1JI_ .tesi!)11 •• f I.I-icl( 111:ls.nlr\' ~~_IIIIIIIIS.

Donald E. Anderson and Edward S. Hoffman, Engineers Col/aborative, Chicago, Illinois

Introduction

The use of reinforced and nonreinforced brick masonry as a structural material has recently been given new freedom which permits the design of such building elements to be based on rational and scientific procedures. This was accomplished by the introduction in May 1966 of "Recommended Building Code Requirements for Engineered Brick Masonry" by the Structural Clay Products Institute (SCPI). These recom­mendations have subsequently been adopted by the National , BOCA, and Southern Building Codes.

The ultimate strength design method is not being used by present-day masonry codes. Future codes will give even further freedom to the design of reinforced brick masonry.

Standard working stress masonry codes in effect prior to SCPI 1966 and previously and presently being used in most cities for reinfprced and nonreinforced brick masonry are:

1. ASA - A41.2 - 1960 - National Bureau of Standards Handbook 74 - "Building Code Requirements for Reinforced Masonry."

2. ASA - A41.1 - 1953 - National Bureau of Standards Publication 211 - "American Standard Building Code Requirements for Masonry."

Working stresses allowed by the ASA-A41.1 - 1953 co de have not been changed for some time and are essentially the same as those of the . 1944 edition and similar to those specified in the "1931 Modifications in Recommended Minimum Requirements for Masonry Construction of the Building Code Committee of the U. S. Department of Com­merce". An exception in the 1931 code was an attempt to consider the importance of workmanship as a factor in establishing working stresses by permitting a 50% increase for properly constructed and inspected masonry. This was omitted in 1944.

94

Purpose

To the writer's knowledge, there have been no tests conducted on eccentrically-Ioaded, reinforced brick columns. Present-day masonry code requirements for reinforced brick columns with moment are not based on test data. This study purposes to determine by exploratory tests on eccentrically· loaded-reinforced-brick (RBM) columns if these test data indicate that the Ultimate Strength Design (USD) method of the American Concrete Institute (ACI), ACI 318-63, for reinforced concrete columns (RCC) is applicable as a design method for reinforced brick masonry columns.

This study also purposes to compare the test results with results calculated by the present-day working stress code requirements of SCPI-66, ASA 41.1 and 41.2, for reinforced and non-reinforced brick masonry and the Working Stress Design Method (WSD) of ACI 318-63 for reinforced concrete columns. It also examines the resulting factors of safety or ratio of test load to calculated load.

The ultimate-strength design theory of reinforced concrete is a consistent method for designing columns in accordance with actual behavior. This is an advantage, because it removes most of the ambiguity now attending the design of eccentri· cally loaded columns. This concept simplifies the design of members subject to combined bending and axial load and clarifies the effect of the load-thrust relationship.

Design Method

The fundamental difference between the working stress or straight-line and the ultimate-strength theories is in the stress-strain relationship assumed for stresses near the ultimate

load. The straight-line theory assumes that stress and strain are proportional up to the ultimate capacity. The ultima te· strength theory is based on the hypothesis that, as the ultirnate

capacity is approached, the plastic range of the material tS

reached in which stress and strain are no longe r proportional. At ultimate load, the stress distribution in the compressioo

zone is not triangular but curvilinear, which can be approX1· mated by various shapes.

Design of Brick Masonry Columns 95

7r~ i 2fl GEOMETillC PROPERTlES

2.: 2, ' Hql<.)ht 1.2.5 In .

'Wldt~ 3 . "3 In. I

I 7.88

----1------------ Lqnqth In.

-, at'I~ Anlb'S . .

----------~

2. B.~ inl.

~ Gro!a~

Nlti 2.4.6 in4

PHYSICAl PROPERTlES.

li­ComprC551VQ Strcnqlh

)I­Modulu5 01 Ruptunz

x·: 13,500 pSI ~ y= ~ %

x- 1f.~B pSI, ya~ %

Color Buff

Vfl..lour

Figure 14-1. Unit information sheet.

For this study, a rectangular stress-distribution block has been chosen as a basis for calculating ultirnate loads.

When working stresses are based on the straight-line theory , the actual factor of safety against failure cannot be calculated. With ultimate-strength design , the actualload and moment are multiplied by a specific load facto r which approximates the safety facto r to obtain the ultimate load used for designo

Tests

Compression tests were made on 12" x 16" x 10'2" long RBM columns hinged at the top and fixed at the bottom with eccentricities of O to 34% of their depth. Tests were conducted at the Structural Clay Products Research Laboratories in Geneva, Illinois. Test columns were constructed with inspected workmanship from the following components:

1. Brick: 13,500 psi minirnum compressive strength 2. Mortar: ASTM 270, Type S 3. Grout: 3197 psi minirnum and 3616 psi average com­

pressive strength 4. Masonry : 5250 psi average compressive strength as

determined by prism tests 5. Reinforcing Steel : ASTM AI5-4- # 5 intermediate grade

deformed bars with minirnum yield strength of 40,000 psi

'w'c.'CJ h t lbs.

Unit information sheet (Figure 14-1) gives the physical properties and geometry of the brick units used in the test.

Figure 14-2 shows a sketch of the RBM test columns with the instrumentation dials. CI, 2, 3 and 4 represent the dials measuring shortening of the four corners. The dials C6 and C8 measure the lateral movement about the major axis. After the first test , the C5 and C7 deflection readings were discontinued because of little or no deflection about the minor axis. The deflection dials were placed 1/3 the column height from the top as this is the point of maximum deflection for a column hinged at the top and fixed at the bottom.

Table 14-1 contains a summary of the test results taken from data submitted by SCPI.

Table 14-2 contains load and average strain measurements for test WUGIA which was conducted with a concentric load. Strain measurements were taken at each comer of the column and averaged. The measurements were stopped at 70% of the ultimate load.

Test WUGIA, conducted as a concentric load , did not fail in the million pound machine when the machine's capacity was reached. Consequently, the first three specimens were tested with an eccentricity of 1/20 (specimens WUGIl, WUGI2 and WUGI3). Preloading the first specimen, WUGIA, did not appear to affect its strength when tested as WUGI I with an

96 Designing, Engineering, and Constructing with Masonry Products

C.OL.UMt:I T~'T

x."

~ - 1---= r-f.iI'. ·iI.'IJ)J ~.41-

'I- 1.3,-Fit K".s;~,~t) ,

w ... 1t roun.d p. T· ~.§g: -~.tl."$.·41

0.00 wn -Lo ...

1 8- 4·

'"

&IC/I. /3,Séc PSI

~ /I:.rH 2~ 7Y'PE S

"'. S2SDN

'" I . : ;;

04 ~ . v

IL~ <t .,

I ~

CI\ oi

Figure 14-2. RMB test arrangement.

eccentric load. The last three tests conducted, with an eccentricity of 5.43 inches, are coded WUGI X, Y and Z as there are not normally twelve specimens in a given test series.

Discussion

The results of ultimate test loads at various eccentricities are compared with calculated ultimate and working stress capacities as determined by ACI 318.{j3 and ASA-41.1 and ASA41.2 in Table 14-3 and Figure 14-3.

ACI 318-63 USD

Ultimate loads for RBM columns were calculated according to the USD (ultimate strength design) method for short reinforced concrete columns and compared with the test results. Figure 14-4. These calculations were based on assumed maximum strain at the extreme compression fiber at ultimate strength equal to .003, a stress intensity at ultimate strength equal to 100% f~ I/J = 1, k, = .80, fy = 40,000 psi and Es = 29,000,000 psi.

Table 14-1 Summary of Test ResuIts

on 12" x 16" x 10'2" Reinforced Brick Masonry Columns

f' (average from prism tests) = 5250 psi m

-Load Ultimate Average

Eccentricity Load Mortar Grout Test No. in

S Strength Cylinder e psi Strength inches kips psi

WUGIA O (974.2)* 1805 3522 WUGIl .80 928 .0 1805 3522 WUGI2 .80 928 .0 2353 3522 WUGI3 .80 921.0 1512 3522 (Average) (.80) (925.7) (1890) (3522)

WUGI4 1.60 839.0 2415 3522 WUGI5 1.60 791.5 2032 3898 WUGI6 1.60 792.5 2143 3898 (Average) (1.60) (807.7) (2197) (3773)

WUGI7 3.36 603.0 2103 3898 WUGI8 3.36 608.0 2404 3898 WUGI9 3.36 549.2 2227 3429 (Average) (3.36) (586.7) (2245) (3772)

WUGIX 5.43 406.0 1782 3429 WUGIY 5.43 418.5 1733 3429 WUGIZ 5.43 426.0 2153 3429 (Average) (5.43) (416.8) (1889) (3429)

* No failure at 974.2 kips

Table 14-2 Load-Strain ResuIts for Test WUGI A

=== A verage S tress Average

Load kips Based on Strain Gross Masonry Arca psi inches/inch

-52.0 271 .00006 104.0 542 .00012 156.0 813 .00020 210.2 1095 .00027 262.8 1370 .00033 312.2 1630 .00041 359.7 1872 .00048 406.0 2120 .00056 451.0 2330 .00063 499.0 2600 .00072 547.0 2850 .00080 591.5 3080 .00088 974.2 5070 --?

Design of Brick Masonry Columns 97

Table 14-3 Tabulation and Comparison of Ultimate Test Loads and Calculated Loads

for Eccentrically Loaded Brick Masonry Columns

====================================================================

e

inches

O .80

1.60 2.38 2.67 3.36 5.33 5.43

e/t

O .050 .100 .149 .167 .210 .333 .340

TEST

PT Average

kips

926 808 710** 670** 587 425** 417

Reinforced Concrete Method

ACI 318-63 USD ACI318-63WSD

Pu PT/ Pc PT/ kips Pu kips Pc

1051 363* 943 .98 296* 313 837 .97 250* 3.23 733 .97 217 3.27 694 .97 605 .97 386 1.10 377 1.11

CALCULATED BY

Reinforced Brick Masonry Non-Reinforced Brick Masonry

SCPI-66 ASA-41.2-60 SCPI-66 ASA-41.1-53

Pc PT/ Pc PT/ Pc PT/ Pc PT/ kips Pc kips Pc kips Pc kips Pc

214 215 192 67 186 4 .98 181 5.12 192 4 .82 52 17.8 165 4.90 156 5.18 166 4.87 42 19.2 148 4.80 138 5.14 140 5.06 36 19.7 143 4.68 132 5.08 130 5.15 34 19.7 131 4.48 120 4.89 111 5.29 107 3.97 96 4.43 60 7.08

* These values were obtained using the basic formula, but exceed the arbitrary code limitation of (Formula 14-1) which would make the maximum alJowable load = 231 kips.

** Extrapolated from test resuIts.

120,*---,-

.001----If.f-==:::::.:::j~~-+++

.0ot----t+--+-+-+.::t::::±~-+-

_ONI"' IN' 01 Kl

BRteK 13500 PS I MORTAR "'STM 270 T'I'PE- S

Hlt. s t7~2)(122} s7.&2

·200'----.L...---.l----.l----.l----.l---_

Figure 14-3. Comparison of brick masonry column [oad­tnoment interaction diagrams.

The assumed stress intensity of f fi= 5250 psi at ultimate strength as shown in Figure 14-5 based on tests ma de by SCPI, indicates that there is no dip in the compression stress strain curve of brick masonry after ultimate stress is reached as there is for concrete. An examination ofTable 14-3 and Figure 14-3 shows that the ultimate loads calculated by the USD method compare very favorably with the ultimate test loads, the ratio of PT/P u varying from .97 to 1.11.

ACI 318-63 WSD

Working loads for short RBM columns were calculated according to the WSD (working stress design) method (Figure 14-6) for reinforced concrete and compared with the test results. These calculations were based on an uncracked section in the compression failure zone and a modular ratio of 2n for alI vertical reinforcing. The factor of safety or ratio of PT/P c ranged from 3.13 to 3.27 and neglected the ACI 318-63 WSD code limitation for maximum design load at minimum eccentricity e = .10t.

Reinforced Masonry Columns

Working 10ads for RBM columns were calculated by the ASA 41.2 and SCPI-66 codes and compared with the test results. These similar codes are based on P = AG [.20f ~ + .65 P g f s] , el t = 1/3, an uncracked section in the compression failure zone , a modular ratio equal to 2n, and appropriate length reduction factors for long columns. No reduction for length is required by the ASA 41 .2 co de (Figure 14-7) for

98 Designing, Engineering, and Constrncting with Masonry Products

;;; ...

e. (CONCRETE) •. .0.038 s.o.o

. .00264 .0.0132 . .0.0132 r.1

I 5250 PSI I _L ~PROPOSED IRICK IIASONR

5.0.0 .o I V

~/ 5- ~ONCRE

t -' Er l 5,200 • r,;-. ~,.o.o.o,.oO SI

4.000

I- o; o; EST A .. ... .. ...

SI .o N

,/ ;;; : N

C N .. ..

~ .... -,Jl -~ .. O .... O! q

V

3000

1.0.0

.o .o . .0.01 • .0.02 . .003 • .004

IN/IN.

.0.05

STIUIII f

y

Figure 144. Proposed stress-strain diagram for brick masonry.

f' m 5250 psi fy 40,000 psi k1 .80 u .003 b = 12" t = 16"

Eccentricity Pu Mu Inches kips Ft.-kips

O 1051 O .80 943 63

1.60 837 112 238 733 145 2.67 694 154 336 605 169 533 386 171 5.43 377 170 6.49 268 145 7.44 197 122 8.46 146 103

O 31 O - 49 O

Figure 14-5. Calculations for reinforced brick columns by USD method from IBM 1130 compu ter.

columns with an H/h ~ 10. The SCPI-66 code (Figure 14-8) reduction factor was equal to .993. The essential difference in these two methods, in addition to the length reduction factol is in the value of Emand Es' SCPI specifies that Em= 1000 f : rt; but 3,000,000 psi max. and E s = 29,000,000 psi. ASA 41.2 ~ re 1960 specifies that Em= 1000 f~and Es = 30,000,000 psi. C(

The factor of safety or ratio of PT/P c for SCPI-66 varied fu from 3.97 to 4.98 and for ASA41.2 from 4.43 to 5.12. ti"

Nonreinforced Brick Masonry Piers

[li

s1 (,

Because there is a great difference in the SCPI-66 and ASA41.1 codes for nonreinforced brick masonry piers and r because the test columns were reinforced with a neal minimum steel percentage P g = .645%, it was thought that a comparison of calculated working loads by code with tes1 loads might be of interest .

The SCPI-66 code recognizes brick strengths in excess of ( 14,000 psi and permits eccentricities up to 1/3 the depth of ( the section. The ratio of ultimate test loads for RBM columns to working loads for non-reinforced piers calculated by I

SCPI-66 varied from 4.80 to 7.08. The ASA41.1 code permits no tension in brick masonry. In

addition, the maximum masonry strength permitted for Type S mortar is 350 psi and no increase in masonry strength is allowed for brick compressive strengths in excess of 8,000 psi. The ratio of ultimate test loads for RBM columns to working loads for non-reinforced brick piers calculated by ASA· A41.1-53 (Figure 14-9) varied from 17.8 to 19.7.

b,12· -.---p

• 192 + 2(9.18)( .3 1)(4) 192 + 22 . 8

.a 214.8 in 2

Ix - *(16)3 + (22.8)(3~)2 .. 4096 + 260

4356 ié

4356/8 '"' 544 in3

Pmax'" 192 [2125f~ + (.85)(000645)(16000)J

(192) [1115 + 87.6) • (192) (1203)

Pmax '"' 231k

For ele <- 2 i~8 ,. .1485

Fa Ag .. (1.89) (192) "" 363

363

1 + (2~~~~l;44) 363

,. 1 + .282e

iy 40,000 psi

f~ :> 5250 psi

H • (.75)(22) - 91.5

Ec • (W1,'3 .J525o • .,f(i2õ)J" (33) (.J5250)

• Jl,728,OOO(J3)(.J5250) < (1318)(33)(72.6) < 3,160,000

29,000,00013,160,000" 9.18

Py .00645

Fb .. (.45)(f~) '" 2360 psi

(0.67 pgm + .17) d

H •

e b • [(.67)(.0064 5)(8.98) + .17] !1~ • (.0388 + .17)(1l~ )« . 209)("\

.. 2.38"

Fa ... 34 (L + (.00645)(8.98)] 5,250

.. 1890 psi

~ o" ': O O 363 but 231 ~~ .

. 050 .80 296 but 231 Q8.~ •

.100 1.60 250 but 231 IIII~ '

.149 2.38 217

Figure 14-6. Calculations for reinforced brick masonry, ACl 318-63 WSD.

Design of Brick Masonry Columns 99

Conclusions

Based on the explOIatory tests on RBM columns, and this study, it appears that the ACI 318-63 USD method for reinforced concrete columns could be applicable for RBM columns. Before it can be adopted in masonry codes, however , further research is needed. It is necessary to establish by test the true shape of the compression stress-strain curve for brick masonry and determine the ultimate strain at ultimate strength, the intensity of maximum stress at ultimate strain (whether OI not there is a dip in the stress-strain curve after ultimate stress is reached), the action of RBM columns in the tension failure zone and the efíect of different percentages of reinforcing. Any code recommendation for design of rein­forced brick masonry coIumns by the USD method should include appropriate load factors for dead and live Ioad to provide an adequate factor of safety and an appropriate capacity reduction factor r/J that recognizes the variations in quality of materiaIs and toIerances in bar and coIumn dimensions as well as approximations inherent in the anaIysis.

The USD method shouId enabIe us to design more economicaI structures if we can predict failure loads and establish appropriate factors of safety. This work needs to be done now in order that RBM can follow the recent advances of reinforced concrete.

Technical Notation

a Depth of equivalent rectangular stress block a = k lC

Ag Gross column are a

.. 192 + 2n Ast • 192 + (2)(5.73)(.31)(4) .. 192 + 14.2

.. 206.2 1n2

Ix .. 12g6)3 + (l4.2)(3~ )2

- 4096 + 162 .. 4258 ié

.. 4258 / 8 " 533 in)

-x

• 192 PO 'm' + '( . 55)(.00645)(15000:] (192) [l050 + 57 . 2]

• 192 (1117) .. 21S k

For e / c : 1/ 3

pk .. 1 !1~15e FbS

215

- 1 + (1. ;~~(533)

fy .. 40,000 psi

fm' - 5250 psi

Brick 13,500 pai

H • ( . 75)(122)' 91.5

Em. 1000 fm' .. 5,250 , 000

ti ., ll..:.1 .. 7 . 62 O.K ... 10 b 12

n .. 3~.~~.~~ .. 5.13

Pg a ('3a~4) ... 00645

fm •• 33 fu' .. 1730 pli

215 p.. 1 + .233e

~ ~

O O . 05 . 80

. 100 1.60 , 149 2.38 .157 2 .67 .210 3 . 35 .333 5. 33

!

215.0 181.2 155.3 138.2 132.2 120 . 4 96.0

Figure 14-7. Calculations for reinforced brick masonry, ASA-A41.2.

p

-!:::!t---+--x

z 192 + 2n Alt

• 192 + (2)(9.57)(.31)(4) ,. 192 + 23.9 - 215.9

I, • *(15)3 + (23.9)(3~ )2

- 4096 + 272 .. 4368 ié _ 4368 _ 546 in3

8

- (.993)(192) G20fm'+( . 65)( . 00(45)(16000) ]

- (.993)(215)

214k

For ele : 1/3

214 - 214e

1 +(2.10)(546)

214 -~

fy .. 40,000 psi

fm' .. 5250 pl i

Brick 13,500 pai

- (.75)(122) - 91.5

1000 fm' but 3,000,000 max .

H .. ll.:..2 .. 1.62 b 12

p' - p ~-(MJ- pH(4~~(:2l} - p (1- . 007) - .993P

s 2~,:::= .. 9 . 67

"" - .40 ",,' - (.40)(5250) - 2100 p.!

el , L !

O O 214.0 .05 . 80 186 . 0

.100 1.50 165.0

.149 2 . 38 148 . 0

.157 2 . 67 142 . 8

. 210 3 . 36 131.2

.333 5.33 107.1

Figure 14-8. Calculations for reinforced brick masonry, SCPI-66.

P 192

411[4 .112 CI"

ASA A41.1-53

fm - 350 psi

e/t max. - .1667

el t !L: o o

.050 .80

.100 1.60

.149 2.38

.167 2 , 67

+ ~~2 • 35j 67200

1 + .375.

,

•

k I'...

67.2 51.6 42.0 35.5 33.6

when e/ t : 1/6

m' - 5250

Bn.ck 13500

H

!! t

A

- (.75)(122) - 91.5"

ll,2 12

= 7.62

- 192 in2

- .í.ill.illll - 512 in3 6

fm' - 5250

e/t max. - .3333

e/t ~ i: o .955 192.5

.05 .955 192.5 .100 .820 165.5 .149 .692 139.2 .167 .645 130.0 .210 .552 111.0 .333 .297 60.0

p - .20 fm' AgC

- (.20)(5250) (192)C

202,000 C

Figure 14-9. Ca/culations for non-reinforced masonry piers.

100 Designing, Engineering, and Constructing with Masonry Products

As Effective cross sectional area of steel b Width of column c Distance from extreme compression fiber to neutral axis

at ultimate strength e Eccentricity of load measured from centerline of

symmetrical column El Initial modulus of elasticity Em Modulus of elasticity of the brick masonry Es Modulus of elasticity of the steel fé Ultimate compressive strength of concrete fm Allowable compressive stress in extreme fiber of

masonry in flexure fm Ultimate compressive strength of masonry fs Allowable steel stress = AO fy fy Yield strength of reinforcement H Column length

k1 Fraction of distance from extreme compression fiber to neutral axis that equals depth of equivalent rectangular stress block at ultimate strength

Mb Ultimate moment at balanced failure Mu Ultimate moment n Es/Em Pg AsI Ag P Column load Pb Ultimate column load at balanced failure Pc Calculated column load PT Ultimate column test load Pu Ultimate column load as calculated by USD method t Overall depth of column <P Capacity reduction factor Eu Strain in masonry at ultimate load Es Strain in steel