STATIC AND DYNAMIC OUT-OF-PLANE RESPONSE OF BRICK MASONRY WALLS

S.A. Adham Agbabian Associates, El Segundo, California

ABSTRACT Responses of brick masonry walls subjected to out-of-plane seismic forces were studied as part of two separate programs. The first program included static testing of reinforced brick masonry walls. The study indicated that there was no evidence of elastic or inelastic lateral instabilíty for the load ranges tested. The tests demon­strated that there is no need to impose fixed height-to-thickness límits. However, they did reveal the need for limits to control potential residual deflection in panels where service loads are exceeded. The second program included dynamic testing of unreinforced brick masonry walls. The three seismic hazard levels of the 1978 ATC provisions were used to develop the seismic input for both analysis and testing. A survival críterion for unreinforced brick masonry walls was developed for different height-to-thickness ratios. The two programs resulted in the developrnent of guidelines and code provisions for the analysis, design, and retrofit of brick masonry wall systems.

1. INTRODUCTION

Out-of-plane static responses of brick masonry walls was studied by the Struc­tural Engineers Association of Southern California (SEASC) in conjunction with the Southern California Chapter of the American Concrete Institute (ACI) (1,2,3). The study was part of a major research program on the vertical and lateral load capacities of tall slender walls.

In past years, walls have been constructed thinner and taller, and many sug­gestions have been made to relax code slenderness height-to-thickness (h/t) limits for load bearing concrete tilt-up-walls and masonry walls. As a result, the above program was conducted to assess the capabilities of brick masonry of a slenderness ratio (h/t) far in excess of 1981 code limitations.

The second program was conducted on unreinforced masonry (URM) walls subjected to dynamic out-of-plane motions. The program is one of several tasks in an overall research program, sponsored by the National Science Foundation, whose objective is to establish bounds on the seismic resistance of URM walls and develop a methodology for mitigation of seismic hazards in existing unreinforced masonry buildings.

Full-scale tests on URM walls subjected to dynamic out-of-plane motions were designed and conducted on 8 clay brick and block masonry wall specimens. The test specimens were subjected to seismic dynamic motion sequences covering the full range of seismicity in the United States from an Effective Peak Accelera­tion (EPA) 0.10 g to 0.4 g (4).

2. ACI-SEASC SLENDER WALL PROGRAM

Test Program

A total of 32 slender walls were built. Thirty-one of the walls were 1.22 m (4 ft) wide and 7.52 m (24 ft-8 in.) high. Among these walls, nine were brick masonry (Table 1). These nine panels were 139.7 mm (5-1/2 in.), 190.5 mm (7-1/2 in.), .241.3 mm (9-1/2 in.), resulting in h/t ratios of 52.4,38.4, and 30.3 respectively. An additional panel, built with hollow bríck uníts was 88.9 mm (3-1/2 in.) thick and 5.08 m (16 ft-8 in.) high for an h/t of 55. The f' for brick ranged between 21,083 KPa (3060 psi) and 42,994 KPa (6240 psi). m

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TABLE 1. SLENDER WALL TESTS (BRICK MASONRY) (2)

Strength Actual Vertical Maximum Maximum Date Wall Thickness Lateral Lateral Vertical No. inches Material f'm h/t Load Load Deflection Reinf . Tested

psi Ratio p.l.f. p.s.f. inches (1981)

1 9-1/2 Brick 3060 30.3 320 150.8 15.6 5114 4-20

2 9-1/2 Brick 3060 30.3 320 164.1 16 . 8 5114 4-17

3 9-1/2 Brick 3060 30.3 320 89.4 14.6 51/4 5-11

4 7-1/2 Brick 3440 38.4 320 59.8 19.6 5114 5-8

5 7-1/2 Brick 3440 38.4 320 57 . 2 15 . 9 5114 5-7

6 7-1/2 Brick 3440 38 . 4 320 78.0 14 . 8 5114 5-6

7 5-1/2 Hollow Brick 6243 52.4 320 86.6 19.3 5114 4-15

8 5-1/2 Hollow Brick 6243 52 . 4 320 86.5 17.0 5114 4-16

9 5-1/2 Hollow Brick 6243 52 . 4 320 61.6 11.1 5114 5-4

1 1nch - 25.4 mm p . l . f. - 14.59 N/m p . S . 1. - 6.89 KPa 114 - 12.7 mm Rebar

The walls were built on a 12.7 mm (1/2 in.) steel plate 1.22 m (4 ft) long on which was welded one-half of a 101. 6 mm (4 in.) diameter pipe. This detail provided a pin support and allowed full free rotation . The top of the wall was restrained by a device with a spherical roller bearing that permi tted vert i cal movement and rotation but prevented horizontal translation, thus providing a pin support.

All masonry walls were reinforced vertically with five 12.7 mm (1/2 in.) bars Grade 60 with an f = 482,300 KPa (70 ksi) and f = 757,900 KPa (110 ksi), located in the cente~. Horizontal bars were spaced u1.22 m (4 ft) apart. The loading frame allows for both lateral load and eccentric vertical load to be applied simultaneously to the panel. The vertical load was provided by water­filled drums whose weight was applied through a lever system to the ledger angle (Fig. 1). The vertical load simulated the actual loading from the roof on a building and was varied from 3282.75 N per m (225 plf) to 4668.8 N per m (320 plf) for the 88.9 mm (3-1/2 in.) thick walls and the remainder of .brick panels respectively.

An airbag 1.22 m (4 ft) wide and 7.32 m (24 ft) high was placed between the wall and the loading frame. The airbag imposed a lateral load aga i nst the wall, which caused a movement in the same direction as that created by the eccentric load on the ledger angle (Fig. 1). The lateral deflection was measured at 11 vertically distributed locations on the wall. During the test, the lateral pressure imparted by the airbag to the panel was incrased unt i l deflections reached two to three times the panel thickness . Up to the yield level, the relationship between lateral load and midheight lateral deflection resembled a bilinear form (Fig. 2).

Test Results

Typical load-deflection test results and idealized load-deflection curves for the brick masonry panels are given in Table 1 and Figures 3 and 4. The results indicate a low modulus of rupture followed by a flat slope. The flexural strength of the panels was limited by the yield and ultimate strength of rein­forcement. The cracking was confined to the bed joints, but there were a few exceptional cracks throilgh the bricks. The load deflection results indicate that these slender panels can resist 50 to 90% of their weight laterally. In addition, the lateral resisting load was increasing even when deflections were extremely large. The results also indicate the panels are flexible and there­fore reach large deflections before yield occurs. The plots ofaxial load or force versus moment (Figs. 5, 6) indicate that the applied axial loads are

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VERTICAL LOAD ON TO LEDGER ANGLE

FIGURE 1. LOADING FRAME AND WALL SPECIMEN SETUP

YIELDING OF STEEL

CRACKING OF MASONRY WALL

o YIELDING OF REINFORCING STEEL

r:~~ ____ -:-_L __ E.::.3:...-_____ 4

E2 COMPOSITE BEHAVIOR OF STEEL AND

MASONRY (SLOPE DEPENDS ON AMOUNT OF REINFORCING STEEL)

VIRGIN STIFFNESS OF MASONRY WALL

lL-----------------------------------------~

FIGURE 2. IDEALIZED COMPOSITE STRESS/STRAIN RELATIONS FOR PANEL

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...... rv ...... cn

2,874 (60)

~ 2,395 ... (50) '" a

1,916 ~ (40)

'" 1,437 ~ (30) --'

--' « a::

"" >-S

TEST RESULTS

A PANEL 1 O PANEL 2 O PANEL 3

A

O

O

O

o

A O

A O

o

LOAO-OEFLECTION RELATION

~ - 0.9, fr - 2.0..rr:: '\R • Sfr

o • ! I , , , !

O 12.7 25.4 38.1 50.8 63.5 76.2 88.9

FIGURE 3.

V> a.

~

....... o

x z

V> Z o I­~ Lo.! Z

Z

<:> « o -' -' « x «

2,225 (500)

o O

FIGURE 5.

(0.5) (1.0) (1.5) (2.0) (2.5) (3.0) (3 . 5)

MIDHEIGHT DEFLECTION, mm -(in.)

MCR

AND ~, 241.3mm (9.6 IN.)

TWO WYTHE BRICK MASONRY

27.2 (20)

(7.5") (9.6") 190.5mm 241.3mm

54.4 (40)

81.6 (60)

108 .8 (80)

BR.I CK

136.0 (100)

MOMENT IN m - NEWTONS X 103 , (KIPS)

163.2 ( 120)

SHORT COLUMN P-M CURVES BRICK

... '" ~

2,395 (50)

1,916 (40)

1,437 ~ (30)

'" « '" --' --' « a::

"" >-« --'

958 (20)

479 (10)

TEST RESULTS

A PANEL 7 O PANE L 8 O PANEL 9

[lo

O O A

O

A O O

O A CJ

A O O

AO

O

O

O

O

LOAO-OEFLECTION RELATION

~ = 0 . 9, fr - 2.5...p;;: M

CR • Sfr

O ~' __ ~ ____ L-____ L-____ L-____ ~ ____ ~ __ ~~ __ ~

012.7 38.1 63.5 88.9 114.3 139.7 165.1 190.5 (0.5) (1.5) (2.5) (3 . 5) (4 . 5) (5.5) (6.5) (7.5)

MIDHEIGHT OEFLECTION, mm -(in.)

FIGURE 4. ~ SELECTION 139.7mm (5.5 IN.) HOLLOW BRICK MASONRY PANEL

~

V> a.

~ (5.5") (l.5") (9.6") BRICK 139.7mm 190.5mm 241.3mrn

66.75 I "<:: ,''''' I I i ,''''' I il i'

"6 (15)

x z

V> Z o I­~ Lo.! Z

Z

<:> « o -'

44.5 ( 10)

22 . 5 (5)

TEST

• •

-' « o I I I " 1« I I, I J

x « O

FIGURE 6.

6 .8 13.6 20.4 27.2 34.0 40.8 47.6 (5) (10) (15) (20) (25) (30) (35)

MOMENT IN m - NEWTONS X 106 , (KIPS)

SHORT COLUMN P-M CURVES EXPANDED-BRICK

FIGURE 7. 0.2 4m (9 1/2") BRICK WALL BRACED AFTER TESTING

FIGURE 8. 88.9mrn (3 1/2") HOLLOW BRICK WALL, 5.08m (16'-8") HIGH, DEFLECTED LATERALLY O.43m (17") UNDER A 2,682 Pa (56 PSF)

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considerably lower than axial loads that may cause instability of the walls. Therfore, stability of these walls even under very large lateral deflections was clearly demonstrated (Figs. 7, 8).

Conclusions of the Slender Wall Program

lt was concluded from the tests that (1) there was no evidence of elastic or inelastic lateral instability for the load ranges tested; (2) the significance of the eccentric moment from the applied simulated light framing roof load was small; (3) the significance of the P-~ moment was most pronounced in the thinner panels but did not produce lateral instability in the load ranges tested (panel weight was the largest component of the secondary moments. However, secondary moments accounted for less than 20% of the total moment at yield of the rein­forcement); (4) the interaction P-M (Load-Moment) curves for short columns pro­vided an adequate predicted moment capacity envelope for brick masonry panels when loaded with relatively low axial loads that are less than balance point on the P-M curve; (5) although the panels exhibited adequate strength at and beyond the yield point, the rebound study indicates that a midpoint permanent deflec­tion of 76.2 mm (3 in.) to 127 mm (5 in.) can be expected for panels loaded to the yield leveI of the reinforcement; (6) the tests demonstrated that there is no need to impose fixed height-to-thickness limits. However, they did reveal the need for deflection limits to control potential residual deflection in panels after service loads experience.

It was recommended that (1) moment capacity of cracked section at yielding of reinforcement should be at least greater than the moment capacity of the uncracked section based on a gross section tensile strength; (2) the adoption of deflection control as a new feature to be used to assure a wall of reasonable straightness after a service leveI loading. (lt should prevent excessive deflection at service load leveI, and also prevent use of a panel with excessive flexibility. The committee recommended that midheight deflection be limited to height divided by 100, that is ~ < h/lOO); (3) A phi (q» factor should be introduced to reflect effective quality control relating to material and con­struction practices. (It is suggested the q> factor be used to account for the differences in construction with continuous and noncontinuous inspection, for both concrete and masonry construction); (4) the maximum flexural steel ratio, P , based on gross area should be limited to the value given in Table 2. This l~mitation on the amount of steel is to assure that there will be a ductile yielding condition and never a brittle failure of masonry.

TABLE 2. BALANCED RElNFORCEMENT PERCENTAGES (1)

Type

5-1/2 in. Hollow Brick

5-1/2 in. Hollow Brick

9-5/8 in . 2-Wythe Brick

Design Values, F = 60,000 psi Y

f' or Balanced Balanced If'

c~ Pb Pgbt ps~

2500 1. 78 0.89

5000 3 .56 1. 78

1800 1.28 0.64 . '-npercentage based on d d~stance, ~.e., d = t/2 tPercentage based on t

ttp < 0.32% in. g= 25.4 mm plf = 14.59 N/m

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Maximum Design Pgtt

0.40

0.40

0.40

Maximum Pb/Pgb

0.45

0.22

0.63

3. ABK UNREINFORCED MASONRY TEST PROGRAM

General

The full-scale tests on URM walls were designed to account as closely as possi­ble, for the nonlinear, dynamic interaction between the walls and diaphragms of typical URM buildings. This interaction was included in the component tests by defining the kinematic environment at the top and base of the walls from non­linear dynamic analyses using analytical models of typical URM buildings that included the nonlinear, hysteretic characteristics of the diaphragms and the diaphragm/wall mass system (5). The kinematic environments were obtained for buildings with both stiff and soft diaphragms. The kinematic input motions for a ground leveI wall element consists of a ground motion at the base of the wall and a compatible diaphragm response at the top of the wall. In addition, the wall s were tested with various leveIs of overburden mass attached to the top of the wall to simulate additional wall or parapet mass above the wall section being tested.

Specimens

The wall specimens tested included 3 Wythe common brick, grouted clay block, in addition to concrete block walls (6). The test program included one form of retrofit that consisted of applying a wire mesh and a plaster covering to both sides of the wall spe cimens . The URM wall specimens were 1.8 m (6 ft) wide and 3.0 to 4.9 m (10 to 16 ft) high , and had height-to-thickness ratios that ranged from 14 to 25. Table 3 gives a brief summary description of the brick mas onry wall specimens.

TABLE 3. WALL SPEClMEN DESCRIPTION (6)

Wall Number 1 2 3 4 5 6 7 8

Overburden 907 1,814 3,628 907 1,814 3,628 907 907 kg (tons) (1) (2) (4) (1) (2) (4) (1) (1)

Wall Weight 6,658 3,084 4,010 kg (tons) (7.34) (3.40) (4.42)

Height 4.9 4.9 4.9 m (ft) (16.0) (16.0) (16.0)

Thickness .35 .19 .25 m (in.) (13.75) (7.63) (9.75)

H/T Ratio 14.0 25.2 19.7

3 Wythe Brick Clay Block Clay Block';"

Material Grouted Grouted Solid Solid

Wall 7: Nominal 203 mm x 203 mm x 406 mm (8 in. x 8 in. x 16 1n.) clay block grouted solid, 25.4 mm (1 in.) plaster each side with 50 . 8 mm x 50.8 mm (2 in. x 2 in. x 14 ga) reinforcing wall 8; double 50.8 mm x 50.8 mm (2 in. x 2 in. x 14 ga) reinforcing. 1 inch = 25.4 mm; 1 foot = 305 mm; 1 ton = 907 kg.

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Test Set-up and Instrumentation

A schematic of the test set-up is shown in Figure 9. The URM wall specimens were installed in a test fixture that allowed the base and top of the wall tobe moved independently in the out-of-plane direction by servocontrolled hydraulic actuators. The wall rested on a low friction, roller supported base and the overburden mass was applied through a mechanical header that was attached to the top of the wall. The wall specimens were instrumented with load cells, accel­erometers, and displacement sensors (Fig. 10). Data from each instrument were recorded on magnetic tape in digital formo

Test Modes and Sequences

Each wall with its prescribed overburden mass (Table 3) was subjected to a sequence of dynamic input motion sets that consisted of a compatible pair of kinematic motions, one for the base and one for the top of the wall (6). The kinematic motions used in the test program are based on actual earthquake ground motion records that correspond to seven major geographical regions of the United States. The ground motion records were scaled to an effective peak acceleration (EPA) of 0.10 g to 0.4 g. For a given wall, the dynamic testing starts with motion sets of low intensity and proceed to higher intensity leveIs of motion until the wall collapses.

Construction of Specimens

Mortar was "O" type and job mixed using sacked portland cement, sacked lime, and washed sand purchased from local sources. The "O" mortar is 1 part portland cement, 2 parts lime, and 9 parts sand mixed on the job site to a workable mortar. Three-wythe masonry units, walls 1 through 3, were solid with the interior-wythe floated in place and the collar joints were slushed with mortar.

Two walls ( i.e., wall 7 and 8) identical to walls 4, 5 and 6 were constructed for application of retrofit methods. Reinforcement of a single or double layer of fabriction wire mesh 50 mm spacing x 2 mm dia (2 x 2, 14 ga) was applied to each face and furred with spacers from the face of the masonry. Portland cement plaster was applied over the mesh to embed the reinforcement in the first plas­ter coat. The plaster proportions were 2 volume cement to 4 parts sand maximum. Lime was also added to plaster.

For walls 1 through 3, the wythes were bonded with continuous header courses at 610 mm (24 in.). Header courses were lapped in the center wythe to complete a tie through the wall. This practice is 2 consistent 2with existing URM. The weight of the wall constructed was 746 Kg/m (153 lb/ft ).

Walls 4, 5, and 6 were 1.8 m wide x 4.9 m high (6 ft x 16 ft) 203.2 mm (8 in.) nominal hollow clay units laid with head and bed mortar joints equal to the face shell thickness. Internal cells of the hollow u'2its were filfed with mortar type "N." Unit weight of these walls were 346 Kg/m (70.9 Ib/ft ).

Results of ABK Test Program

Test resuIts included measured data from instrumentation, still photographs, motion pictures, and visual observations. The dynamic testing of the unrein­forced wall specimens provided data for determining the probability of survival of typical URM walls.

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FIGURE 9.

SERVO VALVE (60 GPH)

&ACK STOP

ACTUATOR 76 mm x 559 mm (3" x 22")

LOAO CE LL (TYP) (5,000 LB)

HECHANICAL HEAOER

URH WALL

SERVO VALVES (25 GPH)

o

ACTUATORS (2) 51 mm x 559 mm (2" x 22")

SCHEMATIC TEST SETUP FOR DYNAMIC TESTING OF WALLS

WF:

WO:

WA:

'" } WOl

j WAl *W01A

W01B

W02~1~

LOAO

"'-1-+-O I SPLACEHENT

ACCELERATION WA3 _____ S

"'~+-' wM S

W05 --1--WA5

I S

W06 ~i~ I!.

WFW7 } 'WOI/7A

WOW7B

NOTE: 1 f t - 305 mm

GAGE LOCATION S 3.05 m WALL - 0 . 61 m (10 FT) (2 FT)

~ . 88 m WALL - 0.915 m (16 FT) (3 FT)

* FEEOBACK CONTRO L NOT RECORO EO OIG ITALLY

FIGURE 10. WALL INSTRUMENTATION

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v o ~

c

1. 778 r---,.,.-----,---,-----,-----,,-----, (lo)

1.52~1_---+-_'<_~..>.ct---+_---+---__1f__-~ (60)

5.0

- 1 . 0161_---+--~--+----'~-+_---"".....",.-+----"""'"""":__If__"_'~ .",-0 _--+ ~ (~o) 3.0

-.: 2.0

0.5

~ . 762 f---+----'~c-"'~:__+-----="'+=_--:Jf--1...:...0 --1 ,; (JO)

0.1

0.0 (~gfl----+-----+---+----+-----11-0:-:/"...W ---+

V. SRSS ~ SQUARE ROOT OF SUM OF PEAK VElOC 1 TI ES AT .25~f--- TOP ANO BOnOM DF WAll SQUAREO 1-- --1 (10) H/T - HE1GHT TO TH1CKNESS RATIO OF WAll

O/W '"' OVERBURDEN WEIGHT TO WAlL WEIGHT RATlO

I I 10 15 20 25 lO l5

H/ T

FIGURE 11. UNREINFORCED MASONRY WALL STABILITY CRITERIA, 98% PROBABILITY OF SURVIVAL

The response of wall 1 is typical of almost all of the walls tested. Wall 1 (Table 3) cracked above its midheíght and one course above base (Figs. 12, 13). In general, the walls would develop these cracks and would respond as two rock­íng blocks, cyclíng on the cracks. The walls approached ínstabílíty under earthquake motíon sets wíth hígh velocity contento Walls wíth smaller overburen loads such as wall 1 had larger excursíons than those wíth larger overburden loads such as walls 2 and 3.

The retrofít specímens utilízed a síngle straíghtforward method of modifying URM walls. The retrofít system díd not exhíbít extended ínelastíc response due to the unexpected strength of the bonding portland cement plaster. However, observatíons of the dynamíc performance of URM specímens gíve insíght into other símple retrofít methods that modífy one or more of the test parameters used. The effect of retrofít on increasing the earthquake resistance of the walls was clearly demonstrated.

Conclusíons of ABK Test Program

The dynamíc testíng of the unreínforced wall specímens provided data for the determination of the survíval of well anchored URM walls. The tests produced valuable data for establíshíng bounds on the resístance of URM walls to collapse when subj ected to dynamíc, out-of-plane motíons. The tests showed that the resístance of the walls to collapse was more dependent on the peak velocities ínput at the base and top of the walls than on the peak relatíve deformatíons índuced between the top and bottom of the walls. Moreover, íncreasíng of the overburden íncreases the collapse resístance of the walls. The tests also demonstrated that the retrofít procedure substantíally enhanced the resistance of the walls to collapse, and províded some ínsíght for the design of other retrofít methods.

1222

FIGURE 12. TEST WALL

FIGURE 13. CRACKS ABOVE MIDHEIGHT AND DETERIORATION AT BASE OF WALL

1223

The information obtained in these dynamic tests is believed to be applicable in alI seismic zones within the United States. Since the geographic United States spans the total range of seismic intensity the information gained can be uti­lized outside its boundaries.

3. CONCLUSIONS OF BOTH STATIC AND DYNAMIC PROGRAMS

Both studies were in general agreement and indicate that brick masonry walls have sustained loads much higher than those causing initial cracking. Unrein­forced brick masonry walls have an inherent stability to dynamic out-of-plane motions, even though they have . little or no tensile strength. Reinforced brick masonry walls exhibit rather large ductilities and displacement controls should be imposed. The two programs established stability criteria for reinforced and unreinforced masonry walls under static and dynamic loads. It remains to study the stability of thin reinforced brick walls when subjected to seismic dynamic loads.

4. REFERENCES

1. Simpson, W.M. (Chairman) et aI. (1982) Report of the Task Committee on Slender Walls, American Concrete Institute - Southern California and Struc­tural Engineers Association Southern California, Los Angeles.

2. Amrhein, James E. (1981) "Slender Walls Research Program by California Structural Engineers," The Masonry Society Journal, 1:2.

3. Adham, Samy A. (1982) "The Slender Wall Test Program Conclusion and Recom­mendations," Proceedings Structural Engineers Association of California.

4. Applied Technology Council (1978) Tentative Provisions for the Development of Seismic Regulations for Buildings, ATC 3-06, PaIo Alto, CA.

5. ABK, A Joint Venture (ABK) (1981) Methodology for Mitigation of Seismic Hazards in Existing Unreinforced Masonry Buildings: Diaphragm Testing, ABK-TR-03, El Segundo, CA: Agbabian Associates, Dec.

6. ABK (1981) Methodology for Mitigation of Seismic Hazards in Existing Unreinforced Masonry Buildings: Wall Testing, Out-of-Plane, ABK-TR-04, El Segundo, CA: Agbabian Associates, Dec.

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