Codes referenced
Table 1 lists unit weights of building materials and Table 2 stored materials.
Floor and roof live loads by occupancy. Table 1 gives uniform distributed loads per m².
Basic wind speed map (Fig. 1); Vz = Vb × k1 × k2 × k3; terrain categories 1-4.
Ground snow load map for hill states; shape factors for roofs.
Temperature (§4), creep/shrinkage (§5), seismic coefficient method (§6), construction loads, accidental loads.
Cl 18.2.2 references IS 875 Pt 5 for load combinations in limit state design.
Part 1 — Dead loads (unit weights)
Dead loads are calculated from unit weights given in IS 875 Pt 1 Table 1. IS 456 Cl 19.2.1 gives plain concrete 24 kN/m³ and RCC 25 kN/m³ as default values.
| Material | Unit weight (kN/m³) |
|---|---|
| Plain cement concrete | 24 |
| Reinforced cement concrete (RCC) | 25 |
| Prestressed concrete | 25 |
| Structural steel | 78.5 |
| Brick masonry (brick + mortar) | 20 (IS 875 Pt 1 Table 1 item 36) |
| Cement mortar / plaster | 20-24 |
| Sand (dry / wet) | 15.5-16 / 18-19 |
| Aggregate (coarse, loose) | 15-17 |
| Floor finish (screed 25 mm) | 0.5 kN/m² for a 25 mm screed — IS 875 Pt 1 Table 2 gives 0.21 kN/m² for 10 mm mortar screeding. 1.0 kN/m² is the usual working allowance once the finish and any false ceiling are included. |
| Partition allowance | 1.0 kN/m² for movable partitions (IS 875 Pt 2) |
Quick rule for slabs: 1 m² of 150 mm RCC ≈ 0.15 × 25 = 3.75 kN/m². Add floor finish (1 kN/m²) and partition allowance (1 kN/m²) → total ~5.75 kN/m² dead load before live load.
Part 2 — Live (imposed) loads by occupancy
2.0 kN/m² — uniformly distributed. No of floors reduction permitted (Pt 2 §3.2) when supported on more than one floor.
2.5-4.0 kN/m² — depending on fixed seating vs mobile workstations; Pt 2 Table 1.
3.0 kN/m² ward; 5.0 kN/m² OT — higher due to equipment (mobile C-arm, etc.).
4.0-5.0 kN/m² — Pt 2 Table 1 categories 8-10.
5.0 kN/m² — public assembly corridors 4.0-5.0; stair treads also check 1.5 kN concentrated load.
2.5 kN/m² for cars up to 2.5 t; 5.0 kN/m² for heavier vehicles.
Part 3 — Wind load: Vz = Vb × k1 × k2 × k3
The 2015 revision of IS 875 Pt 3 uses the formula:
Vz = Vb × k1 × k2 × k3
- Vb — basic wind speed, read from Fig. 1 (Appendix A gives city-wise table). Pune = 39 m/s, Mumbai = 44, Chennai = 50, Kolkata = 50, Delhi = 47, Bhuj = 50.
- k1 — risk coefficient (Figure 2, based on building importance and design life). For ordinary buildings = 1.0; for important = 1.08; for post-disaster = 1.20.
- k2 — terrain, height and structure size factor (Table 2). Categories 1-4 (1 = sea coast; 4 = city centre). Increases with height and decreases with class A/B/C/D structure.
- k3 — topography factor (Table 3). 1.0 for flat terrain; up to 1.36 for crests of hills or cliffs.
Design wind pressure: Pd = 0.6 × Vz² (in N/m² when Vz in m/s) = 0.6 × Vz² / 1000 in kN/m². The pressure is then multiplied by external (Cpe) and internal (Cpi) pressure coefficients for each surface.
Part 4 — Snow loads (hill states)
Snow load applies where ground snow load is significant — Jammu & Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh, and hill parts of West Bengal and Assam above 1500 m altitude.
- Ground snow load: 0.5-5+ kN/m² by altitude and location (Pt 4 Appendix A).
- Roof snow load = ground snow load × shape factor.
- Shape factor for flat roof = 0.7; pitched < 30° = 0.8; pitched 30-60° = 0.8 + 0.4 × (angle-30)/30; pitched > 60° = 1.2 (drift on lee side).
- Snow load not critical in plains or coastal cities — apply 0 kN/m².
Part 5 — Special loads & load combinations
Climatic temperature variation ±10°C on concrete and ±20°C on exposed metal. Required for chimneys, silos, long exposed members, un-insulated roofs in hot climates.
Long-term deformation effects. Required for tall buildings (high vertical members) and indeterminate structures; Pt 5 §5 gives coefficient method.
Applies to simple, low-rise, regular buildings. Seismic coefficient α = β × α₀; importance factor I (1.0-1.75); combination factor. For all major buildings, use IS 1893 (Pt 1):2016 instead — it is the dedicated code.
Allowance during construction: stacked materials, equipment, formwork shores. Pt 5 specifies values for typical projects (e.g. 1 kN/m² for stacked materials on slab during construction).
Limit-state load combinations (6 combos)
Every RCC element is checked against these combinations per IS 456 Table 18 (partial safety factors for loads), with the combination sets of IS 875 Pt 5 Cl 8.1:
- 1) 1.5 × DL + 1.0 × IL
- 2) 1.5 × DL + 1.5 × WL
- 3) 1.2 × (DL + IL + WL)
- 4) 0.9 × DL + 1.5 × WL — the stability case
- 5) 1.5 × DL + 1.5 × EL
- 6) 1.2 × (DL + IL + EL)
Partial safety factors γf applied to characteristic loads; γf applied to characteristic loads (IS 456 Cl 36.4.1 / Table 18), γm applied to material strength (IS 456 Cl 36.4.2).
Worked example — G+5 residential building in Pune
| Parameter | Value |
|---|---|
| Site | Pune (open terrain, Category 2) |
| Basic wind speed Vb | 39 m/s (IS 875 Pt 3 Appendix A) |
| Building class | Class B (residential, 18 m tall) |
| Importance (k1) | 1.0 (ordinary residential) |
| k2 (terrain Cat 2, Class B, 18 m) | ~1.08 (Table 2) |
| k3 (topography) | 1.0 (flat site) |
| Vz = Vb × k1 × k2 × k3 | 39 × 1.0 × 1.08 × 1.0 = 42.1 m/s |
| Design pressure Pd = 0.6 × Vz² | 0.6 × 42.1² = 0.6 × 1773 = 1063 N/m² = 1.06 kN/m² |
| Force on 15 m × 6 m face | 1.06 × 90 = ~95 kN per face (un-factored) |
| Load combination 1.5 DL + 1.0 IL controls | For a 5 kN/m² IL floor + 5.75 kN/m² DL, 1.5 × 5.75 + 1.0 × 5 = 13.6 kN/m² design load |
Common deviations seen on Indian sites
Practitioner uses Mumbai's Vb = 44 m/s in Pune's design — over-designs by 27%. Or uses Pune's Vb in coastal Gujarat project — under-designs by 28%. Use the city-specific table in Pt 3 Appendix A.
Pt 2 §3.2 allows reduction for columns carrying 5+ floors — but each floor must be 100% loaded per the worst-case pattern. Applied blindly on ground floor = under-design.
Gujarat or Pune engineer designs a hotel in Manali with no snow load = roof collapse under typical 1.5-2 kN/m² ground snow. Pt 4 Appendix A gives city-wise values.
A 30 m exposed RCC slab roof in summer — top surface 50°C, bottom 30°C — gradient 20°C causes bowing. Pt 5 §4 specifies ±10°C variation; expansion joints at 30-45 m centres.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a unit weight, a wind speed and a snow load decide whether the numbers on the drawing describe the building on site.
Questionnaire — ask before the loads are entered into the analysis model
- Has every dead load been built up from the IS 875 Pt 1 Table 1 unit weights (plain concrete 24 kN/m³, RCC 25, structural steel 78.5, brick masonry 20) rather than from rule of thumb?
- Does the slab dead load include the floor finish and the 1.0 kN/m² partition allowance, and does a typical 150 mm RCC slab come out at 0.15 × 25 = 3.75 kN/m² before those additions?
- Is the live load taken from Pt 2 Table 1 for the actual occupancy (2.0 kN/m² residential, 2.5 to 4.0 office, 3.0 ward and 5.0 OT, 4.0 to 5.0 retail, 5.0 corridors and stairs, 2.5 to 5.0 car parking), and are stair treads also checked for the 1.5 kN concentrated load?
- Where live-load reduction is taken for a column carrying five or more floors, has it been applied under Pt 2 §3.2 with each floor loaded 100% in the worst-case pattern?
- Which basic wind speed Vb has been read from the Pt 3 Appendix A city table for the actual site, and are k1 (importance), k2 (terrain, height and structure size) and k3 (topography) each recorded, giving Vz = Vb × k1 × k2 × k3?
- Is the site in a snow region (J&K, Ladakh, Himachal, Uttarakhand, Sikkim, Arunachal, and the hill parts of West Bengal and Assam above 1500 m), and if so what ground snow load from Pt 4 Appendix A and what roof shape factor (0.7 flat, 0.8 pitched under 30°) have been used?
- Have the special loads of Pt 5 been screened for, including temperature variation of ±10 °C on concrete and ±20 °C on exposed metal, creep and shrinkage on tall or indeterminate structures, and construction loads such as 1 kN/m² of stacked materials on the slab?
- Are all five limit-state combinations from IS 456 Cl 18.2.2 / IS 875 Pt 5 Table 1 being checked, including 0.9 DL ± 1.5 WL and 1.5 (DL ± EQ), rather than only 1.5 (DL + LL)?
Inspector's checklist — what to verify in the load take-off and on site
- Unit weights traced back to IS 875 Pt 1 Table 1 on the dead load sheet, with the source table cited next to each figure.
- Screed thickness on site measured and the floor finish weight matched to it, since the same 25 mm screed is not the same weight as a 44 mm one.
- Partition allowance of 1.0 kN/m² included where movable partitions are possible, per IS 875 Pt 2.
- Live load schedule checked against the occupancy drawings, and the reduction pattern for multi-storey columns recorded as the worst-case floor loading.
- Wind speed Vb read off the city table for the site (Pune 39 m/s, Mumbai 44, Delhi 47, Chennai and Kolkata and Bhuj 50), with k1, k2 and k3 shown and Vz computed on the sheet.
- Terrain category and topography factor checked against the actual site, including whether the site sits on a crest where k3 rises above 1.0.
- Snow load either applied from Pt 4 or formally recorded as not applicable, rather than left blank on a plains site and copied on a hill site.
- Construction-stage loading checked before the pour, including stacked materials, equipment and formwork shores.
- Combination list in the design matches all five cases, with the governing combination identified for each member.
What happens if… three worked examples
What happens. The G+5 Pune building takes Vb = 39 m/s, k1 = 1.0, k2 = 1.08, k3 = 1.0, so Vz = 39 × 1.0 × 1.08 × 1.0 = 42.1 m/s and Pd = 0.6 × 42.1² = 1,063 N/m² = 1.06 kN/m², about 95 kN on the 15 m × 6 m face. Substitute Mumbai's Vb = 44 m/s and Vz = 44 × 1.08 = 47.5 m/s, Pd = 0.6 × 47.5² = 1.35 kN/m², and the same face takes 1.35 × 90 = 122 kN.
Likely outcome. The frame is sized for wind that this site does not see, since the pressure scales with the square of the speed: (44 / 39)² = 1.27, so 27% of extra wind load runs through every column and beam, paid for in steel and concrete. The same mistake in reverse, on a coastal site, is worse: at Bhuj, where Vb = 50 m/s, the wind speed is 50 / 39 = 1.28 times Pune's, so a design carried out on Pune's number is 28% short on the speed before any pressure factor is applied. Read Vb from the city table every time, and write the city and the table on the sheet.
What happens. Ground snow at that altitude runs 1.5 to 2 kN/m². On a flat roof the Pt 4 shape factor is 0.7, so the roof snow is 1.05 to 1.4 kN/m²; on a 45° pitch the factor is 0.8 + 0.4 × (45 − 30)/30 = 1.0, giving 1.5 to 2 kN/m². None of that reaches the members because no snow load was ever entered in the take-off.
Likely outcome. The roof carries a load it was never designed for, at its worst on the day it is most heavily loaded. The roof members, the trusses and the connections all need rechecking, and for a structure sized on DL + LL alone this is the deviation that shows up as a collapse rather than as a review comment.
What happens. In summer the top surface of a 30 m exposed RCC slab reaches 50 °C while the bottom sits at 30 °C, a 20 °C gradient through the depth. Pt 5 §4 specifies ±10 °C on concrete, and the practical answer to it is an expansion joint at 30 to 45 m centres. A 30 m slab is the shortest length that still needs one.
Likely outcome. The slab bows, and because the gradient is through the depth rather than along it, the roof drains toward the middle and holds water in the low spots. Chasing this with a waterproofing layer is the wrong fix: the joint has to go in, sized and detailed for the movement, or the slab has to be shortened or restrained by design.
Frequently Asked Questions
What is basic wind speed Vb in IS 875 Part 3?
When is snow load critical for design?
What does IS 875 Part 5 say about seismic loading?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For IS 875 / IS 456 / IS 1893 design review or wind-load calculation check, contact amit@concreteinfo.in.
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