C CS Poland Sept08

8/13/2019 C CS Poland Sept08

1/4STRUCTURE magazine September 2008

updates

and

discussions

related

to

codes

and

standards

CODESA

ND

STAND

ARDS

14

ASCE 41-06: Seismic Rehabilitation of Existing BuildingsA New Tool for Achieving Seismic SafetyBy Chris D. Poland, S.E., F. SEAOC

Earthquakes wreak havoc on our lives,our businesses and our communities.For the last 140 plus years, scientistsand engineers have been working tounderstand where they can occur andhow to best mitigate their effects. What

started as a concern centered along theWest Coast has become known as a realthreat that affects the vast majority ofstates in the United States. Thanks tothe work of the United States GeologicalSurvey, we now have a science-basedunderstanding of earthquake hazardsnationwide, which has become the sourcefor national earthquake hazard maps.For over 50 years, engineers have focused

on protecting lives and the vitality of ourcommunities by writing building codesand seismic provisions. Engineers are

identifying and rehabilitating danger-ous buildings nationwide, and the needfor conversations about community resil-ience are refining how we approach design.Unfortunately this progress achieved fullstride just 20 years ago and has left us

with an inventory of buildings nation-wide where more than 80 percent areunable to meet the recognized seismicstandards where they are located. Theseismic rehabilitation of the exist-ing building stock is a key elementin the process of achieving seismic

safety and turning our cities intoresilient communities.Unsurprisingly, when owners un-

derstand and accept the risk of anearthquake, they want it broughtunder control and mitigated to anappropriate level. Often it takesa personal, life-changing earthquakeexperience to bring home the realityof an earthquakes consequences.For those owners who lack suchpersonal experiences, the conse-quences of earthquakes must bedescribed in intuitive terms in orderto appreciate the importance ofseismic mitigation. Another barrierto mitigation is cost; many interest-ed owners have done nothing becausethey think that the investmentrequired to bring older buildingsup to code is ridiculously high.Finally, we structural engineers aretoo often reluctant to speak up anddeclare what is going to happenbecause we sense that somehow

we will be held responsible when it does.Our personal liability fears often stop usfrom speaking up, even though our si-lence leaves the impression that there isnot a problem.If you are reading this from a practice

along the West Coast, you recognizethe situation and the need. From the

Wasatch Front in Utah, many of you

know this same situation exists in youregion, but it remains a tough sell. FromMemphis to the eastern seaboard, thskepticism runs high among ownerand their design professionals and onllimited recognition is evident.Fortunately, the American Society o

Civil Engineers Standard Number 41Seismic Rehabilitation of Existing Build

ings (ASCE 41) provides usefutools to assist design professionals in tackling all three of thes

barriers; risk perception, cost orehabilitation, and professionaliability. It provides the next generation of tools needed to achievseismic safety nationwide. ASCE41 is the culmination of over 2years of work that began with FEMA sponsored dream for a library of guidelines and standardrelated to the seismic evaluatioand rehabilitation of buildingsHaving successfully completed thballoting process, ASCE 41 inow approved by the AmericaNational Standards Institute (ANSIas a standard that can be referenced without amendment blocal codes. As a standard, it provides the liability shield that we aneed to practice with confidenceNo one expects perfection; they jusexpect us to practice at the state othe practice level, and ASCE 4clearly defines what that means.

Target Building Performance Levels and Ranges

Expected Post-Earthquake

Damage State

Operational (1-A)

Backup utility services maintain

functions; very little damage.

(S-1 & N-A)

Immediate Occupancy (1-B)

The building remains safe to

occupy; any repairs are minor.

(S-1 & N-B)

Life Safety (3-C)

Structure remains stable and has

significant reserve capacity;

hazardous nonstructural

damage is controlled.(S-3 & N-C)

Collapse Prevention (5-E)

The building remains standing, but

only barely; any other damage or

loss is acceptable.

(S-5 & N-E)

higher performance

less loss

lower performance

more loss

The wide range of damage at the epicenter of the 1994 Northridge earthquake illustrates that notall existing, non-compliant buildings are dangerous. One building partially collapsed while othersof similar vintage remained usable. Courtesy of Lloyd Cluff, 1994.

ASCE 41 defines ranges for performance levels for structural andnon-structural elements that serve as the basis of a rehabilitationobjective. This chart illustrates the range from minimum safetyto high performance. Courtesy of ASCE 41, 2005.



Predicting PerformanceBuildings respond to earthquakes in a wide

variety of ways, but always within the samepatterns. As the ground shaking begins, thebuilding moves and develops distortionsin proportion to its mass, stiffness, andthe frequency of the ground motion. Theaccelerations that are experienced join withthe mass that is present and generate forcesthat the building needs to resist. If the forces

are within the strength of the building, littledistortion develops and only minor damageoccurs. If the forces that are developed exceedthe capacity of the building, then the buildingstarts to break up, the distortions increase, anddamage becomes more severe. As the buildingbreaks up, it destroys its lateral strength and thebuilding experiences extensive distortions and,in some cases, partial or complete collapse.The design guidelines for new buildings are

set to prevent the breaking up process fromstarting in moderate earthquakes, and toprevent the building from distorting to the

point of pulling apart in extreme earthquakes.The code uses the base shear to achieve theminimum strength required, and requiresspecific details of construction to contain thebreaking up process and achieve the desiredperformance for the buildings usage. Therequired details provide the ductility neededto control the level of damage. The newbuilding code has achieved its purpose througha thoughtful balance of strength and ductilitydefined as prescriptive requirements.Existing buildings may not have the required

balance of strength and ductility that new

buildings possess. Attempts to apply theprescriptive requirements of the new code willlead engineers to the conclusion that mostold buildings need new lateral force resistingsystems. That is, unfortunately, the key driverof the high rehabilitation costs that often result.However, earthquake damage patterns illustratethat new lateral systems are not always necessary,and ASCE 41 provides the tools to distinguish

when strengthening is actually needed and towhat extent. It provides the ability to judge theadequacy of the ductility that is available giventhe strength that is present. As such, ASCE 41is a toolbox of procedures that can be appliedas appropriate for the detailed evaluation andrehabilitation of existing buildings in orderto minimize the cost of strengthening. Itprovides a systematic process that defines targetperformance levels, considers earthquakes ofvarious sizes, and provides four distinct analysistechniques and a wide variety of modelingtechniques to guide the evaluating engineer intoan appropriate conclusion about a buildingsrehabilitation needs.

ASCE 41 ProcessRegardless of whether ASCE 41 is being used

as an evaluation tool in conjunction with theAmerican Society of Civil Engineers StandardNumber 31, Seismic Evaluation of ExistingBuildings (ASCE 31) or for a rehabilitationproject, it begins with the seismic hazard andthe desired performance levels. Unlike theprescriptive requirements for new buildings,

ASCE 41 makes recommendations for bsafety, enhanced, and limited performalevels, and detailed options can be applas needed to match the project neePerformance levels are defined that rafrom collapse prevention to operational

ASCE 41 that, when combined with hazlevels, yield the rehabilitation objective.The ASCE 41 process continues w

requirements for gathering data and

carrying out the evaluation. Instructions obtaining as-built information are given, alowith adequate default values for use whspecific information is not available. Detamaterial testing requirements, approprfor existing buildings, are also includ

Additionally, two rehabilitation methods provided: Simplified and Systematic. TSimplified Method is aimed at small, regusimple buildings and follows a processcorrecting the deficiencies identified us

ASCE 31 procedure in order to achievlife safety performance level. The Systema

Method is a step-by-step process that usesto four levels of analysis to accurately predperformance of a rehabilitation plan and in process minimize the cost of rehabilitatiThe process of developing and validata rehabilitation plan using the SystemaMethod addresses the deficiencies fouby ASCE 31 by showing they have bcorrected or that they are acceptable.In the process of defining the requireme

for rehabilitation, Chapter 1 of ASCE explains the background and importaof each step of the evaluation process, a

Residence in Paso Robles, California that successfullywithstood the 2003 San Simeon Earthquake withlimited damage due to its inherent strength andductility as a wood building and the moderate level ofshaking that occurred. The first level of analysis, theLSP, is sufficient. Courtesy of Chris Poland, 2003.

1950s Firehouse located in Paso Robles that also withstood the 2003 earthquake. The second level of analythe LDP, is suitable to demonstrate the systems adequacy since there is only a single lateral load path at the

first level. Courtesy of Chris Poland, 2003.



provides the most definitive statementon performance based seismic evaluationand design currently available. Within thesection on Performance Levels, there are aseries of tables that describe the expecteddamage for each of the four levels. Thisinformation is particularly useful indescribing the expected performance ofbuildings to owners.The four levels of analysis provided by

ASCE 41, which are explained below, give

progressively detailed information aboutthe need for, and extent of, strengthening.The first two levels match the model codestyle of force-based design, and cannotbe used on buildings with long periodsor significant irregularities. The secondtwo are based on displacement and serve todirectly determine the post yield capability ofthe buildings. In addition to a set of generalanalysis requirements, each analysis methodis defined in terms of specific modelingrequirements and procedures. A commonacceptance criterion is provided for linear

methods (force based) and also for the non-linear methods (displacement based). Thecriteria are extensive, organized by materialtype, and based on the amount of availableinformation, including applicable test results.The four analysis methods for judging the

need for strengthening or the suitability of arehabilitation plan come with the idea thatthe simplest technique should be used toshow adequacy. Buildings deserve a break andowners should not be required to strengthenperceived deficiencies that are not truly aproblem. Buildings should not be considered

in need of strengthening until all fourtechniques are considered, and possibly used,

that a building is okay, not to determinthe extent of strengthening it might needIf a building is immediately strengthenedto meet the LSP requirements, it will misthe beneficial, cost saving opportunitiethat ASCE 41 provides.The second level of analysis is the Linea

Dynamic Procedure (LDP) that usemodal analysis and site-specific responsspectra to determine the force demandsThe LDP also includes modeling rule

that encourage consideration of soil structure interaction and appropriate acceptanccriteria. It is much more beneficial thanthe LSP analysis since it utilizes a sitespecific response spectra, calculated building periods, and the beneficial effects o

multiple modes. It does have a serious limitation in that it cannot properly evaluate building with significant redundancy, that issignificant strength even after damage beginto occur. The rules for judging the buildinto be adequate still triggers unacceptablperformance when the first significant elemen

within the stiffest lateral system exceeds itlimits. For some buildings, this technique isatisfactory since once significant yieldinoccurs, there is nothing else to step in andprovide resistance.The first of the displacement-based pro

cedures is the Non-linear Static Procedur(NSP), commonly referred to as the pusover method. Using analytical techniques, accessible in commercially available advancecomputer programs, a model of the building literally and analytically subjected to increasindeflection while the impact on the lateral forc

resisting elements is monitored. As the yiellimits are exceeded, the elements are alloweto yield and the computer program tracks theipost yield displacement to determine when thbuilding loses its lateral force resisting abilityIn the process, first significant yield does nosignal a problem; instead, it signifies that otheelements need to step up and take over. Using series of approximations, a target displacemenis calculated based on site-specific responsspectra. If there is a lateral system within th

to minimize the extent of work needed andthe cost of correcting the deficiency.The first level of analysis is the Linear Static

Procedure(LSP) that provides an equivalentlateral force, vertical distribution of forcesand rules for modeling, and acceptancecriteria. A first glance, it looks like the

equivalent lateral force procedure from thecodes of the 1970s and 1980s, except that thebase shear is much higher and the ductilityfactors, the m factors, are much smaller. Itis intended to be simple and very conservativeto allow one and two story buildings ofregular configuration to pass because of theirexcessive strength. Unfortunately, manyfirst time users of ASCE 41 gravitate tothis section and base their work on it alonebecause it looks familiar. They never takethe time to explore the other processes. Fewexisting buildings, except wood frame or light

metal construction, can pass this conservativetest. The LSP should only be used to show

1960s University Library located near Palo Alto, Californiathat was rehabilitated using the third level of analysis, theNSP. By relying on the new cantilever concrete walls locatedat each corner to arrest the deflection of the building, theexisting concrete frame was held to within its limits for safe

performance. Courtesy of Chris Poland, 2003.

1960s Concrete Weather Station in Taiwan successfully resisted over 125 percent ground shakingwithout collapse because of the multiple lateral load resisting systems that includes concrete framesand masonry walls. Only the fourth level of analysis, NDP can come close to predicting suchperformance. Courtesy of Chris Poland, 1999.



building that can arrest the movement towithin the target displacement, the buildingis judged adequate. If not, then there is onemore level of analysis, if the building is worththe cost of running it. This process of analysismatches the way buildings behave in earth-quakes since it estimates the buildings actualmovement and resulting damage.The second displacement based method is

a Non-linear Dynamic Procedure (NDP)that uses time history records to represent

the possible shaking that the site couldexperience. By working with a full time seriesof motion, it is possible to take advantageof the beneficial effects of shaking since theground motion is applying as much restoringforce to the building as it is applying forcesthat cause damage. The frequency contentof the record is used directly to determinethe displacement demand and gives a moreaccurate representation. Also, the numberof cycles of non-linear behavior can bemonitored and used to more accuratelypredict the extent of damage that will result

from the non-linear behavior. Buildings thatneed to rely on a high level of non-linearbehavior to achieve their target displacementbenefit most from the NDP. Buildings thatare heavily damaged in earthquakes, butremain standing straight up, illustrate thebeneficial effects of the time history record.Only the NDP can come close to predictingsuch behavior.

Summary and ConclusionHaving a new state-ofthe-art standard to

evaluate and rehabilitate buildings is onlypart of what is needed to achieve seismicsafety. ASCE 41 allows the use of a varietyof earthquake threats and defines three per-formance levels: immediate occupancy, lifesafety and collapse prevention. The ASCE 41process assists the engineer in determining theexpected performance, but seismic safety isachieved when a proper evaluation is done,the resulting options are understood, andappropriate answers selected and imple-mented. Owners need to know what is goingto happen in terms of life threatening injuries,

what it will cost to repair their buildings, andhow long the buildings will not be usable.Fortunately, this information can be deducedfrom the ASCE 41 analysis. When reportingthe results to owners, it is best to focus on thedescription of expected damage before andafter the proposed rehabilitation, as opposedto the minutia of the evaluation itself. In that

way, owners will be able to relate the impactthe performance will have on their use of thebuildings and they will have a basis for deter-mining what to do.

As structural engineers, we need to advocateseismic safety in our communities and carry outthe role of earthquake engineers on every project.

We need to develop a clear understanding of thedifference between designing a new buildingfollowing the prescriptive requirements of thecode and design existing buildings followingthe performance-based evaluation approaches.The nations inventory of existing buildingsthat do not meet minimum seismic safetylevels is very large, and the cost to replace

them is out of the question, not to mentionunnecessary. ASCE 41 is the best tool kit ofprocedures available that allows for site specificand deliberate performance based evaluation

for rehabilitation. And, when applied propethe necessary rehabilitation can be compleat a minimum cost for the owners and withliability for the structural engineers.

New building code requirements are calling foradhesive anchors that perform in cracked-concrete

conditions. To meet this need Simpson Strong-Tieis proud to introduce SET-XP anchoring epoxy.SET-XP adhesive has been formulated and tested toperform to 2006 IBC standards for rebar dowelingand anchoring using threaded rod or our newIXP anchor, which offers higher loads with lessembedment for cracked-concrete applications. Andto make designing under the new code easier, weoffer our free Anchor Designer software. Whencode changes leave you looking for answers, lookto a trusted partner, look to Simpson Strong-Tie.

For more information, or to download the AnchorDesigner, visit www.simpsoanchors.comor callone of our Field Engineers as (800) 999-5099.

Is your adhesive ready?2008SimpsonStrong-T

ieCompanyInc.

SET

XP08-S

IBC2006

TESTEDTO

MEET

IN THE SPECS

ON THE JOB

AT YOUR SERVICE

Chris D. Poland, S.E., F. SEAOC is theCEO of Degenkolb Engineers. Mr. Polanda Senior Principal, is Chair of the ASCESEI Standards Committee on SeismicRehabilitation of Existing Buildings

Committee and Chair of the NEHRPAdvisory Committee on EarthquakeHazards Reduction. He can be reached viaemail at [email protected].

Documents

C CS Poland Sept08