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REINFORCEO BPICK MASONRY CURTAIN WALL TEST ANO OESIGN

JOHN G. TAWRESEY Vice President, KPFF Consulting Engineers

1201 Third Avenue Seattle, Washinglon 98101, USA

ABSTRACT

Reinforced brick masonry curlain wall systems have replaced mosl of lhe conventional veneer systems in the northwesl Uniled Slales. Inilially driven by seismic consideralions, reinforced brick masonry is now an alternative based on cosi and performance. These developmenls are allribuled lo lhe advancemenl in lhe underslanding of reinforced brick masonry behavior by the slruclural engineer.

The firsl reinforced brick curlain wall was erecled in Portland, Oregon in 1975. Since Ihal lime Ihere have been more than fifty such inslallations. The syslems have been lesled in full-scale curtain wall mockups. Their performance has been superior lo more convenlional veneer syslems tesled in the same fashion.

The developmenls began with a tesl program sponsored by Western Stales Clay Producls Association on a four-inch hollow reinforceable unit. The lest program verified conventional reinforced design procedures and indicaled sufficienl lensile slrenglh to allow panelizalion. This paper summarizes the original test program and dala, and discusses the use of reinforced brick masonry for curtain wall applicalions.

INTROOUCTION

For mosl of history, brick was used as a slruclural material. In lhe twenlielh

century, sleel and concrele frames replaced loadbearing masonry as lhe principal

melhod of conslruclion. Brick, while mainlaining dominance in the fireplace

market, fell in stature to a levei slightly higher than Ihal of wallpaper. Recently, in

the U.S., mosl of the brick applied lo buildings hasJ>een in the form of veneer.

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This pape r is not about veneer. II is about an alternative to veneer. It is

about using brick masonry as a structural material. It is about a test program that

gave rise to a new structural application for brick masonry. It is about reinforced

brick masonry curtain wall.

A curtain wall is an exterior non-Ioadbearing skin of a building. The

materiais for the curtain walls generally include glass and aluminum and either

concrete, stone, steel or brick. The curtain wall is normally isolated from the

structural frame. Nevertheless, it carries considerable loads from wind or seismic

acceleration.

Unlike loadbearing applications where the engineer focuses on compression

capacity, in curtain wall applications the engineer focuses on the flexural, shear and

torsional behavior. Also, the engineer focuses on the design of connections where

large stress gradients result from concentrated loading. Reinforced brick masonry,

because of the flexural capacity has seen increasing use as a building curtain wall.

THE WALL SYSTEM

Bricks may be reinforced in many ways. The bricks used in the northwestern U.S.

are hollow bricks having cells greater than 1-1/2 square inches in area. The cells

align vertically when placed in the wall allowing placement of reinforcement in one

direction. A typical brick dimension is shown in Figure 1.

Figure 1. Example dimensions for hollow brick for reinforcement.

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There are many other possible shapes. Reinforcement in lhe olher direction

is normally accomplished in one of Iwo ways. Either a channel is cut Ihrough the

cross webs, or wire reinforcement is placed in the mortar bed joint.

Normally, the walls are one unit in thickness and grouted solid. The grout

employed is normally a very high slump ASTM C 476 fine grout.

WHY REINFORCE

The obvious reason for reinforcing brick masonry, is the same reason for reinforced

concrete, Le. low tensile strength and high compressive strength.

However, if one contemplated the invention of a new material to combine with

steel reinforcement, what would the ideal properties be? High on the list would be

high compression, chemical compatibility and capacity; also non-reversible

expansion. Why? 11 is simple. If the material expands, it puts the material in

compression and the steel in tension.

11 is well documented that brick masonry expands [1]. As an example of the

effect, a 3-1/2 inch thick brick wall reinforced with #4 reinforcement at 24

inches on center would likely result in a compression stress in the brick of

approximalely 21 psi and a tension stress in the reinforcing of 8,700 psL These

values assume a non-reversible moisture expansion of .00003 in/in and a modulus

of elasticily for the brick of 2,000,000 psi. The value of .00003 in/in has been

adopted by the new ACI-ASCE 530 standard. The value, however, is time dependent

and no standard has been set for designo Moreover, the amount of moisture expansion

varies considerably from brick to brick and the data is not readily available. 11

seems that the majority of brick producers view this property as adverse behavior

and tend not to test and publish the numbers. Nevertheless, the behavior exists and

it is beneficiai to reinforced brick performance.

The design methods currentiy employed for reinforced brick masonry are the

same as those used for working stress design in concrete, with a few minor

differences. However, the majority of the design experience and testing is for walls

6 inches or greater in thickness. With the development of the 3-1/2 inch hollow

..

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brick unit, it was necessary to develop material properties and verify the design

methods currently used for the thicker walls.

THE TEST PROGRAM

lhe goal of the test program was to obtain data in support of acode change to allow 3-

1/2 inch reinforced brick masonry in loadbearing construction. More specifically,

the following program objectives were seI:

1. Determine the compressive strength of the brick, mortar, grout and

fabricated prisms.

2. Determine the tensile strengths of the brick, mortar and grou!.

3. Determine the shear strengths of the fabricated prisms.

4 . Determine the bending strengths of walls with and without compression

loading.

5. Determine the column strengths of wide columns with height to thickness

ratios of 27 loaded concentrically and eccentrically.

6. Determine the shear strength of 1/2-inch diameter "J" bolts embedded 2-

1/2 inches.

The statistical test design and planning was a major phase of the programo

lhe test design was configured to maximize, in a statistical fashion , the information

to be obtained. The objective was to maximize the correlation of object variables

while controlling nuisance variables known to hide potential correlation.

Testing was divided into four phases. Phase 1 identified the fundamental

properties of compressive strength , tensile strength, shear strength and flexural

strength both parallel and perpendicular to the bed joint. Phase 2 assessed the

effects of changing brick strengths and fully grouting the brick cells. Phase 3

evaluated the performance of the masonry columns. Phase 4 evaluated the shear

strength of the 1/2-inch diameter "J" bolts.

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Each testing phase contained several testing blocks or replications. The

variables held constant within the block were the mortar batching and proportions,

and the euring conditions. Phase 1 had six replications in addition to one replication

lor testing check-out. The remaining phase had three replications.

The testing program was designed to correlate both within and between testing

blocks. For example, within a given block, shear test results can be compared with

compression prism test results. Since the mortar used to labricate the compression

prism was Irom the same batch as that used to labricate the shear test, the nuisance

variable 01 mortar strength was removed and paired statistical methods could be

applied. Correlation between test blocks related the strengths 01 similar test types

in difterent blocks. For example, a comparison 01 the shear tests in the six blocks 01

Phase 1, which were labricated Irem dillerent batches, provided inlormation about

the elleets 01 morta r strength on the shear strength. Moreover, these comparisons

estimated the lundamental variability 01 the given test set-up and type.

In order to obtain additional inlormation on the basic materiais 01 mortar and

grout, three tests were perlormed on each for both compression strength and tensile

strength within each replication 01 a testing block. This additional testing was

included because the sensitivity 01 masonry strength to these variables was believed

high.

The materiais used were as lollows:

1. Bricks conlormed to International Conlerence 01 Building Ofticials Research

Recommendation #2730 and were as dimensioned in Figure 1. Two strengths

were tested.

2. Mortar was specified as Type S with proportions 01 1 part cement to 1/2 part

lime to 4 parts sand by volume. A low levei 01 control 01 proportions was

applied to simulate actual lield conditions.

3. Grout proportions were 1 part cement to 2-1/4 parts sand. Again a low levei

01 control 01 these proportions was applied to simulate actual lield conditions.

4. Reinlorcement conlormed to ASTM A 615 Grade 40. Joint reinlorcement was

standard "Iadur" type manufactured by Dur-O-Wal, constructed 01 No. 9

gauge cold drawn steel wire conlorming to Unilorm Building Code Standard No.

24-16-73.

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The test type designations were as follows:

1 . BV - brick compressive test conforming to ASTM e 112-60.

2. BT - brick modulus of rupture test conforming to ASTM e 67-66.

3. BA - brick absorption test conforming to ASTM e 67-66.

4. Me - mortar compression tests conforming to use Standard No. 24-23-73.

5. MT - mortar tensile strength conforming to ASTM e 496-71.

6. Ge - grout compressive strength conforming to UBe Standard 24-23-73.

7. GT - grout tensile splitting test conforming to ASTM e 496-71 .

8. ePRE - compression prism tes!.

9. SP R E - shear prism tes!.

10. FA - vertical bending test (exemplified by tensile cracking of the mortar bed

joint).

11. FB - horizontal bending test (exemplified by tensile cracking of the head

joints and brick).

12 . e - column tests.

1 3 . B - bolt shear tests.

14 . W - wall tests.

Object variable designations were as follows:

1 . -H and -L for high and low strength brick.

2 . -NGH, NGL and GH, GL for non-grouted high and low strength brick and

grouted high and low strength brick.

3 . -FV and -FH for bolts embedded in the face shell loaded perpendicular to the

bed joint and horizontal to the bed join!.

4. -JV and -JH for bolts embedded in the mortar bed joint loaded perpendicular

and horizontal to the bed join!.

Actual test specimen dimensions were used to calculate test results. Deflection data

was taken as loads were applied so load deflection plots could be made. A summary of

Phase 1 failure stresses is presented in Table 1. The complete report is available

from Western States elay Products Association [2].

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TABLE 1 Phase 1 failure test stresses for Blocks T, 1 through 6

~ T 1 2

TEST

-1 2330 2360 4010 MC -2 2200 2150 4290

(PSI) -3 2220 2740 3700

-1 549 479 799 MT -2 441 441 752 (PSI) -3 407 562 700

-1 3690 3690 2130 GC -2 3610 3520 2750 (PSI) -3 --- 3540 3050

-1 663 724 506 GT -2 678 765 492 (PSI) -3 658 732 321

CPRE-NGH (PSI-GROSS) 2799 2685 4412 (PSI-NET) 4467 4196 7094

SPRE-NGH (PSI-GROSS) 240 322 154 (PSI - NET) 505 670 320

FA MOMENT 1200 1430 1050 (# inlin)

FB

MOMENT --- 888 691 (# inlin)

---

1 AVERAGE NET AREA FROM TESTS B-HC PHASE I & 11

A = 25.80

C = .3547

3

4650 4420 4240

866 945 758

3740 3650 4100

608 547 527

4741 7665

260 548

1050

806

- ----- --

2 AVERAGE WALL THICKNESSES FROM TESTS B-HC PHASE I

T = 1.679

0=.01912

4

4020 3610 3600

851 609 696

3770 3320 3650

443 332 265

4102 6570

168 356

827

592

5 6

3340 2720 3060 2790 3380 2460

536 471 506 418 530 347

3520 2790 4010 2900 3590 2960

228 168 253 153 248 143

3891 3193 6299 5097

255 221 535 463

827 903

526 608

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TEST CONCLUSIONS

The statistical analysis of the test data was conducted to assess the possible

relationships between the different object variables and test types. For example, in

Phase I the correlation coefficient was calculated between the mortar compression

test results (MC) and the other test types (MT, GC, GT, CPRE, SPRE, FA, and F8).

The results of this particular correlation are as follows:

Correlation Varjables

MCto MT

MCtoGC

MCtoGT

MCto CPRE

MCto SPRE

MCto FA

MCto FE

Correlation Coefticients

.943

- . 027

-.192

.983

- .453

- .444

- .036

The statistical significance of the coefficient is related to the number of test

points. Considering this, and using ali phases of the test program, several

significant results were obtained.

1 . Mortar compressive strengths and mortar tensile strengths are related. A

correlation coefficient of .815 is obtained from 48 data pairs. There was a

99% chance that the true value of the correlation coefficient lies between the

limits of .64 and .91.

2. Mortar compressive strengths and compression prism strengths are related.

A correlation coefficient of .968 was obtained from 10 data pairs. There is a

99% chance that the true value of the correlation coefticient lies between the

limits of .77 and 1.0.

3. The grout tension strengths are related to the wall vertical bending strengths.

A correlation coefficient of .899 was obtained for 7 data pairs. There is a

99% chance that the true value of the correlation coefficient lies between the

limits of .15 and .99.

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4. lhe wall horizontal bending strength (FB) increases with increasing axial

load. A correlation coefficient of .749 was obtained from 6 data pairs. lhere

is a 95% chance that the true value of the correlation coefficient lies between

the limits of -.16 and .97.

In addition to the correlation coefficients, an analysis was performed to

assess the possible differences between the mean test values. For example, the high

strength brick compression results were compared statistically to the low strength

brick compression results to determine if there is a significant difference between

their compression strengths. lhe result of this particular evaluation was that there

is a 99.9% chance that the true mean value of compressive strength for the high

strength brick exceeds the true mean value of the compressive strength for the low

strength brick. Or, there is a 95% chance that the true difference between the mean

values of compressive strengths lies between the limits of 1,294 psi and 2,989 psi.

While this result may not be surprising, a complete analysis of the test data

produced six additional significant relationships.

1. lhe BI-H test results exceed the BI-L test results by 119 psi with a 95%

confidence that the true difference between the mean values lies between the

limits of 335 psi and 497 psi. A 20% decrease in the brick compression

strengths corresponds to a 38% decrease in the brick modulus of rupture

strengths.

2. lhe BA-L test results exceed the BA-H test results by 4.2% with a 95%

confidence that the true difference between the means lies between 3.5% and

4.8%.

3. lhe CPRE-NGH test stresses exceed the CPRE-NGL test stresses by 995 psi

with a 95% confidence that the true difference between the means lies

between -714 psi and 2,706 psi.

4. lhe CPRE-NGH test stresses exceed the CPRE-GH test stresses by 1,481 psi

with a 95% confidence that the true difference between the means lies

between -186 psi and 3,149 psi.

5. lhe SPRE-NGH test stresses exceed the SPRF-NGL test stresses by 191 psi

with a 95% confidence that the true difference between the means lies

between 5 psi and 376 psi.

'.

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6. The FB-H test stresses exceed the FB-L test stresses by 214 pound inches

per inch with a 95% confidence that the true difference between the means

lies between the limits of 87 pound inches per inch to 341 pound inches per

incho

An analysis of the variability in the mortar tension and compression tests and

the grout tension and compression test data was performed. The results are as

follows:

1 . .9% of the variation in the mortar compression results is traceable to

variations in the sampling and testing procedures.

2 . 5.3% of the variation in mortar tension results is traceable to variations in

sampling and testing.

3. .13% of the variation in the grout compression results is traceable to

variations in sampling and testing procedures.

4. .54% of the variation in the grout tension results is traceable to variations

in the sampling and testing procedure.

5 . 88% of the variation in the mortar compression results is traceable to

variations between test blocks due to variations between mortar batches.

6. 86% of the variation in mortar tension results is traceable to variations

between test blocks due to variations between mortar batches.

7. 93% of the variation in grout compression results is traceable to variations

between test blocks due to variations between grout batches.

8. 92% of the variation in grout tension results is traceable to variations

between test blocks due to variations between grout batches.

There are also some interesting results obtained that were not expected.

These include:

1 . The correlation between mortar compression strength and mortar modulus of

elasticity was only .67.

2 . The correlation between grout compression strength and grout modulus of

elasticity was -.195.

3. The correlation between compression prism strength and compression prism

modulus of elasticity was only .29.

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4. The correlation between compression prism strength and shear prism

strength was -.328 for 10 data pairs. This indicates a possible inverse

relationship between compression and shear strengths.

DESIGN CONCLUSION

The test program verified that conventional design methods employed by the

structural engineer for nominal 6- and 8-inch reinforced brick could be used for

normal 4-inch r~inforced brick with adequate factors of safety. Since the weight and

thickness of the 4-inch reinforced system approximately match that of most veneer,

building designers could substitute the reinforced system for the veneer system.

Additionally, it provided the opportunity to build panels similar to those for

reinforced concrete.

There is nothing unique about this system except that it uses the structural

properties of reinforced brick masonry as verified by the test programo Further

validation of the approach has resulted from the curtain wall testing conducted on

several projects. For those not familiar with curtain wall testing, the Architectural

Aluminum Manufacturers Associalion Aluminum Curtajn Wall Design Gujde [3) is

recommended.

Figure 2 shows the curtain wall lesl for lhe KOIN Center project located in

Portland, Oregon. Figure 3 shows the actual failure of a veneer curlain wall les!.

CONCLUSION

Reinforced brick masonry is an ideal slructural material for exterior non­

loadbearing skins of buildings. Because of the moislure or curing expansion of

brick, the brick masonry is placed in compression while the reinforcemenl is placed

in tension. This provides for control of cracking and prevenls waler penetration. A

summary of the tesling prepared in support of nominal 4-inch reinforced

loadbearing design has been presented. The lesling plan allowed for maximum

slatislical correlalions and resulled in several interesling relationships that should

be verified by future tesling. The test program demonslrated lhat convenlional

slruclural design melhods could be used for lhe design of nominal 4-inch reinforced

brick masonry. This provided lhe opporlunily to develop a slructural alternative to

masonry veneer.

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Figure 2. Curtain wall test for reinforced panels.

Figure 3. Failure of brick veneer.

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REFERENCES

1. Grimm, C.T. "Probabilistic Design of Expansion Joints in Brick Cladding" Proceedings of the 4th Canadian Masonry Symposium, University of Fredericton, Canada, 1986.

2 . 4-lnch Reinforced Hollow Unit Masonry Test Report, Western States Clay Products Association, 44 New Montgomery Street, Suite 501, San Francisco, California 94105, U.S.A.

3. Aluminum Curtain Wall oesign Guide Manual, Architectural Aluminum Manufacturers Association, 2700 River Road, Suite 118, Des Plaines, IIlinois 60018, U.S.A.