HORIZONTAL POST-TENSIONED CONNECTIONS FOR . PRECAST CONCRETE LOAD-BEARING SHEAR WALL PANELS

R.L. Hutchinson ', S.H. Rizkalla', M. Lau3, and S. Heuvel4

ABSTRACT

Precast load-bearing shear wall panels are used extensively for high-rise construction because of the ease and speed of assembly, and the high quality of the precast panels. The connections between panels are extremely important since they affect both the speed of erection and the overall integrity of the structure. This paper presents the results of a research program conducted to investigate the behaviour and the capacity of post-tensioned horizontal connections typically used for precast load-bearing shear wall panels subjected to monotonic shear loading.

Nine prototype specimens with three different connection configurations were tested. The first two wnfigurations modelled the connections which support the hollow-core floor slab. Post­tensioning was included in the second configuration. The third configuration consisted of post­tensioned connections without hollow-core slab. Two different levels of load normal to the connection were used to determine the effects of dead load.

Rational mathematical models were developed to predict the shear capacity of the connections at the maximum and ultimate limit states. These models were found to be in good agreement with the experimental results.

, Structural Engineer, Crosier Kilgour & Partners, Winnipeg, Manitoba 2 Professor, Civil Engineering Dept., University of Manitoba 3 Engineering Manager, Con-Force Structures, Winnipeg, Manitoba 4 Principal, ARC+CADD Architectural CAD Services and Structural Engineering, Winnipeg,

Manitoba

HORIZONTAL POST-TENSIONED CONNECTIONS FOR I'RECAST CONCRETE LOAD-BEARING SHEAR WALL PANELS

R.L. Hutchinson', S.H. Rizkalla', M.Lau3 and S. Heuvel4

INTRODUCTION

Precast load-bearing shear wall panels are used extensively for high-rise construction, due to the high quality control that can be achieved with off-site fabrication, the reduction of construction time and, consequently, reduction in cost. In addition, precast construction is seldom interrupted by adverse weather conditions which is an important factor for the Canadian climate.

Large precast panel buildings consist of precast concrete load-bearing shear walls in both the longitudinal and transverse directions, designed to resist lateral loads, as shown in Figure l. The major difference between the longitudinal and transverse shear walls is mainly the type of the horizontal connection. Transverse shear walls are normally used to support the hollow-core slab of the noor system, whereas the longitudinal shear walls run parallel to the hollow-core slab and do not include the hollow-core slab at their connection, as shown in Figure 2.

The connections between the panels are extremely important. A well designed simple connection normally minimizes the construction time, requires minimal falsework, and maintains the overall integrity of the structure.

A recent innovation in horizon tal connections for load-bearing shear wall panels is the use of vertical post-tensioning tendons. The strands pass through galvanized ducts from the top panel to the base of the structure. While the panels are temporarily braced, the gap between panels which is necessary for alignment purposes, is packed with drypack grout. After the erection of several stories, the tendons are post-tensioned, the ducts are grouted, and the temporary braces are removed.

, 3

4

Structural Engineer, Crosier Kilgour & Partners, Winnipeg, Manitoba Professor, Civil Engineering Dept., University of Manitoba Engineering Manager. Con-Force Structures. Winnipeg, Manitoba Principal. ARC+CADD Architectural CAD Services and Structural Engineering, Winnipeg, Manitoba

The post-tensioned horizontal connection, with or without hollow-core slab, has not been examined for a thorough unaerstanding of its shear behaviour under the various limit states. The available literature is not directly applicable to this type of connection .. Therefore, this study was undertaken to investigate the shear behaviour and capacity, under the various limit states. of post-tensioned horizontal connections typically used for precast concrete load-bearing shear wall panels.

LITERATURE REVIEW

Horizontal connections are typically reinforced with a combination of continuity bars and mechanical shear connectors. In a previous study conducted at the University of Manitoba [1]. it was found that the shear capacity of the horizontal connection may be predicted as the sum of the contributions from the shear resistance of the continuity bars, the shear resistance of the mechanical shear connectors. and the shear friction resistance of the concrete interfaces. To improve the shear capacity of this type of connection, some fabricators introduced the use of multipk shear keys along the horizontal portion of the joint surface of the panel. The presence of these shear keys was proven to enhance the shear capacity in comparison to the plain surface connection [2].

Several tests have been conducted in Europe to determine the shear capacity of horizontal connections supporting hollow-core slab, however, the details of the European connections differ from those used in the North American practice, and their results are not directly applicable.

Figure 3 shows a typical "North American type" horizontal connection supporting hollow­core slab. The majority of tests conducted on this type of connection were designed to investigate the vertical load carrying capacity of the connection. It was found that the total applied vertical stress. 0 , . across the connection is distributed to 0" and 0 ,2' to the hollow-core and the concrete fill respectively. as shown in Figure 4 [3) where:

0" = 0, 3E,!(2E, + E2 )

0 '2 = 0 , 3EJ (2E, + E.)

Where E, and E2 are the elastic moduli of the hollow-core slab and the concrete fill, respectively.

Harris and Abboud [4] tested sixteen 3/32 scale models of a prototype horizontal connection under reversed cyclic shear load with an applied load normal to the connection ranging from zero to 2 MPa. Hanson [5] conducted full-scale tests on the North American type horizontal connection under cyclic shear load with load normal to the connection varying from 3.5 to 21 MPa.

No effort was made to separate the contributions of the various connection components to the total shear capacity of the connection. The shear friction coefficients determined from these two investigations were significantly varied. Harris and Abboud obtained shear friction coefticients ranging from 0.7 to 1.6. while the shear friction coefficients obtained by Hanson varied from 0.2 to 0.4.

EXPERIMENTAL PROGRAM

A total of nine full scale specimens were tested in this experimental program. The first two groups of specimens were designed to simulate the transverse interior shear wall connections supporting the hollow-core slabs. The third group is designed to simulate the horizontal connection of the elevator shaft and stair well shear walls. For each category, the presence of post-tensioning prestressing was investigated and two levels of load normal to the connection were considered to simulate the effects of gravity.

In this paper, the first digit of the specimen mark given in Table 1 represents the specimen number. The following two characters indicate the particular combination of parameters as follows:

HD: hollow-core slab and drypack only; HP: hollow-core slab and post-tensioning; and PD: post-tensioned with drypack only

The last digit represents the level of stress, 4 MPa and 8 MPa, normal to the connection.

A typical specimen with hollow-core slab and post-tensioning is shown in Figure 5. The hollow-core slab rests on "Korolath" bearing pads on the bottom panel. The cores of the hollow­core slab and the gap between the ends of two slabs are filled with a flowable concrete fill. The gap between the hollow-core and the top panel is packed with drypack grout. Load was applied normal to the connection to simulate the effects of gravity, and the shear load was applied directly through the centre of the connection, as shown in Figure 5.

To produce equivalent pressure on the bearing pad as produced by the long span hollow­core units of the actual structure, a loading apparatus consisting of threaded bars and load cells was used. Only a short portion of hollow-core was used to model the actual connections since only this portion of the cores of the hollow-core is typically filled with concrete.

The strength of the concrete fill was comparable to that of the precast panels and the hollow-core slab, and is much stronger than the drypack.

Seven days after drypacking, the strands were post-tensioned and the ducts were filled using an expansive grout. Twenty-eight days later, the specimen was moved to the testing machine where the horizontal connection was tested in a vertical orientation, as shown in Figure 6.

The edges of the panels were post-tensioned with a system of dywidag bars to prevent premature cracking of the panels. The load normal to the connection was applied through a system of rollers to allow relative displacement of the two panels in the direction of the applied shear load.

Vertical and horizontal relative displacements from panel to panel and across the various interfaces were measured as schematically shown in Figure 7. Electronic L VDT's were used on one surface of the panel. while mechanical dial gauges were used on the other.

TEST RESULTS

The measured maximum and ultimate load carrying capacity of all the tested connections are given in Table 1. In general, the predominant mode of failure was due to slip along the drypack to panel interface, with the exception of one specimen where slip occurred between the concrete fill and bearing pad to panel interface.

For specimen IHD4, without post-tensioning and subjected to stresses of 4 MPa normal to the connection, the ultimate shear capacity of the connection was reached without any damage to the hollow-core slab. For specimen 4HP4, which is identical to IHD4 except for an additional 1.2 MPa due to post-tensioning stresses, some minor cracking of the hollow-core at the bottom of the connection was observed at failure. This cracking could be due to the incomplete concrete filling of the bottom core of the hollow-core slab, since the cracks did not propagate along the connection with increasing of the applied shear load.

For the specimens tested under higher levels of stress normal to the connection, specimens 2HDS, 3HDS, 5HPS, and 6HPS, an extensive cracking of the hollow-core slab was observed at failure of the connection, as shown in Figure S. The cracking began just prior to the maximum applied shear load, and the cracks propagated by increasing the applied shear load.

For specimen 5HPS, a post-tensioned hollow-core specimen tested at the higher load level normal to the connection, it was found that damage occurred in the wall panel, as shown in Figure 9. Crushing and spalling of the concrete in this zone is attributed to the eccentricity of the applied shear load due to the presence of the hollow-core slab which induces a higher level of compressive stress in this vicinity.

DISCUSSION OF TEST RESULTS

The effect of load normal to the connection on the shear resistance of the connection is shown in Figure 10. The two drypacked post-tensioned specimens, SPDS and 7PD4, were subjected to Sand 4 MPa stresses normal to the connection respectively, in addition to the 1.2 MPa due to post-tensioning stresses. The increase in shear capacity is directly proportional to the increase of the load normal to the connection. This behaviour is consistent with the shear friction theory presented in a previous study (1).

To demonstrate the effect of post-tensioning, Figure 11 compares two specimens with hollow-core slab, 4HP4 and IHD4, with and without post-tensioning, respectively, subjected to 4 MPa stresses normal to the connection. The increase in the maximum and ultimate shear capacities is proportional to the increase in the load normal to the connection due to post­tensioning stresses. This results suggest that the post-tensioning enhances the friction resistance of the connection and may be accounted for by simply adding the post-tensioning effect to the gravity load.

At the low stress level, 4 MPa, normal to the connection, the presence of hollow-core slab had no effect on the behaviour or the capacity of the post-tensioned connections. This is clearly evident in Figure 12 which compares load versus slip curves for two post-tensioned specimens, 4HP4 and 7PD4, with and without hollow-core slab.

However, at higher levels of stresses normal to the connection, the presence of the hollow­core significantly affects the behaviour and the capacity of the connection as shown in Figure 13. The two prestressed specimens, 5HP8 and 8PD8, with and without hollow-core slab show a significant reduction in the maximum and ultimate shear capacity due to the presence of hollow-core slabs. The stiffness and ductility of the connection is also reduced with the presence of hollow-core slab at high levels of load normal to the connection.

Based on the observed behaviour after cracking, the failure mechanism in horizontal connections with hollow-core slab appears to be controlled either by the shear capacity of the hollow-core slab, as in those specimens tested at higher load levels normal to the connection, or by the shear friction resistance of the connection, as in those specimens tested at the lower load level. Shear failure in the hollow-core slab causes a rapid degradation of the shear capacity of the connection, resulting in a failure mode which is less desirable than the more ductile shear friction failure.

PROPOSED MATHEMATICAL MODEL

(i) Maximum Shear Capacity

Based on this investigation, the following mathematical model is proposed to predict the maximum shear capacity of post-tensioned horizontal connections with or without hollow-core slab. The maximum shear capacity could be predicted as the lesser of that determined by the shear friction model, V" equation (la), and the cracking capacity of the hollow-core, Vh, equation (lb):

V, = J.Wo~ (la)

Vh = 2/3 (~,F" + ~2F12) (lb)

F" = j t;,(f" +0'01)

F t2 = j ft2( f" +0'02)

The shear friction resistance, V" can be predicted using the shear friction model which is related to the area of the concrete in terface, ~, and the coefficient of friction, J.L, of the drypack to panel interface. A coefficient of friction of 0.7 is proposed based on the test results of this investigation and the previous studies conducted at the University of Manitoba [1].

The cracking capacity of the connection, V" based on the capacity of the hollow-core slab with concrete fill, can be predicted using the areas 'of the hollow-core 'and concrete fill in contact with the drypack, ~, and ~2' respectively. The magnified tensile capacities of the concrete of the hollow-core and the concrete fill, F" and Ff2, respectively, are based on the tensile strength of each material, 1'" and ff2, and the compressive normal stresses, 0'01 and 0'02' including the effect of prestressing.

Figure 14 compares the maximum shear capacity of all nine specimens tested in this program and the capacity as predicted by the proposed model. The predicted maximum shear capacities are in good agreement with the experimental results.

(ii) Ultimate Shear Capacity

At ultimate, the normal and shear stresses could be resisted only by the area of the web at the mid-height of the hollow-core slab. This assumption is based on a complete loss of bond between the concrete fill and the hollow-core. Loss of bond may be caused by incomplete filling of the cores with the concrete fill during construction, the effects of shrinkage, or the propagation of cracks through the hollow-core and along the surface of the concrete fill in contact with the cores as was observed in several of the specimens tested.

Based on a complete loss of bond, the ultimate shear capacity of horizontal connections with hollow-core could be predicted as the lesser of that determined by the shear friction model, VI' equation (la), and the ultimate capacity of the hollow-core, Vh", equation (2):

(2)

F" = {r"(f,,+a,,,)

Where A", is the summation of the area of the webs at mid-height of the hollow-core beneath the contact surface area A",. For the given geometry of the hollow-core tested in this investigation, A", = A" ,/4.

In this model, the contribution from the centre concrete fill remains unchanged while the contribution from the hollow-core slabs are modilied to reflect the loss of bond. The area of the hollow-core in contact with the drypack, A"" is reduced by a factor of four, however, the normal stress a I" is increased. also by a factor of four, due to the reduction of the contact area at ultimate.

Figure 15 compares the measured and the predicted ultimate shear capacity of all nine specimens tested in this program. The comparison suggests that the proposed model provides a conservative lower bound for the ultimate shear capacity of horizontal connections with hollow­core slab.

SUMMARY AND CONCLUSIONS

Nine specimens, including two hollow-core slab connection configurations with and without post-tensioning, and one plain surface connection with post-tensioning, were tested under monotonic shear loading conditions to investigate the various limit states behaviour of horizontal post-tensioned connections. The effect of the presence of hollow-core, load normal to the connections, and post-tensioning were investigated.

Based on the results of this study, the following conclusions could be drawn: 1. An increase in the level of load normal to the connection increases the maximum shear

capacity of the connection. 2. The effect of post-tensioning may be accounted for by adding the applied post-tensioning

stresses to the normal gravity load.

3. For connections supporting hollow-core slab, the failure mechanism could be controlled by shear friction resistance or by the shear capacity of the hollow-core slab.

4. For connections supporting hollow-core slab at high levels of normal load, the stiffness and ductility 'of the connection are reduced in comparison to the same type of connection without hollow-core slab. Shear failure in the hollow-core slab results in rapid degradation of the shear capacity of the connection.

5. The maximum shear capacity of the connection with hollow-core slab is governed by the lower magnitude of the shear friction capacity and the hollow-core shear capacity.

6. The ultimate shear capacity of the connection with hollow-core slab is based on complete loss of bond. The predicted values provide a conservative lower bound.

ACKNOWLEDGEMENT

This study was performed in the Department of Civil Engineering at the University of Manitoba with financial assistance from the National Research Council of Canada, Con-Force Structures Limited. and the Prestressed Concrete Institute. The help of Messrs E. Lemke, M. McVey. Dong X .. and K.A. Soudki is greatly appreciated.

REFERENCES

1. Foerster. H.R.. Rizkalla. S.H.. and Heuvel, J.S.. "Behaviour and Design of Shear Connections for Load-Bearing Shear Wall Panels", Prestressed Concrete Institute Journal, Vol. 34. No.1. January/February 1989. pp. 100-119.

2. Serrette. R.. Rizkalla. S.H.. Attiogbe. E.. and Heuvel. J.S., "Multiple Shear Key Connections for Precast Load-Bearing Shear Wall Panels", Prestressed Concrete Institute Journal, Vol. 34. No.2. March/April 1989. pp. 104-120.

3. PCI Committee on Precast Bearing Wall Buildings. "Considerations for the Design of Precast Concrete Bearing Wall Buildings to Withstand Abnormal Loads", Prestressed Concrete Institute Journal, Vol. 21. No.2, March/April 1976, pp. 18-51.

4. Harris. H.G .• and Abboud. B.E .. "Cyclic Shear Behaviour of Horizontal Joints in Precast Concrete Large Panel Buildings", Proceedings of a Workshop on Design of Prefablicated Concrete Buildings for £m1hquake Loads, Applied Technology Council, April 27-29, 1981, pp.403-438.

5. Hanson, N.W. "Supplemental Report 'C'o Seismic Tests of Horizontal Joints", Design and Constntpion of Large Panel Concrete Stntctures, Oftice of Policy Development and Research. Department of Housing and Urban Development, Washington, D.C., January. 1979.

TABLE 1 Experimental Program and Test Results

Joint Specimen Load Normal to Maximum Ultimate Type Mark connection Load Load

(MPa) (leN) (leN)

HD IHD4 .. 550 535 2HD8 8 905 750 3HD8 8 885 675

HP 4HP4 4 704 660 5HP8 8 957 680 6HP8 8 947 880

PD 7PD4 4 690 680 8PD8 8 1160 1130 9PD8 8 1157 1125

Figure I

• ~ LONGITUDINAL ~ WIND LOAD

LONGITUDINAL STAIR WELL SHEAR WALLS

7;~~ __ HOLLOW-CORE

TRANSVERSE WIND LOAD

Load-bearing Shear Wall Panel System

SLABS

TRANSVERSE INTERIOR SHEAR WAI LS

HOllOW-CORE SLAB

DR'rPACK

LONGITUDINAL

SHEAR WALL

PANEL

DR'rPACK

Figure 2 Horizontal Connections ror Longitudinal and Transverse Shear Walls

TRANSVERSE INTERIOR SHEAR WALL PANEL

DETAILS IN FIGURE 3

HOllOW-CORE SLAB

DRY?ACK

CAST IN PLACE

CONCRETE FlLL

CONTlNUOUS

BEARING PAD

PRECAST INTERIOR

SHEAR WALL PANEL

HOLLOW-CORE SLAB

PAPER OR PLASTlC DAM

Figure 3 Typical North American Horizontal Joint with Hollow-core Slab

HOLLOW-CORE

CONCRETE FlLL PORTION E:,

In this investigation:

b, = b, = 50 mm

~ DEAD LOAD

1\, = 2 [ 50 x 1200 1 = 12000 mm'

~, = 50 x 1200 = 6000 mm'

1\, = :2 [ 50 x L web 1

DISIRIBUTION OF VERTICAL LOAD

= :2 [ 50 x ( 5x40 + 2x50 ) 1 = 3000 mm'

For Specimen 5HP8:

G, = 8 + 1.2 = 9.2 MPa

G" = 6.5 MPa

G" = 14.6 MPa

G' " = ~,1~" G"

= 12000/3000 G"

= 26 MPa

Figure 4 Distribution of Vertical Load and Dimensions of Typical Horizontal Connection with Hollow-core Slab

E E o CD CD

APPLIED NORMAL LOAD (GRAVITY)

II II II II II II II I II II II II

0000 I II 1200 mm

I II II II II

2300 mm

___ Drypack

---APPLIED SHEAR LOAD

Post-tensioning Strands

Figure 5 Typical Configuration of Test Specimen

Figure 6 T~st Set-LOp

I

Figure 7 Instrumentation

f

}':;.1 ~ . . ;~' - .. ,. -;

'"

.:J ;: ;:)

• ~ -.. -= -

.~ :.r.

-" ~ :J

1 ~

( , .~

• ) ·r · -Q :;)

( :;:;

~~"" '<-' . '- .,

, ::c ... . ..:J;,;. .• :J

:::> oJ) ,.

1200 ........ ~ 1000

o 800 < g 600

~ 400 w ~ 200

... 8PD8

. 7PD4

0._---,----,-----,----,----, o 1 2 3 4 5

SLIP (mm)

Figure 10 Effect of the Load Normal to the Connection

1200 ........ ~ 1000 -....;

0 800 < 0

600 ~

0::: 400 < w ... 4HP4

:I: 200 . lHD4 (/)

0 0 1 2 3 4 5

SLIP (mm)

Figure 11 Effect of Post-tensioning

1200 "....,

~ 1000 '-'

0 800 « 0

600 ...J

a:: 400 «

w :::c

200 Vl

0

Figure 12

1200 "....,

~ 1000 '-'

0 800 « 0

600 ...J

a:: 400 « w

:::c 200 Vl

... 4HP4

.7PD4

0 1 2 3 4 5 SLIP (mm)

Effect of the Presence of Hollow-core Slab at Low Stress Level Normal to the Connection

... 8PD8

.5HP8

0 4 5

Figure 13

0 1 2 3 SLIP (mm)

Effect of the Presence of Hollow-core Slab at High Stress Level Normal to the Connection

>. .... . ~ <J <!l

r; U

I-<

MAXIMUM SHEAR CAPACITY

unconservative

BPOB POB

1:

<!l 800 QJ

..q Ul

't1 QJ .... <J :a QJ I-< Il.

Figure 14

~

Z ~ ~

>. .... . ~ <J <!l r;

U

7P 4HP4

• conservative

<OO~--,---,---,---.---,---,---,---, <DO aDo fZOO

Experimental Results (kN)

Relationship Between the Measured and Predicted Maximum Shear Capacity of the Connection

f200 ULTIMATE SHEAR CAPACITY

unconservative

1:

a aDo QJ

conservative

..q Ul

't1 Q) .... <J .~

't1 Q) I-< Il.

Figure 15

7PO

5HPB • 3H08.

4HP4 H04 •

•

6HPB

• .2HOB

400~--.---.---.---.---.---.---.---, 400 800 f200

Experimental Results (kN)

Relationship Between the Measured and Predicted Ultimate Shear Capacity of the Connection