Page 17 - Advanced Design Examples of Seismic Retrofit of Structures

8 Advanced Design Examples of Seismic Retrofit of Structures

the structure in question. It is thus an extensive task to develop guidelines for
such selection. Conceptual design techniques, on the other hand, can be system-
atically categorized and design strategies formulated.
The primary focus of determining a viable retrofit scheme is on vertically
oriented systems because of their significance in providing either lateral stabil-
ity or gravity load resistance. Deficiencies in vertical elements are caused by
excessive inter-story deformations that create either unacceptable force or
deformation demands.
Given an initial understanding of the importance of the nontechnical issues
described in the previous section, alternate retrofit schemes can be developed.
Retrofit actions can be classified into three types:
l connectivity, consisting of ensuring that individual elements do not become
detached and fall, ensuring a complete load path, and ensuring that the mod-
eled force distributions can occur;
l modification of global behavior, usually decreasing deformations; and
l modification of local behavior, usually increasing deformation capacity.
These three types of actions balance one another in that employing more of one
will mean less of another is needed. It is obvious that providing added global
stiffness will require less local deformation capacity, but it is often less obvious
that careful placement of new lateral elements may minimize a connectivity
issue such as a diaphragm deficiency.

1.5.1 Connectivity
Connectivity deficiencies are within the load path: wall out-of-plane connection
to diaphragms; connection of diaphragm to vertical elements; connection of
vertical elements to foundation; and connection of foundation to soil.
A complete load path of some minimum strength is always required, so connec-
tivity deficiencies are usually a matter of degree. A building with a complete,
but relatively weak or brittle load path might be a candidate for seismic isolation
design to keep the superstructure in the elastic range. Yielding in connections
within the basic load path can create profound complications on the overall
building model. An early decision has to be made concerning modeling such
behavior or preventing it by reducing demand or strengthening of the local con-
nections. Demand can be reduced by adding vertical load resisting elements
(reducing individual collector or foundation loads) or by seismic isolation.

1.5.2 Improvement of Global Behavior

Modification to global behavior normally focuses on deformation. Overall seis-
mic deformation demand can be reduced by adding stiffness in the form of shear
walls or braced frames. A significant period shift is normally required to protect
deformation sensitive elements in this way. New elements may be added, or