---
title: "Earthquake loads in RCC design"
subject: "Design of RCC Structure"
chapter: 12
key: "semester_7/design_of_rcc_structure:earthquake-loads-in-rcc-design"
source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/earthquake-loads-in-rcc-design
site: BCT Engineering
---

# Earthquake loads in RCC design

Design of RCC Structure · Chapter 12: Introduction to Earthquake Resistant Design and Provisions for Ductile Detailing

- Asked in: 2072 Kartik, 4 marks
- Video: [Seismic Analysis by Equivalent Static Analysis Method Using IS:1893 (Part-1) 2016](https://www.youtube.com/watch?v=AxIx-2KCpcU) — Visual Civil
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/earthquake-loads-in-rcc-design
- Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-12

## Question

How do you consider earthquake loads while designing RCC structures? Explain briefly.

## Short answer

Seismic weight W = dead load + percent live load; time period T = 0.075 h^0.75 for bare RC frames; design base shear Vb = Ah W with Ah = (Z/2)(I/R)(Sa/g); distribute over height as Qi = Vb Wi hi^2 / Σ Wj hj^2; apply combinations (1.5(DL±EL), 1.2(DL+LL±EL), 0.9DL±1.5EL) and detail per IS 13920.

## Answer

Earthquake loads are considered as equivalent static lateral forces (or by response spectrum analysis for tall or irregular buildings), as per IS 1893 (Part 1) or, in Nepal, NBC 105:2020.

#### Steps
1. **Seismic weight $W$** = dead load + a percentage of the live load (25% if the live load is $\le 3$ kN/m$^2$, 50% if it is more; roof live load is neglected).
2. **Time period** of a bare RC frame: $T = 0.075\,h^{0.75}$ (s), with $h$ in m.
3. **Design base shear:**

$$
V_B = A_h W,\qquad A_h = \frac{Z}{2}\frac{I}{R}\frac{S_a}{g}
$$

with $Z$ from the zone map, $I$ the importance factor, $R$ the response reduction factor (5 for special moment resisting RC frames, 3 for ordinary ones) and $S_a/g$ from the spectrum for the soil type.
4. **Distribute over the height:** $Q_i = V_B \dfrac{W_i h_i^2}{\sum W_j h_j^2}$, and apply at each floor in both directions (plus 30% of the orthogonal force).
5. **Load combinations** (IS 1893 cl. 6.3; partial safety factors from IS 456 Table 18): $1.5(DL+LL)$, $1.2(DL+LL\pm EL)$, $1.5(DL\pm EL)$, $0.9DL\pm 1.5EL$.
6. Design and detail the members for the worst combination, and provide ductile detailing as per IS 13920.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Seismic weight W includes dead load and:
   A. 100% live load
   B. 25% live load if LL <= 3 kN/m2 (50% if more), roof live load neglected
   C. No live load ever
   D. Roof live load fully
   Answer: B. Percentage of live load as per IS 1893. (Key point: Seismic weight)

2. Design base shear is:
   A. VB = Ah W
   B. VB = W/Ah
   C. VB = Z W
   D. VB = R W
   Answer: A. VB = Ah W with Ah = (Z/2)(I/R)(Sa/g). (Key point: Base shear)

3. Base shear is distributed over height as:
   A. Equally to each floor
   B. Qi = VB Wi hi^2 / sum(Wj hj^2)
   C. Proportional to hi only
   D. Qi = VB/ n
   Answer: B. Parabolic distribution with weight and height squared. (Key point: Vertical distribution)

4. Which is a seismic load combination?
   A. 1.5(DL - LL)
   B. 0.9DL + 1.5EL
   C. 2.0 DL
   D. 1.0 LL
   Answer: B. Combinations include 1.2(DL+LL±EL), 1.5(DL±EL) and 0.9DL±1.5EL. (Key point: Load combinations)

## Related in Design of RCC Structure

- [Earthquake design philosophy, strength and ductility](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aearthquake-design-philosophy-strength-and-ductility) — Design of RCC Structure, chapter 12
- [Beam reinforcement for given loads, 6 m span](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Abeam-reinforcement-for-given-loads-6) — Design of RCC Structure, chapter 4
- [Beam design, 250x475 section](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Abeam-design-250x475-section) — Design of RCC Structure, chapter 4, asked 2×
- [Two-way slab on RCC beams, 4x6 m](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Atwo-way-slab-on-rcc-beams) — Design of RCC Structure, chapter 9, asked 2×
- [Design simply supported beam, 6 m span](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-simply-supported-beam-6-m) — Design of RCC Structure, chapter 4
- [Shear design with torsion, 250x500](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Ashear-design-with-torsion-250x500) — Design of RCC Structure, chapter 5

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