Special Considerations and Challenges in Seismic Design of Tall Buildings

Asian Institute of Technology | Thailand

1-2 June 2018

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Buildings and Structures are expected to be

• Safe

• Secure

• Serviceable

• Reliable

• The contents of the structures are often much more valuable than structure itself

• The loss of service/operations/business is a often larger than repair costs

• Protective

• Friendly

• Sustainable

• Affordable

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How does CTBUH look at Tall

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Relatively Tall. Both for public and the professions

who design and construct

ProportionSlenderness, in plan and in

elevations

Systems and TechnologiesUses something “different” than

ordinary buildings

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Key Challenges in (Tall) Buildings

• Taller

• Slender

• Twisting

• Unusual forms

• Multi Use

• Changing Plans

• Larger column free spaces

• Smaller Cores

• Minimizing Floor Height

• Minimize floor depth

• Minimize column size

• Minimize structural cost

• Inclined columns

• Free form

• Unusual requests

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Main Challenges !

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Win

d

Earthquake

Gravity

emaze.com

Optimizing for one, may de-optimize for others !

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Focus of the Talk – “Tallness Range”

Low Rise

>500 m>300 m>200 m>100 m<50 m >150m

Source: CTBU Report, 2018

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Main Structural Concerns

Stability and integrity

01

Strength and Servivbility

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Deformation

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Drift

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Ductility

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Energy Dissipation

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Motion Perception

07

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Choosing the “Right” Gravity Load

Resisting System

• Direct Load Transfer Systems

• Flat Slab and Flat Plate

• Beam-Slab

• Waffle Slab

• Wall Joist

• Indirect Load Transfer System

• Beam, Slab

• Girder, Beam, Slab

• Girder, Joist

• Materials

• Steel/ Composite Deck

• Reinforced Concrete

• Post-tensioned slab systems

Least weightFast Construction cycleLeast structural depth

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4 Commandments for Lateral Load Systems

Resist overturning forces due to lateral loads by using vertical elements placed as far apart as possible

1Channel gravity loads to those vertical elements resisting overturning forces

2Link these vertical elements together with shear-resisting structural elements with minimum shear lag to activate entire perimeter of the building

3Axial loaded members in compression to resist overturning forces

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10

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Seismic LoadWind Load

Depend on •focus of earthquake

•Shaking intesity

•ground conditions

•Mass and stiffness

distribution

Depend on•Wind speed

• terrain

• topography of the location

• Force increases with height

•Geometry and exposed area

m

ügv

A

Excitation is an applied displacement

at the base

force will be distributed along interior

and exterior lateral load resisting

elements

Excitation is an applied pressure or

force on the facade

force will act mainly on exterior

frames then transferred to floor

diaphragms

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Basic Physics of Dynamics

• Newton’s View, for rigid bodies

F = ma

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Structural engineer’s View

for linear elastic, deformable bodies

𝑀𝑢 + 𝐶𝑢 + 𝐾𝑢 = 𝐹

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Typical Linear Dynamic Response of Tall Building

Animation

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Dynamic Equilibrium

Damping-Velocity

Mass-Acceleration Stiffness-Displacement

Nonlinearity

External Force

The basic variable is displacement and its derivatives

𝑀𝑢 + 𝐶𝑢 + 𝐾𝑢 + 𝐹𝑁𝐿 = 𝐹

𝑀𝑢 + 𝐶𝑢 + 𝐾𝑢

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Nonlinear and Analysis for PBD

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Building Industry relies on Codes and Standards

• Codes Specify requirements

• Give acceptable solutions

• Prescribe (detailed) procedures, rules, limits

• (Mostly based on research and experience but not always rational)

Spirit of the code isto help ensure Public Safety and provide formal/legal basis for design decisions

Compliance to letter of the code is indented to meet the spirit

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Seismic Response

Linear Time History Analysis

EQNL FFKu

Free Vibration

Pushover

Analysis

EQFKu Equivalent

Static Analysis

EQFKu

Response Spectrums

Response Spectrum

Analysis

Acceleration Records

Nonlinear

Time History

Analysis 𝑀𝑢 + 𝐶𝑢 + 𝐾𝑢 + 𝐹𝑁𝐿 = 𝐹

𝑀𝑢 + 𝐶𝑢 + 𝐾𝑢 = 𝑀𝑢𝑔

𝑀𝑢 + 𝐾𝑢 = 0

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The “Arbitrary Factors” in Codes

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For most buildings, dynamic wind response may

be neglected

Gust factor approach predict dynamic

response of buildings with reasonable accuracy

Structures are designed to respond elastically

under factored loads

Structures are designed to respond inelastically

under factored loads

it is not economically feasible to design structures

to respond elastically to earthquake ground

motion

Design for Seismic EffectsDesign for Wind Load

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Structures are designed

to respond inelastically

under factored loads

it is not economically

feasible to design

structures to respond

elastically to earthquake

ground motion

Design for Seismic Effects

Introducing AIT Solutions

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0

5

10

15

20

25

30

35

40

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0 10 20 30 40 50 60

The Problem with R Factor

The elastic forces obtained from the

standard RSA procedure

The RSA elastic forces reduced by 𝑅

The inelastic forces obtained from the

NLRHA procedure

The actual reduction in RSA

elastic forces. The “reward”

of making a nonlinear model

The underestimation causing a “false

sense of safety” due to directly reducing

the RSA elastic forces by 𝑅 factor

Story Shear (x106 N)

Sto

ry L

evel

• The R factor may vary from 2 to 8

depending on definition of structure

type

• R factor could “off” by a factor of 2

to 4

• Other names for R factor are

Response Factor, Behavior Factor

(q), Structure Type factor (K) etc.,

Fawad Najam, 2017

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Effect of Modes on Story Moment

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Effect of Modes on Story Shear

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Are All Buildings Codes Correct ?

• All codes have different values of R and otherfactors

• If they differ, can all of them be correct ?

• Did we inform the structures to follow whichcode when earthquake or hurricane strikes ?

• Codes change every 3 or years, should weupgrade our structures every 3 or 5 years toconform ?

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Code Comparison for Seismic Performance

• Compare Performance of buildings designed to different codes

• ACI 318-14 + ASCE 7-10

• BS 8110-1997 + EURO-8

• EURO-2-2004 + EURO-8

• For low-seismic and high seismic zone

• Manila > Very High

• Bangkok > Low to medium

• All produce different level or performance in different components !!

Two MS Thesis, 2016 at AIT

Shift From Prescriptive to Performance Based Approach

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A Move Towards Performance-based Approach

• Prescriptive Codes restrict and discourage innovation Objective Requirements

Prescribed

Solution

Objective RequirementsAlternate

Solution

• Performance Based approach encourages and liberates it

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Design Approaches

Intuitive Design

Prescriptive Code Based Design

Performance Based Design

>>

>>>

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Looking at some Design Challenges

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Providing (Hiding) the Outriggers

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Outrigger Effects

K: 1+1 = 2 K: 1 +1 =8

1 + 1 2

32Effectiveness of Outriggers

Reduce the natural period – Good forwent responseReduce

Reduce top displacementReduce

Reduce driftReduce

Reduce moment in shear wallsReduce

Follow the All 4 Commandments Follow

Do not reduce shear in shear wallsDo not reduce

Need space to implementNeed

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Real Vs Virtual Outriggers

• Virtual Outriggers are more acceptable” from architectural planning and circulation viewpoint

• They are nearly as effective as “real” outriggers

Direct or “real” Outriggers In-direct or “viryual” Outriggers

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Adding Belts

• More even distribution of axial loads on perimeter columns

• Reduces possibility of tension in columns or foundatons

• Provides virtual outrigger effect in both directon

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Buckling Restraint Braces, BRB

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BRB – An efficient Outrigger and Damper

37N1-S1Core Only N1-S2 N1-S3 N2-S3 N3-S3

Flag Walls – an Alternative to Outriggers

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Core Only Config 1 Config 2 Config 3 Config 4 Config 5

Flag Walls – an Alternative to Outriggers

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Staggered Walls as Outriggers

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The Diaphragm Design Challenges

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Podium Floor Diaphragm Behavior

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Diaphragm Transfer Forces

Large diaphragm transfer forces

should be anticipated at offsets

or discontinuities of the vertical

elements of the seismic-force-

resisting system.

(a) Setback in the building profile

(b) Podium level at grade.

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Podium and Backstay EffectsBackstay Effects

Title: Effects of podium interference on shear force distributions in tower walls supporting tall buildings

Author: Mehair Yacoubian, Nelson Lam, Elisa Lumantarna, John. L. Wilson, 2017

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Typical Diaphragm Components

Chord (Diaphragm)

Chord (Diaphragm) Collector(Support)

Shear Friction (Support)

Shear (Diaphragm)

Shear Wall

Diaphragm

1

2

4

3

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Realistic Model - Finite Element Model

• Finite element modeling of a diaphragm can be useful for assessing the force transfer among

vertical elements, force transfer around large openings or other irregularities.

Shear WallsShear Walls Shear Walls

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EFFECT OF COMMON PODIUM ON THE SEISMIC PERFORMANCETOWERS

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Modeling Options

• Individual design of tower and podium separately in practice

• Restraint of resources such as software, processing time, understanding and references

Single tower

without podiumSingle tower with

half podium

Single tower with

whole podium

Twin tower with

whole podium

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• Restraint of resources such as software, processing time, understanding and references

• To study the effect of various options on seismic response estimation

Problem Statement

Single tower with

whole podium

Twin towers with

whole podiumActual Building

Single-tower less than

Multi-tower

Single-tower greater than Multi-tower

Design ResultsUNECONOMICAL

Design

UNSAFE

Design

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Towers and Podiums

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Effect of Soil-Structure Interaction on Seismic Responses of Tall Buildings

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A

B

C

Site effects

Soil-structure interaction

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MODELS

Without SSI With SSI

Reference model

Equivalent Linear Nonlinear

Model 3BModel 3A

FE (Direct Approach)Code-based(Substructure Approach)

Model in practice

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Model 2A Model 2B

Equivalent Linear

Model 1

Fixed-base

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Continuous improvements in our understanding, research, learning and practice

Way Forward

Thank you