Ex­pert re­ac­tion com­ment

DEMM Engineering & Manufacturing - - EARTHQUAKE -

THE VI­A­BIL­ITY OF NEW ZEALAND’S I NFRASTRUCTURE By Pro­fes­sor John Tookey, Head of Depart­ment Built En­vi­ron­ment En­gi­neer­ing, Auck­land Univer­sity of Tech­nol­ogy.

The struc­tural fail­ure of mod­ern build­ings, built with mod­ern struc­tural sys­tems and to mod­ern earth­quake de­sign codes is hor­ri­fy­ing to many. Given that en­gi­neers rou­tinely over de­sign struc­tural so­lu­tions to a safety fac­tor of be­tween 1.5 and 2.5 times the ‘worst case’ load­ing. Thus, it in­stan­ta­neously puts into ques­tion the vi­a­bil­ity of so much of our in­fra­struc­ture.

Not un­rea­son­ably the gen­eral public and en­gi­neers want an­swers and want them quickly. When the dust set­tles in Welling­ton and sur­rounds, in­ves­ti­ga­tions will be thor­ough and ex­haus­tive.

In com­mon with any en­gi­neer­ing disas­ter – or near disas­ter – of this type, the sus­pects are al­ways the same. It will come down to ei­ther de­sign fail­ure, ma­te­ri­als fail­ure, con­struc­tion process fail­ure or flawed en­gi­neer­ing as­sump­tions – very of­ten a com­bi­na­tion of these fea­tures. This last fea­ture – flawed as­sump­tions - is in­struc­tive.

In truth, en­gi­neer­ing as­sump­tions are our Achilles Heel as en­gi­neers. Lines have to be drawn some­where. We can’t de­sign for ev­ery­thing be­fore it be­comes pro­hib­i­tively ex­pen­sive. The de­sign­ers of the Ti­tanic as­sumed flood­ing of four com­part­ments was worst case – the ice­berg breached five.

The case study of the Cit­i­group Cen­tre in New York in the late 1970s is sim­i­larly dis­turb­ing – and worth read­ing. En­gi­neers de­signed the build­ing to cope with wind act­ing per­pen­dic­u­lar to the struc­ture as per build­ing de­sign reg­u­la­tions. But de­tails of the de­sign meant that the build­ing was vul­ner­a­ble to hur­ri­cane force winds act­ing at oblique an­gles. Quite lit­er­ally the build­ing could have been blown over with mass ca­su­al­ties. Rec­ti­fi­ca­tion took sev­eral months of work and was kept from the public for nearly 20 years.

I could go on and on. Ahead of the find­ings of any fu­ture in­quiries on this cat­a­strophic event, huge lessons will be learned. Pro­fes­sional en­gi­neers will stop and rig­or­ously eval­u­ate what went wrong. De­sign codes will be re­assessed and re­de­fined. Ex­ist­ing build­ings will be as­sessed in the light of the find­ings. Rec­ti­fi­ca­tion works will be un­der­taken on af­fected build­ings. Cur­rent en­gi­neers will be ret­ro­spec­tively up­skilled to re­flect the find­ings. En­gi­neer­ing ed­u­ca­tion will be amended to re­flect the new knowl­edge – mak­ing en­gi­neers of the fu­ture that much more knowl­edge­able.

The dam­age is still fresh and the wounds are still raw. Per­versely the out­comes of these events in the fu­ture is a bet­ter, safer, more sus­tain­able built en­vi­ron­ment.

CON­CEPT FOR MOD­ERN BUILD­ING DE­SIGN FOR SE­VERE EARTH­QUAKE By Dr Charles Clifton, As­so­ciate Pro­fes­sor of Civil En­gi­neer­ing and Struc­tures Group Leader, Univer­sity of Auck­land.

We de­sign mod­ern build­ings for de­fined con­di­tions of per­for­mance in three lev­els of earth­quake. These lev­els are called Limit States:

• The Ser­vice­abililty Limit State (SLS) earth­quake is ex­pected to oc­cur with around

90 per cent prob­a­bil­ity in the 50-year de­sign life of a typ­i­cal build­ing.

• The Ul­ti­mate Limit State (ULS) earth­quake is ex­pected to have 10 per cent

prob­a­bil­ity of oc­cur­rence 50-year de­sign life of a typ­i­cal build­ing.

• The Max­i­mum Con­sid­ered Event (MCE) earth­quake is ex­pected to have ap­prox­i­mately a two per­cent prob­a­bil­ity of oc­cur­rence in 50-year de­sign life of a typ­i­cal build­ing.

The earth­quake ac­tions gen­er­ated by these limit states, for build­ings up to around 10 to 15 storeys high, are very large com­pared with those of other lat­eral load­ing con­di­tions such as wind. The SLS earth­quake ac­tions, for ex­am­ple, will be higher than the ULS wind (which has a 400 year re­turn pe­riod); the ULS earth­quake ac­tions will be some five times higher than the SLS earth­quake ac­tions and the MCE earth­quake ac­tions some eight times higher than the SLS earth­quake ac­tions.

The ULS and MCE earth­quakes are low prob­a­bil­ity events. To de­sign a typ­i­cal build­ing to re­main un­dam­aged in these events is a ma­jor cost and so build­ings are typ­i­cally de­signed to un­dergo con­trolled dam­age in these lev­els of earth­quake, pro­tect­ing the oc­cu­pants at the ex­pense of con­trolled struc­tural dam­age. Through this trade­off of strength for dam­age, the ULS de­sign ac­tions for the build­ing struc­tural sys­tem can be low­ered to the SLS level, but at the ex­pense of struc­tural dam­age be­ing gen­er­ated when the earth­quake ex­ceeds the SLS level.

A good anal­ogy for this is de­sign of a car:

• The SLS earth­quake is the most likely mi­nor ac­ci­dent in a su­per­mar­ket car park or the lo­cal high street. There may be some panel dam­age but the car re­mains fully func­tional.

• The ULS earth­quake is the less likely ac­ci­dent on a ma­jor sub­ur­ban road. The car will be dam­aged and will need re­pair or at worst re­place­ment but the oc­cu­pants will be safe and ex­pect­edly un­in­jured.

• The MCE earth­quake is a head-on crash on the open road. Hope­fully the oc­cu­pants will sur­vive OK; the build­ing will be dam­aged be­yond re­pair. We have con­sid­er­able ex­pe­ri­ence now in New Zealand on how well our mod­ern build­ings per­form in the dif­fer­ent lev­els of earth­quake in­ten­sity. Gen­er­ally, the per­for­mance is demon­stra­bly bet­ter than these lim­its. For ex­am­ple, the Christchurch 2010/2011 se­ries of six dam­ag­ing earth­quakes (i.e. above SLS level) in the CBD com­prised a MCE event in in­ten­sity and du­ra­tion, but which took place over some 18 months in­stead of in one ini­tial very large event with a se­ries of typ­i­cal af­ter­shocks. Only the CTV build­ing col­lapsed; some mod­ern medium to high rise build­ings sur­vived with no struc­tural re­pair needed. Other re­cent earth­quakes have shown that for most build­ings the dam­age thresh­old is some two to three times higher than the mod­els would in­di­cate.

How Se­vere Was the Kaik­oura Earth­quake in Welling­ton?

The best mea­sure of dam­age po­ten­tial is the in­ten­sity, ex­pressed by the Peak Ground Ac­cel­er­a­tion (PGA). Du­ra­tion also mat­ters for se­vere earth­quakes but is less crit­i­cal.

The ULS de­sign level PGA for a typ­i­cal build­ing in Welling­ton is 0.40g, where g is the ac­cel­er­a­tion due to grav­ity. (Air­craft tur­bu­lence on a very rough flight is around 0.2g to 0.25g.) The ser­vice­abil­ity level is around 0.1g. Dam­age thresh­old for a mod­ern build­ing de­signed for max­i­mum con­trolled dam­age is around 0.15g.

“The Kaik­oura earth­quake gen­er­ated PGA val­ues in Welling­ton of be­tween 0.2g and 0.3g. So the wide­spread level of struc­tural dam­age in mod­ern build­ings de­signed for con­trolled dam­age in a se­vere earth­quake is to be ex­pected.”

Why are some build­ings more af­fected than oth­ers?

This de­pends on the size and shape of the build­ing and the type of ground it is on. Soft re­claimed ground moves more vi­o­lently in an earth­quake than solid ground, hence is more dam­ag­ing to build­ings. If the ground shakes it starts to de­velop a res­o­nant shak­ing fre­quency, such as you can get shak­ing a bowl of jelly, and this matches the vi­bra­tion re­sponse of the build­ing, the dam­age will be higher.

Also earth­quakes ex­pose weak­nesses in de­sign or con­struc­tion of a build­ing, such as in­ad­e­quate ty­ing to­gether of struc­tural com­po­nents. All this leads to com­plex and con­sid­er­able dif­fer­ences in build­ing re­sponse, even in ar­eas of close prox­im­ity. This wide vari­a­tion is es­pe­cially the case in an earth­quake of the in­ten­sity some­where be­tween the SLS and ULS limit states, as some build­ings will re­main elas­tic and oth­ers will un­dergo dam­age. When the earth­quake gets very se­vere – e.g. as in the 22 Fe­bru­ary, 2011 Christchurch earth­quake – ev­ery­thing is dam­aged to some ex­tent.

Re­sponse spec­tra for soils

It is clear that for build­ings on class D soils, which is all the Lambton Quay and down­town area and much of the Taranaki St and Courte­nay place area, if the build­ing pe­riod (i.e. the time it would take if you pulled the top of a build­ing over side­ways, let it go and mea­sured how long it took to sway to the other side and come back again) is be­tween about 0.9 to 1.5 sec­onds, this has been closer to the ULS event in in­ten­sity than to the SLS event. Mod­ern, medium rise, flex­i­ble build­ings are in that range so this ex­plains why dam­age is con­cen­trated into those build­ings on soft soils.

The above para­graph refers to the re­sponse spec­tra from EERI here:

Newspapers in English

Newspapers from New Zealand

© PressReader. All rights reserved.