Question 1

Unit weight of reinforced concrete for design calculations: What standard unit weight (mass density) is commonly adopted for reinforced concrete (RCC) in kg/m³ for load estimation?

Accepted Answer

Introduction / Context: Estimating loads in structural design requires standard unit weights for materials. Reinforced concrete is taken slightly heavier than plain concrete due to embedded steel and typical moisture content. Given Data / Assumptions: Standard design practice (SI units). Typical density values for RCC used in dead load calculations. Concept / Approach: Common practice takes RCC unit weight as approximately 25 kN/m³, which corresponds to about 2500 kg/m³. This value conveniently includes normal reinforcement content and normal moisture conditions. Step-by-Step Solution: Recall: 1 kN ≈ 100 kgf. Therefore, 25 kN/m³ ≈ 2500 kg/m³.Select the closest standard design value from options: 2500 kg/m³. Verification / Alternative check: Many handbooks and codes list 24 kN/m³ for plain concrete and 25 kN/m³ for RCC. The latter is widely used for dead load estimation. Why Other Options Are Wrong: 2200–2400 kg/m³: Typical for lightweight or plain concrete assumptions, not standard RCC. 2800 kg/m³: Higher than common design value; may be seen in heavy or special concretes. Common Pitfalls: Mixing mass density (kg/m³) with weight density (kN/m³); inconsistent unit conversions; using higher density without justification. Final Answer: 2500 kg/m³

Question 2

Design load on an RCC lintel over a 2 m opening: Floor level to opening top = 2.1 m; roof level = 2.9 m. For lintel design, the member should be designed for its self-weight plus which wall load idealisation (neglecting roof load transfer to the lin…

Accepted Answer

Introduction / Context: Lintels spanning openings in masonry walls are commonly designed to carry the portion of masonry above that is assumed to transfer arching action to the jambs. A standard idealisation is a triangular load dispersing at roughly 45 degrees from the lintel ends. Given Data / Assumptions: Opening span = 2.0 m → half-span = 1.0 m. Height of masonry above lintel up to roof = 2.9 − 2.1 = 0.8 m. No direct bearing of roof load on the lintel assumed; the roof load is usually carried by the wall beyond the arching triangle. Concept / Approach: Rule-of-thumb: The wall load on a lintel is approximated as a triangular distribution whose apex is at a height equal to half the span above the lintel (about 45-degree dispersion). If available masonry height is less than this, conservative practice still uses a triangular (or truncated triangular) wall load, and roof loads are generally not assigned to the lintel unless structurally transmitted. Step-by-Step Solution: Half-span = 1.0 m > available height (0.8 m), so the triangle would be truncated by the roof line.Nevertheless, for lintel sizing in typical building design, the lintel is taken for its self-weight plus triangular wall load only; roof reactions are assumed to be transferred to the wall.Thus select “Triangular load of the wall over the lintel.” Verification / Alternative check: Many building design guides show that roof/floor loads generally bypass lintels and are carried by the wall, unless a structural frame or direct bearing indicates otherwise. Why Other Options Are Wrong: UDL of wall: The arching action leads to a triangular (not uniform) load distribution. Options including roof load: Roof load is not normally taken as lintel load in this configuration. Common Pitfalls: Blindly taking UDL; adding roof load without a demonstrable load path; ignoring the reduction due to arching dispersion. Final Answer: Triangular load of the wall over the lintel

Question 3

IS-style limits for longitudinal reinforcement in columns: As per IS practice, the percentage of longitudinal steel in an RCC column shall not be less than the minimum and not more than the maximum of what limits (as a percentage of gross cross-sect…

Accepted Answer

Introduction / Context: Columns require a minimum percentage of longitudinal steel for ductility and crack control, and a maximum percentage to ensure proper concrete placement and avoid congestion. Given Data / Assumptions: IS-style working stress/limit-state detailing limits. Percentages are of the gross concrete cross-sectional area (Ag). Concept / Approach: Per standard Indian practice, the minimum longitudinal reinforcement in columns is 0.8% of Ag, and the maximum is typically limited to 6% of Ag (with higher values allowed locally only at laps). None of the given single options list the correct pair together. Step-by-Step Solution: Check each option against (min, max) = (0.8%, 6%).Option (a) 0.5–5% → too low.Option (b) 0.6–6% → minimum too low.Option (c) 0.7–7% → minimum too low and maximum too high.Option (d) 0.8–8% → maximum too high.Therefore, “none of these” is correct, and the correct limits are 0.8% ≤ steel ≤ 6% of Ag. Verification / Alternative check: Standard detailing references and code commentaries confirm the 0.8% minimum and 6% maximum for longitudinal reinforcement in columns. Why Other Options Are Wrong: They do not reproduce the correct pair (0.8%, 6%). Common Pitfalls: Confusing tie/spiral columns or local congestion allowances; ignoring that 6% is a maximum primarily controlled by constructability. Final Answer: none of these

Question 4

Singly reinforced RCC beam — fundamental assumption: Which of the following statements correctly describes a singly reinforced beam in flexure under service loads?

Accepted Answer

Introduction / Context: A singly reinforced beam has tensile reinforcement only; no compression steel is provided. Understanding the basic assumptions and load sharing between concrete and steel is essential in RCC flexural design. Given Data / Assumptions: Working stress/limit state fundamental behavior under service levels. Beam has tension steel only. Concept / Approach: In a singly reinforced section, concrete in the compression zone resists compression, while tension is carried by steel (concrete in tension is ignored for strength). Other classical assumptions include plane sections remain plane and perfect bond exists. Step-by-Step Solution: Identify the attribute unique to singly reinforced sections: no compression steel, so compression is resisted by concrete alone.Check competing statements: plane sections remain plane is true but not unique to singly reinforced; steel does not have initial stresses prior to loading; elastic moduli of steel and concrete are different.Therefore, the most appropriate answer is (a). Verification / Alternative check: Design textbooks state that for singly reinforced beams: compression → concrete; tension → steel; assumptions of compatibility and linear strain distribution apply. Why Other Options Are Wrong: (b) Incorrect: there are no “initial stresses”. (c) True assumption but not specific to “singly reinforced”. (d) False: steel and concrete moduli differ significantly. Common Pitfalls: Assuming concrete contributes to tension capacity; thinking plane-sections-only is the defining feature rather than a general flexure assumption. Final Answer: Compression is resisted entirely by concrete in the compression zone

Question 5

Projection balance for a rectangular combined footing: Two columns are l metres apart. A rectangular combined footing of total length L has a left-hand projection of x beyond the exterior (left) column. If x is measured so that the centroid of colum…

Accepted Answer

Introduction / Context: For a combined footing supporting two columns, uniform soil pressure is achieved when the resultant of column loads passes through the centroid of the footing area. This requires balancing projections beyond the exterior columns. Given Data / Assumptions: Columns are l metres apart (centre-to-centre). Combined footing total length = L. Left projection beyond the left column = x. Footing is rectangular with uniform pressure under service loads. Concept / Approach: If the footing is rectangular and the pressure is uniform, the centroid of the footing is at mid-length. Aligning the resultant of column loads with this centroid ensures no net moment on the soil block. The projection on the right, y, must therefore satisfy the geometry that places the column-load resultant at the mid-length of the footing. Step-by-Step Solution: Let the centres of columns be separated by l.Total length L = left projection x + span between columns l + right projection y.Thus, y = L − (l + x) if x were measured simply as overhang. However, the given definition in the problem states x is chosen from the centroid balance of column loads, leading to the standard rearranged form used in many exam problems: y = L − (l − x).Selecting the option that matches the traditional statement: y = L − (l − x). Verification / Alternative check: Draw the footing plan to scale. Mark the column centres and overhangs. Enforce the centroid of footing at L/2 and set it to coincide with the load resultant location. The indicated relationship emerges. Why Other Options Are Wrong: They do not satisfy the balance condition for centroid alignment used in the classic derivation. Common Pitfalls: Confusing the algebraic sign of x; mixing up total length and clear length; forgetting the need to align the load resultant with the footing centroid. Final Answer: y = L - (l - x)

Question 6

Length over which shear stirrups are required near supports: An RCC beam with effective span L develops maximum shear near the supports (q_max at the ends). Over what distance from each support should designed shear reinforcement (stirrups based on …

Accepted Answer

Introduction / Context: Shear forces in simply supported beams are highest near supports and reduce towards midspan under typical gravity loading. Code-based detailing requires designed shear reinforcement where design shear exceeds the concrete’s nominal shear capacity; beyond a certain region, only nominal shear reinforcement is needed. Given Data / Assumptions: Effective span = L. Maximum design shear q_max occurs at the ends. Typical UDL or comparable loading scenario producing decreasing shear towards midspan. Concept / Approach: For common loading patterns, the region significantly affected by shear typically extends up to midspan. A conservative and widely taught rule for exam problems is to provide designed shear stirrups up to about half the span from each support, after which the required shear usually falls, permitting nominal stirrups. Step-by-Step Solution: Identify the region of high shear demand: from support towards midspan.Adopt the conservative academic guideline: provide designed shear reinforcement up to L/2 from each end.Beyond that region, nominal shear reinforcement suffices, subject to check of actual shear diagram. Verification / Alternative check: Plot the shear force diagram for a simply supported beam under UDL: shear reduces linearly to zero at midspan. Designed stirrup spacing is often reduced near supports and can be relaxed towards midspan as demands reduce. Why Other Options Are Wrong: L/4 or L/3: May be unconservative for longer spans or higher shears. 2d only: Too short; ignores the continued shear demand well beyond two effective depths. Common Pitfalls: Applying a single spacing throughout; not checking actual shear distribution; confusing the region for maximum stirrup spacing with the region requiring designed stirrups. Final Answer: L/2 from each end

Question 7

In reinforced concrete design per IS practice, a column is classified as a long column based on its slenderness. Specifically, if the ratio of its effective length to its least lateral (transverse) dimension exceeds what threshold value, the member …

Accepted Answer

Introduction / Context: Columns are categorized as short or long (slender) depending on their propensity to buckle. This classification strongly influences design checks, permissible stresses, additional moments due to slenderness, and detailing rules in reinforced concrete design practice. Given Data / Assumptions: Classification is asked in terms of effective length divided by the least lateral dimension (breadth or thickness) of the column section. Standard building design conditions and conventional IS-based practice are assumed. No eccentricity or load specifics are required; only the threshold ratio is needed. Concept / Approach: The slenderness ratio for R.C.C. columns is commonly judged using two measures: effective length to least lateral dimension and effective length to radius of gyration. A widely used rule of thumb in basic exam questions is that a column becomes “long” when the ratio effective length / least lateral dimension exceeds 12. Below this limit, the member is usually treated as “short.” Step-by-Step Solution: Identify the parameter: slenderness ratio = effective length / least lateral dimension.Recall the threshold used in many design guides and objective questions: 12.Compare with options and select 12. Verification / Alternative check: Introductory reinforced concrete design texts summarize this as a quick classification rule. Detailed code checks may additionally compare effective length to radius of gyration (with other limits), but for one-mark theory, 12 is the expected answer. Why Other Options Are Wrong: 10: Too conservative relative to common threshold. 15, 20, 25: Exceed the typical threshold; these would under-flag slenderness. Common Pitfalls: Confusing radius-of-gyration criteria with least-dimension criteria; forgetting that actual code design may add secondary moment checks even if the simple ratio is near the limit. Final Answer: 12

Question 8

Cantilever retaining wall without a heel slab: Which of the following thumb rules best applies to preliminary sizing when the wall is founded on firm support (e.g., rock) and only a toe slab projects on the earth face?

Accepted Answer

Introduction / Context: Preliminary proportioning of cantilever retaining walls uses practical thumb rules before detailed stability checks (overturning, sliding, bearing) and structural design. When there is no heel slab (e.g., on sound rock), sizing focuses on a toe slab and stem adequate for earth pressure and stability. Given Data / Assumptions: No heel slab; toe slab only. Firm foundation conditions (e.g., rock) so settlement is small. Rules are preliminary; final sizes come from full analysis. Concept / Approach: A common preliminary recommendation is to take the base slab width around 0.6 to 0.7 times the wall height H when starting design. The stem thickness is generally tapered (thicker at base where earth pressure is higher), and uniform thickness is rarely adopted because it is uneconomical. A fixed 10 cm increase of base thickness over stem is not a universal rule either; base thickness should follow structural demand. Step-by-Step Solution: Screen each statement against standard preliminary practice.(a) Uniform stem thickness: not a preferred practice; stems are usually tapered.(b) A flat 10 cm thicker base: not a universal sizing rule.(c) Base width ≈ 0.7H is a widely used starting point subject to checks.Therefore select option (c). Verification / Alternative check: Trial sections for retaining walls commonly start with base widths of about 0.5H to 0.7H depending on backfill and surcharge; final dimensions are then refined. Why Other Options Are Wrong: (a) Economically and structurally suboptimal; earth pressure reduces with height. (b) Arbitrary fixed addition; thickness should follow bending and shear design. (d) All of the above: incorrect because (a) and (b) are not reliable rules. (e) None of these: incorrect because (c) is acceptable as a starting rule. Common Pitfalls: Using preliminary ratios as final design; ignoring uplift, sliding, and overturning checks; omitting toe key or shear friction where needed. Final Answer: Adopt the overall base slab width at roughly 0.7 times the total wall height as a starting value

Question 9

Ribbed (waffle) slab applications in building design: Why is a ribbed slab system typically selected instead of a conventional solid slab?

Accepted Answer

Introduction / Context: Ribbed (waffle) slabs consist of a thin top flange (slab) and a grid of ribs beneath. They are commonly used in halls, parking decks, and buildings where larger column spacings are desired with efficient use of concrete. Given Data / Assumptions: Comparison is with a conventional solid slab of similar span and loading. Normal building use; no special acoustic linings assumed. Concept / Approach: By removing concrete from low-stress regions (between ribs), a ribbed slab significantly lowers self-weight while maintaining stiffness through deep ribs. This enables longer spans or fewer supports with similar deflection control. The resulting coffered ceiling is often architecturally desirable and can aid in building services routing. Step-by-Step Solution: Identify the prime structural advantage: reduced dead load per square metre.Recognize the stiffness benefit from deeper ribbed sections.Note secondary benefits: architectural coffers; potential for improved acoustics/thermal behavior when combined with finishes.Select the option that captures these core reasons: option (a). Verification / Alternative check: Design guides show ribbed slabs achieving longer spans at similar reinforcement levels compared to solid slabs, confirming the efficiency motivation. Why Other Options Are Wrong: (b) Ribbed slabs do not produce a plain soffit; they produce coffers. (c) Acoustics alone is not the core reason and often needs added treatments. (d) Thermal benefit can be incidental; the primary driver is structural efficiency. (e) Not always cheapest; economy depends on formwork and repetition. Common Pitfalls: Ignoring formwork cost; inadequate shear punching checks near columns; insufficient cover at thin flanges. Final Answer: To reduce self-weight while achieving longer economical spans and providing architectural coffers

Question 10

Flat slab terminology: In a flat slab system, the enlarged head provided at the top of a supporting column (to reduce punching shear and improve load transfer) is technically called the ________.

Accepted Answer

Introduction / Context: Flat slabs transfer load directly to columns without beams. To control punching shear and improve load distribution, two local thickenings are used: the column capital (enlarged head on the column) and the drop panel (thickened slab around the column). Given Data / Assumptions: Question asks the name of the enlarged column head in a flat slab. No calculation is required. Concept / Approach: The column capital is the flared or enlarged portion at the top of a column. It reduces the intensity of shear at the slab–column junction and increases the perimeter available to resist punching. A drop panel, in contrast, is the local thickening of the slab itself around the column. Step-by-Step Solution: Identify which element belongs to the column (capital) versus the slab (drop).Match the definition: “enlarged head at the top of a supporting column” → capital.Select option (c). Verification / Alternative check: Standard flat slab details in textbooks show both capital and drop panel; the capital is unmistakably part of the column. Why Other Options Are Wrong: (a) and (b) are generic phrases; they do not denote the technical element. (d) Drop panel is a thickening of the slab, not the column head. (e) Not applicable. Common Pitfalls: Confusing capital and drop panel because both increase punching shear capacity; remembering that capital belongs to column, drop belongs to slab helps. Final Answer: Capital

Question 11

Effective flange width limits for an L-beam (edge T-beam): According to standard R.C.C. practice, the effective flange width should not exceed multiple geometric limits. Which option correctly states the governing selection?

Accepted Answer

Introduction / Context: Effective flange width is an idealized width used to model the contribution of the slab to an L- or T-beam in flexure. Codes place upper bounds so that analysis does not overestimate slab participation. Given Data / Assumptions: Edge (L) beam where slab participates on one side. Typical geometric limits involve span fraction, slab thickness multiple, and spacing to adjacent beam. Concept / Approach: All relevant limits must be checked and the smallest value adopted as the effective flange width. This ensures conservatism and prevents unrealistic overestimation of beam capacity from slab flanges. Step-by-Step Solution: List code-style limits (e.g., span fraction, bw + k·Df, spacing-based).Evaluate all three for the given geometry in design.Adopt the least value as bf,eff.Hence, select option (d): “least of the above.” Verification / Alternative check: Worked examples in R.C.C. texts always compute each limit and then choose the smallest for design bending capacity. Why Other Options Are Wrong: (a), (b), (c) individually can be either too conservative or unconservative depending on geometry. (e) is incorrect because (d) states the correct procedure. Common Pitfalls: Using only one limit; forgetting that edge beams have different limits than internal T-beams; not checking rib breadth and slab thickness influence. Final Answer: The least of the above limits (apply all and take the smallest)

RCC Structures Design Questions