11-09-2017, 04:47 PM
Seismic adaptation of important structures is essential in seismic active zones. The importance is doubled when it comes to some buildings like schools, hospitals, bridges, etc., because it is required to continue their service even after a major earthquake. In general, seismic adaptation codes have paid little attention to the rehabilitation of foundations due to their construction complexity. In this paper, different methods for the seismic adaptation of high-rise foundations will be discussed and evaluated. Foundations are considered in three different categories. First, foundations are in danger of liquefaction of their underlying soil. Second, foundations located on slopes in active seismological regions. Third, foundations designed according to previous design codes and may show structural defects under earthquake loads. After describing the different methods used in different countries for the adaptation of existing foundations in the active seismological regions, a complete comparison between these methods is made with respect to the categories mentioned above.
Retrofitting of existing foundations involves changing the structural aspects of existing foundations and / or improving the condition of the underlying soil. In many parts of developing countries such as Pakistan, Iran, and China, unreinforced or, in some cases, reinforced masonry is the predominant form of residential and rural housing structures. Masonry was also a common form of construction in the early part of the 20th century, implying that a substantial number of these masonry structures at risk would have a significant patrimonial value. Masonry walls that are not reinforced are especially dangerous. Such structures may be more suitable for replacement than for retrofit, but if walls are the main load bearing elements in modest sized structures, they may be appropriately reinforced. It is especially important that the floor and ceiling beams are firmly attached to the walls. Additional vertical supports may be added in the form of steel or reinforced concrete.
In the western United States, much of what is seen as masonry is actually brick or stone veneer. Current construction rules dictate the amount of tie-back required, consisting of secured metal straps to vertical structural elements. These straps extend into the mortar courses, securing the sheet to the primary structure. Older structures may not secure enough for seismic safety. A weakly secured sheet in an interior of the house (sometimes used to face a fireplace from the floor to the ceiling) can be especially dangerous for occupants. Older masonry chimneys are also dangerous if they have a substantial vertical extension over the roof. These are prone to breakage in the roof line and can fall into the house in one large piece. For adaptation, additional supports may be added; however, it is extremely costly to reinforce an existing masonry fireplace to conform to contemporary design standards. It is best to simply remove the extension and replace it with lighter materials, with special metal chimneys replacing the fireplace and a wooden structure replacing the masonry. This can be paired against existing brick using very thin veneer (similar to a tile, but with the appearance of a brick).
Retrofitting of existing foundations involves changing the structural aspects of existing foundations and / or improving the condition of the underlying soil. In many parts of developing countries such as Pakistan, Iran, and China, unreinforced or, in some cases, reinforced masonry is the predominant form of residential and rural housing structures. Masonry was also a common form of construction in the early part of the 20th century, implying that a substantial number of these masonry structures at risk would have a significant patrimonial value. Masonry walls that are not reinforced are especially dangerous. Such structures may be more suitable for replacement than for retrofit, but if walls are the main load bearing elements in modest sized structures, they may be appropriately reinforced. It is especially important that the floor and ceiling beams are firmly attached to the walls. Additional vertical supports may be added in the form of steel or reinforced concrete.
In the western United States, much of what is seen as masonry is actually brick or stone veneer. Current construction rules dictate the amount of tie-back required, consisting of secured metal straps to vertical structural elements. These straps extend into the mortar courses, securing the sheet to the primary structure. Older structures may not secure enough for seismic safety. A weakly secured sheet in an interior of the house (sometimes used to face a fireplace from the floor to the ceiling) can be especially dangerous for occupants. Older masonry chimneys are also dangerous if they have a substantial vertical extension over the roof. These are prone to breakage in the roof line and can fall into the house in one large piece. For adaptation, additional supports may be added; however, it is extremely costly to reinforce an existing masonry fireplace to conform to contemporary design standards. It is best to simply remove the extension and replace it with lighter materials, with special metal chimneys replacing the fireplace and a wooden structure replacing the masonry. This can be paired against existing brick using very thin veneer (similar to a tile, but with the appearance of a brick).