Shear Walls for Soft-Story Retrofitting

Reinforcement concrete shear walls

Shear walls are one of the most useful lateral load resisting systems in multistoried buildings. Shear walls can be provided in buildings with soft-story to resist the lateral load induced by an earthquake. It is highly recommended by earthquake engineers to use shear walls for soft-story retrofitting.
Shear wall is a structural member used to resist lateral forces i.e. parallel to the plane of the wall. For slender walls where the bending deformation is more, Shear wall resists the loads due to Cantilever Action. In other words, Shear walls are vertical elements of the horizontal force resisting system.

Soft-Story Building

A soft-story building is a multi-level structure placed over a ground level with relatively great size, in which and often more floors have windows, traditionally wide doors, some commercial areas where shear walls can normally be required for firmness used to be directed to earthquake engineering design.
The weakest parts of the buildings are structurally reinforced to resist seismic movement during earthquakes and so often l-beans and structural steel connectors are used. They mostly are used to carry strengthen the weak sections of structures.

Reinforced Concrete Buildings

Reinforced concrete or RC buildings often have vertical plate-like RC walls called Shear Walls in addition to beams, columns, and slabs. These walls generally start at the foundation level and are continuous throughout the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise buildings. Shear walls are usually provided along both the length and width of buildings. Shear walls are like vertically-oriented wide beams that carry earthquake loads downwards to the foundation.
Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes.

Shear Walls in Seismic Regions

Shear walls in high seismic regions require special detailing. However, in past earthquakes, even buildings with a sufficient amount of walls that were not especially detailed for seismic performance (but had enough well-distributed reinforcement) were saved from collapse. Shear wall buildings are a popular choice in many earthquake-prone countries, like the United States and others. Shear walls are easy to construct, because reinforcement detailing of walls is relatively straight-forward and therefore easily implemented at the site. Shear walls are efficient, both in terms of construction cost and effectiveness in minimizing earthquake damage in structural and non-structural elements, such as glass windows and building contents.

Shear walls are especially important in high-rise buildings subject to lateral wind and seismic forces. Generally, shear walls are either plane or flanged in section, while core walls consist of channel sections. They also provide adequate strength and stiffness to control lateral displacements.
The shape and plan position of the shear wall influences the behavior of the structure considerably. Structurally, the best position for the shear walls is in the center of each half of the building. This is rarely practical since it also utilizes the space a lot, so they are positioned at the ends. It is better to use walls with no openings in them. So, usually, the walls around lift shafts and stairwells are used. Also, walls on the sides of buildings that have no windows can be used.


Soft-Story Buildings With Shear Walls

Most reinforced concrete soft-story buildings with shear walls have columns that primarily carry gravity loads. Shear walls provide large stiffness and strength to buildings in the direction of their orientation, which significantly reduces the lateral sway of the building and thereby reduces damage to the structure as well as its contents. Since shear walls carry large horizontal earthquake forces, the overturning effects on them are large. Therefore, the design of their foundations requires special attention. Shear walls should be provided along preferably both length and width. However, if they are provided along only one direction, a proper grid of beams and columns in the vertical plane must be provided along the other direction to resist strong earthquake effects. Door or window openings can be provided in shear walls, but their size must be small to ensure the least interruption to force flow through walls. Moreover, openings should be symmetrically located. Special design checks are required to ensure that the net cross-sectional area of a wall at an opening is sufficient to carry the horizontal earthquake force. Shear walls in buildings must be symmetrically located in plan to reduce ill-effects of twist in buildings. They could be placed symmetrically along one or both directions in plan. Shear walls are more effective when located along the exterior perimeter of the building – such a layout increases the resistance of the building to twisting.

Ductile Design of Shear Walls

Just like reinforced concrete beams and columns, ductile design shear walls can perform much better. Overall geometric proportions of the wall, types and amount of reinforcement, and connection with remaining elements in the building help in improving the ductility of walls.
However, The city of Los Angeles Department of Building and Safety has designed the Non-Ductile Concrete Retrofit Program aiming to help reduce injury or loss of life from the effects of an earthquake on non-ductile concrete properties and structures. The recent earthquakes all over the world have shown the weakness of such kind of structures and the devastating loss of lives because of them. In California, non-ductile concrete buildings that were constructed prior to the Los Angeles City Buildings Code provisions are at risk of collapse and pose notable life safety hazards.

Ductile concrete building

Non-Ductile Concrete Retrofit Program

The non-ductile concrete retrofit program provides a guide for property owners to meet the less standard in improving the performance of these kinds of structures. Non-ductile concrete buildings are considered dangerous for two main reasons:

  • Most of them were built before 1977 (before seismic building codes were created)

  • The concrete used in their frames has become brittle over time and it is at risk of cracking and collapsing during any seismic activity.

The floors and roofs of most of these buildings have made of concrete that is only supported by the concrete walls. There is a big chance, in case any of the floor or walls crack, the structure can completely and quickly collapse. Unfortunately, non-ductile concrete buildings are built across the world and they are considered to be the most dangerous structures which lead to more fatalities during an earthquake, than any other.

When the city of Los Angeles presented its mandatory seismic retrofit program, most of the non-ductile concrete structures were involved. Over thousands of these types of buildings are located all around the city - those are the buildings that most likely can collapse on themselves in an earthquake event. Retrofit contractors are needed to make the target structures safer and more resistant to seismic activities.