Did you know what Are the Most Important Seismic Resistance Considerations When Designing a New Building? There are many. However, there are four key considerations that are often regarded as the 4 virtues of Seismic Resistance. These seismic considerations are called as virtues since the fact that, if the building possess these results or qualities after the considerations, the building will be seismic resistant. As we all know these qualities or virtues will come in the building when the structural engineer adopts these considerations while designing the structure.
Seismic resistance is something which needs to be planned for in the structural design of any type of structure. This structure can be anything from a small single-story dwelling to a high-rise building.
The first step towards seismic resistance is to make sure that the design of the structure is as per the building codes. In India, the codes for seismic resistance are IS 1893 2016. Though it is simple to state this, adhering to IS 1893 involves application of experience.
Let us quickly discuss what is seismic resistance first before discussing the virtues.
What is Seismic Resistance?
The ability of a building or a structure to perform during earth quake is what we all structural engineers desire. The ability of structure to withstand an earth quake is what we call as performance.
Designing an earth quake PROOF building is not possible. There are 2 reasons for this.
We have limitation in understanding the intensity of expected Earthquake as it is a natural phenomenon and we don’t have any control on it. So how do we make a building seismic proof?
Even if we knew the expected intensity, if we try to make a building earthquake proof, it won’t be economic or functionally feasible as the member size will be extremely high to meet the seismic demand. Because we cannot have an earthquake proof building, engineers arrived at a design philosophy that if followed will give the building certain amount of seismic resistance with acceptable level of damages. This design philosophy based on experiments, trials and studies considers less than 15% of expected seismic forces. If you do this, you can achieve the below mentioned conditions
Mild Earthquakes - No cracks and damages
Moderate Earthquakes – Reparable cracks may occur.
Severe Earthquakes – Irreparable cracks may occur but no collapse
Very severe Earthquakes – Collapse but gradual so that occupants can be evacuated.
What are the 4 Virtues of Seismic Resistance?
I have listed the 4 virtues below.
Good Structural Configuration
In fact, the better performance starts with the structural scheme of the building. A good structural scheme is possible when the Architectural scheme is great as well. This can happen when the structural consultant is on board early in the design development stage and the architect and the structural engineer work together in achieving this robust scheme. Robust scheme means a scheme that performs well during a seismic event. This can be achieved by adopting the below mentioned points
Regular geometry:
Regular geometry is really important for a good performance during earth quakes. I have already mentioned about significance of geometry is another blog. You can see it here
Avoiding cantilevers:
Cantilevers perform poorly during earth quakes and should be avoided as much as possible in the design. This is a shared responsibility with client and the architect. Another reason that a structural engineer should be on board the project during architectural scheme development stage itself.
Proper position and sizes of structural members:
Proper scheme of the building is extremely important for the overall wellness of the structural configuration. The structural scheme decides how the members interact and behave during earth quakes.
Proper orientation and position of columns and shear walls:
Orientation of columns properly is important to resist seismic forces. Most large buildings will need additional bracings in the form of shear walls to relieve the columns from seismic forces. All this needs to be carefully planned and configured.
I have already mentioned about shear walls in another blog. You can see it here
Lateral Strength
The building structure should possess minimum lateral strength in each direction in plan. This shall be uniformly distributed as well. This is to ensure no damages during minor earth quake and to help to meet the design philosophy that is mentioned in the start of this blog.
Adequate Stiffness
Adequate initial stiffness needs to be ensured in the building. This will ensure that the occupants don’t feel any discomfort and the contents in the buildings are not damaged.
The building and the structural members may crack and loose some stiffness after the building experiences the earth quake forces. However initial stiffness is very much significant for earthquake resistance of building structures. A structural engineer should carefully ensure this vurtue.
Ductility achieved by Ductile Detailing
It is a bit challenging to define ductility or ductile detailing in a simple language. However, it is easy to explain it as the property opposite to brittleness. So, if you understand what is meant by brittle, it is easy to appreciate ductility. It is related to the ability of a material to plastically deform without fracture. It is an important virtue for seismic resistance. Many get confused between ductility and elasticity. The ductility of a building allows it to absorb seismic forces. I will try to explain it in a bit more by mentioning about seismic design philosophy.
As we all know, earth quake is ground movement and earth quake force is the resulting inertial force generating the displacement type of load. It is not possible to design for this huge force and make the building earth quake proof as it will need enormous member sizes as explained in the initial sections of this blog. This will result in poor functional usage and uneconomic building. For this reason, we design it for a seismic resistance only and not for the full seismic forces and that is the design philosophy all about. Also note that we cannot exactly predict the acceleration of the ground as it is a natural disaster and we have no control on it. For these 2 reasons mentioned, we are designing for a very less seismic force than actual. Now what if the actual earth quake is far more than the considered forces?
The buildings gain energy from the ground movement and this needs to be dissipated. The building will dissipate this energy in various ways by generation of heat, by deflecting, by generation of sound and so on. However, the materials being brittle will also crack up and release energy. It’s important that the cracking and failing is prevented or reduced or differed by a few seconds to ensure less damages and collapse. Here is where ductility is seen to be an extreme virtue helping in a better building performance. Ductility allows the building to accommodate the imposed lateral deformation between the base and the roof of the building.
There is a code of practice for achieving ductility and the code IS 13920 is dedicated to ductile detailing and designing. If the building is designed and detailed by this IS 13920 provisions, the building is ductile and will perform better during earth quakes.
7 FACTORS THAT INCREASE DUCTILITY
Using a simple geometry
Adopting a simple geometry will reduce chances of developing forces that generates torsion and other brittle forces.
Having more redundancy on lateral resisting systems like shear walls.
More redundancy in lateral resisting mechanism will help other members relieve undesirable forces and also achieve more ductility there by reducing the chances of failure.
Avoiding hinge formation in columns
This means avoiding column failure by adopting ‘’weak beam – strong column’’ principle in structural design.
Avoiding all brittle failures in the building
Examples are failures like that due to shear, anchorage or compression failure.
Providing confinement reinforcement
Ductile detailing as in 1S 13920 2016 helps in confining the concrete at critical sections.
Ensuring beams are designed under reinforced.
Under reinforced beams ensure that the steel reaches its ultimate tensile capacity before the concrete reaches its ultimate capacity in compression. The steel failure will be ductile and people can be evacuated in case of a failure as it is a gradual failure.
Avoiding foundation failures.
Foundation failure can result in a lot of brittle forces in the building.
Is ductile detailing mandatory? Here is a discussion on this topic.
Summary
Ensuring these 4 virtues is a very important for seismic resistance and this needs a strong understanding from all stake holders of the project. In India, an architect is a project leader for the building in many cases. So, it becomes a necessity that they appreciate the need. The structural engineers should be on board at the scheme development stage itself so that there can be a meaningful scheme that performs well during earth quake. If the scheme is not proper then ensuring seismic works is challenging and it becomes a kind of damage control.
The 4 virtues of seismic resistance also need to be looked in to in the main 4 stages of the building life cycle too namely planning stage, design stage, construction stage and maintenance.
The authorities also should bring in this requirement in the building approval stages there by ensuring incorporation of these virtues in all the stages.
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