2017 SEAOC CONVENTION PROCEEDINGS
1
Prestandard for Seismic Assessment and Retrofit of One- and Two-Family Dwellings (ATC-110 Project)
Colin Blaney, Structural Engineer,
Buehler & Buehler, San Carlos, California Kelly Cobeen, Structural Engineer
Wiss Janney Elstner Assoc., Emeryville, California Andre Filiatrault, Professor
State University of New York, Buffalo, New York David P. Welch, Post-Doctoral Researcher
Stanford University, Stanford, California Michael Stoner, Graduate Student
Clemson University, Clemson, South Carolina Weichiang Pang, Professor
Clemson University, Clemson, South Carolina Taylor Vincent, Graduate Student
Washington State University, Pullman, Washington
Abstract The California Earthquake Authority (CEA) and the Federal
Emergency Management Agency (FEMA), through the
Applied Technology Council’s ATC-110 Project series, have
sponsored development of a prestandard for seismic
assessment and retrofit of cripple wall, house-over-garage, and
hillside vulnerabilities in residential dwellings. Although
simplified for the purposes of implementation, methodologies
are being developed using best available numerical tools and
performance objectives consistent with the philosophies of
current seismic codes and standards. After more than two
years of work, the project has generated retrofit concepts,
preliminary retrofit criteria, and an extensive set of nonlinear
analysis results for wood light-frame dwellings. The project is
currently developing prestandard provisions and prescriptive
plan sets for assessment and retrofit. This paper discusses the
project methodology, illustrates retrofit approaches and
interim numerical study results, and shares insights gained.
Introduction
Wood light-frame buildings are the most common type of
dwelling in the United States. Although generally providing
good performance in past earthquakes, there are well-known
vulnerabilities in wood light-frame dwellings that have led to
a notable number being rendered uninhabitable or even
unrepairable following earthquakes. Improved seismic design
and seismic retrofitting of vulnerable configurations will
increase the probability that a high percentage of homes are
available to provide shelter immediately following moderate
to large seismic events. Current model building codes and
available seismic retrofit standards do not adequately address
the specifics of assessing and retrofitting light-frame one- and
two-family wood dwellings. The ATC-110 project has
undertaken development of a prestandard addressing practical
assessment and retrofit of common seismic vulnerabilities in
wood light-frame dwellings, with significant emphasis put on
practicality and the intent to encourage wide spread
implementation.
Overall Objectives and Methodology
The prestandard being developed by the ATC-110 Project is
intended to provide a single stand-alone resource for
addressing assessment and retrofit of selected structural and
nonstructural seismic vulnerabilities. Both engineered and
prescriptive retrofit design methodologies are being developed
for all vulnerabilities being addressed. The prescriptive
methods will address dwellings that fall within defined limits
of applicability; the engineered methods will be available for
dwellings falling outside of those limits. It is anticipated that
the prestandard will be subject to further development in an
ANSI-approved consensus standard process, after which it will
be made available as a standard.
2017 SEAOC CONVENTION PROCEEDINGS
2
In addition, plan sets containing prescriptive retrofit design
methodologies are being developed where practical. The plan
set designs are derived from and complying with the
prestandard. The plan sets are being developed based on
experience suggesting that prescriptive retrofit designs,
presented in plan set form and not requiring the involvement
of an engineer, effectively encourage wide-spread
implementation of dwelling seismic retrofit.
The prestandard will not include triggers for use; policy
directing voluntary or mandatory use is outside of the scope of
this project.
The scope of the project is wood light-frame dwellings, with a
primary emphasis on one- and two-family dwellings, and
consideration given to buildings with three or more dwelling
units, where vulnerabilities and retrofits are similar. The scope
includes dwellings located in moderate to high seismic hazard
areas, including Seismic Design Categories C and higher. The
approach taken by the project focuses on identification and
retrofit of specific vulnerabilities, with the objective of risk
reduction. This is believed to provide a notably higher benefit-
to-cost ratio and less invasive than with systematic assessment
and retrofit of the entire dwelling. The prestandard is intended
to permit addressing an individual vulnerability, multiple
vulnerabilities or all identified vulnerabilities.
The vulnerabilities addressed include cripple walls and
anchorage to the foundation for dwellings with a cripple wall
configuration (Figure 1), weak stories and open fronts in
dwellings with a house-over-garage configuration (Figure 2)
or room-over-garage configuration (Figure 3), and vulnerable
anchorage to the foundation in dwellings with a hillside
configuration (Figure 4). Also included in the project are
assessment and retrofit of masonry chimneys.
Figure 1. Cripple wall dwelling configuration.
Figure 2. House-over-garage dwelling configuration.
Figure 3. Room-over-garage dwelling configuration.
Figure 4. Hillside dwelling configuration.
2017 SEAOC CONVENTION PROCEEDINGS
3
Previously available guidance on assessment and retrofit of
these vulnerabilities has had little or no rigorous study of the
improvement in performance resulting from retrofit. While
still using previously available retrofit guidance as a starting
point, this project has taken the approach of using numerical
studies to quantify performance improvements resulting from
retrofit.
Figure 5 provides a flowchart indicating the overall approach
to development. Consistent with the analytically-informed
approach, Step 1 is the definition of performance criteria used
to guide numerical studies. Step 2 involves development of
representative index buildings and initial retrofit designs, and
Step 3 involves numerical studies to determine if performance
criteria are met. Ideally Step 4 would allow derivation of
assessment and engineered retrofit criteria directly from
numerical results, but it has been found that some cycling back
to the performance criteria is required. As the performance
criteria and retrofit criteria are resolved, the end products of
development are the assessment method of Step 5, the
engineered retrofit methodology, captured in prestandard
language in Step 6, and the prescriptive plan sets of Step 7,
derived from the Step 6 engineered methodology.
Figure 5. Flowchart of project approach to development of retrofit solutions.
In Step 1, the initial target for the primary performance
criterion was chosen as ten percent probability of collapse in
the risk-based maximum considered earthquake (MCER), as
determined using the FEMA P-695 (FEMA, 2009)
methodology with explicit numerical modeling of collapse.
This criterion was chosen to be consistent with current
building code performance targets for new buildings, and to
make use of the best available numerical modeling tools. The
probabilities of collapse being numerically predicted by
projects currently using this methodology are higher than is
generally anticipated based on observed performance in past
earthquakes. Based on this there is a general acknowledgement
by the project that probabilities of collapse reported by the
project numerical studies might be somewhat high. This
understanding is included in judgements made by the project
regarding acceptable retrofit performance. In addition, the
numerical results are emphasized more as a relative measure
of improved performance rather than an absolute measure of
performance. The ten percent probability of exceedance was
chosen realizing that it would serve as a starting point, to be
adjusted as necessary as information from the numerical
studies became available; this adjustment is currently being
determined.
Two secondary criteria have been considered in evaluating the
analysis results. These are intended to inform the choice of
retrofit criteria, while not necessarily being a deciding factor.
The first secondary criterion uses drift as an indicator of the
level of repair. This involves tracking transient drift in both
upper occupied stories and stories with retrofit. These are
compared to drifts identified by research to create increments
in repair type and repair cost. The purpose is to understand
possible increased damage as a function of level of retrofit. The
bases of the selected drift are FEMA P-58 (FEMA, 2012a)
fragility functions and CUREE EDA-02 (CUREE, 2010). The
criterion uses a transient drift ratio of 0.75% at a seismic
demand level corresponding to 30% probability of exceedance
in 50 years (140-year mean return period).
The other secondary criterion uses drift as an approximate
indicator of structural safety for continued occupancy. This
relates to possible post-earthquake safety assessment tagging
of the building, based on expert judgement. Bases for the
selected drift include CUREE EDA-02 and FEMA P-807
Appendix D.9 (FEMA, 2012b). The criterion uses a transient
drift ratio of 1.5 % at a seismic demand level corresponding to
10% probability of exceedance in 50 years (475-year mean
return period).
The extent to which either of the secondary criteria will affect
the prestandard is being decided for each vulnerability as
numerical study information becomes available. In recent
discussions, it has been suggested that the retrofit criteria be
chosen first based on the primary performance criterion.
Where possible, the retrofit will be modified to improve the
secondary performance criteria, provided it does not notably
reduce performance at the primary criterion or notably increase
retrofit cost.
In Step 4, the retrofit design methodology selected is based on
International Building Code (IBC) (ICC, 2015) and ASCE 7
(ASCE, 2010) equivalent lateral force seismic design methods.
The use of this methodology is felt to best serve the segment
of the engineering community thought to be the target for use,
and will keep the cost and complication of retrofit design from
becoming too burdensome. Further, the use of performance
based studies by the project to develop the retrofit means that
2017 SEAOC CONVENTION PROCEEDINGS
4
the retrofits will have the benefit of performance-based design
principles without requiring that performance-based design be
performed for each dwelling. As part of the IBC/ASCE 7 approach, R-factors are being
developed for retrofit of each vulnerability and are expected to
differ between vulnerabilities due to significant differences in
dwelling seismic response. R-factors are being determined
based on numerical studies. It has not yet been decided whether
it is necessary to develop overstrength factors, bounding
displacements, or stiffness criteria as part of the engineered
methodology for retrofit design.
Load path connections for retrofit elements are being
developed using capacity methods as part of the engineered
design methodology. It is necessary that the load path develop
the peak capacity of the retrofit elements in order to make valid
the probability of collapse studies. If the load path did not
develop the retrofit element capacity, probability of collapse
could be expected to increase above that reported by the
analyses.
As the project has progressed, it has found that the numerical
studies develop a wealth of information relative to both
primary and secondary criteria. The information has been
found to vary widely enough between the three vulnerabilities
that it has become necessary to revisit and revise the
performance criteria separately for each vulnerability. At the
time of writing, performance criteria have not been finalized.
The discussions that follow capture interim performance
considerations for the vulnerabilities studied.
Numerical Methodology
A primary task of the numerical studies is to generate data used
to measure the performance of dwellings before and after
retrofit, thereby allowing the team to compare results to
performance criteria and judge the improvement in
performance with retrofit. Equally important to project team
members developing the assessment and retrofit
methodologies is the determination of both global seismic
demand and variation in the distribution of seismic demand in
the dwellings and their load path. In order to serve these
several purposes, three analysis teams (one team studying each
vulnerability) generated a range of numerical analysis results
including backbone curves and IDAs, and where needed
extracted detailed information on load path forces and
displacement histories.
The numerical studies have used the Timber3D analysis
program, a three-dimensional (3D) program originally
developed as part of the NEES-Soft project (van de Lindt et al.
2012) to capture the non-linear dynamic response and seismic
collapse mechanisms of light-frame wood buildings. This 3D
program is an extension of detailed 2D programs developed
earlier for the collapse analysis of light-frame wood shear
walls (Pang and Shirazi 2012; Christovasilis and Filiatrault
2010, 2013).
The Timber3D program operates on the Matlab platform using
a co-rotational formulation and large displacement theory.
The horizontal floor and roof diaphragms are modeled using
co-rotational 3D, two-node, 12-DOF elastic beam elements,
which account for geometric non-linearity. Using a co-
rotational formulation allows proper consideration of the in-
plane and out-of-plane motions of the diaphragms under large
deformations.
The elastic flexural and axial stiffness of vertical wall studs are
modeled using 3D, two-node, 12-degrees-of-freedom (DOF)
elastic frame elements. The vertical wall panel-to-framing
assemblies are modeled using 6-DOF, Frame-to-Frame (F2F)
link elements. Only one (lateral) DOF of the F2F link element
is activated to model the lateral non-linear cyclic response of
vertical walls sheathed with wood panels and other (non-
structural) materials.
The non-linear lateral cyclic response of vertical walls is
captured by the CUREE hysteretic rule (Folz and Filiatrault
2001), as illustrated in Figure 6. The loading force-
deformation paths OA and CD follow a non-linear exponential
monotonic envelope curve, while all other unloading and re-
loading paths exhibit a linear relationship between force and
deformation. This hysteretic rule allows for stiffness and
strength degradation as well as post-capping reducing strength.
The CUREE hysteretic rule is completely determined by ten
physically identifiable parameters, as illustrated in Figure 6.
Figure 6. CUREE hysteretic rule for modeling force-displacement response of wood shear walls under cyclic loading.
2017 SEAOC CONVENTION PROCEEDINGS
5
A modified version of the CUREE hysteretic rule is also
available within Timber3D in order to introduce a user-defined
residual strength of vertical walls. The post-capping strength
stiffness (r2K0) is replaced by a reversed S-shaped curve
anchored at a displacement Dx and converging to pre-
determined residual strength level at large displacements, as
shown in Figure 7. This modification was used in all of the
project numerical studies.
Figure 7. Modification of CUREE hysteretic rule for modeling residual strength.
Figure 8 illustrates a single-story light-frame wood building
modeled using the Timber3D analysis program with two
different levels of modeling details, namely simplified and
intermediate models. In the simplified model, the horizontal
diaphragms are modeled using co-rotational 3D, two-node, 12-
DOF elastic beam elements with high stiffness resulting in
rigid behavior. The intermediate models incorporate more
detailed representation of horizontal diaphragms, an
explanation of which follows. Both models include vertical
wall elements composed of standard non-linear wall “building
blocks”.
Using the Timber 3D tool, analytical studies included both
initial push-over analysis to provide understanding of peak
capacities, and incremental dynamic analyses (IDAs) using
non-linear response history analysis.
Consistent with the FEMA P-695 methodology, IDAs are
conducted in order to obtain reliable estimates of median
collapse intensity. Intensities are selected to give feedback for
secondary (drift) criteria and the MCER level. Additional
intensities are then conducted in order to better estimate the
median collapse intensity if necessary. Currently, the FEMA
P-695 far-field set (22 pairs of horizontal ground motions) is
used to represent seismic input across the ATC-110 project.
Uncertainties due to record-to-record variability, material
properties and modeling assumptions are accounted for using
pre-defined dispersion factors according to FEMA P-695.
Further, a set of consistent assumptions are used specifically
for the collapse performance of the wood light-frame
dwellings within ATC-110. This includes using a constant
intensity measure of the spectral acceleration at a period of
0.25s; the lowest period allowed by FEMA P-695.
Adjustments in median collapse intensity are made to account
for using 3D analysis and the absence or presence of large
ductility capacity (i.e. large period elongation) according to
FEMA P-695. The adjusted median collapse intensity is used
in combination with the pre-determined dispersion factors to
obtain the final probability of collapse at an intensity of interest
(e.g. the MCER intensity level).
Figure 8. Schematic illustration of one-story, light-frame wood building model in Timber3D (top) simplified model, and (bottom) intermediate model.
The level of complexity of numerical models used within
ATC-110 is governed by balancing the ability to capture
pertinent physical behavior while minimizing the
computational onus wherever possible. The models used for
cripple wall dwellings represent the most simplified of the
different structural types considered. An illustration of a single
story cripple wall dwelling and the equivalent Timber3D
model is shown in Figure 9. The figure shows that the model
is comprised of two stiff (essentially rigid) diaphragms
representing the floor of the occupied space and the roof. These
are comprised of a series of rigid beam elements with
diaphragm masses applied. The diaphragms are separated by
pinned stud elements that allow for the vertical geometry
between diaphragms to be represented (i.e. cripple wall height
Stud(Beam Element +Rigid F2F Connection to Diaphragm + Pin-connection to ground)
Diaphragm (Beam Element with linear in-plane stiffness)
Wall (F2F Element with calibrated hysteretic behavior)
Stud(Beam Element +Rigid F2F Connection to Diaphragm + Pin-connection to ground)
Diaphragm (Beam Element with linear in-plane stiffness)
Wall (F2F Element with calibrated hysteretic behavior)
Strut (Rigid Beam Element Pin-connected at corners)
2017 SEAOC CONVENTION PROCEEDINGS
6
and first story height). Stud elements also allow for the mass
of the vertical wall materials to be accounted for appropriately.
The horizontal non-linear force-displacement behavior of
different sections and materials of wall elements (between stud
elements) are included with 1D frame-to-frame (F2F) elements
exhibiting the modified CUREE hysteretic rule (Figure 7),
with each element given properties calibrated to available
material test data considered within ATC-110.
The rather simple modeling assumptions considered for
cripple wall dwellings focuses on gaining a better
understanding of how differences in global strength and
ductility capacities between the cripple wall and the
superstructure affect seismic performance; both for existing
conditions and dwellings incorporating structural retrofit. By
minimizing the complexity of these models, numerous
archetype models were able to be studied in order to better
define appropriate design considerations that will be
implemented in retrofit plan sets for cripple wall dwellings (i.e.
R-factors, general limitations of applicability, etc.).
Figure 9. Illustration of simplified Timber3D model used for analysis of cripple wall dwellings. In the cases where house-over-garage, room-over-garage, and
hillside dwelling configurations were studied, additional
information, including chord forces and calibrated diaphragm
stiffness, was desirable for a full understanding of the behavior
of these configurations. To achieve the increased level of
detail in the diaphragm, beam elements calibrated to the
desired in-plane elastic shear stiffness were modeling with
rigid pin-connected boundary members that could be used to
determine boundary member (chord) forces. This
configuration allowed for the same transfer of vertical load to
pinned stud elements and utilized the same 1D frame-to-frame
(F2F) elements exhibiting the modified CUREE hysteretic rule
as the cripple wall dwelling. An illustration of a house-over-
garage configuration and the equivalent Timber3D model is
shown in Figure 10.
Figure 10. Illustration of intermediate Timber3D model used for analysis of house-over-garage dwellings.
Cripple Wall and Anchorage Vulnerability
The cripple wall and anchorage vulnerability is found in wood
light-frame dwellings with a crawlspace or basement below the
first occupied level, including crawlspaces enclosed by wood-
frame cripple walls, concrete or masonry stem walls, basement
walls, or combinations thereof, on flat to low slope sites.
Included in the scope of assessment and retrofit methods
addressing this vulnerability are dwellings with cripple walls
with heights from 0’ (wood floor framing sits directly on
foundation or foundation stem wall) to 6’-0”. The scope of
cripple wall dwelling studies is limited to dwellings in which
2017 SEAOC CONVENTION PROCEEDINGS
7
the difference in height between its tallest and shortest cripple
walls does not exceed 4’-0” (Figure 11). Dwellings with a
difference in height greater than this are addressed in the
hillside dwelling studies.
The cripple wall working group is developing assessment and
retrofit methods for the cripple wall vulnerability. Retrofit
includes plywood sheathing of existing cripple walls studs,
connection of the cripple wall to the structure above, and
anchorage of the cripple wall to the foundation system.
Preliminary provisions have also been included to address
replacement of foundation systems where existing foundations
are not present or not continuous.
Figure 11. Limits of applicability of cripple wall vulnerability assessment and retrofit methods.
The primary approaches for flat and low slope sites remain
very much the same as that included in the current
International Existing Building Code (IEBC) (ICC, 2015b)
Appendix Chapter A3 provisions, and the similar provisions
adopted into various plan sets, including the FEMA P-
1024RA2 plan set (FEMA, 2015) (Figure 10).
Figure 12. Cripple wall vulnerability retrofit concept.
Although code prescriptive provisions and plan sets addressing
cripple walls and anchorage to foundations are available,
outstanding questions regarding this retrofit type have
remained. One question is the seismic force level appropriate
for retrofit design. When thought of from the standpoint of
ASCE 7 R-factors, the appropriate R-factor could be implied
to be 2, based on ASCE 7 treatment of vertical combinations
of systems, or 6-1/2 based on the materials such as plywood
typically used for retrofitting. These different design
parameters would result in significantly different solutions in
terms of extent, cost, and practicality.
As previously mentioned, an initial target of 10% probability
of collapse in the risk-targeted maximum considered
earthquake (MCER) as defined in ASCE 7-10 was chosen as
the primary performance criterion. While the cripple wall
studies originally planned to focus on the cripple wall level, it
quickly became evident that the performance of the combined
superstructure and cripple wall needed to be considered.
Project team consensus also favored that collapses reported by
the numerical studies should largely occur within the cripple
wall level rather that the occupied stories for a better safeguard
to life-safety. In addition the project team was concerned that
over-strengthening the cripple wall level could potentially lead
to propagation of damage to weaker occupied stories, even
under earthquakes with low intensities.
Due to the large variation in the configuration of existing
cripple wall dwellings, the project team conducted an
extensive study with the goal of characterizing a median
superstructure in terms of strength and weight, for use in the
numerical studies. Representative one and two-story home
plans were studied and grouped into six different decades
(1900 through 1960) and subsequently categorized in terms of
peak lateral strength to weight (V/W)Avg, peak lateral strength
to area, (V/A)Avg, baseline weight to area (WBL/A), and strong
to weak direction strength ratios. The subscript “Avg” in these
performance parameters relate to the average values
considering both principal directions of the building. Three
one-story and two two-story plans were selected from each
decade and evaluated with combinations of existing finishes
including exterior stucco and wood siding, and interior plaster
on wood lath and gypsum board. In total, 140 unique
combinations from available home plans were analyzed to
establish trends of median properties between 1900 and 1960.
The analysis developed push-over curves to determine the
peak story shear capacity, V. A sample of resources used for
this study are shown below in Figure 13.
The main results for the one-story median study are shown in
Figure 14. These results were used to calibrate the numerical
models for all analysis runs.
2017 SEAOC CONVENTION PROCEEDINGS
8
Figure 13. Sample of Collected Resources
Based upon the results of the study, shown in Figure 14, it
became evident that both a stronger “median” and a weaker
“median minus beta” one and two-story superstructure were
required to appropriately capture the population of target
dwellings. Current thinking is to use the results from the
“median” superstructures to establish the primary performance
objective. However, the weaker “median minus beta”
superstructures are being evaluated to investigate level of
damage to the superstructure. In general, the median
superstructure is anticipated to largely occur in pre-1950
dwelling and correspond with the presence of interior walls
with plaster over wood lath or plaster over gypsum lath (button
board), and the weaker “median minus beta” superstructures
and largely influenced by the presence of interior walls
consisting of gypsum board mainly found in post-1950
dwellings.
Figure 15 provides a snap-shot of interim results from the
numerical studies that are being used to establish the final
performance criteria and retrofit design criteria. This chart
plots the probability of exceedance of primary and secondary
criteria for a 2’-0” cripple wall below a one-story median
superstructure. Similar plots have been developed for 4’-0”
and 6’-0” tall cripple walls for both median and median minus
beta superstructures. For the configuration shown, it is
anticipated that the predicted probability of collapse under
MCER ground motions will be reduced up to 80% relative to
the unretrofitted dwelling. Preliminary results also suggest
that seismic performance improves modestly as the cripple
wall height increases from 2’-0” up to 6’-0”, due to added
displacement ductility. For taller cripple wall heights, it is
anticipated that P- effects will start to control collapse
probabilities.
(a)
(b)
(c)
Figure 14. Results from the one-story median study (a) average strength to seismic weight ratios of era specific materials, (b) average strength to weight ratios of era specific materials, and (c) total weight to area ratios of era specific materials.
2017 SEAOC CONVENTION PROCEEDINGS
9
Figure 15. Interim results for the one-story 2’-0” high median cripple wall
While a reasonable estimate of existing cripple wall finishes
has been embedded within the numerical models and will
influence the choice of an overall R factor, both assessment
and retrofit methods will ignore the contribution of existing
cripple wall bracing materials (other than existing wood
structural panel sheathing) for purposes of retrofit design. This
is primarily because the condition of finishes can be widely
varying, will be unknown, and will not be practical to
determine short of destructive testing.
Numerical studies are leading the project team to recommend
an R-factor, or R factors that are lower than those used for
wood structural panel retrofits in recent retrofit standards and
plan sets. This is based on the numerical study predicted
probabilities of collapse under MCER ground motions, and
brings predicted probabilities more in line with expectations
for hazard reduction or collapse prevention performance
objectives in current standards. The final selection of the R
factor will include a reasonable balance of anticipated
improvement of collapse probability under MCER ground
motions over a wide range of cripple wall dwellings, with the
economics and practicality of the strengthening solution.
House- or Room-over-Garage Vulnerability
The house-over-garage and room-over-garage working group
is developing assessment and retrofit methods for
vulnerabilities found in wood light-frame dwellings with
living space over the garage, where the garage front is
unbraced or has minimal lateral bracing. Included are single or
multi-level dwellings over a first story consisting of a garage
or a combination of a garage and living spaces (Figure 2). Also
included are two-story ranch-style configurations, which
include bedrooms or other occupancies directly above or
partially above a garage (Figure 3). Typical damage modes are
anticipated to include excessive drift in the lower story relative
to the upper story resulting in significant damage, and possibly
a partial or complete story collapse.
Included in the scope of assessment and retrofit methods
addressing this vulnerability are dwellings with up to 9’-0”
story clear height in the ground story (Figure 16).
Figure 16. Limits of applicability of house- or room-over-garage vulnerability assessment and retrofit methods.
The primary approaches to retrofit at the front of house-over-
garage configurations include solutions with wood structural
panel shear walls where there is enough wall length at the front
to allow this retrofit (Figure 17a), and a cantilevered steel
column just inboard of the front wall otherwise (Figure 17b).
In addition to bracing at the front, these retrofit approaches
include transverse wood structural panel bracing at the back
wall of the ground story, and wood structural panel bracing on
the longitudinal walls. Where house-over-garage
configurations have offices or in-law units built into the back
of the ground story, alternate designs have been developed to
locate retrofit work outside of the built-out spaces.
Rather than looking at a range of superstructure capacities, as
was done in the cripple wall analytical studies, the house-over-
garage working group numerical studies have primarily
focused on study of a representative dwelling, consistent with
the cripple wall working group’s median home.
Figure 18 provides a snap-shot of interim results from the
numerical studies that are being considered in developing final
performance criteria and design criteria for retrofit. This chart
plots the probability of collapse under MCER ground motions
and the probability of exceedance of the two secondary criteria
previously discussed for a representative house-over-garage
with one occupied story. This chart directly illustrates the
2017 SEAOC CONVENTION PROCEEDINGS
10
significant reduction in probability of collapse that can occur
if any of the studied retrofits are provided. The chart also
shows a beneficial decrease in the probability of exceeding the
secondary criteria. The wood structural panel retrofit with R=4
is shown to be a desirable retrofit solution. For the cantilevered
steel column solutions, the R=4 retrofit satisfies the primary
performance criterion. The R=3 retrofit might provide a
somewhat better balance between primary and secondary
criteria. The difference in cost between these retrofit solutions
is believed to be nominal.
(a)
(b)
Figure 17. House- or room-over-garage vulnerability retrofit concepts at building front (a) wood structural panel retrofit and (b) cantilevered steel column retrofit.
Of interest in comparing the results for the R=2, R=3 and R=4
steel cantilevered column retrofit solutions is that the
probability of collapse increases slightly with decreased R-
factor. While this seems counter-intuitive, as the R-factor is
reduced more inelastic response is being pushed into the
occupied story, slightly increasing reported collapses in that
story. This is consistent with the sweet-spot concept of the
FEMA P-807 methodology (FEMA, 2012b), which identified
an optimum range of retrofitting above which the benefits of
retrofit started to decrease. While this behavior is being
reflected in a general way in this project through selection of
an R-factor, further optimization is not practical due to the
variability inherent in the strength and stiffness of the building
stock, and the simplified engineering and prescriptive design
methodologies to be used for retrofit design.
Figure 18. Interim results from house-over-garage working group analysis.
Hillside Dwelling Vulnerability
The hillside dwelling working group is developing assessment
and retrofit methods for vulnerabilities found in wood light-
frame dwellings sited on low to steep sloped hillsides with
unoccupied space below the lowest framed floor. The
unoccupied space in hillside dwellings might be enclosed with
crawlspace walls, be open with wood light-frame post and
beam systems that have no bracing, wood or steel diagonal
bracing, or have skirt walls. Side walls may occur on stepped
or sloped continuous foundations. Foundation systems may
include shallow continuous foundations, shallow isolated
foundations, or deep foundations (such as drilled piers) with
or without connecting grade beams.
Included in the scope of assessment and retrofit methods
addressing this vulnerability are dwellings with cripple walls
between zero-height (wood floor framing sits directly on
foundation or foundation stem wall) and 16’-0”. Use is limited
to dwellings in which the difference in height between tallest
and shortest cripple walls is 4’-1” or greater (Figure 19). This
is meant to dovetail with the cripple wall retrofit provisions,
which apply when the difference in wall height is 4’-0” or less.
The approach to retrofit of dwellings with a hillside
vulnerability is conceptually very different than for cripple
wall dwellings. The primary retrofit approach builds from an
2017 SEAOC CONVENTION PROCEEDINGS
11
approach developed by the City of Los Angeles Hillside Task
Group following the 1994 Northridge Earthquake, and
included in City of Los Angeles Building Code Division 94.
This method recognizes that seismic forces will be attracted to
the stiffer load path of the uphill foundation (Figure 20). As a
result it is necessary to make the strength of the anchorage to
the uphill foundation high enough to resist forces that cannot
be reduced due to ductility, and to provide a load path stiff
enough that shear anchorage to the uphill foundation is not
damaged. The project has arrived at a retrofit approach that
includes substantial primary anchors at each end of the uphill
foundation, as well as secondary anchors, uniformly
distributed between primary anchors.
Figure 19. Limits of applicability of hillside dwelling vulnerability assessment and retrofit methods.
Figure 21 shows an isometric of the retrofit concept and
Figures 22 shows an example detail of a secondary anchor,
attaching the floor diaphragm to the uphill foundation. Figure
23 provides a plan view of the dwelling floor, showing the
concept of primary and secondary anchor placement at the
dwelling uphill foundation.
An unexpected finding of the numeric studies is that even with
primary and secondary anchors to the uphill foundation,
significant performance benefits occur with wood structural
panel sheathing on the downhill crawlspace wall. The seismic
response of the first occupied story is highly torsional, with the
forces concentrating in the occupied story walls immediately
on top of the uphill foundation, due to this wall line providing
a stiffer load path. Numerical studies show high drifts
associated with significant damage in this occupied story wall
line when crawlspace wall sheathing is not provided, and drifts
notably reduced when crawlspace wall sheathing is provided.
For this reason, prescriptive requirements for retrofit will
include both anchorage to the uphill foundation and sheathing
of crawlspace walls.
(a)
(b)
Figure 20. Hillside dwelling seismic demands (a) loading away from the hill pulls diaphragm away from uphill foundation (b) cross-hill loading pulls corner of diaphragm away from uphill foundation. Figure credit FEMA 547.
Figure 21. Hillside dwelling retrofit concept with anchorage to the uphill foundation. Figure credit FEMA 547.
2017 SEAOC CONVENTION PROCEEDINGS
12
Figure 22. Hillside dwelling vulnerability retrofit concept - secondary anchor.
Figure 23. Hillside dwelling vulnerability retrofit concept - plan of dwelling floor showing placement of primary and secondary anchors at the uphill foundation.
Figure 24 provides a snap-shot of interim results from the
numerical studies that are being considered in developing final
performance criteria and retrofit design criteria. Significant
reductions in probability of collapse and probability of
exceeding secondary criteria can be seen for all of the retrofit
methods, with the exception of R1, second from the left. The
high probability of collapse of the R1 retrofit is attributed to
the retrofit solution with only secondary anchors not being able
to effectively resist torsion. Additional analyses are being run
prior to selection of the R-factor for retrofit design. It is
currently envisioned that the R-factor will be close to one.
Figure 24. Interim results from hillside dwelling working group analysis.
Anticipated Next Steps
The ATC-110 Project will complete its work in June 2018. It
is anticipated that the developed prestandard and plan sets will
move forward into an ANSI Standard process. At this time it
is not known whether the prestandard and plan sets will be
made publically available while the ANSI standard process is
ongoing. Information on intended publication will be made
available through ATC.
Acknowledgements
The authors would like to thank project sponsors the
California Earthquake Authority and the Federal Emergency
Management Agency, the Applied Technology Council
project managers, and the project steering committee. The
work discussed in this paper has been developed by a number
of project team members, who we thank for their
contributions. Included are Project Technical Committee
Members Vikki Bourcier, Michael Cochran, Dan Dolan, Brian
McDonald, John Osteraas and Tom Anderson, and the many
members of the project working groups.
The work forming the basis of this publication was conducted
pursuant to a contract with the California Earthquake
Authority and the Federal Emergency Management Agency.
2017 SEAOC CONVENTION PROCEEDINGS
13
Work on the ATC-110 Project is ongoing, and the numerical
study results and retrofit design concepts presented in this
paper are interim, and not final conclusions or
recommendations of the project. Users of information
contained in this publication assume all liability arising from
such use.
References
ASCE, 2010. Minimum Design Load for Buildings and Other
Structures (ASCE 7-10) American Society of Civil Engineers,
Reston, Virginia.
Christovasilis, I.P. and Filiatrault, A. 2010. “Two-
Dimensional Seismic Analysis of Multi-Story Light-Frame
Wood Buildings,” 9th US National & 10th Canadian
Conference on Earthquake Engineering: Reaching Beyond
Borders,” Toronto, Canada, Paper No. 69, 10 p.
Christovasilis, I.P. and Filiatrault, A. 2013. “Numerical
Framework for Nonlinear Analysis of Two-Dimensional
Light-Frame Wood Structures,” Ingegneria Sismica:
International Journal of Earthquake Engineering.
CUREE, 2010. General Guidelines for the Assessment and
Repair of Earthquake Damage in Residential Woodframe
Buildings, (CUREE EDA-02), Consortium of Universities for
Research in Earthquake Engineering, Richmond, California.
FEMA, 2009. Quantification of Building Seismic
Performance Factors (FEMA P695), Federal Emergency
Management Agency (FEMA P695, Washington, D.C.
FEMA, 2012a. Seismic Performance Assessment of Buildings
(FEMA P-58), Federal Emergency Management Agency,
Washington, D.C.
FEMA, 2012b. Seismic Evaluation and Retrofit of Multi-Unit
Wood-Frame Buildings with Weak First Stories (FEMA P-
807), Federal Emergency Management Agency, Washington,
D.C.
FEMA, 2015. Earthquake Strengthening of Cripple Walls in
Wood-Frame Dwellings (FEMA P-1024RA2), Federal
Emergency Management Agency, Washington, D.C.
Folz, B., and Filiatrault, A. 2001. “Cyclic Analysis of Wood
Shear Walls”, ASCE Journal of Structural Engineering,
127(4), 433-441.
ICC, 2015a. International Building Code (IBC), 2015 edition,
pp. 2-161 to 2-163, International Code Council, Country Club
Hills, Illinois.
ICC, 2015b. International Existing Building Code (IEBC),
2015 edition, pp. 2-161 to 2-163, International Code Council,
Country Club Hills, Illinois.
Pang and Shirazi 2012; Pang, W., and Shirazi, S.M. (2012) “A
Co-rotational Model for Cyclic Analysis of Light-frame Wood
Shear Walls and Diaphragms,” ASCE J. of Structural
Engineering.
van de Lindt, J., Symans, M.D., Pang, W., Shao, X., and
Gershfeld, M. 2012. “Seismic Risk Reduction for Soft-story
Woodframe Building: The NEES-Soft Project,” 121th World
Conference on Timber Engineering, Auckland, New Zealand.