URM / UMB
In the early 1980’s, the State of California passed a state bill 547 for the seismic retrofitting of existing buildings for the purpose of Public Safety. The State of California mandated a program for all cities which are located in the Seismic Zone 4 of the USGS map and a voluntary program for the cities which are located in the Seismic Zone 3.
The first phase was concentrated on Un-reinforced Masonry Buildings which were built prior to 1933, had at least one un-reinforced masonry bearing wall and was either commercial or residential with 5 or more units. All the cities in Zone 4 adopted the state bill for these types of buildings.
Under the Public Safety bill, the owners may choose not to retrofit, as long as their buildings stay vacant.
The advantages of retrofitting are:
• Saving on Earthquake Insurance due to high premium and deductable amounts.
• Banks do not underwrite the loans on brick buildings unless they are retrofitted or the retrofitting is a condition of the loan in the event of a sale or refinancing.
The concept behind this Bill is very simple. In case of an earthquake, brick buildings may collapse due to a separation of wood framing of roof or floors which were simply inserted in the free-standing brick wall holes called Joist Pockets.
The seismic retrofitting can be summarize as:
• Anchoring every few feet around the perimeter of the building to the exterior walls at roof and all floor levels.
• Bracing the brick parapet to roof diaphragm.
• Stiffening the roof diaphragm by applying plywood over the existing roof sheathing.
• Installing shear elements on both directions of the building to carry the horizontal (earthquake) forces which are generated by an earthquake. These shear elements can vary from steel moment frame, gunite, shotcrete of the existing walls, additional concrete walls, to closing windows in brick walls.
Every Masonry (Brick) building is unique and the retrofitting should be tailored for its condition. Although the structural engineers may design for retrofitting, it takes an experienced hands on retrofit seismic engineer to design the building (value engineering) and build it economically.