One of the biggest reasons as to why a building collapses due to seismic activity, is because of its inability to counter lateral loads. Particularly susceptible, are buildings whose lowest or lower stories are not properly reinforced to counter the lateral forces produced by phenomena such as earthquakes. These types of buildings are known as Soft Story buildings/structures. The soft stories in these buildings are usually the parking lots, lobbies, ground floor units etc. – large openings that aren’t always fully reinforced. To counter this vulnerability, civil engineers and architects can incorporate structural strengthening that can reinforce existing buildings and reduce the pressure induced by lateral forces. This process is known as a Soft Story retrofit.
Structures must be designed considering the impact of lateral loads. Lateral loads are essentially forces that act across a structure parallel to the ground. Examples of lateral forces can include wind loads, seismic loads or water pressures; however, lateral loads are not very easy to predict. An inherently complex topic, the prediction of the spread of lateral loads across designed structures is difficult to assess when compared with vertical loads. This article will cover the basics surrounding common lateral force resisting systems and how they can be used for soft story retrofits.
Now that we have a basic understanding about what lateral loads and soft story retrofits are, let’s take a look at the basic types of lateral force resisting systems:
Types of Lateral Force Resisting Systems (LFRS):
Engineering teams will want to design a lateral force resisting system that is both effective and inexpensive. Each lateral force resisting system has some specific advantages and disadvantages:
1. Moment resisting frames: This type of LFRS is made up of columns and beams that resist lateral loads by having the members of the structure flex under pressure and via the stiffness of joints connecting the columns and beams.
- Can be modified for various types of architectural design and/or layouts.
- Can drift and deflect more when compared to other types of LFRS.
- Joints can become points of accumulated stress.
- Precise construction of the frame is essential to resist lateral loads effectively.
- Can be expensive.
2. Braced frames: This type of LFRS uses a combination of trusses made up of steel beams/members. Due to the nature of construction of trusses – diagonal beams connecting parallel beams - lateral loads get transferred into axial stresses via compression or tension. There are two basic types of braced frames: concentric and eccentric.
Concentric frames have beams connecting diagonally at the ends of elements. This develops truss action, creating stiffness that can resist lateral loads.
Eccentrically braced frames use beams to support axial beams eccentrically (not connected to the joint). This essentially creates a frame with increased flexibility.
Can be located internally or externally to provide flexibility to architectural design
Accommodates service penetrations
Can be located within partition walls
No need for moment connections
- Produces problems for layout of windows and doors due to obstruction by bracings
- Requires fireproofing materials for steel members which takes up space
- Need for large gusset plates for bracing connections between the beams and columns
3. Shear walls: Shear walls resist lateral forces due to cantilever action principle. Shear walls must be symmetrically placed in any design. This is done to remove or minimize torsional effects produced by lateral forces.
This type of LFRS is usually constructed using concrete or stonework. Supporting beams can provide vertical reinforcement and reduce flexure; conversely, gaps, spaces and holes in shear walls are the weakest points in a shear wall.
Shear walls can be used for other purposes apart from load bearing – they can be used as stairs or walls in compartment areas for fire, sanitation or maintenance, or even in lift shafts etc.
They are stiff and are inherently resilient structures when placed under heavy loads.
If placed in a symmetrical manner, shear walls can provide torsional and rotational resistance in designed structures.
Shear walls can be considered relatively cost-effective as there are usually fewer required to be constructed for a safe design.
Are usually used in conjunction with design and layout of the structure and are therefore part of the structure’s aesthetic.
- Considering shear walls are used to support entire sections of buildings, stress can be pronounced in this LFRS.
- The presence of gaps, openings, and spaces can result in lower levels of strength and stiffness.
- The shear walls themselves can weigh a lot. This weight can put pressure on the foundations of the buildings.
The table below compares the three types of Lateral Force Resisting Systems based on a few features:
|Feature||Moment Resisting Frames||Braced Frames||Shear Walls|
|Strength||Moderately Strong||Moderately Strong||Strong|
|Suitability for Design||Very Suitable||Slightly Suitable||Not Very Suitable|
Soft story retrofits can usually incorporate any combination of the above-mentioned Lateral Force Resisting Structures. These retrofits can be expensive and, for some larger buildings, can cost upwards of $100,000. While the upfront cost can be daunting, retrofits can help save on more expensive damages due to seismic events, and needless to say, be lifesaving. In some places across the state of California, buildings need to be assessed for their structural integrity. In the case of apparent structural weaknesses, building owners are urged to make the necessary changes or incorporate a retrofit. In the event building owners decide not to, clear warnings must be given to guests and/or tenants, indicating that the building is likely unsafe during an earthquake. In other areas, fines and tariffs encourage owners to have a retrofit done for buildings constructed after 1978. You can find out more about this ordinance here.
A soft story retrofit can be a lifesaving, and in the long term, a cost-effective way to protect your building, your tenants, and yourself from the dangers of unexpected seismic activity as well as municipal scrutiny. Have your building assessed for structural resilience and, if you find that you may require a soft story retrofit done, get in touch with Design Everest. Call us at 877-704-5687 to get a free consultation and learn more about soft story retrofits, and we can have the experts on our team help protect your building.
-Anne R, past client with a seismic retrofit project
*Note: The content published above was made in collaboration with members of Design Everest.