IS 875 Explained: Structural Loads as per IS 875 & IS 1893

Structures are subjected to different types of loads throughout their service life. Indian Standards such as IS 875 and IS 1893 define how these loads are calculated, distributed, and combined to ensure safety and serviceability. The primary loads considered in building design are dead load, live (imposed) load, wind load, and earthquake (seismic) load.

Types of Loads in Structures as per IS 875 & IS 1893

Structures are designed to resist various types of loads during their lifetime. The key categories, as defined in Indian Standards, include:

• Dead Load (IS 875 Part 1)
• Live or Imposed Load (IS 875 Part 2)
• Wind Load (IS 875 Part 3)
• Earthquake or Seismic Load (IS 1893)

Dead Load

Dead loads are the permanent, static weights of the structure and its fixed components. They include the weight of beams, columns, slabs, walls, finishes, roofing, fixed equipment (e.g. HVAC, plumbing) and any permanently attached fixtures. Dead load is calculated simply as:

• Dead Load (kN or N) = Unit Weight (kN/m³) × Volume (m³) of each element

For example, a 0.2 m-thick reinforced concrete slab (unit weight ≈24 kN/m³) has a dead load of 24×0.2=4.8 kN/m². 

The unit weights of materials are given in IS 875-1 (1987) as tabulated values (e.g. concrete ≈24 kN/m³, brickwork ≈20 kN/m³). IS 875 Part 1 lists standard weights of materials and stored materials for dead load calculations

Design considerations for dead load include: ensuring all permanent elements are accounted for (including “superimposed” dead loads like finishes and partition walls), and treating dead load as uniformly distributed gravity loads. Dead loads cannot be reduced or shifted; they are applied in ultimate limit state combinations Dead loads are applied with appropriate partial safety factors as specified in IS 875 and IS 456, depending on the governing load combination.

Basic Structural Load Terms (Fundamentals)

1. Unit Weight (kN/m³): The weight of a material per unit volume. It’s used to calculate dead loads. Example: Concrete ≈ 24 kN/m³.
2. Volume (m³): The three-dimensional space occupied by an object, calculated for beams, slabs, columns, etc., when determining dead load.
3. Superimposed Dead Load: Additional permanent loads that are not part of the structural frame—e.g., finishes, tiles, or non-load-bearing partition walls.
4. Uniformly Distributed Load (UDL): A load that spreads equally across a length or area, like the self-weight of a slab or floor finishing applied over the entire surface.
5. Gravity Load: Any vertical load that acts due to gravity—includes dead and live loads. It contrasts with lateral loads like wind and seismic forces.
6. Ultimate Limit State (ULS): A condition beyond which the structure may fail or collapse. In ULS design, safety factors are applied to ensure the structure can handle extreme conditions.
7. Serviceability Limit State (SLS): A condition beyond which the structure may become uncomfortable or unusable (e.g., excessive deflection or cracking), even if it hasn’t collapsed.
8. Load Factor: A safety multiplier applied to loads in design. Example: Dead Load × 1.2 in ULS. It accounts for uncertainties in load estimation and ensures conservative design.
9. IS Code / IS 875 / IS 1893: Standards set by the Bureau of Indian Standards (BIS) that guide load calculations and combinations in structural design. Each part of IS 875 addresses a specific load type.
10. Fixed Equipment: Non-removable systems that are considered in dead load calculations—like HVAC ducts, large water tanks, solar panels, or plumbing pipelines.

Live (Imposed) Load

Live loads are the variable or transient loads due to occupancy and use. They include people, furniture, movable equipment, vehicles (for bridges), and stored materials.

By definition these do not act simultaneously everywhere and can change in magnitude and location, dead loads remain constant, while live loads “vary with respect to the occupancy and activities” in the structure. Live loads are given in IS 875 (Part 2) as minimum specified loads for different occupancies.

For example, Residential floors: 2.0 kN/m² (as per IS 875 Part 2), office may be 3.0 kN/m², and shopping malls 4.0 kN/m² (values from IS 875-2 tables). Roof loads depend on access: IS 875-2 specifies 1.5 kN/m² for accessible flat roofs or0.75 kN/m² if no access.

Live load calculations follow the code procedure:

• Use the minimum characteristic value from IS 875-2 for the given occupancy (e.g. offices, assembly, warehouse).
• Assume live load as uniformly distributed load (UDL) unless concentrated loads are specified (e.g. machinery).

Live load reduction: For large floor areas or multiple floors, IS 875-2 permits reduction. Only the floors with live load are assumed loaded simultaneously. For multistory buildings, code allows reduced live load on columns depending on how many floors they support (e.g. 20% reduction if 3 floors above) (See IS 875-2 Table 3 for reduction factors.) However, IS 1893 (seismic) treats live loads differently for seismic weight calculations.

Live loads are combined with appropriate factors in design. In ultimate load combinations (e.g. 1.5D+1.5L+ …), live load is factored (often 1.5) as per IS 875. For serviceability (deflections, vibrations), unfactored live load (1.0×) is used. Impact and dynamic effects are included via additional factors if applicable (e.g. train loads, crane loads).

Wind Load (As per IS 875 Part 3:2015)

Wind load refers to the lateral (horizontal) force exerted by wind on structures. In Indian standards, wind loading is governed by IS 875 Part 3 (2015), which provides a static equivalent method for calculating design wind pressure based on basic wind speed and modifying factors.

Key Parameters for Wind Load Calculation:

1. Basic Wind Speed (Vb​)
   • Obtained from the wind speed map provided in IS 875 Part 3.
   • Measured at a height of 10 m above ground level in open terrain.
   • Expressed in m/s, varies across different zones in India.
2. Risk Coefficient (k1)
   • Adjusts for the probability of occurrence (return period of wind events).
   • Typically ≥1.0; higher for essential or critical structures.
3. Terrain, Height and Structure Size Factor (k2)
   • Accounts for terrain category (I to IV), height of structure, and size.
   • Increases with height and openness of the surrounding terrain.
4. Topography Factor (k3)
   • Considers effects of hills, ridges, cliffs, and escarpments.
   • Taken as 1.0 for flat terrain.
5. Importance Factor for Cyclonic Region (k4)
   • Applied in cyclone-prone areas, usually 1.0 otherwise.
   • Applicable for buildings with ≤100 m height.

Design Wind Speed at Height z

V_z = V_b \cdot k_1 \cdot k_2 \cdot k_3 \cdot k_4

Where:

• Vz​: Design wind speed at height z (in m/s)
• Vb​: Basic wind speed (from IS 875 wind map)
• k1​: Risk coefficient (importance factor)
• k2​: Terrain, height and structure size factor
• k3​: Topography factor
• k4​: Cyclone factor (typically 1.0 outside cyclone regions)
• k₁, k₂, k₃, k₄ are respective modifying factors.

Design Wind Pressure:

p_z = 0.6 \cdot V_z^2

Where:

• Pz: Wind pressure at height z (in N/m²)
• Vz​: Design wind speed at height z (in m/s), This pressure is applied perpendicular to the exposed surfaces of the structure.

Additional Modifiers:

• Kd – Directionality factor
• Kc– Combination factor
• Area Averaging Factors – For cladding and small elements

These modifiers ensure accurate distribution of wind loads on various components of the structure.

Example Calculation:

For a 30 m tall building in Terrain Category II (urban environment):

• Vb = 39 m/s
• k1 = 1.0
• k2 = 1.0
• k3 = 1.0
• k4 = 1.0

V_z = 39 \cdot 1.0 \cdot 1.0 \cdot 1.0 \cdot 1.0 = 39 \, \text{m/s}

p_z = 0.6 \cdot (39)^2 = 0.6 \cdot 1521 = 912.6 \, \text{N/m}^2

p_z \approx 912.6 \, \text{N/m}^2

This design wind pressure is applied to the windward face and used for assessing uplift forces on roofs.

Design Considerations:

• Wind loads act perpendicularly to structural surfaces (walls, roofs).
• Induce bending in columns and shear in beams.
• Taller and lighter structures are more susceptible to wind effects.
• IS 875-3 provides shape factors (Cf) for various structures and uplift factors for roofs.
• Wind and seismic loads are not considered simultaneously; the more critical of the two is used for design, as per IS 875.

Earthquake (Seismic) Load (IS 1893: 2016)

Earthquake loads are lateral inertial forces caused by ground motion. IS 1893 (Part 1, 2016) governs the seismic design of structures in India. The country is divided into Seismic Zones II to V — from low to very high risk.

Factors Affecting Seismic Load

The total design seismic load depends on:

• Zone Factor (Z): Reflects the seismic zone risk.
• Importance Factor (I): Higher for essential buildings like hospitals.
• Response Reduction Factor (R): Accounts for ductility and overstrength.
• Spectral Acceleration (Sa/g): Varies with soil type and building period.
• Seismic Weight (W): Total mass of the structure contributing to seismic forces.

Design Seismic Base Shear

The equivalent static method is applicable to regular buildings within the height and plan limits specified in IS 1893; taller or irregular structures require dynamic analysis.

IS 1893 uses the Equivalent Static Method to estimate the design base shear Vb

V_b = A_h \cdot W

Where:

• Vb​: Design base shear (in kN)
• W: Seismic weight (in kN)
• Ah​: Design horizontal seismic coefficient

Seismic Coefficient Formula

A_h = \frac{Z I}{2 R} \cdot \frac{S_a}{g}

Where:

• Z: Zone factor
• I: Importance factor
• R: Response reduction factor
• Sa/g​​: Spectral acceleration (dimensionless)

Fundamental Natural Period

The code provides empirical formulas to estimate the fundamental period T of a building. For reinforced concrete moment-resisting frames:

T = 0.075 \cdot h^{0.75}

Where:

• T: Time period (in seconds)
• h: Height of the building (in meters)

Distribution of Seismic Forces

• The base shear Vb​ is distributed over the height of the structure.
• Typically done linearly or as per code-specified lateral force distributions.
• IS 1893 requires the load to be applied in each orthogonal horizontal direction, one at a time.

Vertical Seismic Load

Vertical earthquake effects are usually ignored, unless the structure is:

• Near the epicenter
• Irregular in mass or geometry
• Designed for special functions (e.g., bridges, lifelines)

Load Combinations

Design combinations typically include:

1.2 D + 1.2 E + 0.5 L

Where:

• D: Dead load
• E: Earthquake load
• L: Live load

Other combinations may also be applicable as per IS 1893 and IS 456.

Example Calculation

For a building in Seismic Zone V, with:

Z = 0.36 \\
I = 1.5 \\
R = 5 \\
\frac{S_a}{g} = 2.5 \\
W = 10{,}000 \, \text{kN}

Step 1: Calculate Ah

A_h = \frac{0.36 \cdot 1.5}{2 \cdot 5} \cdot 2.5 = 0.135

Step 2: Calculate Vb

V_b = 0.135 \cdot 10{,}000 = 1{,}350 \, \text{kN}

This base shear of 1,350 kN is distributed among the floors based on their mass and height.

Design Considerations

• Structures must follow ductile detailing as per IS 13920.
• Torsional effects, masonry infills, and irregularities must be accounted for.
• Earthquake load is inherently dynamic, but IS 1893 allows static equivalents for most buildings.
• Buildings in higher zones or of greater importance face higher seismic design forces.

Load Combinations and Design Considerations

In design, these loads are combined per IS 875/1893. The general ultimate load combination for buildings is:

Serviceability combinations (for deflection, vibration) often use 1.0D + 1.0L + 0.5W/E. Special considerations (impact, temperature, snow) are addressed in IS 875-5 but are beyond the main four loads.

Design and Code Terminology (IS 875 & IS 1893)

1. Characteristic Load: The value of a load that is not expected to be exceeded during the life of a structure. For live loads, it’s typically taken as a representative value specified in IS 875 Part 2 based on occupancy.
2. Uniformly Distributed Load (UDL): A load that is spread evenly across a surface or length, measured in kN/m² (for area) or kN/m (for length). Most live loads are assumed to be UDL unless otherwise mentioned.
3. Concentrated Load: A load applied at a specific point, such as the weight of a heavy machine or equipment on a small area. Requires special treatment in design.
4. Live Load Reduction: A reduction in the total live load considered on structural members like columns or foundations, especially in multi-story buildings. It accounts for the probability that not all floors will be fully loaded simultaneously. IS 875 Part 2 Table 3 provides guidelines.
5. Impact Load / Dynamic Load: These loads arise from moving objects like cranes, vehicles, or machinery. They cause additional forces due to motion, vibration, or acceleration, and are added using special factors as per IS codes.
6. Seismic Weight: The effective weight of the structure considered for earthquake load calculations. It includes full dead load and a portion of the live load, depending on the occupancy and usage type, as per IS 1893.
7. Ultimate Load Combination: A set of combined loads (dead, live, wind, earthquake, etc.) used to check the structure under extreme or worst-case conditions. For example, 1.5D + 1.5L. Factors are defined in IS 875 Part 5 and IS 1893.
8. Serviceability Limit State (SLS): Design criteria that ensure comfort and usability (e.g., limiting deflection, vibration, or cracking). Loads are taken without factors (1.0×) under this state.

Key design notes:

• Dead loads are well-known and must be taken as given; live loads are variable but governed by occupancy (and reducible).
• Wind and seismic loads act laterally and often govern design of tall, slender or flexible structures.
• The codes require engineers to assume the worst-case of wind vs seismic (whichever produces larger demand) since both are not assumed to act simultaneously

FAQs: Structural Loads as per IS 875 & IS 189

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