Skip a proper load calculation and a slab thickness becomes a guess. It might “look” fine on the drawing, but cracks and serviceability problems tend to show up after occupancy, when they’re far more expensive to fix. Here’s a step-by-step walkthrough of how load calculation actually works for a simple building.
The four load types you’re working with
Structural loads break down into a small set of categories:
- Dead load (DL) — the permanent, static weight of the structure itself: slabs, beams, columns, walls, plus permanent finishes. Dead loads are calculated from the weights and volumes of actual materials, so they’re the most predictable load type.
- Live load (LL) — temporary or movable loads: people, furniture, equipment, vehicles. These vary by occupancy type and are far less predictable, which is part of why they carry a higher safety factor in design.
- Wind load (WL) — a lateral, dynamic force from air movement, calculated from basic wind speed, exposure, and pressure coefficients (per codes such as IS 875 Part 3 or equivalent). For low-rise buildings, gravity loads usually govern overall design, but wind still matters for slender elements and cladding.
- Seismic load (EL) — the force a structure must resist during an earthquake, derived from the building’s mass (dead load plus a portion of live load, commonly around 25%) and a design coefficient tied to location, soil type, and code. This can be critical even for low-rise buildings in active seismic zones.
Follow the load path
Every structure follows the same logical path: loads first act on slabs, slabs transfer them to beams, beams pass them to columns, and columns finally transfer everything down to footings and the soil. Calculating loads means working through this chain step by step, member by member.
A worked walkthrough: dead load on a slab
Take a typical 150 mm RCC slab. Self-weight is calculated from concrete’s unit weight (commonly ~25 kN/m³), so a 150 mm slab works out to roughly 25 × 0.15 ≈ 3.75 kN/m² (this figure varies a little by source and exact thickness). Add floor finish and plaster — commonly assumed around 1–1.5 kN/m² — and you get a total dead load on the slab somewhere around 5–5.25 kN/m², depending on the specific finishes used.
For live load, typical assumed values are around 2 kN/m² for residential floors and roughly 1.5 kN/m² for roofs — though always check the value your governing code specifies for the actual occupancy type rather than assuming. Adding dead and live load for a residential floor slab gives a combined value commonly in the range of 7–7.5 kN/m² in worked examples — the exact number depends on your specific slab thickness and finishes.
Passing the load to beams and columns
Once the slab load (in kN/m²) is known, it gets distributed to the beams supporting it based on the area each beam carries — for a slab supported on all sides, each beam typically picks up roughly half the slab width feeding into it, converting the area load into a line load (kN/m) along the beam. That beam load is then totalled at each support to give a point load (kN) transferred down into the column below, and the column load, in turn, becomes the load the footing must spread into the soil.
Load combinations: why you don’t just add everything up
Real design doesn’t simply sum every load at full intensity simultaneously — codes specify load combinations with factors reflecting how loads behave and how likely they are to coincide at their peak. Common combinations include something like 1.2DL + 1.6LL for ordinary gravity design, a wind combination such as 1.2DL + 1.0WL + 0.5LL, and a seismic combination along similar lines. Dead loads get a lower factor because they’re predictable; live loads get a higher factor because they’re variable; rare extreme events like earthquakes are factored differently again. Exact factors depend on your governing code (IS 875/1893, ASCE 7, Eurocode, etc.) and should always be taken from the current standard, not memorized.
A word of caution
This walkthrough shows the logic of load calculation, not a substitute for it. Real design requires checking pattern loading (alternate spans loaded differently), different combinations for strength versus serviceability checks, and code-specific provisions that vary by region and structure type. Use this as the conceptual foundation, then have an actual structural engineer carry out and stamp the final calculations — this is not a step to shortcut on a real building.
Part of our Structural & RCC series. Next: working stress vs limit state design.