15 Durability and
16 Fire Resistance
Dr S R Satish Kumar, IIT Madras
1
Metal Connection
C
A
Electrolyte
Mechanism of corrosion as a
miniature battery
Anode
Drop of water
A
C
Cathode
Metal bar
Mechanism of Corrosion in steel
2
Methods of prevention corrosion - Simple
procedures
Detailing to enhance air
movement between
joints
Simple orientation of members
Simple rule:
•Eliminate the electrolyte
•Avoid simultaneous
presence of water and oxygen
3
Is Corrosion a real Problem?
• Indian designers feel that steel corrodes most in
India. Is it true?
• Steel corrodes all over the world! But they are
better managed in the western countries!
• Excellent protective coatings which retain their
life even up to 20 years are available!
• Corrosion-where does it matter? Normal inland
there is no problem! Exposed conditions
ofcourse do need attention.
• Corrosion is no more a disincentive for not using
steel in housing sector!
4
SECTION 15 DURABILITY
15.1
General
15.2
Requirements for Durability
– Shape, Size, Orientation of Members, Connections and
Details
– Exposure Condition (Table 15.1)
– Corrosion protection methods
– Surface protection
– Protective coating requirements (Table 15.2)
– Special steel
Dr S R Satish Kumar, IIT Madras
5
TABLE 15.1 ENVIRONMENTAL EXPOSURE CONDITIONS
Environmental
Classifications
Mild
Moderate
Severe
Very severe
Extreme
Dr S R Satish Kumar, IIT Madras
Exposure conditions
Surfaces normally protected against exposure to weather
or aggressive condition as in interior of buildings, except
when located in coastal areas
Structures steel surfaces exposed to
i) condensation and rain
ii) continuously under water
iii) non-aggressive soil/groundwater
iv) sheltered from saturated salt air in coastal areas
Structural steel surfaces exposed to
i) severe frequent rain
ii) alternate wetting and drying
iii) severe condensation
iv) completely immersed in sea water
v) exposed to saturated salt air in coastal area
Structural steel surface exposed to
i) sea water spray
ii) corrosive fumes
iii) aggressive sub soil or ground water
Structural steel surfaces exposed to
i) tidal zones and splash zones in the sea
ii) aggressive liquid or solid chemicals
6
TABLE 15.2 PROTECTION GUIDE FOR STEEL WORK APPLICATION
(a) Coating System Desired Life in Different Environments (In Years)
Atmospheric
Condition
Coating
System
1
Coating
System
2
Inland
Normal
(Rural and Urban
areas)
12
years
18
years
Inland
*Polluted
airborne
(High
sulphur dioxide)
10
years
15
years
Coastal
Normal
(As normal inland
plus high airborne
salt levels)
Coastal
Polluted
(As polluted Inland
plus high airborne
salt levels)
10
years
12
years
8 years
10
years
Dr S R Satish Kumar, IIT Madras
Coating
System
3
20 years
12 years
20 years
10 years
Coating
System
4
About
20 years
About
18 years
About
20 years
About
15 years
Coating Coating
System system
6
5
About
Above
20
years
20
years
15-20
years
Above
20
years
About
Above
20
years
20
years
15 - 20
years
Above
20
years
7
TABLE 15.2 (b) Specification for Different Coating System
(i) Shop Applied Treatments
Coating
System
1
2
Surface
Preparation
Blast Clean
Blast Clean
Prefabrication
primer
Zinc
Phosphate
Epoxy 20 m
2 pack Zincrich Epoxy 20
m
Post
fabrication
primer
High-build Zinc
Phosphate
modified Alkyd
60 m
2 pack Zincrich Epoxy
20 m
Intermediate
coat
Top coat


Dr S R Satish Kumar, IIT Madras
3
Blast
Clean
4
Blast Clean

2 pack Zincrich Epoxy 20
m
Hot tip
Galvanise
85 m
2 pack Zincrich Epoxy 25
m
High-build
Zinc
Phosphate 25
m



5
Girt Blast

6
Blast Clean
Ethyl Zinc
Silicate 20
m
Sprayed Zinc Ethyl Zinc
Silicate 60
or Sprayed
Aluminium
m
2 pack Epoxy
Micaceous Iron
oxide
Sealer
Chlorinated
Rubber
Alkyd 35 m
2 pack Epoxy
Micaceous Iron
Oxide 85 m
Sealer

8
TABLE 15.2
(b) Specification for Different Coating System
(ii) Site Applied Treatments
Coating System
Surface
Preparation
Primer
Intermediate
Coat
Top Coat
1
As necessary
Touch in

High-build
Alkyd Finish
60m
Dr S R Satish Kumar, IIT Madras
2
As
necessary
Touch in
Modified
Alkyd
Micaceous
Iron Oxide
50 m
Modified
Alkyd
Micaceous
Iron Oxide
50 m
3
No site
treatment



4
As necessary

Touch In
High-build
Chlorinated
Rubber
5
No site
treatment

6
As necessary
Touch in

High-build
Micaceous Iron
Oxide
Chlorinated
Rubber
Micaceous 75 m

High-build Iron
Oxide
Chlorinated
Rubber75 m
9
FIRE PROTECTION
Positive points of steel as a
construction material under fire
• Damage to strength of steel due to fire is
reversible in most of the cases
• Using the principle “ if the member is straight
after the fire - the steel is O.K” many of the
members could be salvaged.
• Up to about 2150C steel retains its strength
• In the case of concrete, at 2350C turns pink;
5900C turns red and irreversible damage after
6000C
• Steel exposed to 6000C could be strengthened
and reused.
10
Typical fire loads and behaviour of steel
under fire
Examples of fire load in various structures
Type of steel structure
Kg wood / m2
School
15
Hospital
20
Hotel
25
Office
35
Departmental store
35
Textile mill show room
>200
11
Typical fire loads and behaviour of steel under fire
0C
1000
Furnace
temperature
Unprotected steel
500
Fire protected steel
temperature
0
30
60
90
Time (Minutes)
12
1.5
Coeff. of thermal
expansion (x 105)
1.0
0.5
Young’s modulus
ratio
Yield stress ratio
200
400
600
800
1000
Temperature 0C
Mechanical properties of steel at elevated
temperatures
13
Fire Engineering of steel structures
D
Hp =2D+B
t
Hp
=2D+3B-2t
B
High Hp / A
Value
Low Hp / A
Value
The section factor
concept
Hp =2D+2B
Hp
=2D+4B-2t
Some typical values of HP of
fire protected steel sections
14
Methods of fire protection
• Spray protection
• Board protection
• Intumescent coatings
• Concrete encasement?
15
SECTION 16 FIRE RESISTANCE
16.1
Requirements
16.2
Definitions
16.3
Fire Resistance Level
16.4
Period of Structural Adequacy (PSA)
16.5
Variation of Mechanical Properties of Steel with Temp.
16.6
Limiting Steel Temperature
16.7
Temperature Increase with Time in Protected Members
Dr S R Satish Kumar, IIT Madras
16
Fire Protection Criteria
• period of structural adequacy (PSA) greater than or equal to the
required fire-resistance level (FRL) in minutes attained in the
standard fire test
• FRL shall be prescribed by other standards depending on the use
of the structure and the time required to evacuate.
• The period of structural adequacy (PSA) shall be determined using
one of the following methods:
(a) By calculation
– (i) By determining the limiting temperature of the steel (Tl) in
accordance with 16.6 and then.
– (ii) By determining the PSA as the time (in minutes) from the
start of the test (t) to the time at which the limiting steel
temperature is attained in accordance with 16.7 for protected
members and 16.8 for unprotected members.
(b) By direct application of a single test in accordance with 16.9 or
• (c) Calculation of the temperature of the steel member by, using a
rational method of analysis confirmed by test data or by methods
available in Specialist literature.
Dr S R Satish Kumar, IIT Madras
17
16.5 Variation of Mechanical Properties of Steel with T
(a) Yield stress
905  T

f y (20)
690
f y (T )
1.2
1
0.8
0.6
0.4
(b) Modulus of elasticity
0.2
0




E (T )
T

 1.0  

E (20)
  T  
2000

ln  1100  
 
 

T 

6901 

 1000 

T  53.5
Dr S R Satish Kumar, IIT Madras
0
200
400
600
800
1000
1200
0C  T  600C
600C  T  1000C
18
16.6 Limiting Steel Temperature
limiting steel temperature (Tl) in degree Celsius shall be calculated as
Tl= 905-690 rf
where
rf rf = ratio of the design action on the member under fire to the design
capacity of the member (Rd = Ru/m) at room temperature
Rd, R Rd , Ru = design and ultimate strength of the member at room
temperature
m = partial safety factor for strength
The design action under fire shall consider
a) The reduced bond likely under fire.
b) The effects of restraint to expansion of the elements during fire.
Dr S R Satish Kumar, IIT Madras
19
Temperature Increase with Time
Protected Members
16.7.1 The time (t) at which the limiting temperature (Tl) is attained shall
be determined by calculation on the basis of either
a suitable series of fire tests and regression analysis in accordance
with 16.7.2 or
from the results of a single test in accordance with 16.7.3.
Unprotected Members
calculate using the following equations.
a) Three-sided fire exposure condition
 0.433T 

t   5.2  0.0221T  
 k sm 
 0.213T 

t   4.7  0.0263T  
 k sm 
b) Four-sided fire exposure condition
where
t = time from the start of the test, in minutes
T = steel temperature, in degrees Celsius, 500oC  T  750C
ksm = exposed surface area to mass ratio, 2103 mm2/kg  ksm  35
103 mm2/kg
Dr S R Satish Kumar, IIT Madras
20
Fire resistant steels
Chemical composition of fire resistant steel
FRS
Mild
Steel
C
 0.20
%
 0.23
%
Mn
Si
 1.50  0.50
%
%
 1.50  0.40
%
%
S
 0.04
0%
 0.05
0%
P
 0.04
0%
 0.05
0%
Mo+Cr
 1.00
%
-
•Very cost effective compared to structural steel
•FRS are available in India
•Very popular and cost effective - Japanese
experience
21