The core concepts of seismic-resistant structural design have two main points. Do you know what they are?

The seismic design of building structures includes two design categories: conceptual design and parameter design. The conceptual design of seismic resistance in building structures mainly addresses the uncertainty of earthquakes and the approximation of finite element analysis. Let's take a look together.

From a conceptual perspective, especially considering the overall structure, engineering decisions for seismic resistance; the parameter design of building structures mainly adopts a two-stage seismic design method (calculating seismic effects, verifying component strength, and checking structural deformation, etc.) to meet the three levels of seismic fortification requirements.

The two are complementary. A correct seismic design must emphasize the conceptual design of seismic resistance and flexibly and reasonably apply seismic design ideas to avoid falling into blind calculations.

1. Main content of structural conceptual design

01 Reasonable building shape and structural form:

1) Minimize wind load effects;

2) Minimize seismic effects.

02 Reasonable structural selection:

1) Should have a clear calculation diagram and a reasonable seismic force transfer path.

2) Should avoid loss of seismic resistance or gravity load-bearing capacity due to damage to part of the structure or components.

3) Should have necessary seismic bearing capacity, good deformation capacity, and the ability to dissipate seismic energy.

4) Should have multiple lines of seismic defense.

5) Should have a reasonable distribution of stiffness and bearing capacity to avoid weak points due to local weakening or sudden changes, which can cause excessive stress concentration or plastic deformation concentration.

6) The dynamic characteristics of the structure in the two principal axis directions should be similar.

03 Reasonable structural layout:

The structural plane layout should be regularly symmetrical.

1) Vertical components should be symmetrically arranged along the perimeter;

2) Shear walls should have flanges;

3) Vertical components should be arranged in areas with high bearing capacity.

The lateral stiffness of the structure should vary uniformly along the vertical direction.

The cross-sectional dimensions and material strength of vertical lateral force-resisting components should gradually decrease from bottom to top to avoid sudden changes in lateral stiffness and bearing capacity.

1) If the stiffness is too small, the lateral displacement during an earthquake will be large, leading to increased damage to non-structural and structural components; if the stiffness is too large, the seismic force will increase.

2) The strength of the structure in terms of shear, bending, compression, tension, and torsion should meet seismic requirements.

3) Four strong and four weak:

① Strong columns and weak beams; ② Strong shear and weak bending; ③ Strong joints and weak components; ④ Strong compression and weak tension.

The overall structure should deform by bending rather than shear.

Try to avoid torsional deformation.

04 Use high-strength, lightweight materials.

Fully utilize the material properties of components: ductility (deformation capacity) should be strong. Under certain strength conditions, if the structure has stable plastic deformation capacity, it can dissipate the seismic energy input into the structure during an earthquake, reducing seismic forces and preventing severe brittle failure or collapse.

05 Good integrity:

Integrity can prevent structural and non-structural components from being shaken loose and falling during an earthquake, and it is also a basic condition for the structure to exert its spatial function.

06 Reasonably set seismic joints, expansion joints, and settlement joints.

07 Reasonably select foundation types.

08 Construction quality must be excellent.

09 Cost must be reasonable.

2. Conceptual design of site and foundation.

The site affects the seismic response of the structure, and the magnitude of the seismic response determines the seismic damage to the structure. Generally, on deep soft soil layers, the seismic response of high-rise buildings is more intense; on shallow hard soil layers, structures with shorter natural periods have a stronger seismic response. Therefore, when designing buildings on soft soil foundations, attention should be paid to the response of flexible structures; conversely, when designing buildings on hard soil foundations, attention should be paid to the response of rigid structures.

In the design of foundations and bases, care should be taken not to place the same structural unit on fundamentally different foundation soils or use different types of foundations. When the foundation has weak clay, liquefied soil, recently filled soil, or severely uneven soil layers, the integrity and rigidity of the foundation should be strengthened.

3. Conceptual design of structural layout.

Investigations and theoretical analyses of seismic damage after earthquakes have proven that structures with simple shapes, consistent centers of stiffness and mass, and uniform distribution of mass and stiffness in both the plane and vertical directions have better seismic resistance and less damage. Therefore, in conceptual design, attention should be paid to the following points regarding the layout of the structure:

01 The plane layout of the building should be simple, symmetrical, and have the mass center coincide with the stiffness center.

From a seismic perspective, simple shapes such as squares, rectangles, circles, and regular polygons are most favorable. Wings that extend too far, have concave angles, and are asymmetrical in one direction belong to irregular planes. During an earthquake, the torsional effect will cause large displacements and cracks or collapses at the ends of wings far from the stiffness center, or cause stress concentration and cracking at concave angles.

It is important to note that even for buildings with regular and symmetrical planes, the distribution of mass and stiffness must ensure that the stiffness center coincides with the mass center; otherwise, torsional effects may still occur, causing significant seismic damage to components far from the stiffness center.

02 The vertical stiffness distribution of the building should be uniform and continuous.

There should not be sudden reductions in stiffness at certain levels; otherwise, during an earthquake, weak layers may form due to concentrated plastic deformation. Additionally, the stiffness of the top few layers should not be reduced too much; otherwise, it may lead to a whipping effect during an earthquake, exacerbating damage.

When the layout of the building's plan and elevation is very complex, consider using seismic joints to separate the structure into several independent units.

But two points should be noted:

1) The purpose of setting seismic joints is to divide complex plan and elevation buildings into simpler shapes. Since the setting of joints also changes the structure's natural vibration period (generally, the period becomes longer), it may approach the inherent period of the foundation soil; moreover, the seismic components of each separated unit must be arranged properly, with uniform and symmetrical stiffness, otherwise it may also increase seismic damage.

Therefore, whether to set seismic joints should be decided based on the magnitude of seismic response, structural layout, and the characteristics of the foundation soil, and if necessary, a comparative seismic response analysis should be conducted.

2) If it is decided to set seismic joints, the structure must be disconnected from top to bottom (above the foundation), with double columns or double walls on both sides of the joint, and with sufficient joint width. Care must be taken to ensure that it is not blocked by construction debris and rendered ineffective, nor can it fail due to rigid sealing materials jamming the joint. Relevant provisions for seismic joints can be found in "Anti-Regulations" - Article 3.4.5 and "High Regulations" - Article 3.4.9.

4. Utilization of large deformations of components and structures

In seismic conceptual design, one should not only focus on not being damaged by small earthquakes but also consider not collapsing during large earthquakes. Long-term practice has made people realize that it is unwise to rely solely on the elastic stage of materials to resist rare large earthquakes; instead, the plastic deformation capacity of components and structures should be utilized to dissipate the energy input into the structure during an earthquake.

Steel and reinforced concrete flexural components have obvious yield points and can dissipate input seismic energy through large deformations, while reinforced concrete shear components, columns with high axial compression ratios, short columns, joints, and masonry do not possess these characteristics. Therefore, in conceptual design, one should leverage strengths and avoid weaknesses, or modify them to improve their properties.

01 In reinforced concrete frame structures, the relationship between beams, columns, and joints:

When any part yields, the other two parts can receive a certain degree of protection. Therefore, considering the differences in the plastic performance of beams, columns, and joints, the design should consider the relationship of strong columns and weak beams, strong shear and weak bending, strong joints and weak components, encouraging beams to undergo large deformations in the form of bending yield to dissipate the energy input during an earthquake, thereby avoiding difficult-to-repair damage to columns and joints and preventing collapse as much as possible.

02 Masonry is a brittle material, but it still has a certain deformation capacity after reaching its ultimate bearing capacity.

To utilize this deformation capacity, internal reinforcement can be used, that is, reinforcing bars can be placed in horizontal and vertical mortar joints; or external reinforcement can be used, that is, steel mesh and cement mortar layers can be applied on both sides of the wall to fully utilize the large deformation of masonry, allowing it to maintain a certain strength during large deformations. In addition, setting structural columns within the masonry can constrain the masonry, thereby fully utilizing its original deformation capacity.

5. Uneven and asymmetric distribution of planar stiffness

Resulting seismic damage

Complex building plans and asymmetric structural stiffness can easily cause torsion and local stress concentration during an earthquake (especially at corners). If corresponding strengthening measures are not taken, it will lead to severe seismic damage.

For example:

A six-story cast-in-place reinforced concrete frame structure in Tianjin, 27m high, with an L-shaped plan. Due to insufficient consideration of the impact of torsion during the design, it was in an 8-degree zone during the Tangshan earthquake. Post-earthquake investigations found severe damage to the corner columns on the second and third floors, horizontal cracks at the window sills of edge columns, and numerous cracks in both the exterior walls and interior infill walls.

Another example is Factory No. 11 of Tianjin 754 Factory, with a rectangular plan, featuring a five-story cast-in-place reinforced concrete frame in the middle, connected at both ends to elevator shafts with very high stiffness made of brick masonry, with a symmetrical overall layout.

However, due to the building's length of 110m, a seismic joint was set in the center, dividing the entire building into two independent units with uneven and asymmetric stiffness distribution. During the Tangshan earthquake, the factory experienced significant torsion, causing severe twisting and cracking of the frame columns, and serious cracking and displacement of the stairwell walls.

6. Conclusion

Seismic conceptual design is very important in structural design; it is a key link that showcases the advanced design ideas and profound design skills of structural designers. Structural designers should integrate seismic conceptual design throughout the entire structural design process, continuously think and innovate, while grasping the major principles of design, to ensure the scientific and rigorous nature of the design results.