dr s r satish kumar, iit madras1 15 durability and 16 fire resistance
TRANSCRIPT
Dr S R Satish Kumar, IIT Madras 1
15 Durability and
16 Fire Resistance
2
Electrolyte
C
Metal Connection
Mechanism of corrosion as a miniature battery
A
AnodeDrop of water
Metal bar
CA
Cathode
Mechanism of Corrosion in steel
3
Simple orientation of members
Methods of prevention corrosion - Simple procedures
Detailing to enhance air movement between joints
Simple rule:
•Eliminate the electrolyte
•Avoid simultaneous presence of water and oxygen
4
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!
Dr S R Satish Kumar, IIT Madras 5
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 6
TABLE 15.1 ENVIRONMENTAL EXPOSURE CONDITIONS
Dr S R Satish Kumar, IIT Madras 7
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
Coating System
3
Coating System
4
Coating System
5
Coating system
6
Normal Inland (Rural and Urban areas)
12 years
18 years
20 years About
20 years
About
20 years
Above
20 years
*Polluted Inland (High airborne sulphur dioxide)
10 years
15 years
12 years About
18 years 15-20 years
Above
20 years
Normal Coastal (As normal inland plus high airborne salt levels)
10 years
12 years
20 years About
20 years
About
20 years
Above
20 years
Polluted Coastal (As polluted Inland plus high airborne salt levels)
8 years 10
years 10 years
About
15 years 15 - 20 years
Above
20 years
Dr S R Satish Kumar, IIT Madras 8
TABLE 15.2 (b) Specification for Different Coating System(i) Shop Applied Treatments
Dr S R Satish Kumar, IIT Madras 9
TABLE 15.2
(b) Specification for Different Coating System
(ii) Site Applied Treatments
10
FIRE PROTECTIONPositive 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.
11
Examples of fire load in various structuresType of steel structure Kg wood / m2
School 15Hospital 20Hotel 25Office 35Departmental store 35Textile mill show room >200
Typical fire loads and behaviour of steel under fire
12
Typical fire loads and behaviour of steel under fire
Time (Minutes)
1000
500
0
0 C
30 60 90
Furnace temperature
Unprotected steel
Fire protected steel temperature
13
Mechanical properties of steel at elevated temperatures
0.5
1.0
1.5
200 400 600 800 1000
Young’s modulus ratio
Coeff. of thermal expansion (x 105)
Yield stress ratio
Temperature 0C
14
High Hp / A Value
Low Hp / A Value
The section factor concept
Some typical values of HP of fire protected steel sections
B
D t Hp =2D+3B-2t
Hp =2D+B
Hp =2D+2B Hp =2D+4B-2t
Fire Engineering of steel structures
15
Methods of fire protection
• Spray protection
• Board protection
• Intumescent coatings
• Concrete encasement?
Dr S R Satish Kumar, IIT Madras 16
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 17
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 18
16.5 Variation of Mechanical Properties of Steel with T
690
905
)20(
)( T
f
Tf
y
y
(a) Yield stress
(b) Modulus of elasticity
1100ln2000
0.1)20(
)(
T
T
E
TE
5.531000
1690
T
T
CTC 6000
CTC 1000600
Dr S R Satish Kumar, IIT Madras 19
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 20
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 eithera 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
b) Four-sided fire exposure condition wheret = time from the start of the test, in minutesT = steel temperature, in degrees Celsius, 500oC T 750C ksm = exposed surface area to mass ratio, 2103 mm2/kg ksm 35
103 mm2/kg
smk
TTt
433.00221.02.5
smk
TTt
213.00263.07.4
21
Chemical composition of fire resistant steel
C Mn Si S P Mo+CrFRS 0.20
% 1.50
% 0.50
% 0.04
0% 0.04
0% 1.00
%MildSteel
0.23%
1.50%
0.40%
0.050%
0.050%
-
Fire resistant steels
•Very cost effective compared to structural steel
•FRS are available in India
•Very popular and cost effective - Japanese experience