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[Econf_Moderator]

Pl find attached welcome note from Prof Swaminathan Krishnan


regards,
SEFI Admin�

Posted via Email

[thirumalaichettiar]

While going through the welcome address by Prof.Swaminathan Krishnan I observed the following words:
On the face of it, this seems to be a ratification of the ACI-�‐318 seismic provisions and suggests that such a code could be adopted for tall building design the world over.

But in the Recommendations for the Seismic Design of High-rise Buildings� by Michael Willford Andrew Whittaker Ron Klemencic from council of Tall Building and Urban Habitat is stated as below:

Whilst these codes( IBC, ICBO,1997) are referenced for the design of high-rise buildings in many countries, in part because the UBC still forms the basis for many national building codes, theyare not suitable for the design of high-rise buildings for the followingreasons:

1) They were developed for application to low and medium-rise buildings.
2) They permit only a limited number of structural systems for
建筑比49米高的身高,这不是economic for buildings of significantly greater height, and do not include systems that are appropriate for many high rise buildings.
3) Rules appropriate at or below 49m are not necessarily valid at 100+m in height.
4) The use of elastic response analysis with force reduction factors (denoted R in the United States) for strength design is inappropriate for buildings where several modes of vibration contribute significantly to the seismic response along each axis of a building.

I request Dr.S.Krishnan's Comment on this.

T.RangaRajan.

[swamikrishnan]

I will try to be as brief as possible (for greater details, pleaserefer to the documents given below). I will start by describing thecurrent building codes in the US, then give some background on thedevelopment of the "Alternative Design Guidelines (ADG)" that youreference, then list some important differences between the ADG and thecodes, and finally close with my thoughts on ADG vs Prescriptive Codesand questions to SEFI about the suitability of adopting either forIndian conditions.

1. US Codes:The established US codes are prescriptive,i.e., they define a set of rules on how to perform structural design --"thou shalt do this; thou shalt NOT do this, etc.". The most recent UScodes fall into three categories:

(a) ASCE 7 (Standard): Lays out the minimum design loads for buildingsand other structures, in addition to describing seismic designrequirements in detail (describes the various structural systems andthe limits of applicability of these systems).

(b)国际建筑规范(IBC):我这段代码s a "national"conglomeration of several regional codes of the past--the UniformBuilding Code, the Southern Standard Building Code, the BOCA NationalBuilding code, etc. The IBC describes how to perform basic structuralanalysis and design methods (static analysis, dynamic response spectralanalysis, or time history analysis), defines occupancy categories andseismic design categories (seismic design category corresponds to aspecific combination of seismic zone and occupancy category), loads andload combinations, classification of soils, design of foundations, andthen goes on to outline rules of design for structures using variousmaterials including steel, concrete, masonry, wood, glass, plasticetc. IBC references and relies heavily on the ASCE-7 standard forloads and load combinations and seismic design requirements, as well asother material-specific codes such as the ACI-318 for concrete, AISCManual of Steel Construction for steel, etc.

(c) Material-specific codes such as ACI-318 for concrete and AISCManual of Steel Construction for steel: These codes go into extremedetails of how concrete (or steel) structures must be designed anddetailed. Special requirements for seismic detailing are outlined ingreat detail in these documents (for steel, there is a separatedocument called "Seismic Provisions for Steel Buildings ANSI/AISC341-10" that handles seismic detailing; for concrete a separate chapterwithin ACI-318 lists the seismic detailing requirements in variousseismic zones).

2.背景交替德西的发展gn guidelines for tall buildings:在经济繁荣时期(也许更多的来往ctly, the real-estatebubble years), as house (and land) prices were sky-rocketing, there wasa great market for tall residential buildings in the US. Butarchitects called for an open exterior, free of "structure", tomaximize the sale price of the units and make tall buildingconstruction a profit-making proposition for the owner. This became aproblem, because the prescriptive codes have strict height limits. Inparticular, ASCE 7 requires a "dual-system" to be employed for allbuildings greater than 160 feet (BTW, the IBC classifies any buildingwith an occupied floor located more than 75 feet above the lowest levelof fire department vehicle access as a high-rise building; nominally, Ithink, this would work out to >10 stories). Dual systems can beconfigured by combining a braced-frame or a shear wall core and aperimeter moment frame; the moment frame should be configured toresist 25% of the prescribed seismic forces. To get around thisproblem, engineers decided to draft "alternate design guidelines (ADG)"with contributions and input from several prominent academics. Thefirst document was for San Francisco city followed closely by a similardocument for Los Angeles by the Los Angeles Tall Building StructuralDesign Council (LATBSDC). Subsequently, the CTBUH drafted its ownguideline (along the lines of the LATBSDC document) which is thedocument you reference.

3.关键区别ADG g(替代设计uidelines for Los Angeles) and IBC (prescriptive code):

(a) The ADG eliminates height limits on all structural systems. Itpermits the use of any structural system for any tall building as longas the Engineer of Record performs three checks -- a serviceabilitycheck [no damage in an earthquake with a 43-year return period (50%probability of being exceeded in 30 years), i.e., structure shouldremain elastic], a life-safety check [this is the code-level seismicevaluation, some damage is acceptable, but intent is to protect life inan earthquake with a 475-year return period (10% probability of beingexceeded in 50 years)] and a collapse prevention check [safeguardagainst collapse in an earthquake with a 2475-year return period (2%probability of being exceeded in 50 years)]. The collapse preventioncheck must be done using nonlinear time history analysis of thebuilding subjected to 7 or more appropriate ground motion timehistories. The design must be reviewed by a recognized panel ofexperts. Prescriptive codes do not require the serviceability check orthe collapse prevention check or peer review.

(b) For the code-level check in the ADG, the prescriptive codeprovisions were adopted, but with certain key exclusions: (i) The codeprovides a simple formula to determine the time period for variousstructural systems as a function of the height (termed Method A period,Ta). One can also create a 3-D structural model and compute thefundamental period using an eigen-value analysis. This period istermed Method B period, Tb. In calculating the design base shear, V,which is inversely proportional to the fundamental period, the value ofT cannot exceed 1.3Ta (most engineers do not include the stiffness ofpartitions in their structural models, thus typically overestimatingthe structural natural period; this criterion is to safe-guard againstgross over-estimation of periods, and under-estimation of the baseshear). So even if Tb is demonstrably much larger, it cannot be usedin calculating V. Thus there is a lower limit on V that is based on1.3Ta. The ADG eliminates this requirement, i.e., a larger value for Tcan be used in calculating (a lower) V if backed up by a structuralmodel. (ii) The ADG eliminates two other minimum design base shearrequirements that were first introduced in the 1997 UBC to account fornear-source effects, replacing it with a hard lower bound of 2.5%instead. (iii) ADG eliminates the drift limit imposed by theprescriptive code. The result of these three "relaxing" exceptions isthat the ADG makes it possible to conceive buildings that are moreflexible than what the prescriptive code would allow. In fact, the ADGin San Francisco was used in justifying the design of the 64-story OneRincon Hill Tower in SFO with a structural system consisting of ashear-wall core, connected to perimeter mega-columns in one directionthrough 2 sets of outrigger BRB (buckling-restrained brace) trussesfrom levels 26 to 32 and 51 to 55. There are no beams on the perimeter(http://continuingeducation.construction.com/article_print.php?L=5&C=415).

4. My thoughts about ADG:While the ADG certainly elevates structural engineering to a higher plane, several questions come to mind:

(a) Eliminating the hard lower bound on the fundamental period (1.3Ta)in the base shear computation: Can we believe the natural periodspredicted by our structural models that typically do not includestiffness associated with "non-structural elements" such as partitionsand facades as well as the stiffness contributed by the partialrestraint inherent in simple (non-moment) connections of the gravitysystem?

(b)冗余:没有显式冗余requirements in theADG unlike the prescriptive code which calls for two systems that aredistributed in plan and over the height for buildings >160 feet. Will it lead to an over-reliance on discrete elements such as themega-column or the outrigger truss in the case of One Rincon Hill? What about the lateral stability of the slender shear wall core shouldeither of these elements fail?

(c) Nonlinear analysis software: Are our nonlinear analysis softwarecapable of accurately predicting collapse? Do they have the ability tomodel true shear wall behavior including axial, flexural and shearinteraction and cracking? Can they capture link beam behaviorproperly? What about soil compliance, soil nonlinearity, andsoil-structure interaction?

(d) Ground motion selection: We do not have enough earthquake recordsin the large magnitude, close-distance regime. This means we have toscale existing records from small earthquakes to get to the "2475-year"event. Such scaling is riddled with pitfalls. Large earthquakesgenerate ground motions that are significantly different in frequencycontent as well as intensity and duration. Addressing these is a bigchallenge in the scaling process. The alternative is to use syntheticground motions from computational models of the earth and seismic wavepropagation calculations. But most engineers are still reluctant toembrace these synthetic time histories.

(e) The last point has to do with the "subjectivity" of the designprocess that is afforded within the ADG. When there are immensepressures on architects and engineers to go for the lowest commondenominator, what is the guarantee that the principle of naturalselection will not result in geotechnical engineers that provide the"smallest ground motions", peer reviewers that are "easy-going" andstructural engineers that can "make it work through modelingassumptions" getting all the projects on offer? We have seen thishappen in the US in the real-estate sector. During the housing bubble,appraisers and lenders were in cahoots to artificially boost the valueof homes, everybody (except the homeowners) benefiting from the bubble!

5. Questions to SEFI in the Indian context:

(a) Flexible vs stiff (see my original post)?
(b) ADG vs Prescriptive?

Swaminathan Krishnan
California Institute of Technology
http://krishnan.caltech.edu

[thirumalaichettiar]

Dear Dr.S.K Sir,

While going through your comments I would like to know more on this:

Dual systems can be configured by combining a braced-frame or a shear wall core and a perimeter moment frame; the moment frame should beconfigured to resist 25% of the prescribed seismic forces.

Can you explain how to model in any software like STAAD etc so that the frame takes at least 25% of base shear?

这may be silly but need to know the means.

Also browsing your web page come to know thatFRAME 3Dprogram is only on line use or available separately on price?

Your comment on this is welcome.

T.RangaRajan

[thirumalaichettiar]

Dear Dr.S.K Sir,
In your comments on US codes you have listed the three codes.
Why there is no comment on UBC(Uniform Building Code) as adopted in US?

T.RangaRajan.

[akadakia]

I agree with Mr. Swamikrishnan regarding US codes. Below is a comment on one of the earlier email about Chile codes and drift Vs Stiffness.

� From my conversations with our staff returning from New Zealand, properly designed and detailed modern structures typically performed well. The issues there seem concentrated on
o actual quake values roughly double what had been anticipated locally, a geotechnical issue
o diaphragm load path strengths inconsistent with shear wall or frame strengths in some cases, a detailing issue
o uncertainty of post-quake available strength to resist future events, a policy issue
� From SEAOC presentations on Chile the issues there seem to be
o buckling and post-buckling strength of thinner, more-stressed walls following newer code rules; this is often presented as the ratio between shear wall area and total wall area, a measure Mr. Sheth mentions.
o inadequate confinement of shear walls, particularly at ends and transitions
� The Tohoku, Japan event was more about tsunami damage than shaking damage, at least in the discussions I’ve seen.

I wonder if the point is a disconnect between strength and drift limits in some cases.
� US codes had ‘minimum base shear’ requirements, reinstated by ASCE7-05 Supp2 and kept in ASCE7-10. For tall flexible structures they result in greater shear and overturning design forces than one would find using ELF or RSA formulas and design spectra alone. This was done to reduce the risk of ‘ratcheting’ failure in which nonlinear deformations are too great to recover from, where deformations keep increasing in one direction until collapse.
� ASCE7 allows designers to ignore one of the ‘minimum base shear’ limits (but not both) when checking for building drift limits.
o This was a perennial question and source of confusion under previous codes. It is now clarified.
o ASCE7-10 section 12.8.6.1 says, “Eq. 12.8-5 [the general minimum of 0.044 I Sds] need not be considered for computing drift.”
o Note that Eq. 12.8-6 [the ‘near field’ minimum of 0.5 I S1 / R] is not excluded so it still gets considered in Equivalent Lateral Force method drift check
o ASCE7-10 section 12.9.4.2 specifically requires considering Eq. 21.8-6 [near field effect] in the Response Spectrum Analysis method drift check where it applies.
� Period limits (conservatively keeping periods short so shear is higher) are not required for computing RSA drift, per ASCE7-1- section 12.8.6.2.
o It is apparently used to avoid applying a double (or triple) penalty to many tall buildings: min shear AND drift limit using min shear AND higher shear from keeping T small.
� Drift limits are only one way to improve building stability by reducing the P-Delta effect.
o Apparently ASCE7 figures that a higher design story shear force from ‘minimum base shear’ is another, acceptable way to address the same concern.
o What about the ‘stability coefficient’ Theta intended to minimize adverse P-Delta effects (ASCE7-10 section 12.8.7)?
o Note that for ‘near field’ conditions ASCE7 still wants the conservatism of a large minimum base shear when checking drifts, apparently to address the big ‘pulse’ force that can occur near a fault.





Anjana S. Kadakia, P.E., LEED AP BD+C
Senior Principal
Thornton Tomasetti
744 Broad Street
Newark, NJ 07102
T973.286.6100F973.286.6101
D973.286.6116M917.612.6836
AKadakia@ThorntonTomasetti.com
www.ThorntonTomasetti.com
From:swamikrishnan [mailto:forum@www.buonovino.com]
Sent:Tuesday, November 20, 2012 6:58 PM
To:econf34289@www.buonovino.com
Subject:[E-CONF] Re: Welcome Note From Prof Swaminathan Krishnan to the Econferen



I will try to be as brief as possible (for greater details, pleaserefer to the documents given below). I will start by describing thecurrent building codes in the US, then give some background on thedevelopment of the "Alternative Design Guidelines (ADG)" that youreference, then list some important differences between the ADG and thecodes, and finally close with my thoughts on ADG vs Prescriptive Codesand questions to SEFI about the suitability of adopting either forIndian conditions.

1. US Codes:The established US codes are prescriptive,i.e., they define a set of rules on how to perform structural design --"thou shalt do this; thou shalt NOT do this, etc.". The most recent UScodes fall into three categories:

(a) ASCE 7 (Standard): Lays out the minimum design loads for buildingsand other structures, in addition to describing seismic designrequirements in detail (describes the various structural systems andthe limits of applicability of these systems).

(b)国际建筑规范(IBC):我这段代码s a "national"conglomeration of several regional codes of the past--the UniformBuilding Code, the Southern Standard Building Code, the BOCA NationalBuilding code, etc. The IBC describes how to perform basic structuralanalysis and design methods (static analysis, dynamic response spectralanalysis, or time history analysis), defines occupancy categories andseismic design categories (seismic design category corresponds to aspecific combination of seismic zone and occupancy category), loads andload combinations, classification of soils, design of foundations, andthen goes on to outline rules of design for structures using variousmaterials including steel, concrete, masonry, wood, glass, plasticetc. IBC references and relies heavily on the ASCE-7 standard forloads and load combinations and seismic design requirements, as well asother material-specific codes such as the ACI-318 for concrete, AISCManual of Steel Construction for steel, etc.

(c) Material-specific codes such as ACI-318 for concrete and AISCManual of Steel Construction for steel: These codes go into extremedetails of how concrete (or steel) structures must be designed anddetailed. Special requirements for seismic detailing are outlined ingreat detail in these documents (for steel, there is a separatedocument called "Seismic Provisions for Steel Buildings ANSI/AISC341-10" that handles seismic detailing; for concrete a separate chapterwithin ACI-318 lists the seismic detailing requirements in variousseismic zones).

2.背景交替德西的发展gn guidelines for tall buildings:在经济繁荣时期(也许更多的来往ctly, the real-estatebubble years), as house (and land) prices were sky-rocketing, there wasa great market for tall residential buildings in the US. Butarchitects called for an open exterior, free of "structure", tomaximize the sale price of the units and make tall buildingconstruction a profit-making proposition for the owner. This became aproblem, because the prescriptive codes have strict height limits. Inparticular, ASCE 7 requires a "dual-system" to be employed for allbuildings greater than 160 feet (BTW, the IBC classifies any buildingwith an occupied floor located more than 75 feet above the lowest levelof fire department vehicle access as a high-rise building; nominally, Ithink, this would work out to >10 stories). Dual systems can beconfigured by combining a braced-frame or a shear wall core and aperimeter moment frame; the moment frame should be configured toresist 25% of the prescribed seismic forces. To get around thisproblem, engineers decided to draft "alternate design guidelines (ADG)"with contributions and input from several prominent academics. Thefirst document was for San Francisco city followed closely by a similardocument for Los Angeles by the Los Angeles Tall Building StructuralDesign Council (LATBSDC). Subsequently, the CTBUH drafted its ownguideline (along the lines of the LATBSDC document) which is thedocument you reference.

3.关键区别ADG g(替代设计uidelines for Los Angeles) and IBC (prescriptive code):

(a) The ADG eliminates height limits on all structural systems. Itpermits the use of any structural system for any tall building as longas the Engineer of Record performs three checks -- a serviceabilitycheck [no damage in an earthquake with a 43-year return period (50%probability of being exceeded in 30 years), i.e., structure shouldremain elastic], a life-safety check [this is the code-level seismicevaluation, some damage is acceptable, but intent is to protect life inan earthquake with a 475-year return period (10% probability of beingexceeded in 50 years)] and a collapse prevention check [safeguardagainst collapse in an earthquake with a 2475-year return period (2%probability of being exceeded in 50 years)]. The collapse preventioncheck must be done using nonlinear time history analysis of thebuilding subjected to 7 or more appropriate ground motion timehistories. The design must be reviewed by a recognized panel ofexperts. Prescriptive codes do not require the serviceability check orthe collapse prevention check or peer review.

(b) For the code-level check in the ADG, the prescriptive codeprovisions were adopted, but with certain key exclusions: (i) The codeprovides a simple formula to determine the time period for variousstructural systems as a function of the height (termed Method A period,Ta). One can also create a 3-D structural model and compute thefundamental period using an eigen-value analysis. This period istermed Method B period, Tb. In calculating the design base shear, V,which is inversely proportional to the fundamental period, the value ofT cannot exceed 1.3Ta (most engineers do not include the stiffness ofpartitions in their structural models, thus typically overestimatingthe structural natural period; this criterion is to safe-guard againstgross over-estimation of periods, and under-estimation of the baseshear). So even if Tb is demonstrably much larger, it cannot be usedin calculating V. Thus there is a lower limit on V that is based on1.3Ta. The ADG eliminates this requirement, i.e., a larger value for Tcan be used in calculating (a lower) V if backed up by a structuralmodel. (ii) The ADG eliminates two other minimum design base shearrequirements that were first introduced in the 1997 UBC to account fornear-source effects, replacing it with a hard lower bound of 2.5%instead. (iii) ADG eliminates the drift limit imposed by theprescriptive code. The result of these three "relaxing" exceptions isthat the ADG makes it possible to conceive buildings that are moreflexible than what the prescriptive code would allow. In fact, the ADGin San Francisco was used in justifying the design of the 64-story OneRincon Hill Tower in SFO with a structural system consisting of ashear-wall core, connected to perimeter mega-columns in one directionthrough 2 sets of outrigger BRB (buckling-restrained brace) trussesfrom levels 26 to 32 and 51 to 55. There are no beams on the perimeter(http://continuingeducation.construction.com/article_print.php?L=5&C=415).

4. My thoughts about ADG:While the ADG certainly elevates structural engineering to a higher plane, several questions come to mind:

(a) Eliminating the hard lower bound on the fundamental period (1.3Ta)in the base shear computation: Can we believe the natural periodspredicted by our structural models that typically do not includestiffness associated with "non-structural elements" such as partitionsand facades as well as the stiffness contributed by the partialrestraint inherent in simple (non-moment) connections of the gravitysystem?

(b)冗余:没有显式冗余requirements in theADG unlike the prescriptive code which calls for two systems that aredistributed in plan and over the height for buildings >160 feet. Will it lead to an over-reliance on discrete elements such as themega-column or the outrigger truss in the case of One Rincon Hill? What about the lateral stability of the slender shear wall core shouldeither of these elements fail?

(c) Nonlinear analysis software: Are our nonlinear analysis softwarecapable of accurately predicting collapse? Do they have the ability tomodel true shear wall behavior including axial, flexural and shearinteraction and cracking? Can they capture link beam behaviorproperly? What about soil compliance, soil nonlinearity, andsoil-structure interaction?

(d) Ground motion selection: We do not have enough earthquake recordsin the large magnitude, close-distance regime. This means we have toscale existing records from small earthquakes to get to the "2475-year"event. Such scaling is riddled with pitfalls. Large earthquakesgenerate ground motions that are significantly different in frequencycontent as well as intensity and duration. Addressing these is a bigchallenge in the scaling process. The alternative is to use syntheticground motions from computational models of the earth and seismic wavepropagation calculations. But most engineers are still reluctant toembrace these synthetic time histories.

(e) The last point has to do with the "subjectivity" of the designprocess that is afforded within the ADG. When there are immensepressures on architects and engineers to go for the lowest commondenominator, what is the guarantee that the principle of naturalselection will not result in geotechnical engineers that provide the"smallest ground motions", peer reviewers that are "easy-going" andstructural engineers that can "make it work through modelingassumptions" getting all the projects on offer? We have seen thishappen in the US in the real-estate sector. During the housing bubble,appraisers and lenders were in cahoots to artificially boost the valueof homes, everybody (except the homeowners) benefiting from the bubble!

5. Questions to SEFI in the Indian context:

(a) Flexible vs stiff (see my original post)?
(b) ADG vs Prescriptive?

Swaminathan Krishnan
California Institute of Technology
http://krishnan.caltech.edu

[rohitkapoor]

Its lots of emails these days..??

Subject: [E-CONF] Re: Welcome Note From Prof Swaminathan Krishnan to the Econferen
From:forum@www.buonovino.com
Date: Wed, 21 Nov 2012 05:27:53 +0530
To:econf34289@www.buonovino.com

I will try to be as brief as possible (for greater details, pleaserefer to the documents given below). I will start by describing thecurrent building codes in the US, then give some background on thedevelopment of the "Alternative Design Guidelines (ADG)" that youreference, then list some important differences between the ADG and thecodes, and finally close with my thoughts on ADG vs Prescriptive Codesand questions to SEFI about the suitability of adopting either forIndian conditions.

1. US Codes: The established US codes are prescriptive,i.e., they define a set of rules on how to perform structural design --"thou shalt do this; thou shalt NOT do this, etc.". The most recent UScodes fall into three categories:

(a) ASCE 7 (Standard): Lays out the minimum design loads for buildingsand other structures, in addition to describing seismic designrequirements in detail (describes the various structural systems andthe limits of applicability of these systems).

(b)国际建筑规范(IBC):我这段代码s a "national"conglomeration of several regional codes of the past--the UniformBuilding Code, the Southern Standard Building Code, the BOCA NationalBuilding code, etc. The IBC describes how to perform basic structuralanalysis and design methods (static analysis, dynamic response spectralanalysis, or time history analysis), defines occupancy categories andseismic design categories (seismic design category corresponds to aspecific combination of seismic zone and occupancy category), loads andload combinations, classification of soils, design of foundations, andthen goes on to outline rules of design for structures using variousmaterials including steel, concrete, masonry, wood, glass, plasticetc. IBC references and relies heavily on the ASCE-7 standard forloads and load combinations and seismic design requirements, as well asother material-specific codes such as the ACI-318 for concrete, AISCManual of Steel Construction for steel, etc.

(c) Material-specific codes such as ACI-318 for concrete and AISCManual of Steel Construction for steel: These codes go into extremedetails of how concrete (or steel) structures must be designed anddetailed. Special requirements for seismic detailing are outlined ingreat detail in these documents (for steel, there is a separatedocument called "Seismic Provisions for Steel Buildings ANSI/AISC341-10" that handles seismic detailing; for concrete a separate chapterwithin ACI-318 lists the seismic detailing requirements in variousseismic zones).

2.背景交替德西的发展gn guidelines for tall buildings: During the economic boom years (perhaps more correctly, the real-estatebubble years), as house (and land) prices were sky-rocketing, there wasa great market for tall residential buildings in the US. Butarchitects called for an open exterior, free of "structure", tomaximize the sale price of the units and make tall buildingconstruction a profit-making proposition for the owner. This became aproblem, because the prescriptive codes have strict height limits. Inparticular, ASCE 7 requires a "dual-system" to be employed for allbuildings greater than 160 feet (BTW, the IBC classifies any buildingwith an occupied floor located more than 75 feet above the lowest levelof fire department vehicle access as a high-rise building; nominally, Ithink, this would work out to >10 stories). Dual systems can beconfigured by combining a braced-frame or a shear wall core and aperimeter moment frame; the moment frame should be configured toresist 25% of the prescribed seismic forces. To get around thisproblem, engineers decided to draft "alternate design guidelines (ADG)"with contributions and input from several prominent academics. Thefirst document was for San Francisco city followed closely by a similardocument for Los Angeles by the Los Angeles Tall Building StructuralDesign Council (LATBSDC). Subsequently, the CTBUH drafted its ownguideline (along the lines of the LATBSDC document) which is thedocument you reference.

3.关键区别ADG g(替代设计uidelines for Los Angeles) and IBC (prescriptive code):

(a) The ADG eliminates height limits on all structural systems. Itpermits the use of any structural system for any tall building as longas the Engineer of Record performs three checks -- a serviceabilitycheck [no damage in an earthquake with a 43-year return period (50%probability of being exceeded in 30 years), i.e., structure shouldremain elastic], a life-safety check [this is the code-level seismicevaluation, some damage is acceptable, but intent is to protect life inan earthquake with a 475-year return period (10% probability of beingexceeded in 50 years)] and a collapse prevention check [safeguardagainst collapse in an earthquake with a 2475-year return period (2%probability of being exceeded in 50 years)]. The collapse preventioncheck must be done using nonlinear time history analysis of thebuilding subjected to 7 or more appropriate ground motion timehistories. The design must be reviewed by a recognized panel ofexperts. Prescriptive codes do not require the serviceability check orthe collapse prevention check or peer review.

(b) For the code-level check in the ADG, the prescriptive codeprovisions were adopted, but with certain key exclusions: (i) The codeprovides a simple formula to determine the time period for variousstructural systems as a function of the height (termed Method A period,Ta). One can also create a 3-D structural model and compute thefundamental period using an eigen-value analysis. This period istermed Method B period, Tb. In calculating the design base shear, V,which is inversely proportional to the fundamental period, the value ofT cannot exceed 1.3Ta (most engineers do not include the stiffness ofpartitions in their structural models, thus typically overestimatingthe structural natural period; this criterion is to safe-guard againstgross over-estimation of periods, and under-estimation of the baseshear). So even if Tb is demonstrably much larger, it cannot be usedin calculating V. Thus there is a lower limit on V that is based on1.3Ta. The ADG eliminates this requirement, i.e., a larger value for Tcan be used in calculating (a lower) V if backed up by a structuralmodel. (ii) The ADG eliminates two other minimum design base shearrequirements that were first introduced in the 1997 UBC to account fornear-source effects, replacing it with a hard lower bound of 2.5%instead. (iii) ADG eliminates the drift limit imposed by theprescriptive code. The result of these three "relaxing" exceptions isthat the ADG makes it possible to conceive buildings that are moreflexible than what the prescriptive code would allow. In fact, the ADGin San Francisco was used in justifying the design of the 64-story OneRincon Hill Tower in SFO with a structural system consisting of ashear-wall core, connected to perimeter mega-columns in one directionthrough 2 sets of outrigger BRB (buckling-restrained brace) trussesfrom levels 26 to 32 and 51 to 55. There are no beams on the perimeter(http://continuingeducation.construction.com/article_print.php?L=5&C=415).

4. My thoughts about ADG: While the ADG certainly elevates structural engineering to a higher plane, several questions come to mind:

(a) Eliminating the hard lower bound on the fundamental period (1.3Ta)in the base shear computation: Can we believe the natural periodspredicted by our structural models that typically do not includestiffness associated with "non-structural elements" such as partitionsand facades as well as the stiffness contributed by the partialrestraint inherent in simple (non-moment) connections of the gravitysystem?

(b)冗余:没有显式冗余requirements in theADG unlike the prescriptive code which calls for two systems that aredistributed in plan and over the height for buildings >160 feet. Will it lead to an over-reliance on discrete elements such as themega-column or the outrigger truss in the case of One Rincon Hill? What about the lateral stability of the slender shear wall core shouldeither of these elements fail?

(c) Nonlinear analysis software: Are our nonlinear analysis softwarecapable of accurately predicting collapse? Do they have the ability tomodel true shear wall behavior including axial, flexural and shearinteraction and cracking? Can they capture link beam behaviorproperly? What about soil compliance, soil nonlinearity, andsoil-structure interaction?

(d) Ground motion selection: We do not have enough earthquake recordsin the large magnitude, close-distance regime. This means we have toscale existing records from small earthquakes to get to the "2475-year"event. Such scaling is riddled with pitfalls. Large earthquakesgenerate ground motions that are significantly different in frequencycontent as well as intensity and duration. Addressing these is a bigchallenge in the scaling process. The alternative is to use syntheticground motions from computational models of the earth and seismic wavepropagation calculations. But most engineers are still reluctant toembrace these synthetic time histories.

(e) The last point has to do with the "subjectivity" of the designprocess that is afforded within the ADG. When there are immensepressures on architects and engineers to go for the lowest commondenominator, what is the guarantee that the principle of naturalselection will not result in geotechnical engineers that provide the"smallest ground motions", peer reviewers that are "easy-going" andstructural engineers that can "make it work through modelingassumptions" getting all the projects on offer? We have seen thishappen in the US in the real-estate sector. During the housing bubble,appraisers and lenders were in cahoots to artificially boost the valueof homes, everybody (except the homeowners) benefiting from the bubble!

5. Questions to SEFI in the Indian context:

(a) Flexible vs stiff (see my original post)?
(b) ADG vs Prescriptive?

Swaminathan Krishnan
California Institute of Technology
http://krishnan.caltech.edu

Posted via Email

[Dr. N. Subramanian]

Dear Dr Swaminathan,

I appreciate your informative and interesting posting!

Regards,
NS

[gautam chattopadhyay]

how to simulate hystogram for 475 yrs return period and 2475 yrs return period? do brick masonry walls offer any effective resistance during oscillation of a structure? particularly the partition walls? In modern internal decoration people are more and more adopting artificial timber partitions which do not have any structural stiffness at all.

On Wed, Nov 21, 2012 at 5:27 AM, swamikrishnan forum@www.buonovino.com)> wrote:

[swamikrishnan]

1. Dual system proportioning question by Er. Rangarajan: If you are performing linear elastic static or dynamic analysis of the structure, you can ensure that the moment frame is resisting 25% of the forces, by comparing the sum of the column shear forces in each story against the total story shear force in that story (sum of column shear forces and shear wall shear forces).

2.FRAME3D can only be accessed through the Caltech Virtual Shaker site: http://virtualshaker.caltech.edu (Er. Rangarajan's question).

3.Er. Rangarajan's question on UBC: As I mentioned in my posting, the International Building Code is a conglomeration of all the former regional codes in the US, one of which is the Uniform Building Code (UBC). The IBC supercedes UBC and the other regional codes.

4. Dr. N. Subramanian: Thank you for your contributions to this conference and this forum in general as well.

5. Er. Gautam's query on DBE (475 year return period) Vs MCE (2475 year return period):

US: DBE = 2/3*MCE
India: http://www.iisc.ernet.in/currsci/mar102007/639.pdf

6. Er. Gautam's comment on partitions: I am not familiar with partitions used in residential towers in India. From my experience in Indonesia, Taiwan, and S. Korea, hollow concrete blocks seem popular for residential units in highrise buildings; gypsum boards partitions are common in office towers. The stiffness from partitions may be significant at low levels of shaking when they are intact; under intense shaking they may be damaged/separated (decoupling/sliding) and, perhaps, cannot offer much by way of stiffness. The elastic design base shear must be computed with the true fundamental natural period (which by definition is a characteristic of the elastic dynamic response). This must account for partitions which do contribute some stiffness at low-amplitude excitation that induces elastic response.

Swami