Experimental study on behaviour of structural elements made of hot-rolled and cold-formed steels in fire

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Experimental study on behaviour of structural elements made of

hot-rolled and cold-formed steels in fire

NATIONAL RESEARCH COUNCIL O F CANADA DIVISION O F BUILDING RESEARCH

EXPERIMENTAL STUDY ON BEHAVIOUR O F STRUCTURAL ELEMENTS MADE O F

HOT-ROLLED AND COLD-FORMED S T E E L S IN FZRE

by L. Konicek F i r e Study No. 35 of the D i v i s i o n of Building R e s e a r c h OTTAWA November 1974

EXPERIMENTAL STUDY ON BEHAVIOUR OF STRUCTURAL ELEMENTS MADE OF

HOT-ROLLED AND COLD-FORMED STEELS IN FIRE

L. Konicek

ABSTRACT

Two types o f open-web s t e e l j o i s t s have been designed f o r t h e same span and t h e same loading conditions. The f i r s t type of j o i s t was made of h o t - r o l l e d s t e e l shapes. Cold- formed s t e e l was used f o r c o n s t r u c t i o n of t h e second type.

The

e f f e c t of t h i s d i f f e r e n c e on t h e behaviour of j o i s t s exposed t o f i r e , i s i n v e s t i g a t e d i n t h i s experimental study

.

EXPERIMENTAL STUDY ON BEHAVIOUR OF STRUCTURAL ELEMENTS MADE OF

HOT-ROLLED AND COLD-FORMED STEELS IN FIRE

L . Konicek

I d e n t i c a l r o o f assemblies supported by a p a r t i c u l a r t y p e of s t e e l j o i s t were b u i l t . Each o f them was t e s t e d according t o "Standard f o r F i r e T e s t s o f Building Construction and ~ a t e r i a l s "

.

'

Results o b t a i n e d , t h e i r e v a l u a t i o n , and a comparison of t h e behaviour of both samples t e s t e d , a r e p r e s e n t e d i n t h i s r e p o r t .

SPECIFICATIONS OF MATERIALS USED

1. Open-web s t e e l j o i s t , 16 i n . deep ( h o t - r o l l e d chords) S t e e l : Chord, G 40.12

Web, A 36

Applied load s p e c i f i e d by manufacturer, 1020 p l f T o t a l c r o s s - s e c t i o n a r e a o f chords, 1.920 i n . 2 Weight, 8.9 l b / f t

For d e t a i l s e e Figure 1.

2. Open-web s t e e l j o i s t , 16 i n . deep (cold-formed chords)

S t e e l : Yield s t r e n g t h , 55000 p s i

T e n s i l e s t r e n g t h , 67000 p s i , minimum

Applied load s p e c i f i e d by manufacturer, 1138 p l f T o t a l c r o s s - s e c t i o n a r e a o f chords, 1.868 i n . 2 Weight, 7.7 l b / f t

For d e t a i l s s e e Figure 2.

3 . Roof deck: 6 f t 2 i n . countersunk one end t o provide l a p j o i n t

3 f t 7 i n . countersunk one end t o provide l a p j o i n t

3 f t 7 i n . p l a i n ends

Units f a b r i c a t e d from 16-gauge wiped coated (galvanized) s h e e t s t e e l

4. Thermal i n s u l a t i o n : 1 - i n . o r g a n i c f i b r e i n s u l a t i n g board

5. F i r e p r o t e c t i o n : Cementitious sprayed p r o t e c t i o n , t h i c k n e s s 2 i n . average d e n s i t y 16.2 l b / f t 3 .

CONSTRUCTION OF A ROOF ASSEMBLY

Two simply-supported open-web s t e e l j o i s t s were placed i n t h e c o n c r e t e frame which forms p a r t o f t h e f l o o r furnace. An arrangement of j o i s t s i n t h e frame with d e t a i l o f j o i s t b r a c i n g and support can b e s e e n i n Figure 3 . The s t e e l roof deck was plug welded t o t h e s u p p o r t i n g s t r u c t u r e and covered by thermal i n s u l a t i o n with a s b e s t o s paper on i t s e x t e r n a l s u r f a c e . The method o f c o n s t r u c t i o n of t h i s p a r t of t h e assembly can be s e e n i n Figure 4 . The unprotected roof deck i s shown a t t h e t o p o f t h e p i c t u r e , t h e roof deck p r o t e c t e d by thermal i n s u l a t i o n i n t h e middle, and t h e completed e x t e r n a l s u r f a c e o f t h e r o o f a t t h e bottom and i n Figure 5 .

The thermocouples were peened t o t h e s u r f a c e of open-web s t e e l j o i s t s i n p r e s c r i b e d p l a c e s ( s e e Figure 6 ) and a t t a c h e d by screws t o t h e s t e e l roof deck ( s e e Figure 7 ) . S u r f a c e temperatures on t h e unexposed s i d e were measured by a s e t o f thermocouples covered by s t a n d a r d a s b e s t o s pads ( s e e Figure 8)

.

A f t e r mounting a l l thermocouples and s t r a i n gauges, t h e s u r f a c e o f t h e c o n s t r u c t i o n exposed t o f i r e was sprayed with cementitious f i r e p r o t e c t i o n . Incomplete f i r e p r o t e c t i o n o f t h e assembly can be s e e n i n Figure 9 . The

completed assembly i s shown i n Figure 10.

A f t e r c a r e f u l c u r i n g , during which t h e moisture c o n t r o l i n t h e sprayed f i r e p r o t e c t i o n reached a s t e a d y l e v e l , with only very small weight d e v i a t i o n s

(caused by v a r i a t i o n i n t h e r e l a t i v e humidity of t h e a i r i n t h e l a b o r a t o r y ) , t h e assembly was ready f o r t e s t i n g . The changes i n d e n s i t y o f t h e sprayed f i r e p r o t e c t i o n i n r e l a t i o n t o time a r e p l o t t e d i n Figure 11. Both completed roof assemblies were s t o r e d i n t h e l a b o r a t o r y f o r approximately two months p r i o r t o t h e f i r e t e s t s .

INSTRUMENTATION

Furnace temperature was measured by n i n e chromel-alumel thermo-

couples enclosed i n s t a n d a r d weight 1 / 2 - i n . black wrought-iron p i p e . I n d i v i - dual temperatures, a s well a s t h e average recorded temperature o f t h e n i n e thermocouples, were recorded.

A t y p i c a l arrangement of thermocouples measuring t h e temperature

course i n an open-web s t e e l j o i s t c r o s s - s e c t i o n can be s e e n i n Figure 6 . The l o c a t i o n s o f t h e s e t y p i c a l c r o s s - s e c t i o n s A-F a r e shown i n t h e s k e t c h o f t h e roof assembly p l a n , a g a i n i n Figure 6 .

Temperatures on t h e s t e e l r o o f deck were measured i n l o c a t i o n s G and H by thermocouples 1-4 ( s e e Figure 7 )

.

Unexposed s u r f a c e temperatures were measured a t n i n e l o c a t i o n s shown i n Figure 8; t h e thermocouples were covered with s t a n d a r d a s b e s t o s pads.

S t r a i n on t h e loaded c o n s t r u c t i o n was measured b e f o r e t h e f i r e t e s t , by s t r a i n gauges, which were a t t a c h e d t o t h e upper chord, t h e lower chord and t h e diagonal o f each j o i s t , i n t h e middle o f i t s span. This measurement, t o g e t h e r with measured d e f l e c t i o n s a t t h i s s t a g e o f t h e experiment, proved c o r r e c t load d i s t r i b u t i o n .

The load permitted by t h e manufacturer's s p e c i f i c a t i o n was a p p l i e d by means of 30 h y d r a u l i c jacks.

V e r t i c a l d e f l e c t i o n s were measured a t t h e c e n t r e of each open-web s t e e l j o i s t and i n t h e middle o f t h e t e s t e d r o o f assembly.

FIRE TESTS AND READINGS

The furnace temperature s a t i s f a c t o r i l y approached t h e s t a n d a r d time-temperature curve1 throughout both t e s t s ( s e e Table 1 and Figure 1 2 ) . Maximum d e v i a t i o n was always w i t h i n t h e p e r m i s s i b l e l i m i t s and no c o r r e c t i o n f a c t o r was r e q u i r e d .

Temperatures measured on t h e assembly made o f h o t - r o l l e d s t e e l a r e p r e s e n t e d i n Table I1 and Figure 13. When a reading on a thermocouple was h i g h e r a t one l o c a t i o n than a t another comparable l o c a t i o n ( f o r example

A = 1 6 2 5 ' ~ ) ~ t h e r e a d i n g was marked on t h a t p a r t i c u l a r Table, and p l o t t e d i n t h e corresponding f i g u r e a s a course o f maximum temperature i n t h e measured p l a c e .

D e f l e c t i o n measurements f o r t h e l1Northl1 open-web s t e e l j o i s t u s i n g h o t - r o l l e d s t e e l a r e shown i n Table I11 and Figure 14. The j o i s t c o l l a p s e d a t 98.5 min.

The d a t a o b t a i n e d from t h e f i r e t e s t o f t h e r o o f assembly, where cold-formed s t e e l i s used i n t h e c o n s t r u c t i o n o f t h e s t e e l j o i s t s , a r e s i m i l a r l y p r e s e n t e d . Temperatures a r e shown i n Table IV and Figure 15; d e f l e c t i o n s a r e i n Table I11 and Figure 14. The "North" j o i s t c o l l a p s e d a t 95.5 min.

A p a r t i c u l a r comparison o f measured d e f l e c t i o n s on open-web s t e e l j o i s t s made o f cold-formed s t e e l and h o t - r o l l e d s t e e l was made and i t s ' r e s u l t s can b e found again i n Table I11 and Figure 14.

OBSERVATIONS

Thermal p r o t e c t i o n of both assemblies was checked b e f o r e t h e f i r e t e s t s . There was no v i s i b l e damage i n t h e f i r e p r o t e c t i o n .

During t h e f i r s t hour o f t h e f i r e t e s t s no change i n t h e q u a l i t y o f t h e f i r e p r o t e c t i o n was n o t i c e d . L a t e r on, s e v e r a l very t h i n cracks were observed. Some o f t h e cracks began t o develop a t 40 min, a s i n d i c a t e d by

s e v e r a l thermocouple r e a d i n g s .

A f t e r 70 minutes s e v e r a l cracks developed t o a much l a r g e r e x t e n t . These d i s t u r b a n c e s caused t h e roof assembly using h o t - r o l l e d s t e e l t o c o l l a p s e a f t e r 98.5 min. The assembly using cold-formed s t e e l withstood 95.5 min o f f i r e t e s t . D e t a i l s o f t h e c o l l a p s e d roof assemblies a r e shown i n Figure 16 (a) t o (e)

.

A p i c t u r e o f t h e s u r f a c e o f t h e roof assembly exposed t o t h e f i r e t e s t i s shown i n Figure 16 ( a ) . D e t a i l of t h e ruptured lower chord o f t h e open-web s t e e l j o i s t i s i n Figure 16 ( b ) . Deformed s t r u c t u r a l s t e e l and r e s u l t i n g r u p t u r e o f t h e lower chord o f t h e open-web s t e e l j o i s t i s shown i n Figure 16 (c)

,

(d)

,

and (e)

.

Figure 16 (a) t o (e) shows t h e r u p t u r e d open-web s t e e l j o i s t where h o t - r o l l e d s t e e l was used. Figure 17 shows a r u p t u r e d lower chord o f open-web s t e e l j o i s t made o f cold-formed s t e e l .

When sprayed f i r e p r o t e c t i o n was taken o f f t h e j o i s t s a f t e r t h e f i r e t e s t s , t h e s u r f a c e o f t h e j o i s t s was corroded. Figures18 and 19 show t h e change i n c o l o u r o f t h e s u r f a c e o f t h e sprayed p r o t e c t i o n i n c o n t a c t with t h e s t e e l ( t o p p a r t of Figures 18 and 1 9 ) . Samples with marks o f c o r r o s i o n a r e compared t o a sample not exposed t o t h e f i r e t e s t (lower p a r t o f Figures 18 and 1 9 ) .

RESULTS

I n t h e case o f t h e assembly u s i n g h o t - r o l l e d s t e e l , t h e average temperatures measured on t h e lower chord, upper chord and diagonals of t h e open-web s t e e l j o i s t , reached t h e ASTM c r i t i c a l s t e e l temperature,

-

tc

-

1 1 0 0 ° ~ , a f t e r 80 min. The c r i t i c a l temperature was exceeded i n s e v e r a l p l a c e s much sooner ( s e e Figure 13, Table 1 1 ) . Actual c o l l a p s e occurred a t 98.5 min, (when t h e lower chord o f t h e open-web s t e e l j o i s t r u p t u r e d . )

The temperature measured on t h e upper chord of t h e open-web s t e e l j o i s t made o f cold-formed s t e e l reached i t s c r i t i c a l v a l u e i n 60 min. Maximum measured temperatures i n p l a c e A ( s e e Figure 15) exceeded llOO°F a t 60 min o f t h e f i r e t e s t . A r u p t u r e o f t h e lower chord occurred a t 95.5 min.

The d e f l e c t i o n s on an open-web s t e e l j o i s t made o f cold-formed s t e e l were b i g g e r t h a n those measured f o r t h e h o t - r o l l e d s t e e l j o i s t . The measured d e f l e c t i o n values with p a r t i c u l a r comparisons a r e given i n Table

I11 and Figure 14.

During t h e t e s t s described i n t h i s r e p o r t , t h e formation of l a r g e cracks caused a s t e e p i n c r e a s e i n l o c a l s t e e l temperatures. The load- bearing c a p a c i t y o f t h e s t e e l was decreased with r e s u l t i n g r u p t u r e of lower chords f o r both t e s t e d open-web s t e e l j o i s t s .

CONCLUSION

The t e s t s have shown t h e following r e s u l t s :

Measured d e f l e c t i o n s i n t h e middle o f t h e span on t h e open-web s t e e l j o i s t s made o f cold-formed s t e e l were g r e a t e r t h a n corresponding d e f l e c t i o n s of open-web s t e e l j o i s t s made of h o t - r o l l e d s t e e l .

T o t a l c o l l a p s e of a j o i s t made o f cold-formed s t e e l came a t 95.5 min; a j o i s t made of h o t - r o l l e d s t e e l c o l l a p s e d a t 98.5 min. This d i f f e - rence is s o small t h a t performance o f both j o i s t s can be r a t e d e q u a l .

On t h e b a s i s of t h e s e r e s u l t s we can conclude t h a t t h e r e i s no s i g n i f i c a n t d i f f e r e n c e i n t h e performance of open-web s t e e l j o i s t s made of h o t - r o l l e d o r cold-formed s t e e l s when exposed t o f i r e . The f i r e r e s i s t a n c e c l a s s i f i c a t i o n of both assemblies according t o ASTM El19 was 1-1/2 h r .

ACKNOWLEDGEMENT

This p r o j e c t was undertaken a t t h e request o f t h e S t e e l I n d u s t r i e s Fellowship Committee. The work was c a r r i e d o u t under a cooperative program between DBR/NRC and t h e Canadian S t e e l I n d u s t r i e s Construction Council, known a s t h e S t e e l I n d u s t r i e s Agreement.

The author wishes t o thank Messrs. J . E . Berndt and E.O. Porteous, who c a r r i e d o u t t h e f i r e t e s t s and a s s i s t e d i n a n a l y s i s of t h e d a t a developed.

REFERENCE

1. Standard f o r F i r e T e s t s of Building Construction and ASlM El19 Materials, Second Edition, September 1971,

T A B L E I

FURNACE TEMPERATURES,

PRESCRIBED A ~ D EXPERIMENTAL

*

TIME,

PRESCRIBED EXP.

-

HOT-ROLLED E X P .

-

COLD-FORMED min. S T E E L S T E E L

T A B L E I1

T E M P E R A T U R E S ON HOT-ROLLED S T E E L O P E N - W E B JOIST (AVERAGE, MAXIMUM)

( s e e Fig. 13)

T.EMP JOIST "SOUTH"

U P P E R LOWER U P P E R 1 LOWER DmGONAL DIAGONAL CHORD . - CHORD I 0 70 7 0 70 70 70 70 40 49 0

_-

490 525 615

1

580 560 530

5

6

0

690 610 50 620 725 730 825 7 60 9 50 5 5 550 8 50 850 825 _- I 790 900 60 560 975 860 79O -, ' 820 8 2 5 I 6 5 800

1

A ~ 1 0 6 0 880 700 870 7 60 830 _I 70 0 A31160 70

'

530

0

."10-k--600

looo

,

7 50 Az1260 7 5 940 9 70

I

790 9 80 _I 8 0 8 5 1060 1120 1150 1325 I 90 9 5 h 1 3 3 0 1 I 1110 875

1

1 0 6 0

1

850 950

i

A=1625

)

1250 1280

1

,

1 0 7 5

1

1085 A r 1525 1135 I 1225

1

ziioo

1220 - 1100 9 60 1180 A t 1600 1220 I 1340

i

TABLE 111

DEFLECTIONS ON OPEN-WEB S T E E L JOISTS,

WHERE COLD-FORMED AND HOT-ROLLED S T E E L S ARE USED,

COMPARISON WITH CALCULATED DIFFERENCE

*

TIME DEFLECTION DEFLECTION

( min) COLD-FORMED S T E E L HOT-ROLLED S T E E L DIFFERENCE

0. 60 0.73 0. 82 0. 87 0.89 0. 89 0. 89 0.93 1.02 1.14 1. 31 1.54 1.72 2.02 2. 18 2.42 2.94 3. 83 5. 44 COLLAPSE 9 5. 5 min 0.80 0.80 0. 81 0.85 0.94 1.08 1.22 1.41 1. 56 1. 82 2.04 2. 24 2.48 3.04 COLLAPSE 98. 5 min - . -. - - --

*

s e e Fig. 14

T A B L E IV

TEMPERATURES ON COLD-FORMED OPEN-WEB S T E E L JOISTS (MAXIMUM T E M P E R A T U R E )

*

J O I S T PARAMETERS S U P P L I E D B Y PRODUCER S P A N : 1 5 ' - 6 " W = 1 0 2 0 PLF STEEL: C H O R D S - G 4 0 . 1 2 F I G U R E 1 S K E T C H O F H O T - R O L L E D O P E N - W E B STEEL J O I S T W I T H TECH N l C A L PARAMETERS

J O I S T PARAMETERS SUPPLIED BY PRODUCER S P A N : 1 5 ' 6 " W = 1 1 3 8 P L F R R = R L = 8 8 2 0 L B STEEL: T E N S I L E S T R E N G T H : 6 7 0 0 0 PSI M I N I M U M Y I E L D S T R E N G T H : 5 5 0 0 0 P S I M I N I M U M F I G U R E 2 2 A6 = 1 . 0 3 2 ( I N . ) A 5 = 0 . 8 3 6 ( I N . 2 ) SKETCH O F C O L D - F O R M E D O P E N - W E B STEEL J O I S T W I T H T E C H N I C A L PARAMETERS

D E T . 1

h

S U P P O R T O F STEEL R O O F D E C K - -

t

.I__- A S B E S T O S PAPER T H E R M A L I N S U L A T I O N STEEL R O O F D E C K J O I S T T H E R M A L I N S U L A T I O N S U P P O R T O F J O I S T S C O N C R E T E F R A M E W A L L O F T H E F U R N A C E TESTED STEEL J O I S T t _--- --* - - - A Q ---- C O N T O U R O F S P R A Y E D FIRE P R O T E C T I O N O F STEEL PARTS S U P P O R T O F STEEL R O O F D E C K

---

F R A M E O F THC F U R N A C E D E T . 1 F I G U R E 3 l

-

h A S B E S T O S PAPER T H E R M A L I N S U L A T I O N STEEL R O O . F D E C K D E T . i L O C A T I O N O F J O I S T S I N THE F U R N A C E W I T H T W O D E T A I L S O F R O O F A S S E M B L Y ( S K E T C H )

Fig. 4 Roof a s s e m b l y under c o n s t r u c t i o n

7 -

-

I

PLAN OF T H E RGOF ASSEMBLY

-

TEMPERATURE I N L O C A T I O N S A-F WERE RECORDED

CROSS

-

SECTICN A-F WITH THERMOCGUPLES 1

-

4

'r"

NORTH

FIGURE 6

TEMPERATURE MEASUREMENT O N S T E E L J O I S T . L O C A T I O N O F THERMOCOUPLES

PLAN OF THE ROOF ASSEMBLY

-

NORTH

S T E E L ROOF DECK CROSS SECTION

FIGURE 7

NORTH

L

FIGURE 8

F i g . 9 Uncompleted s p r a y e d f i r e protection of roof a s s e m b l y

1 0 0 4 8 1 2 16 20 24 28 32 36 N U M B E R O F D A Y S I I I 1 1 I I I S P R A Y E D F I R E P R O T E C T I O N : - 2" T H I C K N E S S O F M E A S U R E D S A M P L E - + - - - - - - - - L I 1 I I I I I I F I G U R E 1 1 C H A N G E I N D E N S I T Y O F S P R A Y E D M A T E R I A L W I T H R E L A T I O N T O T I M E

-

P R E S C R I B E D F U R N A C E T E M P E R A T U R E E X P E R I M E N T A L T E M P E R A T U R E , H O T R O L L E D S T E E L

--

E X P E R I M E N T A L T E M P E R A T U R E , C O L D - F O R M E D S T E E L 30 4 0 5 0 6 0 T I M E , M I N F I G U R E 12 F U R N A C E T E M P E R A T U R E S . P R E S C R I B E D A N D E X P E R I M E N T A L

0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 T I M E , M I N 1.1

-

U P P E R C H O R D ( A V E . , M A X . A T " A " ) 2

--

LOWER C H O R D 3

----

D I A G O N A L F I G U R E 1 3 T E M P E R A T U R E M E A S U R E D O N N O R T H O P E N - W E B STEEL J O I S T ( H O T - R O L L E D STEEL) I I I DEFLECT1 O N S I + C O L D F O R M E D STEEL I I -

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H O T R O L L E D STEEL

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I I I I I I 0 10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 100 T I M E , M I N F I G U R E 1 4 D I A G R A M C O M P A R I N G T H E D E F L E C T I O N S OF C O L D - F O R M E D A N D H O T - R O L L E D O P E N - W E B STEEL J O I S T S

0 0 10 20 30 40 50 60 70 80 90 100 T I M E , M I N - 1, 1 U P P E R C H O R D ( A V E . , M A X . A T " A 1 0 2 --LOWER C H O R D T 3

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D I A G O N A L * - - - - - - - - I - - 98.5 M I N 1- I 1 I I I I I I I F I G U R E 15 T E M P E R A T U R E M E A S U R E D O N N O R T H C O L D - F O R M E D S T E E L O P E N W E B J O I S T

(a) Exposed surface of the assembly after fire test

( b ) D e t a i l of r u p t u r e d l o w e r c h o r d of s t e e l j o i s t

( c ) D e t a i l s of r u p t u r e d l o w e r c h o r d of o p e n - w e b s t e e l j o i s t

( d ) Change in d i m e n s i o n s of lower chord c r o s s - s e c t i o n a f t e r f i r e t e s t

( e ) Detail of r u p t u r e d lower chord of open-web s t e e l joist

Fig. 17 Deformation and r u p t u r e of open-web s t e e l joint made of cold-formed s t e e l

Fig. 1 8 C o r r o s i o n of s t e e l j o i s t , hot-rolled s t e e l .

F i r e p r o t e c t i o n with visible m a r k of c o r r o s i o n , w h e r e in contact with s t e e l ( t o p p i c t u r e ) . F i r e p r o t e c t i o n not exposed t o f i r e ( l o w e r p i c t u r e )

Fig. 19 Corrosion of s t e e l joist

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cold-formed s t e e l . F i r e protection with visible mark of corrosion, where in contact with s t e e l (top picture). F i r e protection not exposed to f i r e (lower picture).