Shear wall is a structural element that resists lateral forces such as wind and seismic loads. Many young engineers are able to answer this much. They know that a shear wall needs to provide lateral load resistance. This is absolutely right. However still that is not a complete understanding of what a shear wall is.
All RC walls are not shear walls. It needs to behave as a shear wall to call it a shear wall. So, what should be the behavior of a shear wall? This is what we will discuss first in this structural Blog.
In fact, this blog is a result of a question asked by a civil engineering student in my forum. I thought I will extend the question and write a bit more on shear walls and its other structural significance.
Is any wall a shear wall?
IS 456 clause No.32 mentions about walls. Many thinks that all points in this clause is related to shear wall. That is not correct. Most of the points in this clause is related to a concrete wall like a side wall or a compound wall in RC. Necessarily it is not a shear wall. Shear wall should be designed more carefully, not just by clause 32 in IS code.
Is 200x900 size qualify as a shear wall?
Many think that if the ratio of length to thickness of a column is more than 4 then it is a shear wall.
IS 13920 Cl No. 10.1.3 says that if a shear wall needs to be ductile its ratio shall be 4. This also is a reason many decides if a wall is a shear wall or not based on this ratio.
Where is the code IS 13920 saying that, for a wall to be a shear wall the ratio has to be 4?
IS 13920 is mentioning about the ductility requirement alone and not defining what a shear wall is. What if the building is in zone 2 and we don’t need to be detailed ductile?
Ratio 4 is not the definition of a shear wall; the ratio only qualifies it to be called ductile shear wall. So, the ratio is mandatory to qualify it as ductile.
What if the wall is massive but not properly connected to the building? It is not taking the shear as it is near a large cut out. a kind of free standing. then? So, there is something more than its dimensions alone?
What is a shear wall?
Shear wall meaning is much sought out. Shear wall meaning is not some thing that can be defined. We need to have clear understanding of its purpose and behavior. Once we know what is a shear wall technically, then we will realize that looking for shear wall meaning is not much sensible.
Basically, its connected to the sway factor. If the wall does reduce the sway index, we can fairly conclude that the wall is effective. You can also check the share of lateral load in the wall too and then decide. There is no clear definition in IS 456 or even in IS 1893. However American code tie it to the sway and also give a rough thumb rule based on the stiffness of wall to that of all columns put together. If the stiffness or the inertia of wall is many times more than that of the columns put together then it qualifies as a shear wall. A point to note is that the position of the shear wall also matter. A massively stiff wall may not be resisting any base shear unless it is positioned correctly.
Many consultants also check the bending in the wall and if it’s behaving as vertical cantilever, then consider it as a shear wall.
Position of a shear wall also matters?
Ideal position of a shear wall should be carefully decided in larger projects. Center of lateral mass and if the Center of stiffness matches the building won’t be subjected to TORSION when the ground moves. A wrong stiffness concentration around corner of a building can result in undesirable torsion in the building during earth quakes. The position needs to be finalized based on careful analysis. Deciding position of a shear wall is infact part of deciding the structural configuration which is one among the 4 virtues of seismic resistance.
Are the shear walls always around lift?
Most provides a shear wall around the lift since that’s a place where architects and clients are ok to give you a huge wall. However, if the lift is at a corner of a building, it’s not always useful. It induces torsion in the building in the initial modes and results in brittle failure of the building during earth quakes. Shear wall is not decided based on if there is a lift existing in a building project or not. You may need a shear wall in a building to resist lateral forces even if you don’t have a lift. Lift has nothing to do with the need of a shear wall.
And it is also not right to believe that shear walls are always around the lift.
Large opening and stairs around shear wall
If a wall is surrounded by cutouts, then it may not do the job a shear wall is supposed to do. This is since other parts of the building is not connected to the wall. The wall may not be effective in resisting lateral shear. This also results in horizontal irregularity. Though this is slightly a different structural point, if you want to read about it, I have a blog about it here.
How much is the minimum percentage in Shear wall?
Many asks, what is the minimum percentage of steel in a shear wall needed. Is it 0.12% as in a slab or 0.25% as in a wall or 0.8 % as in a column? The main point to state is, if your shear wall is needing only minimum % of steel, probably it is not attracting enough of forces and is not effective. Or it is oversized. Any ways a shear wall is to be treated as a vertical compressive member like a column carrying loads, moments and shear. It is never a slab.
Is shear wall needed to resist Wind?
Shear walls may be needed to resist wind loads too. Many thinks that it is only for resisting seismic forces. It is true that in many midrise buildings, seismic forces may govern unless the building is slender. However, if the building is slender or if it is tall, even wind can be critical and may be more critical than the seismic forces. So, the shear wall acts as a bracing for any lateral load.
Is shear wall needed for controlling deflection alone?
Controlling story drift and deflection is one of the major functions of a shear wall. Many times, you will need to provide shear walls, even if the deflections and drifts are with in limits with out a shear wall. This is when the column members sizes result to be massive in design or heavily reinforced and it becomes un economical or nonfunctional. By providing shear walls, other structural members are relieved from lateral forces there by its size, reinforcement or both can be reduced.
Shear wall design is similar to columns?
Shear wall is a vertical member carrying all forces similar to columns. The lateral shear will be much higher. Though the design procedure is interactive as in columns, there are many other careful considerations that needs to be adopted for shear wall design.
Shear wall types
There are different shear wall types too which I will write in some other blog. Shear wall types are decided based on various factors mainly the building seismic demand based on its configuration and height. Shear wall technology evolved over a period of time. Evolution of shear wall technology and various shear wall types can be a different blog topic and I will write about it in another blog some time.
Is shear wall needed for 2 to 4 floor buildings?
A shear wall may be needed even for a low-rise building depending on various factors. The factors deciding this is the seismic zone, number of columns and the position, size and orientation of columns. If the total lateral design shear is not resisted by columns, then shear walls are needed. Its true that we can plan and design a low-rise building by avoiding shear walls. However, it is not true that we can just rule out its requirement.
Does presence of shear wall avoid collapse of a building?
Its not a guarantee that a shear wall will avoid collapse of a building by all means. What we do is a seismic resistant design. We are designing only for a part of the actual expected seismic forces. If we try to make a building earth quake proof, it results in massive sizes for the structural members. Even if we want to do this earth quake proof design, we never know what the magnitude of actual earth quake will be. So, it is quite impossible to do a seismic proof design. If the actual seismic force during ground movement is more than the capacity of the members, it is likely that the structure will collapse. A careful design will only ensure that the collapse is gradual.
A shear wall is not just a piece of RC wall that can be called so by some ratio or size or shape or just by being around a lift core. A wall can be considered as a resisting shear, if it is attracting shear due to its stiffness and position and is designed to withstand shearing stresses, which are horizontal forces The purpose of a shear wall is to resist lateral forces as a vertical cantilever and relieve other column members from lateral forces.
Please comment your questions and share your views as comments in the blog section or discuss this in the forum.