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STRENGTH TESTS ON SLENDER REINFORCED CONCRETE WALLS IN ONE

AND TWO-WAY ACTION

Jeung-Hwan Doh1, Sam Fragomeni

2and Yew-Chaye Loo

3

ABSTRACT : A total of nine reinforced (normal and high strength) concrete walls with and without sidesupports have been tested under eccentric loading. The slenderness ratios (H/tw) of these half-scale walls varied

from 30 to 40. This paper highlights the experimental set-up and reports on the failure loads and typical crack

patterns of the reinforced concrete walls. In addition, the strength results are compared with those predicted

using the design formulas recommended in the Australian Standard AS3600-2000 and American code ACI318-

1999. The shortcomings of the code methods are identified.

KEYWORDS: code methods, normal and high strength concrete, reinforced concrete walls

1. INTRODUCTION

The simplified wall design methods prescribed in the Australian Standard AS3600-2000 [1] and

American code ACI318-1999 [2] are considered restrictive and conservative. The methods areintended for axially loaded concrete walls supported at top and bottom only (one-way action); no

guidance is given for walls supported on all four sides (two-way action). Other limitations include:

restrictions on slenderness ratios (H/tw 30), no extra load capacity being allowed for reinforcingsteel, and that the methods are not applicable to high strength concrete walls. To rectify the

inadequacies that exist in these national codes, more experimental studies may be required.

Recent research has focused on easing these limitations. Swartz et al [3] tested 24 rectangular concrete

walls in two-way action under concentric loading. Based on their experimental results, Saheb and

Desayi [4, 5] proposed two formulas to determine the ultimate strength of one-way and two-way

walls. Fragomeni [6] carried out extensive testing on high strength concrete panels and proposed some

modifications to the AS3600 wall design equation to allow for the inclusion of high strength concrete

parameters. Sanjayan and Maheswaran [7] tested high strength concrete panels in two-way action with

different load eccentricities.

This paper focuses on tests undertaken on normal and high strength concrete walls in one- and two-

way action, and with various slenderness ratios (H/tw) between 30 to 40. These tests cover an area

where previous research is limited. Apart from highlighting the experimental set-up, failure loads and

typical crack patterns of the test panels are also reported. Experimental results are then compared withthose predicted by the Australian Standard and ACI code recommendations.

1Griffith University, Queensland, Australia, BE(Hons), ME(Hons), PhD Scholar

2Griffith University, Queensland, Australia, PhD, MIEAust, CPEng, Senior Lecturer3Griffith University, Queensland, Australia, PhD, FIEAust, FIStructE, FICE, CEng, CPEng, Professor and Head

The Eighth East Asia-Pacific Conference on Structural Engineering and Construction

5-7 December 2001, Nanyang Technological University, Singapore

Paper No.: 1302

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2. DESIGN OF WALLS USING CODE METHODS

Reinforced concrete walls in Australia are designed using the guidelines given in AS3600-2000 [1].Section 11 of the Standard specifies a simplified equation that can be used for the design of walls

when certain loading and constraint conditions are met. The Standard also allows any wall to be

designed as a column using the provisions detailed in Section 10.

In the simplified design method, the ultimate design axial strength per unit length (in N/mm) of a

braced wall in compression is given as

'

cawu f6.0)e2e2.1t(N = (1)

where the strength reduction factor is 0.6, twis the wall thickness (mm), e is the load eccentricity (inmm) which has a minimum value of 0.05tw, fc(in MPa) is concrete strength and ea= Hwe2/(2500tw).For computing ea, Hwe is the effective height of the wall and is the lesser of kH or kL, where H is

height of wall, L is length of wall, and k is 0.75 for walls restrained against rotation at the top and

bottom ends and 1, for walls not so restrained.

Equation (1) applies to walls where H/tw30 (but if the ultimate design axial force N*0.03fc Agthen Hwe/ tw50). A practice sometimes adopted in Australia is to use H we/ tw20 when large axialloads are encountered. The walls are required to have a minimum vertical reinforcement ratio v =0.0015, and a minimum horizontal reinforcement ratio h= 0.0025.

The empirical design formula recommended in ACI 318-1999 is given as

=

2

w

w

'

cut32

kH-1tf55.0N (2)

where is taken as 0.7, k = 0.8 for walls restrained at the top and bottom against lateral translationsand against rotation at one or both ends, and k = 1.0 for walls not so restrained against rotation.

Equation (2) applies to walls where Hwe/tw25 or L/tw25 whichever is less for load-bearing walls.The minimum allowable thickness is 100mm. The resultant axial load must be in the middle third of

the gross thickness of the wall allowing for a maximum eccentricity of tw/6. The walls are required to

have a minimum reinforcement ratio v= 0.0015 and h = 0.0025. These values can be reduced to0.0012 and 0.002 respectively if the reinforcing bars are less than 16 mm in diameter or if mesh is

used.

It should be noted that Equations (1) and (2) are limited to Hwe/t 30 and do not include the effects ofreinforcement content, high strength concrete and two-way action.

3. TEST SPECIMENS AND TEST SET UP

3.1 Test Panels

A total of nine wall panels were cast and tested. The dimensions and material properties of the walls

are summarised in Table 1. The panels designated as OWNS and OWHS were tested in one-way

action and panels TWNS and TWHS, in two-way action. Further, NS refers to normal strength

specimens and HS refers to high strength ones.

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A local ready-mix concrete supplier batched normal and high strength concrete for all the specimens.

The maximum aggregate size was 10 mm. The high strength concrete mix consisted of silica fume and

super-plasticiser in addition to the normal constituents. No admixtures were used in the normal

strength mix. Three test cylinders for each test panel were cast to obtain the average compressivestrength of concrete.

All wall panels were reinforced with a single F41 mesh, placed centrally in the panel cross-section.

The F41 mesh had a design yield strength of 450MPa and the minimum tensile strength was 500 MPa.

The reinforcement ratios vand hwere 0.0031 for all panels, satisfying the minimum requirements inthe Australian Standard and the ACI code.

Table 1 Reinforced concrete wall panel dimensions and concrete strengths

MODEL H

(mm)

L

(mm)

tw

(mm)

f'c

(MPa)

H/tw

OWNS2 1200 1200 40 35.7 30.00

OWNS3 1400 1400 41 51.9 34.15

OWNS4 1600 1600 40 51 40.00

OWHS2 1200 1200 40 78.2 30.00

OWHS3 1400 1400 40 63 35.00

TWNS2 1200 1200 40 37 30.00

TWNS4 1600 1600 40 45.8 40.00

TWHS3 1400 1400 40 60.1 35.00

TWHS4 1600 1600 40 70.2 40.00

3.2 Test Set-Up

An overview of the test set-up is shown in Figure 1. The test frame was designed to support three

independent hydraulic jacks each of 80 tonne capacity. The jacks were required to transmit a

uniformly distributed load across the top through a loading beam at an eccentricity of tw/6.

The top and bottom hinged support conditions were each simulated by placing a 23 mm diameter high

strength steel rod on a 50 mm thick steel plate of 150 mm width and varying lengths which

corresponded to the different test panel dimensions. Two 20 mm 20 mm angle sections wereclamped to the thick plate by bolts and the 23 mm diameter rod was welded along the length of the

plate. Details of the simply supported top hinged edge are shown in Figures 2 and 3.

To achieve the hinged support conditions for two-way action, the edges of the panels had to be

effectively stiffened so that rotation about the x-axis was prevented while they were free to rotate

about the y-axis. To achieve this, two parallel flanged channel sections extending along the height of

each of the wall panels were placed on both sides of the panel. They were separated by a square

hollow section as shown in Figure 1.

Dial gauges were used to record the out-of-plane displacements or deflections. They were placed at

quarter and three-quarter heights, and at the centre of the panels as indicated in Figure 1.

3.3 Test Procedure

The walls were loaded in increments up to failure. At each load increment, crack patterns and

deflections were recorded. The latter allowed the load-deflection history to be accurately traced.

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Figure 1 Test frame arrangement and side supports

Eccentricity t/6

23roller

20x20EA

40 mm thickness test panel

150x50 plate

Welding

Figure 2 Details of top hinged edge Figure 3 Details of steel plate

4. RESULTS

Table 2 presents the experimental failure load as well as the peak-load central deflections of the wall

panels. Also given are the experimental axial strength ratio, N u/fcLtwand the ultimate loads computed

using Equations (1) and (2) due to AS3600 and ACI318 respectively.

It can be seen in the table that the failure load predictions by the two code formulas are poor in that

they either give very conservative results for H/tw=30, or zero capacity (i.e. negative values) for those

specimens with H/tw>32. Figure 4 depicts this fact further where the code methods predict zero loadcapacity for cases beyond H/tw = 32. Also shown in Figure 4 are the experimental values of Nu. All

the nine test panels (6 of which have H/tw>32) possessed significant load bearing capacities.

Table 2 and Figure 4 also confirm the beneficial effects of side (two-way) supports the difference in

axial wall strengths between two-way and one-way wall panels. The panels in two-way action were

approximately 3 times stronger than their one-way counterparts. Differences between the normal

strength panels and their high strength counterparts are more difficult to decipher. The high strength

panels, as can be seen in Figure 4, yield slightly lower axial strength ratios, when the slenderness is

held constant.

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Table 2 Failure load and deflection of panels

MODEL H/tw Deflection

(mm)

Load

(kN)

Nuf'cLtw

AS (kN)

Equation (1)

ACI (kN)

Equation (2)

OWNS2 30.00 14.97 247.21 0.144 82.25 114.13

OWNS3 34.15 6.36 426.73 0.143 -237.65 -227.47

OWNS4 40.00 8.97 441.45 0.135 -940.03 -1009.80

OWHS2 30.00 8.85 482.65 0.129 180.15 249.96

OWHS3 35.00 6.61 441.45 0.125 -380.90 -380.76

TWNS2 30.00 9.18 735.75 0.415 85.18 118.19

TWNS4 40.00 7.32 1177.2 0.402 -844.00 -906.64

TWHS3 35.00 10.2 1294.92 0.385 -363.61 -363.47

TWHS4 40.00 6.14 1677.51 0.373 -1293.93 -1389.96

Figure 4 Axial strength ratio versus slenderness ratio

Table 2 also indicates that, in general, deflections are slightly smaller in two-way panels than in the

corresponding one-way specimens. Finally, the crack patterns for four of the panels are recorded inFigures 5 to 8 for illustrative purposes. The figures show that walls in one-way action are

characterised by horizontal cracking at midspan while those in two-way action feature biaxial

cracking. Note also that TWNS4 and TWHS4 sustained a brittle failure.

5. CONCLUSION

Laboratory strength tests have been carried out on nine reinforced normal and high strength concrete

wall panels in one- and two-way action. Loaded with an eccentricity of tw/6, these half-scale

specimens had high slenderness ratios, i.e. with H/tw of between 30 and 40.

A comparative study indicates that the design formulas recommended in the Australian Standard

(AS3600-2000) and the ACI code (ACI318-1999) are inadequate in that their strength predictions are

very conservative for the specimens with H/tw30; for those with a higher H/tw ratio, the formulasyield negative strength values which indicate zero load-bearing capacity.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50

H/tw

Nu/(f'cLtw)

One-way (NSC)

One-way (HSC)

Two-way (NSC)

Two-way (HSC)

AS3600 (Equation 1)

ACI (Equation 2)

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The study finds that side supports (or two-way action) greatly increase the strength of the wall panels.

The Standard and code formulas do not account for these beneficial effects.

In view of the significant shortcomings, there is a need to amend these design formulas.

Figure 5 Crack pattern of OWNS2 Figure 6 Crack pattern of OWHS3

Figure 7 Crack pattern of TWNS4 Figure 8 Crack pattern of TWHS4

6. REFERENCES

[1] Standard Australia, Australian Standard 3600, Concrete Structures, Draft, Sydney, 2000, pp.108-109.

[2] ACI Committee 318, Building Code Requirements for Reinforced Concrete ACI318-99,

American Concrete Institute Detroit, 1999, 111 p.

[3] Swartz, S. E., Rosebraugh, V. H., Berman, M. Y., Buckling test of rectangular concrete panels.

ACI Journal, Vol.71, No. 1, Jan., 974, pp. 33-39.

[4] Saheb, S. M., and Desayi, P., Ultimate Strength of R.C. Wall Panels in One-way in-plane

action, Journal of Structural Engineering, ASCE, Vol.115, No.10, Oct., 1989, pp. 2617-2630.

[5] Saheb, S. M., and Desayi, P., Ultimate Strength of R.C. Wall Panels in Two-way in-planeaction, Journal of Structural Engineering, ASCE, Vol.116, No.5, May., 1990, pp. 1384-1402.

[6] Fragomeni, S., Design of normal and high strength reinforced concrete walls, Ph.D Thesis, The

University of Melbourne, 1996.

[7] Sanjayan, Jay G. and Maheswaran, T., Load Capacity of Slender High-Strength Concrete Walls

with Side Supports, ACI Structural Journal, Vol.96, No. 4, July-August, 1999, pp. 571-576.