DESIGN OF PRESTRESSEDCONCRETE FOR FIRERESISTANCEArmand H. GustaferroConsulting EngineerThe Consulting Engineers Group, Inc.Glenview, Illinois

Presents an introduction to rational methods forcalculating the fire resistance of prestressed concretestructures.Reviews current fire test methods and shows howdesign methods are developed using laboratory tests.Specifically, fire endurance is estimated for simplysupported and continuous prestressed concrete slabsand beams.Data are given in the case of fire under the floor of aninterior bay.Also, the important effects of restraint on amember are discussed.A study of heat transmission through floor androof slabs is included.There is also some data on two-course assemblies.It is concluded that in the near future structuraldesigners will have the necessary data to designprestressed concrete structures with any desireddegree of fire resistance.

The purpose of this paper is to presentan introduction to a subject that we re-fer to as "rational (analytical) methodsfor calculating the fire resistance ofconcrete structures."

During the past several years thePortland Cement Association (PCA)and other laboratories around the world

have conducted major research projectson fire resistance of concrete. Much ofthe data that have been developed willpermit a structural engineer to design abuilding for fire resistance in about thesame manner as he now designs forgravity loads, wind, or earthquake.

A logical question may be asked:

102

Time in Hours

"Why burden a structural engineerwith the added task of designing forfire resistance?" The answer to thatquestion is not obvious, but I think thatthere are several reasons for developingrational design procedures.

First, the methods now used for as-signing fire resistance classifications arebeing seriously challenged. Thesemethods are based on results of stan-dard fire tests of building materials andconstructions, i.e., ASTM E-119.

Nevertheless, many building officials,testing agencies, as well as architectsand engineers have become disillu-sioned with the classifications derivedfrom standard fire tests. In fact, ASTMCommittee E-5, which is responsiblefor E-119, has been trying to revise thestandard to make it more realistic.However, the committee appears to befacing an insurmountable task becauseno single standard fire test can accu-rately reflect the behavior of an assem-bly under the many different ways theassembly can be used in buildings.

Secondly, it is often possible to pro-vide added fire resistance more eco-nomically through structural designthan by other more conventional meth-ods. An illustration of this came tolight in a large government office build-ing in Washington, D.C. By using someof the data that have been developed,the structural engineer was able to in-crease the fire endurance of 600,000 sqft of prestressed concrete double teesfrom 1 to 2 hrs for less than 50 per sqft.

REVIEW OF FIRE TEST METHODS

It might be useful to begin by re-viewing briefly, the standard fire testmethods for materials and constructionsused in the United States and Canada(ASTM E-119). Specimens of floors,roofs, beams, columns, and walls, hav-ing dimensions exceeding certain spec-ified minimum values, are subjected to

W

dd

dV

3aaE

Fig. 1. Furnace time-temperaturecurve (ASTM E-119).

a standard fire while supporting theirdesign loads. For example, floor or roofspecimens must be representative of theconstruction they simulate and the areaexposed to fire must be at least 180 sqft, with neither dimension less than 12ft.

Throughout the fire test, the speci-men must support gravity loads equiv-alent to the maximum permissible loadapplied in the building which the spec-imen represents. The underside of afloor or roof is exposed to a "standard"fire. The fire is standard by virtue ofthe fact that the temperature of the fire(measured 12 in. from the specimen)is specified in terms of a time-tempera-ture curve (see Fig. 1). The fire endur-ance of a specimen is defined as theelapsed time during a fire test until anend point is reached.

Prior to 1969 the end point criteria(ASTM E-119) for floors and roofswere:

1. The specimen must support its de-sign load (structural end point).

2. Flames or gases hot enough to ig-nite cotton waste must not pass throughthe specimen (flame passage endpoint).

3. Unexposed surface temperaturemust not rise more than 250 F average,or 325 F maximum at any point (heattransmission end point).

PCI Journal/November-December 1973 103

Fig. 2. PCA control roomfor test furnaces.

In 1969 additional end point criteriawere tentatively included in ASTM E-119. These criteria deal with steel tem-peratures and are discussed under thesection "Recent Developments."

To study the behavior of concretestructures subjected to fire, the PCAbuilt a Fire Research Laboratory in1958 in Skokie, Illinois. It is probablythe largest and best equipped labora-tory in the world devoted exclusively toresearch on fire resistance of concrete.The laboratory houses three fire re-search furnaces.

Furnaces are operated from a controlroom, shown in Fig. 2. Fire tests areprogrammed and controlled from theside of the console shown. Output dataare printed by a number of strip chartrecorders located behind the control

panel. Furnace temperatures, hydraulicpressures, and furnace draft pressuresare controlled automatically or manual-ly from the control room.

Fig. 3 shows PCA's beam furnace.The fire chamber is 40 ft long and 6 ftwide. Natural gas is burned for fuel.Loads are applied with hydraulic rams.By moving the end walls and using onlycertain burners, it is possible to vary thelength of the fire chamber between 10and 40 ft.

FIRE TESTS OF SIMPLYSUPPORTED SLABS

Let us now turn to a specific researchproject which serves to illustrate howdata for rational design methods are de-

Fig. 3. Furnace for firetesting beams.

104

veloped. This project dealt with fire en-durance of simply supported pre-stressed concrete slabs. Fig. 4 shows asimply supported concrete slab, eitherreinforced or prestressed (note therocker and roller bearings).

Assume that a fire is applied to theunderside of the slab. As soon as thefire starts, the underside of the slab willbecome warmer than the top, and willtend to expand more than the top. Theslab will deflect toward the fire, and thedeflection will increase until the tem-perature gradient through the slab sta-bilizes, i.e., until the temperature dif-ferential between the top and bottomremains constant.

Gradually the temperature of the re-inforcement will increase and thestrength of the steel will be reduced.When the steel strength approaches thesteel stress, the deflection will increaserapidly and a structural end point willoccur.

Strength of prestressing steel athigh temperatures

Fig. 5 shows the effect of tempera-ture on the strength of cold-drawn pre-stressing steel. At room temperature,the strength is shown as 100 percent.The shape of this curve is typical ofthose of other steels. However, hot-rolled steels have somewhat higher rel-ative strengths at temperature aboveabout 600 F.

Strength of prestressed membersduring fire test

The moment capacity of a pre-stressed concrete beam or slab can becomputed from the formula:

aM. =Ap8fps( d— (1)

in whichAP8 = area of prestressing steelfr8 = stress in the steel at ultimated = distance between the steel

centroid and the extreme fiber

PCI Journal/November-December 1973

^'rSSX^^^u

Fig. 4. Simply supported slab subjectto fire from underneath.

a = depth of an equivalent com-pressive stress block at ulti-mate

Values of f2,, and a can be calculatedby the formulas in ACI 318-71. Eq. (1)is the same formula that appears inthe Commentary to ACI 318-71 withcp (capacity reduction factor) equal toone. When the temperature of the steelis increased to some temperature 0,values of f,8 and a must be reduced toreflect the new temperature. M

the moment capacity with the steel attemperature 0, and is termed the "re-tained" or "residual" moment capacity.Theoretically, during a fire test, astructural end point will occur whenthe residual moment capacity is re-duced to that of the applied moment.

Fig. 6 shows moment diagrams for a

100

fpu9 75%of 50

fp u25

070 300 600 9006, TEMPERATURE,°F

Fig. 5. Strength-temperature relationof cold-drawn prestressing steel.

105

MOMENT CAPACITY, Mu

APPLIED MOMENTM D, IL Mu=Apsfps ( d _ 2)

Steel at 70°FM U e 08

Mue ApsfPs9 ( d- 2Steel at 9°F

Fig. 6. Moment capacityof a simply supportedconcrete slab subject to

fire.

simply supported slab with a uniformlydistributed load. The applied momentdiagram is parabolic, representing auniform load. With straight and uni-form prestressing steel, the moment ca-pacity diagram is a horizontal line witha height of M. During a fire test, thesteel will become heated and itsstrength will be reduced. Simultane-ously, the moment capacity will be re-duced. It appears reasonable to assumethat when the M se line reaches theapplied moment diagram, the slab willfail.

Fire tests to verify theoryTo verify the theory, a series of pre-

stressed concrete slabs (Fig. 7) werefire tested while simply supported onspans of either 12 ft or 20 ft. All slabswere 27.4 in. wide and 6.5 in. thick.

Some specimens were prestressed withfive %i6-in. diameter strands and otherswith fifteen 1/4-in. diameter strands. Allstrands in a specimen had the same cov-er, 1, 2, or 3 in. Some specimens weremade of normal weight concrete andothers of lightweight concrete. Load in-tensities (MD + L) during the testsranged between 40 and 60 percent ofthe calculated ultimate capacities. Dur-ing the fire tests, the steel temperatureswere monitored. The temperature ofthe steel when collapse was imminentwas used in calculating the residualmoment capacity.

A comparison of calculated residualmoment capacity and applied momentis shown in Fig. 8. Note that the valuesare almost equal. This clearly illustratesthat the moment capacity during a firecan be predicted and that behavior dur-

P P P P APi

IF -- 12' or 20' Au= COVER = I", 2", or 3"

STRANDS, 6V2^^5-- 746-in.

OR --27.4"15-- /4 -in.ҟSEC. A-A

Fig. 7. Details of simply supportedprestressed concrete slab specimens.

300

APPLIEDҟ200MOMENT

M D*Lҟl00< M DI Mue

in-kips• 0 100 200 300

CALC. RESIDUAL MOMENTM U e in -kips

Fig. 8. Applied moment versus cal-culated moment capacity at end of

fire tests.

106

ing fires follows basic engineering prin-ciples.

As noted above, steel temperatureswere monitored throughout the firetests. Fig. 9 shows temperatures for 1,2, and 3-in, cover of the normal weightconcrete specimens. Similar data wereobtained for lightweight concrete. If,for a particular load intensity, the "criti-cal" steel temperature is 900 F and ifthe cover is 2 in., the fire endurance ofa simply supported prestressed concreteslab would be slightly less than 3 hr.Similarly, it is possible to estimate thefire endurance for various cover thick-nesses and load intensities.

Design aids

The graphs in Fig. 10 were preparedto assist designers in estimating fire en-durance of simply supported pre-stressed concrete slabs. These graphsshow the inter-relations among load in-tensity, cover thickness, fire endurance,and concrete type. Note that fire endur-ance is improved by decreasing loadintensity or increasing cover. Also, forthe same cover and load intensity, fireendurance is longer for lightweight con-crete than for normal weight concrete.

One may ask, "Why be concernedwith load intensity if ASTM E-119 re-quires that a specimen support its de-sign load during a fire test?" By usingrational design methods, the effect ofload intensity can be evaluated. In cer-tain cases, such as precast prestressedslabs, it may be more economical toprovide additional load capacity by us-ing more strands than to provide addi-tional cover. By evaluating the effectsof cover and load, an economical andrational solution can be obtained.

Fig. 10 shows how results of fire testscombined with rational design proce-dures can be combined to form rela-tively simple design aids. Modified de-sign aids can be used for simply sup-ported beams.

900

//

o^STEEL 700 ^o

TEMPERATURE, ti

°F

//500

30 60 90 120 180 240

TEST TIME, minutes

Fig. 9. Steel temperatures in normalweight concrete specimens during fire

tests.

FIRE TESTS OFCONTINUOUS BEAMS

Let us now turn to a more complexcase, namely, that of continuous beams.Fig. 11 shows a portion of a continuousbeam, depicting the location of theprincipal reinforcement. Note that atmidspan the reinforcement is locatednear the bottom of the beam while overthe supports, the reinforcement is nearthe top. Let us assume that this type ofconstruction continues for several addi-tional bays. Let us also assume that afire occurs beneath one span as shown.

As the underside of the beam heats

3 4hr/

COVER

4 hr.3

Z /2 2THICKNESS,

in.ҟINormal Wt LightweightConcrete Concrete

Cold-drawnҟ00.3 0.5 0.7 0.3 0.5 0.7

wire or strandҟLOAD INTENSITY, MD,LMD

Fig. 10. Influence of cover thicknessand load intensity on fire enduranceof simply supported prestressed con-

crete slabs.

PCI Journal/November-December 1973ҟ 107

T' iFig. 11. Continuous reinforced concretebeam subject to fire from underneath.

up, the heated span tends to deflect.This deflection is accompanied by a ro-tation of the beam over the supportsand since the adjacent spans are con-tinuous, the upward deflection is re-sisted.

To visualize the behavior, considerthat the portion of a beam shown inFig. 11 was subjected to a fire test. Toprevent the "cantilever" ends from ris-ing, additional load would have to beapplied at the ends of the cantilevers.Additional load would increase the mo-ment over the supports, but since theload between the supports is un-changed, the midspan moment will de-crease. This change in moments is oftencalled redistribution of moments.

Moment diagramsIn terms of moment diagrams, Fig.

12 shows the behavior during a fire of acontinuous concrete beam. This is, ofcourse, a simplified case, with straight

continuous bars top and bottom. Notethat prior to fire, the moment capacitiesare about twice the applied moments.

During a fire some important chang-es in moments and moment capacitiesoccur;

1. Redistribution of moments in-creases the moments over the supportsand decrease the moment at midspan.

2. The effect of fire decreases thepositive moment capacity much morerapidly than it decreases the negativemoment capacity because the top bars,on which the negative moment capacityis dependent, are well protected fromthe fire.

Thus, the fire endurance of a contin-uous beam is generally much longerthan that of a simply supported beamwhich has the same cover thickness.Furthermore, it is apparent that thetemperature of the reinforcement isonly of secondary importance.

Fire tests on continuous beamsA series of tests was initiated at the

PCA to investigate the behavior of con-tinuous beams exposed to fire. To simu-late continuity in beams, the specimen(Fig. 13) had cantilever ends project-ing beyond the fire exposed span whichrepresented portions of the adjacentspans.

The principal reinforcement consist-

A5.

AS2

—MҟFig. 12. Moment capacitySTEELҟ of a continuous reinforced

a 'ҟ aҟconcrete beam subject to70°Fҟ Mҟ+Mu`As2fy(d- —)

o+L fire.

A Z STEEL MD+L Mu°tҟ /ҟae

g °Fҟ +MuB=AS,(d- 2)

108

TEST TIME, HR.

ed of straight No. 6 bars. The four cor-ner bars extended through the lengthof the specimen but the others werecutoff at various locations. Two top barswere cut off 2 ft 2 in. from the supportsand two others 3 ft 6 in. from the sup-ports. Two bottom bars were cut off 4ft 2 in. from the supports.

One specimen was tested as a simplysupported beam, i.e., the cantileverloads Pi and P3 were omitted. The ap-plied moment (dead plus live load)was equal to 50 percent of the calcu-lated ultimate moment capacity at mid-span and the fire endurance was about1 hr 25 min.

In the second test, loads were ap-plied on the cantilevers as well as mid-span so that the resulting applied mo-ments were 50 percent of the ultimateover the supports as well as at midspan.In the first test (simple support) theP2 loads were each about 4½ kips. Inthe second test, the P2 loads were 111/4kips, and the cantilever loads at the be-ginning of the test were 13% kips.

During the tests the cantilever ends(Points A and B) were kept at a con-stant elevation by changing the loadsPl and P. This was done to simulatethe behavior of a continuous beam sub-jected to fire in one span.

Fig. 14 shows the changes in canti-lever loads during the test. Note thatearly in the test, Pl and P3 increasedsharply and then leveled off. Note alsothat P2 loads were kept constant. Youwill also note that the fire test was con-tinued for 3 1/2 hrs.

The moment diagrams in Fig. 15show graphically the behavior of thespecimen during the fire test. At the be-ginning of the test, the maximum ap-plied moments were one-half the ulti-mate moment capacities. Note that themoment capacity diagrams are stepped.These steps are shown at the cutoffpoints, and do not take into account thereduction in moment capacity withinthe bar anchorage length.

P^ P2 P2 P2 P2 P3

A B

L.-FIRE EXPOSURE= IBS

5-+------------- j - 5IFI2"^

I j

I BARS" COVER

AT SUPPORT AT MIDSPAN

Fig. 13. Details of experimental beamsubject to fire exposure.

Note also that at 3'/z hrs the appliednegative moment has greatly increased,and the applied positive moment hasdecreased. The negative moment ca-pacity had not decreased very much,but the positive moment capacity wasapproaching the applied positive mo-ment. The test was stopped when themidspan deflection began to increaserapidly.

It is of interest to note that theamount of redistribution that occurredwas limited by the length of the cutoffbars. If the two top bars that were cut-off 3 ft 6 in. from the supports had ex-tended another foot or so, there wouldhave been a greater redistribution ofmoments, and there would have been

LOADIN

KIPS

Fig. 14. Variation of cantilever loadwith time for beam shown in Fig. 13.Change in Pl and P3 to keep points

A and B at constant elevation.

PCI Journal/November-December 1973ҟ

109

At 3%2 HR

-ISOa -100

-500

+50•100

z-1500

f -100- 50

050

Fig. 15. Moment diagrams of speci-men shown in Fig. 13.

little or no positive moment at midspan.In that case, the fire endurance mighthave been extended an additional 1 or2 hrs.

The important lesson learned fromthese tests is that a designer can pro-vide for any required fire resistance ofcontinuous members through the appli-cation of established engineering prin-ciples.

Design aidsFrom the data we are developing on

continuous members, we believe that itwill be possible to develop design aidssimilar to that in Fig. 16 for estimating

fire endurance. Note that Fig. 16 re-sembles the design aid for simply sup-ported prestressed concrete slabs (Fig.10). However, it deals with reinforcedconcrete slabs and the moment intensityratio extends from 0 to 0.7.

If the moment intensity for a simplysupported slab is 0.5 and the cover is3/4 in., the fire endurance would beabout 1 1/2 hrs. If the slab is continuous,and if after redistribution the momentintensity ratio is 0.25, the fire endur-ance would be more than 3 hrs.

We believe that charts such as Fig.16 will enable the designer to estimatethe amount of redistribution requiredfor any required fire endurance. It willalso permit the designer to evaluatewhether an increase in cover or an in-crease in redistribution of moments willbe more economical.

FIRE UNDER FLOOR OFINTERIOR BAY

Let us now proceed to a still morecomplex problem—that of a fire beneaththe floor in the interior of a large build-ing, as shown in Fig. 17. As the portionof the floor becomes heated, it tends toexpand. However, this expansion is re-

COVER,INCHES

-1Z

Hot-Rolled ReinforcementNormal Weight Concrete

w -0.17

4 Hr.

3

2

00ҟ0.2ҟ0.4ҟ0.6

S6MD+L AFTER REDISTRIBUTIONa

Fig. 16. Influence ofcover and moment inten-sity on fire endurance ofreinforced concrete slabs.

110

listed, or restrained, by the adjoiningconstruction.

Section 23(b) of ASTM E-119 im-plies that a floor or roof assemblyshould be restrained during a fire testin the same manner in which the flooror roof is restrained in the constructionthat the specimen represents. Comply-ing with that requirement has been, atbest, most difficult since the magnitudeand nature of such restraining forceshave not been understood.

To study the nature of restraint inconcrete floors and roofs during fires,the PCA built a unique floor furnace.The cut-away drawing of the floor fur-nace (Fig. 18) shows the main compo-nents. The specimen measures 14 x 18ft. A simulated uniform live load is ap-plied through 16 hydraulic rams. Thefire chamber is located below floor leveland natural gas is burned as fuel.

What makes the furnace unique arethe elements on the four sides of thespecimen which can serve to supportthe specimen and provide restraint tothermal expansion. The four elementsbear against horizontal hydraulic ramsand are free to rotate or move in thedirection of the rams. As a specimen isheated it tends to expand. To restrain

O

O

Fig. 17. Fire under floor of an interiorbay of a multi-bay building.

the expansion the pressures in the hy-draulic rams must be increased.

By knowing the hydraulic pressures,it is possible to calculate the magnitudeof the restraining force. The capacity ofthe restraining system is about 1,100,-000 lb along the 14-ft dimension, andabout 1,500,000 lb along the 18-ft di-mension.

Fig. 19 shows the restraining systemdiagrammatically. The south and westsupporting elements are actuated bytwo sets of hydraulic rams, upper rams,referred to as restraint rams (R) and

Fig. 18. Cutaway view of

PCA floor furnace.

PCI Journal/November-December 1973 1 _i

TEST SPECIMEN

R-1R

M 18 FT. M

Fig. 19. Schematic drawing of re-straining mechanism in PCA floor fur-

nace.

10in.

3/g-in. 4strands

Fig. 20. Test specimen used to studyeffects of restraint during fire tests.

lower rams, called moment rams (M).By knowing the magnitude and locationof R and M (the upper and lower ramsare 2 ft apart) it is possible to calculatethe magnitude and location of the re-sultant restraint force, or "thermalthrust."

When PCA's floor furnace was firstbuilt, three floor specimens were firetested in a series of shake-down tests.The first specimen was tested withoutend restraint, i.e., it was simply sup-ported and the structural end point wasreached at about 1 hr and 20 min. Thesecond specimen was supposed to betested "fully restrained," i.e., no expan-sion was to be allowed.

Very early in the test the capacity ofthe restraining system was reached, andat 40 min. a compression type of failureoccurred within the slab resulting in ahole through the slab.

600RESTRAINING

, Ta, 400 — Normal WeightFORCE, C

kips 200

oncrete

XX

0

0ҟ0.2ҟ0.4 0.6

EXPANSION e, %

Fig. 21. Maximum measured restrain-ing force versus expansion for pre-

stressed concrete specimens.

In the third test, an "intermediate"degree of restraint was attempted. Dueto a malfunction of the restraining sys-tem, quantitative data were not ob-tained, but after 4 hrs the specimen wasstill supporting its load with little or novisible deflection or damage.

EFFECTS OF RESTRAINTDURING FIRE

To study quantitatively the effects ofrestraint during fire tests, a series ofspecimens shown in Fig. 20 were castand tested. Some of the specimens werepretensioned with 3/s in. strand asshown here; others were reinforcedwith bars. Some specimens were madeof lightweight concrete and others ofnormal weight concrete.

Live load was applied to the speci-men and the fire test begun. Each spec-imen was permitted to expand a givenamount (ranging from 0.04 to 1.40 in.in 18 ft) and further expansion wasprevented. After expansion wasstopped, the restraining force (or ther-mal thrust) increased to a maximumvalue and then either diminished or re-mained relatively constant.

Fig. 21 shows the maximum mea-sured restraining forces for the pre-stressed concrete specimens. Note thatfor small expansions these forces arevery large. A small increase in expan-sion is accompanied by a large reduc-

112

tion in restraining force. Note also thatthe forces are larger for normal weightconcrete than for lightweight concrete,probably because both the modulus ofelasticity and coefficient of expansionare greater for normal weight concrete.What is most significant, however, isthat all of these tests were conductedfor more than 3 hrs, despite the widerange in allowed expansions.

Fig. 22 summarizes the effects of re-straint on fire endurance in a qualita-tive manner. Zero percent of full re-straint means a simply supported condi-tion where thermal expansion cannot berestricted. In that case, a structural endpoint will occur when the moment ca-pacity is reduced to the applied mo-ment.

With 100 percent restraint, a com-pression type of failure will occur.However, it is doubtful that full re-straint could ever occur in a real build-ing because the forces involved are solarge that some movement will occur,and that movement will be accompa-nied by a great reduction in the re-straining force.

Between no restraint and full re-straint, almost any degree of restraintwill be sufficient to increase the endur-ance enough that the heat transmissionend point will occur. So the amount ofrestraint determines the type of endpoint; in Zone 1, structural end point;in Zone 2, heat transmission; and inZone 3, flame passage. Most concretefloors lie in Zone 2.

HEAT TRANSMISSIONTHROUGH SLABS

To study the heat transmissionthrough floors and roofs, the PCA hasconducted a large number of tests on itssmall slab furnace. Data on heat trans-mission through 3 x 3 ft concrete slabscorrelate well with data from full-scaletests.

The graph in Fig. 23 was developed

FIREENDURANCE

PERIODҟeat Trans.

End Point

0'ҟJ0ҟ100% OF FULL RESTRAINT

Fig. 22. Effect of restraint on fire en-durance of beams and slabs.

INSULATING4 CONCRETE

FIRE L/w

ENDURANCE,

hr. 2 /w

^I 3 5 7SLAB THICKNESS, in.

Fig. 23. Fire endurance of concreteslabs as determined by criteria fortemperature rise of unexposed surface.

I IV SU L AT I NG :.GONG::;4 ':NW„CONCRETE e-

OVER LAYTHICKNESS, 4 Hr.

in.

I 3 5 7BASE SLAB THICKNESS, in.

Fig. 24. Typical design aid for de-termining fire endurance of two-courseassembly (normal weight concretebase slab with insulating concrete over-

lay).

PCI Journal/November-December 1973 113

from a series of tests conducted on thesmall slab furnace. The relation be-tween slab thickness and fire enduranceis shown for three types of concrete,normal weight (145 lb per cu ft), struc-tural lightweight (110 lb per cu ft),and insulating concrete (30 lb per cuft dry). Minor differences occur fordifferent normal weight concretes—car-bonate aggregate concrete gives slight-ly longer endurance periods than silice-ous aggregate concrete.

The fire endurance of lightweightand insulating concretes is influencedlargely by two factors, unit weight andmoisture content. A decrease in unitweight or an increase in moisture con-tent is accompanied by an increase infire endurance.

FIRE TESTS OF TWO-COURSEASSEMBLIES

There has been a need for data onfire endurance of two-course floors androofs. To fulfill this need, the PCA con-ducted a number of fire tests of vari-ous combinations of materials. From theresults, design aids such as the oneshown in Fig. 24 were developed. Thedata apply only to the heat transmis-sion criteria.

The design aid (Fig. 24) is only ap-plicable for a two-course assembly con-sisting of normal weight concrete baseslab with an insulating concrete over-lay. For a 3-in, base slab with a 2-in.overlay, the fire endurance is about 3hrs.

Design aids were also developed forcombinations of normal weight andlightweight concretes, concrete slabsundercoated with insulating materials,concrete roofs with rigid board insula-tion, and terrazzo floors.

RECENT DEVELOPMENTS

During the past 13 years, the PCI

has sponsored an extensive series of firetests at Underwriter's Laboratories. Inaddition, a number of manufacturershave sponsored fire tests not only atUL, but at other laboratories. As a re-sult, fire ratings for a wide variety ofprestressed concrete sections are listedby UL, the American Insurance Asso-ciation, and in building codes. Listingsinclude double-tees, single-tees, mono-wing slabs, hollow-core slabs, invertedtee beams, I-beams, joists, and solidslabs.

The 1970 version of ASTM E119contains a tentative revision which, ineffect, adds new end point criteria tothose already specified. The revisionimposes temperature limits of 1100 Ffor structural steel and hot-roIIed rein-forcing bars, and 800 F for cold-drawnprestressing steel in unrestrained as-semblies.

For restrained beams spaced morethan 4 ft on centers, the same tempera-ture limits must not be exceeded duringthe first half of the fire endurance pe-riod. There are no temperature limitsfor the steel in restrained slabs or beamsspaced 4 ft or less on centers.

The same tentative revision also in-cludes an appendix which is intendedto serve as a guide for determiningwhether an assembly is restrained orunrestrained.

Underwriters' Laboratories includesthe dual ratings, i.e., restrained and un-restrained, in the Fire Resistance Index.The January 1973 issue lists about 80prestressed precast concrete designsthat qualify for various fire resistanceratings.

SUMMARY

The structural fire endurance of pre-stressed concrete is affected mainly bythe method of framing—simply sup-ported and unrestrained, continuous, orrestrained against thermal expansion.

For simply supported and unre-

114

strained members, the cover to the re- 4.inforcement and the load intensity gov-ern the fire endurance.

For continuous beams and slabs, theamount and location of negative andpositive moment reinforcement deter-mine how much moment redistributionwill occur, which in turn determinesthe fire endurance. 5.

For restrained members, structuralfire endurance is affected only by themember's ability to withstand the re-straining forces, and this is seldom aproblem.

The fire endurance of restrained 6.floors or roofs is generally governed bythe criteria for temperature rise of thetop surface.

Within the past few years, we havemade great strides in developing datafor use in rational methods of designingfor fire resistance. I am optimistic that

within a few years structural engineers 7.will have the data necessary to designconcrete structures for any required de-gree of fire resistance.

SELECTED BIBLIOGRAPHY

1. ASTM E-119-71, "Standard Meth-ods for Fire Tests of Building Con-struction and Materials," Part 14,

ASTM Book of Standards, Ameri- 8.can Society for Testing and Mate-rials, Philadelphia, Pennsylvania.

2. Carlson, C. C., "Function of theNew PCA Fire Research Labora-tory," PCA Research DepartmentBulletin 109, Portland Cement As-sociation, Skokie, Illinois.

3. Abrams, M. S., and Cruz, C. R., 9."The Behavior at High Tempera-ture of Steel Strand for PrestressedConcrete," Journal of the PCA Re-search and Development Laborato-ries, Vol. 3, No. 3, September1961, pp. 8-19; PCA Research De-partment Bulletin 134, PortlandCement Association, Skokie, Illi-nois.

Gustaferro, A. H., and Selvaggio,S. L., "Fire Endurance of SimplySupported Prestressed ConcreteSlabs," PCI JOURNAL, Vol. 12,No. 1, February 1967, pp. 37-52;PCA Research Department Bulle-tin 212, Portland Cement Associa-tion, Skokie, Illinois.Gustaferro, A. H., "TemperatureCriteria at Failure," Fire Test Per-formance, ASTM STP 464, Ameri-can Society for Testing and Mate-rials, Philadelphia, Pennsylvania,1970, pp. 68-84.Ehm, H., and von Postel, R., "Testsof Continuous Reinforced Beamsand Slabs Under Fire," Proceed-ings, Symposium on Fire Resis-tance of Prestressed Concrete,Translation available at S.L.A.Translation Center, John Crerar Li-brary, Chicago, Illinois.Selvaggio, S. L., and Carlson,C. C., "Effect of Restraint on FireResistance of Prestressed Con-crete," Fire Test Methods, ASTMSTP No. 344, American Society forTesting and Materials, Philadel-phia, Pennsylvania, 1962; PCA Re-search Department Bulletin 164,Portland Cement Association, Sko-kie, Illinois,Issen, L. A., et al., "Fire Tests ofConcrete Members: An ImprovedMethod for Estimating RestraintForces," Fire Test Performance,ASTM STP 464, American Societyfor Testing and Materials, Philadel-phia, Pennsylvania, 1970, pp. 153-185.Abrams, M. S., and Gustaferro,A. H., "Fire Endurance of Con-crete Slabs as Influenced by Thick-ness, Aggregate Type, and Mois-ture," Journal of the PCA Researchand Development Laboratories,Vol. 10, No. 2, May 1968, pp. 9-24; PCA Research DepartmentBulletin 223, Portland Cement As-sociation, Skokie, Illinois.

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10. Abrams, M. S., and Gustaferro,A. H., "Fire Endurance of Two-Course FIoors and Roofs," ACIJournal, Vol. 66, No. 2, February1969, pp. 92-102.

11. Gustaferro, A. H.; Abrams, M. S.;and Litvin, A., "Fire Resistance ofLightweight Insulating Concretes,"ACI Special Publication 29, Light-weight Concrete, American Con-crete Institute, Detroit, Michigan,1971, pp. 161-180.

12. "Fire Resistance Index," Under-writers' Laboratories, Inc., North-brook, Illinois, January, 1973.

13. "Fire Resistance Ratings," Ameri-can Insurance Association, NewYork, New York.

14. Gustaferro, A. H., and Carlson,C. C., "An Interpretation of Re-sults of Fire Tests of PrestressedConcrete Building Components,"PCI JOURNAL, Vol. 7, No. 5, Oc-tober 1962, pp. 14-43.

Note: This paper was initially presented at the 37thAnnual Convention of the Structural Engineers Associationof California when the author was manager of the PortlandCement Association's Fire Research Laboratory. The texthas been modified to bring the information up-to-date.

Discussion of this paper is invited.Please forward your discussion to PCI Headquartersby April 1, 1974, to permit publication in theMay-June 1974 PCI JOURNAL

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