Publisher’s version / Version de l'éditeur:
Construction Technology Update, 1999-05-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Seismic evaluation and upgrading of buildings
Allen, D. E.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=3c00d851-1e60-4ba8-be51-8ba6dec39146 https://publications-cnrc.canada.ca/fra/voir/objet/?id=3c00d851-1e60-4ba8-be51-8ba6dec39146by D.E. Allen
This Update briefly review s the main factors that determine the extent of
building failure and loss of life during earthquakes. It also describes
guidelines for evaluating and upgrading existing buildings w ith regard
to earthquake resistance.
C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 2 6
Requ irem en ts in Part 4 of th e Nation al Bu ild in g Cod e of Can ad a (NBC) relatin g to earth qu ake-resistan t d esign are w ritten p rim arily for n ew bu ild in gs an d can n ot easily be ap p lied to existin g bu ild in gs. How ever, th ere are m an y old er bu ild in gs w ith stru ctu ral system s, com p on en ts or m aterials th at are n ot ad d ressed by th e NBC. Attem p ts to ap p ly th e Part 4 requ
ire-m en ts to ire-m ake th ese bu ild in gs earth qu ake resistan t h ave often resu lted in m od ifica-tion s th at w ere in vasive, im p ractical an d exp en sive.
Several seriou s earth qu akes in North Am erica over th e p ast d ecad e or so h igh -ligh ted th ese d ifficu lties an d in d icated th e d earth of in form ation available to con su ltan ts for evalu atin g an d u p grad in g bu ild in gs. To
ad d ress th is lack of in form ation , th e In stitu te for Research in Con stru ction , w orkin g w ith p artn ers th rou gh ou t Can ad a, d evelop ed an d p u blish ed a set of th ree gu id elin es. Tw o of th e th ree gu id elin es are referen ced in Com m en tary K of th e User’s
Gu id e – N BC 1995 S tru ctu ral Com m en taries (Part 4), w h ich
p rovid es gu id an ce on th e ap p li-cation of NBC Part 4 requirements to existin g bu ild in gs.1 Th e scop e
of th e th ree IRC gu id elin es an d a n ew CSA gu id elin e n ow bein g p rep ared are d escribed in th is Up d ate.
Ea rt hqua k e s a nd Buildings
An earthquake is caused by a sudden grinding slippage between two parts of the earth’s crust, which propagates motions in the surrounding ground. These ground motions, which occur in all directions, shake buildings and
Seismic Evaluation and
Upgrading of Buildings
2
can lead to collapse or cause building compo-nents to fall, either of which can be life threat-ening. Buildings can also be damaged to the point where they are unusable or prohibitively expensive to repair.
Main Factors that Determine Building Failure
Wh eth er or n ot a bu ild in g su rvives an earth qu ake d ep en d s p rim arily on h ow it beh aves w h en su bjected to th e grou n d m otion s gen erated by th e earth qu ake. Th e m ain factors th at con trol th is beh aviou r are d iscu ssed below.
Se ism icity. Th is term refers to th e exp ected
seism ic grou n d m otion s, w h ich are d eter-m in ed by th e eter-m agn itu d e of earth qu akes an d th eir frequ en cy of occu rren ce in variou s region s of Can ad a (see Figu re 1). For each location , th e NBC sp ecifies a m agn itu d e of grou n d m otion th at h as a 10% p robability of occu rrin g on ce in 50 years; it is th is m agn itu d e of sh akin g, categorized in term s of seism ic zon es ran gin g from 0 (low m agn itu d e) to 6 (h igh m agn itu d e), th at a bu ild in g m u st be d esign ed to w ith stan d .
Inte grity. Th is term refers to th e d egree to
w h ich com p on en ts of a bu ild in g are in ter-con n ected an d th u s able to p reven t th e bu ild in g from bein g sh aken ap art by an earth qu ake. Th e com p on en ts th at affect a bu ild in g’s in tegrity in clu d e n ot on ly th e stru ctu ral com p on en ts (e.g., beam s, colu m n s, w alls an d fou n d ation s), bu t also th ose su p p orted by th e bu ild in g stru ctu re (e.g., h eavy p artition s an d equ ip m en t). For a bu ild in g on firm grou n d in a location of low seism icity, lack of in tegrity is likely to be its on ly seism ic d eficien cy, i.e., th e on ly factor th at cou ld lead to d am age or collap se.
La te ra l stre ngth/d uctility . Horizon tal
sh akin g p rod u ces h orizon tal forces
th rou gh ou t th e bu ild in g th at are tran sferred th rou gh th e floors to th e vertical stru ctu re an d d ow n in to th e grou n d . Th e critical p rop erty in term s of p reven tin g failu re is th e vertical stru ctu re’s ability to resist h orizon tal forces ap p lied to each storey (i.e., its lateral stren gth ).
Equ ally im p ortan t in areas of m ed iu m to h igh seism icity, w h ere very large earth -qu ake forces can occu r, is th e ability of th e vertical stru ctu re to yield u n d er th e forces (d u ctility) w ith ou t com in g ap art, an d to tran sfer force from overload ed com p on en ts to oth er com p on en ts (red u n d an cy). Som e bu ild in g com p on en ts, su ch as clay-tile
p artition s in fram e stru ctu res, h ave n o d u ctility an d m ay fail su d d en ly an d exp losively, releasin g en ergy, w h ich p rom otes collap se of th e bu ild in g.
La te ra l stiffne ss. Lateral forces from an
earth qu ake d istort th e vertical stru ctu re betw een floors, w h ich can cau se d am age to bu ild in g com p on en ts attach ed to th e stru ctu re (e.g., p artition s an d service lin es) an d ren d er th e bu ild in g u n u sable. Th e lateral stiffn ess of th e vertical stru ctu re con trols d istortion , w h ich is critical in p re-ven tin g th e failu re of attach ed com p on en ts. Often th is m ean s th at sh ear w alls are requ ired , as th ey are m u ch stiffer u n d er lateral forces th an colu m n s.
Build ing irre gula ritie s. A bu ild in g w ith ou t
irregu larities is on e w h ose vertical stru ctu re is sym m etrical in p lan w ith con tin u ou s colu m n s or w alls from top to bottom so th at earth qu ake forces are tran sferred d irectly to th e grou n d . Som e of th e irregu larities th at can p rom ote d am age or collap se are sh ow n in Figu re 2.
S oft or unsta b le ground cond itions.
Bu ild in gs on rock u su ally su rvive earth -qu akes m u ch better th an th ose w ith fou n d a-tion s on soft or u n stable soil. Soft grou n d sh aken by th e rock below vibrates like a bow l of jelly, am p lifyin g th e seism ic m otion in th e rock, an d resu ltin g in greater d istortion s an d forces in th e bu ild in g. In th e Sagu en ay earth qu ake of 1988, for exam p le, th e d istortin g fram e stru ctu res im p acted on con crete-block p artition s, cau sin g th em to fractu re an d collap se.
Soft grou n d m ay also be u n stable, an d can liqu efy (like qu icksan d ) or slid e d u rin g an earth qu ake, resu ltin g in large grou n d distortions and severe damage to the building. Se ism ic Eva lua t ion a nd
U pgra ding of Buildings
Th e follow in g gu id elin es are recom m en d ed to h elp stru ctu ral con su ltan ts an d bu ild in g m an agers carry ou t seism ic m itigation of bu ild in gs at m in im u m cost an d d isru p tion .
Screening Buildings for Seismic Evaluation
Th e IRC d ocu m en t, Man u al for S creen in g
of Bu ild in gs for S eism ic In vestigation ,2is
recom m en d ed as a tool to h elp p rop erty m an agers 1) d eterm in e w h ich bu ild in gs n eed an en gin eerin g evalu ation an d 2) ran k th em w ith resp ect to th eir n eed for atten tion . Th e m eth od is based on a rap id in sp ection (ap p roxim ately an h ou r) of each bu ild in g or its d raw in gs. Th e in sp ector u ses a form to
Seismic Evaluation
Engineering evalu ation s can be d on e u sin g IRC’s Gu id elin es for S eism ic Evalu ation of
Ex istin g Bu ild in gs.3Th is d ocu m en t can p
ro-vid e th e m ean s for con d u ctin g con sisten t an d cost-effective en gin eerin g evalu ation s of all bu ild in gs excep t sm all bu ild in gs fallin g w ith in th e scop e of Part 9 of th e NBC. It can be ap p lied to m ost bu ild in gs w h ere th e p reven tion of collap se an d loss of life is th e p rim ary con cern , e.g., ap art-m en t an d office bu ild in gs. It can also be u sed to evalu ate p ost-d isaster bu ild in gs su ch as h osp itals; h ow ever, ad d ition al requ irem en ts m u st be m et to en su re th at th e bu ild in g can be u sed for p ost-d isaster services.
Th is p u blication en ables a qu ick evalu ation u sin g a ch ecklist of p oten tial d eficien cies based on life-th reaten in g failu res d u rin g p ast earth qu akes, m ain ly in Californ ia an d Alaska. Som e of th e obtain a “score” for each bu ild in g based
on th e follow in g seism ic risk factors: • seism icity
• grou n d con d ition s
• type and age of construction (both of which influence integrity, strength and ductility) • bu ild in g irregu larities
• u se (e.g., h osp ital or office)
• p resen ce of h eavy or d an gerou s n on -stru ctu ral bu ild in g com p on en ts, w h ich m ay fall, or bu ild in g services lin es an d equ ip m en t, w h ich m ay fail.
Th e m an u al p rovid es:
• gu id an ce on h ow to organ ize an d carry ou t a seism ic screen in g;
• in form ation , or in form ation sou rces (e.g., for grou n d con d ition s), n eed ed to com p lete an evalu ation ;
• a consistent approach for use by inspectors. Th is gu id elin e sh ou ld n ot, h ow ever, be u sed to con d u ct an en gin eerin g evalu ation of a bu ild in g.
item s on th e ch ecklist requ ire on ly a “back-of-th e-en velop e” calcu lation , follow ed , if n ecessary, by a m ore d etailed evalu ation of item s th at are u n certain or bord erlin e. Th is p roced u re p rovid es a w ay of d eter-m in in g a bu ild in g’s d eficien cies an d ran k-in g th em at m k-in im u m cost.
For th e m ost p art, th e criteria u sed for th e stru ctu ral evalu ation of an existin g bu ild in g m u st follow Part 4 of th e NBC. How ever, th e NBC-sp ecified seism ic load is red u ced by 40% for existin g bu ild in gs becau se of th e large cost associated w ith stru ctu ral in terven tion com p ared to th e sm all extra cost of ach ievin g seism ic safety in n ew con stru ction . Wh en th e calcu la-tion s sh ow th at th e bu ild in g com p on en ts are n ot able to w ith stan d th is red u ced (40% ) seism ic load , th ey sh ou ld , u n d er m ost circu m stan ces, be u p grad ed an d d esign ed for th e fu ll seism ic load sp ecified by th e NBC.
A sp ecial p roced u re is in clu d ed for th e evalu ation of u n rein forced m ason ry bu ild -in gs w ith w ood floor an d roof stru ctu res, a form of con stru ction n o lon ger p erm itted by Part 4 of th e NBC in earth qu ake-p ron e region s.
A n ew stan d ard on th e seism ic evalu a-tion of existin g bu ild in gs, in clu d in g p ost-d isaster bu ilost-d in gs, is bein g ost-d evelop eost-d in th e Un ited States.4 It w ill con tain an
u p d ated ch ecklist th at takes in to accou n t
th e exp erien ce gain ed from recen t earth -qu akes in Mexico, th e Un ited States an d Jap an . Of sp ecial con cern are w eld ed steel m om en t fram es, m an y of w h ich fractu red d u rin g th e Los An geles earth qu ake of 1994 (see Referen ce 5 for gu id an ce). In ord er to be able to ap p ly th is n ew stan d ard in Can ad a, h ow ever, ad ju stm en ts w ill h ave to be m ad e to th e U.S.-based criteria.
Seismic Upgrading
Th e IRC d ocu m en t, Gu id elin e for S eism ic
Up grad in g of Bu ild in g S tru ctu res,6
d escribes variou s seism ic retrofits an d p rovid es gu id an ce on m akin g th e righ t ch oices for sp ecific p rojects.
Most of th e retrofits are con ven tion al con stru ction tech n iqu es, an d in clu d e: • an ch orin g m ason ry an d oth er h eavy
com p on en ts to th e bu ild in g stru ctu re (Figu re 3);
• p lacin g con n ectors betw een existin g stru ctu ral com p on en ts;
• con n ectin g n ew stru ctu ral com p on en ts (m em bers, overlays, an d in fills) to existin g com p on en ts (Figu re 4); an d • bu ild in g n ew su b-system s su ch as sh ear
w alls, bracin g system s or ad d ition al fou n d ation elem en ts, an d con n ectin g th em to th e existin g stru ctu re (Figu re 5).
Sp ecial retrofits in clu d e th e ad d ition of d am p in g d evices to red u ce d istortion s an d forces d u e to earth qu akes; th e ad d ition of
4 Construction Technology Update No. 26
flexible bearin g p ad s betw een th e fou n d ation an d th e su p erstru ctu re (base isolation ) to red u ce th e tran sm ission of h orizon tal grou n d m otion s to th e stru ctu re; an d soil-stabilization tech n iqu es, su ch as vertical gravel d rain s, to p reven t soil liqu efaction .
Th e ch oice of retrofits an d th eir location in th e bu ild in g d ep en d s n ot on ly on correctin g stru ctu ral d eficien cies (see “Main Factors th at Determ in e Bu ild in g Failu re” above) bu t also on th e follow in g issu es.
A cce ssib ility. Th is refers to th e ease or
d ifficu lty w ith w h ich th e con tractor is able to gain access to th e bu ild in g com p on en ts in ord er to carry ou t th e retrofit. Th e m ajor con sid eration s are as follow s:
• typ e, qu an tity an d location of retrofits; • n eed for scaffold in g, cran es or oth er
sp ecial equ ip m en t; an d
• sp ace available to p erform th e w ork. Th e m ore d ifficu lt th e access, th e greater th e cost an d d isru p tion , an d th e less ch oice th ere is w ith resp ect to retrofits. Fou n d ation u p grad in g is p articu larly exp en sive becau se access is u su ally very d ifficu lt; h ow ever, it can
often be avoid ed by in corp oratin g oth er elem en ts, su ch as sh ear w alls or bracin g, in to existin g fram es.
Disrup tion. If th e bu ild in g m u st be u sed
d u rin g th e u p grad in g, d isru p tion becom es a m ajor con sid eration . For th is reason , seism ic retrofits are best carried ou t d u rin g a m ajor ren ovation , w h en th e bu ild in g is sch ed u led to be u n occu p ied . If th is ap p roach is n ot an op tion , retrofits m u st be carried ou t in stages, sh iftin g p eop le an d op eration s, or u n d ertakin g w ork ou tsid e bu sin ess h ou rs, all of w h ich in crease th e cost. Altern atively, exterior retrofits (bracin g or fou n d ation system s) are less d isru p tive th an in terior retrofits. In th e case of h osp itals, for exam p le, exterior retrofit w ou ld likely be th e p referred ap p roach .
Build ing function. New stru ctu ral com p
o-n eo-n ts, su ch as sh ear w alls or bracio-n g, cao-n n egatively affect th e layou t of th e bu ild in g (an d h en ce traffic flow ), d ayligh t, or aes-th etics. For aes-th is reason , m om en t fram es m ay be p referable to bracin g or sh ear w alls in certain location s.
Figure 4.Overlays added to walls
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca © 1999
Nation al Research Cou n cil of Can ad a May 1999
ISSN 1206-1220
A e sthe tics/he rita ge v a lue . Th e p
reserva-tion of a bu ild in g’s aesth etics an d its h eritage valu e is esp ecially ch allen gin g.
Th e en gin eer m u st w ork closely w ith th e ow n er, th e arch itect, th e con tractor an d an y sp ecialists (e.g., a h eritage con su ltan t) to select a retrofit ap p roach th at best ad d resses an d resolves all th ese issu es.
New Guideline for Non-Structural Components
A n ew d ocu m en t, Gu id elin e for S eism ic
Risk Red u ction of Op eration al an d Fu n ction al Com p on en ts of Bu ild in gs,7
w h ich d eals w ith th e seism ic evalu ation an d u p grad in g of n on -stru ctu ral bu ild in g com p on en ts, is n ow bein g p rep ared by th e Can ad ian Stan d ard s Association . It w ill recom m en d p roced u res an d criteria to m itigate seism ic risk at m in im u m cost an d d isru p tion .
A sep arate gu id elin e on n on -stru ctu ral bu ild in g com p on en ts is n eed ed becau se n on -stru ctu ral retrofits can often be carried ou t as p art of a regu lar m ain ten an ce p ro-gram w ith little d isru p tion to bu ild in g activities. In areas of low to m ed iu m seism icity, th e failu re of n on -stru ctu ral bu ild in g com p on en ts d u rin g an earth qu ake often p oses a greater risk th an stru ctu ral failu re. Th e 1988 Sagu en ay earth qu ake, in w h ich m ost of th e d am age w as d u e to th e failu re of con crete-block p artition s, is a recen t exam p le of th is.
Re fe re nc e s
1. User’s Gu id e — NBC 1995 Stru ctu ral Com m en taries (Part 4). Can ad ian
Com m ission on Bu ild in g an d Fire Cod es, Nation al Research Cou n cil of Can ad a, Ottaw a, 1996. 135 p . (NRCC 38826). 2. Man u al for Screen in g of Bu ild in gs for
Seism ic In vestigation . In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a, Ottaw a, 1993, 88 p . (NRCC 36943).
3. Gu id elin es for Seism ic Evalu ation of Existin g Bu ild in gs. In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a, Ottaw a, 1993, 150 p .
(NRCC 36941).
4. FEMA 310: Han d book for Seism ic
Evalu ation of Bu ild in gs — A Prestan d ard . Fed eral Em ergen cy Man agem en t Agen cy, Wash in gton , DC, Jan u ary 1998 (d raft of an Am erican Society of Civil En gin eers Stan d ard to be p u blish ed in 1999).
5. FEMA 267. In terim Gu id elin es: Evalu ation , Rep air, Mod ification an d Design of Weld ed Steel Mom en t Fram e Stru ctu res. Fed eral Em ergen cy Man agem en t Agen cy, Wash in gton , DC, 1995. FEMA-267A. In terim Gu id elin es, Ad visory No. 1. Su p p lem en t to FEMA 267. Fed eral Em ergen cy Man agem en t Agen cy, Wash in gton , DC, 1997.
6. Gu id elin e for Seism ic Up grad in g of Bu ild in g Stru ctu res. In stitu te for Research in Con stru ction , Nation al Research Cou n cil of Can ad a, Ottaw a, 1995, 47 p .
(NRCC 38857).
7. Guideline for Seismic Risk Reduction of Operational and Functional Components of Buildings. Draft CSA Standard S832-2000. Canadian Standards Association, Etobicoke, Ontario (to be published in 1999).
Dr. D.E. A lle nis a gu est research officer in th e Bu ild in g En velop e an d S tru ctu re Program at th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
