Page 97 - Advanced Design Examples of Seismic Retrofit of Structures

Example of a Two-Story Unreinforced Masonry Building Chapter 2 89

2.8.2 Incremental Seismic Rehabilitation
As mentioned in the first part of this chapter, the present example deals with
retrofitting of a typical two-story unreinforced masonry building. Speaking
of school buildings, some challenges arise for their retrofit in addition to those
in ordinary residential buildings. Although vulnerable school buildings need to
be replaced with safe new construction or rehabilitated to correct deficiencies,
for many school districts new construction is limited (at times severely) by bud-
getary constraints, and seismic rehabilitation is expensive and disruptive. How-
ever, an innovative approach that phases a series of discrete rehabilitation
actions implemented over a period of several years, incremental seismic reha-
bilitation, is an effective, affordable, and nondisruptive strategy for responsible
mitigation action. It can be integrated efficiently into ongoing facility mainte-
nance and capital improvement operations to minimize cost and disruption. The
strategy of incremental seismic rehabilitation (ISR) makes it possible to start
now on improving earthquake safety in a school district [1].
The benefits of seismic rehabilitation of a building are [1]:
l reduced risk of death and injury of students, teachers, and staff;
l reduced building damage;
l reduced damage of school contents and equipment; and
l reduced disruption of the delivery of school services.
Incremental rehabilitation phases seismic rehabilitation into an ordered series of
discrete actions implemented over a period of several years, and whenever fea-
sible, these actions are timed to coincide with regularly scheduled repairs, main-
tenance, or capital improvements. Such an approach, if carefully planned,
engineered, and implemented, will ultimately achieve the full damage reduction
benefits of a more costly and disruptive single-stage seismic rehabilitation
(SSR) [1].
An initial prioritization of seismic rehabilitation increments should be estab-
lished primarily in terms of their respective impact on the overall earthquake
resistance of the structure. Facility managers will begin with these priorities
when determining the order of seismic rehabilitation increments to be under-
taken. However, the final order of increments may deviate from this priority
order depending on other planning parameters [1]. When following the ISR con-
cept, a “worst first” approach should be used. For example, in case of the URM
buildings, the walls usually pose a great danger to the overall stability of the
building; as a result, retrofitting of walls should be placed as a high priority.
In the next phases, the roofs and foundation may be considered for retrofit.
A generalized life-cycle benefit analysis shows that incremental approaches
can return a substantial portion of the expected benefits of single-stage seismic
rehabilitation carried out now. The schematic diagram shown in Fig. 2.44 illus-
trates such a life-cycle benefit analysis. The three wide arrows represent the
benefits of SSR occurring at three points in time: now, in 20years, and in