Chapter 22Fire Resistance

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 1055

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22.1 Requirements

The requirements shall apply to steel building elements designed to exhibit a required fire-resistance level (FRL) as per the relevant specifications. The provisions of this section are quite rudimentary and simplistic. Specialist literature for design of steel structures for fire resistance and design based on testing may be necessary for a more comprehensive design of steel buildings to resist fire.

22.1.1 For protected steel members and connections, the thickness of protection material $(h_{\mathrm{i}})$ shall be greater than or equal to that required to give a period of structural adequacy (PSA) greater than or equal to the required FRL.

Table 29(b) (i) Protection Guide for Steel Work Application — Specification for Different Coating System (Shop Applied Treatments)

Sl No.	Protection	Coating System
Sl No.	Protection	1	2	3	4	5	6
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
i)	Surface preparation	Blast clean	Blast clean	Blast clean	Blast clean	Girt blast	Blast clean
ii)	Pre-fabrication primer	Zinc phosphate epoxy, 20 μm	2 pack zinc-rich epoxy, 20 μm	-	2 pack zinc-rich epoxy, 20 μm	-	Ethyl zinc silicate, 20 μm
iii)	Post fabrication primer	High-build zinc phosphate modified alkyd, 60 μm	2 pack zinc-rich epoxy, 20 μm	Hot dip galvanized, 85 μm	2 pack zinc-rich epoxy, 25 μm	Sprayed zinc or sprayed aluminium	Ethyl zinc silicate, 60 μm
iv)	Intermediate coat	-	High-build zinc phosphate 25 μm	-	2 pack epoxy micaceous iron oxide	Sealer	Chlorinated rubber alkyd, 35 μm
v)	Top coat	-	-	-	2 pack epoxy micaceous iron oxide, 85 μm	Sealer	-

Table 29(b) (ii) Protection Guide for Steel Work Application – Specification for Different Coating System (Site Applied Treatments)

(Clause 21.2.4.1)

Sl No.	Protection	Coating System
Sl No.	Protection	1	2	3	4	5	6
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
i)	Surface preparation	As necessary	As necessary	No site treatment	As necessary	No site treatment	As necessary
ii)	Primer	Touch in	Touch in	-	-	-	Touch in
iii)	Intermediate coat	-	Modified Alkyd Micaceous iron oxide, 50 μm	-	Touch In	-	High-build micaceous iron oxide Chlorinated rubber Micaceous, 75 μm
iv)	Top coat	High-build Alkyd finish, 60μm	Modified Alkyd Micaceous iron oxide, 50 μm	-	High-build chlorinated rubber	-	High-build iron oxide Chlorinated rubber, 75 μm

22.1.2 For unprotected steel members and connections, the exposed surface area to mass ratio $(k_{\mathrm{sm}})$ shall be less than or equal to that required to give a PSA equal to the required FRL.

22.2 Fire Resistance Level

The required FRL shall be as prescribed in Part 4 ‘Fire and Life Safety’ of the Code as appropriate or in building specifications or as required by the user or the city ordinance. The FRL specified in terms of the duration (in min) of standard fire load without collapse depends upon;

a) the purpose for which structure is used; and

b) the time taken to evacuate in case of fire.

22.3Period of Structural Adequacy (PSA)

22.3.1 The calculation of PSA involves:

a) Calculation of the strength of the element as a function of temperature of the element and the determination of limiting temperature;

b) Calculation of the thermal response of the element, that is calculation of the variation of the temperature of the element or the parts of the element with time, when exposed to fire; and

c) Determination of PSA at which the temperature of the element or parts of the element reaches the limiting temperature.

22.3.2 Determination of Period of Structural Adequacy

The period of structural adequacy (PSA) shall be determined using one of the following methods:

a) By calculation:

determining the limiting temperature of

the steel $(T_{\mathrm{j}})$ in accordance with 22.5; and

determining the PSA as the time (t) (in min) from the start of the test to the time at which the limiting steel temperature is attained, in accordance with 22.6 for protected members and 22.7 for unprotected members.

b) By direct application of a single test in accordance with 22.8; or

c) By 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.

22.4Variation of Mechanical Properties of Steel with Temperature

22.4.1 Variation of Yield Stress with Temperature

The influence of temperature on the yield stress of steel shall be taken as follows for structures of mild steels and high strength low alloy steels:

\frac {f _ {\mathrm{y}} (T)}{f _ {\mathrm{y}} (2 0)} = \frac {9 0 5 - T}{6 9 0} \leq 1. 0

where,

f _ {\mathrm{y}} (T) = \text {yield stress of steel at} T ^ {\circ} \mathrm{C},

$f_{y}(20) = \text{yield stress of steel at } 20^{\circ}\mathrm{C} \text{(room temperature), and}$

$T$ = temperature of the steel, in ${}^{\circ}\mathrm{C}$ .

This relationship is shown by Curve 1 in Fig. 24.

For temperature less than $215^{\circ}\mathrm{C}$ no reduction in the yield stress need to be considered.

22.4.2 Variation of Modulus of Elasticity with Temperature

The influence of temperature on the modulus of

PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL

Chart

FIG. 24 VARIATION OF MECHANICAL PROPERTIES OF STEEL WITH TEMPERATURE

elasticity shall be taken as follows for structures of mild steels and high strength low alloy steels:

\left| \frac {E (T)}{E (2 0)} = 1. 0 + \left[ \frac {T}{2 0 0 0 \left[ \ln \left(\frac {T}{1 1 0 0}\right) \right]} \right] \right|

when $0^{\circ}\mathrm{C} < T \leq 600^{\circ}\mathrm{C}$

= \frac {6 9 0 \left(1 - \frac {T}{1 0 0 0}\right)}{T - 5 3 . 5} \text {when} 6 0 0 ^ {\circ} \mathrm{C} < T \leq 1 0 0 0 ^ {\circ} \mathrm{C}

where

$E(T) =$ modulus of elasticity of steel at $T^{\circ}\mathrm{C}$ , and

$E(20) = \text{modulus of elasticity of steel at } 20^{\circ}\mathrm{C}$ (room temperature).

This relationship is shown by Curve 2 in Fig. 24.

22.4.3 For special steel with higher temperature resistance, such as TMCP steels, the manufacturer's recommendation shall be used to obtain the variation of yield strength and modulus of elasticity of steel with temperature.

22.6Temperature Increase with Time in Protected Members

22.6.1 The time $(t)$ at which the limiting temperature $(T_{\mathrm{l}})$ is attained shall be determined by calculation on the basis of a suitable series of fire tests in accordance with 22.6.2 or from the results of a single test in accordance with 22.6.3.

22.6.1.1 For beams and for all members with a four-sided fire exposure condition, the limiting temperature $(T_{1})$ shall be taken as the average of all of the temperatures measured at the thermocouple locations on all sides.

22.6.1.2 For columns with a three-sided fire exposure condition, the limiting temperature ( $T_{l}$ ) shall be taken as the average of the temperatures measured at the thermocouple locations on the face farthest from the wall. Alternatively, the temperatures from members with a four-sided fire exposure condition and having the same surface area to mass ratio may be used.

22.6.2 Temperature Based on Test Series

Calculation of the variation of steel temperature with time shall be by interpolation of the results of a series of fire tests using the regression analysis equation specified in 22.6.2.1, subject to the limitations and conditions of 22.6.2.3.

22.6.2.1 Regression analysis

The relationship between temperature $(T)$ and time $(t)$ for a series of tests on a group shall be calculated by least-square regression as follows:

\begin{array}{c} t = k _ {0} + k _ {1} h _ {\mathrm{i}} + k _ {2} \left(\frac {h _ {\mathrm{i}}}{k _ {\mathrm{sm}}}\right) + k _ {3} T + k _ {4} h _ {\mathrm{i}} T \\ + k _ {5} \left(\frac {h _ {\mathrm{i}} T}{k _ {\mathrm{sm}}}\right) + k _ {6} \left(\frac {T}{k _ {\mathrm{sm}}}\right) \end{array}

where

$t$ = time from the start of the test, in min;

$k_{0}$ to $k_{6}=$ regression coefficients from test data (see 22.6.2.2);

$h_{\mathrm{i}}$ = thickness of fire protection material, in mm;

T = steel temperature, in °C obtained from test as given in 22.6.1, T > 250°C; and

$k_{sm}$ = exposed surface area to mass ratio, in $10^{3}$ mm $^{2}$ /kg.

22.6.2.2 In-lieu of test results, the values for coefficients in Table 30 may be used in the equation 22.6.2.1 when the test satisfies the conditions specified in 22.6.2.3.

Table 30Regression Coefficients, k

$k_{0}$ (1)	$k_{1}$ (2)	$k_{2}$ (3)	$k_{3}$ (4)	$k_{4}$ (5)	$k_{5}$ (6)	$k_{6}$ (7)
-25.90	1.698	-13.71	0.030 0	0.000 5	0.514 4	6.633

22.6.2.3 Limitations and conditions on use of regression analysis

Test data to be utilized in accordance with 22.6.2.1, shall satisfy the following:

a) Steel members shall be protected with board, sprayed blanket or similar insulation materials having a dry density less than $1\ 000\ kg/m^{3}$ ;

b) All tests shall incorporate the same fire protection system;

c) All members shall have the same fire exposure condition;

d) Test series shall include at least nine tests;

e) Test series may include prototypes which have not been loaded provided that stickability has been demonstrated; and

f) All members subject to a three-sided fire exposure condition shall be within a group in accordance with 22.9.

The regression equation obtained for one fire protection system may be applied to another system using the same fire protection material and the same fire exposure condition provided that stickability has been demonstrated for the second system.

A regression equation obtained using prototypes with a four-sided fire exposure condition may be applied to a member with a three-sided fire exposure condition provided that stickability has been demonstrated for the three-sided case.

22.6.3 Temperature Based on Single Test

The variation of steel temperature with time measured

PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL

in a standard fire test may be used without modification provided:

a) Fire protection system is the same as the prototype;

b) Fire exposure condition is the same as the prototype;

c) Fire protection material thickness is equal to or greater than that of the prototype;

d) Surface area to mass ratio is equal to or less than that of the prototype; and

e) Where the prototype has been submitted to a standard fire test in an unloaded condition, stickability has been separately demonstrated.

22.6.4 Parameters of Importance in the Standard Fire Test

a) Specimen type, loading, configuration,

b) Exposed surface area to mass ratio,

c) Insulation type, thermal properties and thickness, and

d) Moisture content of the insulation material.

22.7Temperature Increase with Time in Unprotected Members

The time $(t)$ at which the limiting temperature $(T_{1})$ is attained shall be calculated using the following equations:

a) Three-sided fire exposure condition

t = 5. 2 + 0. 0 2 2 1 T + \left(\frac {0 . 4 3 3 T _ {1}}{k _ {\mathrm{sm}}}\right)

b) Four-sided fire exposure condition

t = 4. 7 + 0. 0 2 6 3 T + \left(\frac {0 . 2 1 3 T _ {\mathrm{l}}}{k _ {\mathrm{sm}}}\right)

where

$t =$ time from the start of the test, in min;

$T =$ steel temperature, in ${}^{\circ}\mathrm{C}, 500^{\circ}\mathrm{C} \leq T \leq 750^{\circ}\mathrm{C}$ , and

$k_{sm} = \text{exposed surface area to mass ratio, } 2 \times 10^{3} \text{ mm}^{2}/\text{kg} \leq k_{sm} \leq 35 \times 10^{3} \text{ mm}^{2}/\text{kg}.$

For temperatures below $500^{\circ}$ C, linear interpolation shall be used, based on the time at $500^{\circ}$ C and an initial temperature of $20^{\circ}$ C at t equals 0.

22.8 Determination of PSA from a Single Test

The period of structural adequacy (PSA) determined from a single test may be applied without modification provided:

a) Conditions, specified in 22.6.3 are satisfied,

b) Conditions of support are the same as the

prototype and the restraints are not less favourable than those of the prototype, and

c) Ratio of the design load for fire to the design capacity of the member is less than or equal to that of the prototype.