Seismic performance assessment of a conventional Multi-storey building

Giuseppe marcantonio del gobbo’-Martin S Williams –Anthony bakebotough’

Recent earthquakes such as the 2010 ‘Canter bury earthquake’ and ‘chile earthquake’
have demonstrated that buildings that incur minimal structural damage frequently experience
extensive nonstructural damage (Dha- kal 2010; Miranda et al). These systems are essential
to building functions and comprise the majority of building investment. An improved
understanding of the expected structural and nonstructural seismic performance of
conventional code-compliant buildings is therefore needed. This study assesses the seismic
performance of a structural design that is representative of conventional buildings constructed
in seismic regions. The results of the performance assessment provide a benchmark on which
to evaluate retrofit alternatives for existing buildings or design options for new structures. A
case study building was designed to represent conventional structures in seismic regions. A
finite element model of the design was created in OpenSees (PEER 2015) to determine the
nonlinear response of the structure. A FEMA P-58 (ATC2012) building performance model
was produced to convert the structural analysis results into probable seismic performance.
Nonlinear time history analyses of the finite element model were conducted using the ground
motion suites.The results of the seismic performance assessment were calculated and
analyzed.

By evaluating the seismic performance of the case study building, conclusions can be
inferred about the seismic performance of conventional structures with similar designs. It is
useful for an example of possible structural and nonstructural damage distributions to be
determined in detail. The case study structure is a 16-storey steel office building. This PGA is
bounded by the two strongest seismic zones in Greece and in Turkey. Case Study Building
2D model of the structure was created in the finite element program OpenSees (PEER
2015).The first period was elongated to account for anticipated brace buckling, mitigating the
generation of artificial damping forces (Charney 2008).As the lateral load resisting system of
the structure is CBFs, the behavior of the braces will control the response of the building
during earthquakes. The braces were model using the procedure proposed by Uriz et al.Brace
buckling in compression and yielding in tension are accurately predicted, capturing the
hysteretic behavior. Ground Motion Scaling and Selection Suitable earthquake ground
motion records were selected in order to investigate the seismic response of the case study
building. Nonlinear time history analysis of the structural model can then be per- formed
using these ground motions. Records were obtained from the PEER ground motion database
(PEER 2013).Ground motion suites representing the ultimate limit state (ULS) and the
serviceability limit state (SLS) as defined by Eurocode 8 were created. A linear scale factor
was applied to each record, which minimized the mean squared error (MSE) between the
ground motion spectrum and the target Eurocode spectrum over the period range of 0.2 7, to
2 T, Figure 4 compares the ground motion suite spectrum and the Eurocode 8 elastic response
spectrum for both the ULS and SLS.

The FEMA P-58 performance assessment procedure (ATC 2012) was used to
evaluate the seismic performance of the case study building. Seismic performance is
expressed by probable repair costs. Each type of building component is associated with
fragility functions and repair cost functions. Table 2 provides a summary of the critical
fragility information used in the case study from FEMA P-58 (ATC 2012). Peak structural
response parameters from the time history analyses are used in combination with fragility
functions to determine damage states for the building components. FEMA P-58 enables the
seismic performance of a structure to be measured in repair costs rather than a set of
structural parameters. A nonlinear time history analysis of the case study building model was
conducted in OpenSees for each of the 50 ground motion records for both the ULS and SLS.
Peak absolute floor velocity results were omitted as the mean values are comparable over the
building height, with a ULS mean of 4.2 m/s and a SLS mean of 2.5 m/s. Figure 5 shows that
the ULS results have a greater spread than the SLS results. This result coincides with the
significant nonlinear behavior exhibited by the braces during the larger intensity ULS
earthquakes. The same ground motion record of Imperial Valley-02 1940 was used.

The results of the seismic performance assessment are expressed by repair costs in
2011 US dollars. Direct repair costs resulting from damage to building assets are calculated,
while indirect costs due to building downtime are out of scope. Although the indirect costs
are significant, they are difficult to accurately assess for a case study building. Cumulative
distribution functions of the ULS and SLS total repair costs for the case study building are
shown in Fig. The ULS repair costs are greater than the SLS repair costs, as expected, due to
the difference in earthquake intensity. However, the assessment results indicate that extensive
SLS damage is expected. These results imply it is probable that similar buildings designed to
current structural codes may be demolished and replaced following a ULS earthquake. Repair
costs from the seismic performance assessment were grouped based on the EDP that
generated the damage. The mean repair costs of the case study building considering the
related EDP are shown in Fig. Acceleration sensitive damage and drift-sensitive damage
comprise the majority of repair costs for both the ULS and SLS. Fifty three percent of the
ULS repair costs are drift-sensitive and 40% are acceleration-sensitive, while IDR and
acceleration each account for 42% of the SLS repair costs. In order to further investigate the
relationship between EDPs and repair costs, mean repair costs were calculated for each floor
in the case study building. The levels of acceleration-sensitive and velocity-sensitive repair
costs on each office floor are comparable. The total repair cost for the building is the sum of
the damage experienced by all fragility groups. The mean repair costs were arranged by the
responsible structural and nonstructural fragility groups. Figure 10 indicates that significant
repair costs are produced by the fragility groups of structural components. That the structural
system has the greatest repair cost of the fragility groups. The structural system accounts for
26% of the total repair cost. Structural damage is anticipated for the ULS event as
conventional seismic design relies on structural members experiencing inelastic
deformations. However, 74% of the ULS repair costs can be attributed to these systems.
Nonstructural systems account for 87% of the SLS repair costs.

The case study exposed limitations of the Eurocode-8 damage mitigation

methodology. Eurocode-8 Cl 4.4.3 provides damage limitation requirements as drift limits
based on the nonstructural systems of the building. The most stringent requirement to protect
nonstructural systems is a maximum allowable inter storey drift of 0.5% during the SLS. The
0.5% drift limit was inadequate in preventing to drift-sensitive nonstructural systems in the
case study building. If the 0.5% drift limit is respected, a wall partition is still expected to
sustain damage. It is expected that the nonstructural repair costs would increase for buildings
with more vulnerable systems. In addition to the drift-sensitive costs, extensive acceleration-
sensitive nonstructural repair costs were also generated. These results highlight the need for
further review of the Eurocode nonstructural provisions.

This study assesses the seismic performance of a structural design that is

characteristic of conventional multi-storey buildings constructed in seismic regions. The
FEMA P-58 procedure was used to determine the expected earthquake repair costs for a
modern Eurocode CBF structure considering nonstructural systems. The performance
assessment indicated it is probable that similar buildings designed to modem struc tural
standards may be demolished and replaced following a ULS earthquake due to the high repair
costs. The substantial repair costs for the SLS scenario were concerning because damage
should be limited during this more frequent event. These results suggest that modem building
standards do not accomplish earthquake resilience: the ability of a community to recover
quickly after an earthquake. Limitations of Eurocode damage mitigation methodology were
revealed in a novel manner using FEMA P-58.The prescribed 0.5% drift limit to minimize
nonstructural damage did not prevent the generation of large nonstructural repair The
nonstructural systems used in the case study are of the highest seismic design category-repair
costs would increase for more vulnerable systems. Extensive acceleration-sensitive
nonstructural repair costs were also generated, as accelerations are only considered in
Eurocode 8 for calculations of anchorage strength. These results draw attention to the need
for structural design procedures that enhance nonstructural seismic performance. This shift in
seismic design philosophy is required in order to minimize the societal impacts of
earthquakes and achieve a rapid return to building occupancy after an Fragility groups that
significantly contribute to repair costs were identified as structural components, glass curtain
wall cladding, wall partitions, suspended ceilings, HVAC equipment, and office equipment.
This novel finding will enable future seismic performance assessments to include only the
fragility groups critical to repair costs in order to decrease computational effort. The results
of the seismic performance assessment provide a benchmark on which to evaluate alternative
building designs. The final aim of the project is to evaluate fluid viscous damper placement
strategies that minimize nonstructural repair costs and building service disruptions. The
capability of fluid viscous dampers to improve nonstructural seismic performance will be
assessed and damper placement optimization will be explored.