In 2015, the Los Angeles Municipal Council enacted Ordinance No. 183893, which introduced a mandatory earthquake retrofit program for wood-frame soft-story and non-ductile concrete buildings. If you own or manage a concrete building erected before 1977, chances are you’ve heard about this program, and if you’ve already received an Order to Comply from the city, you may be wondering what to do next.
One of the first things you have to do is submit a completed Non-Ductile Building Checklist to the City. The guide below walks you through the checklist and explains its various sections and requirements. Some of these points may seem a bit complicated at first; you can choose to hire an experienced engineer to help you with the same.
By following this section of the checklist, engineers can establish some basic information related to the construction of the building.
The first item inquires whether the building was constructed to the 1977 Building Code. If this is the case, the building may be exempt from the Earthquake Retrofit ordinance, but adequate documentation will have to be provided to support this claim.
Further, this section identifies the materiality of the vertical and horizontal structural members. As the Earthquake Retrofit ordinance is specifically aimed at buildings with an all-concrete structure, evidence of structural concrete walls and columns will confirm that the building is indeed non-ductile.
Lateral force resisting system
Using the checklist, engineers can assess the building’s ability to resist seismic lateral forces. The checklist identifies moment frames and shear walls, as well as diaphragms - the horizontal members that transfer seismic loads to their vertical counterparts.
The Non-Ductile Checklist also aims to pinpoint structural irregularities that may act as failure mechanisms during a seismic event. These include weaknesses in the configuration of the horizontal and vertical lateral force-resisting elements, as well as the uneven mass distribution between the stories of a building.
When a building is subjected to lateral forces, its stories “drift”, or displace relative to the stories above, below, and the ground. A torsional irregularity exists when one section of a story is expected to drift more than average for the story in question. This condition can be rectified by increasing the lateral stiffness along the line of columns where drift is expected to exceed the average.
Reentrant corner irregularity
This irregularity is present in T-, L-, H, and U-shaped buildings at the point where the horizontal diaphragms of each wing connect. During earthquakes, the wings’ capacities for resisting inertia forces are different, and this condition can cause damage to the horizontal diaphragms of the building. Engineers can remedy the condition by adjusting the relative stiffness of each wing, or by separating the wings into independent structures.
Diaphragm discontinuity irregularity
Diaphragms are horizontal structural elements that transfer lateral forces to the vertical resisting members, such as shear walls. In concrete buildings, these are the slabs that make up every floor. Diaphragms are often penetrated to accommodate stairwells and MEP services; if the penetrations are too large, the diaphragm may not be able to transfer lateral seismic loads to the shear walls.
Out-of-plane offset irregularity
An out-of-plane offset happens when a lateral force-resisting element such as a shear wall is on a different vertical plane than its counterpart on an adjacent story. This condition causes a discontinuity of the lateral load path and weakens the structure’s ability to resist earthquakes. If this weakness is present in your building, an engineer may recommend installing beams, known as collectors, to transfer lateral forces from one resisting element to another.
Non-parallel system irregularity
A non-parallel system irregularity exists if your building’s vertical lateral force-resisting elements, such as shear walls or moment frames, are not symmetric or parallel with the building’s main orthogonal axes (aka the X-Y axes). This configuration can lead to torsion and localized damage during earthquakes and may be difficult to address in existing buildings without significant alterations to the structure.
One of the most common and dangerous conditions, a soft-story compromises the stiffness of the building’s ground story and leaves the building prone to collapse during earthquakes. This weakness is often caused by tuck-under parking lots, large lobbies, or open retail spaces on the ground level of a building. If a soft-story condition is identified during the structural assessment, you will have to reinforce the weak wall line. Shear walls, braced walls, or moment frames may be a suitable solution.
A weight irregularity exists where the weight of one story exceeds the weight of an adjacent one by 150%. When an earthquake strikes a building with a weight irregularity, the building’s gravity-resisting elements may be weakened by the earthquake’s lateral forces. The engineer performing the structural evaluation will recommend the best course of action if this condition is found in your building, but an even distribution of live loads across the floors may resolve the problem.
Possible vertical geometric irregularity
A vertical geometric irregularity is present when the horizontal dimension of the seismic-force-resisting system is 130% greater than that of an adjacent story, resulting in a vertical setback. If this condition exists in your building, engineers may add redundancy to the lateral resistance system.
In-plane discontinuity in vertical lateral force-resisting element irregularity
An in-plane discontinuity refers to the condition produced when vertical lateral force-resisting members are on the same horizontal plane but do not align vertically. In this situation, the lateral load path must be continued through collectors.
Weak story irregularity
Where a soft-story is less stiff, or more flexible, than those above, a weak story has less strength. A weak story may be on the ground floor or at the intermediate levels of a building. As with a soft story, this condition leaves buildings vulnerable to collapse during earthquakes and requires reinforcement with shear walls, braced walls, or moment frames.
Checklist submittal and exemptions
If the city sent you an Order to Comply, you must return the completed Non-Ductile Checklist within 3 years of receipt of the order. If your building was constructed to 1977 or later Los Angeles Building Code, and you believe it should be exempt from the retrofit requirements, you have to submit a building permit copy showing an application date after January 13th, 1977. Likewise, you may get an exemption by producing the building’s original construction drawings or schematic diagrams, if they show that construction did not include concrete floors and/or roofs supported by concrete walls, columns, or frames with or without masonry infills, or any combination thereof. This will be followed by the city carrying out a thorough inspection of your building, usually visual and non-destructive in nature, to confirm the building was built as per the schematic drawings.
Having submitted the Non-Ductile Checklist, you can move on to the next steps of compliance. Within 10 years of receiving an Order to Comply, you must hire a licensed civil or structural engineer to perform a comprehensive evaluation of the building to establish whether it meets or exceeds the requirements of the Earthquake Retrofit ordinance. If the evaluation determines that the building does not meet the ordinance standards, you need to present plans for an adequate alteration to the structure, or plans for demolition. The retrofit or demolition must be completed within 25 years of receiving the Order to Comply.
We can help
Have you received an Order to Comply from the city? If you’re not sure where to start, our team can help. The engineers at Design Everest are licensed and experienced and can guide you through the retrofit process from start to finish. If you believe that your building should be exempt from the Retrofit Ordinance, we can help you gather the necessary evidence and documents to prove your claim. Call us at (877) 704-5727 or email us at email@example.com to discuss your project and receive a quote today.
*Note: The content published above was made in collaboration with members of Design Everest.