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Dowel Action in reinforced concrete construction (beam-column connections) Kazakoff, Ely E. 1974

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C I DOWEL ACTION IN REINFORCED CONCRETE CONSTRUCTION (BEAM-COLUMN CONNECTIONS) by ELY E . KAZAKOFF B . A . S c , University of Br i t i sh Columbia, 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE In the Department of CIVIL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 197^ In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree that p e r m i s s i o n f o r ex t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l ' n o t be allowed without my w r i t t e n permission. E l y ». Kazakoff Department of CIVIL ENGINEERING The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada A p r i l 1974 i ABSTRACT The t r a n s f e r of shear i n a beam-column j o i n t by dowel a c t i o n alone was experimentally and a n a l y t i c a l l y s tudied. The l a b o r a t o r y work in v o l v e d the shear capacity determination of i n d i v i d u a l r e i n f o r c i n g s t e e l dowels embedded i n concrete. Two main s e r i e s of experimental t e s t s were conducted on bottom and top dowels - component parts of a beam-column j o i n t . A l l e x p e r i -mental r e s u l t s were compared to a t h e o r e t i c a l a n a l y s i s . The t h e o r e t i c a l a n a l y s i s c o n s i s t e d of choosing a r a t i o n a l p h y s i c a l model, i . e . , a mode of behaviour f o r each of the two component parts of the j o i n t . No c u r v e - f i t t i n g to the experimental r e s u l t s was done. These r e s u l t s do show, however, that the model provides a safe lower bound on the shear capacity of the j o i n t . A l s o , the model permits reasonable e x t r a p o l a t i o n to other design problems where the c o n d i t i o n s of the problem are not e x a c t l y the same as those imposed during the experimental t e s t s . A design example of p r e d i c t i n g the shear capacity of a beam-column j o i n t on the b a s i s of dowel a c t i o n of the r e i n f o r c i n g s t e e l i s presented f o r any combination of top and bottom dowels. TABLE OP CONTENTS Abstract i Table of Contents i i Lis t of Figures i i i List of Tables iv Acknowledgements V i Page Chapter 1 Introduction 1 Chapter 2 Laboratory Program 7 2.1 Material 7 2.2 Fabrication of Test Specimens 7 Chapter 3 Foundation Modulus of Concrete K 10 Chapter 4 Bottom Dowel Tests 19 4.1 Experimental Procedure 19 4.2 Analysis 20 4.3 Comparison of Results with Previous Work 35 Chapter 5 Top Dowel Tests 40 5.1 Laboratory Test Program 40 5.2 Analysis 45 Chapter 6 The Joint: Sum of Top and Bottom Dowels 60 Chapter 7 Conclusions ..,.<•' 66 References 67 Appendix 1 Bottom Dowel Experimental Graphs 69 Appendix 2 Top Dowel Experimental Graphs'; 79 i i i LIST OF FIGURES Figure No. Page 1.1a Beam - Column Joint 2 1.1b Bottom Dowel 2 1.1c Top Dowel 2 1.2a Transverse Crack in Reinforced Concrete Specimen 4 1.2b Shear-friction in Cracked Concrete Specimen 4 3.1a Foundation Modulus Test Specimen 11 3«lb Test Specimen in Baldwin 11 3.2 Foundation Modulus Test 12 3.3 Failure of Test Specimen 12 3.4 Foundation Modulus Test Graph 14 3.5 Foundation Modulus K vs. Dowel Diameter 16 3.6 Foundation Modulus for Varying Concrete Strengths 18 4.1 Bottom Dowel Specimen 19 4.2 Loading Apparatus for Bottom Dowels 21 4.3 Bottom Dowel Test 22 4 . 4 a Bottom Dowel Specimen 23 4.4b Bottom Dowel as a Beam-on-elastic Foundation 23 4 . 5 a Bottom Dowel #4 27 4.5b Bottom Dowel #7 28 4 . 5 c Bottom Dowel #11 29 ,4.6 Bottom Dowel Shear 31 4.7 Bottom Dowels: P = 2g 3 EIy 4.8 Bottom Dowels: Shear at Ultimate and 0 . 0 3 " Deflection 34 4.9 Comparison with ACI-ASCE 36 4.10 Bottom Dowel Specimens at Ultimate Load 37 i v Figure No. Page 4 . 1 1 Crack Pattern at Ultimate Load 38 5 . 1 Top Dowel Specimen 4 l 5 . 2 Top Dowel Test Apparatus 43 5 . 3 Top Dowel Test 44 5.4 Shear V acting on Top Dowel 46 5-5 Transformed Section 46 5 . 6 a Top Dowels: Shear at 0 . 0 3 " Deflection 48 5 . 6 b Top Dowels: Normalized Shear 50 5-7 Crack Propagation in Top Dowel Test 52 5 . 8 Yielding of the F irs t Stirrup 55 5 . 9 Top Dowels: Experimental and Analytical Results 56 '5 .10 Shear Failure of #4 Dowel 57 5 . 11 Rupture of F irs t Stirrup 57 5 .12 Crack Pattern in Top Dowel Test 58 5 . 13 Specimen at Ultimate Load 58 6 . 1 Bottom and Top Dowels 6 l 6 . 2 Design Beam-Column Joint 62 6 . 3 Ultimate Shear: Experimental Result 65 V LIST OF TABLES Page Table 2.1 Concrete and Steel Properties; ; 9 Table 3.1 Foundation Modulus Tests 15 Table 4.1 Bottom Dowel Variables 26 Table 4.2 Shear at 0 . 0 3 " Deflection 30 Table 5.1 Top Dowel Test Specimens 42 Table 5.2a Transformed Section Properties 47 Table 5.2b Normalized Experimental Results 49 Table 5 . 3 Direct Tensile Force 54. Table 5.4 Tension, at Stirrup Yield 55 v i ACKNOWLEDGEMENTS'-I wish to express my appreciation to Professor S. L . Lipson for his guidance and help throughout this thesis. Also, I wish to thank Messrs. R. Postgate, B. Merkl i , W. Schmitt, and J . Sharpe for their assistance in making the test equipment and carrying out the tests. F ina l l y , I wish to •acknowledge f inancial assistance from the National Research Council of Canada and the Computer Centre of the University of Br i t i sh Columbia. A p r i l 1974 Vancouver, B. C. 1. CHAPTER 1. , : .INTRODUCTION In order to determine the shear capacity of a reinforced concrete beam-column jo int , where a l l the shear is transferred solely by dowel action, of the reinforcing steel bars crossing the beam-column interface, an experimental test program was conducted and the results compared with a theoretical analysis. This type of joint could be made by forming and pouring a cast-in-place beam against a precast column which already has the necessary bottom and top dowels protruding from i t . The reverse case is also possible. A precast beam with bottom and top dowels pro-truding from i ts end could be positioned against a formed column and the column subsequently cast-in-place. The effect of f r i c -t ional shear between the beam-column interface has not been considered in this investigation. Only the dowel action of the steel bars is considered. The beam-column joint as shown in F i g . 1.1a can be broken down into i ts component parts - bottom and top dowels (Fig. 1.1b and 1 . 1 c ) . -fV-—Pd Fig . 1.1a Beam-Column Joint i 1 i 1 1 i •— 1 Fig . 1.1b Bottom Dowel I I I I F i g . 1.1c Top Dowel The experimental investigation'consisted of two main series of tests. The f i r s t one involved a column specimen and bottom dowel to determine i ts shear capacity (Fig. 1.1b). The second one involved a beam specimen and a top dowel (Fig. 1.1c). In each series of tests the variable was dowel'diameter with sizes ranging from 3/8" to 1-3/8". Once the shear-deflection history of each component part was obtained, the shear capacity of the joint was predicted. A mode of behaviour (physical model) is presented for each test series and this model is used to predict the behaviour of the steel dowels in shear. The bottom dowel has been modelled as a beam-on-elastic foundation. The top dowel model uses a 3. transformed section for the steel-concrete interactive behaviour. The details of the experimental program and theoretical analysis w i l l be discussed in the following chapters. The model for the bottom dowel behaviour required the value for the foundation modulus of concrete K as a function of dowel size. An auxiliary laboratory test program was carried out to establish;.the K value as a function of dowel diameter. This program is described in detai l in Chapter 3. This investigation on beam-column joints is a continua-tion of previous work that has been done by Kratz^ 1 1 ^ and Peter^1^.. Peter's experimental work covered only #3> #5 and #6 dowels and the method of theoretical analysis was in certain cases different from that being presented here. Also, the experimental procedure was different in certain respects. Peter concluded that significant shear capacities are obtained from the dowel action of steel bars. Birkeland and B i r k e l a n d i n t r o d u c e d the concept that shear between a concrete to concrete interface is developed by f r i c t ion and not by bond. The reinforcement across the interface is stressed in tension, thereby providing the normal force which is required across the interface to develop the f r i c t iona l force. to) 1 MastA ' uses the shear-friction theory in predicting shear transfer across shear planes. The method is applicable to many design problems. A brief description of his method is pre-sented here. 4. F i g . 1 . 2 a shows a concrete specimen with a crack running perpendicular to the reinforcement. V F i g . 1 . 2 a Transverse Crack i n Reinforced Concrete Specimen F i g . 1 . 2 b S h e a r - f r i c t i o n i n Cracked Concrete Specimen Under the a c t i o n of a shear force V , one surface tends u 5 to s l i p r e l a t i v e to the other. As the two surfaces t r y to separate, the s t e e l i s s t r e s s e d i n t e n s i o n . From the freebody of F i g . 1 . 2 b , the f o l l o w i n g expression can be formulated: V u = A s f y t a n * ( 1 - 1 ) or v u = pf tan* ( 1 - 2 ) As where the s t e e l r a t i o p = r—r ^ bd and <J> = angle of i n t e r n a l f r i c t i o n f o r concrete. Mast recommends a range of values f o r tan<j> as 0 . 7 (concrete to concrete and .5. smooth interface) to 1.4 (concrete to concrete and rough inter-face) . The equations presented above assume that sufficient separation occurs at the interface to strain the steel to the yield point. As an example, for #5 bars of intermediate grade steel , a separation of 0.01 inches is required to stress the bars to their y ie ld point. Mast also notes some limitations on his theory in order to prevent unsafe extrapolation beyond current knowledge. The value of <j> has been assumed to be independent of concrete strength and the stress level at the cracked interface. Since this may not actually be so, Mast l imits the term pf to i 15% of the concrete cylinder strength f c . He also recommends that #6 bars (intermediate grade steel) be taken as an upper l imit in shear f r i c t ion design. Hofbeck, Ibrahim and Mattock^' investigated the shear transfer strength of reinforcing dowels (stirrups) crossing a shear plane. Concrete specimens with and without i n i t i a l cracks along shear planes were experimentally tested. When the concrete specimens had an i n i t i a l crack along the shear plane, there was considerable contribution to shear transfer strength by dowel action. For uncracked specimens, the reinforcement is put into tension as a truss- l ike action develops, i . e . , a saw-tooth action as one face tr ies to s l ip relative to the other. The shear-friction design concept, as proposed by Mast, has been successfully applied to several design situations of which the author is famil iar. In one instance, a precast load bearing beam-panel was dowelled into a cast-in-place column. Due to shrinkage, i t was feared that the two concrete surfaces may separate and fr i c t ion would not develop between the two surfaces. In the hope of preventing th is , the cast-in-place column was revibrated within 90 minutes of the i n i t i a l pour in order to "squeeze-out" the excess water and thus minimize shrinkage. In such cases as described above, i t may be useful to consider the dowel action of the steel bars and design the beam-column connection on that basis. The additional work and expense of revibration could be avoided. Also, in order to insure shear-fr i c t ion action, stringent construction tolerances necessitate that the precast units be positioned snugly against the forms of the cast-in-place units. In certain situations, shear keys are provided in columns against which a beam is later cast. Some design engineers consider this a very s t i f f connection and an ideal area for stress concentrations. On the other hand, the design of such beam-column joints on the basis of dowel action provides for a ductile joint as characterized by the shear-deflection behaviour of i n d i -vidual dowels (Appendix 1 and 2). Many connections are subjected to forces aris ing from settlement, creep, and shrinkage. These forces are generally unknown and therefore the connection must possess duct i l i ty in order to accommodate the additional stresses imposed by these forces. The following chapters present an experimental and analytical study of dowel action in a beam-column joint and the results of this work are intended to fac i l i ta te the design of such connections. 7. CHAPTER 2. LABORATORY PROGRAM The laboratory work of forming, casting and curing followed a standard procedure for each test series. This chapter describes the methods involved. 2.1 MATERIAL A l l the concrete was delivered by truck from a local ready-mix plant. Type III (High Early) Portland Cement and 3/4" maximum size aggregate was used in the mix. A slump of 3" was specified for each mix. The deformed bar reinforcing steel was of the type used on construction projects (40 and 60 grade) and was obtained from a local supplier - cut and bent to the required shape. Steel samples were tested in tension to determine y ie ld stress f and ultimate stress f . Three concrete cylinders were tested at the beginning of each test series and three at the end. The value of the compressive strength f* which was used in the analysis of the test results was an average of the six tests. The concrete and steel properties for each test series are tabulated in Table 2.1. 2.2 FABRICATION OF TEST SPECIMENS After the plywood forms were coated with o i l , the pre-fabricated cages of reinforcement were positioned in the forms. During pouring, the concrete was consolidated with a vibrator. 8. Six companion cylinders (4" x 8") were poured with each test series./ _ < . -Wet burlap sacks were placed over the poured specimens and everything was covered with.a plastic sheet to prevent mois-ture loss. The burlap sacks were repeatedly moistened everyday. The forms were stripped two days after pouring, but moist curing continued for a total duration of 10 days, after which the plastic and burlap sacks were removed and the specimens left to dry cure on the laboratory f loor. Table 2 . 1 Concrete and Steel Properties TEST SERIES • f i (Ksi) f (Ksi) f u (Ksi) K TESTS 6.33 for a l l specimens BOTTOM DOWEL TESTS Bar Sizes #3 54 79 #4 56 80 #5 66 101 #6 4.2 for all- 71 103 #7 specimens • :. 73 110 #8 69 97.5 #9 69 112 #10 66.4 102 #11 66.4 93 TOP DOWEL TE Bar Sizes STS #4 5.675 65 79,5 #5 3 . 13 70.5 110 #6 3.13 66.4 100 #7 5.675 6 9 .5 110 #8 3.13 64 104 #9 5.675 6 2 .7 109 #10 5,675 62 .7 87.5 #11 6.0 62 .7 87 .5 CHAPTER 3. FOUNDATION MODULUS OF CONCRETE K As previously mentioned, the theoretical analysis for the bottom dowels required the value for the foundation modulus of concrete K as a function of dowel size. To determine K for each dowel s ize, i t was decided to test 4 dowel sizes and inter-polate for the others. Three specimens were cast for each of dowel sizes #4, #6, #8 and #11. Pouring and curing of concrete followed the standard procedure as described in Chapter 2. A typical specimen is shown in F i g . 3.1a. Only the bottom-half of the dowel was embedded in concrete. The specimens were tested in a Baldwin loading machine with load and deflection simultaneously recorded on a X-Y plotter. F ig . 3- lb is a schematic representation of the laboratory set-up. F ig . 3-2 shows a specimen in the Baldwin just before the beginning of a test . The deflection of the steel dowel was measured with l inear transformers positioned at each end of the dowel. The x-y plotter recorded the average of the two deflections and also the load which was applied continuously at an average rate of 6 Kips per minute. There were no v is ib le signs of distress in the concrete specimen unt i l a substantial load was applied'. Crushing and spalling of the concrete immediately below the dowel were the f i r s t v is ib le signs of progressive fa i lure . For bar sizes #4 and #6, the extent of fai lure was only crushing of the concrete below the dowel. For the #8 and #11 dowels, the usual crushing and spalling occurred at the i n i t i a l stages of loading. Also, a 1.1. LMWM6 PLATE *3 B E f N F O R C I N G S T E E L , Pig. 3:la Foundation Modulus Test Specimen T o JC-Y PLOTTER LIME LOAD LINEAR TRANSFORMER S T E E L P L A T E S GLUED TO S P E C I M E M L c * ° $ ^ Fig . 3-lb Test Specimen In Baldwin F i g . 3.3 F a i l u r e o f T e s t S p e c i m e n 1.3. hairl ine crack began to propagate vert ica l ly downwards and at the completion of the test , the crack had progressed to the base of the specimen. An "explosive" type of fai lure was prevented by the horizontal #3 reinforcing bars (Fig. 3 . 1 a ) . F ig . 3 . 3 shows the specimen at the end of the test. The load-deflection graphs for the #8 dowel tests are presented in F ig . 3 - 4 . This set of graphs is typical of the other series. In order to amplify the straight line portion of the graphs, the vert ical scale on tests 2 and 3 was doubled. This fac i l i ta ted in establishing the value for the slope of the graph. The foundation modulus K is determining the slope of the straight-l ine portion of the load-deflection graphs and dividing the value by the width of the specimen which was 8 inches. slope = Kl£ s l o p e A6 i n . K = ^Lope Ksi Therefore the constant K denotes the reaction per unit length of the beam (dowel) where the deflection is equal to unity (Timoshenko ). -The results of a l l the tests are tabulated in Table 3 . 1 . and F ig . 3 . 5 is a plot of the average K value for each dowel size. The graph was drawn by joining the experimental points.and extrapolating to the #3 dowel size. At each averaged point is a heavy dark line which gives the range in the experimental values. Table 3 . 1 Foundation Modulus Tests DOWEL SIZE TEST NO. FOUNDATION MODULUS K Ksi AVERAGE K Ksi 1 512 #4 2 536 457 3 323 1 820 #6 2 875 787 3 665 1 925 #8 2 • 795 863 3 870 1 1 ,010 #11 2 986 1 ,005 3 1 ,020 CONCRETE: f 1 c = 6 , 3 3 0 psl The graph in F ig . 3 . 5 is for a concrete strength f* of 6 , 3 3 0 psi as determined from the standard cylinder tests. This graph can be scaled for other values of concrete strengths by the following method. The modulus of e las t ic i ty of concrete E c is a function of^jfTas gi ven by t h e e m p i r i c a l e q u a t i o n : E = 3 3 w ^ 2 J f * " , \ (w = u n i t weight o f h a r d e n e d concrete in c c pcf) and the foundation modulus K varies direct ly with E K2 ; Therefore = K l The factor for scaling the graph of F ig . 3-5 to other concrete strengths is or This has been done for several concrete strengths as shown in F i g . 3 . 6 . The foundation modulus K is not too sensitive to varying concrete strengths since the curves of F ig . 3-6 l i e in a narrow band. i M ! 1 I 'M I M i " L i n LLLLUJ-.i.:..-..j...i.. : CHAPTER 4 . BOTTOM DOWEL TESTS 4 . 1 EXPERIMENTAL PROCEDURE In order to determine the shear capacity of the bottom dowels, 36 concrete specimens, as shown in Pig. 4 . 1 , were formed and cast. The variable involved in this study was the dowel size Pour specimens were cast for each dowel size ranging from #3 to #11. -N PLAN #3 T I E S a Z • # S BARS BOTTOM DovvEL E L E V A T I O N ') F i g . 4 . 1 Bottom Dowel Specimen The method of pouring and curing of concrete was as described in Chapter 2. It was desired to load the protruding steel dowels in shear only. For this purpose, a wide flange beam was clamped to 20. the steel dowels and the load applied at the mid-point of the beam. F i g . 4.2 and 4.3 show the positioning of the test specimens (two per test) and the method of load application. The load was applied with an Amsler hydraulic jack. The deflection of the steel dowel was measured at the column face (positions 1 and 2, F ig . 4.2).- Since the deflection probes from the transformers were positioned on the dowel i t s e l f , the steel clamps were attached 1/4" away from the column face to provide the necessary space for the probes. As a result of this set-up, some bending moment would be developed in the dowel at the column face. This is considered in the theoretical analysis; Linear transformers were again used to measure the deflections and both deflections and load were simultaneously recorded on punched paper tape on a Digi ta l Data Acquisition unit. A computer program converted the paper tape data into the shear-deflection graphs which are presented in Appendix 1. Four curves were obtained for each dowel size. For simulating the actual column conditions, the concrete column specimens were compressively stressed to 1 Ksi with the tension rods (Fig. 4.2). The force in each tension rod was de-termined with a strainsert bolt . 4.2 ANALYSIS The behaviour of the bottom dowel embedded in the con-crete column specimen was modelled as a beam-on-elastic foundation. F ig . 4.4b shows a semi-infinite beam on an elastic foundation as discussed in Timoshenko ' . This model is assumed to represent the section shown in F ig . 4.4a. T E S T S P E C I M E N TENSION ROD S T E E L CLAMP BRONZE S H I M TRANSFORMER PROBE FOR MEASURING 1/ DEFLECTION AT COLUMN F A C E [ L O A D P i z LOAOING B E A M T7777T / / / / / / / / / i i / / / / STRAINS6KT BOLT I /T7T Fig . 4.2 Loading Apparatus For Bottom Dowels 2 3 . BOTTOM DOWEL CONCRETE COLUMN SPECIMEN RESIOM MODELLEP AS A BEAM-OH-ELASTIC FoUMPATIOM Fig . 4.4a Bottom Dowel Specimen BOTTOM OOWEL E L A S T I C F O U N D A T I O N " Fig . 4.4b Bottom Dowel Specimen as a Beam-on-elastic Foundation The solution to the di f ferent ia l equation for a semi-inf in i te beam on an elastic foundation as shown in Pig. 4 . 4 b is y(x) = e" e x (Pcosgx --pMc fcosgx-sinBx-]•) ( 4 - 1 ) where 3 = h ,^ K = Foundation Modulus E = modulus of e last ic i ty of the beam I = moment of inert ia of the beam The values obtained for the foundation modulus in Chapter 3 were used in calculating the g term. Since the bottom dowel specimens had a concrete strength of 4 , 2 0 0 p s i , the values for the foun-dation modulus were scaled by using a factor of •^ H§- = 0 . 8 1 5 . (Refer to page 17) The units of K are Ksi and the value for E in a l l the analysis was 2 9 , 0 0 0 Ksi (modulus of e last ic i ty of the steel dowels). To determine the deflection at the column face, the value x = 0 must be substituted into equation 4 - 1 . y;(x=0) = 2 ^ (P- PM o) ( 4 - 2 ) or rearranging P = 2g 3 EIy 4. 6 M ( 4 - 3 ) O • As previously mentioned, since some room had to be pro-vided forL-positioning the deflection probes onto the dowels, the steel clamps were not snug against the column face. Hence, the bending moment that is developed in the dowel at the column face is opposite in sign to that shown in Fig; 4 .4b. With the change in sign, equation 4-3 becomes P = 2g3EIy - g M Q . (4-4) When M Q is zero, equation 4-4 reduces to P = 2g3EIy (4-5) The two extreme values of "M • are zero and M - the ° P plast ic moment of the steel dowel. Equations 4-4 and 4-5 were superimposed on the shear-deflection curves of dowel sizes #4, #7 and #11, as shown in Pig. 4 .5a , 4.5b and 4 .5c. The experi-mental shear-deflection curve is an average of the 4 curves as shown in Appendix 1 for the corresponding dowel size. Table 4.1 l i s t s the variables involved in this analysis. As can be noted from the graphs, the theoretical curves are below the experimental curve for the #11 dowel up to a de-f lection of 0.04". As the dowel size is reduced the two theoreti-cal curves shift closer to the experimental curve u n t i l the upper l ine (equation 4-5) begins to exceed the experimental results at a deflection of 0.02" (#4 dowel s ize) . Pig. 4.6 is a plot of the shear at 0.03" deflection for the range of bar sizes tested. Equations 4-4 and 4-5 are also plotted with the value of "y" equal to 0.03". The majority of the experimental points are the two extreme equations. (Heavy dark vert ica l lines show the range in the experimental results . ) Table 4.2 l i s t s the values required in plotting F ig . 4 .6 . The 3 term was evaluated for a concrete strength of 4,200 ps i . Table 4 . 1 Bottom Dowel V a r i a b l e s Dowel Siz e Diameter d ( i n . ) ' foment of I n e r t i a I ( i n . 4 ) -Foundation Modulus K ( K s i ) ( f o r f i = 4200 p s i ) 3 = \f~P ( 1 ) / V 4 E I ( i n . ) (E = 2 9 0 0 0 K s i ) P l a s t i c moment M = 0.l67f a 3 p. y (KIP-IN j #3 0 . 3 7 5 0 . 0 0 0 9 7 180 1 . 1 3 0 . 4 8 #4 0 . 5 0 . 0 0 3 0 6 372 1 . 0 1 1 . 1 7 #5 0 . 6 2 5 0 . 0 0 7 5 520 0 . 8 8 2 . 7 #6 0 . 7 5 0 . 0 1 5 5 640 0 . 7 7 5 . #7 0 . 8 7 5 0 . 0 2 8 6 670 0 . 6 7 8 . 1 5 #8 1 . 0 0 . 049 7 0 0 0 . 5 9 1 1 . 5 #9 1 . 1 2 0 . 0 7 7 5 730 0 . 5 3 1 6 . 2 #10 1 . 2 5 0 . 1 2 7 7 0 0 . 49 2 1 . 6 #11 1 . 3 8 0 . 1 7 8 815 0 . 4 5 2 9 . 2 Table 4.2 Shear at 0.03 Deflection f* = 4200 psi • E = 29000 Ksi y = 0.03" Dowel Size Average Shear at 0 . 0 3 " Deflection (Experiment) (KIPS) P = 2g3EIy - BMp (Kips) P = 2$3EIy (Kips) #3 2.5 1.9 2.4 #4 3.6 4.3 5.5 #5 4.9 6.5 8 .9 #6 10. 4 8.6 12. 4 #7 10.4 9.5 15. #8 13 .6 10.9 17.7 #9 18 .4 11.9 20.5 #10 27 .4 13.3 2 3 . 8 #11 30. 14.4 27 .4 3 2 . The "knee" of the shear-deflection curves occurs (in most cases) at around the 0 . 0 3" value of deflection with the con-crete s t i l l in the elastic range. At this deflection there were no vis ible signs of crushing or spalling' of the concrete around the dowel. Thus, for this reason the theoretical beam-on-elastic foundation equation was compared to the 0 . 0 3 " value. Extrapolating the equation to higher values of deflection would result in over-estimating the shear capacity, since the concrete under the dowel begins to crush and crack and the shear-deflection curves assume a shallower slope. Nevertheless, the experimental and theoretical values are in close agreement, at the 0.03" value for the entire range of dowel sizes, with some sizes experiencing more deviation than, others. Pig. 4 . 7 is a plot of equation 4-5 for varying values of concrete strengths. As was shown in Pig. 3 . 6 , the foundation modulus K is not very sensitive to differences in concrete strength. Hence the 3 term is also rather insensitive to con-crete strength, with the result that the two graphs (Pig. 4 . 7 ) do not have much variation. For a 50$ increase in concrete strength the maximum increase in shear capacity (for a #8 bar) is about 17%. The ultimate shear for each dowel was taken to be the stage at which the concrete was crushing under the dowel and no increase in load was possible. The ultimate shear and the shear at 0 . 0 3 " deflection is plotted in F ig . 4 . 8 . In most cases the ultimate shear is double that at 0 . 0 3 " deflection. A design based on the 0 . 0 3 " deflection curve would provide a safety factor of 2 in most cases. 3.5. 4 . 3 . COMPARISON OF RESULTS WITH PREVIOUS WORK The results of these tests were compared to previous work which has been done with reinforcing steel dowels and metal studs. F i g . 4 . 9 presents the experimental results and two expressions from the ACI-ASCE C o m m i t t e e . T h e r e the allowable shear for reinforcing steel dowels is given by the expression V = A/(A f cose) 2 + ( 1 . 5 d 2 f ^ i n e ) 2 * ( 4 - 6 ) <v s s c where d = sum of the diameter of bars or dowels 9 = angle between beam-column interface and the dowel. For 6 = 90°. (as is the case in this study), the expression reduces to V = 1 . 5 d 2f£. <4-7) For a metal stud embedded in concrete, the allowable shear is given by V = H O d 2 / ^ ( 4 - 8 ) where d = diameter of stud. ' Equations 4-7 and 4-8 are plotted and the relative positions of the graphs show that there is a considerable safety factor inherent in these expressions. (2) Also plotted is Mast's v expression . V u = Asfytan<l>. ( 4 - 9 ) Although the method of testing the specimens did not have any shear-friction action, the expression was nevertheless compared to the experimental results by using the lowest value of P i g . 4 . 1 0 Bottom Dowel Specimens at Ultimate Load 3.8. tancj) = 0 . 7 as suggested by Mast. There is excellent agreement between Mast's expression and the experimental results up to the #6 dowel. However, extrapolating the equation to the larger dowel sizes results in overestimating the ultimate shear as obtained in this experiment. Some of the larger dowel.sizes reached ultimate deflect tions of 0 . 5 " to 0 . 7 " (Appendix 1) while the smaller ones ranged between 0 . 2 " to 0 . 4 " . An "average" duct i l i ty factor y for an individual dowel (based on the 0 . 0 3 " deflection value as an elastic or y ie ld l imit) would be calculated as p 0 . 0 3 ~ J - J . . • Pig. 4 . 1 0 shows several specimens at ultimate load and the extent of damage to them. The test specimens had only two column ties (Pig. 4 . 1 ) and there was substantial diagonal cracking and spalling of the concrete at ultimate load (Fig. 4 . 10 and 4 . 11 ) for the larger dowel sizes. The smaller sizes experienced only local crushing and spalling under the dowel. DIAGONAL CRACKS STEEL DOWEL Fig . 4 . 1 1 Crack Pattern at Ultimate-Load 3.9. More column ties should be provided at such beam-column connections to prevent excessive cracking and spalling of the concrete. This hoop reinforcement would provide additional con-finement to the concrete and as such increase the ultimate capacity of the dowel. A study of beam-column connections Was conducted by (4) Hanson and Connorx . They found that confinement of the concrete at c r i t i c a l sections such as beam-column connections increases the duct i l i ty of the jo int . The hoop reinforcement resists the tendency of the joint to expand under multiple reversals of beam loading. For joints that are confined on at least three sides by beams or spandrels, hoop reinforcement in the joint region is not required. For uhcdnfined or isolated beam-column joints , hoop reinforcement is most beneficial . 40. CHAPTER 5• TOP DOWEL TESTS 5.1 LABORATORY TEST PROGRAM To determine the shear capacity of the top dowels in a beam-column jo in t , 16 beam specimens were tested. Pig. 5-1 shows a typical beam specimen. Again the variable was dowel diameter. Two such specimens were cast for each bar size - #4 to #11 i n -clusive. Forming, pouring and curing of the test specimens were done by the standard procedure as outlined i n Chapter 2. The distance from the beam end to the f i r s t stirrup was kept constant at 1 inch. A previous study by P e t e r s h o w e d that the shear capacity was signif icantly influenced by the distance to the f i r s t stirrup.. He varied the distance from 1 inch to 3 inches and obtained the maximum shear with the 1 inch position. His tests were done for a #5 top dowel only. To determine i f the beam stirrup spacing has any effect on the shear capacity of the top dowel, one-half of the beam had the stirrup spacing as required by the ACI code (318-71) and the other half had double the specified spacing. The testing apparatus is shown in F ig . 5.2 and F ig . 5-3. Not.shown in F ig . 5.2 are two end r o l l e r restraints placed against the sides of the test beam and clamped to the end supports. (These are v is ible in the photographs, F ig . 5.3.) The rol lers prevented the beam from rotating lateral ly as the load was applied. Deflections were measured with l inear transformers at STIRRUPS TOP DOWEL d (REFER To TABLE Fig . 5.1 Top Dowel Specimen Table 5 . 1 Top Dowe 1 Test Specimens Top Dowel Size S (In.) h (in.) Stirrup Size #4 5 . 1 2 #3 #5 12 #3 #6 • 5 *\ 12 #3 #7 12 #3 #8 16 #3 #9 6 , 16 #4 #10 3k 16 #4 #11 3k 16 #4 S E C . A - A •771 T O P D O W E L LOAD T E S T B E A M DEFLECTION (f) DEFLECTION 0 Pig. 5.2 Top Dowel Test Apparatus 4 4 . F i g . 5-3 Top Dowel T e s t p o s i t i o n s 1 and 2 ( F i g . 5 - 2 ) and bo t h l o a d and d e f l e c t i o n s were r e c o r d e d on punched paper t a p e . As b e f o r e , a computer program c o n v e r t e d the punched paper tape d a t a i n t o the s h e a r - d e f l e c t i o n graphs which a r e p r e s e n t e d i n Appendix 2 . 5 . 2 ANALYSIS As i n t h e bottom dowel a n a l y s i s , a model was chosen f o r the top dowel b e h a v i o u r . F i g . 5 . 4 shows the end r e g i o n o f the t e s t beam w i t h a shear f o r c e V a p p l i e d t o t h e dowel at the beam end. I n o r d e r t o a n a l y z e t h i s ' end r e g i o n as a u n i t , the s e c t i o n i s t r a n s f o r m e d as shown i n F i g . 5 . 5 . I f t h e s e c t i o n shown i n F i g . 5 - 5 i s assumed t o a c t as a 1" l o n g c a n t i l e v e r beam, the moment t h a t i s developed b e f o r e the s e c t i o n r u p t u r e s i n t e n s i o n I s f r J t M = - f — i ( 5 - 1 ) ^b where the modulus o f r u p t u r e o f c o n c r e t e f r . = 7 . 5 A J f J \ 1^ = moment o f i n e r t i a o f t r a n s f o r m e d s e c t i o n and = d i s t a n c e from t h e n e u t r a l a x i s o f t h e t r a n s f o r m e d s e c t i o n t o the extreme f i b e r i n t e n s i o n . With t h e c o n c e n t r a t e d l o a d V at t h e end of the c a n t i l e v e r M = V * l " or V * l " = f r T t . — ( 5 - 2 ) T a b l e 5 . 2 a l i s t s a l l t h e v a r i a b l e s r e q u i r e d t o p l o t e q u a t i o n 5 - 2 . F i g . 5 . 6 a shows the e x p e r i m e n t a l r e s u l t s and a Fig . 5.4 Shear V acting on Top Dowel h = A R E A O F T o p D O W E L MODULAR RATIO > i = J a . a | o F ig . 5.5 Transformed Section Table 5.2a Transformed Section Properties Top Dowel Size f r • 7 - 5 ^ T i (Ksi) ! h ( i n . 4 ) Yb (in.) V = (Kips) Yb #4 0.565 3.86 0.75 2 .9 #5 . 0.42 4.8 0.72 2.8 #6 0.42 6 .08 0.71 3.6 #7 0.565 7.73 0 .69 6.3 #8 .0.42 8.46 0.63 5.65 #9 0.565 9.14 0.57 9. #10 0.565 11 .9 0.55 12.2 #11 0.580 13-57 0.54 14.6 Table 5.2b Normalized Experimental Results Dowel Size V = KIPS) b •: f 1 c Ksi V c Vexp. Shear at 0 .03" Defln. (KIPS) Vexp. f 1 c #4 2.9 5 . 6 7 5 0.511 2.34 0.412 #5 2.8 3.130 0 .894 2.34 0.747 #6 3 . 6 3.130 1.15 3 O.96 #7 6 . 3 5 . 6 7 5 1.11 4.5 0.79 #8 5 . 6 5 3 . 1 3 1 .81 5.1 ,1.63 -#9 9 5 . 6 7 5 1.58 7.2 1.27 #10 12.2 5 . 6 7 5 2.15 7 .5 1.32 #11 14.6 6.0 2.43 9 . 3 1.55 plot of equation 5 - 2 . Due to varying concrete strengths and hence varying f , the graph of equation 5-2 is not a: smooth and constantly increasing curve. In F ig . 5 . 6 b , equation 5-2 and the data points have been normalized. Each value of F ig . 5 . 6 a has been divided by the corresponding concrete strength f* for that particular case (Table 5 . 2 b ) . Both analytical and experimental curves exhibit similar shapes. This model is reasonably accurate up to the #8 dowel size and begins to deviate substantially for the larger dowels. The cantilever model requires that the end condition be fixed, i . e . , a fixed condition at the f i r s t st irrup location. This condition holds for the smaller dowels where the f i r s t stirrup does not y ie ld and bending occurs in the top dowel within the 1" cantilever distance. On the other hand, the large dowels simply w i l l not bend in a 1" distance and hence tend to yie ld the f i r s t stirrup in direct tension. Thus the fixed condition at the f i r s t stirrup would not hold true. The above model does not take into account any direct tensile stresses or the effect of yielding of the f i r s t s t irrup. These two points w i l l be considered now. The f i r s t v is ible sign of any cracking in the top dowel tests was as shown in F ig . 5-7> where a longitudinal crack pro-pagated from the top dowel out towards the beam sides and then horizontally along the beam. The area over which direct tension occurs is a rectangle 6" (beam width) by 1" (distance to f i r s t s t irrup) , i . e . , 6 square inches. With the tensile strength of 5 2 . P i g . 5 . 7 C r a c k P r o p a g a t i o n i n Top Dowel T e s t .5.3. concrete taken as 7 . 5A^^J the shear force required to crack the section can be calculated -directly as V = .7.5AFT *6 (5-3) The results of this calculation are l i s ted in Table 5.3 and F i g . 5-9 shows a plot of equation 5-3 in relation to other experi-mental values. The effect of the f i r s t stirrup yielding shall be con-sidered next. F ig . 5 .8 i l lustrates the condition at the f i r s t stirrup where the shear force V is resisted by the tension in the s t irrup. The values given in Table 5.4 are plotted in F i g . 5.9,as two discontinuous straight lines (stirrup sizes #3 and #4). These two lines agree reasonably well with the ultimate values obtained from experiment. In this test series, i t was d i f f i cu l t to compare the experimental and model deflections. Since the f i r s t st irrup w i l l strain and therefore extend under the application of load, the deflection that is measured at positions 1 (or 2.) is not identical ly the same as the deflection of the top dowel vert ica l ly above position 1 (or 2 ) . (Refer to F ig . 5 . 2 . ) As shown in F ig: 5 . 9 , the ultimate shear is considerably higher than that obtained at 0 . 0 3 " deflection.. The #4 dowel fai led in shear at ultimate (Fig. 5 . 1 0 ) . In the case of #7 and #8 dowels, the f i r s t st irrup ruptured at ultimate. (Fig. 5 . 1 1 ) . The #9, #10 and #11 test specimens had #4 size stirrups and in these three cases the concrete beam fai led in shear (at the end with the larger stirrup spacing - F ig . 5 . 1 3 ) , Table 5-3 Direct Tensile Force Dowel Size f r = 7 ' 5 / f c ' ( K s l ) V = 7 . 5 A ^ * 6 (KIPS) 0 . 5 6 5 3.39 #5 0 .42 2 . 5 2 #6 0 .42 2 . 5 2 0 .565 3.39 #8 0 .42 2 . 5 2 #9 0 .565 3.39 0 . 5 6 5 3.39 #11 O.58O 3.48 F i g . 5 . 8 Y i e l d i n g of the F i r s t S t i r r u p Table 5 .4 Tension at S t i r r u p Y i e l d Dowel Size S t i r r u p S ize A s S t i r r u p 2 Area (in.) f ( K s i ) 2T = A s f KIPS J #4 #5 #6 #3 0.22 54 11.88 #7 #8 #9 #10 #4 0. 40 60 24 #11 <b •! : ; • .7. ; I : j -; • M 0OW£L\ P/AM£TER(J/a /NCH) /7G. SIS : I I ' j 1 I ! I 1. I 5 7 . 5 8 . 59. Even though the stirrup spacing was varied on both halves of the test beams, no significant differences in the behaviour or cracking patterns was observed between the two ends. Pigs. 5.12 and 5.13 show the crack patterns during testing and at ultimate load. 6.0. CHAPTER 6. THE JOINT:SUM OF TOP AND BOTTOM DOWELS The previous two chapters have discussed bottom and top dowel tests and analysis. In this chapter the results are com-bined and the shear capacity of a beam-column joint is calculated. The graphs in F ig . 6.1 are the theoretical values obtained previously, plotted for the same deflection (0.03 in . ) for both bottom and top dowels. As can be noted from the graphs, the bottom dowels contribute most to the shear capacity of a jo int . In nearly a l l cases, the shear for a top dowel is between 33 - 44% of that for the same size bottom dowel. For the bottom dowel curve, the value for the foundation modulus K (and hence g) corresponds,!; to a Concrete strength of f* equal to 4,000 ps i . Similarly , the modulus of rupture f is calculated for the same concrete strength and equation 5-2 plotted. The joint shown in F ig . 6.2 could be considered as a design problem. A l l the shear is to be transferred by dowel action and hence the shear capacity of this joint can be calculated by using the graphs presented in F ig . 6.1. In determining the shear capacity of the bottom dowels in this case, the expression P = 2g3EIy is used. The bending moment term gM is neglected in this case because i t is extremely doubtful that the plast ic bending moment could be developed in the bottom dowels at the beam-column interface. The deflection of the bottom dowels is symmetric about the beam-column interface with the point of inf lect ion occurring at the beam-column interface. t I ! ! ! BOTTOM, AND TOP ; POWELS 6 .! i ! ' .7,..; • j ! : ; 8 : DOWEL D/AMEfERfl/B /#CH) /0 6.2. P i g . 6.2 D e s i g n Beam-Column J o i n t \ 63. From the graphs of F i g . 6 . 1 : Bottom Dowel shear: 2 x 17.4 34.8 k Top Dowel shear: 2 x 3.0 6.0 k Total 40.8* The shear capacity of this jo int , assuming dowel action only, is 40 kips (service load shear). As shown in F ig . 6 . 2 , the f i r s t st irrup should be placed around each top dowel individual ly . This provides the necessary tie-down force to the top dowels and hence a greater contribution to the shear capacity. As was mentioned before, the shear capacity of the top dowels is sensitive to the distance to the f i r s t s t irrup. Also, since the deflection of the bottom dowels is assumed to be symmetrical about the beam-column interface, the net beam deflection would be 0.03 + 0.03 - 0.06" at a shear force of 40 kips. At such small deflections, the stress pattern around one dowel is assumed not to influence the behaviour of i t s neighbouring dowel. Hence the shear capacity of the dowels is assumed to be additive d irect ly . At large deflections, the inter-action and overlapping of stress patterns between neighbouring dowels may be s ignif icant. .Therefore the ultimate shear capa-c i t ies would not be additive d irect ly . bending moment at the beam-column interface. These effects, however, w i l l only help to increase the shear capacity of the joint and therefore a design based only on the dowel action of the reinforcing bars provides a lower bound on the joint capacity. This analysis has neglected the effect of f r i c t ion and Fig . 6.3 shows the relative positions of the graphs for ultimate shear for!both top and bottom dowels. These graphs, however, can riot be used to accurately predict the ultimate shear capacity for a combination'-.of top and bottom dowels because of interactive stress effects between dowels. 6.6. CHAPTER 7. CONCLUSIONS 1. The beam-on-elastic foundation analogy forms a reasonable method of modelling the behaviour of the bottom dowel and i ts appl icabi l i ty could be confidently extrapolated to other s i tu -ations which are not exactly the same as those presented here. 2. The beam-on-elastic foundation model should not be extrapolated to large values of deflection; 0.03" deflection is a recommended upper l imi t . 3. A beam column joint designed solely on the basis of dowel action of the reinforcing steel.bars may provide adequate shear capacity. 4. The bottom dowels are the major shear-carrying components of a beam-column jo int . * 5. The top dowel should be well anchored by the f i r s t stirrup i f i t is to contribute to the shear capacity of the jo int . 6. The variation of stirrup spacing in the beam specimens did not have any effect on the shear capacity of the top dowels. 7. There is a wide range of values for the foundation modulus between the small and large dowels - 200 to 1,000 Ks i . 67. REFERENCES 1. Peter, B.G.W., M.A.Sc. Thesis, University of Br i t i sh Columbia, Vancouver, 1971. 2. Mast, R. F . , "Auxiliary Reinforcement in Concrete Connec-tions", J . Str . Div. ASCE, June 1968, pp. 1485-1503-3. Hofbeck, J . A . , Ibrahim, I . O . , and Mattock, Alan H . , "Shear Transfer in Reinforced Concrete", ACI Journal, February 1969, pp. 119-128. 4. Hanson, Norman W. and Connor, Harold W., "Seismic Resistance of Reinforced Concrete Beam-Column Joints", J . Str . Div. ASCE, October 1967, pp. 533-560. 5. Timoshenko, S. , "Strength of Materials - Part II", Princeton, N . J . , D. Van Nostrand Company Inc. , March 1956. 6. Birkeland, Phil ip W. and Birkeland, Halvard W., "Connections in Precast Concrete Construction", ACI Journal, March 1966, PP. 345-368. 7. K r i z , L . B. and Raths, C. H . , "Connections in Precast Con-crete Structures - Strength of Corbels", PCI Journal, February 1965, pp. l 6 - 6 l . 8. Gaston, J . R. and K r i z , L . B . , "Connections in Precast Con-crete Structures - Scarf Joints", PCI Journal, June 1964, PP. 3 7 - 5 9 . 9. ACI-ASCE Committee 512, "Suggested Design of Joints and Connections In Precast Structural Concrete", ACI Journal, August 1964, pp. 921-937-10: Prestressed Concrete Institute, "PCI Design Handbook Precast and Prestressed Concrete", Chicago, I l l i n o i s , 1971. 11 . Kratz, R. D . , M.A.Sc. Thesis, University of Bri t i sh Columbia, Vancouver, 1970. 1 2 . Dulacska, Helen, "Dowel Action of Reinforcement Crossing Cracks in Concrete", ACI Journal, December 1 9 7 2 , pp. 7 5 4 - 7 5 7 . 1 3 . Anderson, Arthur R. , "Composite Designs in Precast and Cast-in-Place Concrete", Progressive Architecture, September I 9 6 0 , pp. 1 7 2 - 1 7 9 . 6 9 . APPENDIX 1. BOTTOM DOWEL EXPERIMENTAL GRAPHS The f o l l o w i n g g r a p h s a r e t h e e x p e r i m e n t a l s h e a r - d e f l e c -t i o n r e s u l t s f o r t h e b o t t o m d o w e l t e s t s . E a c h g r a p h i s l a b e l l e d a c c o r d i n g t o t h e n o t a t i o n u s e d i n F i g . 4.2 i n C h a p t e r 4. F o r e x a m p l e , a c u r v e l a b e l l e d as 1-A i n d i c a t e s t h e de-f l e c t i o n a t p o s i t i o n 1 o f t e s t s e r i e s A. 79. APPENDIX 2. TOP 'DOWEL. EXPERIMENTAL GRAPHS The following graphs are the experimental shear-deflec-tion results for the top dowel tests. Each graph is labelled according to the notation used in Pig. '5.2 in Chapter 5. For example, a curve labelled as 1-A indicates the de-f lection at position 1 of test series A. *08 •18 0.0 3.0 S H E A R ( K I P S ) 6.0 9.0 12.0 J L 15.0 18.0 _L 21.0 •£8 *98 


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