design of tall buildings

1. Special Considerations and Challenges
in Seismic Design of Tall Buildings
Asian Institute of Technology | Thailand
1-2 June 2018

2. Buildingsand Structuresare expected tobe
2
• 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

3. Howdoes CTBUHlook at Tall
3
Relatively Tall.
Both for public and the professions
who design and construct
Proportion
Slenderness, in plan and in
elevations
Systemsand Technologies
Uses something “different” than
ordinary buildings
3

4. Key Challenges in(Tall) Buildings
4
• Taller
• Slender
• Twisting
• Unusual forms
• Multi Use
• Changing Plans
• Larger column freespaces
• Smaller Cores
• Minimizing Floor Height
• Minimize floor depth
• Minimize column size
• Minimize structural cost
• Inclined columns
• Free form
• Unusual requests

5. Win
d
Gravity
Earthquake 5
5
emaze.com
Main Challenges !
Optimizing for one, may de-optimize for others!

6. 6
Focus oftheTalk–“Tallness Range”
Low
Rise
>500m
>300m
>200m
>100m
<50m >150m
Source:CTBUReport,2018

7. Main Structural Concerns
01 02 03 04 05 06
Stabilityand
integrity
Strengthand
Servivbility
Deformation Drift Ductility Energy
Dissipatio
n
7
07
Motion
Perception

8. • Direct LoadTransfer Systems
• Flat Slaband Flat Plate
• Beam-Slab
• Waffle Slab
• Wall Joist
• Indirect LoadTransferSystem
• Beam, Slab
• Girder, Beam, Slab
• Girder, Joist
• Materials
• Steel/ Composite Deck
• Reinforced Concrete
• Post-tensioned slabsystems
8
Choosing the
“Right” Gravity Load
Resisting System
Least weight
Fast Construction cycle
Leaststructuraldepth

9. 4Commandments forLateralLoad Systems
1
Resistoverturning forces
due to lateral loads by
using vertical elements
placed asfar apart as
possible
2
Channel gravity loadsto
those vertical elements
resisting overturning
forces
3
Link these vertical
elements together with
shear-resisting structural
elements with minimum
shear lag to activate
entire perimeter of the
building
4
Axial loaded membersin
compression to resist
overturning forces
9

10. 10
10
SeismicLoad
WindLoad
Depend on
•focus of earthquake
•Shaking intesity
•ground conditions
•Mass and stiffness
distribution
Depend on
• Wind speed
• terrain
• topography ofthe location
• Force increases withheight
• Geometry andexposed area
v
A
üg
m
 Excitation isan applied displacement
at the base
 force will be distributed along interior
and exterior lateral load resisting
elements
 Excitation isan applied pressure or
force on thefacade
 force will act mainly on exterior
frames then transferred to floor
diaphragms

11. Basic Physicsof Dynamics
1
1
• Newton’s View, for rigidbodies
F= ma

12. Structuralengineer’s View
𝑀
𝑢
+ 𝐶
𝑢+ 𝐾
𝑢=𝐹
for linear elastic, deformablebodies
12

13. Typical Linear Dynamic Response of Tall Building
13
Animation

14. Dynamic Equilibrium
Damping-
Velocity
Mass-Acceleration Stiffness-
Displacement
External
Force
The basic variable is displacement and its derivatives
14
𝑀
𝑢
+ 𝐶
𝑢+ 𝐾
𝑢+ 𝐹
𝑁
𝐿=𝐹
Nonlinearity
𝑀
𝑢
+ 𝐶
𝑢+𝐾
𝑢

15. Nonlinear and Analysis for PBD
15

16. BuildingIndustryrelieson Codes and Standards
• CodesSpecify requirements
• Giveacceptable solutions
• Prescribe (detailed) procedures, rules, limits
• (Mostly based on research and experience but not always rational)
Spirit of the codeis
to help ensure Public Safety and
provide formal/legal basis for design
decisions
16
Compliance to
letter of thecode is
indented to meet thespirit

17. Seismic Response
Linear Time History
Analysis
Ku  FNL  FEQ
Pushov
er
Analysi
s
Ku  FEQ
Equivalent
Static
Analysis
Ku  FEQ
Response
Spectrums
Response
Spectrum
Analysis
Acceleration
Records
Nonlinear
Time
History
Analysis
17
𝑀
𝑢
+ 𝐶
𝑢+ 𝐾
𝑢+ 𝐹
𝑁
𝐿=𝐹
𝑀
𝑢
+ 𝐶
𝑢+ 𝐾
𝑢=𝑀
𝑢
𝑔
Free Vibration
𝑀
𝑢
+ 𝐾
𝑢=0

18. The“ArbitraryFactors”inCodes
18

19. 19
 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 SeismicEffects
Design for WindLoad

20. 20
 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 SeismicEffects

21. 21
0
5
10
15
20
25
30
35
40
45
0
TheProblemwithRFactor
The elastic forces obtained from the
standard RSAprocedure
The RSA elastic forces reduced by 𝑅
The inelastic forces obtained from the
NLRHAprocedure
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 𝑅factor
10 20 30 40 50 60
Story Shear (x106 N)
Story
Level
• 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, BehaviorFactor
(q), Structure Type factor (K)etc.,
Fawad Najam,2017

22. Effectof Modes on Story Moment
22
22

23. Effectof Modes on Story Shear
23
23

24. Are All
Buildings
Codes
Correct ?
24
• All codes have different values of R and other
factors
• If they differ, can all of them be correct ?
• Did we inform the structures to follow which
code when earthquake or hurricane strikes ?
• Codes change every 3 or years, should we
upgrade our structures every 3 or 5 years to
conform ?

25. Code
Comparisonfor
Seismic
Performance
25
• Compare Performance of buildings designed
todifferent codes
• ACI 318-14 +ASCE 7-10
• BS 8110-1997 +EURO-8
• EURO-2-2004 +EURO-8
• Forlow-seismic and high seismiczone
• Manila >VeryHigh
• Bangkok >Low tomedium
• All produce different level orperformance in
differentcomponents !!
Two MS Thesis, 2016 at AIT

26. Shift From Prescriptive to
Performance Based Approach

27. A Move TowardsPerformance-based Approach
• Prescriptive Codes restrict
and discourage innovation Objective Requirements
Prescribed
Solution
Objective Requirements
Alternate
Solution
• Performance Based approach
encourages and liberates it
27

28. Design Approaches
Intuitive Design
Prescriptive
Code Based
Design
Performance
Based Design
>>
>>>
28

29. 29
Looking at some Design Challenges

30. Providing (Hiding)the Outriggers
30

31. Outrigger Effects
1 + 1
K:1 +1=8 31
K:1+1=2
2

32. 32
Effectivenessof Outriggers
Reduce Reducethe natural period – Good forwent response
Reduce Reducetop displacement
Reduce Reduce drift
Reduce Reducemoment in shearwalls
Follow Follow the All 4Commandments
Donot reduce Donot reduce shear in shearwalls
Need Need spaceto implement

33. 33
Real VsVirtual Outriggers
• Virtual Outriggers are more acceptable” from
architectural planning and circulation viewpoint
• Theyare nearly aseffective as“real” outriggers
Direct or “real”Outriggers In-direct or “viryual”Outriggers

34. 3
Adding Belts
• More evendistribution
of axial loads on
perimeter columns
• Reduces possibility of
tension in columns or
foundatons
• Provides virtual
outrigger effect inboth
directon
4

35. Buckling RestraintBraces, BRB
35

36. BRB–An efficient Outriggerand Damper
36

37. 37
N1-S1
CoreOnly N1-S2 N1-S3 N2-S3 N3-S3
Flag Walls–an Alternative to Outriggers

38. 38
CoreOnly Config1 Config2 Config3 Config4 Config5
Flag Walls–an Alternative to Outriggers

39. Staggered Wallsas Outriggers
39

40. TheDiaphragm DesignChallenges
40

41. PodiumFloor
Diaphragm
Behavior
41

42. Diaphragm TransferForces
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.
42

43. Podium and Backstay Effects
Backstay 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
43

44. Typical Diaphragm Components
Chord (Diaphragm)
Chord (Diaphragm)
Shear (Diaphragm)
Shear Wall
Diaphragm
1
2
4
Shear Friction
(Support)
3
Collector
(Support)
44

45. Realistic Model - FiniteElement 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 Walls
45
Shear Walls Shear Walls

46. EFFECTOF COMMON PODIUMON
THESEISMICPERFORMANCE
TOWERS
46

48. 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 podium
48
Single tower with
half podium
Single tower with
whole podium
Twin tower with
whole podium

49. • 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 podium
Actual Building
Single-tower
less than
Multi-tower
Single-tower
greater than
49
Multi-tower
Design Results
UNECONOMICAL
Design
UNSAFE
Design

50. 50
TowersandPodiums

51. 51
Effectof Soil-Structure
Interactionon Seismic
Responsesof Tall Buildings

52. 52
A
B
C
Siteeffects
Soil-structure interaction
52

53. 53
MODELS
Without SSI With SSI
Referencemodel
Equivalent Linear Nonlinear
Model 3B
Model 3A
FE(DirectApproach)
Code-based
(SubstructureApproach)
Model inpractice
Model 2A Model 2B
Equivalent Linear
Model 1
Fixed-base
53

54. 54
Continuous improvements in our
understanding, research, learning and
practice
Way Forward

55. T
h
a
n
ky
o
u