There are different types of structural systems for buildings. Many entry level engineers are confused what is a structural scheme and what a structural system is. Also, they are confused on what system needs to be adopted. Many focuses and give more emphasis to ETABS modelling with out even developing a scheme. Its very important to have a robust scheme for a building to ensure a proper load path for the stability of the building.
A structural system is the arrangement of load transfer. It could be a beam and slab system, Flat plate, Flat slab and so on. A structural system could be even a non-frame system like that of a load bearing wall system. All this needs to be carefully decided and designed by an experienced structural engineer and constructed by an experienced civil construction engineer.
A structural scheme means a more detailed arrangement of structural members including the column orientation, shear wall positions, beam positions and so on.
Load Bearing Building System
A load bearing walls system is generally adopted for 1 to 2 floor buildings and when the soil conditions are good. Here the load is transferred from slab to walls and from walls to a random rubble masonry foundation generally. You don’t need columns as the walls are taking the loads.
Some times when there are more open architectural plans, a few columns may be needed. Also in places like sit outs, porticos and courtyards, you may need some columns. Generally, this load bearing system will be economic as there are no much RCC and reinforcement needed. Foundations are only RR masonry.
Very rarely in places like coastal areas where the soil is sandy, you may need under reamed piles or a compaction under reamed piles and a plinth beam over which you can provide a masonry which is load bearing. The substructure forms a frame where the walls are supported
Framed Structure
A framed building is one in which there are columns and beams. The slab is supported on beams and the beams on columns. You will need separate footings for the columns. The foundations could be isolated, combined footing, raft foundation or pile foundation depending on number floors, the load and the soil type.
In a beam and slab system, you may or may not have shear walls, depending on the seismic demand of the building. If the building is in higher seismic zones or if the number of floors is more, you will need a bracing system. Shear wall is a bracing system to transfer the lateral or base shear.
Single System & Dual System
This is a different categorisation of the structural system just based on the seismic resistance mechanism. If a building has a shear wall as well as columns, and if the entire lateral load is designed to be taken by only the shear walls, it is called a single system. If the columns also participate in the lateral analysis and takes earth quake forces, then it is a dual system. As per IS 1893, it is mandatory that the columns put together has the capacity to take 25% of the base shear.
Flat Plates and Flat slabs
Flat plate is a beam less structural arrangement. The slabs are directly supported on the columns of the building. These are generally adopted in smaller buildings like residential houses. The slab thickness will be larger than normal buildings as the punching shear will be critical. The seismic resistance of such buildings will be lesser as there are no beams to tie the columns effectively. The columns are generally placed closer to reduce the forces.
Flat slab is also a beam less structural system like flat plate. However, the soffit of the slab is not flat but will have drop panels at the columns. This is needed to reduce the punching mainly. Also, the moment capacity and normal shear capacity will be more if there is a larger thickness in the form of a drop panel near the column. Most commercial buildings in India are built as flat slabs. This will help to increase the floor heights of the buildings.
In older times, flat slabs and flat plates were manually designed. It was not recommended when the number of bays were less than 3 as it was difficult establish thumb rules for manual design of these buildings. Now with more sophisticated method of analysis using software like Staad Pro and ETABS, it is possible to do advanced analysis and even check deflections of these slabs by shell modelling and analysing. Also, for more head rooms and neat soffits, many modern buildings are built using this system. It is generally not recommended to adopt flat slab system or flat plate system in higher seismic zones. However, many do these methods by considering a single system and ensuring a load path to the shear wall through peripheral beams and by providing sufficient ties.
Precast Buildings
Precast buildings are in fact a beam and slab system in most cases. Most of the times in India, being a seismic zone, the structural joints are wet jointed, which means joint is monolithic. The analysis remains fairly same as in a normal building. Only the construction differs. The structural elements are built in the factory or at site and then jointed at site. In non-seismic zones, even slotted connection as in a hollow core slab is possible. This is popular in Middle East countries like UAE and Bahrain
MAIVAN System
Mivan is in fact a formwork system. It generally uses aluminium slip formwork which is an extremely speedy construction method. This is very popular for low-cost housing projects. All walls including internal and partition walls are in RCC. This increases the speed of the construction and also offsets any cost increase due to the speedy construction.
Since all the walls are in concrete, this needs proper planning to avoid any later changes in the location of any ducts, openings and even switch boards. Structural analysis and design of mivan buildings also needs special attention. Some times the analysis is tricky when you have a lower parking where you need columns for the open stilt. The upper level will have RC walls and lower-level stilt which will create soft storey and weak storey issues.
Steel Buildings
In India, Steel buildings are more popular for industrial structures. These buildings are generally trusses and are long span. Steel can be used for normal buildings too but then the cost of construction and maintenance will be high. Also, the fire proofing needs additional attention. Stee buildings will be more susceptible to sound transmission also. In most cases steel buildings will be critical for wind loading as they are generally lighter.
If it is a tall building in steel, the challenges are the cost, the bracing adequacy and so on. In many cases a shear wall takes the seismic shear as a single system and the steel members generally takes the gravity loads.
Summary
A structural system needs to be carefully chosen based on various loading and structural parameters. Cost aspects, architectural aspects also some times decides the structural system. Also, the local practice of design and construction rarely decides the structural arrangement of a building.for more civil engineering online courses and civil engineering training please visit civilera.
