CMU Building Seismic Retrofitting
Concrete Masonry Unit (CMU) buildings often consist of a reinforced concrete masonry unit perimeter walls and interior walls, with a wood or metal deck flexible diaphragm and/or rigid concrete diaphragm.
The cells of the masonry block unit typically are fully grouted in high seismic hazard and partially grouted in non-high seismic hazard. The exterior walls are commonly bearing, with an interior post and beam system of steel or wood. Older buildings of this type are generally small and used for a wide variety of occupancies.
Recently, CMU buildings have become commonly used for one-story warehouse and wholesale or retail occupancies.
Common Seismic Deficiencies in CMU Buildings
CMU buildings have performed relatively well in past earthquakes and are considered less hazardous to life safety. The primary deficiencies in CMU buildings are due to:
- Inadequate or missing shear and tension ties between the masonry bearing wall and the diaphragms. This can lead to loss of vertical support to the diaphragms.
- Inadequate bearing length for the girder seat or insufficient reinforcement at the top of the pilaster.
- Cross-grain bending of ledger under out-of-plane tension loading.
- Inadequate anchorage of brick veneer to the masonry unit, creating a falling hazard.
- Insufficient in-plane strength and stiffness of wood diaphragm.
- Inadequate chord capacity and stress concentrations at large-diaphragm openings and re-entrant corners.
- CMU walls having a fairly open street façade at the ground level, leading to a weak and soft first story and torsional irregularities.
Applicable Rehabilitation Technique
The strength of a CMU building is dependent primarily on the strength of the wall to diaphragm connection, and the in-plane capacity of the wood diaphragm. The seismic rehabilitation of a CMU building can be addressed in the following ways:
The connection between walls and diaphragms is the single most significant deficiency. This can be strengthened by adding tension ties between walls and diaphragms for out-of-plane forces and shear ties for in-plane forces along with the typical wall-diaphragm interface.
Wood diaphragm deficiency can be addressed by adding new interior shear elements to cut the diaphragm span or by enhancing the diaphragm itself with wood structural panel overlays.
Rehabilitation at re-entrant corners requires the provision of adequate chords and collectors, shear transfer to the in-set wall panels, and possibly the strengthening of the wall panels and connection to the foundations. A similar provision is required for large openings in the diaphragms.
The open façade in commercial buildings can be rehabilitated by adding new shear walls, enhancing existing shear, or the addition of steel braced frames.
Why Alpha Structural is Your Southern California Earthquake Retrofit Specialist
Alpha Structural has extensively studied the 2015 FEMA paper and the subsequent changes to building designs and resultant codes changes. Our engineers and construction specialists are uniquely qualified to assess your building’s roof and determine if additional seismic protection is required. Our seismic inspections and estimates are thorough and use the latest guidance to ensure your building will be protected from Southern California’s shifting landscape.
If our engineers determine that your building could benefit from seismic protection, our teams can provide you with an innovative retrofitting process that guides the diaphragm to the existing structure. This system connects the roof to the rigid wall in such a way that in the event of an earthquake, they move independently, giving the connectors more strength, security, and flexibility.
The Benefits of Value Engineering
At Alpha Structural, we believe that process matters. As a design-build company, Alpha Structural can provide you with end-to-end project management, from concept to construction to completion. This streamlined approach ensures that your project doesn’t get bogged down in consulting or approvals or inspections. We know exactly what we’re doing - and because of that, we can innovate in-project, reducing costs without sacrificing quality. This is the core of Value Engineering, and we believe it is what separates Alpha Structural from our competitors.
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