URM Building Seismic Retrofit
Unreinforced Masonry (URM) building consists of unreinforced bearing walls, usually at the perimeter and usually brick. Solid clay-brick unit masonry is the most common type of masonry unit. The floors and roof are typically of wood joists and wood sheathing supported on the walls and on interior post and beam construction. This building type is common throughout the United States, and was built for wide variety of uses, from one-story commercial or industrial occupancies to multistory warehouses to mid-rise hotels.
Common Seismic Deficiencies in URM Building.
URM buildings are considered to be one of the most hazardous building types. Significant property damage and loss of life have occurred in URM buildings during earthquakes around the world and United States. The primary deficiencies in URM buildings are due to
- Unbraced parapets and chimneys which can fail on adjacent pedestrian through-fares.
- Inadequate or missing shear and tension connections between the unreinforced masonry bearing wall and the wood floor or roof. This can lead to wall failure and loss of vertical support for diaphragms.
- Inadequate wood diaphragms strength and stiffness.
- Inadequate out-of-plane bending resistance of an unreinforced masonry wall.
- Inadequate unreinforced masonry in-plane wall strength due to window openings.
- Masonry walls having 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 URM building is dependent on the in-plane shear capacity of walls, out-of-plane bending capacity of the walls, and strength of wall to diaphragm connection. The seismic rehabilitation of URM building can be addressed in the following ways,
- URM wall capacity can be enhanced by adding concrete or shortcrete overlays. Steel moment frames can be added for wide open storefronts.
- Wood diaphragms 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.
- Walls susceptible to out-of-plane bending can strengthen by strongbacks placed against them either on the outside or more commonly on the interior face.
- 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 force.
- URM parapets can be braced or removed to minimize the falling hazard. Bracing is usually done with steel angle brace.
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.