On seismic structures, what is the ideal structure for to avoid the building failing laterally?
I read some of Fema 454 and I understand there are moment connection, braced, and shear walls can be used, is there one that is better than the others for seismic design? Does it depend on the scenario, like a tall building should have braced where a small building can just a shear wall?
Also what are important concepts to consider for wind loads on a building? I understand that pressure is higher the taller you go, so more lateral resisting force is needed.
When thinking about structures and how they behave in seismic situations a key word is flexibility. Taller buildings are designed to be more flexible, this is why when you go to an observation tower often you can feel the building move. This flexibility (often from steel structures) allows for the building to allow the waves of energy from a seismic situation to move through it, bending and flowing. A smaller stiffer building tends to have a much harder time in these scenarios.
In terms of materials, wood and steel are more flexible and have more give. Unreinforced masonry or concrete on the other hand are not flexible and tend to preform poorly in these seismic zones.
Another solution apart from strengthening the building against shock waves is to reduce the amount of force that the building is expected to sustain. They do this through “base isolators” which do exactly as they describe, isolate the base of the building from the earth’s movements.
There are two forms of these types of isolators. The first type looks a bit like a hockey puck made of flexible material, they absorb some of the energy of the shaking and allow the building to rock independently above them. The other type is similar to a sandwich of metal plates that are lubricated in-between. As the earthquake strikes the bottom plate can then move independently of the top plate which holds the building. I would reccomend looking up these two systems because the graphics really help illustrate how they work.
In regards to your question about wind loading you are correct, the pressure increases the higher and taller you go so more lateral resisting force is needed.
Many taller buildings are designed to move with the wind loads, however this can be very uncomfortable for the occupants. One solution is to install concrete weights that counter the movement of the building. Another technique is to make the concrete floors of the building thicker as they move towards the top of the building, thus shifting the center of gravity and reducing movement. Finally another strategy would be to create openings on the façade, which are areas that are designed to allow wind through the structure thus reducing the load.
I would reccomend that you study both types of connections; moment and shear. Understand where they are used and why. This way you can then loop back to the original question, and find that it depends on how the building settlement occurs. The important thing to know is the connections and how they preform under different stresses.
I did some study on google and assume it’s better to use moment connection if differential settlement happens? To keep building more stiff to resist deflections as image below?
Hi @whuang - differential settlement is a defect that can occur if a foundation is not properly designed, therefore it’s not a situation that one plans for by using one type of connection in the structural frame vs another.
Differential settlement can be caused by a number of factors (vibration, nearby excavation, large tree roots, etc.). It can be remediated in a number of ways as well - as the architect, you should work with your geotechnical and structural engineers to come up with a solution, such as underpinning, micro piles, etc. The point is to avoid the differential settlement in the first place, not plan the building to account for it. This is different from the strategy that one would take for seismic or wind loads, which are unavoidable and need to be planned for.
I hope that was helpful and good luck on your next exam!
