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Precast Concrete Buildings : Structural Design Considerations


Precast concrete is a very popular construction method in the modern world. As we all know and the name suggests, Precast concrete construction method is the way to cast the concrete in a factory or at site and then assemble the structural members at site. This necessarily need not be the work site. After undergoing a proper procedure, these precast structural members or components are transported and assembled at the site in their respective locations. There are proven examples of bridge construction and many high-rise buildings using precast. Precast buildings are getting popular due to the ease of construction especially if the site has a lot of mobility and transportation related challenges. In such cases, it may be a good idea to precast the structural members and assemble at site. Also for repetitive forms, it is faster and sometimes cheaper to go for precast concrete.


Structural Design of Precast Buildings

Many structural engineers think that the analysis and design of a precast building is drastically different from a normal RCC building. This is not really correct. Though there are nitty-gritty related to precast connections that would be different from normal cast in situ construction practices, most of the structural analysis conditions remain the same. I will mention some of the important structural design considerations below.

India being a seismically active zone, we cannot have slotted connections for beams and columns. Slotted is a kind of simply supported condition where the joints are not restrained and hence frame action will not be existing. All precast joints have to be doweled in. This is always a grouted cast in situ wet joint and not a slotted connection. Maybe that some of the joints can be simply supported i.e., slotted connection but then there should be a bracing system like that of a shear wall that will take care of the seismic demand of the building or any other form of bracing that will take the lateral load down to the ground. If not, there shall be some other frames in the building that are not slotted connections that will provide lateral stability and lateral resistance.

Sometimes the slabs can be slotted. However, for diaphragm action to kick in, it is better to have slabs and beams also connected by dowels. If not, there shall be proper screeding or topping above the slab with minimal rebar as per IS 1893 to ensure diaphragm action of the slabs. As per 1893 clauses, minimum rebars are also needed in the topping for the diaphragm action to kick in.


Is there a difference in Modelling in ETABS?

Many engineers have this confusion if the ETABS modelling has to be different for a precast building. If the construction is wet grouted and dowelled then the frames would behave very similarly as in a cast in situ condition. Hence the ETABS modelling will be the same as normal buildings. However if there are any differences in the connections and hence behavior, then that shall be idealized in the ETABS model as well. As discussed in previous sections, precast structures cannot be dry jointed as in a slot as it will not generate a frame action needed for seismic resistance.


Precast construction - Other considerations


Handling Stresses

There will be additional handling stresses to be considered for precast construction. During transportation as well as while lifting the precast members, there will be different kinds of forces developed in precast members. This handling of precast members shall be planned and the information on the method of lifting and transportation shall be well coordinated between the precast vendor and the structural engineer and all consultants. These handling stresses shall be considered in the design and construction of the members shall be done precisely by providing hooks etc. as needed for lifting the members.


Site Restrictions

Whether a precast construction can be adopted or not depends on various factors. One of the main advantages of precast buildings is the speed of construction. However this also needs a lot of space if the precast factory is set up at site. If the site does not have enough space, this will not be possible. Same way, if the precast factory is set up remotely, then handling may sometimes become a challenge depending on the site conditions. This needs to be planned and well coordinated to avoid last minute issues.


Multidisciplinary Coordination

Mechanical, Electrical and Plumbing services will have a lot of specific requirements like cutouts and sleeves. This can be easily incorporated in conventional construction. However when all members including walls are precast, then these MEP requirements shall be meticulously planned and coordinated so that core cutting is avoided. This also reduces flexibility of changes. The client may not be able to make changes in the requirement once it is planned and construction is done in the factory. It is also important to have meticulous multidisciplinary coordination to avoid clashes between the needs of different disciplines.


Precast Houses

Precast house construction is very popular in the west and the middle east. Precast houses can be constructed in India as well. The seismic demand of precast houses is not very large as the weight and height are not very high like that in a high rise building. All the more, we already have load bearing houses very much popular in India and we design and detail it with necessary reinforced concrete bands at lintel levels and sil levels with adequate cross walls to make it stable against lateral forces like earthquake and wind.. Same can be adopted for precast buildings. It is nothing different from a load bearing house.


Other Precast members

Precast construction is very popular for secondary members. Stair cases, compound walls and retaining walls are all built using precast technology. Even sump tanks, septic tanks and many similar structures like drains and soak pits can be conveniently precast. It is already a vendor built and supplied items. The standardization in this area has helped to speed up the construction of secondary structures and infrastructure development related to the buildings. These secondary structures are done lastly in most structures and availability as a prebuilt item with standard sizes helps in avoiding the last minute delays in project completion.


Precast Technology vs Mivan Technology

Mivan and pre-cast concrete technologies are a boon in the current construction industry. In a variety of ways, both techniques can be compared, and let us look into some of the factors that can be compared with. I have written a separate blog on Mivan Technology here which is more comprehensive on Mivan.


Work Quality

Both of the methods offer a perfect quality for their products. An extra-quality finish with less wastage of materials is an additional advantage for both techniques.


Work Speed

When compared to normal construction techniques, the Mivan and pre-cast concrete technology are faster. Mivan is a cast in situ method. But then it will not need setting up of a factory like that in precast construction. Both need adequate planning and coordination between disciplines to avoid discrepancies.


Workforce needs

Mivan technology requires more work force when compared to the pre-cast technique. This is because the Mivan technology is cast in situ.


Formwork Usage

Aluminium formwork usage has limitations for larger projects because of its lesser weight. But there are no such limitations for precast concrete technology.


Cost of Construction

Cost of construction is related to the complications in the project. For Mivan, the cost of Aluminium shuttering used can be a bit higher than the pre-cast technique. But precast may incur higher transportation and machinery costs. Finally, it narrows down to the project situation that decides the economy.


Summary

Precast technology is a convenient method of construction. As discussed, it has advantages and disadvantages. One can decide to adopt precast construction considering the points discussed in this blog. The structural analysis and design considerations remain almost the same. If you want to learn more about it, you can comment below this blog and discuss.

If you are keen on learning structural aspects and shift to high paying structural roles, you can consider my mentoring programs.



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