stress–strain curve: unloading from within the elastic region retraces the loading path and returns to zero strain. Why Other Options Are Wrong: Plasticity involves permanent set, not full recovery. Ductility and malleability describe modes of plastic deformation, not recovery. Toughness is an energy-absorption measure and unrelated to shape recovery upon unloading. Common Pitfalls: Using ductility or malleability as catch-all terms for “good” mechanical behavior; they are specific to plastic deformation, not elasticity. Final Answer: Elasticity

Question 15

Strength of materials – simple vs. derived strains In elementary mechanics of solids, which one of the following is NOT classified as a simple (elementary) strain? Choose the most appropriate option, keeping the standard definitions used in civil an…

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Introduction / Context: Strain measures deformation per unit dimension. In strength of materials, we commonly distinguish between simple (elementary) strains and derived strains. Knowing which strain type is simple helps in selecting the correct constitutive relations and in interpreting laboratory tests and field measurements. Given Data / Assumptions: Small deformations and linear elastic behavior are assumed. Standard engineering strain definitions are used. Material can be considered homogeneous and isotropic for basic classification. Concept / Approach: Simple strains act along a single direction or plane: tensile strain (elongation per unit length), compressive strain (shortening per unit length), and shear strain (change in angle between originally orthogonal line elements). Volumetric strain is a derived measure equal to the algebraic sum of normal strains in the three orthogonal directions; it quantifies the relative change in volume and is not a single-direction elementary strain. Step-by-Step Solution: Identify elementary strains: tensile, compressive, shear.Recognize volumetric strain as a resultant: epsilon_v = epsilon_x + epsilon_y + epsilon_z.Therefore, among the listed options, volumetric strain is the one that is not a simple strain. Verification / Alternative check: In a uniaxial test, a simple strain is measured directly along the loading direction (tension or compression). Volumetric strain requires summation of three mutually perpendicular normal strains; hence it is inherently derived. Why Other Options Are Wrong: Tensile, Compressive, and Shear strains each refer to a single primary deformation mode; all are simple strains. Common Pitfalls: Confusing volumetric strain with lateral strain or Poisson's ratio. Volumetric strain concerns total volume change; Poisson's ratio relates lateral to longitudinal strain in uniaxial loading. Final Answer: Volumetric strain

Question 16

Theory of simple bending – core assumptions Which of the following statements are core assumptions in the classical (Euler–Bernoulli) simple bending theory for beams used in civil and mechanical engineering design?

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Introduction / Context: The Euler–Bernoulli (simple) bending theory forms the backbone of beam analysis in structural engineering. Its assumptions limit the theory's applicability but also make closed-form solutions possible for a wide range of practical problems. Given Data / Assumptions: Linearly elastic response, small deflection theory. Prismatic beam unless stated otherwise. No significant shear deformation (plane sections remain plane). Concept / Approach: Key assumptions include: (1) material linearity and identical modulus in tension and compression, (2) plane sections remain plane and perpendicular to the neutral surface after bending, (3) the material is homogeneous and isotropic, and (4) no net axial force on a pure bending section (resultant of normal stresses over the section is zero), so only bending moment acts. Step-by-Step Solution: Assess option A: Same E in tension and compression supports symmetric linear stress–strain behavior.Assess option B: Plane sections remain plane is the cornerstone assumption, eliminating shear strain in bending.Assess option C: Homogeneous and isotropic material ensures uniform properties and a centroidal neutral axis.Assess option D: In pure bending, the algebraic sum of normal stresses over the section is zero, giving zero resultant axial force.Conclusion: All statements align with the simple bending theory. Verification / Alternative check: Standard derivations of the flexure formula sigma = M y / I explicitly invoke these assumptions to obtain a linear stress distribution. Why Other Options Are Wrong: Each individual statement is correct; therefore selecting any one of A–D alone would be incomplete relative to the complete set of assumptions. Common Pitfalls: Applying the simple bending theory to deep beams or materials with different tensile/compressive moduli; such cases require Timoshenko theory or nonlinear material models. Final Answer: All the above

Question 17

Three-force member equilibrium – angle between two equal inclined forces A jointed member is balanced at its end by two other members that carry equal forces and are symmetrically inclined. For equilibrium of the three-force member, what must be the…

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Introduction / Context: Many truss and frame joints are three-force members, meaning only three non-collinear forces act on the pin. Recognizing the geometry of force equilibrium for equal forces is a fast way to size members and check joint stability. Given Data / Assumptions: Three concurrent forces act at a joint. Two of the forces are equal in magnitude and act along the two inclined bars. The third force acts along the balanced member. No external moments; pin (frictionless) joint. Concept / Approach: For three forces to be in equilibrium, they must be concurrent and their vector sum must be zero. With two equal forces, the force polygon becomes an equilateral triangle when all three forces are equal in magnitude and separated by 120° angles. The resultant of two equal forces F at an included angle alpha is R = 2 F cos(alpha/2). For zero net force, the third force must be equal and opposite to R. Symmetry and equality lead to alpha = 120°. Step-by-Step Solution: Let two equal forces be F and F with included angle alpha.Resultant magnitude: R = 2 F cos(alpha/2).For all three forces to be equal in magnitude (common in symmetric arrangements) and sum to zero, require R = F.Solve: F = 2 F cos(alpha/2) → cos(alpha/2) = 1/2 → alpha/2 = 60° → alpha = 120°. Verification / Alternative check: Construct a force triangle with three equal sides; internal angles are 60°, which correspond to 120° separations between the line-of-action directions at the joint. Why Other Options Are Wrong: 60° gives R = 2F cos 30° = 1.732F ≠ F. 90° gives R = 1.414F. 45° and 3° are inconsistent with equilibrium for equal member forces. Common Pitfalls: Forgetting that at a joint the angles between the force directions are 120° when the magnitudes are equal; mixing up internal angles of the force polygon with angular separations at the joint. Final Answer: 120°

Question 18

Planar truss determinacy – member count for a stable, statically determinate frame For a stable, statically determinate planar truss (simple frame), how many members m are required in terms of the number of joints j?

Accepted Answer

Introduction / Context: Before analyzing a truss, engineers quickly check determinacy and stability using counting rules. A classic relation links members, joints, and reaction components for planar trusses. Given Data / Assumptions: Planar truss, pin-jointed, loads applied at joints. Two reaction points providing three reaction components in total. No internal multi-member rigidities beyond pin connections. Concept / Approach: For a statically determinate and stable planar truss, the member count is governed by the relation m = 2j − 3. This arises by equating the number of unknown bar forces plus reactions to the available independent equilibrium equations (2 per joint, or 3 for the whole truss with method of joints approach). Step-by-Step Solution: Total unknowns: bar forces m + reaction components r (typically r = 3).Independent equations of static equilibrium for the whole truss do not suffice; instead use method of joints: each joint contributes 2 equations, giving 2j equations.Set unknowns equal to equations for determinacy: m + r = 2j → m + 3 = 2j → m = 2j − 3. Verification / Alternative check: For a basic triangular truss (j = 3), m = 2*3 − 3 = 3, which matches the three sides of a triangle. Why Other Options Are Wrong: 3j − 3, 2j − 2, and 2j − 1 would render the truss indeterminate or unstable relative to the counting rule. Common Pitfalls: Confusing the planar truss relation with space frames (which have m = 3j − 6 for simple determinate cases); forgetting to count actual support reactions. Final Answer: m = 2j − 3

Question 19

Core (kernel) of a circular column – location of load for wholly compressive stress To keep the stress distribution wholly compressive in a circular masonry/RC column of diameter d, the eccentric load must lie within a concentric circle (core). What…

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Introduction / Context: The core (or kernel) of a section is the locus of points where a resultant compressive load can act without causing tensile stress anywhere on the section. For masonry and plain concrete, keeping the stress wholly compressive is essential. Given Data / Assumptions: Uniform circular cross-section of diameter d. Linear elastic stress distribution (no cracking in compression-only design). Eccentric compressive load applied. Concept / Approach: For a circular section, the core is also circular with radius r_core = d/4. Therefore, the core diameter is 2 * (d/4) = d/2. Any load applied within this core produces a linear stress block that remains non-tensile across the section. Step-by-Step Solution: For any section, core boundaries are defined by the condition that the extreme fibre stress just reaches zero.For a circle, r_core = I / (A * c) with c = radius = d/2, I = (pi/64) d^4, A = (pi/4) d^2 → r_core = [(pi/64) d^4] / [ (pi/4) d^2 * (d/2) ] = d/4.Hence core diameter = 2 * d/4 = d/2. Verification / Alternative check: Many design handbooks directly list the circular section core as a circle of radius d/4, confirming the computation. Why Other Options Are Wrong: d/3, d/4, d/8, d/10 do not match the derived size. d/4 is the core radius, not the diameter. Common Pitfalls: Mistaking core radius for core diameter; applying rectangular-section kernel formulas to circular sections. Final Answer: d/2

Question 20

Kernel (core) of a square masonry pillar – side of the core square A short masonry pillar has a 60 cm × 60 cm square cross-section. The kernel (core) for zero-tension loading is a square (rotated 45°) inside the section. What is the side length of t…

Accepted Answer

Introduction / Context: For masonry or plain concrete compression members, the resultant load must lie within the kernel (core) to avoid tensile stress at the edges. For a rectangular section b × d, the kernel is a rhombus with diagonals b/3 and d/3. A square section makes this rhombus a square rotated 45°. Given Data / Assumptions: Square section: b = d = 60 cm. Kernel (core) rhombus diagonals = b/3 and d/3. Linear elastic stress distribution and compression-only requirement. Concept / Approach: For a square, both kernel diagonals are equal: D = 60/3 = 20 cm. A rhombus with equal diagonals is a square. The side s of a square relates to its diagonal D by D = s * sqrt(2). Therefore, s = D / sqrt(2) = 20 / sqrt(2) ≈ 14.14 cm. Step-by-Step Solution: Compute diagonals: D_1 = D_2 = 60/3 = 20 cm.Recognize rhombus becomes a square (equal diagonals).Convert diagonal to side: s = 20 / sqrt(2) = 14.142 cm (≈ 14.14 cm). Verification / Alternative check: Kernel coordinates for a rectangle are within ±b/6 and ±d/6 from the centroid along principal axes; for b = d = 60 cm, limits are ±10 cm. The inscribed square's half-diagonal equals 10 cm → full diagonal 20 cm → side 14.14 cm. Why Other Options Are Wrong: 17.32, 20.00, 22.36, 25.22 cm do not satisfy D = s * sqrt(2) for D = 20 cm. Common Pitfalls: Confusing kernel bounds (±b/6, ±d/6) with the side of the inscribed square; mixing up diagonal and side relations. Final Answer: 14.14 cm

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