Page 186 - Dynamic Loading and Design of Structures

Page 161

of achieving two or three performance objectives: to resist minor earthquakes without
significant damage, moderate earthquakes with repairable damage, and major earthquakes
without collapse. However, as a rule, design checks are only explicitly performed for one
performance objective, typically for the ULS (corresponding to either life safety or no
collapse requirements). The Eurocodes recognize explicitly two limit states (ULS and SLS),
whereas, interestingly, the American code (ICBO, 1997) explicitly states that ‘the purpose of
the provisions is primarily to safeguard against major structural failures and loss of life, not to
limit damage or maintain function’.
One problem with all existing seismic codes is that their criteria for evaluating adequacy of
performance are not always directly tied to specific measures of performance. Moreover, the
actions used for checking these criteria are typically based on one design earthquake (the ULS
action); of course, as discussed in Section 4.3.4, in serviceability related checks the lower
intensity of the SLS earthquake is implicitly accounted for (v factor in EC8, resulting in the
SLS displacements being 1/2 to 1/2.5 the ULS displacements).
The intent of PBD is to ensure that structures perform at appropriate levels for all
earthquakes, and is deemed to provide engineers with the ability to design structures capable
of providing controlled and predictable performance for multiple performance objectives
(Fajfar and Krawinkler, 1997). The difficulty, of course, lies in the quantification of this
attractive concept.
One recent attempt to quantify performance levels and corresponding hazard levels has
been done in the new NEHRP Guidelines for Seismic Rehabilitation (strengthening) of
Buildings (FEMA 1997b). The hazard levels are expressed by probabilities of exceedance in
50 years, or the corresponding mean return periods T r; that is:

●50 per cent/50 year (T =72 year);
t
●20 per cent/50 year (T r=225 year);
●10 per cent/50 year (T r=475 year) ;
●2 per cent/50 year (T r=2,475 year).

Structural performance levels are quantified both in a qualitative sense (description of the type
of damage associated with each one) and in a quantitative sense. At building level a
convenient global measure of damage is the inter storey drift. Appropriate drift values are
suggested by FEMA (1997b) for various types of structural systems; significantly higher
values are applicable to flexible and ductile structures, such as frames, compared to stiffer and
more brittle structures, such as walls (particularly masonry walls). Requirements
(performance levels) are also included in FEMA (1997b) for non-structural elements
(partitions, cladding, mechanical and electrical installations, plumbing, contents and
finishings). It is pointed out that depending on the type of structure to be designed (or
assessed), different combinations of hazard and performance levels would be appropriate. For
a normal structure (e.g. an apartment building) the ‘immediate occupancy’ level would
normally be associated with an earthquake with a 50 per cent/50 year probability,