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Tilt-Up

Non-Ductile

 

Soft Story

The City of Los Angeles recently passed Ordinance 183893, which requires the retrofit of pre-1978 wood-frame soft-story buildings and non-ductile concrete buildings. The goal of the mandatory retrofit programs, under the ordinance, is to reduce these structural deficiencies and improve the performance of these buildings during earthquakes. Without proper strengthening, these vulnerable buildings may be subjected to structural failure during and/or after an earthquake.

The City of San Francisco Mandatory Soft Story Retrofit Program (MSSP) was created in 2013 as a multi-year community-based effort led by the Earthquake safety Implementation Program and enforced by the Department of Building Inspection to ensure the safety and resilience of San Francisco's housing stock through the retrofit of older, wood-framed, multi-family buildings with a soft-story condition.

 

UMB/URM

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

 

Tilt-Up/Non-Ductile

The Tilt-Up Buildings are referred to as Non-Ductile Concrete Buildings, mostly one or two story, where the exterior walls consist of concrete panels and the roof and floor diaphragms are wood members. The popularity of this kind of building is because the construction cost is low with the sub-standard roof system. They mostly serve as warehouses or industrial buildings. Recently due to architectural influence you observe the tilt up buildings as office buildings, car dealerships, wholesale stores such as Costco, Wall-Mart, etc. The tilt up buildings which were constructed prior to 1976 can be categorized as non-ductile concrete buildings which are subject to seismic retrofitting in most cities in Southern California. The seismic retrofitting of these types of buildings are a straight forward procedure. Since the concrete walls are attached to the slab on grade and the foundation, therefore it is necessary to tie the top of the wall to floor and roof diaphragms. This can be achieved by anchoring the concrete walls to floor or roof diaphragm and also tying the opposite walls together thru the floor and roof systems. The cost of retrofitting is low. It is actually based on the number of connections at the ceiling level. There is no need for relocation of the tenant for seismic retrofitting. The connections can be done under the roof at the ceiling line. There is no need to disturb the roofing materials. Consult with a Structural (Seismic Retrofit) Engineer for proper design and Value Engineering and avoid the need for a plywood diaphragm nailing check which would disturb the roof.

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