bearing capacity tests on precast concrete thin-wall ... journal...refabricated thin-wall ribbed...

Bearing Capacity Tests onPrecast Concrete Thin-Wall

Ribbed Panels

Zenon A. ZielinskiProfessor of EngineeringConcordia UniversityMontreal, Quebecand President,UCOPAN Enterprises Inc.

Michael S. TroitskyProfessor of EngineeringConcordia UniversityMontreal, Quebec

Elie EI-ChakiehStructural EngineerLecompte Engineering Ltd.Consulting EngineersOttawa, Ontartio

88

refabricated thin-wall ribbed rein-forced concrete modular panels are

widely used in the building industry.For example, panels under the name ofUCOPAN (Universal Concrete Panel)have been used for low and moderatecost housing construction. 1 ' 2 •3 In thissystem the same standard modularpanels are used as flexural elements forboth floors and roofs, and as loadbear-ing components for walls and partitions.

In the past several years, the strengthand behavior of thin-wall ribbed panelshave become the subject of ongoing re-search work at the structural laboratoryof the Civil Engineering Department atConcordia University in Montreal.

A series of tests were conducted ear-lier on full-scale panels working asbeam panels (deep beams).'' s Thestrength and behavior of such panelswere studied using both conventional¢and finite element analysis5 methods.

Later research work includedstrength tests of panels working asuniformly loaded bearing walls. Aseries of tests and strength analysis wereconducted on full-scale loadbearingpanels of unsymmetric channel shaped(U) cross section, 6 identical to thoseused for low cost housing.1.3

The purpose of this research was tofurther investigate the bearing capacityof full-scale wall panels. However, thetest models used in this research werespecially designed. Unlike the previousresearch, these panels had a thin wallmembrane of 1.5 in. (38 mm) thicknessand were stiffened with perimeter ribsplaced across the center of the mem-brane to form a ribbed wall of symmet-ric I-shape cross section with very thinweb. The overall behavior and ultimatebearing capacity of the wall, the stabil-ity of the thin membrane, and the ap-plicability of the ACI Code provi-sions 7 for conventional walls werestudied in this research program. Theinvestigation was limited to wall panelsof the following proportions:

1. Ratio of membrane thickness to

overall panel width (approxi-mately equal to spacing of verticalrib): tlw = 1/30.

2. Ratio of overall length of side rib(equal to overall panel height) todepth of rib (equal to maximumwall thickness); l,lh = 15.

3. Ratio of depth of rib to membranethickness: hit = 4.

EXPERIMENTAL PROGRAMThe panels tested in this research

program, as shown in Fig. 1, had thefollowing dimensions:

Overall height l = 89 in. (2260 mm)Widthw = 44 in. (1120 mm)Membrane thickness t = 1.5 in.

(38 mm)The 3 x 6 in. (76 x 152 mm) perimeter

rib provided each panel with the re-quired stiffness during transportation,erection, and serviceability as a load-bearing wall.

The reinforcement consisted ofwelded wire mesh 6 x 6-6/6 (152 x152—MW 18.7 x MW 18.7) of yieldstrength 78.4 ksi (541 MPa) in themembrane, and 2 #3 (2 #10 M) of yieldstrength of 60.1 ksi (414 MPa) in theperimeter rib.

These panels were produced in 1975;some had been used in earlier researchas beam panels. 5 Three panels weretested in this program as bearing wallelements in a steel frame, as shown inFig. 2. A line grid was drawn on thesurface of each panel to facilitate theobservation of the exact location ofcracks occurring during the loading.Hard masonite boards, 6 x 1/4 x 44 in.(152x 6 x 1120 mm) in size, were used asbearing pads on the top and bottom ofeach panel. In addition, a layer of /4 in.(6 mm) fast hardening fresh mortar wasplaced between the panel's surface andthe masonite board to assure a uniformload transfer.

Dial gages accurate to 1/1000 in. (1/40

mm) were used to measure the hori-

PCI JOURNAUMay-June 1983 89

89'(226) Total

CD0

6(15)1°(2.5) b4

1 b3

16.4)

WeldingTensile Relnf. biL 3-1/2" x2-1/2° x l/2'-'

(9x6x1.3)

1 (2.5 )_

s°2.1/2 (6.4)

1/2"(13)

b4

44°(112)

2° (3.8)

4"(5.7)

1/2(1.3)

2.1/2(6.4)

REINFORCFMFNT SCHFOtJI F

Fy "50000 psi

Bar Type and N° LengthSize of of of

N° Bars Bars Bars

b I WVariable 2 88"(224)

*3 87••b2 2(22))

b3 *3 2 42"(I 07)

b4 6x6 - 6/6 87"x 42"Wire mesh (221x107)

Spacer I /2' thick

II -]I

Fig. 1. Dimensions and reinforcement details of a typical test panel.

Loading Jack.

Loading CoilRigid Beam 11

1

2-I/2

masoniteboard andmortar

1.1/2"

89..

f- b-side side

11/2

t717177 77T7T

- I.I 11 I 2-1/2"

44° ---• I F61

• ,-6^Dial gageposition

Fig. 2. Arrangement of typical panel tested as a bearing wallelement in a steel frame.

zontal displacements. Gages wereplaced at every intersection of gridlines (see Fig. 2). Additional dial gageswere used to determine the panel'sshortening along its centerline. Detailsof the loading arrangement are shownin Figs. 3 and 4.

The loading was done incrementally.An initial load of 46 kips (205 kN) wasapplied, followed by load increments of23 kips (102 kN). Cracks were observedand marked on the panels as they oc-

curred at every load increment. Allthree panels tested in this programwere loaded in a similar manner. Eachtest was performed in two stages. In thefirst stage, panels were loaded up toabout one-half of the predicted failureload. Afterwards, the load was removedand the panel was left unloaded for oneday. In the second stage, i.e., the nextday, the panels were loaded from zeroload up to failure.

Two types of cracks (horizontal and


Fig. 3. Arrangement of loading jack and load cell attachment.

Fig. 4. Overall view of load testing.

vertical) occurred during loading. Thedevelopment of these cracks was se-quential. Horizontal cracks occurred onthe vertical ribs under the load of 46 to114 kips (205 to 507 kN), which werethen followed by vertical cracks on thewall membrane.

Vertical cracks were predominant inall panels. They were observed first onthe horizontal top and bottom ribs, onrib faces away from the bearing surfaceat loads from 92 to 229 kips (409 to 1019kN), and in the wall membrane startingat 69 kips (307 kN) and continuing up tofailure. The vertical cracks in the mem-brane slab occurred suddenly, audibly,and over an extensive length.

The panels had some cracks presentin the membrane before testing whichmight have occurred from shrinkage orcareless handling. These cracks wereidentified and marked. These existingcracks did not influence significantlythe appearance of new cracks under

92

Bock -Side

itii

it

Face -SideF ° 238 Kips. (1059 kN.)

---- Existing Shrinkage (or Handling)Cracks

Fig. 5. Recorded crack patterns for Panel C1. Numbers indicate the load, in kips, atwhich the corresponding cracks occurred (Note: 1.0 kip = 4.448 kN).

loading. It was also observed thatexisting horizontal cracks closed underincreasing loading.

In the final loading stage, the failureoccurred suddenly, first along horizon-tal (or slightly inclined) lines close tothe horizontal rib, where stress redis-tribution between the wide rib and thinmembrane slab takes place. Afterwards,failure propagated vertically along thejunction between the slab membraneand the vertical ribs, causing their sep-aration and final wall collapse. Re-corded crack patterns are illustrated inFigs. 5, 6 and 7. Note that the numbersin these figures indicate the loadingforce (in kips) at which a correspondingcrack occurred.

ANALYSIS OFTEST RESULTS

Failure LoadsThe ultimate bearing capacity of a

concentrically loaded wall of uniformthickness (h) can be calculated usingthe empirical equation given in ACI318-77:7.8'

P..=0.55 f^Aa I1– (40h)2] (1)

The above equation assumes a mini-mum ratio of vertical and horizontalreinforcement of 0.0025 and 0.0015 ofgross area, respectively. However, inthin-wall panels like the ones tested,

PCI JOURNAL/May-June 1983 93

160206 29

69- TT 15^ 27S 1

a ^ ^

78 1

`

JIGO 232114 ` ^ 1i. 14,p--

^ 297 `137 114 `

2871 229-,1114252^ 20ÎK

111 229137 2291. `3?^_

137297 252 206

Y A

52 206 a

A I297

!2 112 192

229

229 1 137

_-+- -7X29

9 1371 1

2291

252

297 297

29

Back-Side Face-SideF = 297 Kips. (132( kN.)

---- Existing Shrinkage (or HandlingCracks

Fig. 6. Recorded crack patterns for Panel C2. Numbers indicate the load, in kips, atwhich the corresponding cracks occurred (Note: 1.0 kip = 4.448 kN).

the ratio of reinforcement may be greater and should be taken into account. Thus,it is proposed to modify Eq. (1) as follows:

P. =0.55O.fÂ,,

L

1 -(40h )2][ 1+(n-1)PR +(m-1)Pm ] (2)

Eq. (2) can be further modified as follows, assuming that all reinforcements willreach yield:

Pu =0.55 -Of,'A,[1—( 40h )2] I

1+ (m - 1 )Pm] (3)

A comparison of measured and calculated failure loads is given in Table 1.Column 7 of Table 1 shows values of the capacity reduction factor 4 measured

in tests which were estimated from Eq. (2) after replacing the value of P. with thevalue of P,, 4 = F and using the equation:

r 2 l

0.55fÂg l 1—( 40h / J

[1+(n-1)p.+(m-1)p. J

(4)

94

(114 1717 OB206 137

\206,

137183

275206

275 27569 137

252 re13137 137 183 13114 f.

27

jh03

275 1137\ 137 275 1

137183 275

921 275 1275Face - Side Back - Side

F = 275 Kips. ( (223 kN.)---- Existing Shrinkage (or Handling

Cracks

Fig. 7. Recorded crack patterns for Panel C3. Numbers indicate the load, in kips, atwhich the corresponding crack occurred (Note: 1.0 kip = 4.448 kN).

Table 1. Summary of Test Measurements and Calculated Failure Loads.

Panel No. Concrete Test failure Ultimate design Ratio Calculated Measuredstrength load capacity calcu- bearing capacity

lated using capacity reductionEq. (2) using Eq. (4) factor

f,', psi (MPa) F, kips (kN) P,,, kips (kN) F/P„ P„ = P„ /4, 4 = F/P„kips (kN)

1 2 3 4 .5 6 7C1 6055 (41.75) 238(1059) 192.5(856) 1.24 275(1223) 0.865

C2 6055 (41.75) 297 (1321) 192.5 (856) 1.54 275 (1223) 1.079

C3 7252 (50.0) 275 (1223) 228 (1019) 1.21 327 (1454) 0.842Average 6454 (44.5) 270 (1201) 204.3 (910) 1.32 292 (1300) 0.929

NOTE: Values of P. and P. were calculated assuming that # 3 bars in side ribs reached yield strength, butwelded wire fabric steel worked in elastic range under stress corresponding to strain (contraction)measured during testing.


RMATI01

_ACEMEIP

C3

P (Kips)

P(Kips)

-.003 -.002 -.001 0 +.00I p (in.) -.003 -.002 -.001 0 +.001 0 (in)

Fig. 8. Horizontal displacement and deformation of center point (Node C3) for PanelsC2 and C3 (Note: 1.0 kip = 4.448 #N; 1 in. = 25.4 mm).

It can be observed that the measuredaverage value of the capacity reductionfactor was 0 = 0.929, which is greaterthan 0 = 0.7 recommended for wallsand columns by ACI 318-77.

Thus, it can be noticed that the re-corded strength of the panels is about:

0.929 x 100 = 1320.7

or 32 percent greater than that calcu-lated by ACI 318-77.

Lateral Deformation of PanelsThe horizontal displacement and

deformation of the center point of Pan-els C2 and C3 (Node C3 ) are shown inFig. 8. The term "deformation" denotesthe measured relocation from the origi-nal position of the panel and the term"displacement" denotes the net valueof the deformation,

All the panels tested showed a simi-lar mode of deformation. The maximumdisplacement recorded for center nodes(C 3 ) was in the range of 0.003 to0.00375 in. (0.076 to 0.095 mm), whichis very small.

Note that a very small (but not recov-erable) plastic deformation of 0.0002 in.(0.005 mm) was recorded after reload-ing. Overall deformation patterns indi-cate that there was no loss of stability ofthe membrane wall. Overall displace-ment and deformation curves of thepanels are shown in Figs. 9, 10, and11.

Vertical Load/Strain RelationshipThe load-strain (contraction) relation

that was measured for the centerline ofthe panels is illustrated in Fig. 12.Curves I and II correspond to the firstand second stages, respectively. All

96

LPANEL Cl

C FAIL= 238 K(ps(+10) CURVES AT 229 Kips

R

(-I54) (-9 )^

(-228)

(-6) (-178

(-143)89"

(-53) (-78)

(-169)

(-4)/

44"

Fig. 9. Overall displacement and deformation of Panel C1under load 226 kips (1078.6 kN). Displacements are shownin 1 x 10-3 in. (Note: 1.0 in. = 25.4 mm).

panels tested showed elastic behaviorin the initial stage of loading (Curve I)up to about 137 kips (609 kN).

The elastic behavior was also contin-ued in the second stage of loading up to252 kips (1121 kN), which is 85 to 90percent of failure load, beyond whichthere was an abrupt increase in strain.Maximum strain values recorded atfailure were between 0.00225 and0.0026. Such strain values put #3 bars,used as rib reinforcement, into yield,but wire mesh in the membrane re-

mains in an elastic state. It should benoted here that the ACI provisions oncolumns and beams are based on a limitstrain in compression of e = 0.003.

Stability of Panel and MembraneThe critical buckling strength of a

thin-wall precast panel is evaluatedconsidering the stiffness of the overallsystem combining the membrane slaband the perimeter ribs. It is assumedthat the vertical side ribs of each panel


L PANEL C2

FAIL.= 297 KipsC CURVES AT 275 Kips

-8J) /

R(- II

-7S)/ (-88)

/

(..I(

(0) (-16) I

c•2or,I

(+4) (+18) f I

(+25Y'

(+22)I

Fig. 10. Overall displacement and deformation of Panel C2under load 275 kips (1223 kN). Displacements are shown in1 x 10-3 in. (Note: 1.0 in. = 25.4 mm).

act as lateral elastic supports to themembrane, and displacement at top andbottom is restrained by the horizontalribs and the friction between these ribsand the supporting surface.

The vertical side ribs could deflectunder the applied load either in thesame direction or in opposite direc-tions. According to Timoshenko,'Q ifside ribs were assumed to deflect inopposite directions, a transcendentalequation would be obtained whichwould give critical strength valueshigher than when ribs deflect in thesame direction. Thus, it is reasonable to

analyze first the case where both ribsdeflect in the same direction. If such amodel shows that the critical load isgreater than the static carrying capacityof a panel, then it would be not neces-sary to check the transcendental equa-tion, and buckling would not be critical.Such an analysis was carried out.

In the analysis, torsional rigidity ofthe panel was neglected because ofsymmetry in both directions, and theload was applied purely axially.

The critical buckling strength of aplate stiffened with longitudinal edgeribs (parallel to the direction of load-

98

L PANEL C3

FAIL= 275 KipsC

(-52) CURVES AT 275 Kips

(-9)

(—17)(+14)

(+57)/

(+24) (+3)

(+7) 8 9"

(-9)

(+99)

4 4"

Fig. 11. Overall displacement and deformation of Panel C3under load 275 kips (1223 kN). Displacements are shown in1 x 10-3 in. (Note 1.0 in. = 25.4 mm).

ing), and having end conditions along of applied load) with uniaxial uniformthe top and bottom edges as bearing load, can be defined, followingsupports (perpendicular to the direction Timoshenko, 10 by Calisev's equation as:

^— CD [ 'T + (1 — v) 4)] 2 tan 1/2 ^ — (D2

+ + ['P — (1 — v) 4)] 2 tanh 72 '1t 02 = 2 5I2 4r (5)

where

b E I A 'i's= m ar a B — b Dr bt (Dz

(6)

'P = b t Ocr v = Poisson's ratio = 1D r6


.001 .002 .003Strain E (in./in.)

p. -

.001 .002 .003Strain E (in./in.)

PANEL C3F 275 Kips

.001 .002 .003Strain E (in./in.)

P(Ki

300

200

100

ips

0

[oil]

00

30

Fig. 12. Recorded load-strain (contraction) relationships measured on centerlines ofpanels (Note: 1.0 kip = 4.448 kN).

A transformed cross section is con-sidered for calculating the stiffnessproperties, recognizing also the ratio ofthe modulus of elasticity of steel toconcrete. For Panel C1, the criticalbuckling strength was obtained fromEq. (5) using a trial and error proce-dure.

7.515 ksi (51.8 MPa)

The transformed cross-sectional areaof the panel is:

A t = 94.32 sq in. (60852 mm2)

Hence, the analytical critical • load forbuckling failure of Panel C1 is:

'c, = QcrA t = 7.515 x 94.32= 708.8 kips (3153 kN)

Assuming a capacity reduction factor

of 0.7 for concrete columns, the actualcritical load can be evaluated as:

Pc,. = 0.7 x 708.8 = 496 kips (2207 kN)

The above critical buckling load ismuch higher than the failure load mea-sured. This leads to the conclusion thatbuckling is not a critical phenomenonfor the panels tested. Also, there was noneed to consider the transcendentalequation in evaluating the criticalbuckling strength or to apply a moreexact analysis, particularly recognizingthe positive influence of other contrib-uting parameters. For Panel C2, theanalytical buckling stress would be thesame as for Panel CL. However, thecritical stress for Panel C3 is higher be-cause of the higher concrete compres-sive strength.

100

Fig. 13. Overall view of panels tested, shown after failure. Numbers indicate the load, inkips, at which the corresponding cracks occurred (Note: 1.0 kip = 4.448 kN).

CONCLUSIONS1. The panels tested showed a higher

ultimate loadbearing capacity than val-ues calculated according to ACI 318-77provisions for walls. Thus, the currentACI Code design provisions for con-ventional axially loaded bearing wallscan be extended to thin-wall ribbedpanels which have dimensional param-eters within the limits tested:

-<15;t-<30; --->4

hAdditional tests would be required to

establish whether the above conclusioncould be further extended to panelswith larger rib spacing and smaller di-mensional parameters than tested inthis research program. Also, the effectof eccentric loading, including the ef-fect of construction eccentricity, shouldbe considered in future tests.

2. The panels showed differentmodes of deformation under loading. Insome panels, both ribs and the mem-brane deflected in the same direction,and in others, side ribs deflected in op-posite directions. The maximum re-corded side deformation of the wholewall was 1,/385. No loss of stabilityunder loading was observed for themembrane plate.

3. No significant discrepancy be-tween individual panels in load-strainrelation under loading was observed.Strains were measured almost up tofailure. The maximum recorded strain(contraction) was about 0.0026, which isless than the limit strain of 0.003 usedby the ACI Code for the ultimate de-sign of columns and flexural members.The panels behaved elastically almostup to 85 percent of failure load, andthen the strain increased suddenly.

4. All panels showed almost the


same cracking pattern under increasingload. Vertical cracks along the compres-sion direction started to occur at loadsclose to 137 kips (609 kN), which isabout 65 percent of the ultimate load.

5. The failure of the panels occurredin all cases subsequent to crushingalong a horizontal line near the junction

of the membrane with the bottom or tophorizontal rib. In the final stage, afterhorizontal-line crushing, the failurecracks developed along the vertical ribscausing separation of the vertical sideribs from the membrane plate. An over-all view of the panels after failure isshown in Fig. 13.

ACKNOWLEDGMENTThe study reported here was made

possible through research grants fromthe National Sciences and EngineeringCouncil of Canada, Grant No. A1017,Le Program de Formation de Cher-cheurs et d'Action Concertee, GrantNo. FQ-1645 (Gouvernement de Que-bec), and Concordia University.

The concrete panels tested were pro-duced by Central Precast Co. of Ottawa,Ontario.

The tests were done in the structurallaboratory of the Department of CivilEngineering, Concordia University,This study formed a part of the master'sdissertation of Mr. El-Chakieh.

REFERENCES

1. Zielinski, Z. A., "Building Industriali-zation Trends in India," Industrializa-tion Forum, V. 3, No. 2, December 1971,pp. 43-52.

2. Zielinska, C., and Zielinski, Z. A., "Pro-posed Ribbed Panel System for PrecastConcrete Housing," PCI JOURNAL, V.27, No. 3, May-June, pp. 92-115.

3. Zielinska, C., Zielinski, Z. A., andTroitsky, M. S., "Thin-Wall RibbedConcrete Building Systems—New Ap-plications and Research," Housing Sci-ence Journal, V. 6, No. 4, 1982, pp. 279-300.

4. Zielinski, Z. A. and Abdulezer, A., "Ul-timate Strength and Diagonal Splittingof Reinforced Concrete Thin-Wall Pan-els," Canadian Journal of Civil Engi-neering V. 4, No. 2, June 1977, pp. 226-239.

5. Taner, .N., Fazio, P. P., and Zielinski,Z. A., "Strength and Behavior of Beam-Panels — Tests and Analysis," ACI

Journal, V. 74, No. 10, October 1977, pp.511-520.

6. Zielinski, Z. A., Troitsky, M. S., andChristodoulou, H., "Full Scale BearingStrength Investigation of Thin-WallRibbed Reinforced Concrete Panels,"ACI Journal, V. 79, No. 4, July-August1982, pp. 313-321.

7. ACI Committee 318, "Building CodeRequirements for Reinforced Concrete(ACI 318-77)," American Concrete In-stitute, Detroit, Michigan, 1977.

8. Kripanarayanan, K. M., "Interesting As-pects of the Empirical Wall DesignEquation," ACI Journal, V. 74, No. 20,May 1977, pp. 204-207.

9. Oberlender, G. D., and Everard, N. J.,"Investigation of Reinforced ConcreteWalls," ACI Journal, V. 74, No. 28, June1977, pp. 256-263.

10. Timoshenko, S., and Gere, J. M., Theoryof Elastic Stability, McGraw-Hill BookCo., 2nd Edition, 1961.

102

APPENDIX — NOTATION

P

trans- P.

A = area of transformed rib crosssection

a = membrane plate lengthA9 = gross cross-sectional areaAt = overall transformed cross-sec-

tional areab = membrane plate widthDE = flexural rigidity of

formed membrane plateE, = modulus of elasticity of con-

creteE, = modulus of elasticity of steelF = test failure loadf = compressive strength of con-

cretef„ = yield strength of reinforce-

menth = overall thickness of wall panel

equal to depth of side ribI, = moment of inertia of trans-

formed rib cross sectionh = overall height of wall panel

equal to length of side ribm' = number of half wave length

(m' = 1 is assumed)

m = fv/fc = ratio of yield strengthof steel-to-concretestrength

n =E$/E, = ratio of modulus ofelasticity of steel-to-concrete

= applied load= 4P9 = ultimate design bear-

ing capacityP. =P9/.0 = nominal bearing ca-

pacityt = thickness of membrane platew = overall width of wall panel

equal to spacing at vertical ribsoe = limit compressive strain in

concrete0 = term defined by Eq. (6)u = Poisson's ratio for concrete

= ratio of yielded steel-to-grosscross-sectional area

pxi = ratio of non-yielded steel-to-gross cross-sectional area

crc,. = critical buckling strength= capacity reduction factor

4','1' = terms defined by Eq. (6)

r

NOTE; Discussion of this paper is invited. Please submityour discussion to PCI Headquarters by January 1, 1984.


bearing capacity tests on precast concrete thin-wall ... journal...refabricated thin-wall ribbed...

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