R E I N F O R C ED CONCRETE B E A M - C O L U MN J O I N TS FOR S E I S M IC L O A D I NG P a rt II - E x p e r i m e n t al Results

R E I N F O R C E D CONCRETE B E A M - C O L U M N J O I N T S FOR S E I S M I C L O A D I N G

P a r t II - Experimental Results

R. C. Fenwick and H. M. Irvine*

ABSTRACT

The results of tests on four beam-column joint specimens are reported.

It is shown that a joint which contained bond plates to prevent a bond failure of the flexural reinforcement in the joint, and was proportioned to limit yielding of the steel in this zone, had a markedly superior performance to specimens designed to comply with the ACI-318-71 code, or the proposals of May 19

76

for the revision of the N. Z. Ministry of Works and Development code of practice for the design of public buildings

(PW 81/10/1).

1. BEAM-COLUMN JOINT TEST PROCEDURE The theory relating to the earthquake resistant design of beam column joints was reviewed and extended in an earlier p a p e r ^ and need not be repeated here.

The four tests reported herein were designed to compare the performance of joints proportioned in accordance with existing practice with a special unit in which yield in the joint zone was controlled and bond slip was prevented by the use of bond plates welded to the flexural steel.

In designing the tests a careful watch had to be kept on the costs involved. For this reason the units' dimensions were kept small. This economic constraint, together with the size of the existing testing frame, resulted in a unit with a rather long column which gave a relatively flexible structure requiring a high section to displacement ductility ratio.

The column axial load was taken as zero to simplify the tests and to create a severe loading condition for the joint.

With no axial load, small joint dimensions (thereby accentuating the bond problem) and the high ratio of section to displacement ductility, the test conditions were severe compared with previous tests reported in the literature (see References 3-8 in Reference 2 and References 13-37 , 39-44 in Reference 3 ) .

The test set-up is illustrated in Figure 1. The units were tested in a horizontal position in a frame that was bolted down on to the test floor. The joint zone reinforce- ment for the four units is shown in Figure 2.

Unit 1 was designed to comply with the ACI 318-71 code. Unit 2 was the first of two units in which bond plates were detailed to prevent slip. Unit 3 was designed by Mr.

I. C. Armstrong (District Structural Engineer, M.W.D., Auckland) to comply with the draft revision PW 81/10/1 - May 1976. Finally, Unit 4 was an improved version of Unit 2.

The main features of these are listed in Table 1. In all cases the columns had an appreciably greater theoretical ultimate

* Senior Lecturers in Civil Engineering, University of Auckland.

flexural strength than the beams.

The concrete for the tests was supplied by a local ready mix company. The specified cylinder strength was 35 MPa with a 75 mm slump and a maximum aggregate size of 10 mm.

The concrete strengths listed in Table 1 are the averages of three 300 mm x 150 mm cylinder crushing tests for each mix.

1.1 Bond Plate Design Details (Units 2 and 4) The units with bond plates were designed to transmit all the joint shear by diagonal strut action (see Figures 3 and 2 (b) and

(d)). No bond transfer was assumed for the main flexural steel in the joint zone and, consequently, the complete change in force in the bar, from yield in compression on one side to yield in tension on the other, was taken out on the bond plate which

supported the diagonal compression force on the concrete.

The plates were proportioned so that the bearing stress on the concrete was approximately at the cylinder crushing strength. There was no evidence of local crushing adjacent to the plate so that it is likely that the procedure is conservative, possibly overly so bearing in mind the con- finement of the diagonal strut.

To reduce the possibility of a local brittle failure in the bar due to high welding

shrinkage stresses, the bond plates were cut in two and each half was then butt welded to the bar. The junctions between the two halves of the plate were then lightly welded together.

If the bar between the bond plates yields (in this region of the bar only yield in tension is possible), then, on load reversal, the bar would slip back through the concrete to allow the second bond plate to bear against the concrete.

Clearly, to prevent this slippage and the consequent degradation of stiffness, it is necessary to prevent, or at any rate limit, the inelastic extension of the flexural reinforcement in the joint zone.

In Unit 2 an attempt was made to limit the yield extension between the bond plates

B U L L E T I N OF THE NEW Z E A L A N D N A T I O N A L SOCIETY FOR E A R T H Q U A K E ENGINEERING, V O L . 10, NO. 4, DECEMBER 1977

by the addition of extra flexural reinforce- ment in the beam. This took the form of four 12 mm bars which were placed on the inside of the main flexural reinforcement cage.

These bars extended between the bond plates, but away from this location they were bent into the centre of the beam (see Figure 2(b)). The test results showed that this was only partially successful. At the higher ductility levels these bars appear to have become debonded allowing extensive yielding of the main steel in the joint and, therefore, slip of the bars through the joint.

Yielding of the main beam steel in the joint of Unit 4 was prevented by the addition of two 10 mm bars fillet welded onto each 24 mm bar between the bond plates (see Figure 2(d)). This arrangement worked extremely well and no slippage was apparent.

With diagonal strut action as the principal method of load transfer in the joints of Units 2 and 4, joint shear rein- forcement was not required. However, some joint steel is needed to control the diagonal splitting which occurs in the joint, as indicated in the previous p a p e r . The areas of joint reinforcement actually used to control this cracking (1584mm

²

and 999mm for units 2 and 4 respectively) were markedly less than those required for joint shear resistance in units 1 and 3 (see Table 1 ) . In the case of Unit 4 only half the steel specified by equation (7) and Reference (1) was used (i.e. y

^s hi see equation (8)).

It was felt that once diagonal cracks form the tension stiffness across the diagonal decreases and the forces required to be

transmitted across the crack are substantially reduced. The experimental results bear out this conclusion.

1.2 Load Sequence and Experimental Measure- Men ts

The loading sequence was the same for all units, being similar to that used in recent MWD tests(4). In the first two complete cycles of the test on Unit 1, the load was increased in each direction to three-quarters of the level calculated to give first yield of the reinforcement. The resulting load-deflection relationship was plotted and a straight line was fitted to the points and extended to the calculated first yield level. The displacement corres- ponding to this was then used as a reference value of "ductility one". From this stage on displacements were imposed on the beams as indicated in Figure 4 ( a ) . All subsequent units were subjected to the same displacements as Unit 1.

Readings of the beam deflections were taken at the two points shown in Figure 4 ( b ) . In addition, the deflected shapes of the complete units were picked out using a theodolite to measure the movement of

targets attached to the unit. These measure- ments were taken at only a few representative load increments.

Electrical resistance strain gauges were attached to the main flexural steel in Unit 1. In all, eighteen gauges were used, their positions being as shown in Figure 7.

Demec gauge readings were taken on all

units. However, extensive yielding of the reinforcement in the joint zones of Units 1, 2 and 3 rapidly caused the strains to exceed the gauge capacities. By comparison, the cracks in the joint zone of -Unit 4 were well controlled and this allowed readings

to be taken to the end of the ductility six displacements.

In Units 3 and 4 measurements were taken of the expansion of the central 1.6 m of the beam under cyclic loading. At low ductility levels these values were found, using demec gauge readings, but at higher levels the expansion was measured directly with a tape.

2. EXPERIMENTAL RESULTS

The main experimental results are recorded in Figures 5-14.

2.1 Units 1 and 3

Unit 1 was designed to comply with the current ACI code (318-71), and Unit 3 was proportioned to meet the draft provisions of PW 81/10/1 of May 1976, with the exception that as the gap between the flexural steel in the column was small no intermediate column bars were used. The effect this may have had on the joint is unknown. (See Table 1 in Reference 1.) The flexural tension steel percentage in the beam in Unit 3 was considerably less than in the other units (1.3% as against 1 . 8 % ) , as it was limited by the calculated shear strength of the joint zone corresponding to the maximum quantity of stirrup reinforce- ment that could be fitted into this region.

The behaviour of both joints was similar in that both suffered severe load and stiffness degradation. The considerable increase in the ratio of shear reinforcement to flexural steel increased the toughness of Unit 3, but the gain appears to be small (Figures 5, 6, 9, and 10) .

Both joints cracked diagonally at a load of 18 kN in the jacks. These cracks opened up when the units were taken into the inelastic region. The inelastic

extension of the steel was concentrated in the joint and in the beam close to the joint.

For load cycles involving ductilities greater than two slippage of the beam rein- forcement through the joint zone was apparent. Inspection of the joint of Unit 1 showed clear evidence of bar slip, with the deformations on the steel moving relative to the indentations in the concrete by as much as 3 - 5 mm. Such slippage gives rise to beam displacements as is apparent from the flat portions of the load deflection curves at higher ductilities (see Figure 5 ) . In the joint of Unit 3 the concrete contained around and between the six bars in each face of the beam was extensively broken up.

It was apparent in this case that the bond

was destroyed not so much at the steel-

concrete interface as by disruption of the

concrete surrounding the bars in the joint

region. For this type of bond failure the

reduction in bar diameter from 20 mm in

Unit 1 to 12 mm in Unit 3 did not appear

to help because it was the concrete inter-

face right round the group that failed.

In t h e l o a d c y c l e s i n v o l v i n g d u c t i l i t i e s of four and more t h e s t i f f n e s s e s of t h e u n i t s w e r e s e v e r e l y r e d u c e d d u e t o t h e s l i p o f t h e b a r s . T h e s h e a r r e s i s t a n c e w a s t h e r e s u l t o f d i a g o n a l s t r u t a c t i o n . E x t e n s i v e y i e l d of t h e b e a m r e i n f o r c e m e n t (see F i g u r e 7 ) , c o u p l e d w i t h t h e h i g h c y c l i c b o n d s t r e s s e s , d e s t r o y e d t h e b o n d f o r c e s in t h e j o i n t s . W i t h e a c h l o a d c y c l e t h e f l e x u r a l t e n s i o n s t e e l w a s a n c h o r e d f u r t h e r b a c k i n t o t h e c o m p r e s s i o n z o n e o f t h e b e a m o n t h e f a r s i d e o f t h e j o i n t . T h e e x t e n s i v e b a r s l i p a t low l o a d s , w h i c h w a s a c h a r a c t e r i s t i c o f t h e s e t w o j o i n t s , g a v e p o o r e n e r g y a b s o r p t i o n c h a r a c t e r i s t i c s (see F i g u r e s 5, 6, a n d 1 0 ) .

S o m e o f t h e s t r a i n g a u g e r e a d i n g s w h i c h are t a b u l a t e d in t h e r e p o r t n o t e d b e l o w * a r e s h o w n in F i g u r e s 7 a n d 8. T h e r e a d i n g s i n d i c a t e d t h a t t h e b e a m r e i n f o r c e m e n t i n U n i t 1 y i e l d e d , b u t t h e c o l u m n s t e e l r e m a i n e d in the e l a s t i c r a n g e . It is i n t e r e s t i n g t o n o t e t h a t t h e b o n d d i d n o t f a i l o n t h e c o l u m n b a r s e v e n t h o u g h t h e s e s t r e s s e s w e r e v e r y h i g h and of a c y c l i c n a t u r e . F i g u r e 8 s h o w s t h a t t h e f l e x u r a l t e n s i o n f o r c e s in t h e c o l u m n s t e e l w e r e e f f e c t i v e l y a n c h o r e d b y b o n d o n the f a r s i d e o f t h e j o i n t . T h i s c o n f i r m s t h e d i a g o n a l s t r u t m e c h a n i s m (see F i g u r e 3 (c) o f R e f e r e n c e ( 1 ) ) . T h e e x p e r i m e n t a l m e a s u r e - m e n t s s h o w t h a t s u s t a i n e d c y c l i c b o n d s t r e s s e s of a t l e a s t 6.0 M P a , a n d p r o b a b l y 9 M P a , w e r e s u s t a i n e d . It is b e l i e v e d t h a t t h i s m a g n i t u d e o f b o n d r e s i s t a n c e c o u l d b e m a i n t a i n e d o n l y b e c a u s e :

(i) t h e c o l u m n b a r s h a d s t i r r u p s a n c h o r e d r o u n d t h e m ; t h e r e b y i n c r e a s i n g b o n d r e s i s t a n c e ^ , a n d

(ii) t h e b a r s d i d n o t y i e l d .

E x t r e m e s p a l l i n g o f t h e c o v e r c o n c r e t e in t h e j o i n t s o f U n i t s 1 a n d 3 o c c u r r e d d u r i n g t h e f i r s t c y c l e o f d u c t i l i t y s i x . 2.2 U n i t s 2 a n d 4 - B o n d P l a t e U n i t s

B o t h t h e s e u n i t s c o n t a i n e d b o n d p l a t e s , and t h e y w e r e d e s i g n e d t o r e s i s t s h e a r b y d i a g o n a l s t r u t a c t i o n . E x t r a f l e x u r a l s t e e l w a s a d d e d to t h e j o i n t s t o l i m i t t h e y i e l d e x t e n s i o n o f t h e b a r s b e t w e e n t h e b o n d p l a t e s on e a c h s i d e o f t h e j o i n t . T h e m a i n d i f f e r e n c e in t h e s e u n i t s l a y in t h e w a y in w h i c h t h e e x t r a s t e e l w a s a d d e d .

In U n i t 2 t w o e x t r a 12 m m b a r s w e r e p l a c e d b e l o w t h e b o n d p l a t e s in e a c h f a c e of t h e b e a m , a s s h o w n i n F i g u r e 2 ( b ) . T h i s g a v e r i s e t o a t h e o r e t i c a l i n c r e a s e in m o m e n t c a p a c i t y o f t h e j o i n t o f 1 6 % a t f i r s t y i e l d o f t h e s t e e l . In f a c t , t h e a c t u a l i n c r e a s e in b e n d i n g m o m e n t a f t e r f i r s t y i e l d of t h e s t e e l w a s 2 1 % a n d , c o n s e q u e n t l y , i n e l a s t i c e x t e n s i o n o f t h e s t e e l w o u l d h a v e o c c u r r e d in t h e f i r s t c y c l e o f d u c t i l i t y f o u r . It is b e l i e v e d t h a t t h e d e t e r i o r a t i o n o f j o i n t p e r f o r m a n c e a f t e r t h e c o m p l e t i o n of t h e c y c l e s o f d u c t i l i t y f o u r w a s d u e t o :

(i) t h e i n e l a s t i c e x t e n s i o n o f t h e b a r s m e n t i o n e d a b o v e , a n d

(ii) t h e i n a d e q u a t e a n c h o r a g e o f t h e 12 m m b a r s in t h e b e a m s . U n d e r c y c l i c l o a d i n g

* F e n w i c k , R . C . , a n d I r v i n e , K . M . , " B e a m - C o l u m n J o i n t s f o r S e i s m i c L o a d i n g " , U A S E R e p o r t N o . 1 4 2 , M a r c h 1 9 7 7 .

t h e s e a d d i t i o n a l b a r s b e c a m e d e b o n d e d m a k i n g t h e m i n e f f e c t i v e i n l i m i t i n g y i e l d o f t h e r e i n f o r c e m e n t b e t w e e n t h e b o n d p l a t e s .

In s p i t e o f t h e s e w e a k n e s s e s , U n i t 2 p e r f o r m e d m a r g i n a l l y b e t t e r t h a n U n i t s 1 a n d 3, e v e n t h o u g h it h a d a m u c h s m a l l e r a r e a o f j o i n t r e i n f o r c e m e n t (see T a b l e s 1 a n d c o m p a r e F i g u r e s 6, 9, 10 a n d 11) .

In U n i t 4 t h e a r e a o f b e a m r e i n f o r c e m e n t in t h e j o i n t z o n e w a s i n c r e a s e d b y w e l d i n g t w o 10 m m b a r s t o e a c h o f t h e 24 m m b a r s in e a c h f a c e . T h i s g a v e a l o c a l i n c r e a s e in b e a m s t e e l a r e a t h r o u g h t h e j o i n t o f 3 5 % . As t h e p e a k m o m e n t c a p a c i t y w a s 3 3 % a b o v e t h e c a l c u l a t e d f i r s t i n e l a s t i c e x t e n s i o n o f t h e s t e e l , y i e l d in t h i s z o n e s h o u l d h a v e b e e n a v o i d e d . B y t a c k w e l d i n g t h e 10 mm. b a r s t o t h e m a i n s t e e l , a n d b u t t w e l d i n g t h e m t o t h e b o n d p l a t e s , t h e p o s s i b i l i t y o f a n c h o r a g e f a i l u r e w a s p r e v e n t e d .

T h e p e r f o r m a n c e o f U n i t 4 s t a n d s o u t f r o m t h e o t h e r s (see F i g u r e s 1 4 , 1 2 , 9 a n d 1 0 ) . T h e c r a c k s in t h e j o i n t w e r e f i n e a n d w e l l c o n t r o l l e d u p t o t h e s e c o n d c y c l e o f d u c t i l i t y s i x . A t t h e f i r s t c y c l e o f d u c t i l i t y e i g h t t h e c o n c r e t e in t h e j o i n t z o n e s t a r t e d to s p a l l , a n d it w a s n o t u n t i l t h e s e c o n d c y c l e o f d u c t i l i t y e i g h t t h a t t h e u n i t s t a r t e d t o d e g r a d e . H o w e v e r , r i g h t t h r o u g h t h e w h o l e l o a d i n g s e q u e n c e t h e p e r f o r m a n c e o f t h e j o i n t w a s v e r y m u c h b e t t e r t h a n t h e o t h e r s . It s h o u l d b e n o t e d t h a t t h i s m a r k e d i m p r o v e m e n t i n p e r f o r m a n c e o c c u r r e d i n s p i t e o f t h e f a c t t h a t t h e a m o u n t o f s h e a r r e i n f o r c e m e n t in t h e j o i n t w a s v e r y s u b s t a n t i a l l y l e s s t h a n in a l l t h e o t h e r t e s t s (see T a b l e 1 ) .

It is b e l i e v e d t h a t t h e d e g r a d a t i o n of t h e j o i n t a r o s e f r o m y i e l d i n t h e f l e x u r a l s t e e l o f t h e c o l u m n . T h e c o l u m n w a s d e s i g n e d w i t h an u l t i m a t e s t r e n g t h 4 0 % g r e a t e r t h a n t h a t o f t h e b e a m , b u t t o f i r s t y i e l d o f s t e e l t h e c o r r e s p o n d i n g v a l u e w a s o n l y 2 4 % . T h i s d i f f e r e n c e w a s i n s u f f i c i e n t t o p r e v e n t y i e l d i n g o f t h e c o l u m n b a r s w h e n d i s p l a c e m e n t d u c t i l i t i e s o f s i x w e r e

i m p o s e d . M e a s u r e m e n t s m a d e o n t h e u n i t (at d u c t i l i t i e s o f e i g h t ) i n d i c a t e d t h a t t h e c o l u m n h a d e x p a n d e d b y 3 m m o v e r t h e c e n t r a l 600 m m o f t h e j o i n t z o n e . T h i s is a c l e a r i n d i c a t i o n o f y i e l d o f t h e c o l u m n s t e e l . 2.3 B e a m E x p a n s i o n a n d Sliding S h e a r in B e a m s

U n d e r c y c l i c l o a d i n g i n v o l v i n g i n e l a s t i c e x t e n s i o n s t h e r e i n f o r c e m e n t e x t e n d s in

t e n s i o n , t h e r e b y o p e n i n g u p t h e c r a c k s i n t h e c o n c r e t e , b u t u n d e r l o a d r e v e r s a l t h e s e c r a c k s t e n d n o t t o c l o s e c o m p l e t e l y in p l a s t i c h i n g e z o n e s . F u r t h e r m o r e , i n j o i n t z o n e s w h i c h a c t b y t h e f o r m a t i o n o f d i a g o n a l s t r u t s , i n e l a s t i c t e n s i l e s t r a i n s m a y b e e x p e c t e d t o e x i s t s i m u l t a n e o u s l y in b o t h t h e t o p a n d b o t t o m s t e e l t h r o u g h t h e j o i n t (see F i g u r e 3 (c) o f R e f e r e n c e (1) ) . T h i s b e h a v i o u r w a s c o n f i r m e d i n U n i t 1 (see F i g u r e 7 ) . T h e i n e l a s t i c t e n s i l e e x t e n s i o n of t h e r e i n f o r c i n g s t e e l , w h i c h is n o t r e m o v e d b y s u b s e q u e n t c o m p r e s s i o n , r e s u l t s in a g r o w t h i n t h e l e n g t h o f t h e b e a m s . O f c o u r s e , in a c t u a l s t r u c t u r e s t h e g r o w t h in l e n g t h w i l l b e r e s t r a i n e d by s l a b s , s h e a r w a l l s o r f i x i n g s (the s l a b

e f f e c t ) .

The increase in length of the beams was recorded for Units 3 and 4 and the results are shown in Figure 13. It will be noticed that these increments are far from negligible, particularly in Unit 4, where extensive plastic hinging occurred over an appreciable length of the beams.

The total extension at the last cycle on Unit 4 was 30 mm, which is approximately 1%.of the total beam length! The reason for the extension being greater in Unit 4 than in Unit 3 is because bar slip occurred in Unit 3, thereby allowing the large ductilities to be achieved without such high plastic strains as were necessary in Unit 4 (where the joint behaved itself).

The continuous opening of the flexural cracks in the plastic hinge zones of the beams gave rise to the problem of sliding shear. Units 1 and 3, which failed in the joint zone and did not develop major plastic hinges in the beams, showed no distress in shear. The beams of Units 2 and 4, in which the shear reinforcement was proportioned to carry 70 kN (cf> = 1) , did show distress in shear. In Unit 2 this arose in the form of wide steep cracks (steeper than 45°).

Additional external stirrups were fitted to the beams to prevent a premature shear failure.

Unit 4, in which very wide flexural cracks developed in the plastic hinging zone of the beams, showed distress in sliding shear. To prevent this external stirrups were clamped around the critical cracks.

Without these it is almost certain a sliding shear failure would have occurred. The shear reinforcement in Unit 4 consisted of 6.5 mm diameter two-leg stirrups spaced at 60 mm centres. The shear stress, based on the peak load (occurring at the first cycle of ductility s i x ) , was

I .

1 9 MPa, or 0.19 /f£ MPa. This is well below the limit at which sliding shear is normally expected.

Sliding shear in beams is the subject of a current thesis project under the supervision of the senior author. Results from this study will be available at a later date.

2.4 Crack Widths in the Joint Zone

The opening of cracks in the joint zone was indirectly measured from displacement readings made across this region by mechanical strain gauges. A few of the results are shown plotted in Figure 14. It can be seen that the crack widths in the joint zones of Units 1, 2 and 3 increased rapidly once they were taken into the inelastic region.

By contrast, the crack widths in the joint of Unit 4 remained small and well controlled until the second cycle of ductility six.

3. CONCLUSIONS

The test results from Unit 4 clearly indicate that beam-column joints may be designed to resist joint shear by diagonal strut action. Such joints require that:

(i) Extra beam flexural steel be placed through the joint to control the amount of yielding in the joint zone and to keep the plastic behaviour mainly in the beams. This additional reinforcement must be adequately anchored in the beams

to prevent debonding.

(ii) Bond plates be attached to the flexural steel in the column and beams to transmit the bar forces into the diagonal struts which form in the joint (alternatively, the flexural steel could be cranked through the joint but this appears to be a difficult detail to effect in practice and is, therefore, mentioned here only in passing).

(iii) Stirrups and longitudinal bars be placed in the joint zone to control the width of the main diagonal cracks.

The area of steel required to achieve this appears to be of the order of a quarter to one-third of that currently being used.

Beam-column joints designed as described above appear to have a higher sustained strength and a better ductile performance than the more conventional details, in spite of the fact that they have a very much lower shear steel area in the joint. It appears that higher shear stresses may be applied to the joint and, consequently, beam sizes may be reduced. At present, beam depths are large and steel percentages are kept low in order to find room for the joint steel and to reduce flexural forces. This leads to substantially increased costs due to material content, the labour involved in fixing steel in the highly congested joint zones, the difficulty of placing concrete in these zones and the increased height of construction required to accommodate the larger beam sizes.

With the bond plate joint detail the size of the joint zones may be reduced so that they no longer dictate the beam sizes.

The next limit on beam size arises from considerations of sliding shear in beam plastic hinge zones. However, the authors believe that this potential failure mode may be prevented, as described below, thus allowing small beams containing up to 2.5%

of flexural tension steel to be used with confidence, as compared with current practice that requires approximately 1.3%.

Mr. B. W. Buchanan, M.W.D., Wellington, has suggested that sliding shear failures could possibly be prevented by providing an element with a high dowel shear capacity in the centre of the beam.

A scaffolding tube filled with concrete and placed in a debonded state in the middle of the beam in the plastic hinge zones might satisfy this requirement. Alternatively, the authors believe it may be possible to control sliding shear by the use of some unstressed prestressing strands which are added to the normal flexural mild steel in the beam. This, it is thought, may change the character of the plastic hinge and increase the sliding shear capacity of the hinging region.

With the bond plate detail difficulties would arise with site placing of reinforcement, and it would be important to avoid this

problem by having the steel cages prefabricated in a workshop. From discussions with contractors it appears that just this approach has been

used to considerable advantage on several

recent structures with conventional joints.

To reiterate, the mode of resistance of

a b o n d p l a t e d j o i n t is c l e a r and a r a t i o n a l design may b e m a d e . T h e s a m e d o e s n o t a p p e a r to b e t r u e o f d e s i g n s m a d e t o A C I - 3 1 8 - 7 1 , o r t h e MWD's PW 8 1 / 1 0 / 1 . T h e A C I c o d e d o e s n o t r e q u i r e a n y v e r t i c a l r e i n f o r c e m e n t in the j o i n t z o n e , a n d t h e d r a f t M W D c o d e PW 8 1 / 1 0 / 1 r e q u i r e s o n l y n o m i n a l v e r t i c a l s t e e l i n t h a t it is n o t e d t h a t c o l u m n s t e e l s h o u l d b e d i s t r i b u t e d r o u n d t h e p e r i m e t e r o f t h e c o l u m n (see T a b l e 1 i n R e f e r e n c e 1 ) .

C o n s e q u e n t l y it i s d i f f i c u l t t o d e t e r m i n e j o i n t s t i r r u p r e q u i r e m e n t s b y a l o g i c a l a n a l y s i s o f j o i n t f o r c e s in c o l u m n s w i t h l o w a x i a l l o a d s . T h e e x p e c t e d m o d e of a c t i o n is n o t c l e a r , a n d t h e p o o r p e r f o r m - a n c e o f U n i t s 1 a n d 3 u n d e r t h e s e c o n d i t i o n s is not s u r p r i s i n g . F i n a l l y , it s h o u l d b e n o t e d t h a t t h e c o n d i t i o n s f o r t h e s e t e s t s w e r e s e v e r e in t h a t t h e r e w a s n o a x i a l l o a d

(a c o n d i t i o n w h i c h l e a d s t o a h i g h v e r t i c a l s t i r r u p requirement), a n d t h e r a t i o o f s e c t i o n to d i s p l a c e m e n t d u c t i l i t y w a s h i g h .

4. A C K N O W L E D G E M E N T S

T h e a u t h o r s a r e g r a t e f u l t o M r . I. C . A r m s t r o n g , M . W . D . A u c k l a n d , a n d M r . B . W . B u c h a n a n , M . W . D . W e l l i n g t o n , for t h e i r h e l p w i t h t h i s p r o j e c t . In a d d i t i o n , Mr. W . S t r i n g e r a n d Mr. K. C u r r y o f t h e M . W . D . A u c k l a n d p r o v i d e d v a l u a b l e a s s i s t a n c e . D o w n e r a n d C o . L t d . p r o v i d e d m a t e r i a l a s s i s t a n c e .

5. R E F E R E N C E S

(1) F e n w i c k , R. C . a n d I r v i n e , H . M . ,

" R e i n f o r c e d C o n c r e t e B e a m - C o l u m n J o i n t s for S e i s m i c L o a d i n g , P a r t I - T h e o r y " , B u l l e t i n o f t h e N . Z. N a t i o n a l S o c i e t y for E a r t h q u a k e E n g i n e e r i n g , S e p t . 1 9 7 7 . V o l . 1 0 , N o . 3.

(2) A C I - A S C E C o m m i t t e e 3 5 2 , " R e c o m m e n d a t i o n s for D e s i g n o f B e a m - C o l u m n J o i n t s in

Monolithic R e i n f o r c e d C o n c r e t e S t r u c t u r e s " , P r o c . A C I , V o l . 7 3 , N o . 7, J u l y , 1 9 7 6 . (3) P a r k , R. a n d P a u l a y , T . , " R e i n f o r c e d

C o n c r e t e S t r u c t u r e s " , J. W i l e y a n d S o n s , 1 9 7 6 .

(4) P r i e s t l e y , M . J . N . , " T e s t i n g o f T w o R e i n f o r c e d C o n c r e t e B e a m - C o l u m n A s s e m b l i e s U n d e r S i m u l a t e d S e i s m i c L o a d i n g " , R e p o r t N o . 5 - 7 5 / 1 , M W D C e n t r a l L a b o r a t o r i e s , W e l l i n g t o n , 1 9 7 5 . (See, a l s o , B u l l e t i n o f t h e N . Z . N a t i o n a l S o c i e t y f o r E a r t h q u a k e E n g i n e e r i n g , V o l . 8, N o . 1, M a r c h , 1 9 7 5 , p p . 3 8 - 7 0 . ) (5) A C I C o m m i t t e e 4 0 8 - " B o n d S t r e s s - T h e

S t a t e o f t h e A r t " , A C I P r o c e e d i n g s , V o l . 6 3 , N o . 1 1 , 1 9 6 6 .

P a p e r r e c e i v e d 26 S e p t e m b e r , 1 9 7 7 .

TABLE 1

BASIC DATA FOR BEAM-COLUMN JOINT UNITS

ITEM UNIT 1

(ACI-318-71) UNIT 2

1ST WITH BOND PLATES UNIT 3

PW 81/10/1 - MAY 76 UNIT 4 2ND WITH BOND

PLATES.

Cylinder

strength f^ 42.9 MPa 35.6 MPa 39.3 MPa 40.4 MPa

Beam flexural

steel (T & B) 3 - D20 mm 3 - D20 mm 6 - D12 mm 2 - D24 mm

f

y 280 MPa 317 MPa 318 MPa 291 MPa

A . = A * st sb 942 m m

²

942 m m

²

678 mm

²

2

905 mm

width 200 200 200 200

effective depth 260 260 254 252

Beam steel % p 1.81 1.81 1.34 1.80

Joint shear steel horizontal (EA^)

5 - R12 - 2 leg 5 - R10 -

2 leg 1916 m m

²

11 - R6.5 - 2 leg +4 - D12 bars

1182 m m

²

5 - R12 - 2 leg 5 - R10 - 2 leg 5 - R6.5 - 2 leg

2248 mm

²

12 - R6.5 - 2 leg

597 m m

²

Joint vertical

steel 2 - D16 mm 2 - D16 mm ' Nil 2 - D16 mm

in jt. zone (ZA

^v

) 402 m m

²

2

402 mm 402 m m

²

Column - steel (yield f as for beams)

^y

4 - D20 nun 4 - D16 mm

4 - D20 mm 4 - D12 mm

16 - D12 mm each

side 4 - D16 mm 2 - D16 mm 2 - D16 mm

central 2 - D16 mm central 2 - D16 mm

central A

^g

(total) in

columns

2

2463 mm 2463 m m

²

1810 m m

²

2614 m m

²

width 250 250 250 250

effective depth 260 260 254 252

Steel ratios

Z

V

^A

s t 2.03 1.25 3.32 0.66

(EA

^h

+ Z A

^v

) / A

^{s t}

2.46 1.68 3.32 1.10

*L

^u

-ACI-71 cj) = 1.0 50.1 kN 55.5 kN 40.8 kN 52.4 kN

* L-^ load at 1st yield of

flexural steel 46.3 kN 52.4 kN 37.0 kN 44.9 kN

* L« measured load

at ± 2D 49.4 kN 58.6 kN 43.9 kN 52.6 kN

Col. flexural ult.

beam

(calculated) 1.31 1.31 1.33 1.44

* Notes

L

^u

- calculated ultimate flexural load on each beam (ACI-318-71 <f> = 1.0) L

^x

- calculated load on each beam at 1st yield of beam steel

L

²

- measured load on each beam at ductility two 1st cycle clockwise direction A - flexural steel bottom of beam, A . - flexural steel top of beam

sb st

U25 U25

RACKING

30 TON JACK 30 TON LOAD

CELL BEAM

L J

30 TON LOAD CELL

1

r i I. J

475.

SUPPORT BEARINGS ON PACKNG

r i BEAM

L J

vol I J ^fj^

I t - ^ 1

' Mr

200

30 TON LOAD CELL

30 TON JACK H—PACKING COLUMN

PACKING

- ROLLER BEARING

F I G U R E 1: PLAN VIEW OF TEST A R R A N G E M E N T FOR BEAM-COLUMN

J U N C T I O N SPECIMENS.

O i CO CD

N

4

f ^

1 4?5

- 6 - S E T S R 6 5 DOUBLE T I E S - 1 0 0

J1X

3Qj

6 - D 2 0 -

- B E A M

6 - R 6 - 5 T I E S - 1 5 0 - ^ 3 - 0 2 0 . T & B - 5 - S E T S TRIPLE

R6-5 TIES

5 - 5

" ^ j T I E S IN JOINT - 6 - SETS R6-5

DOUBLE TIES-100

- 2 - 0 1 6

- 4 - 0 2 0 E. F.

- 9 - R 6 5 T I E S - 1 5 0

R = plain round.

D = deformed.

016 = deformed 16mm

nominal bar.

-200I

1&I

JO O

A-A ¥

R12 TIE R10 TIE 4 - 0 2 0

-COLUMN

2 - 0 1 6 ,R6-5 TIES R6 5 TIE

4 - 0 2 0 4 - 0 2 0

2-016

4 - 0 2 0

^Fillet weld R12"

& R10 ties.

STRUCTURE IS ETRICAL ABOUT £ ' s

"-Fillet weld R6-5 tie 300

C-C FIGURE 2 ( a ) : R E I N F O R C E M E N T DETAILS FOR T H E FOUR S P E C I M E N S (?) UNIT 1

Bond plate — -for details s

enlargement.

1 - D16 E.F.

1 425 J 7 5

^H

x = 20mm OUTSIDE BARS x = 45 mm INSIDE BARS

2-012E.F. W]

o

CO

2 - D16

2 - 0 2 0 * - 2 -D16 E.F.

9 - R 6 5 - T I E S - 1 5 0

COLUMN

J

WW

i

/-6-SETS R6-5 / DOUBLE TIES -100

2T

COLUMN O R —

BEAM FACE L x ^ 2 0

2J

c

x , . , 2

5 2

BEAM

6 - R 6 - 5 TIES-150 5 - SETS TRIPLE

TIES IN JOINT

3 - 0 2 0 T. & B.

60 x 60 x 2 0 * .

BOND PLATE DETAIL

D20 OR 016 BAR

- - 6 - SETS R6-5 DOUBLE T I E S - 1 0 0

^ ^ _ ^ ^ 0 016 2-016-v - 6 - 0 2 0 R6-5 TIE — ⁿ

o- ^ , 012 - i

iD"\ r

| R6 5 TIES

•—012

016- 012

-R6-5 TIES 012

4

o o CO'

o4

R6-5 TIES

0 2 0 f

] - 0 1 6 -

0 2 0

• ^ i

^ If

5$

NQTE^:

STRUCTURE IS SYMME

Fillet weld R6-5 ties

B-B

Fillet weld R6-5 t i e

2 X ^ - i _ 4

o

METRICAL ABOUT £ ' s

. 3 0 0

c-c

w

r^5

F I G U R E 2 ( b ) : R E I N F O R C E M E N T D E T A I L S FOR T H E FOUR S P E C I M E N S (b) Unit 2

14.25

i

1 *8

" 7

-10-SETS R6-5 TRIPLE T I E S - 6 0

415,

-3-D12 T . * B .

R6-5 TIE R12 TIE - WO T I E - 16-D12

Fillet weld all

ties.

Note R10 ties to have fillet Weld on both sides of lap.

"XT ffl

EAM

7-SETS R6-5 - TRIPLE TIES -100

^-5-S E T S TRIPLE

TIES IN JOINT

— 6 - SETS R6 5 TRIPLE TIES- 60 -4-D12E.F.

-4-012 E.F 16- SETS R6 5

TRIPLE TIES- 90 12-012-

COLUMN

-3-D12 T. & B.

200

r~ ""1

n

• 4

B-B

300

2 5 ^ 0 3Q, , ^25

16-012-

o-

1

4

* • •

4

s

CM1

A-A R6-5 TIES

C-C NOTE

:

STRUCTURE IS

SYMMETRICAL ABOUT £'s

FIGURE 2 ( c ) : REINFORCEMENT DETAILS FOR THE FOUR S P E C I M E N S (c) U n i t 3

30 n 20 *

6 1

1 425

1 3 - R 6 - 5 T I E S - 80 1 4 - R 6 5 TIES - 80 2 - 0 1 6 - -

5 - R 6 - 5 i T I E S - 1 2 5 |.

COLUMN- 2 - 0 2 4 *

1-016E.-F.

of

Fillet weld to bond plate and bar.

90 m 90 m 24

It -n

BEAM FACE

BEAM

2 - 0 2 4 T. & B.

^ - S E T S TRIPLE TIES IN JOINT

200

45 - ^ ^ 5 r - 4 - 0 2 4

BONO PLATE NOTES

11)024 BEAM BARS - Bond

plates as shown in detail.

(2)D24 COLUMN BARS. - 30 * 20 * 6 ft. each side of bond plate.

(3)016 COLUMN B A R S -

70 « 70 x 20 bond plate

with no fillet plates.

BONO PLATE DETAIL FOR BEAM STEEL

U -

016-

U

-D24-

R6-5 TIES 4 - 0 1 6 4 - 0 2 4

5L ™^™SJ

'mm—^immSr^ .

" 1

N0TE;

STRUCTURE IS SYMMETRICAL ABOUT ^'s

L

R 6 5 TIE

Fillet weld- R6-5 tie

fcJB

R6-5 TIES

o

CM!

I

45. j _ J i l

U 30J ^ C-C

FIGURE 2 ( d ) : R E I N F O R C E M E N T DETAILS FOR THE FOUR S P E C I M E N S (d) U n i t 4

FIGURE 5: LOAD D E F L E C T I O N CURVES FOR U N I T 1

i a } LOAD S E Q U E N C E FOR A L L UNITS

1-250J DIAL o

G A U G E S ^ | J A C K S

, J - 2 5 0 . j

3 2 0 0

lb) S K E T C H SHOWING D I A L GAUGE POSITIONS ON B E A M S

F I G U R E 4

F I G U R E 6: LOAD D E F L E C T I O N CURVES FOR U N I T 3

F I G U R E 7: S T R A I N R E A D I N G S ON BEAM BARS U N I T 1 UP TO D U C T I L I T Y 2

_ YIELD STRAIN (1400 * 10" I _

- — - D 4 i

- D 4 i

7

0 4 i YIELD STRAIN i 1 4 Q 0 j L l Q f L _

F I G U R E 8: S T R A I N READINGS ON C O L U M N BARS U N I T 1 UP TO D U C T I L I T Y 4

(SEE F I G U R E 7 FOR S T R A I N G A U G E LOCATIONS)

Load

UNIT 4

D U C T I L I T Y UNIT 1

U N I T 4

F I G U R E 9: L O A D S U S T A I N E D A T D I F F E R E N T D U C T I L I T I E S (i= first cycle, ii=second cycle)

0-8

0-6

0-4

0-2

0-6 H

0-4

: 0-2

Area "a", a c t u a l curves ri, Displacement

^ ACI Ultimate (0=1) — 7

^ o a d /

±

Displacemeni

eJasto-plastic

2i 4i 6i 8i DISPLACEMENT DUCTILITY - 1st CYCLE

Unit 4

^ Unit 3 - _ \ U n i t 2

~Unit 1

2ii 4ii 6ii 8ii

DISPLACEMENT D U C T I L I T Y- 2 n d CYCLE

F I G U R E 10: E N E R G Y DISSIPATION

VERSUS D U C T I L I T Y

FIGURE 13: EXPANSION OF BEAM IN UNITS 3 A N D 4 UNDER CYCLIC L O A D I N G

M E A S U R E D OVER C E N T R A L 1.600m OF BEAM

. • i - i +ii - h . .+i - i +it - i i • i+» - i +ii - i i . ,+i - i +ii - i i . i+i - i +ii - i i , DUCTILITY

J/ ^ D 2D 4 D 6 D 8 D

F I G U R E 14: C R A C K W I D T H IN J O I N T M E A S U R E D BY

DEMEC G A U G E ON BEAM