Structural Seismic Retrofit Design Guidelines

Introduction

The risk of earthquakes in California is high. Yet many buildings in the state are vulnerable to considerable damage from earthquakes. As you know, earthquakes cause negative impacts like death and extensive damage. To prevent this, all jurisdictions in the state allow voluntary seismic retrofitting. This is also known as structural strengthening. Some cities have mandatory structural retrofit programs, especially for buildings constructed before 1981.

Also, there are minimum structural seismic retrofit design standards that property developers should adopt. However, most building owners choose performance/ prescriptive structural retrofit standards for their buildings. The main aim of these guidelines is to reduce the risk of injury/ death from earthquake effects. They also promote public welfare from buildings that may contain structural deficiencies.

In this article, we’ll look at the basics of these structural retrofit design guidelines. Let’s get into it!

What buildings do these guidelines apply to?

These guidelines apply to concrete structures built before the 1979 Uniform Building Code. They also apply to Steel Moment Frame buildings constructed before December 1995. However, these structures may be exempted if they meet a more recent Building Code.

If a building is within this scope, the seismic retrofit shall be conducted. It should include every earthquake lateral resisting element in that building. Moreover, it may include; foundations, shear transfer connections, collectors/ chords, and diaphragms.

Exemption to these guidelines

There are some instances that a building may be exempted from these guidelines. For instance, the building may be exempted if it’s comprised of a wooden structure over a concrete podium. However, the design professional has to prove that the podium doesn’t have a major deficiency. Besides, if it’s within these guidelines, only the deficiencies will be considered.

Other structures that these guidelines may be exempted are;

• Unreinforced masonry structures previously retrofitted with Steel Moment Frames
• Single story concrete structures, if the lateral system doesn’t have a concrete moment frame.
• Residential wood-framed structures with Steel Moment Frames
• Concrete structures where all roof/ floor diaphragms are flexible. This also applies if the diaphragms consist of plywood, wood sheathing, or bare metal deck.
• Residential community developments

Does your building need a structural retrofit?

Again, these structural retrofit design guidelines apply to older existing Steel Moment Frame structures. They also apply to Non-Ductile concrete buildings. A non-ductile building is any concrete structure built under the 1976 Building Codes. However, it’s always good to ensure your building is earthquake-ready, even if it’s not in these categories.

A seismic retrofit investigation is done by design professionals. In most cases, this is a structural or civil engineer registered and licensed in California. The structural seismic retrofit design engineer shall investigate and perform a structural analysis of existing construction. This will determine if the structure is targeted by these guidelines. Also, they shall recommend the necessary structural alterations. This will ensure that the building meets all the minimum standards.

Major Deficiencies

These are the most important elements of an existing structure during an earthquake. An existing building can have various deficiencies, thus requiring a structural retrofit. They include;

• Load path deficiency. This type of deficiency happens when a building doesn’t have a well-defined load path. This affects the transfer of building elements’ inertial forces to the foundation. The load path connections must be sufficiently strong and complete. This will allow them to resist maximum loads delivered to them.
• Soft/ weak story deficiencies. It’s when the seismic-resisting system’s shear strengths are below 80% of an adjacent story.
• Torsion deficiency. It’s when the distance between the rigidity and mass Story center exceeds the building’s width by 20%.
• Vertical Irregularity deficiency. It’s when the seismic-resisting system’s vertical elements don’t extend to the foundation.
• Captive Column Deficiency. It happens when the height/ depth ratio of some columns is 50% less than the normal ratio of typical columns.

Retrofit plans for mitigating major deficiencies

Retrofit plans for mitigating major deficiencies include MTCAP results for the entire building. MTCAP stands for Material Testing & Condition Assessment Program. Before implementing the program, the structural retrofit engineer reviews the condition evaluation and test locations.

On top of that, a geotechnical report should be submitted. This report may include seismicity data, foundation recommendations, and geological data. This should be accompanied by structural calculations as well as associated computer models. Lastly, you need to submit construction documents associated with the major deficiencies retrofit.

Conclusion

Structural retrofits, especially in older buildings, are important in ensuring public safety. That’s why it’s good to determine if your home, property, or building needs a retrofit. To do that, reach out to the Building & Safety Department.

Alternatively, you can speak with industry experts like Innodez Design and Engineering. We have some of the best structural retrofit engineers in the entire state of California. If you want to learn more about structural retrofit design, call us now!