IS 4991 : 1968Criteria for blast-resistant design of structures for explosions above ground

Link points to Internet Archive / others. Not hosted by InfraLens. Details

UFC 3-340 · ASCE/SEI 59 · IStructE (2010)

CurrentSpecializedCode of PracticeStructural Engineering · Earthquake Engineering

Link points to Internet Archive / others. Not hosted by InfraLens. Details

IS 4991:1968 is the Indian Standard (BIS) for criteria for blast-resistant design of structures for explosions above ground. This standard provides criteria for designing above-ground structures to resist the effects of blast loading. It outlines methods to determine blast wave characteristics, calculate equivalent static loads from dynamic pressures, and perform dynamic analysis, focusing on the inelastic behavior and energy absorption capacity of structures.

Criteria for blast-resistant design of structures for explosions above ground

Overview

Status: Current
Usage level: Specialized
Domain: Structural Engineering — Earthquake Engineering
Type: Code of Practice

International equivalents

UFC 3-340-02 · US Department of Defense (DoD), USA ASCE/SEI 59-11 · American Society of Civil Engineers (ASCE), USA IStructE (2010) · Institution of Structural Engineers, UK

Typically used with

IS 456 IS 800

Also on InfraLens for IS 4991

4Key values 4Tables 4FAQs

Practical Notes

! The code's charts and methods are based on data from the 1950s-60s. Modern practice often uses advanced software (FEA/CFD) for more accurate blast analysis, though the principles of this code are still fundamental.

! The most critical and often uncertain inputs are the design threat, specifically the equivalent TNT charge weight (W) and the standoff distance (R).

! Design for blast resistance is fundamentally about ductility and energy absorption, not just strength. Detailing of reinforcement and connections to prevent brittle failure modes is paramount.

Frequently referenced clauses

Cl. 2.2Blast Wave Characteristics Cl. 2.3Scaled Distance Cl. 3.1Equivalent Static Load for Design Cl. 3.2Dynamic Analysis Cl. 4Design of Elements Appendix A - Charts for Blast Parameters

reinforced concretestructural steelmasonry

International Equivalents

Similar International Standards

UFC 3-340-02US Department of Defense (DoD), USA

HighCurrent

Structures to Resist the Effects of Accidental Explosions

Comprehensive design standard for structures subjected to above-ground explosions.

ASCE/SEI 59-11American Society of Civil Engineers (ASCE), USA

HighCurrent

Blast Protection of Buildings

Provides procedures for the design and analysis of buildings for blast effects.

TM 5-1300US Department of the Army, USA

MediumWithdrawn

Structures to Resist the Effects of Accidental Explosions

Predecessor to UFC, shares a similar chart-based empirical approach with IS 4991.

IStructE (2010)Institution of Structural Engineers, UK

MediumCurrent

Structural design for physical security: State of the art

Guidance document covering blast and other security threats, less prescriptive than a code.

Key Differences

≠IS 4991:1968 is based on simplified charts and an equivalent static load method. Modern standards like UFC 3-340-02 specify advanced dynamic analysis methods like Single-Degree-of-Freedom (SDOF) and time-history analysis using Finite Element Methods (FEM).

≠Modern standards provide extensive tables of Dynamic Increase Factors (DIFs) to account for high strain-rate effects on material strength. IS 4991 offers a far simpler approach, suggesting a general increase in permissible stresses (e.g., by 33%), which is less accurate.

≠UFC 3-340-02 and ASCE 59-11 define explicit performance levels (e.g., Low, Medium, High damage) and corresponding structural response limits (e.g., ductility ratios, rotation angles). IS 4991 lacks this structured performance-based design framework.

≠International standards provide detailed methods for calculating blast loads considering reflections, clearing times, and complex geometries. IS 4991's charts are primarily for a free-field hemispherical surface burst and are less detailed.

≠Modern codes like UFC 3-340-02 include extensive tables for TNT equivalency to convert the mass of different explosives to an equivalent mass of TNT. IS 4991 is based on TNT only and provides no guidance on other explosives.

Key Similarities

≈All standards use the fundamental concept of Scaled Distance (Z = R / W^(1/3)) to correlate blast parameters with the explosive yield and standoff distance.

≈The use of Pressure-Impulse (P-I) diagrams as a tool to assess the level of damage to a structural element is a core concept common to both IS 4991 and modern international standards.

≈Both the Indian and international standards recognize that designing structures to remain within the elastic range is often uneconomical and impractical, emphasizing the critical importance of ductility and energy absorption through inelastic deformation.

≈The fundamental approach of determining the external blast loading on the structure first, and then analyzing the structural response to that load, is a common methodology across all standards.

Parameter Comparison

Parameter	IS Value	International	Source
Scaled Distance (Z) Formula	Z = R / W^(1/3)	Z = R / W^(1/3)	UFC 3-340-02
Peak Pressure Calculation	Determined from empirical charts based on Scaled Distance.	Calculated using Kingery-Bulmash equations or read from more refined charts.	UFC 3-340-02
Dynamic Increase Factor (DIF) for Steel Reinforcement (Yield)	Not explicitly defined; uses a general increase in permissible stress.	~1.17 (for flexure); specific values provided based on grade and use.	UFC 3-340-02
Dynamic Increase Factor (DIF) for Concrete (Flexural Compression)	Not explicitly defined; uses a general increase in permissible stress.	~1.19 (for flexure); specific values provided for flexure, shear, and direct compression.	ASCE/SEI 59-11
Allowable Ductility Ratio (μ) for RC Flexural Members	Not explicitly defined or limited.	Specified based on performance level. E.g., μ = 1 to 3 for 'Low' damage; up to 10 for 'Medium' damage.	UFC 3-340-02
Load Factor on Blast Load	Uses working stress method; for ultimate design, load factor is not specific to blast.	1.0; the design blast load is considered an ultimate load.	ASCE/SEI 59-11
TNT Equivalency Basis	Based on TNT; no conversion factors provided for other explosives.	Based on TNT, but provides a detailed table of equivalency factors for many other explosives (e.g., Comp B, C-4, ANFO).	UFC 3-340-02

⚠ Verify details from original standards before use

Key Values4

Quick Reference Values

Permissible ductility ratio for RC flexural members3 to 10 (depending on damage level)

Permissible ductility ratio for steel flexural members10 to 20 (depending on damage level)

Permissible support rotation for RC one-way slabs (repairable damage)2 degrees

Permissible support rotation for RC beams (repairable damage)2 degrees

Key Formulas

Z = R / W^(1/3) — Scaled distance, where R is distance (m) and W is charge weight (kg TNT)

P_so = f(Z) — Peak overpressure as a function of scaled distance (from charts)

T = 2π * sqrt(m/k) — Natural period of an SDOF system

μ = x_m / x_el — Ductility Ratio, where x_m is max deflection and x_el is elastic limit deflection

Tables & Referenced Sections

Key Tables

Table 1 - Damage Criteria (Implicitly defined by support rotations and ductility ratios)

Fig. 1 - Idealized Pressure-Time Curve

Fig. 2 - Peak Overpressure vs Scaled Distance for Surface Burst

Fig. 4 - Positive Phase Duration vs Scaled Distance

Key Clauses

Clause 2.2 - Blast Wave Characteristics

Clause 2.3 - Scaled Distance

Clause 3.1 - Equivalent Static Load for Design

Clause 3.2 - Dynamic Analysis

Clause 4 - Design of Elements

Appendix A - Charts for Blast Parameters

Frequently Asked Questions4

What is 'scaled distance'?+

It is a dimensionless parameter, Z = R / W^(1/3), that combines the distance from the blast (R) and the charge weight (W) to determine blast effects like peak overpressure from standardized charts (Clause 2.3).

How is the blast pressure converted to a design load?+

The code provides methods to convert the dynamic blast pressure into an equivalent static load, often using a Dynamic Load Factor (DLF), which depends on the blast duration and the structure's natural period (Clause 3.1).

What is a ductility ratio?+

It is the ratio of the maximum permissible deformation to the deformation at the elastic limit. Blast-resistant design allows for controlled inelastic deformation to absorb energy, and this ratio quantifies the allowable damage.

Is this code still valid?+

Yes, it was reaffirmed by BIS in 2018. However, for complex or high-risk projects, it is often supplemented or replaced by more modern international standards (e.g., UFC 3-340-02 from the USA) and advanced numerical analysis.

QA/QC Inspection Templates

📋

QA/QC templates coming soon for this code.

Browse all 300 templates →

IS 4991 : 1968Criteria for blast-resistant design of structures for explosions above ground

PDF Google Compare BIS Portal

Link points to Internet Archive / others. Not hosted by InfraLens. Details

UFC 3-340 · ASCE/SEI 59 · IStructE (2010)

CurrentSpecializedCode of PracticeStructural Engineering · Earthquake Engineering

PDF Google Compare BIS Portal

Link points to Internet Archive / others. Not hosted by InfraLens. Details

Criteria for blast-resistant design of structures for explosions above ground

Overview

Status: Current
Usage level: Specialized
Domain: Structural Engineering — Earthquake Engineering
Type: Code of Practice

International equivalents

UFC 3-340-02 · US Department of Defense (DoD), USA ASCE/SEI 59-11 · American Society of Civil Engineers (ASCE), USA IStructE (2010) · Institution of Structural Engineers, UK

Typically used with

IS 456 IS 800

Also on InfraLens for IS 4991

4Key values 4Tables 4FAQs

Practical Notes

! The most critical and often uncertain inputs are the design threat, specifically the equivalent TNT charge weight (W) and the standoff distance (R).

! Design for blast resistance is fundamentally about ductility and energy absorption, not just strength. Detailing of reinforcement and connections to prevent brittle failure modes is paramount.

Frequently referenced clauses

Cl. 2.2Blast Wave Characteristics Cl. 2.3Scaled Distance Cl. 3.1Equivalent Static Load for Design Cl. 3.2Dynamic Analysis Cl. 4Design of Elements Appendix A - Charts for Blast Parameters

reinforced concretestructural steelmasonry

International Equivalents

Similar International Standards

UFC 3-340-02US Department of Defense (DoD), USA

HighCurrent

Structures to Resist the Effects of Accidental Explosions

Comprehensive design standard for structures subjected to above-ground explosions.

ASCE/SEI 59-11American Society of Civil Engineers (ASCE), USA

HighCurrent

Blast Protection of Buildings

Provides procedures for the design and analysis of buildings for blast effects.

TM 5-1300US Department of the Army, USA

MediumWithdrawn

Structures to Resist the Effects of Accidental Explosions

Predecessor to UFC, shares a similar chart-based empirical approach with IS 4991.

IStructE (2010)Institution of Structural Engineers, UK

MediumCurrent

Structural design for physical security: State of the art

Guidance document covering blast and other security threats, less prescriptive than a code.

Key Differences

Key Similarities

≈All standards use the fundamental concept of Scaled Distance (Z = R / W^(1/3)) to correlate blast parameters with the explosive yield and standoff distance.

≈The use of Pressure-Impulse (P-I) diagrams as a tool to assess the level of damage to a structural element is a core concept common to both IS 4991 and modern international standards.

≈The fundamental approach of determining the external blast loading on the structure first, and then analyzing the structural response to that load, is a common methodology across all standards.

Parameter Comparison

Parameter	IS Value	International	Source
Scaled Distance (Z) Formula	Z = R / W^(1/3)	Z = R / W^(1/3)	UFC 3-340-02
Peak Pressure Calculation	Determined from empirical charts based on Scaled Distance.	Calculated using Kingery-Bulmash equations or read from more refined charts.	UFC 3-340-02
Dynamic Increase Factor (DIF) for Steel Reinforcement (Yield)	Not explicitly defined; uses a general increase in permissible stress.	~1.17 (for flexure); specific values provided based on grade and use.	UFC 3-340-02
Dynamic Increase Factor (DIF) for Concrete (Flexural Compression)	Not explicitly defined; uses a general increase in permissible stress.	~1.19 (for flexure); specific values provided for flexure, shear, and direct compression.	ASCE/SEI 59-11
Allowable Ductility Ratio (μ) for RC Flexural Members	Not explicitly defined or limited.	Specified based on performance level. E.g., μ = 1 to 3 for 'Low' damage; up to 10 for 'Medium' damage.	UFC 3-340-02
Load Factor on Blast Load	Uses working stress method; for ultimate design, load factor is not specific to blast.	1.0; the design blast load is considered an ultimate load.	ASCE/SEI 59-11
TNT Equivalency Basis	Based on TNT; no conversion factors provided for other explosives.	Based on TNT, but provides a detailed table of equivalency factors for many other explosives (e.g., Comp B, C-4, ANFO).	UFC 3-340-02

⚠ Verify details from original standards before use

Key Values4

Quick Reference Values

Permissible ductility ratio for RC flexural members3 to 10 (depending on damage level)

Permissible ductility ratio for steel flexural members10 to 20 (depending on damage level)

Permissible support rotation for RC one-way slabs (repairable damage)2 degrees

Permissible support rotation for RC beams (repairable damage)2 degrees

Key Formulas

Z = R / W^(1/3) — Scaled distance, where R is distance (m) and W is charge weight (kg TNT)

P_so = f(Z) — Peak overpressure as a function of scaled distance (from charts)

T = 2π * sqrt(m/k) — Natural period of an SDOF system

μ = x_m / x_el — Ductility Ratio, where x_m is max deflection and x_el is elastic limit deflection

Tables & Referenced Sections

Key Tables

Table 1 - Damage Criteria (Implicitly defined by support rotations and ductility ratios)

Fig. 1 - Idealized Pressure-Time Curve

Fig. 2 - Peak Overpressure vs Scaled Distance for Surface Burst

Fig. 4 - Positive Phase Duration vs Scaled Distance

Key Clauses

Clause 2.2 - Blast Wave Characteristics

Clause 2.3 - Scaled Distance

Clause 3.1 - Equivalent Static Load for Design

Clause 3.2 - Dynamic Analysis

Clause 4 - Design of Elements

Appendix A - Charts for Blast Parameters

Frequently Asked Questions4

What is 'scaled distance'?+

How is the blast pressure converted to a design load?+

What is a ductility ratio?+

Is this code still valid?+

QA/QC Inspection Templates

📋

QA/QC templates coming soon for this code.

Browse all 300 templates →

IS 4991 : 1968Criteria for blast-resistant design of structures for explosions above ground

Overview

International Equivalents

Key Values4

Tables & Referenced Sections

Related Resources on InfraLens

Frequently Asked Questions4

QA/QC Inspection Templates

IS 4991 : 1968Criteria for blast-resistant design of structures for explosions above ground

Overview

International Equivalents

Key Values4

Tables & Referenced Sections

Related Resources on InfraLens

Frequently Asked Questions4

QA/QC Inspection Templates