Connections In Precast Concrete Structures-Scarf Joints by J. R. Gaston and L. B. Kriz* SYNOPSIS

This study concerns the structural behavior and strength of scarf joints subjected to bending, shear, and axial forces. It was found that the ultimate strength of scarf joints can be evaluated by the ultimate strength design principles of the 1963 ACI Building Code, but reinforcing details must be given particularly careful attention. The effect of scarf joints on the flexural rigidity of assembled members can be evaluated by using an equivalent reduced section. The resistance to slip in scarf joints subjected to axial forces was studied in an auxiliary series of push-off tests. This paper is part of a series of papers concerning design criteria for connections in precast concrete structures. BACKGROUND AND PURPOSE

The Research and Development Laboratories of the Portland Cement Association are conducting a continuing program of investigations of precast concrete connections, the results of which are reported in a series of papers, "Connections in Precast Concrete Structures." The first report concerned the strength and structural behavior of continuity connections in double-tee floor construction1n, and the second dealt with the bearing strength of column heads supporting precast beams 2 • This third part of the series concerns the strength and behavior of bolted scarf joints in beams and columns. Future papers will discuss corbelsbrackets which protrude from the face of a column-and connections between columns and footings. "Former Development Engineer and Development Engineer, respectively, Structural Development Section, Portland Cement Association Research and Development Division, Skokie, Illinois. uNumbers refer to references listed at the end of the paper. June 1964

Use of Scarf Joints in Precast Concrete Structures Scarf joints are useful i11 precast concrete structures at locations where a limited degree of continuity is acceptable. They are commonly used in gable and portal frames, where they may be located near the points of contraflexure. Fig. 1 shows scarf joints in a rigid frame for farm buildings developed by the Department of Agricultural Engineering of the University of Illinois3 • In the structural frame of the British factory shown in Fig. 2, the scarf joints are used at the points of contraflexure of the continuous transverse gable frames and also in the longitudinal frames supporting the gables. Prior Experimental Work While the scarf joint has been used successfully in timber construction for several centuries, only limited information has been available on its efficiency in precast concrete structures. In 1956 Dobbie and Wajda 4 reported on three tests in 37

Fig. 1-University of Illinois Rigid Frame

Fig. 2-Scarf Joints in Gable Frames and ·in Portal Frames A~

TB. +

4fr- 15"----J4fl

Section A·A

6"

I

f

•

I

1..

3bar~

,...-------.±;!!:!

16 • 13

I I

BEAM I-1

~~~~~:::::::::::.:: 240 in.-kips

t6

. .3 r8 I- 6"-.j

6"

I

16 13

+-. I

I

~AMI-2

:4m.·ktps

16

L6~'j

.. 8 6"

I

+-6 • L

"

I

I

•

I

13 16 13 16

BEAM I-3 414 in.-kips

~6":..1

04812in.

Note: All stirrups are 14 bars

Scale

Fig. 3-Pilot Specimens-Group I Beams

38

PCI Journal

which scarf joints were subjected to a bending moment, a bending moment and shear, and an eccentric axial force. In 1960 Larnach5 published a report on tests of six types of joints and six companion monolithic specimens. Four of the specimens included scarf joints; three were tested in pure bending, one in bending and shear. All four scarf joints were over-reinforced, and all failed in compression. 'Ille flexural strength of the connections was 34 to 50 percent of the strength of the companion monolithic beams. Purpose This investigation amplifies the knowledge of the behavior and strength of scarf joints in precast concrete members subjected to various types of loading. Design criteria based on the strength of the joints are also developed. Scope The scarf joints were tested in beams and columns. The Yariables in the beam connections were (a) the detailing of the reinforcement, (b) the concrete strength, (c) the ratio of moment to shear, and (d) the initial tension in the bolts. The variables in the column connections were (a) the angle of scarf, (b) the eccentricity of the load, and (c) the presence or absence of mortar in the end gaps of the connections. BEAM TESTS Nine beams were tested to evaluate the performance of scarf joints subjected to combinations of bending moment and shear, ranging from pure bending to pure shear. Attention· was given to developing satisfactory detailing of reinforcement in joint regions, to the influence of scarf joints on flexural rigidity of beams, June 1964

to the variation of tension in the bolts and to the ultimate flexural stren'gth of the joints. The essential properties of the test beams are summarized in Table 1. Detailing of Reinforcement Details of reinforcement in a scarf joint designed to resist bending were developed from tests of Beams I -1 to I-3. These beams were 6 in. by l.i m. by 10 tt long, with scarf joints at midspan. Nominal concrete strength was f; == 5000 psi'\ and the reinforcement was intermediate grade deformed bars. The reinforcement of the three beams is shown in Fig. 3. The specimens were tested as simply supported beams with a span of 9 ft. Two equal loads were applied at 22 in. on either side of the center of the span, subjecting the scarf joints to pure bending. Only the ultimate loads and the modes of failure were observed in these pilot tests. The failure of Beam I-1 was caused by wide cracking near the cut-off of the two bottom bars, as shown in Fig. 3. After this cracking only the bars extending to the support could develop the yield stress. Considering only these two bars as acting reinforcement, the calculated bending strength of the reduced section where the joint starts is 233 in.-kips. The beam failed at a moment of 240 in.-kips. In Beam I -2 the strength of the bottom half of the joint was improved by placing a stirrup 4 in. from the end of the scarf joint. Nevertheless, insufficient anchorage of the inclined tension reinforcement in the upper half of the joint caused "The notation is given at the end of the paper.

39

TABLE 1-BEAM DATA

Beam No.

I-1 1-2 1-3 11-1 11-2" III-1 III-2 III-3 III-4"

Concrete Strength t;, psi 4930 5300 5020 8200 8050 5290 5100 5010 5150

Yield Strength of Reinforcement Beam Connection f •• ksi f,, ksi t;, ksi 46.9 46.9 48.1 45.8 46.8 49.0 45.4 45.4 50.2 56.5 56.5 56.5 59.0 45.7 45.7 45.7 45.4 45.4 45.4 45.2 45.2 45.2 44.9

Initial Bolt Tension, kips

4 0.5

7 18

• Monolithic beams

TABLE 2-BEAM TEST RESULTS

Beam No.

"d

C1 ...... '-< 0

~

se:..

1-1 1-2 1-3 11-1 11-2" III-1 III-2 III-3 III-4"

Measured Ultimate Moment Connection, Beam, in.-kips• in.-kips•

2560 2890 1650 1500 1330

• Monolithic beams • Loaded as in Fig. 6 (b) c Loaded as in Fig. 12

240 304 414 1055 638 612 754

Calculated Ultimate Moment Connection, Beam, in.-kips• in-kips•

2337 2691 1237 1224 1221 1165

393 1007 582 582 564

Bolt Tension at Ultimate Moment Measured, Calculated, kips kips

Connection

Working Bolt Tension

Mtest

Ttest

Mcalc

Teale

12

40.0

1.05 1.05

0.30

50.0 57.0 44.5

46.6 46.6 40.1

1.10 1.05 1.34

1.07 1.22 1.11

. 0 Section A-A

216

211·:.

t

I

21e 216

218 ' '. '

Some reiniorcement as other half

31n,, 2114,

'

2114"

3112" 2114

216

(o) Beam li .. I

415[]12'"'" 10314

415 •

•

25/8

11

11

Fig, 4-Group II Beams

Section B·B

Li'-'!"-i

7

2188~ ":

2 4""

218

•

•

218

•

•

218

•

•

2-f-o·· 218+ I 17

2" 2" 4" 211

(a) Beams III-1,-2,-3

115"--4--12"1 · A'"l /17cutoff

f

17 cutoff_...

l'-218+ 117 ,r218

l-- 12"

A_J 1 -'-'-i-

B

1

'-218

I!~I 17 IS+

r-

15"

__.J AU stirrups 14, 9"c. to c.

F~2·8·

1j u'" '-+- B

(bl Beom ll!-4

Fig. 5-Group Ill Beams

a bond failure at a moment of 304 in.-kips. In Beam I -3 the inclined tension reinforcement in the upper half of the joint was extended well into the beams, and an additional stirrup was provided. This beam failed at a moment of 414 in.-kips by yielding June 1964

of the tension reinforcement of the upper half of the joint, followed by crushing of concrete near one of the two bolts. The behavior of these three beams demonstrated the importance of adequate anchorage of connection reinforcement. The minor changes 41

]....-t

(a)

of connection

Test I Zero slope

I

t

Af.--48"--+-----9~"

(mointained

~96'''------+t