what we learned from christchurch taking the risk out ... - PlaceMakers

More documents

Recommendations

Info

FEATURESBRANZEarthquake performanceBRANZ research shows that houses built to the current Building Code and standards generally stood upwell to the Canterbury earthquakes. However, more complex, specifically designed houses often had issuesThe earthquakes that have struckCanterbury since 4 September2010 have provided anopportunity for engineers to comparethe actual performance of a widerange of structures against theoreticalexpectations.This has been particularly true forBRANZ structural engineers, whohave been closely involved from theinitial event, first in the emergencyresponses to the main earthquakes of4 September 2010, 22 February 2011and 13 June 2011, and then in therecovery operations centred on theregion’s housing stock.HOUSE AND GROUNDPERFORMANCEFollowing the June 2011 earthquake,BRANZ conducted a comprehensiverandom survey of over 300 housesin Christchurch to gain a betterappreciation of the performance ofthe range of house types, includingthe effects of ground conditions.Houses on the flat had either beenaffected by ground failure resultingfrom the earthquakes (liquefaction)or by earthquake shaking. Whilepredominantly affected by shaking,some hillside houses were also affectedby ground instability such as rock fall,rock roll, and slope and cliff failure.To varying degrees, liquefactionaffected a large proportion of houseson the flat. Of the properties surveyedon the flat, 30% experienced liquefactionof the site, with half of these havingliquefaction in all three major earthquakeevents before the survey.CHRISTCHURCH FOUNDATION STYLESThe survey sample confirmed there havebeen two predominant foundation stylesin Christchurch over the history of thecity. Houses built in the early and middle20th century generally have floors onpiles with perimeter concrete foundationwalls, while most houses built since 1980have concrete slab-on-ground foundations.When the ground liquefied, neitherfoundation style was sufficiently robust,leaving many houses uninhabitable.Many of the perimeter concrete foundationwalls supported heavy brick or blockcladdings and heavy concrete or clay tileroofs. When the liquefaction was severe,these walls settled into the ground at agreater rate than the more lightly loadedpiles, resulting in a badly distorted house.The slab-on-ground floors were also hithard by the soil liquefaction. Many ofthe slabs were unreinforced (acceptablein NZS 3604 construction before theearthquakes), and those that were hadbrittle reinforcing mesh fractured as theslab distorted. The then DBH issued anamendment to its citation of NZS 3604,requiring all new floors in New Zealandbe reinforced with ductile reinforcing.NEED FOR STRONGER FOUNDATIONSThe Engineering Advisory Group(EAG) has developed a range ofmore robust foundation systems fornew houses on properties wherethere is a likelihood of liquefaction infuture earthquakes. Some foundationsare expected to accommodate theassociated ground distortions withminimal distortion of the house –stiff rafts – while others have beendesigned so re-levelling is a relativelyeasy process – suspended timber floors.Other solutions involve treating theground before the house is built tocontain the liquefying soils and providea stiff soil platform for the foundation.Repairs would still be required in manyinstances, but these would be minor andthe disruption to occupants is expectedto be low.Now, Christchurch probably has thebest database of subsoil informationof any territorial authority (TA) in thecountry, allowing a more informedselection of appropriate foundationsystem for the conditions.Other TAs with known areas ofliquefiable soils, particularly where thereis also potentially high seismicity, shouldconsider what house foundations orground treatments are most appropriateto ensure resilient performance.14www.branz.co.nzThis house was effectively destroyed because of damage to foundations resulting from liquefaction
GEOTECHNICAL STUDY FORNEW SUBDIVISIONSThe greatest opportunity comes withnew subdivisions, where global treatmentcan be undertaken to improve the soilcharacteristics of the whole area beforenew houses are built, if required. The EAGhas developed guidance for geotechnicalassessments of proposed new subdivisionsin the Canterbury earthquake regionthat are also likely to be applicable inother areas of the country.BEST STRUCTURAL PERFORMERSThe performance of light timber-framedand steel-framed houses generallyconfirmed predictions that these structuresare resilient to violent shaking. Theauthors are not aware of any of thesestructures – which generally fit withinthe constraints of NZS 3604 – collapsingin any of the earthquakes.Undeniably, damage was sustained,but these houses had sufficient energydissipation capability to resist theearthquakes and remain standing,allowing occupants to escape injury ordeath – the primary performance objectiveof the New Zealand Building Code.BRACING STOOD UPModern, framed houses rely heavilyon plasterboard linings to providethe primary bracing resistance, whileolder houses have diagonal timberbraces fitted into the wall framingbehind the linings.Both methods of bracing providedadequate resistance to lateralWHAT’S THAT ALL ABOUT THEN?When the ground liquefied, neither foundationstyle was sufficiently robust, leaving manyhouses uninhabitableloads, although the stiffness of theplasterboard sheet linings is greaterthan the diagonal braces, which meantlinings were damaged before or asthe diagonal braces took up load.Where liquefaction was notexperienced, there was little observedmajor damage to the sheet bracingsystems, suggesting that no changeswere required to NZS 3604:2011.STIFFNESS ISSUES IN COMPLEX HOUSESGreater levels of damage wereobserved in more complex specificallydesigned houses that fell outside thescope of NZS 3604. This tended to becaused by discontinuities in floor plates– split level styles and irregularity, bothin plan and vertically. It was also clearthat, while houses with uneven stiffnessdistribution did not collapse, there wasoften significant non-structural damage.An area of specific design requiringgreater consideration is the need toTEST YOUR KNOWLEDGE!Attention!check the likely response of structuraldesigns with stiffness irregularities– often the case when houses havebeen built with large windowopenings. These designs needappropriate measures to eithermitigate the stiffness incompatibilityor ensure windows and other nonstructuralitems can accommodatethe expected deflections.CODES AND STANDARDS PASSEDTHE TESTOverall, the performance of the housesof Christchurch and Canterbury wasvery encouraging from the life safetyviewpoint. While there was a lot ofcostly damage and loss of amenity,this sequence of events producedlevels of shaking significantly abovethe design values.The overriding lesson for our industryis a better application of existing Codesand standards rather than wholesalechanges on a countrywide basis.Are you a building contractor who pays levy fees through a consent authority?If so, then you are entitled to a free subscription of BUILD magazine from BRANZ.Simply email publicationsales@branz.co.nz to check that you meet therequired criteria and get your subscription.10) a b c11) a b c12) a b cWhat structures were generally resilientto violent shaking in Christchurch?a) Light timber-framed and steel-framedhouses.b) Houses built of Lego.c) More complex specifically designedhouses.During ground liquefaction, whichfoundation style was sufficiently robust?a) Perimeter concrete foundation walls.b) Concrete slab-on-groundfoundations.c) Neither.Greater levels of damage to housesin Christchurch tended to be caused by:a) Discontinuities in floor plates.b) Godzilla.c) Small window openings.NB: The questions and answers in this section have been produced by the publisher and do not necessarily reflect views or opinions of the contributing organisation.15
Page 2 and 3: FOREWORDLessons to be learnedHistor
Page 5 and 6: BUILDERS’ BUSINESSWe want your in
Page 7 and 8: They work hard, play harder down So
Page 9 and 10: REPAIRING CRACKS INSLAB AND FOUNDAT
Page 11 and 12: elevant act or omission. But even a
Page 13 and 14: At the same time, though, inspector
Page 15: Consented building work picks up in
Page 19 and 20: BUILTIN NZFEATURESWhat PLI doesn’
Page 21 and 22: textures. Information on segmentalp
Page 23 and 24: CHEMICALS AND NOISEChemicals, paint
Page 25 and 26: ) Create a uniform regime betweenre
Page 27 and 28: SPORT REPORTHurricanes get a kick f

what we learned from christchurch taking the risk out ... - PlaceMakers

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?