by There is a cognitive gap between internal and external structural engineering majors about what is structure and what is non-structure.
For those who do not engage in structural engineering, it seems that as long as they can not be bent with bare hands, they are all structures, so that the infilled walls, pipes, aluminum alloy doors and windows…
are all structures.
People engaged in decoration call these “hard decoration”; In the view of the people engaged in structure, they are all “non-structural components”.
Although people who engage in structure have a narrow understanding of structure, they prefer to call everything outside the narrow “structure” as “non-structure”; However, when I first saw “reinforced concrete non-structural wall” in Japanese literature more than ten years ago, I still felt insulted for reinforced concrete: do you call reinforced concrete non-structural wall?! Yes, they do call some cast-in-place reinforced concrete walls non-structural members, and they are eloquent.
Simply put, all reinforced concrete walls that do not have to meet the various checking and structural requirements of shear walls in the reinforced concrete code are non-structural walls.
They are not included in the checking calculation of the bearing capacity of the structure, but only used as the bearing capacity reserve.
They have a variety of forms and a wide range of uses.
They also have names that are pleasant to hear and easy to remember, and Chinese people have no dyslexia (above).
Today I want to tell you a sad story about Fang Libi.
1.
Before the RC infill wall is rigidly connected, the reinforced concrete (RC) square wall is poured with the RC beams on the upper and lower sides, that is, the upper and lower ends are rigidly connected.
However, it was soon discovered that its reinforcement was too small (usually the spacing of 150 mm-thick double-layer round 10 reinforcement in the outer wall was 450 mm, and the spacing of 120 mm-thick single-layer round 10 reinforcement in the inner wall was 300 mm), and its rigidity was very large, so it was particularly vulnerable to brittle failure in the earthquake.
As a RC wall rigidly connected to the main structure, it could have provided a lot of bearing capacity reserves.
But before the main structure of the RC frame can fully exert its ability, they broke down first, and the bearing capacity reserves were gone.
In recent years, several earthquakes occurred in Japan and such earthquake damage occurred.
For example, in the East Japan earthquake in 2011: or in the Kumamoto earthquake in 2016: or in our experiment: 2.
There was an idea to set up a RC infilled wall.
Since these non-structural walls do not participate in the checking calculation of structural bearing capacity and are so easy to be damaged in the earthquake, it is better to simply set a joint between them and the main structure and separate them from the main structure.
For example, as shown in the following figure: In this way, except for the force transmitted by the three limiting bars, the wall limb is almost free of force, and of course it will not be damaged.
As expected, the experiment was done without any shock.
As for why the joint is set at the bottom of the wall instead of the top of the wall, it is mainly for the convenience of construction.
These RC infilled walls are poured together with the main structure.
If you want to set joints on the top of the wall, it is not easy to construct.
3.
Jointed RC infilled wall+energy dissipator However, just like another battle field of the rigid and flexible dispute in the Japanese seismic field in the last century, some people have raised objections to the joint of RC infilled wall: a good reinforced concrete wall, then a considerable safety reserve of bearing capacity, how can we say it is not necessary? How disrespectful of bearing capacity! So the little friend in Japan came up with a way: put some energy dissipators in the seam and concentrate the deformation on the energy dissipators, on the one hand, it can maintain a certain bearing capacity; On the other hand, it will not quit work prematurely due to brittle failure.
This idea, emmm, is not very attractive to me.
But you might as well try it.
What if it’s fun? We think of two ways to arrange energy dissipators.
One is to continue the construction tradition of setting joints under the wall, and place energy dissipators in the bottom gap, which is called “bottom energy dissipation wall”; The other is to place the energy dissipator at the reverse bending point of the wall limb, which is the middle of the wall limb.
Let’s call it the “intermediate energy dissipation wall”.
We believe that both have their own advantages: the bottom energy dissipation wall takes care of the traditional construction technology of setting joints; The stress of the intermediate energy dissipation wall is more reasonable.
But the experiment shows that both have their own shortcomings (now we can start to grieve).
The cantilever of the bottom energy dissipation wall is long, and the deformation ratio of the wall limb after cracking is large, so the role of the energy dissipation device is small.
The damage is mainly concentrated at the root of the wall limb.
First, the bending yield, then the concrete on both sides collapsed, the reinforcement buckled, and finally the shear bearing capacity was directly lost.
The wall limb of the intermediate energy dissipation wall is relatively short, with high rigidity, and the wall limb does not bend and yield, so the proportion of wall limb bending is small; However, the damage of the intermediate energy dissipation wall is mainly manifested in the damage of the connection part of the energy dissipation device.
At the later stage of the test, when the inter-story displacement angle was large, the energy dissipator simply withdrew from work because of the connection failure.
In the bottom energy dissipation wall and the middle energy dissipation wall, the total shear force of the wall limb is similar, and the stud design of the connection node is the same as the reinforcement design of the wall limb.
Why is the connection node of the bottom energy dissipation wall okay, while the connection node of the middle energy dissipation wall is broken into slag? One conjecture is that because the two RC walls of the middle energy dissipation wall have short legs and high stiffness, they provide greater rotational constraints for the energy dissipation device.
On the one hand, it is certainly beneficial to concentrate the deformation on the energy dissipation device and improve the damping efficiency; But on the other hand, it also increases the demand for the flexural bearing capacity of the energy dissipator connection nodes (namely, stud groups).
However, at the beginning of the design, the bearing capacity of the stud group under bending and shear coupling was not considered, but only its shear bearing capacity was checked.
This conjecture is also confirmed by the final cone-shaped failure pattern of the connection part of the energy dissipator of the intermediate energy dissipation wall.
I have to say that this is a design error.
How sad.
However, this is still only a conjecture.
It may explain all the reasons or only part of the reasons.
.
