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2017 SEAOC CONVENTION PROCEEDINGS

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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.


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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.


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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


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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.


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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)


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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


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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.


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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.


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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.


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.


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


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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.


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


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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.


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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.


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.


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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.

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