# What is Stiffness Modifiers – Value As Per IS 16700 & ACI-318

In this article, we explain what is stiffness modifiers, why stiffness modifiers are used, what is the code provision for stiffness modifiers for beams, slabs, columns, wall, etc.

So please read the article till the end and if you get any valuable information from this article then please share it with your friends.

## What is Stiffness Modifiers

Stiffness Modifiers is nothing but a factor value for R.C. sectional property to consider relative stiffness of R.C section in analysis and design of structure.

As per Indian code IS 456, A relative stiffness is based on moment of inertia of section and determined by any one of the following.

• Gross Section
• Transformed Section
• Cracked Section

It is provided to consider R.C. Section as cracked in design of structure. The different value of stiffness modifiers is suggested by various standard and research paper based on their research and study.

Before going to know the value of stiffness modifiers, let’s understand why it is provided?

Also Read: How to Check Axial Compressive Stress using Etabs

## Why Stiffness Modifiers are used in design of R.C.C Structure

The R.C.C structure during its life span, get cracked due to various reason under constant loading condition. Due to the cracking, the cross sectional area of member get reduced which ultimately affect

As a structure designer, we need to design a structure which is remain stable and strong during such cracked condition. By considering cracked section in analysis, we can ensure safer member section to keep the structure safe in terms of serviceability under cracking condition.

So to make structure stable in such condition we need to consider the cracked RC section in design and analysis. In Cracked RC section the effective moment of inertia will be lesser than gross moment of inertia that means the actual stiffness of Cracked RC section is lesser than Gross stiffness of member which affect the deflection and forces.

Hence, To account for the reduced moment of inertia of the cracked section, the concept of stiffness modifiers are introduced by the standards.

Also Read: Deep Beam Design Criteria as per IS456

## Stiffness Modifiers Value as per IS:1893

The value of stiffness modifiers for beam and column in masonry and RC Structure, the moment of inertia shall be taken as given below.

Where, Ig = Gross moment of inertia of member.

## Stiffness Modifiers Value as per IS:16700

The value of cracked moment of inertia is tabulated as below.

Also Read: Criteria for SBC & Types of Soil as per IS:1893

## Stiffness Modifiers Value as per ACI-318

The values of inertia modifiers as per Eurocode is tabulated as below.

Also Read: Ductile Detailing of Column as per IS 13920-2016

## Frequently Asked Questions (FAQs)

### What are stiffness modifiers for slabs?

As per IS 16700, the moment of inertia for slab is 0.25 Ig in Factored loading condition while in case of Un-factored Loading is 0.35 Ig.

### What are the stiffness modifiers for slabs?

As per IS 16700, the moment of inertia for beam is 0.35 Ig in Factored loading condition while in case of Un-factored Loading is 0.7 Ig.

### What are the stiffness modifiers for Walls?

As per IS 16700, the moment of inertia for wall is 0.7 Ig in Factored loading condition while in case of Un-factored Loading is 0.9 Ig.

### What are the stiffness modifiers for Columns?

As per IS 16700, the moment of inertia for column is 0.7 Ig in Factored loading condition while in case of Un-factored Loading is 0.9 Ig.

### Are stiffness modifiers required only for analysis of structure?

Yes, As per IS 1893 clause 6.4.31, reduced moment of inertia considered only for structure analysis.

### Is stiffness modifiers applicable for all load cases?

No, Stiffness modifiers are applicable only for earthquake load cases.

Reference:

• IS 1893: 2016 PART-1
• IS 16700: 2016
• ACI-318 (19)