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1. What is structural design?
Answer:
Structural design is the process of analyzing and proportioning structural members so they can safely carry loads and transfer them to the foundation without failure or excessive deformation, as per relevant codes.
2. What are the main loads considered in structural design?
Answer:
Dead load
Live load
Wind load
Earthquake load
Snow load (where applicable)
3. Difference between analysis and design?
Answer:
Analysis: Finding internal forces (bending moment, shear, axial force)
Design: Providing member size and reinforcement to resist those forces safely
4. What is limit state design?
Answer:
Limit State Design ensures safety against:
Limit state of collapse (strength failure)
Limit state of serviceability (deflection, cracking, vibration)
5. Which code is used for RCC design in India?
Answer:
IS 456: 2000 – Code of practice for plain and reinforced concrete.
6. What is characteristic strength of concrete?
Answer:
The strength below which not more than 5% of test results are expected to fall.
Example: M20 → 20 MPa characteristic compressive strength.
7. What is partial safety factor?
Answer:
A factor applied to loads and material strength to account for uncertainties in loading, material properties, and workmanship.
8. What is working stress method?
Answer:
A design method where stresses under service loads are limited to permissible values.
(It is older and less economical than limit state method.)
9. Difference between one-way and two-way slab?
Answer:
One-way slab: Load carried mainly in one direction (L/B ≥ 2)
Two-way slab: Load carried in both directions (L/B < 2)
10. What is effective depth?
Answer:
Distance from the compression face to the center of tension reinforcement.
11. Why is minimum reinforcement provided?
Answer:
To:
Control cracking
Improve ductility
Prevent sudden brittle failure
12. What is neutral axis?
Answer:
The line in a cross-section where stress changes from compression to tension and strain is zero.
13. What is under-reinforced section?
Answer:
A section where steel yields before concrete crushes, giving ductile failure.
(It is preferred in design.)
14. Why over-reinforced sections are not allowed?
Answer:
Because they fail suddenly by concrete crushing, without warning.
15. What is shear failure?
Answer:
Failure caused by diagonal tension cracks, usually sudden and brittle.
16. Why stirrups are provided in beams?
Answer:
Resist shear forces
Hold main reinforcement in position
Improve ductility
17. Difference between short and long column?
Answer:
Short column: Fails by crushing
Long column: Fails by buckling
18. What is slenderness ratio?
Answer:
Ratio of effective length to least lateral dimension of column.
19. Why lateral ties are provided in columns?
Answer:
Prevent buckling of longitudinal bars
Confine concrete
Improve ductility
20. What is development length?
Answer:
The length required to develop full strength of reinforcement through bond with concrete.
21. Why is cover provided?
Answer:
Protect steel from corrosion
Ensure fire resistance
Provide proper bond
22. What is load combination?
Answer:
Combination of different loads multiplied by safety factors to consider worst-case scenarios.
23. Difference between fixed and hinged support?
Answer:
Fixed: Restrains rotation and translation
Hinged: Allows rotation but restrains translation
24. What is indeterminate structure?
Answer:
A structure where reactions cannot be found using only equilibrium equations.
25. What is stiffness?
Answer:
Resistance offered by a structure or member against deformation.
26. What is deflection control and why is it important?
Answer:
To ensure:
Comfort of occupants
No damage to finishes
Proper serviceability
27. What is creep and shrinkage?
Answer:
Creep: Time-dependent deformation under sustained load
Shrinkage: Volume reduction due to moisture loss
28. What is ductility?
Answer:
Ability of a structure to undergo large deformation before failure, especially important in earthquakes.
29. What is load path?
Answer:
The route by which loads travel from slab → beam → column → foundation → soil.
30. What software are used for structural analysis?
Answer:
ETABS, STAAD Pro, SAFE, SAP2000
(But results must be verified manually.)
When interviewing for a Structural Design role, the focus shifts from general site execution to your understanding of mechanics, load paths, and code compliance (like IS 456, IS 800, or ACI codes).
Here are the most frequently asked technical questions and how to answer them:
1. Fundamental Mechanics & Structural AnalysisThese test your "engineering intuition" before you ever touch a software.
Q: What is the difference between a Fixed Support and a Pinned Support?
Answer: A Fixed support resists three forces: vertical, horizontal, and moment (rotation). A Pinned (Hinged) support resists vertical and horizontal forces but allows rotation (zero moment).
Q: Explain the concept of "Ductility" in a structure.
Answer: Ductility is the ability of a structure to undergo significant plastic deformation before failure. In seismic (earthquake) design, we want ductile structures so they dissipate energy and give occupants time to escape before a collapse.
Q: Draw the Shear Force Diagram (SFD) and Bending Moment Diagram (BMD) for a simply supported beam with a UDL.
Answer: The SFD will be a linear sloping line passing through zero at the center. The BMD will be a parabolic curve with the maximum value at the center (M=8wL2).
Q: Why is steel used as reinforcement in concrete? Why not other metals?
Answer: Concrete is strong in compression but weak in tension; steel provides the necessary tensile strength. Steel is specifically chosen because its Coefficient of Thermal Expansion is nearly identical to concrete, preventing internal stresses during temperature changes.
Q: What is the difference between "Working Stress Method" (WSM) and "Limit State Method" (LSM)?
Answer: WSM is a deterministic approach that assumes materials behave elastically and uses a high factor of safety on material strength. LSM is a probabilistic approach that considers safety factors for both loads (partial safety factors) and material strength, making it more economical and realistic.
Q: What is "Development Length" (Ld)?
Answer: It is the minimum length of a rebar that must be embedded in concrete to ensure a sufficient bond between the two, preventing the bar from "pulling out" when under tension.
Q: What is a "Slenderness Ratio" and why does it matter?
Answer: It is the ratio of the effective length of a column to its least radius of gyration (λ=rLeff). A higher ratio means the column is more likely to fail by buckling rather than crushing.
Q: Why are I-sections most commonly used for beams?
Answer: In bending, the maximum stress occurs at the top and bottom fibers. The I-section concentrates the material (flanges) at these extreme fibers where the stress is highest, making it highly efficient for resisting moments.
Q: In ETABS or STAAD.Pro, what is a "Diaphragm"?
Answer: A diaphragm is a structural element (usually the floor slab) that transmits lateral loads (wind or earthquake) to the vertical resisting elements like columns and shear walls.
Q: What is the purpose of "Clear Cover" vs. "Nominal Cover"?
Answer: Clear Cover is the distance from the concrete surface to the outer surface of the reinforcement (stirrup). Nominal Cover (per IS 456) is the design requirement to protect steel against corrosion and fire.
| Element | Minimum Grade of Concrete | Minimum Steel (as per IS 456) |
| Slab | M20 | 0.12% (HYSD) or 0.15% (Mild Steel) |
| Column | M20 | 0.8% (Min) to 6% (Max) of Gross Area |
| Beam | M20 | Based on As=fy0.85bd |
