fire resistance through concrete filling

7/27/2019 Fire Resistance Through Concrete Filling

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Achieving fire resistance in steel columns through

concrete filling

Kodur, V.

NRCC-47620

A version of this document is published in / Une version de ce document se trouve dans:Concrete Engineering International, v. 8, no. 4, Winter 2004, pp. 50-53

http://irc.nrc-cnrc.gc.ca/ircpubs
http://irc.nrc-cnrc.gc.ca/ircpubshttp://irc.nrc-cnrc.gc.ca/ircpubs


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Achieving Fire Resistance in Steel Columns Through Concrete Fillingby

V.K.R. KodurSenior Research Officer, Institute for Research in Construction,

National Research Council of Canada, Ottawa, Canada

ABSTRACT

Results from experimental and theoretical studies indicate that required fire resistance,in the practical region, can be obtained for hollow structural steel columns filled with three typesof concrete-filling: plain concrete, bar reinforced concrete and steel fibre reinforced concrete.Case studies illustrating the use of concrete-filling, as a means of providing fire protection, tohollow steel columns are presented for the Museum of Flight in Seattle, Washington and aschool building in Hamilton, Ontario.

INTRODUCTION

Steel hollow structural section (HSS) columns are very efficient structurally in resisting

compression loads and are widely used in the construction of framed structures in industrialbuildings. By filling these sections with concrete, the load-bearing capacity of such columns canbe increased substantially. The two components of the composite column complement eachother ideally, in that the steel casing confines the concrete laterally allowing it to develop itsoptimum compressive strength, while the concrete, in turn, enhances resistance to elastic localbuckling of the steel wall.

In addition, a higher fire resistance can be obtained without using external fire protectionfor the steel, thus increasing the usable space in the building. Further, the steel sectionsdispense with the need for formwork and can be prefabricated, thus enabling their erection in alltypes of weather. Properly designed concrete-filled hollow steel columns can also lead, in aneconomic way, to the realization of architectural and structural design with visible steel without

any restrictions on fire safety.

The National Research Council of Canada, supported by the North American steelindustry, has developed innovative practical solutions for obtaining the required fire resistancefor HSS columns, without any external protection, through concrete-filling. Both experimentaland theoretical studies, using numerical techniques, were carried out at the NRC to investigatethe influence of three types of concrete-filling; namely, plain concrete (PC), bar-reinforcedconcrete (RC), and fibre-reinforced concrete (FC), on the fire resistance of HSS columns. Theresults of these studies were used to develop simple design equations for calculating the fireresistance of concrete-filled HSS columns. The calculation procedures were used to providedata on the necessary fire protection measures for the concrete-filled HSS columns used in theMuseum of Flight building in Seattle, WA, and a school building in Hamilton, Ontario.

EXPERIEMNTAL STUDIES

The experimental program consisted of fire tests on full-scale concrete-filled HSScolumns. Both square and circular HSS columns were tested and the influence of factors,including type of filling (PC, RC, FC), concrete strength, type and intensity of loading, andcolumn dimensions was investigated. During a test, the column was exposed, under a load, toheating controlled in such a way that the average temperature in the furnace followed, asclosely as possible, the North American standard temperature-time curve. The furnace,


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concrete and steel temperatures, as well as the axial deformations and rotations, were recordeduntil failure of the column occurred. Figure 1 shows a typical concrete-filled HSS columnimmediately after a fire resistance test.

Results from the tests showed that, at room temperature, the load on a concrete-filledHSS column is carried by both the concrete and the steel1. When the column is exposed to fire,

however, the steel carries most of the load during the early stages since the steel sectionexpands more rapidly than the concrete core. At increased temperatures, the steel sectiongradually yields as its strength decreases, and the column rapidly contracts sometime between20 and 30 minutes after initial fire exposure. At this stage, the concrete filling starts to take overand carries a progressively increasing portion of the load as the temperature rises. Thestrength of the concrete decreases with time and ultimately, when the column can no longersupport the load, failure occurs. The elapsed time that it takes for the column to fail is themeasure of its fire resistance.

The behaviour of concrete-filled HSS columns under fire conditions is illustrated inFigure 2 which shows the variation of the axial deformation with time for the three types ofconcrete-filling. The columns had similar dimensions and loading conditions and the results can

be used to illustrate the comparative fire behaviour of the three types of concrete filling.

As expected, the three columns expand in the initial stages and then contract leading tofailure. The deformation in these column results from several factors such as load, thermalexpansion and creep. While the effect of load and thermal expansion is significant in the earlystages, the effect of creep becomes pronounced in the later stages. It can be seen from thefigure that the deformation behaviour of the FC-filled steel column is similar, during the laterstages of the test, to that of the RC-filled steel column. The initial higher deformations in thefibre reinforced concrete-filled column might be due to higher thermal expansion of fibre-reinforced concrete. The fire resistance of RC-filled column is higher than that of FC-filledcolumn, which in turn is higher than PC-filled HSS column.

EFFECT OF CONCRETE-FILLING ON FIRE RESISTANCE

Results from the tests show that filling the column with plain concrete, without any steelreinforcement, offers the most economical arrangement from the point of view of fire resistance.However, in some cases, especially when the dimensions of the columns are large (323 mm ormore), PC-filled steel columns fail at relatively low loads when exposed to fire. These failurescan be attributed to early cracking initiated by strength loss in the steel casing at elevatedtemperatures1,2 and excessive local stresses in the concrete due to the reduction incompressive strength of the concrete at elevated temperatures.

In the bar-reinforced concrete-filled HSS column, the presence of rebars not onlydecreases the propagation of cracks and sudden loss of strength, but also contributes to the

load-carrying capacity of the concrete core1

. The fire resistances of these columns wereimproved significantly. However, there is the additional cost of steel, and installation of therebars in the column.

The use of fibre-reinforced concrete-filling in HSS columns provided better fire behaviourand resulted in fire resistance values which are comparable to those of RC-filled HSS columns.The load-carrying capacity of the column is also increased to a certain degree. This can beattributed to the fact that the compressive strength of fibre-reinforced concrete increases with


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temperature up to about 400C. The additional cost in the case of FC-filled columns, over thecost of concrete, is the cost of the steel fibres.

NUMERICAL STUDIES

The experimental data were used to validate the computer programs that were developedfor predicting the fire behaviour of concrete-filled steel columns. These programs can take intoaccount the influence of the various parameters that determine the fire resistance performanceof concrete-filled HSS columns. Using the computer programs, data can generated forobtaining alternate, but cost-effective designs (Figure 3)1,3. The computer programs were usedto conduct detailed parametric studies to establish the influence of various parameters on thefire resistance of HSS columns filled with concrete; it was found that the most importantparameters are:

type of concrete filling (plain, bar-reinforced, fibre-reinforced)

outside diameter, or the outside width, of the column

load on the column

effective length of the column

strength of the concrete

type of aggregate

eccentricity of load

DESIGN EQUATION

Data from the tests and the computer-simulated parametric studies were used todevelop a simple equation for calculating the fire resistance of circular and square HSS columnsfilled with any of the three types of concrete. It was possible to express the fire resistance ofthese columns, as a function of the parameters that determine it, by an unified equation:

R f(f 20)

(KL 1000)D

D

C

c

'

2=

+

(1)

where:

R = fire resistance in minutes

= specified 28-day concrete strength in MPafc'

D = outside diameter or width of the column in mm

C = applied load in kN

K = effective length factor as per CAN/CSA-S16.1-M89 Standard

L = unsupported length of the column in mm

f = a parameter to account for the type of concrete filling (PC, RC, and FC), the type ofaggregate used (carbonate or siliceous), the percentage of reinforcement, the thickness ofconcrete cover, and the cross-sectional shape of the HSS column (circular or square).The values of parameter f can be found in References 1 and 2, as can the limits of

applicability for the various parameters for the above equation.

The above design formula, which is based on the results of a large number of computer runsand was verified using the results of full-scale tests, can now be used for calculating the fireresistance of concrete-filled hollow steel columns. Further, the formula calculates fire resistanceas a function of such parameters as the sectional dimensions, load and material properties.Using such formulas, engineers can design the most economical structural members, with the


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required fire resistance, simply by varying the parameters. The fire resistance design can alsobe conveniently integrated with structural design since the fire resistance is expressed in termsof structural design parameters.

This type of design formula can be used as a tool to verify fire resistance for variousstructural members under the upcoming objective-based codes in Canada. Using this

approach, the fire resistance design can be carried out entirely on a rational basis and cost-effective measures can be derived in a consistent manner3.

The fire resistance equations evolving from these studies have been incorporated intothe National Building Code of Canada and other standards such as ASCE-29 ACI 216 and AISCfire standards.

PRACTICAL APPLICATIONS

The research described above was used to calculate the fire resistance of HSS columnsused in actual buildings. The practical applications included:

Museum of Flight, Seattle, WashingtonThe Museum of Flight at King County Airport is the largest air and space museum on the westcoast of the United States and is home to one of the most extensive aircraft collections in theworld. The 13,300 m2 (143,200 ft.2) museum is dominated by a six-storey-high Great Gallery,constructed as part of a three-part extension, and is composed of a main steel-and-glass exhibithall, a library, a 268-seat auditorium, and office and conference space. The irregularly shapedbuilding is 148 m (185 ft.) long, 76 m (250 ft.) wide, and 23 m (75 ft.) high.

The architectural concept adopted for the building was shaped by the need to naturally light theexhibits and visibility needs; the ability to see exhibits from outside (from the street and from theair), as well as to see the sky from inside to provide a natural background for aircraft suspendedfrom the ceiling (Figure 4). Further, the framing members (specially columns) had to be thin so

as not to distract from the exhibits, and not to generate any visible noise. Steel tubes wererecommended by the structural engineers for the columns. The Authority Having Jusisdictionhad adopted the Uniform Building Code which required a 60 minute fire resistance rating for thecolumns supporting the roof. To avoid the bulk and appearance of sprayed on fire protection, itwas decided to try concrete-filling. In addition, four reinforcing bars were added to the concreteto help maintain as small a profile as possible while providing the necessary loading carryingcapacity at the specified fire endure period.

Although the UBC did not specifically recognize the concept of concrete-filled HSS, the localBuilding Official accepted the test data and analytical work done by the National FireLaboratory. Using the same mathemetical model developed to conduct the simplified designequations above, the NRC staff showed that the bar-reinforced concrete-filled HSS columns

could provide a fire resistance rating of 60 minutes or higher under full design loads. Thus theabove research was instrumental in increasing the architectural beauty of Museum of Flightwithout compromising on fire resistance.

St. Thomas Elementary School, Hamilton, Ontario

Owned by the Hamilton-Wentworth Roman Catholic Separate School Board, St. ThomasElementry School is a typical two storey building with an interconnected floor. The open area


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contains two stair cases and is covered with a glass dome. The natural light and slendermembers give the space an airy look. To conform with the Ontario Building Code, the designersrequired the columns to have a one hour fire resistance rating. The National Fire Laboratorywas called upon to use their analytical techniques, and provided solutions using both squareand round HSS with different concrete strengths. The designers selected the more elegantround HSS columns and adopted bar-reinforcing for the ground level columns, in order to carry

higher loads, and plain concrete-filling for second storey supporting the roof. Figure 8 shows atypical two-storey school building, with concrete-filled steel columns, in Hamilton, Ontario.

SUMMARY

Concrete-filling offers an attractive practical solution for providing fire protection to hollowstructural steel columns without any external protection. Results from the experimental andnumerical studies indicate that fire resistance, in the practical region, can be obtained for HSScolumns through three types of concrete-filling. Fire protection of hollow steel columns, throughconcrete-filling, has increased the architectural beauty of the Museum of Flight and hasprovided an efficient design option for a school building.

REFERENCES

1. Kodur, V.K.R., and Lie, T.T. 1995,. Fire Performance of Concrete-filled Hollow SteelColumns. Journal of Fire Protection Engineering, 7(3): 89-98.

2. Kodur, V.R.; MacKinnon, D.H. "Fire endurance of concrete-filled hollow structural steelcolumns" AISC Steel Construction Journal, 37(1), 13-24, 2000.

3. Kodur, V.R. "Performance based fire resistance design of concrete-filled steel columns" Journaof Constructional Steel Research Institute, 51, 21-36, 1999.


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List of Figures

Figure 1 Concrete-filled HSS column immediately after a fire resistance test.

Figure 2. Axial Deformation in Concrete-filled HSS Columns as a Function of Fire ExposureTime

Figure 3. Effect of Concrete-filling on Calculated Fire Resistance of HSSColumns

Figure 4. Display of Aircraft in the Museum of Flight Building

Figure 5. Elevation of Concrete-filled Steel Column Used in Hamilton School Building


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Figure 1 Concrete-Filled HSS Column Immediately after Fire Resistance Test


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Figure 2. Axial Deformation in Concrete-filled HSS Columns as a Function of Fire Exposure

Time


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Figure 3. Effect of Concrete-filling on Calculated Fire Resistance of HSS Columns


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Figure 4. Display of Aircraft in the Museum of Flight Building


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Figure 5. Elevation of Concrete-filled Steel Column used in Hamilton School Building

fire resistance through concrete filling

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