304 CONTRIBUTION OF STEEL REINFORCEMENT IN COLUMNS UNDER SUSTAINED LOAD Hamza Ben-Omran Graduate Student Civil Engineering Department University of Manitoba Winnipeg, Manitoba Canada R3T 2N2 ABSTRACT John Glanville Associate Professor Civil Engineering Department University of Manitoba Winnipeg, Manitoba Cana da R3T 2N2 A research program that examines the effect of sustained loading on the strength of reinforced masonry columns is reported. The behavior under sustained load is predicted using the stress-strain relationships and time-dependent properties of the constituent materiaIs. The experimental part of the program includes testing ten pairs of reinforced and grouted two-core concrete block masonry columns. One column in each pair was preIoaded to about 0.45 f', the other being the non-preloaded control specimen. Pairs of columns tested to failure after about ten months, strains being recorded during increasing load. The variables considered are the size and percentage of vertical reinforcement. Creep and shrinkage are time-dependent properties that have their greatest effect in the early stages. These properties cause a transfer of load from the concrete to the vertical reinforcement in the pre-Ioaded columns. This resuIts in an increase in the efficiency of the vertical reinforcement in resisting the load with a consequent increase in t he ultimate strength. --
Transcript
304
CONTRIBUTION OF STEEL REINFORCEMENT IN ~lASONRY COLUMNS UNDER SUSTAINED LOAD
Hamza Ben-Omran Graduate Student Civil Engineering Department University of Manitoba Winnipeg, Manitoba Canada R3T 2N2
ABSTRACT
John Glanville Associate Professor Civil Engineering Department University of Manitoba Winnipeg, Manitoba Cana da R3T 2N2
A research program that examines the effect of sustained loading on the strength of reinforced masonry columns is reported. The behavior under sustained load is predicted using the stress-strain relationships and time-dependent properties of the constituent materiaIs. The experimental part of the program includes testing ten pairs of reinforced and grouted two-core concrete block masonry columns. One column in each pair was preIoaded to about 0.45 f', the other being the non-preloaded control specimen. Pairs of columns ~ere tested to failure after about ten months, strains being recorded during increasing load. The variables considered are the size and percentage of vertical reinforcement.
Creep and shrinkage are time-dependent properties that have their greatest effect in the early stages. These properties cause a transfer of load from the concrete to the vertical reinforcement in the pre-Ioaded columns. This resuIts in an increase in the efficiency of the vertical reinforcement in resisting the load with a consequent increase in t he ultimate strength.
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305
INTRODUCTION
Although previous studies have been carried out to investigate the resistance of masonry to axial load [5,6,8) the effect of sustained load has received no attention. Since masonry columns are normally in a state of sustained load, their behavior is likely to be different from that of columns tested under short-term loading . The time-dependent shrinkage and creep of concrete block and grout result in a transfer of load to the compression reinforcement and enhances its contribution to the strength of masonry columns.
The research reported in this paper involves the development of a theory to expia in the internai load distribution, and redistribution with time, between the component material, and a program of load testing reinforced concrete masonry columns that have been subjected to sustained load and comparison columns that have noto The experimental results are compared with those predicted by the theory.
THEORETICAL ANALYSIS
An analytical method to predict the ultimate strength of a reinforced masonry column subjected to sustained load and then loaded to failure is developed. The theory utilizes the stress-strain relationships of the significant materiais and the usual principies of equilibrium and compatibility.
The significant materiais considered in the analysis are masonry unit, grout and vertical reinforcement. The mortar and joint reinforcement being small in quantity and lying in plane perpendicular to the load are assumed to have little effect and are excluded from the theory.
The stress-strain relationship for concrete under short-time loading suggested by Desayi and Krishran [4) is assumed for both masonry unit and grout. The masonry unit and grout are assumed to have no tensile strength. The approach to shrinkage and creep of both materiais follows the genera l principies outlined by Balaguru and Nawy [lJ .
The reinforcing steel is assumed to be an idealized elasto-plastic material and does not undergo time-dependent deformation.
Since application be rewritten follows:
the column were cured for a period of time before the of the sustained load, the stress-strain relationship [4J can
to account for the shrinkage of masonry unit and grout as
for masonry unit
Eb( E-E Sb )
fb E-ESb)2 1 + (E
bo and, for grout
f g
E (E- E ) g sg
E-E 1+(~)2
E go
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~ O
O
The unit and grout shrinkage E b and E respectiveiy obtained from the foiiowing reiation [ll~ sg
Esh(t) E (1_e-O.067t) shu
the stress in the reinforcement steei is
f EE s s
~ f y
(1)
(2 )
can be
(3)
(4 )
Appiying the principies of equiiibrium and compatibiiity, the instantaneous strain in the coiumn E . due to the appiication of the sustained ioad P., is caicuiated by soiviàg the foiiowing equation:
1
+ EbAb(E i - Esb ) E A (c - E ) P. IE.E A + g g 1 sg (5 )
1 1 S S E. - E E. - E 1+( 1 Sb)2 1 +( 1 sg) 2
Ebo
E go
Once the ioad P i is sustained for a periodof' time, creep takes piase in both the masonry unit and grout. Consequentiy, the stresses in the constituent materiais wiii change such that:
[ 'eb (c - E b)
Hb Eb 1 s
1 + (Eeb)2 E - E
1 +( i Sb)2 E
bo E
bo
(6)
where
E. + E - E 1 c sb
Eeb (1 + Cb
)
[ , (c - E ) óf E
eg 1 5g g g
1 +(~)2 E . - E
1 + ( 1 Sg) 2
E go go
(7)
where E. + E - E
1 C 5g E
(1 + C ) eg g
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307
6f = c E s c s
( 8 )
The time depcnde nt c r ee p coefficient f o r masonr y block an d g rout, Cb and C respec ti vel y , ca n be ob tained from the fo l l owi n g relat i o n [ lJ.
g
C = K (1 _ e-0.0875]t) (9 )
ko' tRe
The constant k i s k whi c h depend s gfaphs provided by
The s t rain in the substituting equations equation:
equal to the product o f the coeffici e nts kb
, k , o n the column properties and can be obtained usigg CEB []).
c o lumn due (6), (7),
to creep, EC' can be solved for by and (8) in to the follow ing equilibrium
ó fsAs + ó fbA b + ÓfgAg o (lO)
Following the period of sustained load, the procedure adopted in determining the stresses in both the masonry unit and grout is to shift the Desayi/Krishnan (4) relationship by the amoun t of the creep strain.
A computer program based o n these equations has been written. The program calculates load through increments of strain and time to predict the load-strain relationship through to failur e. For example, Figure 1 shows the pred i cted behavior for preloaded and non-preloaded column a nd Figure 1 shows the contribution of each material in carrying the load.
EXPERIMENTAL PROGRAM
The experimental program was designed to investigate the behavior of reinforced masonry columns, particularly the effects of preloading on the contribution of vertical reinfo rcement. The constituent materiais were tested individually for strength, shrinkage and creep properties.
Mat e riais and Test Specimens
Ali materiais used in the test specimens are representative of those used locally in masonry construction. The size of the masonry unit was contro lled by the maximum capacity of the available testing equ ipment . Normal-weight concrete masonry blocks, 140 x 190 x ]90 mm, with an average compressive strength of 29 Mpa and type S mortar were used in constructing ali columns and prism specimens. Properties of grout mixes are described in Table 1. Properties of the vertical reinforcement are given in Table 2.
A total of eleven, two-block high prisms were built at the time of construction of the column specimens . Five of these prisms were grouted. Ten pairs of reinforced masonry columns were built, one of each pair for preloading, the othe r being the non-preloaded control specimen. All columns were ten blocks high and one block wide, built in stack bond. Joint reinforcement was placed within the 10mm mortar joints. Each column is reinforced with two or four bars, one or two in each core. Table ] gives the specimen details.
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Instrumentation
Strains in the column were gauge, buttons being attached to the Figure 3 . Strains in the vertical of 5 mm strain gauges.
measured using a 200 mm Demec strain face shell of the column as shown in reinforcement were measured by means
Testing
The testing included the time-dependent test in which axial compressive load was applied to one of each pair of shrinkage readings were taken on the control specimen. period of pre-load ali columns were tested to failure.
Pre-loading test
the sustained columns while Following the
The sustained load was applied by means of the pre-loading frame shown in Figure 4. The frame was designed so that a column could be tested to failure without removing the preload. The frame consists of two upper and one lower crosshead tied together by four dywidag bars. A rubber compound used as the load sustaining material was placed between the two upper crossheads. Detailed information about the loading frame is available in references [2,7). After completing the placement of the frame, the column was placed under the testing machine and a load of 700 kN applied. The load was kept constant for a period of half an hour before tightening the dywidag nuts. Strain measurements were taken immediately before and after the application of the load. The columns were stored in a relatively constant temperature environment and strain measurements were taken throughout the preloading period.
Ultimate testing
Upon expiry of the preloading time períod, each column pair was tested to failure. A bottom assembly consisting of a hinge was bolted into the testing machine. The column was then aligned concentrically under the testing machine. The load was slowly applied and strain measurements were taken every 100 kN up to about 1500 kN. The top cross heads were then bolted together and plywood sheets were placed around the column for safety. Loading continued to failure with only reinforcement strain and load being recorded. Details of the mode of failure were recorded.
TEST RESULTS
Two major factors that affect column strength were considered in the test program o One was the effect of preloading and the other was the effect of varying the amount of reinforcement. The test results of the ultimate load for the masonry columns are summarized in Table 4.
The type of failure and ultimate load of the masonry columns are dependent on construction variations, reinforcement area and whether ar not the column was preloaded. Since it was necessary to surround the
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309
columns with plywood during ultimate testing in order to contain any flying debris, the post-failure observations formed the basis on which the failure types were identified.
The failure of the pre-loaded columns were the most explosive. Vertical cracking occurred on ali faces with longer cracks appearing on the face shells. The columns ultimately failed in shear accompanied by buckling of the reinforcement bars as shown in Figure 5. The failure occurred near the top of the column, except for columns 5 and 8 where the failure were near the bottom of the column. Preloaded column 3 failed near the middle of the column where poor grouting was observed.
The failure of the non-preloaded columns was less explosive and the vertical cracks shorter than that for the pre-loaded columns. Columns with a small area of steel (200 - 400 mm 2 ) failed at the top of the column due to crushing of the block and grout as shown in Figure 6. Column 4 failed near the middle where poor grouting was observed in the same location. The columns with a larger area of steel (600 - 1000 mm 2 )
failed near the middle. These columns experienced long cracks and buckling of longitudinal reinforcement. Columns 7 and 8 failed due to crushing of the concrete block near the top or the bottom. These columns have comparatively larger area of steel but small bar size.
The ultimate load at failure greatly depends on the grouting of the column. When the columns are completely grouted, the preloaded columns failed at a higher load than the non-preloaded column. The efficiency of the vertical reinforcement in carrying the load was therefore higher for the preloaded column. Figure 7 shows the percentage of load carried by the reinforcement for pre-loaded and non-preloaded columns.
The test results indicated that the value of the stiffness, EA, of column cross section was higher for the preloaded columns. This probably being caused by the reduction of voids and relative increase in steel efficiency due to creep of the masonry block and grout.
DISCUSSION
The theoretical and experimental investigation presented in this paper indicate the significant effect that sustained load may have on the strength of reinforced masonry columns. The experimental results shows an increase in strength up to twelve percent due to sustained load.
Actual load testing is not likely to produce exactly the same loadstrain curves as those shown in Figures 1 and 2 since the theory assumes that neither the block nor the grout has tensile strength and neglects the effect of mortar and joint reinforcement. The failure loads predicted by the model were compared with the experimental results as shown in Table 4. While the results are far from conclusive, since the theory is only one-dimensional and neglects the effect of mortar and joint reinforcement, the model does expiain the increased strength provided by preloading. A more comprehensive modei is currently being developed .
310
The experimental and the theoretical results emphasize the need for proper vibrat(on during the grouting process to reduce the grout shrinkage and consequently reduce the load imposed on the units.
Sustained load results in creep which increases steel efficiency and causes concrete consolidation by reducing the void ratio. The result was different types of ultimate failure for the preloaded and non-preloaded columns.
ACKNOWLEDGEMENTS
The authors are grateful to Ms. Arlene Flatfoot for typing and formatting this paper.
APPENDIX I - REFERENCES
1. Balaguru, P., and Nawy, E.G., "Evaluation of Creep Strain and Stress Redistribution in R.C. Columns". ACI Special Publication SP-76, American Concrete Institute, Detroit, 1982, pp. 309-324.
2. Ben-omran, H .• and Glanville, J., "The Effect of sustained loading on Reinforced Masonry Columns", proc. 4th North American Masonr y Conference, University of California at Los Angeles, Los Angeles, California, August 1987, Vol. I, pp. 21.1 - 21.13.
3. CEB-FIP, International Recommendations Construction of Concrete Structures, p. Concrete Association. 1970.
for 80,
the Design and London, Cement and
4. Desayi, P., and Krishnan, S., "Equation for the Stress-Strain Curve of Concrete", J. Amer. Conr. Inst., 61, pp. 345-50, March 1964.
5. Feeg, C., Longworth. J . , and Warwaruk, J., "Effect of Reinforcement Detailing for Concrete Masonry Columns", Department of Civil Engineering, University of Alberta, Structural Engineering Report No. 76, May 1979.
6. Hatzinikolas, M., Longworth, J., Warwaruk, J., "Concrete Masonry Walls", Department of Civil Engineering, University of Alberta, Structural Engineering Report No. 70, September 1978.
7. Roy, K.G., "An Investigation into the Behavior and Ultimate Strength of Reinforced Concrete Block Masonry Columns under Sustained Loading", M.Sc. Thesis, Department of Civil Engineering, Universitv of Manitoba. June 1986.
8. Sturgeon, G.R., Longworth, J., Warwaruk, J., "An Investigation of Reinforced Concrete Block Hasonry Columns", Department of Civil Engineering, University of Alberta, Structural Engineering Report No. 91, September 1980.
....
311
AI'PENDIX 11 - NOTATION
f Y
fbo
f go
[ bo [
go
[s b E
sg Esh(t)
Eshu
Stress in the block at any strain
Stress in the grout at any strain
Stress in the steel at any strain
Yield stress for the reinforcement
Compressive strength of the block
Compressive strength of the grout
Strain in the block at maximum stress
Strain in the grout at maximwn stress
Shrinkage strain in the block
Shrinkage straín in the grout
time dependent shrinkage strain
ultimate shrinkage strain
of fbo
of f go
t time expressed as square root of the actual time
LOADING Reinforcement ratio Preloaded Non-Preloaded
(ITnll 2 ) 12%
200 0.0037 x 200 0.0037 x
200 0.0037 x 200 0 . 0037 x
400 0.0073 x 400 0.0073 x
400 0.0073 x 400 0.0073 x
600 0.011 x 600 0.011 x
600 0.011 x 600 0.011 x
600 0.011 x 600 0.011 x
800 0.0147 x 800 0.0147 x
1000 0.0183 x 1000 0.0183 x
1000 0.0183 x 1000 0.0183 x
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313
TAfll.E I .
TEST ~ESULT
V ~ . rti ca l keinf orccmenl Ult.ímil LC 10.1<1 (l\N)
Arca Cotur:lIl Nurnber
CI'OUt.
St rcngth IMPa)
Yícld Strenp,th (Mpa)
.;\vercJ.~c
Prcload ( KN)
Experiment.;,l \:: \l c ulated (mm' )
c:rIU 37 200 391 C:P 1L 686
(P2U 31 •. 6 200 396 CP2L 71.6
CP3U 28 . 5 1.00 351 CP3L 698
CP4U 36.1. 400 355 CP4L 707
CP5U 32 .8 600 365 C:P5 L 683
CP6U 43 . 5 600 374 CP6L 688
CP7U 31.2 600 356 CPlL 719
CP8U 29.8 800 351 CP8L 689
CP9U 32.9 1000 350 CP9L 656
CP 10U 34.3 1000 356 CP10L 645
Average compressive strength for masonry bloc k Average strain at failu re for masonry block Average strain at fa illlre f or grout Assumed shrinkage strain for masonry block Assumed shrinkage stra in for grout
Fig. 2. - Contribution of Component Materials to Column Strength
evttoni"l of .. co I ............... , tllc 1&1"'< on bot.h col........, (acc ,hclll. Thc ~ct, .... J1ic .. l diUt'cc Itr.i,., rc...odi"ll '-'cre nUl"bcr<'!d lc(t to ri,hl. top
t:>otto .. . (ro. I &.r\d I'.> to 2a O" t.hc oChrr.
Flg. 1. Demec bullon t.tyout on test coltutln
nn n n{\---
~ ~ ~
-
li U uli
Oywiõa& b.ar nut
PrirYry upper cro:loShead
Po I yuret.h.ane. p l)""'Ood And :loteei plate .rr ... ng~nl
Secondary upper crosshead
Test coll.nl\
Dywida& bAr
8otton cro$she.ad
Flg.~. Pre'!oadlnjt {rAlle \frolS he. 1I
....
-
'" '" o
315
figure ~ figure 6
f.ailure of pre:loaded column 2. Failure of non-preloaded column 2
0.0 0.1 o .• o .• o." 1.0
(Thousands) Column Load (KN)
I.' I.' I." 1.0
figure 7. Contrlbutlon of vertical reinforcernent in colUll\l'l pair I
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