Steel Structures 7 (2007) 201-208 www.kssc.or.kr

Experimental Behaviour of Steel Connections to Concrete

Filled Steel Tubular Columns in Fire

Y. C. Wang* and J. Ding

School of MACE, University of Manchester, UK

Abstract

This paper presents observations and some structural behaviour results of five fire tests on a structural assembly consistingof a steel beam connected to a pair of concrete filled tubular (CFT) columns in a rugby style arrangement. The objectives ofthis study are to obtain thermal behaviour data of joints and to examine large deflection structural behaviour of axiallyrestrained steel beams and robustness of different types of steel joint to CFT column. The five tests include one test using asimple fin plate welded to the steel tube and bolted to the beam web, two tests using a T-stub bolted to the steel tube and thesteel beam, and two tests using a channel welded to the steel tube on the toes and bolted to a flexible end plate welded to theweb of the steel beam. In each test, only the steel beam was loaded by two point loads at approximately 1/3 spans. All firetests continued beyond the limiting temperatures of the steel beams based on pure bending in order to examine the developmentof catenary action and the behaviour of the joint components under combined tension, shear and bending moment in fire. Thetest results indicate that all the joints would survive the fire if the beam temperature does not exceed the beam’s limitingtemperature for pure bending. The joints were sufficiently strong to enable the beams to develop some catenary action so thatthe steel beam failure temperature was 100oC higher than the beams’ limiting temperatures. However, to enable the steel beamto survive much higher temperatures in catenary action, the joint strength in tension would have to be substantially increased.

Keywords: joints, concrete filled tube, catenary action, fire resistance, fire test

1. Introduction

Joints are critical members in structures. Despite extensive

research studies of CFT column behaviour in fire in the

past, very little information is available on joints between

steel beams and CFT columns under fire conditions. In

particular, observations from fire tests and real fire

accidents in other types of steel structure indicate that

there is strong interaction between joints and the

connected beams and columns, and that the behaviour of

the joints in complete structures is different from that in

isolation. In general, steel beams restrained by the

adjacent structure would go through a number of phases

in behaviour when exposed to fire (Wang, 2002),

including combined flexural bending and compression

during the early stage of fire exposure (as a result of

restrained thermal expansion of the beam), through pure

flexural bending at temperatures around the beam’s

limiting temperature for pure bending, and finally to

catenary action at the late stage of fire exposure when the

beam deflection is very large. As a result, the forces in the

joints will change during the course of a fire exposure. In

particular, catneary action in the steel beam can allow the

beam to survive very high temperatures and it is possible

to utlilise catanary action in the steel beam to remove fire

protection (Yin and Wang, 2004, 2005a, b; Wang and

Yin, 2006). However, it is important that the joints have

sufficient resistance to the catenary forces. Isolated joint

tests under fixed loads, either in bending, tension,

compression or their combinations, cannot capture the

changing nature of joint performance and its impact on

the adjacent structural behaviour. Therefore, the fire tests

of this research were carried out in a structural assembly,

consisting of a steel beam and two CFT columns in a

rugby frame. The aim of this research is to provide detailed

experimental information to help quantify temperature

fields in the joint region, and structural performance of the

joints and structural assemblies in fire. In this research,

the beam was loaded and heated to very high

temperatures and very large deflections, so that these new

fire tests can provide information on joint behaviour over

the entire range of steel beam behaviour.

A few previous research studies of a similar nature

examined steel beam to H-column joints (Dong and Li,

2005, Liu et al., 2002). However, these investigations

have mainly been concerned with joint rotational restraint

to the steel beams, with limited information on beam

behaviour during the catenary stage. The joints of Dong

and Li (2005) were outside the test furnace. Furthermore,

these previous studies did not report in detail whether and

*Corresponding authorTel: +44 161 3068968E-mail: yong.wang@manchester.ac.uk

202 Y. C. Wang and J. Ding

how the joints had failed during the tests.

This paper will present the results of 5 tests on joints to

CFT columns, which have recently been completed. This

paper will provide observations of different joint failure

modes as well as the beam behaviour. Preliminary

conclusions will be drawn on design implications.

2. Test Descriptions

The fire tests were carried out in one of the three

furnaces in the Fire Testing Laboratory of the University

of Manchester. This furnace was a rectangular box having

internal dimensions 3000 mm × 1600 mm × 900 mm. The

interior faces of the furnace were lined with ceramic fibre

materials of thickness 200 mm that efficiently transferred

heat to the specimen. An interior full-height honeycomb

ceramic wall, as shown in Fig. 1, was constructed to

ensure uniform heating. A gas burner and an exhaust

were connected to the furnace. The firing and control

equipment was installed with the gas burner. Before

actual fire testing of loaded specimens, a number of trial

fire tests without a test specimen were carried out. In

these tests, the hole positions in the interior wall were

adjusted until the most uniform temperature distribution

inside the furnace was reached. The burner system was

fully computer-controlled to follow any required time-

temperature curve, with automatic recording of the results

and a runtime display.

The furnace consisted of 6 panels bolted at their

junctions for easy assembly. It could accommodate

frames with 2 m beam length and 3m total column height

and take account of the large displacements associated

with high-temperature testing.

The arrangement of the members to be tested was in the

form of a complete ‘Rugby goalpost’ frame as shown in

Fig. 2. The steel beam was mainly unprotected. In order

to simulate the heat-sink effect due to the concrete slab in

realistic structures, the top flange of the beam was

wrapped with 15 mm thick ceramic fibre blanket. The

columns were unprotected. The columns were restrained

from lateral movement (so as to provide axial restraint to

the beam) at the ends and were free to move in the

longitudinal direction at the top. The lateral restraint at

each column end was provided by a bracket bolted to the

strong reaction frame around the furnace. Two transverse

point loads were applied to the beam using two

independent hydraulic jacks connected to the top member

of the strong reaction frame surrounding the furnace to

form a self-equilibrating system. The jacks and the

brackets were protected by ceramic fibre blanket during

the fire test.

Each assembly consisted of two concrete filled tubular

(CFT) columns and a steel beam. The cross-section size

of the beams in all tests was the same, being grade S275

universal beam section 178 × 102 × 19UB. SHS tubes

were used, and the overall dimensions were 200 × 200

mm. The five tests investigated three types of joint: fin

Figure 1. Schematic arrangement of the furnace.

Figure 2. Schematic arrangement of test.

Figure 3. Fin plate joint (Test 1).

Figure 4. T-stub joint (Tests 2 & 3).

Experimental Behaviour of Steel Connections to Concrete Filled Steel Tubular Columns in Fire 203

plate welded to the tubular wall (Fig. 3), T-section bolted

to the tubular wall using Molabolts (Fig. 4), reverse

channel welded to the tubular wall (Fig. 5). The Molabolt

was a new type of blind bolting system for application to

tubular structures (www.molabolt.com). It worked by

expanding inside the tube. In order for the Molabolt to

tightly join the tube, the hole diameter should not exceed

the Molabolt diameter by 0.5 mm. For comparison, the

clearance for conventional bolts is 2 mm. The Molabolts

should be inserted into the tube before concreting. There

was no applied load in the columns. The same load

(30 kN in each jack) was applied in all tests, being about

50% of the lateral torsional buckling load carrying capacity

of the simply supported beam at ambient temperature.

The average temperature inside the furnace was

programmed to rise according to the ISO834 standard

time-temperature curve. However, due to high thermal

mass of the test assembly, especially that of the wet

concrete, in relation to the furnace size, the furnace

temperatures could not follow the standard fire curve.

The furnace temperatures were measured with the aid of

6 control thermocouples, which were placed in two lines

opposite each other at 430 mm apart (Fig. 1). These

thermocouples were embedded in ceramic tubes and their

measuring tips project out-side the ceramic tubes and

were located in the mid height of the furnace.

Due to high temperatures in the fire test, the means of

measuring strains and displacements was very limited.

No strain gauges were placed on the specimen, as they

would be quickly destroyed during the fire exposure.

High temperature strain gauges were considered, but they

would be too expensive and would not be able to provide

reliable information due to different thermal expansions

from those of the structural materials at high temperatures.

Therefore, only the temperature distributions, displacements

and the horizontal reaction forces at the ends of the

columns were measured during the tests. In particular, a

large number of thermocouples were installed on each

test specimen in the beam, joint components and columns

to record detailed temperature distributions, as shown in

Fig. 6.

3. Observations

3.1. Furnace temperatures

In all tests, because the test frames were of the same

size and configuration, temperature distributions inside

the furnace were almost identical. This is confirmed by

Fig. 7, which compares the average recorded furnace

temperature for the five tests. Also due to the large mass

of the test specimen in comparison to the size of the

furnace, the furnace temperatures were slightly lower

than the intended ISO-834 heating curve. As indicated in

Fig. 1, the furnace temperatures were measured by six

control thermocouples. Because only one burner was

used, the six thermocouples inside the furnace recorded

non-uniform temperature distribution in the furnace,

which is shown in Fig. 8. Close examination of the

individual thermocouple results in Fig. 8 indicates that

the highest gas temperature was measured in the middle

of the furnace from thermocouples 2 and 5. Higher

temperatures were recorded towards the left hand side of

the furnace (thermocouples 3&6). Thermocouples 1&4

recorded the lowest temperatures towards the right hand

side of the furnace. The higher temperatures on the left

hand side of the furnace may be used to explain that in all

the tests, failure (or maximum damage) occurred on the

left hand side of the test frames.

Figure 5. Reverse channel joint (Tests 4 & 5).

Figure 6. Locations of measuring devices.

204 Y. C. Wang and J. Ding

3.2. Test 1

Before the furnace was ignited, the loads were applied

to the beam gradually. No visible deformation was found

in the specimen at this stage. After 23 minutes of fire test,

the beam deflection increased rapidly. As a result, the

applied loads failed to catch up with the deflection. At

24.5 minutes of fire test, the displacement transducer at

the mid-span of the beam (Fig. 6) was detached from the

beam, and did not give meaningful readings thereafter.

The sub-frame failed at 31 minutes with fracture at the

weld in the left hand joint, as shown in Fig. 9(a). The

beam experienced large twist and vertical deflections as

shown in Fig. 9(b).

3.3. Test 2

The loads were applied to the beam gradually at room

temperature. No visible deformation was found in the

specimen at this stage. After 23 minutes of fire test, the beam

deflection started to increase very quickly and the applied

loads failed to catch up with the deflection. The sub-frame

failed at 30 minutes with pull-out of the beam web at the left

end, as shown in Figs. 10(a) and 10(b). The beam had large

twist and vertical deflections as shown in figure 10(c). Due

to the large twist in the beam, parts of the T-section bolted to

the beam were also twisted to one side, as shown in Fig.

10(d). Consequently, the bolts between the left T-section and

the beam experienced large rotations as shown in Fig. 10(e).

Parts of the T-sections bolted to the steel tubes were pulled

out slightly. The steel tube walls were also pulled out slightly

around the connection area but there was no indication of

failure in the tubes.

3.4. Test 3

Test 3 used SHS 200 × 12.5 mm tubes and bolted T

joints. The only difference between the test 3 and test 2

configurations is in the thickness of the tube. Also, to

prevent pull-out failure of the beam web which occurred

in test 2, a length of 100 mm of the beam web was

protected by 15 mm thick ceramic fibre blanket at both

end, as shown in Fig. 11(a).

After applying the target loads at room temperature, no

clear deformation was found in the specimen. After 24.5

minutes of fire test, the beam deflection increased rapidly

and the applied loads could not be maintained. After 30

minutes of fire exposure, the beam deflection started to

increase at a lower rate and the applied loads were

increased to the original level. The applied loads were

then maintained until the sub-frame failed at 38.2 minutes

after fire exposure. As shown in Fig. 11(b), failure of this

sub-frame was due to fracture of the stem of the T-section

Figure 7. Average furnace temperatures.Figure 8. Measured temperature distribution inside furnace.

Figure 9. Failure mode of test 1.

Experimental Behaviour of Steel Connections to Concrete Filled Steel Tubular Columns in Fire 205

on the left hand side. Figure 11(c) shows a detailed image

of the failure surface. No noticeable deformation could be

seen in the steel tubes and there was no sign of tube

failure. As shown in Fig. 11(c), there was no noticeable

deformation in T-sections bolted to the steel tubes and the

Molabolts.

3.5. Test 4

The beam was loaded gradually at room temperature to

the target loads. No clear deformation was found in the

specimen before fire ignition. After 23 minutes of fire

test, the beam deflection increased rapidly and the applied

loads could not catch up with the deflection for a short

period. After 27 minutes of the fire test, the applied loads

were successfully increased to the original level and then

maintained until the end of the test. The sub-frame failed

at 33 minutes after fire exposure with fracture in the beam

web on the left hand side, as shown in Fig. 12(a). The

fracture length was the same as the weld length and the

fractured web formed a sharp edge, as shown in Fig.

12(b), indicating tension failure. Due to large deflections

in the beam and flexibility of the connection, the endplates

and the front faces (web) of the reverse channels deformed

significantly, as shown in Fig. 12(c). However, there was

no fracture in the reverse channels, indicating their extremely

high ductility. There was no noticeable deflection in the

steel tubes. Neither was any damage found in the connection

welds.

Figure 10. Behaviour and failure mode of test 2. (a) Jointfailure due to beam web fracture (b) Fractured beam web(c) Deformed beam (d) Deformed T-stub (e) Deformedbolts between T-stub and beam web.

Figure 11. Behaviour and failure mode of test 3. (a) Local fire protection to joint (b) Separation of T-stub from beam(c) Fracture of T-stub stem.

Figure 12. Behaviour and failure mode of test 4. (a) Joint failure due to beam web fracture (b) Fractured beam web (c)Deformed reverse channel web.

206 Y. C. Wang and J. Ding

3.6. Test 5

The only difference between the test 4 and test 5

configurations was the tube wall thickness. Also to

prevent fracture in the beam web which happened in test

4, a length of 100 mm of the beam web was protected by

15 mm ceramic fibre blanket at both ends as shown in

Fig. 13(a).

Before fire ignition, the beam was loaded gradually to

the target loads and no obvious deformation was found in

the specimen. After 25 minutes of the fire test, the beam

deflection increased rapidly and the applied loads

dropped and could not be maintained for a short period.

After 29.5 minutes of fire test, the applied loads were

successfully increased to the original level and were then

maintained until the end of the test. After 41.4 minutes of

fire exposure, the hydraulic jack on the left hand side ran

out of travel and was unable to maintain the load on the

beam. After 46.2 minutes of fire exposure, the hydraulic

jack on the right hand side also ran out of travel. When

the test was finally terminated at 51.5 minutes, no fracture

failure in the sub-frame could be observed from the view

aperture on the furnace door. However, after the specimen

was taken out of the furnace, fracture was found in the

steel tubes where the reverse channels were welded, as

shown in Fig. 13(b). There was also fracture in the weld

between the endplate and the beam web on the left hand

side, as shown in Fig. 13(c). A crack also appeared to

have started in the weld between the reverse channel and

the tube on the right hand side, as shown in Fig. 13(d).

The crack in the steel tube revealed the concrete core,

which did not show any damage at all. Due to the large

catenary force in the beam, the endplates and the reverse

channels distorted significantly, as shown in Fig. 13(e).

4. Beam Structural Behaviour

4.1. Beam deflections

Figure 14(a) compares the beam vertical deflection -

temperature curves for the five tests. The temperature

measured in the beam bottom flange at the mid-span is

used in the figure. The beams were able to sustain the

load without excessive deflection up to a bottom flange

temperature of about 670ºC, which is about the limiting

temperature at a load ratio of 0.5 according to BS 5950

Part 8 (BSI 1990). Further rise in the steel beam temperature

led to a progressive run-away of the beam deflection as

the loss of stiffness and strength of steel accelerated.

Figure 14(b) shows the beam vertical deflection - time

curves for the five tests. It can be seen that the deflections

of test 1, 2 and 4 were very close during the first 24.5

minutes of heating. After that, the displacement transducer

at the mid-span of the beam in test 1 was detached from

the beam, and did not give the correct reading thereafter.

However, the deflections of test 2 and 4 were still very

close until about 27 minutes of fire test. Afterwards, the

rate of beam deflection increase in test 4 was lower than

in test 2. This is clearly linked to the greater catenary

force in test 4 than in test 2, as shown in Fig. 15(a). It can

also be seen in Fig. 14(a) that the run-away of the beam

deflection happened at about 23 minutes in test 1, 2 and

4. For test 3 and 5, the deflections were very close. Due

to fire protections on the joints, run-away deflection of

the beams in tests 3 and 5 occurred at about 25 minutes

of heating, which is a delay of about 2 minutes comparing

with tests 1, 2 and 4,. After about 30 minutes, the rate of

run-away deflection decreased in tests 3 and 5 compared

to other tests.

Figure 13. Behaviour and failure mode of test 5. (a) Beam protection near joint (b) Fractured weld on tube on left handside (c) Fracture of weld on beam (d) Fractured weld on tube on right hand side (e) Deformed end plate.

Experimental Behaviour of Steel Connections to Concrete Filled Steel Tubular Columns in Fire 207

4.2. Beam axial forces

During the early stage of fire exposure, axial compression

force developed when the thermal expansion in the

heated steel beam was restrained by the CFT columns.

The axial compression started to reduce when the steel

temperature approached the beam’s limiting temperature

for combined bending and axial compression. The

compression force continued to decrease until catenary

action started to develop in the beam at the late stage of

fire exposure when the beam deflection was very large.

Figures 15(a) and 15(b) compare the beam axial force -

time curves and beam axial force - temperature curves

recorded by the horizontal load cells on both sides of the

test assembly for the five tests. It can be seen that

catenary action in the steel beam has developed at the late

stage of fire exposure in all the tests. The compression

force in the steel beam of test 1 was relatively small due

to the low stiffness of the fin plate joint. By comparing

test 2 with test 3 and test 4 with test 5, it can be seen that

the beams connected with the thicker tubes developed

higher compression forces due to the higher axial restraint

stiffness (test 3 and 4) provided to the beam by the thicker

steel tubes. By comparing test 2 with test 5 and test 3

with test 4, it can be seen that the beam had higher

compression forces when the reverse channel joint was

used, indicating higher axial stiffness provided by the

reverse channel joint than the T-stub joint. In tests 3 and

5, when parts of the joints were protected, the beams

experienced a considerable period of catenary action so

that the beam failure times were much higher then the

time when the beams reached their limiting temperatures

Figure 14. Beam deflection behaviour. (a) Beam verticaldeflection-temperature relationships (b) Beam verticaldeflection-time relationships.

Figure 15. Beam axial force behaviour. (a) Beam axialforce-time relationships (b) Beam axial force-temperaturerelationships.

Table 1. Summary of fire test specimens

Test Tube size Thickness Joint Type

1 SHS 200 5 mm Fin plate

2 SHS 200 5 mm Bolted T

3 SHS 200 12.5 mm Bolted T with 100 mm long steel beam protection

4 SHS 200 12.5 mm Reverse channel

5 SHS 200 5 mm Reverse channel with 100 mm long steel beam protection

208 Y. C. Wang and J. Ding

under bending. Without the fire protection (tests 2 and 4),

beam failure occurred in the joint region much earlier

than in tests 3 and 5. Therefore, if the beam to column

joints possess sufficient strength, it is possible for the

steel beam to achieve high fire resistance even without

fire protection, through the development of catenary

action.

Table 2 gives a summary of the test results.

5. Conclusions

This paper has described some results of five fire tests

on a rugby type structural arrangement using different

types of joints between steel beam and concrete filled

tubular columns. The following preliminary conclusions

may be drawn:

All the joints behaved well before the steel beam had

reached its limiting temperature for flexural bending,

which is about 670oC.

It is possible for moderately complex joints to concrete

filled tubular columns (T-stub, reverse channel) to develop

substantial catenary action, whereby the steel beams

could survive much higher temperatures than 670oC. The

fin plate joint had little stiffness to develop catenary

action in the beam and the resistance of the joint to

catenary action in the beam is low. The reverse channel

joint appeared to have higher stiffness and strength than

the T-stub joint to resist catenary action in the steel beam.

The failure modes of the test specimens were always in

the joint region and the steel beams were able to

accommodate very large deflections. By appropriate design

and protection of the joint components, it is possible for

the steel beams to develop catenary action and survive

very high temperatures. However, the catenary force and

its distributions in different joint components should be

carefully calculated so that the joints can be properly

designed. These are now being investigated by the authors.

In addition, the authors are also conducting further fire

tests to examine the behaviour of joints under cooling.

Acknowledgments

The work presented in this paper was funded by

CIDECT (Project 15S - 13/05). The authors would like to

thank Mr. Andrew Orton of Corus Tubes for his interests

and practical suggestions. Thanks are also due to Mr. Jim

Gorst and a number of post-graduate students who

assisted with the fire tests.

References

British Standards Institution (BSI 1990). British Standards

BS 5950, Structural use of Steelwork in Buildings, Part 8:

Code of practice for fire resistant design, British

Standards Institution, London.

Dong, Y.L. and Li, X.D. (2005). “The behaviours of H-section

steel beam in fire.” Proceedings of the 8th International

Symposium on Fire Safety Science, Beijing, China.

Liu, T.C.H., Fahad, M.K. and Davies, J.M. (2002).

“Experimental investigation of behaviour of axially

restrained steel beams in fire.” Journal of Constructional

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Wang, Y.C. (2002). Steel and composite structures,

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Wang, Y.C. and Yin, Y.Z. (2006). “A simplified analysis of

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Yin, Y.Z. and Wang, Y.C. (2004). “A numerical study of

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elevated temperatures.” Journal of Constructional Steel

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Yin, Y.Z. and Wang, Y.C. (2005a). “Analysis of Catenary

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Calculation Method, Part 1: Theory and Validation for

Uniform Temperature Distribution.” Journal of

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Table 2. Summary of test results

TestSteel beam

tensile strength(N/mm2)

Max. beambottom flange

temp. (oC)

Max. beam vertical def. at mid-span

(mm)Failure mode

Failure time(min)

1 358.4 753* Not reliable Fin plate weld fracture 31

2 439.1 744 265 Fracture of beam web 30

3 439.1 769 263 Fracture of T-stem 38.2

4 439.1 788 288 Fracture of beam web 33

5 439.1 817 264 No failure (fracture of weld to tube) 41.4