UNIT 2: Advanced Structural Systems for Sustainability

1.0 PAVEMENT DESIGN (Flexible and Rigid)

1.1 Flexible Pavements

Subgrade Strength Assessment

• California Bearing Ratio (CBR) Test:

  • Procedure: A piston of 50 mm diameter is forced into a soaked soil sample at a rate of 1.25 mm/min. The load required to penetrate the sample to 2.5 mm and 5.0 mm is recorded. CBR is the ratio of the measured load to the standard load (for 2.5 mm penetration) multiplied by 100%.

  • Limitations: Highly empirical, sensitive to moisture content and compaction, does not account for repeated loading effects.

• Other Tests: Plate bearing test, R-value (Cohesive soil), California R-value (for base/sub-base).

Load Equivalency Concepts

• Equivalent Single Wheel Load (ESWL): The single wheel load that produces the same vertical stress at a given depth as a group of wheels (e.g., dual wheels, tandem axles).

• Determination (Equal Vertical Stress Criterion):

  For dual wheels, ESWL is calculated at a depth z using:

$$ \text{ESWL} = \frac{P}{2} \left[ 1 - \frac{1}{\pi} \sin^{-1}\left(\frac{s}{2z}\right) \right] $$

Where `P` = load on one wheel, `s` = spacing between wheel centers.

> [!TIP] For tandem axles, ESWL is found by superimposing stresses from each axle pair at the critical depth.

Design Methodologies

• IRC Flexible Pavement Design (IRC:37-2018):

  1. Assess traffic (ESALs - Equivalent Single Axle Loads).

  2. Determine subgrade CBR (or other strength parameter).

  3. Select design reliability and standard deviation.

  4. Use design charts/graphs to find total pavement thickness (D) for given CBR and ESALs.

  5. Distribute D among component layers (BC, DB, GS) based on layer coefficients.

• AASHTO 1993/1998 Method: Uses Structural Number (SN) concept.

$$ SN = a_1 D_1 + a_2 D_2 + a_3 D_3 + ... $$

Where `a_i` = layer coefficients, `D_i` = layer thicknesses (inches). SN is determined from:

$$ \log_{10} W_{18} = Z_1 \times S + Z_2 \times \log_{10} (SN+1) - 0.2 + \frac{\log_{10} \left( \frac{\Delta PSI}{4.2-1.5} \right)}{0.4 + \frac{1094}{SN+1}} $$

(Complex; overview sufficient for exam).

• Design Factors:

  • Load Variables: Wheel load, contact pressure, axle configuration, ESALs.

  • Structural Variables: Layer thickness, material properties (elastic modulus, Poisson's ratio), layer coefficients.

  • Climatic Variation: Rainfall, temperature, frost depth (affects drainage, frost heave).

Overlay Design

• Using Benkelman Beam Deflection (BBD) Data:

  1. Measure initial deflection (δ_i) on existing pavement.

  2. Compute corrected deflection (δ_c) for temperature, season, and load.

  3. Determine allowable deflection (δ_a) for new overlay from design charts (based on CBR/subgrade strength).

  4. Overlay thickness t is found from:

$$ t = C \times (\delta_c - \delta_a) $$

    Where `C` = overlay coefficient (from experience/charts, ~2.5-5.0 cm/mm).

Materials and Components

• Road Aggregates: Crushed granite, basalt, limestone, gravel. Tests: Aggregate Crushing Value (ACV), Los Angeles Abrasion (LAA), Impact Value, Flakiness & Elongation Index.

• Component Layers & Functions:

  | Layer | Function | | :--- | :--- | | Surface Course (BC/SC) | Wear resistance, smoothness, drainage. | | Binder Course (DBM) | Distribute loads, bind layers. | | Base Course (WBM/GSB) | Major load distribution, drainage. | | Sub-base | Additional distribution, prevent frost heave. | | Subgrade | Natural soil foundation. |

• Permissible Deflection: For flexible pavements, often taken as 0.25 cm (for design) under standard load (4200 kg wheel load, 6.0 kg/cm² pressure).

1.2 Rigid Pavements

Stress Analysis Theories (Westergaard)

• Assumptions:

  1. Slab is a homogeneous, elastic, isotropic plate of finite length but infinite in other directions.

  2. Slab rests on a Winkler-type foundation (modulus of subgrade reaction k).

  3. Load is applied as a uniformly loaded circular area.

• Key Equations (Stresses in kg/cm²):

  • Interior Stress (Corner loading): $$\displaystyle \sigma_{int} = \frac{3P}{2\pi h^2} \left( 1 - \mu^2 \right) \left[ \ln \frac{E h^3}{k a^4} + 1.84 \right] $$

  • Edge Stress (Edge loading): $$\displaystyle \sigma_{edge} = \frac{3P}{h^2} \left( 1 - \mu^2 \right) \left[ \ln \frac{E h^3}{k a^4} + 1.84 \right] $$

  • Corner Stress (Corner loading): $$\displaystyle \sigma_{corner} = \frac{3P}{h^2} \left( 1 - \mu^2 \right) \left[ \ln \frac{E h^3}{k a^4} + 1.84 \right] \times \text{corner factor} $$

  Where P = load, h = slab thickness, μ = Poisson's ratio, E = modulus of elasticity, a = radius of loaded area, k = modulus of subgrade reaction.

  [!TIP] Critical Stresses: Edge stress is usually highest for load. Corner stress is critical for curling/warping.

Design Parameters

• Modulus of Subgrade Reaction (k): $$\displaystyle k = \frac{p}{\delta} $$ (Pressure per unit deflection). Determined from plate bearing test (units: kg/cm³).

• Radius of Relative Stiffness (l): $$\displaystyle l = \left[ \frac{E h^3}{12 k (1-\mu^2)} \right]^{1/4} $$. Indicates slab's stiffness relative to subgrade.

• IRC Recommendations (IRC:58-2015): Thickness based on flexural stress (not fatigue). Uses k value and design load. Provides charts/tables for 90% reliability.

Joints in Rigid Pavements

• Types & Functions:

  | Joint Type | Spacing | Primary Function | | :--- | :--- | :--- | | Transverse Expansion | 50-120 m | Allow slab expansion, prevent blow-ups. | | Transverse Contraction | 3-5 m | Control cracking from shrinkage. | | Longitudinal | Along lane lines | Control longitudinal cracking. | | Isolation | At structures | Isolate slab from fixed objects. |

• Tie Bars:

  • Purpose: Hold adjacent slabs together, prevent lane separation.

  • Design:

    • Diameter (φ): Based on bond stress: $$\displaystyle \phi = \sqrt{\frac{2 \times T}{\pi \times \tau_b \times L}} $$

    • Spacing (s): $$\displaystyle s = \frac{A_s \times f_s}{W \times \mu \times h} $$

    • Length (L): Must develop bond on both sides, typically L = 0.7L_1 + 0.3L_2 (anchorage lengths).

  • Installation Difficulties: Misalignment, improper embedment, congestion during concreting.

• Dowel Bars:

  • Purpose: Transfer loads across transverse joints, maintain alignment.

  • Design: Smooth, round bars (typically 20-25 mm dia), placed mid-depth, spaced 30 cm apart. One end greased.

• Joint Filler vs Sealing Compound:

  | Feature | Joint Filler | Sealing Compound | | :--- | :--- | :--- | | Purpose | Fill space, allow expansion | Prevent water/debris ingress | | Material | Pre-molded (cork, foam) | Liquid applied (bitumen, silicone, polysulfide) | | Properties | Compressible, permanent | Adhesive, elastic, durable |

Thermal Stresses

• Seasonal Temperature Variation: Causes warping (curling). Slab expands/contracts uniformly. Stress: $$\displaystyle \sigma_{warp} = \frac{E \alpha \Delta T}{2} $$ (for infinite slab, restrained). Critical at interior/edge/corner.

• Daily Temperature Gradient: Causes curling (non-linear temp. through depth). Top surface hotter/cooler than bottom. Stress: $$\displaystyle \sigma_{curl} = \frac{E \alpha \Delta T_{daily}}{2} \times \text{curling coefficient} $$.

  [!TIP] Critical Combination: Maximum load stress + maximum curling/warping stress (same sign) governs design.

1.3 General Pavement Considerations

• Structural Requirements: Strength to carry loads without excessive deflection/fatigue.

• Functional Requirements: Smoothness, skid resistance, drainage, low noise.

• Lateral Distribution Factor (LDF):

  • Necessity: Load from a wheel is not entirely transmitted vertically down; it spreads laterally through the pavement layers. LDF accounts for this distribution when calculating stress at a depth.

  • Estimation: For a semi-infinite slab, stress at depth z under center of load is used. The effective area at depth z is larger than the loaded area. LDF = (Area at depth z) / (Loaded area). Often taken as 1.5 to 2.0 for design charts.

  [!TIP] With Sketch: Show a wheel load P on surface. At depth z, the stressed area has increased diameter d'. LDF ≈ (d'/d)^2.

• Evaluation of Existing Pavements: Benkelman Beam Method:

  1. Place beam behind a loaded truck (4200 kg wheel load).

  2. Measure initial deflection (δ_i) at pavement surface.

  3. Compute corrected deflection (δ_c) for temperature, moisture, and load.

  4. Compare δ_c with permissible deflection to assess remaining life.

• Climatic Effects: Rainfall → drainage design, water damage. Temperature → thermal stresses, joint spacing. Frost → frost heave, need for non-frost-susceptible sub-base.

2.0 WATER RESOURCES ENGINEERING (Irrigation and Hydrology)

2.1 Irrigation Fundamentals

Irrigation Necessity and Evaluation

• Necessity: To supplement rainfall, ensure water supply during dry periods, increase crop yield & intensity, stabilize production.

• Advantages: Increased yield, multiple cropping, drought protection, weed control, fertilizer application.

• Disadvantages: Waterlogging, salinity, high cost, soil structure deterioration, disease/pest promotion.

Irrigation Methods

• Surface Methods:

  • Free Flooding: Water released from field channels without control. Suitable for close-growing crops on irregular land. Low efficiency, high wastage.

  • Border Flooding: Land divided into long, narrow strips (borders) with low bunds. Water flows down the slope. Better control than free flooding.

  • Check Flooding: Land divided into small, level plots (checks) with bunds. Water retained until infiltrated. High efficiency, suitable for heavy soils.

  [!DIAGRAM]

  DiagramSEARCH: border flooding irrigation sketch, check flooding irrigation sketch

• Sprinkler Irrigation: Water pressurized through nozzles, sprayed into air. Suitable: Uneven terrain, sandy soils, close-spaced crops. Advantages: No land leveling, water saving, fertigation possible.

• Drip Irrigation: Water applied slowly near root zone through emitters. Suitable: High-value crops, arid regions, saline water. Advantages: Highest water use efficiency, fertigation, weed control.

• Comparison (Sprinkler vs Drip):

  | Feature | Sprinkler | Drip | | :--- | :--- | :--- | | Efficiency | 60-75% | 85-95% | | Cost | Moderate | High initial, low operating | | Wind Effect | High | Negligible | | Fertigation | Possible | Excellent | | Suitable Crops | Field, pasture | Row crops, orchards |

Crop Water Requirements

• Consumptive Use (CU): Total water used by crop for transpiration + evaporation from soil + metabolic needs. ≈ Evapotranspiration (ET).

• Determination Methods:

  1. Field Experiments: Soil moisture depletion studies.

  2. Tank/Evaporimeter Method: CU = K × E_pan (K = crop coefficient).

  3. Blaney-Criddle Formula: CU = K × f × T (f = day factor, T = temp).

  4. Penman Formula: Based on energy balance.

• Soil-Plant-Water Relationships:

  • Field Capacity (FC): Water content after free drainage (~2-3 days). Optimum for plant growth.

  • Permanent Wilting Point (PWP): Water content at which plants wilt permanently (~15 bar tension).

  • Wilting Coefficient: Water content at which plants show temporary wilting (recoverable at night).

  • Available Moisture (AM): AM = FC - PWP. Water available to plants.

• Duty, Delta, Base Period:

  • Duty (D): Area irrigated per unit discharge (hectares/cumec).

  • Delta (Δ): Depth of water applied (cm).

  • Base Period (B): Number of days water is supplied to cover crop requirement.

  • Relationship: $$\displaystyle D = \frac{8.64 \times B}{\Delta} $$ (for D in ha/cumec, B in days, Δ in cm).

    \boxed{D = \frac{8.64 , B}{\Delta}}

  • Derivation: Volume of water required = Area × Δ. Discharge required = (Area × Δ) / (B × 86400 sec). Duty = Area / Discharge → D = (B × 86400) / (Δ × 10000) = 8.64 B / Δ.

• Irrigation Scheduling:

  • Frequency: Interval between irrigations = AM / Daily CU.

  • Depth: Usually to bring soil to field capacity.

  • Kor Period: First watering time after sowing (critical for germination).

  • Paleo Irrigation: Pre-sowing irrigation in dry season.

• Crop Terms:

  • Gross Command Area (GCA): Total area that can be irrigated from a source.

  • Culturable Command Area (CCA): Part of GCA actually cultivable (excludes uncultivable land).

  • Crop Ratio: Ratio of areas under irrigated and unirrigated crops.

  • Crop Rotation: Sequential growing of different crops on same land.

  • Cash Crops: High-value commercial crops (e.g., cotton, sugarcane).

2.2 Canal Design and Management

Canal Classification

• Based on Function:

  | Canal | Function | | :--- | :--- | | Main Canal | Direct from source, no direct irrigation. | | Branch Canal | Off-take from main, high discharge. | | Distributary | Off-take from branch, supplies to minors. | | Minor | Supplies to water courses. | | Water Course | Field channel, last unit. |

• Based on Discharge/Importance: Main, Branch, Distributary, Minor.

Canal Design Theories (Regime Channels)

• Kennedy's Theory:

  • Concept: silt is carried in suspension by turbulent flow. Critical velocity V₀ prevents silt deposition/scouring.

  • Design Equation: $$\displaystyle V = 0.55 \, m \, D^{0.64} $$ (for alluvial soils). m = critical velocity multiplier (0.6-1.0).

  • Procedure: Assume m, trial depth D, compute V from equation, check Q = A × V. Iterate.

• Lacey's Theory:

  • Concept: Regime channel has stable perimeter, slope, depth for given discharge & silt.

  • Silt Factor (f): $$\displaystyle f = 1.76 \sqrt{d_{50}} $$ (mm) or from V & R.

  • Design Equations:

    • Perimeter: $$\displaystyle P = 4.75 \sqrt{Q} $$ (if f=1)

    • Area: $$\displaystyle A = \frac{Q^2}{f^2} $$

    • Slope: $$\displaystyle S = \frac{f^{5/3}}{1440 \times Q^{1/6}} $$

  • Drawbacks: Empirical, assumes silt-laden flow is always in regime, f not well-defined.

Canal Lining and Alignment

• Objectives of Lining:

  1. Reduce seepage loss (increase conveyance efficiency).

  2. Increase velocity (reduce canal size).

  3. Prevent weed growth, reduce maintenance.

  4. Protect against erosion.

  5. Enable steeper slopes in stable soils.

• Materials: Earth (puddled clay), cement mortar, concrete (precast/shotcrete), brick, stone, synthetic membranes (HDPE, PVC).

• Factors Influencing Canal Alignment:

  • Topography: Follow contour for gravity flow.

  • Soil & Geology: Avoid unstable slopes, poor soils.

  • Command Area: Serve maximum area with minimum length.

  • Drainage: Avoid crossing drainage lines unnecessarily; plan cross-drainage works.

  • Existing Infrastructure: Avoid roads, buildings.

  • Cost: Balance between earthwork and structure cost.

Hydraulic Structures

• Weirs: Overflow structures for measurement/regulation (e.g., Cippoletti weir for canals).

• Barrages: Similar to weirs but with gates for better control (e.g., Nagarjuna Sagar).

• Aqueducts: Carry canal over a drain/river. Key components: Inlet, trough, outlet.

• Canal Regulation Structures: Gates (head regulator, cross regulator), Escapes (for safety), Outlets (modular/rigid modules).

• Cross-Drainage Works:

  | Type | When Used | Sketch Principle | | :--- | :--- | :--- | | Aqueduct | Canal over drain | Canal on elevated trough. | | Siphon Aqueduct | Canal over drain with high water table | Canal and drain under pressure. | | Super Passage | Drain over canal | Drain on elevated structure. | | Culvert | Canal under road/drain | Simple pipe/box. | | Level Crossing | Equal bed levels | Canal and drain at same level with regulators. |

2.3 Waterlogging and Salinity

• Waterlogging:

  • Causes: Excessive irrigation, poor drainage, high water table, obstruction to flow.

  • Effects: Aeration deficiency, nutrient deficiency, reduced yield, soil structure damage.

  • Prevention: Proper irrigation scheduling, drainage system (surface/subsurface), land leveling.

  • Reclamation: Install drainage system (tile drains, open ditches), use salt-tolerant crops, leaching.

• Salinity:

  • Salt Efflorescence: White crust of salts on soil surface due to capillary rise & evaporation.

  • Effects: Ion toxicity, osmotic stress, nutrient imbalance.

  • Reclamation:

    1. Leaching: Apply excess water to dissolve & percolate salts (requires good drainage).

    2. Scraping: Remove salt crust.

    3. Soil Amendments: Gypsum for sodic soils.

    4. Plant Salt-Tolerant Crops (e.g., barley, sugar beet).

  • Restoring Fertility: Leaching + organic matter addition + balanced fertilization.

2.4 Groundwater Engineering

Aquifers

• Types:

  • Unconfined (Water Table): Upper surface is water table, atmospheric pressure.

  • Confined (Artesian): Between impermeable layers, under pressure.

  • Perched: Local water table above main aquifer due to lens.

• Properties:

  • Coefficient of Permeability (K): Rate of flow under unit hydraulic gradient (m/day).

  • Storage Coefficient (S): Volume of water released per unit area per unit decline in head (confined: dimensionless; unconfined: specific yield ≈ porosity).

Wells

• Types:

  • Open Well: Dug/open to atmosphere. Low yield.

  • Tube Well: Drilled, cased, with strainer. High yield.

  • Artesian Well: Taps confined aquifer, flows without pumping.

• Design Aspects:

  • Strainer Length: Based on aquifer thickness, permeability, desired yield.

  • Diameter: Based on pump size, required discharge, well loss.

• Discharge Calculation:

  • Dupuit's Equation (Artesian Aquifer): $$\displaystyle Q = \frac{\pi k (h_1^2 - h_2^2)}{2.303 \log_{10}(r_2/r_1)} $$

    Where h₁ = head at well, h₂ = head at observation well, r₁ = well radius, r₂ = radius of influence.

  • Theim's Formula (Confined): Similar form, uses S.

  • For Unconfined (Dupuit-Forchheimer): $$\displaystyle Q = \frac{\pi k (h_1^2 - h_2^2)}{\ln(r_2/r_1)} $$

Groundwater Recharge

• Infiltration Galleries: Horizontal tunnels/wells dug below riverbed to intercept seepage. Contribution: Collects subsurface flow from river, increases groundwater storage.

• Improving Recharge Methods:

  1. Check Dams/Percolation Ponds: Store surface water, promote infiltration.

  2. Recharge Wells: Direct injection of surface water into aquifer.

  3. Contour Trenching/Bunding: Slow runoff, increase infiltration.

  4. Land Leveling: Reduce runoff velocity.

  5. Wastewater Reuse: Treated sewage for recharge.

2.5 Hydrology

Hydrological Cycle

[!DIAGRAM]

DiagramSEARCH: hydrological cycle diagram with evaporation, condensation, precipitation, runoff, infiltration, groundwater

Precipitation

• Rainfall Measurement:

  • Non-Recording: Symons Rain Gauge (weekly reading).

  • Recording: Siphon Recording, Weighing Type, Tipping Bucket.

• Estimation of Missing Rainfall:

  • Arithmetic Mean Method: $$\displaystyle P_m = \frac{\sum P_i}{n} $$ (for uniform rainfall).

  • Normal Ratio Method (if stations have different normals): $$\displaystyle \frac{P_m}{N_m} = \frac{\sum P_i / \sum N_i}{} $$

• Average Precipitation Over Area:

  • Arithmetic Mean: Simple average of station values.

  • Thiessen Polygon Method: Weighted average based on area of influence around each station.

  • Isohyetal Method: Draw lines of equal rainfall (isohyets), compute area between them.

• Depth-Area-Duration (DAD) Curves: Show maximum average depth for given area & duration. Significance: Used for design storm estimation (e.g., PMP).

Infiltration

• Definition: Entry of water into soil surface.

• Factors Affecting: Soil type, structure, initial moisture, vegetation, slope, rainfall intensity/duration.

• Infiltration Indices:

  • φ-index (Phi-index): Constant infiltration rate that produces given runoff. (Average rate when rainfall > φ).

  • W-index (W-index): φ-index corrected for initial loss. $$\displaystyle W = \frac{P - R - I_a}{t_r} $$ (where I_a = initial loss, t_r = rainfall duration after I_a).

  [!TIP] Numerical: Given rainfall hyetograph and runoff depth, find φ by trial such that total rainfall - (φ × duration) = runoff (ignoring initial loss for φ-index).

Runoff and Hydrographs

• Unit Hydrograph (UH): Direct runoff hydrograph from 1 cm (or 1 inch) of effective rainfall uniformly over the basin in a specified time (T hours). Assumptions: Linearity, time invariance.

• Derivation from Runoff Hydrograph:

  1. Separate baseflow from total runoff.

  2. Compute direct runoff (DRH).

  3. Compute total effective rainfall (P_eff).

  4. UH ordinates = DRH ordinates / P_eff.

• S-curve Hydrograph: Summation of n unit hydrographs of duration T, lagged by T. Used to derive UH of different duration.

$$ \text{UH}_{mT} = \frac{\text{S-curve of } mT - \text{S-curve of } nT}{m-n} $$

• Instantaneous Unit Hydrograph (IUH): UH of infinitesimal duration. Derived from S-curve by differentiation or from Clark's IUH (using time-area histogram & storage coefficient).

• Factors Affecting Hydrograph Shape: Basin size, shape, slope, soil, vegetation, rainfall intensity/duration, initial loss.

Floods

• Flood Frequency Analysis: Use Gumbel's Extreme Value (EV1) or Log-Pearson Type III distributions to estimate flood magnitude for given return period (T years).

  • Reduced Variate: $$\displaystyle y = -\ln[\ln(T/(T-1))] $$

  • Flood Estimate: $$\displaystyle Q_T = \bar{Q} + K_T \times S $$ (where K_T from frequency factor).

• Design Discharge Computation:

  1. Rational Method: $$\displaystyle Q_p = C_i A $$ (for small catchments < 200 km²).

     Where C = runoff coefficient, i = rainfall intensity (from IDF), A = area.

  2. Flood Frequency Analysis: For large catchments.

  3. Empirical Formulas: (e.g., Ryves, Dicken).

• Flood Control Measures: Structural: Dams/reservoirs, levees, channel improvement, bypass channels. Non-Structural: Flood forecasting & warning, zoning, insurance.

• Channel Routing (Muskingum Method):

  • Concept: Storage S = K [x I + (1-x) O] (linear weighting).

  • Parameters: K = storage time constant, x = weighting factor (0 ≤ x ≤ 0.5).

  • Routing Equation:

$$ O_2 = C_0 I_2 + C_1 I_1 + C_2 O_1 $$

    Where coefficients depend on `C` (routing coefficient) and `x`.

*   **Steps:**

    1.  Determine `K` and `x` from inflow hydrograph & stage-storage-discharge data.

    2.  Compute `C₀`, `C₁`, `C₂`.

    3.  Apply equation sequentially for each time step.

Evaporation

• Analytical Methods for Lake Evaporation:

  1. Energy Balance Method: $$\displaystyle E = \frac{R_n - G - H - \lambda E}{λ} $$ (most accurate, hard to measure all terms).

  2. Aerodynamic Method (Penman): $$\displaystyle E = \frac{\Delta (R_n - G) + \gamma \frac{900}{T+273} (e_s - e_a) u_2}{\Delta + \gamma (1 + 0.34 u_2)} $$

  3. Combined Method (Penman-Monteith): FAO-56 recommended, uses net radiation, temperature, humidity, wind speed.

3.0 PREFABRICATED AND MODULAR CONSTRUCTION

3.1 Introduction and Systems

• Need for Prefabrication: Speed, quality control, material savings, reduced site labor, weather independence, eco-friendly (less waste).

• Systems of Prefabrication:

  | System | Description | Advantages | Disadvantages | | :--- | :--- | :--- | :--- | | Large Panel | 3D room-sized panels (walls, floors). | Fast erection, good insulation. | Heavy, requires heavy crane, transportation difficulty. | | Frame | Precast columns, beams, slabs assembled on site. | Flexible planning, lighter panels. | More connections, potential for differential movement. | | Box/Volumetric | Complete 3D units (rooms). | Maximum factory completion, fastest. | Very heavy, transport size limits. |

• Modular Coordination: Standardizing dimensions in multiples of a basic module (M = 100 mm). Ensures compatibility between components from different manufacturers.

  • Significance: Reduces waste, simplifies design, allows mass production, cost-effective.

• Standardization: Using standard sizes/shapes for components. Benefits: Economies of scale, reduced inventory, easier replacement.

• Degree of Progressivity: Extent to which building is prefabricated. Ranges from semi-progressive (some elements prefabricated) to fully progressive (entire building as 3D modules).

3.2 Production, Transportation, and Erection

• Process Flow (Roof/Floor Slabs):

  [!DIAGRAM]

  DiagramCANVAS: Flow chart: 1. Mold preparation -> 2. Reinforcement placing -> 3. Concrete placing & vibration -> 4. Surface finishing -> 5. Curing -> 6. Demolding -> 7. Stacking/Storage -> 8. Transportation -> 9. Erection & Grouting

• Production Considerations:

  • Quality Control: Material testing, dimensional checks, curing regime (steam curing for speed).

  • Curing: Moist curing (wet burlap) or steam curing (accelerated). Must prevent shrinkage cracks.

• Transportation: Requires careful planning for route survey (overhead wires, bridge capacity). Use specialized trailers. Protect edges, use lifting lugs. Damage Prevention: Cushioning, proper stacking, avoiding overloading.

• Erection: Sequence: Foundations -> Columns -> Beams/Girders -> Walls -> Floors -> Roof. Use temporary props. Alignment checked with theodolite/laser. Connections (grouting, welding) done precisely.

3.3 Structural Components

Wall Panels and Shear Walls

• Classification of Wall Panels:

  • Load-Bearing: Carry vertical loads (self-weight, floors).

  • Non-Load-Bearing (Curtain Walls): Only infill, carry no vertical load.

  • Partition Walls: Internal, non-structural.

• Shear Walls vs Conventional Brick Masonry:

  • Shear Wall (RC): High in-plane stiffness & strength, ductile, good for lateral loads (wind/seismic). Thin, lightweight.

  • Brick Masonry: Brittle, low tensile strength, poor performance under seismic loads. Thick, heavy.

  [!TIP] Benefit: Shear walls provide dual function (gravity + lateral resistance) with less material.

• Design Example (Concrete Wall Panel): Check for:

  1. Axial Load: $$\displaystyle P_u \leq 0.4 f_{ck} A_g $$ (short column) or use interaction diagrams.

  2. Bending: $$\displaystyle M_u \leq \phi M_n $$ (with slenderness effects).

  3. Shear: $$\displaystyle V_u \leq \phi V_c $$ (or with shear reinforcement).

  4. Slenderness: $$\displaystyle \lambda = \frac{l_e}{h} $$ check (l_e = effective length, h = thickness).

Slabs and Columns

• One-way vs Two-way Prefabricated Slabs:

  • One-way: Supported on two opposite edges (like beams). Aspect ratio l_y/l_x > 2. Thinner, lighter.

  • Two-way: Supported on all four edges. Aspect ratio l_y/l_x ≤ 2. Stiffer, can carry more load. Requires more complex connections.

• Manufacturing of Roof/Floor Slabs: Cast on long-line beds (for repetitive elements). Use molds (steel/wood). Include lifting hooks, conduits, recesses for connections. Post-tensioning possible for longer spans.

• Column Structures: Usually precast columns with footing (precast or cast-in-situ). Connections: Column to footing (grouted sleeve, welded base plate), column to beam (dowels, corbels, mechanical connectors).

Special Components

• Box Prefabricates: 3D volumetric units (e.g., toilet pods, kitchen modules). Advantages: Complete with finishes, services. Limitations: Transport size/weight restrictions.

• Earthen Walls: Use stabilized soil blocks (SSB) or rammed earth. Brief: Low embodied energy, good thermal mass, but low strength, moisture sensitive. Requires protection (overhangs, plaster).

3.4 Joints and Connections

• Types of Joints:

  • Expansion Joint: Accommodates thermal expansion (filled with compressible filler).

  • Contraction/Control Joint: Controls cracking from shrinkage (saw-cut or pre-formed).

  • Isolation Joint: Separates structure from fixed elements (columns, walls).

  • Flexibility Joint: Allows differential movement (in modular buildings).

• Joint Filler vs Sealing Compound:

  • Filler: Pre-molded (foam, cork). Function: Fill space, allow compression. Not a seal.

  • Sealing Compound: Liquid-applied (silicone, polysulfide, polyurethane). Function: Seal against water, air, contaminants. Must be adhesive, elastic, durable, weather-resistant.

• Connections for Wall Panels:

  • Mechanical: Bolts, plates, cleats.

  • Welded: Steel plates welded on site.

  • Grouted: Steel protrusions inserted into grouted sleeves/holes.

• Problems due to Joint Flexibility: Weatherstripping failure, water leakage, air infiltration, acoustic issues, differential settlement leading to panel misalignment.

• Disuniting Process (Dismantling for Reuse):

  1. Planning: Document connections, mark components.

  2. Removal of Finishes: Strip non-structural elements.

  3. Cutting Connections: Carefully cut welds, remove bolts, break grout.

  4. Separation: Use jacks/cranes to separate panels.

  5. Inspection & Repair: Check for damage, repair if possible.

  6. Cleaning & Storage: Clean, store for reuse.

  [!TIP] Precautions: Avoid damaging reinforcement/concrete, maintain structural integrity during separation, ensure safe lifting.

3.5 Design Considerations for Sustainability

• General Recommendations for Expansion Joint Design:

  • Width based on max temperature range & coefficient of thermal expansion.

  • Use compressible filler (foam) + sealing compound on surface.

  • Provide drip edge to shed water.

  • Joint should be continuous through all layers.

• IS Code Provisions for Abnormal Effects (IS 1893, IS 13920):

  • Earthquake: Design for lateral forces using response spectrum. Provide ductile detailing (ties, stirrups, beam-column joints). Equivalent Static Load Method for low-rise, Response Spectrum Method for high-rise.

  • Wind: Calculate wind pressure from basic wind speed, importance factor, height, terrain. Design for pressure & suction.

• Seismic Design Aspects:

  • Equivalent Design Load: Base shear V = A_h × W (where A_h = seismic coefficient from code, W = seismic weight).

  • Intensity vs Magnitude: Magnitude (M) = energy released (Richter scale). Intensity (Mercalli) = observed effects at location. Design uses intensity (PGA) from hazard maps.

• Damping: Ability to dissipate vibrational energy.

  • Types: Material Damping (internal friction in concrete/steel), Structural Damping (friction in connections), Added Damping (viscous dampers, tuned mass dampers).

• Modular Planning for Residential Apartments:

  • Use 3.6 m or 7.2 m modules (multiples of 0.3M).

  • Standardize flat sizes (e.g., 1BHK: 45-60 m², 2BHK: 75-90 m²).

  • Repeat core units (stairs, lifts, shafts).

  • Benefits: Reduced design time, mass production of panels, efficient material use.

• Factors Influencing Response of Precast Components:

  1. Connection Stiffness: Rigid vs flexible connections.

  2. Mass & Stiffness Distribution: Torsional effects if irregular.

  3. Damping: Higher in connections (friction).

  4. Foundation Flexibility: Soil-structure interaction.

  5. Construction Sequence: Temporary vs final state stiffness.