Updated: Nov 5, 2021
In this blog I will discuss about two closely related structural topics. Soft storey and weak storey. Though these are 2 different things, the situation that creates these 2 weaknesses are very similar in nature. Please also note that these two issues are the result of absence of one or two or three among the 4 virtues of seismic resistance namely good structural configuration, adequate lateral strength and adequate stiffness.
Both these terms are related to a weakness or an undesirable occurrence in a building structure. This situation is arising due to either a poor Architectural and structural scheming or sometimes due to challenging Architectural and functional requirement. What ever be the reason, these are undesirable irregularities which makes the building weak during any lateral loads like the seismic or wind load.
The best to do for a better structural performance during an earthquake is to have very regular buildings. Having said this, it is important to know what situations can create these undesirable conditions.
Soft storey is a situation when the upper levels of a building is stiffer than the lower storey. This can result in undesirable performance of the building. There are many situations when a storey above can be stiffer than that below it. Some practical examples are discussed in the below mentioned points.
When there is stilt in the building.
A Stilt is a common requirement in many apartments. A stilt is a floor where you have parking. Due to the parking requirement, there won’t be any masonry walls between the frames. In upper levels since the functional usage demands infill walls, it will exist and the stiffness of the walls plays in to action when there is a seismic activity. This is an undesirable situation especially in seismic zones and needs special attention structurally. Please also see the video to understand what code says about soft storey.
When a mivan technology is used in construction.
Mivan technology is a formwork technology that helps to speed up construction. This technology is widely used in India now especially for low-cost housing. Most of the budget apartments that you see in tier 1 cities in India are constructed using Mivan technology. The irregularity of buildings occur due to the same reason as mentioned above. Most apartments will need stilt parking. When you use mivan technology, all walls are made of RCC. Even the partition walls are RCC. This makes the stiffness difference even more.
Floor height variation.
Many buildings like hotels, rail stations, institutional buildings and many more buildings based on its functional use may need more height for bottom floors or some of the upper floors. The need for larger height may be due to the space requirement vertically or many times for light and ventilation due to large people assembling there. As we all know, stiffness is also dependent on the nearness or farness of the support. In this case that is the floor height and hence if the height is more, obviously the stiffness is lesser. This can create stiffness variation across floors and make the building susceptible to seismic or lateral loads. This situation is another form of irregularity and needs special attention as it qualifies in to this soft storey or weak storey case and sometimes in to both.
Mitigating Soft storey complications.
As already mentioned, the best thing to do is to try to scheme it properly and avoid such situations. If unavoidable follow all the code provisions as in 1893-2016, the seismic code of practice.
The older code 1893 2002 was more liberal. However, the new 1893 2016 is more stringent in dealing with soft storey.
The seismic code 1893 2016 Table 6 says that structural plan density needs to be calculated for unreinforced masonry and if it is more than 20% then it needs to be modelled as per clause 7.9 1893 2016. Further the code says that the maximum forces in bare frame or the model in which the effects of unreinforced masonry is captured should be taken.
Clause 7.9.1 1893 2016 elaborately explains how the un reinforced masonry needs to be modelled for analysis. It basically recommends to model a diagonal equivalent of the masonry. The width of the diagonal is all defined in the clause and you can calculate and model it with out a rigid connection at the ends joining the frame.
Clause 7.10 also recommends to provide compensating stiffness in the form of diagonal or shear wall that goes till the roof. This is a recommendation to improve the scheme. Note that Clause 7.9 is providing information on how to model the effects of masonry where as clause 7.10 is mentioning what is the actual scheme needed at site and design. There are many more provisions which needs to be carefully considered and executed in design and detailing. The objective of this blog is to provide basic information about these two weaknesses and not to discuss the entire design methods. That will be covered in our structural design courses.
A weak storey is a storey which is weaker than the storey above it. The situations that will create a weak storey is exactly similar to what you have in soft storey. Therefore, there is no need for me to repeat the reasons for a weak storey. The very same stilt and height differences are the major reasons for weak storey aswell. Also provision of openings in lateral load resisting mechanisms like shear walls can result in undesirable weakness in the building. Some times the shear wall shifts from its continuity in the scheme due to architectural and functional reasons and creates undesirable weaknesses. This should be avoided to ensure more regularity
Mitigating Weak storey complications
7.10 provisions in 1893 2016 is applicable to all open storey structures and hence applicable to weak storey cases too. As mentioned, both weaknesses are created due to similar reasons.
Clause 7.10 asks us to provide RCC walls throughout the height of the building with separate foundations. These walls shall be connected to main frames too. If the building is in seismic zone 3 ,4 or 5 then it has to be ductile detailed too. One should also ensure that the introduction of walls do not induce torsional irregularity. The scheme should ensure that torsional irregularity is not additionally generated. Some of the other important points are mentioned below.
Lateral stiffness of the open storey shall not be less than 80% of the storey above.
Lateral strength in the open storey shall not be less than 90% of the above.
There are many other structural situations that you may want to consider in your structural analysis that can come together with soft story and weak story situations. One such case is that of short column effect. Short columns tucked between two close beams can create very undesirable structural situations. Especially when you have basements and part basements and large openings in some of the walls, it is likely to have the short column effect. A careful planning of structural scheme shall be done to avoid existence and co-existence of such undesirable structural situations.
As we say always, it is always better to design building structures keeping in mind the four virtues of seismic resistance. However sometimes, it is not possible to avoid some irregularity like that arising due to the open floors due to requirement of parking. In such cases, careful considerations as mentioned in IS 1893 2016 shall be adopted.
I also specially suggest to do a key element removal check and then ensure bridging of failure of these key elements. At least this need to be done for unfactored load combinations to rule out accidental collapses that might also become progressive.