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Let’s first look at the provisions of the main specifications, the high specification 3.11.2 note: “key component” means failure of the component may cause continuous damage to the structure or serious damage to life safety.
The provisions of the steel standard: the column base, the frame column less than 1 / 3 of the total height in the multi-storey and high-rise steel structure, the column of the vertical truss of the outrigger structure, the member in the intersection area between the horizontal outrigger and the vertical, and the seismic member that directly transmits the internal force of the conversion member shall be treated as the key member.
Article 3.9.2 of Guangdong high standard: generally, key components are designated by designers.
From the above discussion, it can be seen that the continuous collapse analysis is required to determine the key components in a strict sense, that is, the nonlinear continuous collapse analysis is carried out for the more important components in the mechanical concept one by one, and the components that cause continuous failure or serious failure after demolition are defined as key components.
However, in practical projects, it is too complicated to adopt the above ideas.
In practice, we often directly determine the key components according to the mechanical concept and engineering experience, generally including the transfer components in the horizontal components (such as transfer beams and trusses), connected parts and important vertical components.
It is difficult to determine which vertical members belong to important members.
The steel standard clearly stipulates that the frame column at the bottom is an important member, but which shear wall belongs to important members is not clearly specified.
Some students may intuitively think that the shear wall at the bottom strengthening part is the key component.
Is this reasonable? We need to analyze the seismic design idea of shear wall first.
Paragraph 1 of article 6.2.7 of the anti-seismic Code: the design value of the combined bending moment of the wall limb above the bottom reinforcement of the grade I seismic wall shall be multiplied by the amplification coefficient, and the value can be 1.2.
According to the above provisions, the bending moment diagrams before and after adjustment are drawn as follows: the inner curve is the bending moment diagram before adjustment, and the outer bending curve is the bending moment diagram after adjustment.
The specification enlarges the bending moment of the upper part, so that when the lower part reaches the design bending moment, there is still allowance for the bearing capacity of the upper part, so that the lower part becomes a relatively weak area, and bending failure occurs first, that is, the specification deliberately guides the failure sequence through internal force adjustment.
The key components are the components that want to be damaged finally.
Therefore, setting the bottom shear wall as the key component is contrary to the spirit of code 6.2.7.
If the shear wall at the bottom is to be damaged later than the upper shear wall, the bottom must be multiplied by an amplification factor greater than 1.2, so that the upper part is still a relatively weak part despite being amplified by 1.2 times.
This is logically strange.
If you want the bottom to be damaged later, it is not necessary to multiply the upper part by the coefficient of 1.2 first, because for earthquake resistance, it is meaningless to blindly amplify the internal force of all components.
The adjustment coefficient of the specification is essentially to control the order of damage.
What the reinforced part at the bottom of the shear wall actually strengthens is not the bending capacity, but to improve the bending ductility of the whole wall by making the edge part into a restrained edge member and increasing the volume stirrup ratio.
Since the bottom area first enters the bending plasticity, it is a reasonable idea to make it yield without damage; Since the internal force of the upper area has been amplified and it is impossible to destroy it first, there is no need to specially strengthen the ductility.
Only the structural edge members need to be configured.
Foreign codes generally call the bottom area of shear wall as bottom plastic hinge area, which makes the concept clearer.
It is obvious that the bottom should be plastic first, so there will be no problem whether it should be designated as a key member.
Most of the early super high-rise structures designated the plastic hinge area at the bottom of the shear wall as the key member, which is equivalent to multiplying the design bending moment in the bottom area by a magnification factor greater than 1.2, so that the upper wall will break first during a large earthquake.
From the above, it can be seen that the flexural ductility of the first broken part should be strengthened, that is, the structural edge members of the shear wall in the upper failure area need to be upgraded to restrained edge members, The constrained edge member can be set only in the failure area and does not need to extend continuously to the bottom area.
The 2013 version of Guangdong Provincial high-rise plan clearly puts forward that the bottom strengthening part can not be designated as a key member.
After that, the super high-rise structure has the design that the bottom plastic hinge area is not used as a key member.
The design idea of the American code is to let the plastic hinge first appear at the bottom.
In fact, whether the plastic hinge appears at the bottom or at the top, it can ensure the safety of the structure under a large earthquake through calculation and structural measures.
It is more natural and reasonable that the bottom is destroyed first, because normally, the part with the largest bending moment must appear at the bottom, and the bottom will be destroyed first.
Letting the upper part destroy first violates this natural characteristic, which is easy to increase the structural cost.
In fact, there is no simple dogma for the designation of key components.
The determination of key components is finally determined by the designer’s comprehensive judgment.
The key components, like the heart, lungs and other internal organs of the human body, play a vital role in our life safety.
The task of consuming seismic energy is entrusted to the energy consuming components.
We should ensure that the key components are elastic or at least not yield under the action of large earthquakes..
