Code of Practice For Design Loads (Other Than Earthquake) For Buildings And Structures, Part 5: Special Loads And Combinations

IS 875 Part 5 (1987) - Scope Summary & Key Specifications

Scope:
IS 875 Part 5 covers loads other than wind on structures, including hydrostatic and soil pressures, temperature effects, and other special loads relevant to building design.

Key Points from Clause 3.2 (Lateral Soil Pressure on Columns in Sloping Soils)

• Effective width = Actual width × Ratio (as above)
• Used for calculating lateral soil pressure on column-like members resting on slopes.

Load Combinations (Clause 6.4)

• Load combinations for crane loads are covered under IS 875 Part 2.
• Other loads (temperature, soil, hydrostatic) must be combined as per relevant clauses in IS 875 Parts 1-5.

Temperature Effects (Clause 2.1.3)

• Structural analysis must consider:
  • Change in mean temperature through the section.
  • Temperature gradient through the section.

Additional Notes

• The code is primarily for civil engineers and architects designing buildings.
• It provides guidelines for basic design loads other than wind.
• For detailed load values and combinations, refer to respective IS 875 parts.

flowchart LR
    A[Actual Width of Member] --> B{Width Range}
    B -->|< 0.5 m| C[Ratio = 3.0]
    B -->|0.5 to 1.0 m| D[Ratio = 3.0 to 2.0]
    B -->|> 1.0 m| E[Ratio = 2.0]
    C --> F[Effective Width = Actual Width × 3.0]
    D --> F
    E --> F

For detailed design, always refer to the full IS 875 Part 5 document.

IS 875 (Part 5) - Temperature Effects: Key Points

1. Temperature Effects to Consider

• Mean temperature change (f1, ta): Change through the section relative to initial temperature.
• Temperature gradient (τ1, τ2): Variation of temperature through the thickness of structural elements.
• Applies mainly to single-leaf elements exposed to weather.
• Neglect below ground level unless during construction exposure.

2. Design Considerations

• Use analytical principles to evaluate temperature effects.
• Consider material temperature variation differing from air temperature (Fig. 1 & 2 provide max/min air temps).
• Engineering judgment is essential to decide if temperature effects can be neglected.

3. Basic Formula for Thermal Stress

[ \sigma_t = E \alpha \Delta T ] Where:

• (\sigma_t) = thermal stress,
• (E) = modulus of elasticity,
• (\alpha) = coefficient of thermal expansion,
• (\Delta T) = temperature change (mean or gradient).

4. Load Combinations Including Temperature (Typical)

• Combine temperature loads with other loads (dead, live, wind) as per IS 875 Part 1.

Summary Table: Temperature Parameters

flowchart LR
    A[Exposure to Weather] --> B[Temperature Change (Mean & Gradient)]
    B --> C[Material Thermal Expansion]
    C --> D[Thermal Stresses \(\sigma_t = E \alpha \Delta T\)]
    D --> E[Structural Analysis & Design]

Note: Refer to IS 875 (Part 5) Fig. 1 & 2 for regional max/min air temperatures to estimate (\Delta T). Adjust for material-specific temperature variation.

IS 875 Part 5: Soil and Hydrostatic Pressures – Key Points

1. Design Considerations (Clause 3.1 & 3.1.1)

• For structures below ground level (retaining walls, basement walls), lateral pressures from soil and water must be considered.
• When soil is below free water surface, lateral earth pressure = (weight of soil - buoyancy) + full hydrostatic pressure.
• Foundation slabs/footings under water pressure must resist uniform uplift = full hydrostatic pressure.
• Check overturning using buoyant weight of foundation.

2. Effective Width for Soil Pressure on Columns (Clause 3.2)

• Use this ratio to calculate lateral soil pressure on pillar-like members in sloping soil.

3. Basic Formulae

• Lateral Earth Pressure (below water table):
  [ P = \gamma' \times h + P_w ] where,
  (\gamma' = \gamma_{soil} - \gamma_{water}) (effective submerged unit weight)
  (h =) depth of soil below water table
  (P_w = \text{hydrostatic pressure} = \gamma_{water} \times h_w)

• Hydrostatic Pressure:
  [ P_w = \gamma_{water} \times h_w ] where (h_w) = depth of water.

Summary Diagram: Soil & Hydrostatic Pressure on Basement Wall

graph LR
A[Ground Surface] --> B[Soil Pressure (γ × h)]
B --> C[Below Water Table: Effective Pressure (γ' × h)]
C --> D[Hydrostatic Pressure (γ_water × h_w)]
D --> E[Total Lateral Pressure = Soil + Hydrostatic]

Note: Temperature effects below ground are generally negligible (Clause 2.1.3.2

Stability Considerations per IS 875 Part 5

1. Overturning Stability:

• Restoring Moment (MR) ≥ 1.2 × Overturning Moment due to Dead Load (MDL) + 1.4 × Overturning Moment due to Imposed Loads (MIL)

• If dead load provides restoring moment only, consider 0.9 × Dead Load for restoring moment; ignore imposed loads for restoring moment.

[ MR \geq 1.2 \times MDL + 1.4 \times MIL ]

or if dead load dominant:

[ MR \geq 0.9 \times MDL ]

2. Sliding Stability:

• Factor of safety against sliding ≥ 1.4 under the most adverse load combination.
• Only 0.9 × Dead Load considered for sliding resistance.

3. Soil Pressure on Column-like Members (Clause 3.2):

4. Bearing Pressure:

• Wind load bearing pressure < 25% of combined dead + imposed load may be neglected.
• Otherwise, foundation designed so combined pressure ≤ 1.25 × allowable soil bearing pressure.
• Earthquake effects per IS 1893 considered separately.

flowchart LR
    A[Applied Loads] --> B{Check Overturning}
    B -->|Calculate Overturning Moment| C[MDL, MIL]
    B -->|Calculate Restoring Moment| D[MR]
    C & D --> E{Is MR ≥ 1.2*MDL + 1.4*MIL?}
    E -->|Yes| F[Stable]
    E -->|No| G[Modify Design]

    A --> H{Check Sliding}
    H -->|Calculate Sliding Resistance| I[0.9*Dead Load]
    H -->|Calculate Sliding Force| J[Loads]
    I & J --> K{Is FS ≥ 1.4?}
    K -->|Yes| F
    K -->|No| G

IS 875 Part 5: Expansion and Contraction Key Points

1. Design Considerations (Clause 2.1):

   • Account for temperature-induced expansion/contraction.
   • Use expansion joints per IS 3414-1968 or design for additional thermal stresses.

2. Structural Analysis (Clause 2.1.3):

   • Consider:
     • Mean temperature change through the section, (\Delta T_m = T_{mean} - T_{initial})
     • Temperature gradient through the section, (\Delta T_g)

3. Thermal Strain and Stress:

   • Thermal strain:
     [ \varepsilon_t = \alpha \Delta T ] where (\alpha) = coefficient of thermal expansion (typ. (10^{-5}/^\circ C)).

   • Thermal stress (if restrained):
     [ \sigma_t = E \alpha \Delta T ] where (E) = modulus of elasticity.

4. Expansion Joint Design:

   • Refer IS 3414 for joint spacing and details.
   • Joint spacing depends on material, temperature range, and structural configuration.

5. Temperature Ranges:

   • Use local temperature charts (IS 875 Part 5 Fig.1 & Fig.2) for max/min temperatures to estimate (\Delta T).

Summary Table: Thermal Effects

flowchart LR
    A[Temperature Change \(\Delta T\)] --> B[Thermal Strain \(\varepsilon_t = \alpha \Delta T\)]
    B --> C

IS 875 Part 5 - Accidental Loads: Key Points & Specifications

1. Nature of Accidental Loads (Clause 6.0)

• Definition: Unlikely, short-duration loads caused by human actions such as:
  • Impacts and collisions
  • Explosions
• Characteristics:
  • Not due to normal use
  • Low probability but potentially severe consequences
• Causes:
  • Poor equipment safety (design/maintenance)
  • Wrong operation (negligence, insufficient training)

2. Design Approach

• Accidental loads should be based on risk analysis considering:
  • Magnitude of loads
  • Preventive measures
• Usually, only principal load-bearing systems are designed for accidental loads.
• Most unlikely loads may be neglected after risk evaluation.

3. Erection Loads (Clause 5.1)

• Include all loads from materials, equipment during construction.
• Must consider combined action of dead, wind, and imposed loads.
• Temporary bracings required to keep stresses within permissible limits.

4. Fire-related Accidental Loads (Clause 6.4.1)

• Loads from people on escape routes
• Loads transferred from failing structures during fire

Typical Design Considerations (General Engineering Knowledge)

Simplified Formula for Impact Load (Example)

[ F_{impact} = C_d \times m \times v / t ]

• (C_d) = Dynamic amplification factor (1.5 to 3)
• (m) = Mass impacting (kg)
• (v) = Impact velocity (m/s)
• (t) = Impact duration (s)

flowchart TD
    A[Accidental Loads] --> B[Impacts & Collisions]
    A --> C[Explosions]
    A --> D[Fire Related Loads]
    B --> E[Dynamic Amplification]
    C --> F[Blast Pressure]
    D --> G[Escape Route Loads]
    D --> H[Load Transfer from Failure]

IS 875 Part 5: Impacts and Collisions Key Points

1. Vehicle Collision Load (Clause 6.1.2)

• Fictitious vehicle model:

  • Two masses:
    • ( m_1 = 400 , \text{kg} ), rigidity ( C_1 = 10,000 , \text{kN/m} )
    • ( m_2 = 12,000 , \text{kg} ), rigidity ( C_2 = 300 , \text{kN/m} )
  • Impact speed: ( v = 10 , \text{m/s} ) (36 km/h)
  • Impact height: 1.2 m
  • Mass ( m_1 ) brakes fully before ( m_2 ) starts braking.

• Maximum static forces (non-elastic element):

  • ( F_{max, m_1} = 630 , \text{kN} )
  • ( F_{max, m_2} = 600 , \text{kN} )
  • Use 630 kN as a safe static force for design.

2. Crane Impact Load on Buffer Stop (Clause 6.1.4)

• Horizontal impact load ( P_y ) (tonnes):

[ P_y = \frac{M \cdot V^2}{2F \cdot g} ]

Where:

• ( V ) = crane speed at impact (m/s), assumed half nominal speed
• ( F ) = maximum buffer shortening (m):
  • 0.1 m for cranes ≤ 50 t with flexible suspension
  • 0.2 m otherwise
• ( M ) = reduced crane mass (t·s²/m), calculated as:

[ M = P_h + P_t + kQ ]

Parameters:

• ( P_h ) = crane bridge weight (t)
• ( P_t ) = crab weight (t)
• ( Q ) = crane loading capacity (t)
• ( k = 0 ) (flexible suspension), ( k = 1 ) (rigid suspension)
• ( g = 9.81 , m/s

IS 875 Part 5 - Vertical Load on Air Raid Shelters (Clause 6.3)

Key Specifications:

• Vertical loads on air raid shelters (usually below ground level) depend on building storeys and construction type.
• Applies when floor imposed load ≤ 5.0 kN/m².

Characteristic Vertical Loads (kN/m²):

* Reinforced in-situ concrete structures

Adjustment for Imposed Loads > 5 kN/m² (Clause 5.0):

[ \text{Vertical Load} = \text{Base value} + (\text{Avg imposed load on upper storeys} - 5.0) \quad \text{kN/m}^2 ]

Notes:

• Storeys counted above the shelter.
• Factor against sliding ≥ 1.4 (use 0.9 × dead load).
• Wind load can be neglected if < 25% of combined dead + imposed load.
• Earthquake loads follow IS 1893.

Summary Table for Quick Reference:

flowchart TD
    A[Floor Imposed Load ≤ 5 kN/m²?] -->|Yes| B[Use Base Vertical Load from Table]
    A -->|No| C[Calculate Increase = Avg imposed load above - 5 kN/m²]
    C --> D[Add Increase to Base Load]
    B --> E[Apply Loads to Shelter Design]
    D --> E

IS 875 Part 5: Load Combinations (Clause 8.1)

Key Load Combinations (General Guidance)

When specific code provisions are absent, adopt the following combinations to ensure safety:

• Note: When snow load is present, replace imposed load (IL) with snow load in these combinations.
• Note: Partial imposed load should be considered with earthquake load as per IS 1893.
• Simultaneous maxima of all loads (wind, earthquake, imposed, snow) are unlikely.

Important Specifications:

• Follow IS 1893 for earthquake load factors.
• Use appropriate partial safety factors for Limit State Design.
• For Working Stress Design, use permissible stresses as per relevant codes.

Summary Diagram of Load Combinations:

graph LR
DL[Dead Load]
IL[Imposed Load]
WL[Wind Load]
EL[Earthquake Load]
TL[Temperature Load]
SL[Snow Load]

DL --> C1[Combination 1: DL + IL]
DL --> C2[Combination 2: DL + WL]
DL --> C3[Combination 3: DL + EL]
DL --> C4[Combination 4: DL + IL + WL]
DL --> C5[Combination 5: DL + IL + EL]
DL --> C6[Combination 6: DL + IL + EL + TL]

SL --> ReplaceIL[Replace IL with SL when snow present]
ReplaceIL --> C4
ReplaceIL --> C5
ReplaceIL --> C6

This ensures structural safety under various realistic loading scenarios per IS 875 Part 5.

IS 875 Part 5: Design Considerations for Special Loads

Key Specifications & Notes:

• Sliding Resistance Factor:
  [ \text{Factor against sliding} \geq 1.4 ] Under the most adverse load combination, consider only 0.9 × Dead Load (DL) for sliding checks.

• Bearing Pressure due to Wind:

  • If wind pressure < 25% of (DL + Imposed Load), neglect wind pressure on soil.
  • If > 25%, foundation design should ensure combined pressure ≤ 125% of allowable soil bearing pressure.
  • When earthquake loads included, follow IS 1893-1984 for permissible soil pressure increase.

• Load Combinations:

  • Do not apply reduced imposed load (from IS 875 Part 2) when combined with earthquake forces.
  • Accidental and other special loads should be handled case-wise.

Summary Table: Bearing Pressure Check

Load Combination Example for Sliding Check:

[ \text{Sliding check load} = 0.9 \times DL + \text{(Other loads as applicable)} ]

flowchart TD
    A[Loads on Structure] --> B{Check Sliding}
    B -->|Sliding Factor ≥ 1.4| C[Safe]
    B -->|Sliding Factor < 1.4| D[Redesign Foundation]

    A --> E{Wind Pressure on Soil}
    E -->|≤ 25% of (DL+IL)| F[Neglect Wind Pressure]
    E -->|> 25% of (DL+IL)| G[Design Foundation for 1.25 × allowable pressure]

    A --> H{Earthquake Load Included?}
    H -->|Yes| I[Follow IS 1893-1984 Soil Pressure Limits]
    H -->|No| J[Use IS 875 Part 5 Guidelines]

This

IS 875 Part 5 - Load Combinations: Key Guidance

Load Combinations (Clause 8.1)

Use the following combinations, selecting the most unfavourable effect:

• Replace Imposed Load (IL) by Snow Load when snow is present on roofs.
• For earthquake combinations, use reduced imposed load as per IS 1893.
• Temperature load (TL) included where relevant.
• Factor against sliding ≥ 1.4 under adverse loads, considering 0.9 × DL only.
• Wind load may be neglected if < 25% of DL+IL bearing pressure.

Important Notes

• Follow IS 1893 for earthquake load specifics.
• Use partial safety factors per design method (working stress or limit state).
• Special loads (moisture, shrinkage, accidental) as per relevant codes or performance needs.

Summary Diagram of Load Combinations

graph LR
    DL[Dead Load]
    IL[Imposed Load]
    WL[Wind Load]
    EL[Earthquake Load]
    TL[Temperature Load]
    SL[Snow Load]

    DL --> C1[DL + IL]
    DL --> C2[DL + WL]
    DL --> C3[DL + EL]
    DL --> C4[DL + IL + WL]
    DL --> C5[DL + IL + EL]
    DL --> C6[DL + IL + EL + TL]
    SL --> C7[DL + SL + EL + TL (replace IL with SL)]

This concise guidance aligns with IS 875 Part 5 for safe structural design under combined loads.

IS 875 Part 5: Safety Factors & Stability Requirements

Key Safety Factors:

• Factor of Safety against Uplift:

  • Minimum 1.2 when high water table exists (Note 5, Clause None).

• Factor against Sliding:

  • Minimum 1.4 under the most adverse load combination (Note 5, Clause 1.4).
  • Only 0.9 times dead load is considered for sliding resistance.

Stability Against Overturning (Clause 1.2, Note 4):

• Restoring moment ≥
  [ 1.2 \times M_{overturning, dead} + 1.4 \times M_{overturning, imposed} ]
• If dead load provides restoring moment, consider only 0.9 × dead load and ignore imposed loads for restoring moment.

Design Considerations (Clause 3.3):

• Ignore imposed loads that favor stability (e.g., resisting overturning/sliding).
• Account for possible soil removal (temporary or permanent).

Bearing Pressure (Clause 1.4, Note 6):

• Wind load can be neglected if it causes < 25% of combined dead + imposed load pressure.
• If > 25%, foundation size must limit combined pressure to ≤ 125% of allowable soil bearing pressure.
• For earthquake loads, refer to IS 1893 for permissible bearing pressure increases.

Summary Table:

flowchart LR
    Loads[Applied Loads] --> OverturningMoments[Calculate Overturning Moments]
    Loads --> RestoringMoments[Calculate Restoring Moments]
    OverturningMoments --> StabilityCheck{Restoring Moment ≥ 1.2×Dead + 1.