11 th INTERNA TIONAL BRICKlBLOCK MASONRY CONFERENCE

TONGJI UNlVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

AN EXPERIMENTAL STUDY ON ASEISMA TIC MECHANISM OF FRAME-SUPPORTED WALL-BEAM

Quan xueyou1 Fu Guangyao3 Bai Shaoliang4

1.ABSTRACT

This paper, baseei on the experimental results of two full-scale specimens, studied the aseismatic mechanism of single span frame-supported wall-beam with no opening. Test results showed that, when horizontal force involved, frame-supported wall-beam does its work not in inner-arch mechanism "but in diagonal compression strut mechanism. U nder the effect of horizontal forces, vertical gravitational load would be shifted to one end of span and, together with the compression caused by overturning mement. form an area of compression concentration which would result in shear failure at beam end and compression splitting around the lower corners of brickwork. In addition, tie-coll­umn would be in tension at the bottom end of it even when it is in compression zone corresponding to overturning momento

2. INTRODU CTION

It is a commonplace in China to observe. no matter whether it is in a big city or at a

Keywords: Masonry; Frame-Supported; Wall-beam; Mechanism; Shear.

lLecturer, Faculty oí Civil Engineering, Chongqing Jianzhu U niversity, Shapingba, Chongqing630045,China

2Engineer, Chengdu Surveying and Designing Institute of China Airforce, Cheng­du610041,China

3 Associate professor, Department of Structural Engineering. N orth western Institute of Architectural Engineering , Xian 710061 , China

·Professor and Head, Faculty of Civil Engineering, Chongqing Jianzhu U niversity, Shapingba ,Chongqfng630045 ,China

567

town , that multi-storey buildings near main street are usually designed as shear-wall structure for the lower one or two floors to satisfy commercial purposes, and for the upper part ,designed as masonry structure for dwelling, without exception even in seis­mic regions. It is generally believed that ,load bearing frame beam and the adjacent up­per brick wall of one storey height form a complicated structural element -so-called frame-supported composite wall-beam. For simply supported single span composite wall-beam under pure gravitational load (i. e. no earthquake responses taken into ac­count) , extensive and thorough researches had been carried out in the engineering field of China, and an practical and effective design method, based on inner arch mecha­nism ,had been established. For the past recent years, with the extensive applications of frame-supported masonry structure, the aseismatic behaviours of such a kind of struc­ture have drawn more and more attentions of researchers. The weak-floor problem caused by abrupt changes in rigidity and strength has been dealt with, and the aseis­matic behaviours of supported wall element have been studied too. Nevertherless, be­cause of the complexity of this matter ,an clear and definite understanding of the aseis­matic mechanism of frame supported wall-beam has not been aquired. Great diversity still exists among engineers as how to correctly design such a composite structural ele­ment. It is in consideration of such a situation that the experimental research involved in this paper had been carried out.

3. DESIGN OF SPECIMENS

This paper involves two pieces of fulI-scale single span frame-sup­ported wall-beams with no open­ing on it. The supporting frames were precast in a specialized con­crete factory, whereas the upper confined brick walI were ~on­

structed in laboratory in accor­dance with the working regula­tions. Specimen 1 (FWBI), shown in Fig 1, consists of a brick wall of one storey height and a supp0rting frame designed rota­tionalIessly at the bottom end of the colIumns. Specirnen 2 (FWB2) ,shown in Fig2,consists of a brick wall of two storey height and a supporting frame supported with two hinges.

AlI the two pieces of confined masonr:r walls were designed to

Fig. 1 Specimen FWBl(units in mm)

be made of MulO slaty clay bricks(53 X 115X240mm3)and M5 cement lime motar,so the specimen walls have a width of 240mm. AlI the tie-beams and tie-colIumns were designed to be made of C20 concrete. Frames for FWBl and FWB2 were designed to be made of C20 and C30 concrete respectively. Reinforceing bars,stirrups and section dimensions are also shown in Fig 1 and Fig 2 respectively for both FWBl and FWB2. AlI the actual measured strengths of materiais involved in FWB2 are listed in Table 1.

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The average compression on top of wall for FWBl was O. 8N / mm2

• However, for FWB2, the average compression of O. 28N / mm2

, O. 83N /mm2 and 1. ON / mm2 were used respectively. Cy­

.cling horizontal loads were sup­plied by American MTS struc­ture testing system. For FWBl, the actuator was located at top of the wall. For FWB2, the domi­nant actuator was placed at Loca­tion A and afterwards at Loca­tion B, as. shown in Fig 2. Re­stricted by the installation holes already distributed on the rein­forced load bearing wall, Loca­tion B couldn I t be arranged at the levei of first layer tie-beam.

= = ~

=.,., ~§

::zo: :o:: =>

~

~ >-...... ...... -'

e 240MM

BRICK IJALL

e

LDCATIDN A ~ DDMINANT ACTUATOR

, '_ DCA TlDN B

<==:> DDMINANT

~ ACTUATOR -' = !? .,.,

(ij

~ '" ....

~

The principie in designing these Fig.2 Specimen FWB2(units in mm) two specimens was to ensure that failure of the confined masonry anticipales that of the supporting frame,so,an asistant actuator was supplied at the levei of the frame beam to enhance the supporting frame of FWB2. The confined masonry wall was designed to fail in diagonal shear, and the estimated shear resistances had been worked out based on references to some published methods. [IJ

Table 1 Material Strength for FWB2

A verage Cubic Strength (N / mm') Yielding Strength (N / mm')

C30 C20 M5 $25 <1>12 d>10 <t>8 <t>6

53. O 33.3 9.57 506.6 309.5 :\:\ 1. O 318.3 353. 7

4. CRACK PATTERNS AND FAILURE MODES

4. 1 Specimen l(FWBl)

When vertical compression reached O. 8N /mm2 and then kept constant, the dominant MTS actuator started to exert lateral shear force on top of FWBl. But ,all of a sudden, the test controlling system went out of control and loaded specimen FWBl to failure in about 5 seconds. Damaged FWBl is shown in Fig3.

During the unexpected loading course, three cracks seemed to occurred and developed at lhe same time. The first crack CD and the second crack ® appeared diagonally in the middle part of the wall.The third crack ® emerged at the lower right comer near the right tie-collumn. These three cracks were the most impressive during the whole load-

569

ing history. Cracks CD and CID developed diagonally right-up almost to the top of the wall and left-down to the top of the frame beam . After these two diagonal cracks reached left top surface of the frame beam, several radial diagonal cracks at the left beam end increasingly developed and then one of them (crack @,shown in Fig3) come into being a main diagonal crack about 5mm wide. Almost the same time, bricks around the lower left corner began to split and then the left tie-collumn shifted about 50mm outside the joint at the bottom end as shown in Fig3,which represented the last failure for the specimen.

Fig. 3 Damaged FWBl Fig. 4 Damaged FWB2

All other observed cracks are also shown in Fig3. The most interesting is that an evi­dent horizontal crack ,referred to as @,remaining at the interfaée between the frame beam and the masonry wall with a length more than half of the span ,started from the inner side of the right tie-collumn ,but no recognizable crack could be found at the bot­tom end of right tie-collumn.

4. 2 Specimen 2 (FWB2)

Specimen 2 had been tested for 5 times . The damaged speimen is shown in Fig4. Main parameters are listed in table 2.

Table 2 Maln Test Parameters for FWB2

Dominant Actuator Test series compression q (N/mm2

) cycles

Location MAX.Push MAX.Pull

(KN) (KN)

1 0.28 5 A 450 400

2 0.83 8 A 620 620

3 0.83 12 B 850 650

4

5 0.83 6 B 850 800

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The first crack observed was a smáll one emerged at the lower right comer along the

interface between the frame beam and the brickwork during the first testo Following

cyclic horizontal shear forces brought about horizontal cracks around the bottom ends

of both the right and left tie-collumns and around bolh the lower comers near or along

the interface, brought about vertical flexural cracks and shear diagonal cracks at both

ends of the frame bearn ,also brought about cracks on both of the frame collumns.

It was during the third test, when the dominant actuator reached a push force of

550KN and a pull force of 500KN ,for the vertical crack in the brickwork around the

lower right comer near the right tie-collumn and around the lower left comer near the

left tie-collumn to come in sight respectively.

The diagonal cracks in the brickwork emerged during the flfth testo The first diagonal

crack ,referred to as CD shown in Fig4,emerged when the dominant actuator reached a

pull force of 790KN ,and the second diagonal crack ,referred to as ® shown in Fig 4, e­

merged when the dominant "actuator reached a push force of 840KN. After diagonal

crack reached top surface of the frame beam , the radial cracks on the beam end devel­

oped increasingly and the brickwork began to split gradually arou'1d the lower comer

with the increase of inter-fioor drift ratio. In the last cycle (the sixth) ,when the domi­

nant actuator reached a push foree of 650KN ,shear resistance of the brickwork beg"n

to descend with severe damages around the left beam end and the lower left comer of

the brick wall. At last, the left tie-collumn was pushed aside for about 2Omm.

Through out the whole test series, horizontal cracks across the bottom ends of tie-col­

lumns and along the interface between frame beam and brickwork are the main cracks

devepoed. When the dominant actuator pushing forward ,horizontal crack starting from

the right end of the interface opened wide, and when the dominant actuator pulling

back ,horizontal crack starting from the left end of the interface open wide.

Observations showed that there have been some differences between failure modes of

these two specimens. As for specimen 1 ,right tie-collumn may have undergone a small

tension strain because cracks around the bottom end are very small. While for specimen

2, tie-collumns on both sides endured relatively larger tension strains, no matter

whether the dominant actuator was at Location A or at Location B. Measured results

showed that tie-collumns had ever reached tension strairis more than lOO<4te at bottom

ends when tested for the second and for the fifth time.

But these two specimens shared some equal characteristics which are the niost impor­

tanto First ,horizontal shear forces brought abo)lt long horizontal cracks along ihe inter­

face between frame beam and brickwork, which directly shifted all the vertical loads

and the compression caused by overtuming moment to concentrate in a small area

571

around the lower comer. Second, the above mentioned compression concentration

caused shear failure in the beam end and splitting around the lower comer of the brick­

work.

5 MECHANISM FOR VERTICAL LOAD TRANSFER

Rotations at both left and right frame joints for both FWBl and FWB2 had been mea­

sured. The measured results showed that, with the increases of pure verticalload, joint

rotations were too small to be measured. It was the case even when FWB2 was tested

for the fifth time, which means that the frame beam and the adjacent wall worked well

together ,and the composite element had a very large flexural rigidity.

In addition, vertical strains at the bed of brick wall for both FWBl and FWB2 along

the interface had been measured. Fig. 5 presents the measured results for FWB2 during

tll,e vertical loading process when tested for the fifth time. It can be concluded that

when only vertically loaded even the interface had ever cracked (but till now ,no diago­

nal crack on the wall had been formed) ,inner-arch mechanism still well existed,i. e.

most of the verticalloads direct1y transfered to left and right beam ends.

1000 600 ~0

800 Z

400 . - LEFT JOINT ~ - - - RIGHT JOlNT

7e0 ~ 200 o

w liOO u.

-= 5I:l0 :z

4ee ~ t- 300 '" 200

100 0

a:: 0 -< ~ (f) -200 a:: 8 ~ -400 d: ~ -&00

-100 0 0.5 1 1.5 2 2.5 3 3.5 4

-800 L............L..........J~...J..............J..~.;............J..........J

-40 -30 -20 -le 0 10 2e 30

LOCATION (H) JOINT ROTATION <Xl0-3arc.)

Fig.5 Vertical Strain at Wall Bed Fig. 6 Joint Rotations vs. Inter-Floor Shear

6 MECHANISM WHEN LATERAL SHEAR INVOLVED

6. 1 Deformation Characteristics of Supporting Frame

Fig. 6 shows the joint rotation vs. inter-floor shear hysteretic curves for both !eft and

right joints when tested for the fifth time (FWB2) ,in which inter-floor shear is that of

the supporting floor and is positive when it brought about a leftward drift . And coun-

572

terclockwise rotation is taken as positive. It shows that, under the effect of lateral

forces the frame joints at both ends rotate in the same direction and increase with the

lateral forces. Ali these deformation characteristics are quite similar to those of a com­

moo free frame.

Joint rotation measurements during the first four tests showed the same regular pat­

tern,but the maximum rotations in clockwise and counterclockwise directions were rel­

atively small.

6. 2 Stress State of Tie-Collumns

Measured results showed that when only vertically loaded, tie-collumns got a larger

compressive strain for section at a lower position and even got a compressive strain co­

ordinates with that of adjacent brickwork at bottom ends ,as shown in Fig5. It implies

that tie-collumn constructed on the working regulations does its work in coordination

with the brickwork. This is valuable because it can share a big part of verticalload and

reduces those that the frame beam has to bear.

But when lateralload involved ,bottom end strains of tie-collumns developed in an un­

expected way. Fig •. 7(a) and (b) presents the hysteretic relations of outside bar strain

vs. lateral forces andO inner side bar strain vs. lateral forces at bottom ends of both left

and right tie-collumns when tested for the second time and for the fifth time respec­

tively, in which OL TC means outside bar of left tie-collumn, IL TC means inner side

bar of left tie-collumn and IRTC means inner side bar of right tie-collumn. It can been

seen clearly' that when laterally loaded ,strain distribution across bottom end section are

not unüorm. In Fig. 7(a) ,inner side bar got an small increment of compressive strain

when it was in compression zone corresponding to overturning moment and got at a

large tension strain when it was in tension zone. On the contrary, outside bar reached a

larger tension strain when it was in tension zone and still kept in tension when it

turned back to be in compression zone. In Fig. 7(b) ,inner side bar always got an incre­

ment of tension strain no matter whether it was in tension zone or in compression

zone. Outside bar strain kept in the same pattern as in Fig. 7(a). Either in Fig. 7(a) or

in Fig. 7 (b) ,inner side bar returned to be in compession strain state when lateral forces

released.

Such a strain development pattern for bottom end of tie-collumns may be explained in

Fig. 8. U nder the effect of inter-floor shear, frame joint may rotate and exert tension

increment across the bottom end section of tie-collumn ,with a larger value for outside

bar and a small one for inner side bar. If the joint rotates large enough, both outside

and inner side bars may change into tension strain state, just as what is observed in

Fig. 7(a) and Fig. 7(b).

573

800

GOO IRTC

400

200

~ 0 ~ O- O-

-200

-400

-G00

-800 -1000 -500 0 500 1000 1500

STRA IN (~E)

(a)

Fig.7 Tie-Collumn Strains vs.

TIE -COLLUMN

1/;· @ I I COMPRESSIDN ZONE

FRAME BEAM --H \\1------

bQrs My keep in tension here becuuse of JOlnt rotution

Fig. 8 Consequence of Joint Rotation

2500

2000

1500

\J) 1000 "'-

z 500 ~

<C o:: 0 f-

(f)

-500

-Hl00

-1501a

1000

000

Ge0

400

200

0 -200

-400

-Ge0

-000

-1000 -2000 -1000 e 1000 2000

STRAIN (~E )

(b)

Lateral Shear Force

e 0.5 1.5 2 2.5 3 3.5 4

LOCATION (1"1)

Fig. 9 Vertical Strain at Wall Bed

6. 3 Mechanism For Vertical And Horizontal Load Transfer

Fig. 9 represents the vertical strain distribution along the interface when the dominant

actuator reached each cycle's maximum push force and maximum pull force during the

fifth tcst for FWB2 ,in which a significant strain redistribution can be seen ,i. e. 'when

Iaterally loaded, interface would crack, vertical load and compression casued by over­

turning moment would shift to concentrate at beam end area. The compression stress

of brickwork around the lower corner would increase on a large scale. This compression

conr.entration would cause diagonal shear in beam end and splitting failure around the

lower corner of brickwork.

As mentioned in previous paragraphs ', tie-collumn, even though it is in compression

zone ,may change into a tension strain state because of frame joint rotations. This will

574

intensify the compression concentration of brickwork in the vicinity of it. It ean be de­

duced that tie-collumn in compression zone can be kept in compression by means of in­

creasing the joint rotation stiffness or reducing the inter-floor shear resisted by sup­

porting frame. Thus, tie-collumn may share part of the compression caused by vertical

gravitationalload and overturning moment so as to postpone ,or even to avoid ,diagonal

shear failure at beam end ane splitting failure around lower comer of brickwork.

Based on the experimental results in­

volved in this paper ,load transfer mecha­

nism, when lateral earthquake force is

taken into account ,ean be described as di­

agonal compression StfUt mechanism, as

shown in Fig. 10. Axial force N of tie-col­

lumn is a function of joint rotation and

some other parameters. It may be in ten­

sion or in compression. Checking compu­

tations for shear and flexure resistances of

frame beam and for local splitting of

brickwork should be based on such a

mechanism.

7 CONCLUSIONS

I I I 11 I I I I I I I I I I /I I c

Supporting Frame

Fig. 10 Diagonal Compression StrlJt Mechanism

(l)When only vertically loaded ,frame supported wall-beam works in inner-arch mech­

anism. The working regulation for construction of tie-collumn will ensure it to deform

in coordination with adjacent brickwork and help frame beam in resisting vertical grav­

itational load. The composite wall-beam has so large a flexural rigidity tbat both joints

can hardly rotate.

(2) When laterally loaded, deformation characteristics of supporting frame are quite

similar to those of a common free frame. Frame joints rotate in the same direction.

(3)Joint rotations may lead tie-collumn in compression zone of overtuming moment to

be tensioned at bottom end. Such a stress state of tie-collumn intensifies compression

concentration aound lower comer of brickwori<.

(4)Load transfer mechanism when laterally loaded may be described as diagonal com­

pression strut mechanism shown in Fig. 10. Determined by joint rotation, tie-collumn

may be in tension or in compression. Joint rotations should be limited effectively in or­

der that tie-collumns keep in compression and reduce the compression concentration.

575

Such a mechanism supplies the basis for checking computations in shear and flexure re­

sistances of frame beam ,and in local splitting of brickwork as well.

REFERENCES

1. Liu Xihui, Zhang Hongxi, Liu Jingwei, Liu Liquan, A Study of Aseismatic Charac­

teristics of Masonry Building with Reinforced Concrete Tie-Collumn, Building Struc­

ture, Vol. 6 ,1981,pp47 -55.

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