Glossary

The 28-day cube/cylinder strength below which not more than 5% of test results may fall.

For mix design, the target mean strength is set higher than the characteristic strength by a margin — the greater of 1.65 S and the grade-based X in IS 10262:2019 Cl 4.2 — so that statistically only 5% of cubes fall below f_ck. Concrete grades like M25 mean characteristic cube strength is 25 MPa.

The average strength a mix must achieve at the lab so that the probabilistic 5%-fail margin still meets the characteristic value.

IS 10262:2019 Cl 4.2: the greater of f_ck + 1.65 S and f_ck + X, with S the standard deviation (Table 2 until site data exists) and X the margin for the grade (Table 1).

A label like M25 (IS) or C25/30 (EC) that combines characteristic cube and cylinder compressive strength in MPa.

In IS notation, "M25" means the 28-day characteristic cube strength is 25 MPa. In Eurocode, "C25/30" means cylinder strength 25 MPa and cube strength 30 MPa.

Ratio of water to cementitious binder by mass. The single biggest determinant of concrete strength and durability.

A lower w/c gives higher strength and lower permeability, but reduces workability — so superplasticisers are used to maintain a workable slump while keeping w/c low.

Mass of cement per cubic metre of concrete (typically 280–500 kg/m³).

Codes set both minimum cement content (durability) and maximum cement content (heat of hydration / shrinkage cracking).

A measure of fresh concrete consistency — the height (in mm) by which a moulded concrete cone slumps after the slump cone is lifted.

Typical slumps: 25–50 mm for low-workability mass concrete, 75–100 mm for normal RCC slabs, 150–200 mm for pumped concrete or self-compacting mixes.

The slope of the stress-strain curve of concrete in the elastic range.

The stress at which steel transitions from elastic to plastic behaviour. Common rebar grades: Fe 415 (415 MPa), Fe 500 (500 MPa).

Constant ≈ 2 × 10⁵ MPa for all standard structural and reinforcing steels.

A cross-sectional property that relates bending moment to extreme-fibre bending stress.

The section modulus calculated assuming the entire section yields. Always larger than the elastic Z.

A geometric property that controls how prone a column is to buckling. Smaller r → more buckling-prone.

A failure mode where a slender beam loaded in bending rotates and translates sideways before reaching its in-plane capacity.

LTB capacity drops as the unbraced length increases. Codes use a slenderness parameter λ̄_LT to compute a reduction factor χ_LT applied to the plastic moment capacity.

The length of an equivalent pin-ended column that would buckle at the same load as the actual column.

K depends on end restraints. K = 1.0 for pin-pin, 0.7 for pin-fixed, 0.5 for fixed-fixed, 2.0 for cantilevers.

Plastic / compact / semi-compact / slender — based on flange and web b/t ratios.

Plastic sections develop full plastic moment with rotation capacity. Slender sections fail by local buckling before yield. Higher-class sections allow plastic design.

Ratio of plastic to elastic section modulus (Zp / Z). 1.5 for rectangles, ~1.12 for I-beams.

The maximum pressure soil can carry without shear failure (q_u) or unacceptable settlement (q_s).

Safe bearing capacity (q_s) divides ultimate bearing capacity (q_u) by a factor of safety, typically 2.5 to 3.0.

A failure mode where a column "punches" through a flat slab or footing along an inclined surface around the column perimeter.

Checked on a critical perimeter offset d/2 (IS, ACI) or 2d (EC2) from the column face. Often governs flat-slab and pad-footing design.

Shear failure across a single plane perpendicular to the beam axis. Standard τ = V/(b×d).

The total axial load a single pile can carry, sum of skin friction along the shaft and end bearing at the toe.

Vertical movement of a foundation under load. Has elastic (immediate) and consolidation (time-dependent) components.

The difference in settlement between two foundation elements. More damaging than total settlement.

IS 1904 limits differential settlement to L/300 (typical) for framed structures. Excessive differential causes cracking and rotation of structural members.

Number of blows from a 63.5 kg hammer falling 760 mm needed to drive a standard sampler 300 mm into soil.

Loose sand: N < 10. Medium dense: N = 10–30. Dense: N > 30. Corrections for overburden, rod length, hammer energy and borehole diameter are applied to get N₆₀ and (N₁)₆₀.

Loss of soil shear strength when saturated cohesionless soil temporarily behaves as a fluid under cyclic seismic loading.

Screened by comparing cyclic stress ratio (CSR) to cyclic resistance ratio (CRR). FoS_liq = CRR/CSR < 1.0 indicates liquefiable.

The angle between the normal force and resultant reaction force at soil failure under shear loading.

Loose sand: 28°–32°. Dense sand: 35°–45°. Soft clay: ~0° (cohesion-dominated).

Permanent loads from the structure's own weight + fixed components (walls, finishes, fixed equipment).

Variable loads from occupancy, furniture, people, movable equipment. IS 875 Part 2 sets typical values.

Residential: 2 kN/m². Office: 2.5–4 kN/m². Public assembly: 4–5 kN/m². Storage: 5+ kN/m².

Lateral pressure on a structure caused by wind, dependent on wind speed, height, exposure category and topography.

Lateral inertia force induced by ground acceleration during an earthquake. Computed as base shear V_b = A_h·W with A_h = (Z/2)·(I/R)·(Sa/g) (IS 1893 (Part 1):2016 Cl 6.4.2 / 7.6.1).

A weighted sum of individual loads (DL, LL, WL, EL) used in a single design check. Code-specific factors.

IS 456 typical: 1.5(DL+LL), 1.2(DL+LL+WL), 0.9DL+1.5WL. ACI uses 1.4D, 1.2D+1.6L, 1.2D+1.0W+L+0.5(Lr or S or R).

A numerical method that discretises a continuous structure into many small elements, computes stiffness, and solves for displacements.

A FEM plate element that includes transverse shear deformation — appropriate for thick plates and footings where Kirchhoff thin-plate theory is inaccurate.

Idealisation of soil as an array of independent vertical springs with stiffness k (kN/m³). Simple but ignores soil continuity.

Diagrams plotting shear force and bending moment along a beam's length. Standard output of any beam analysis.

A plot of the column's axial-moment capacity envelope. Any (P, M) point inside the envelope is safe.

The width of cracks in concrete under service loads. Limited by codes for durability and aesthetics.

IS 456 Cl 35.3.2: 0.3 mm in general; 0.2 mm suggested where cracking is harmful (members exposed to moisture or in contact with soil or groundwater); 0.004 × nominal cover to the main steel for particularly aggressive ('severe' and worse) environments. IS 3370 (Part 2) for water-retaining structures: 0.2 mm, tightened to 0.1 mm for the more severe cases it lists.

Code-specified maximum permissible deflection. Span/250 (total), span/350 (after construction) are common limits.

Minimum thickness of concrete from the surface to the nearest reinforcement bar. Protects steel from corrosion and fire.

IS 456: mild = 20 mm, moderate = 30, severe = 45, extreme = 75 mm. Footings always need ≥ 50 mm bottom cover.

Minimum length a reinforcing bar must be embedded in concrete to develop its design tensile stress through bond.

Overlap of two parallel bars to transfer force from one to the other through bond. Length depends on stress level and bar location.

A tabular list of every reinforcement bar in a structure: shape, diameter, length, quantity, total mass.

Used by detailers to fabricate cut and bent rebar. Each bar gets a unique mark number that appears on the drawing.

Code-specified minimum steel area to prevent brittle first-cracking failure. ~0.205% for Fe 415, 0.17% for Fe 500 (IS 456 beams).

A design philosophy that ensures the structure does not exceed any defined limit state — ultimate (collapse) or serviceability (cracking, deflection).

A code-specified factor applied separately to loads and material strengths to account for uncertainty.

IS 456: γ_c = 1.5 for concrete, γ_s = 1.15 for steel. Load factors typically 1.5 for DL+LL.

Ratio of capacity to demand. Single global FoS used in working-stress methods; replaced by partial factors in limit-state design.

A regulatory document setting the rules for structural design (e.g., IS 456 for RCC in India, ACI 318 in the US, Eurocode 2 in Europe).