FRP
FRP materials have been introduced over 50 years in construction industry. The use of FRP in advanced new reinforced concrete beams and columns is superior to traditional retrofitting due to possession of high strength capacity and corrosion resistance and can be employed as the main reinforcements in combination with adhesives and anchorages. Also, the FRP composites can be used to recover the strength of damaged and corroded beams and columns with good durability and insulation performance.. The common problems for reinforced concrete beams and columns are loss of strength. These elements are needed to be strengthened to increase their flexural strength, shear strength, fatigue life, and seismic resistance. Beams and columns should be strengthened after a certain period of loading because damage may well be in the form of structural damage or corrosion. The thin polymer layer in FRP may comprise different fibers. Various typical materials are glass, carbon, aramid and basalt.
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.
