A multistory wall in a space moment resisting frame may yield under the in- plane lateral load, and it may also be subjected to reversal of out-of-plane lateral load. Flexural yielding causes the wall to stand on one column in the compression side only, imposing high level of axial compression on that column. Under the action of out-of-plane loading, the column must behave as an independent column under high compression, possibly leading to failure at relatively small deformation. Thus it is possible that the deformation capacity of a wall would be smaller under bidirectional loading.
An elaborate experimental program was implemented in the New RC project to test structural walls under bidirectional loading. Four specimens were tested. They were about a quarter scale dumbbell type walls, taken from a lower portion of multistory wall, as shown in Fig. 4.37. Center-to-center span of 1.5 m, column size 200 mm square, wall thickness 80 mm, and wall
MWTypc PType Fig. 4.37. Detail of wall specimens.
clear height 2.0 m a r e all c o m m o n t o four specimens. Concrete w i t h specified s t r e n g t h of 60 M P a , D10 re-bars for column axial reinforcement with yield s t r e n g t h of 865 M P a , a n d D6 re-bars for wall reinforcement a n d c o l u m n lateral reinforcement w i t h yield s t r e n g t h of 826 M P a , were used t h r o u g h o u t . Gross column reinforcement r a t i o was 2.14 percent, a n d wall reinforcement ratio was 0.80 percent.
Table 4.6. List of walls under bidirectional loading.
Specimen M35X M35H P35H M30H
Column Spiral
D6@ 60
Column Subhoop
2-D6® 60
—
2-D6® 60
Axial Load Ratio N 2A0aB
0.35
0.30
Axial Load N kN 1800 1960 1900 1500
Loading P a t h in-plane
bidirectional
Table 4.6 lists variables for four specimens. The first letter M or P denotes difference in the column lateral reinforcement. Peripheral spiral of D6 at 60 mm is common to four specimens whose web reinforcement ratio was 0.53 percent, but those with mark M had additional subhoops of 2-D6 at 60 mm in two directions, making the web reinforcement ratio to a doubled value. P is a specimen without subhoops. The next two digits in the specimen mark refer to the axial load level. In terms of axial stress with respect to column area, it was either 35 or 30 percent of concrete strength. As the actual concrete strength fluctuated between 62.6 and 70.0 MPa, amount of axial load on each specimen is shown in Table 4.6, which was kept constant during the testing using four vertical actuators. The average normal stress on the gross wall and column area was 9.8 to 10.7 MPa for the first three specimens and 8.2 MPa for the last specimen.
The last letter X or H in the specimen mark corresponds to the type of loading. M35X was subjected to reversal of in-plane horizontal loading. The load was applied in such a way that the shear span (critical moment divided by shear force) would be 3.0 m, or shear span ratio with respect to center-to- center span of 2.0. Since the wall is 2.75 m high to the top surface of top loading girder, additional moment was produced by a pair of vertical actuators simul- taneously with the loading from horizontal actuators. Other three specimens with the letter H are subjected to bidirectional loading as shown schematically in Fig. 4.38. In-plane loading was similar to M35X. Out-of-plane loading was made so that the wall (actually two side columns) would be in an antisymme- tric bending. For this purpose another pair of vertical actuators were activated
to apply top moment simultaneously with horizontal loading.
The behavior of M35X was quite similar to NW-3 in the previous section, with somewhat greater deformation capacity. This is quite understandable
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Fig. 4.38. Bidirectional loading path.
Kx(S).
starting of yielding of
•column main bar M3 6H
-5 t 5 6 z (mm)
(b) Out-of-plane loading of M35H
Fig. 4.39. Load-deflection curves of walls under bidirectional loading.
•2.0 1200
800 - 400 -
S
•400 800 1200
-41
150 100
8
-50 -100
150 -2.0
•1.0 1.0 **M 2.0
1 j 1
calculated Qmu starting of yielding column main bar
i i i
f...
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I stf2£~ (\
\ failure yielding of all L column main; bars
••••j P-3 6-H
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(c) In-plane loading of P35H
•1.0 0 1.0 JO
Ry<*) T " T
• starting'ofyieWiitgof column main bar
T -
- failure
41 2.0
yielding of all olumn main bars
P 3 B + ằ
_l_ _L-
I I -IS -(0 -S I v 5 I I IS
6x(mm)
(d) Out-of-plane loading of P35H
20
Fig. 4.39. (Continued)
when one sees that the cross section, column reinforcement and level of axial load are similar, with the only difference being the increased wall re-bars in M35X. After sustaining 2 cycles at 1.5 percent deflection, shear compression failure of web wall plate took place at 1.8 percent deflection, leading to a sudden loss of lateral resistance.
Figure 4.39 shows load-deflection relationship of two specimens, M35H and P35H, subjected to the bidirectional loading of Fig. 4.38. M35H failed, after sustaining 1 cycle of 1.5 percent deflection, by the shear compression failure
of web wall plate at second 1.5 percent deflection. P35H had fewer column confining re-bars, and failed in the first 1.5 percent deflection cycle, also by the shear compression failure of wall. M30H with lower axial struss was similar to M35H, except that it failed in a similar way after sustaining 2 cycles of 1.5 percent deflection. The load-deflection relationship of four specimens was quite stable in general before the onset of the failure. As shown in Fig. 4.39, load at the peak in-plane deflection dropped due to the effect of out-of-plane load reversal. Similar load drop in the out-of-plane direction is not observed, as the in-plane loading was applied when the out-of-plane deflection was zero, and only a V-notch was formed near the ordinate of the out-of-plane hystere- sis. Except for this kind of load drop, effect of bidirectional loading was not conspicuous in Fig. 4.39.
The bidirectional effect was more clearly seen in the axial strain measure- ment. Figure 4.40 is a plot of axial strain of a column and nearby wall at the conclusion of out-of-plane loading cycles. For all specimens, compressive strain of column increases as the horizontal drift of the wall increases, and furthermore in case of specimens under bidirectional loading, the column strain increases in the second out-of-plane cycle. On the contrary the wall compressive strain does not increase rapidly while the horizontal drift remains within 1 percent deflection. It is inferred that the sudden wall strain increase at 1.5 percent of M35H and P35H was caused by the high compressive strain of columns at the previous 1 percent deflection. Damage of columns due to out-of-plane loading should have caused the transfer of axial load to the wall plate, thus accelerated the shear compression failure.
To conclude this section, major findings from this experiment were as follows:
(1) Deformation capacity of walls under bidirectional loading was smaller than those under unidirectional loading.
(2) Deformation capacity loss was more remarkable for higher axial stress or poorer column confinement.
(3) Deformation capacity loss of wall should be the consequence of the progress of column axial strain.