Strength of Slab Coloumn Connection

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STRENGTH OF EXTERIOR SLAB-COLUMN CONNECTIONS

S. Teng, Nanyang Technological University, SingaporeJ.Z. Geng*, Nanyang Technological University, Singapore

H.K. Cheong, Nanyang Technological University, Singapore

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29

t

Conference o OUR WORLD IN CONCRETE STRUCTURES: 25

-

26 August 2004 Singapore

STRENGTH OF EXTERIOR SLAB COLUMN CONNECTIONS

S. Teng

Nanyang Technological University, Singapore

J.Z. Geng*,

Nanyang Technological University, Singapore

H.K.

Cheong Nanyang Technological University, Singapore

Abstract

Punching shear strength of exterior slab-column connections, including edge and corner

connections is discussed in detail based on the analysis of available data in t

he

literature

according to the

ACI

318-02. It is found that the interaction between moment and shear for

exterior connections

is not as strong as represented in the ACI 318-02. The interaction is

weak for edge connections,

and

even weaker for corner connections. A reduction f

v

is

proposed based on the analysis. For edge connections, the reduced v is equal to 60

percent of the ACI defined value; for corner connections, it equals

to

10 percent of the ACI

defined value only. Once this reduction of v is considered in the ACI 318-02, the accuracy

of prediction can be improved greatly for the collected data.

Keywords: punching shear strength, slab-column connections, moment transfer, design code.

1. Introduction

The

ACI

318-02

1

presents

an

eccentric shear stress model for predicting punching shear strength

of slab-column connections with moment transfer.

It

assumes that the shear stresses due

to

unbalanced moment can be added directly

to

shear stresses due to shear force. The shear stresses

due

to

unbalanced moment vary linearly along

the

critical section . The interaction between shear and

moment transfer

is

represented by a coefficient Yv which defines the fraction of unbalanced moment

resisted

by

eccentric shear.

This paper begins with a summary of data obtained from numerous experiments

on

exterior slab

column connections, including edge and corner connections. The eccentric shear stress model in the

ACI 318-02

is

reviewed. The predictions according

to

the ACI 318-02 for the collected data are

analyzed and compared with the experimental results. Detailed discussions are provided and the

interaction between shear and moment is studied and emphasized .

2.

Research significance

The present study provides a fresh review of previous experimental data on exterior slab-column

connections, including edge and corner connections. The punching strength of experimental data

is

checked based on the ACI 318-02.

It is

found that the interaction between shear and moment

is

weak

for edge connections, and even weaker for corner connections . Reductions of

y

v are both proposed

based

on

the ACI defined value for edge and corner connections.

3. Review of experimental data

Numerous experimental data

on

slab-column connections are available

in

the literature. Included

in

this study are seventy-four exterior slab-column connections subjected to combined shear and

moment transfer, tested by over 15 research centers around the world. Of the

74

connection

specimens, 46 are edge slab-column connections

and

28 are corner connections. Some details of

each group of specimens are described below.

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Compression surface

Simply supported r . \ i

!

Tension surface

10;10

3 1

Edge slab column connections

Forty-six data involving edge slab-column connections were collected

in

this study. The respective

detailed experimental information can be found in Ref. 2) through [17]. For all included test slabs, the

outer faces of the columns were flush with the slab edge. Most of specimens had columns extended

from

both

above and below the slab, while some other specimens had columns extended from below

the slab

only.

Those data included both isolated single slab-column edge connections (Ref [2] through

[9])

and

non-single specimens (Ref [10] through [17]). The specimens tested by Scavuzzo [10] and

Sherif

and

Dilger [14] comprised both an interior and exterior connections. The specimens tested

by

Regan [11) comprised a slab spanning two edge connections. The subassemblies tested

by

Robertson

and Durrani [15] consisted of two exterior connections and one interior connection each. Falamaki and

Loo [16) tested a series of nine half-scale models representing two adjacent edge and corner panels of

a building floor. Each specimen contained six columns, including three slab-column connections with

spandrel beam or torsion strip. Only the connections without spandrel beam were collected

in

this study.

Typical test specimens are shown

in Fig.1

.

Vertical loads

Roller-supported

edge

Lateral load

Reactions ~

Lateral Load

Restrained against

rotation about edge

t

Reactions

Vertical reaction

a)

Scavuzzo test specimens [10] b)

Specimens tested by Regan [11]

Fig. 1 Typical non-single specimens of edge connections

None of the connections

had

slab shear reinforcement or edge beams, and no moment

transferred parallel

to

the slab edge. The specimens tested

by

Hawkins et

al [5)

were subjected to

inelastic load reversals simulating earthquake effects. The subassemblies tested by Robertson and

Durrani

[15)

were applied cyclic lateral load

on

the top of columns to study the load-drift response and

interaction between interior and exterior connections. All other specimens collected herein were tested

under static loading.

Load

Transverse

Load plate

Reaction

.

Transverse load

(a) Test specimen

by

Zaghlool et

al

[19]

(b) Test specimen by Zaghlool et

al

[22]

Fig. 2 Typical specimens of corner connections

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3.2 Corner slab column connections

Twenty-eight data were collected from the literature (Ref [16], and Ref [19] through [23]) involving

corner slab-column connections. The detailed experimental information can

be

found in the

corresponding references. Normally, gravity-induced biaxial unbalanced moments are transferred from

the

slab

to

the column plus horizontal loads from wind or earthquake forces for corner slab-column

connections. The specimens involved

in

Zaghlool et al [19], Walker and Regan [20], were corner bays

of flat plate floors supported on four corner columns. The specimens tested

by

Ingvarsson [21],

Zaghlool et

al

[22],

and

Hammill

and

Ghali [23], were isolated single corner connections. Typical test

specimens are shown

in

Fig.2.

4

ACI 318 02 for punching strength with moment transfer

According

to

the

ACI

318-02, the punching shear strength of slabs without shear reinforcement

can be

determined from the lowest of the following expressions

in

SI units)

Vc=0.083Xl2+;}f t ; (MPa) (1)

(MPa) (2)

c = 0.083X +

2] Jf7

Vc = 0.083

x

4.JC'

(MPa) (3)

where

f is

the ratio of the longer side

to

the shorter side of the concentrated load (or columns),

a sis 40 for interior column, 30 for edge columns, and

20

for corner columns. b

o

is

the length of critical

shear perimeter taken at a distance of

0 5d

away from the column face

and

has square corners for

square columns and round shapes for circular columns. d is the effective depth of slabs. ( is specific

concrete cylinder strength,

in

MPa unit.

r

2

+O.5d=b

2

1

A

IZ

-

------r-------- C

i g

Column

centroid

Vul

;

t.lM V

g

u u

Critical

section

- _______ 1________

B i .. _C.:..=AB== . : .it--- cC D=---.I

z

I

Fig. 3 Eccentric shear stress model for edge connections

A

Column

centroid

-.2;"

Shear stress

Fig.

4 Eccentric shear stress model for corner connections

531


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The ACI 318-02 presents an analytical method eccentric shear stress model) to calculate the

shear stress when both shear force and unbalanced moment are transferred. It assumes that the shear

stresses on the critical section due to the direct shear force can be added to the shear stresses on the

same section due to moment transfer. The shear stress due to unbalanced moment

is

distributed

linearly on the critical section.

The critical ratio between measured and computed strength for edge connections is the maximum

value of three ratios : vAB vc VCD / v

c

and 1 Yv XM u- Vug}/M

r

where, v S is the shear stress along

critical section AB as shown in Fig 3;

vCD

is the shear stress along critical section CD;

y

v is the

fraction of unbalanced moment resisted by shear;

(Mu - Vug)

is the ultimate unbalanced moment

acting at the centroid of the slab critical section; g is the distance between centroids of the slab critical

section

and

the column critical section; M

r

is the flexural strength of slab reinforcement with a transfer

width of c

1

+ 3h .

The critical ratio between measured and computed strength for corner connections is the

maximum value of three ratios:

vB/v

e

v c / ve

and flexural strength ratio, similar to that for edge

connections, where,

V

is the shear stress at Point

B;

vc is the shear stress at Point C as shown in

Fig . 4.

5. Data analyses for all collected specimens

5.1 Edge slab column connections:

Table 1 lists the overall prediction for the forty-six collected data. Note that due to the space limit ,

the respective prediction of each specimen collected herein is not listed in this paper. The overall

prediction includes the average strength ratio , the standard deviation Stdev) of the strength ratio, and

the coefficient of variation COV).

Table 1 Overall prediction according to AC1318-02 and that with a reduction of Yv

for edQe connections 46 data)

Method

Minimum of

strength ratio

Maximum of

strength ratio

Average of

strength ratio

Stdev

COV

AC1318-02 0.807 2.546

1.464

0.419 0.286

ACI 318-02 with the

proposed reduction of

y

v

0.750 2.277 1.236 0.293

0.237

According to ACI 318-02, analysis of the data collected reveals that calculated strength is

governed by limiting shear stresses on the slab critical section rather than flexural yield for nearly all the

test specimens, except two specimens Specimen 5A tested by Hall and Rangan [12]. and Specimen C

by Rangan [13]). The calculated strengths of those two specimens are governed by flexural yield .

Calculated strengths are almost in all cases conservative, with ratios between measured and

calculated strengths ranging from 0.807 to 2.546, except four specimens, having a mean of 1.464 and

a coefficient of variation of 0.286. It is interesting to note that even for the two specimens with moment

transfer only Specimen M/E/2 tested by Stamenkovic and Chapmen [3] and Specimen Z-V 4) tested

by Zaghlool [4]) the calculated strengths are still governed by the limiting shear stress on the critical

section, not by the flexural yielding .

Moehle [18] suggested that there is no interaction between shear and moment for edge

connections based

on

the analysis

of

27 data he collected. The strong interaction between shear and

moment embodied in the ACI 318-02 is the coefficient of

Yv

the fraction of unbalanced moment

transferred by shear). Analytical work has been done herein to see how the predictions go by reducing

this coefficient y v step by step. The criterion is the value of COV for the data collected in this study. Fig.

5 shows the relationship between C OV and the percent 0 f yv This figure clearly shows that when

reducing

Yv

from 100 percent of ACI defined value, the value of COV becomes smaller and smaller

until

Yv

was reduced to 60 percent of ACI value. After that, the value of COV becomes larger if we

continue to reduce this

Yv

This behavior suggests that the relationship between shear

and

moment for

edge connections is neither as strong as that represented in the ACI 318-02 100 percent of

yv

should

be used), nor as zero as that proposed by Moehle [18]. A 60 percent of ACI defined Yv value should be

used for edge connections. Table 1 also lists the overall predictions when considering the reduction of

Yv

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0.300

c:

.290

.Q

/

;::

m

0.280

m

>

/

....

0.270

0

C

/

)

0.260

'

(3

/

E

Q

0.250

0

/

.240

....

/

0.230

0.00 0.20

0.40

0.60 0.80 1.00

Percent of y v

Fig . 5 Relationship of value of COY and percent

of

yv for edge connections

It shows that the predictions have been improved much after we use a reduced value of

v

(60

percent of ACI defined value), meaning that a less interaction between shear and moment exists for

edge connections. The average strength ratio is 1.236, having a value of COY of 0.237. Note that there

are nine specimens which had strength ratios less than unity when we reduce this y

v

This problem

can

be

easily solved by using a slightly larger strength reduction factor, which will not

be

discussed in

this paper.

5.2 Corner slab column connections:

Table 2 lists the overall prediction for the twenty-eight collected data. The overall prediction also

includes the average strength ratio, the standard deviation (Stdev) of the strength ratio, and the

coefficient of variation (COV).

Table 2 Overall prediction according to ACI 318-02

and

that with a reduction of

v

for corner connections (28 data

Method

Minimum of

strength ratio

Maximum of

strength ratio

Average of

strength ratio

Stdev

COy

AC13180-02 1.067

3.441

1.901

0.638

0.335

ACI 318-02 with the

proposed reduction of v

0.731 1.687

1.160

0.253 0.218

According to ACI 318-02, analysis of the data collected reveals that calculated strength is

governed by limiting shear stresses on the slab critical section rather than flexural yield for all the test

specimens. Calculated strengths are in all cases conservative, with ratios between measured and

calculated strengths ranging from 1.067 to 3.441, and having a mean of 1.901 and COY of 0.335. The

over-conservativeness and scattered trend of the data in Table 2 occurs in part because the analytical

model assumes a significant interaction between shear and moment as we discussed in the previous

section, which is embodied by the coefficient

Yv as

defined in the ACI 318-02. Analytical work has

been

done similar to that for edge connections

to see

how the predictions go

by

reducing this

coefficient yv step by step. The criterion is still the value of COY for the data collected for corner

connections.

Fig.

6 shows the relationship between COY and the percent of

Yv

This figure obviously

shows that when reducing

yv from 100 percent of ACI value, the value of COY becomes smaller and

smaller until

v

was reduced

to 10

percent of

ACI

value. After that, the value of COY becomes larger if

we continue

to

reduce this Yv This behavior suggests that the relationship between shear and moment

for corner connections

is

even less than that for edge connections. In the previous discussion part for

edge connection ,

v

was reduced

to

60 percent of ACI defined value.

533


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0.340

/

.320

c:

o

ii 0 300

.;::

rn

/

:: 0.2BO

o

/

0 260

'

(3

/

EO 0.240

( )

o

/

0 220

0.

200

0.00

0.20 0.40

0.60 O.

O

1.00

Percent of Yv

Fig . 6 Relationship of value of COY and percent of Yv for corner connections

The overall predictions following a reduction of Yv (10 percent of

ACI

defined value) are also listed

Table 2. The strength ratio ranges from 0.

731 to

1.687, having a mean of 1.

160

and a value of COY

about 0.218. The accuracy of prediction

is

highly improved by reducing

v

only. However, there are

nine specimens which had strength ratios less than unity, meaning that their strengths are

overestimated. This problem can also be easily solved by applying for a larger strength reduction factor

in the ACI 318-02, which is not discussed in detail in this paper.

6

Conclusions

Based

on the

analysis of available data for exterior connections, including edge and corner

connections, the following conclusions may

be

drawn.

For exterior connections the interaction between shear and moment is not as strong

as

expected.

The interaction between shear and moment

is

even weaker for corner connections than for edge

connections . A

60

percent of ACI defined Yv value should

be

used for edge connections, and 10

percent of that value should

be

used for corner connections only. Once the reduced value of Yv

is

used

in the ACI 318-02, the accuracy of the strength prediction for exterior slab-column connections can be

improved greatly.

7

Acknowledgements

This research is part of the joint BCA-NTU research

on

fiat plate structures. Research grants from

the Building and Construction Authority - Singapore, and the Nanyang Technological University are

gratefully acknowledged.

8 References

[1J

ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-02) and

Commentary (318R-02)," American Concrete Institute, Farmington Hills, Mich ., 2002, 443pp.

[2J

Hanson, N.W. and Hanson, J.M., "Shear and Moment Transfer between Concrete Slabs and

Columns," Journal

o

the Research and Development Laboratories Portland Cement Association,

Vo1.10, NO .1, Jan . 1968, pp.2-16.

[3] Stamenkovic,

A

and Chapman, J.

C.,

"Local Strength at Column Heads

in

Flat Slabs Subjected

to

A

Combined Vertical and Horizontal Loading,"

Proceedings Institution o Civil Engineers

(London) , Part

2,

V. 57

, June 1974, pp.205-232.

[4]

Zaghlool, E.R.F., "Strength and Behavior of Corner and Edge Column-Slab Connections

in

Reinforced Concrete Flat Plates, " PhD Thesis, Department of Civil Engineering, The University of

Calgary, Calgary, 1971, 366pp.

[5]

Hawkins, N.M.; Wong, C.F.; Yang, C.H. , "Slab-Edge Column Connections Transferring High

Intensity Reversing Moments Normal

to

the Edge of the Slab,"

Structures and Mechanics Report No

.

SM78-1, Department of Civil Engineering, University of Washington, Seattle, May 1978.

[6J

Kane,

K.A., "Some Model Tests on the Punching Action of Re inforced Concrete Slabs at Edge

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[7]

Lim

, F.K. and Rangan, B.v., "Studies on Concrete Slabs with Stud Shear Reinforcement in Vicinity

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,

No

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[8] Mortin, J.D. and Ghali , A., Connection of Flat Plates to Edge Columns, ACI Structural Journal Vol.

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L.,

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in

a Reinforced Concrete Flat

Plate Structure, The Royal Military College, Kingston, 1978.

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Industry Research

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Information Association, London, United Kingdom,

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and

Rangan, B.v., Forces

in

the Vicinity of Edge Columns

in

Flat-Slab Floors,

Magazine

of

Concrete Research Vol. 35, No. 122, Mar. 1983,

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.19-26.

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Strength of Slab Coloumn Connection