Improving seismic performance

Minimum intervention involves utilizing the capabilities of the existing structure to the greatest extent possible. This might be achieved by improving its performance sufficiently by strengthening one or two individual members or connections rather than inserting a whole new structural system. But first, the adequacy of the strength, stiffness and ductility of all structural elements and connections in seismic

CoR

1

CoM

r"

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r

_CoR

Saw cut up wall

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▲ 12.6 By weakening the structural walls, the centre of rigidity (CoR) is brought closer to the centre of mass (CoM), reducing torsional eccentricity.

Rear wall rendered non-structural' to

Rear wall rendered non-structural' to

▲ 12.7 Y direction seismic forces are resisted by long masonry walls but new moment frames are required to resist x direction forces. Frame columns also provide out-of-plane resistance to the masonry walls.

Plan

▲ 12.7 Y direction seismic forces are resisted by long masonry walls but new moment frames are required to resist x direction forces. Frame columns also provide out-of-plane resistance to the masonry walls.

force paths require evaluation. Remember, a continuous force path is required for seismic forces acting in the direction of each of the two main orthogonal axes of a building (Chapter 2). Below is a typical list of force path components a structural engineer checks in detail in each direction:

• Strength of exterior and interior walls against out-of-plane forces (Chapter 2)

• Connections of those walls to diaphragms

• Connection of diaphragms to primary vertical structure like shear walls

• Primary vertical structure, such as shear walls, braced frames or moment frames (Chapter 5)

• Connection of vertical structure to the foundations, and

At this stage of structural review, the existing building is also checked for any horizontal or vertical configuration problems that might compromise its seismic performance (Chapters 8 and 9). Irregularities, such as diaphragm discontinuities in the form of large openings, significant torsional eccentricity or a soft-storey, may be too severe to deal with by merely improving what exists.

If the results of force path and configuration evaluations indicate that a strategy of improvement rather than renewal will be successful, then those members and connections found deficient are upgraded using one or more of the reasonably standard techniques outlined in the following section. Occasionally, some members may even warrant intentional weakening. For example, the torsional response of a building with very strong eccentric boundary walls might be improved by vertical saw cuts to 'soften' them up (Figs 12.6 and 12.7).

The approach of improving an existing structure is particularly appropriate for seismically sub-standard housing. Relatively minor upgrading, like bolting walls down to the foundation or increasing the bracing in the foundation crawl space by strengthening perimeter or cripple walls, is effective. Many publications by public bodies aimed at homeowners are available on the internet.11 At least one US city provides earthquake strengthening workshops, handbooks for homeowners, free house retrofit plan sets, a list of contactors who completed the city's home-retrofit Contractor Workshop, a construction tool lending library and limited financial assistance.I2

Plan

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