asic webinar session 7 - building configuration slides

AISC Night SchoolSession 7: Building ConfigurationApril 21, 2014

Fundamentals of Earthquake Engineeringfor Building Structures

Copyright © 2014American Institute of Steel Construction 1

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© The American Institute of Steel Construction 2014

Building ConfigurationApril 21, 2014

This Building Configuration lecture will focus on load path and the role and components of diaphragms. There will be a discussion of foundation issues. Irregularities and their treatment in steel frame design will be covered. The session will present the treatment of 3-dimensional analysis issues as well as of modal-response-spectrum analysis issues. The concept of deformation compatibility will be presented. This lecture will also include a discussion of issues related to fixity and rotation demand.

Course Description

• Gain an understanding of load path for the design of steel framed structures

• Become familiar with diaphragm behavior and design principles.

• Learn and understand about foundation design concepts for steel framed structures.

• Learn and understand about deformation compatiblity in steel framed structures.

Learning Objectives

8

Fundamentals of Earthquake Engineering for Building Structures

Rafael Sabelli, SE




9

Course outline

1. Seismology and earthquake effects2. Dynamics and response3. Building dynamics and response4. Steel behavior5. System ductility and seismic design6. Steel systems7. Building configuration 8. Building codes

10

Session 7:

Building configuration

Session topics• Load path• Foundations• Diaphragms• Collectors• Deformation compatibility

11 12

Load path




Load path• Connects “point of

application” to “point of resistance”

• In seismic design, every element with mass is considered a point of application

• Foundation is considered point of resistance

13

Load path

14

Lateral framing

Gravity framing

Wind vs. seismic loads• Wind loads

o External• Exposed areas

participate

• Seismic loadso Inertial

• All mass participates

• Load path required between mass and foundation

15

Wind load path

16




Seismic load path

17

Mass without connection to structure:No load path

Seismic load path

• All masses must have positive connection to seismic-load-resisting system

• Magnitude of connection force due tooGround motionoMass of itemo Building dynamics (local acceleration)

• Diaphragms contain the majority of typical building mass

18

Seismic-load-resisting system

• Vertical frameso Beamso Columnso Braces (if any)

• Diaphragmso Decko Chordso Collectors

• Foundations

19

Load path issues

• Continuityo Load path must

be continuous between mass and foundation

20




Load path issues

• EccentricityoHorizontal eccentricity

between mass and frame causes flexure in diaphragm

o Vertical eccentricity between mass and foundation causes overturning in frame

21

CompressionTension

Load path issues

• Change in directiono At a change in

direction load path there is an additional force

• Verticalo Overturning

22

23

Foundations

Foundations

• Shallow foundationso Supporto Lateral resistanceo Stability

• Deep foundationso Supporto Lateral resistanceo Stability

24




Shallow foundations: support• Overturning at

frameso Bearing pressure

• Short-duration increase in resistance

• Idealized as triangular• Or modeled with soil

springso No tension!

25

Shallow foundations: lateral resistance

• Lateral resistanceo “Sliding”o Frictiono Bearing (passive

pressure)o Engagement of

multiple footings

26

Shallow foundations: lateral resistance

• Lateral resistanceo “Sliding”o Frictiono Bearing (passive

pressure)o Engagement of

multiple footings• Relative lateral

movement of footings can be problematic

27

H H H

H

H

H H

H H

HH H

H

Grade beam

Shallow foundations: stability• May be governing

consideration for foundation

• Nonlinearo May be stable under ASD and

unstable under LRFD loads• Minimum requirement: Evaluate

under ASD• Design footings for soil capacity

(amplified)

28




Shallow foundations: stability• Implications of

designing for stability with reduced loadso Rocking may be

governing modeo System above may have

lower ductility demando Displacements may be

larger than anticipated

29

Deep foundations: support• Overturning at

frameso Compression

• End bearing• Friction

o Tension• Friction

o Short-duration increases

30

Deep foundations: lateral resistance

• Lateral resistanceo Pile shear and bendingo Pile-cap bearing

(passive pressure)o Engagement of multiple

footings

oBatter piles

31

Deep foundations: lateral resistance

• Lateral resistanceo Pile shear and bendingo Pile-cap bearing

(passive pressure)o Engagement of

multiple footingso Buildings tied together

• Engage all piles• Prevent relative

movement

32

Grade beam

H H H

H

HH H

H

H

H

H H




Deep foundations: stability• Stability

o Addressed by strength design of piles

o Upper-bound soil strength difficult to establish

o Rocking mechanism not applicable

33 34

Diaphragms

Steel Deck (AKA “Metal Deck”)

Shear load path through steel deck and fasteners.

Steel chords and collectors.

35

Deck and FillShear load path through steel deck and fasteners.

Concrete stiffens deck and prevents buckling.


36




Steel deck with reinforced concrete fill

Shear load path through reinforced concrete and shear studs. Chords and collectors:

Steel members, orReinforcement in deck

37

Shear studs.

Reinforcement

Horizontal truss diaphragm

Shear load path through steel diagonals and framing.


Deck is for gravity only.

38

Truss

Diaphragms• Roles of Diaphragms• Diaphragm Components• Diaphragm Behavior and Design

Principles• Building Analysis and Diaphragm Forces• Diaphragm Analysis and Internal

Component Forces

39

Roles of diaphragms

40

• Support gravity• Deliver forces to

frames• Brace columns

for stability• Transfer forces

between frames• Resist P- thrust




Distribute inertial forces

41

Lateral bracing of columns

KL

(K=1)

42

Resist P- thrust

43

Vertical beam

reaction

Horizontal thrust

Sloped column

axial force

Transfer forces between frames

44




Transfer diaphragms

45

PodiumSetbacks

Transfer diaphragms

46

V M

Stiff “plaza level” diaphragm

Horizontal Force Couple

Vertical Force Couple

Shear reversal at plaza level

Backstay Effect

Demand at backstay

diaphragm

Chord

Collector

Deck(“diaphragm”)

Diaphragm Components

48




Diaphragm Components

Chord

Collector

Deck(“diaphragm”)

49

Diaphragm rigidity

Flexible Rigid

Semi-Rigid

50

Diaphragm types and analysis

• Determinate• Flexible, or• 3 lines of resistance

o Analyze diaphragmo Diaphragm reactions

load frames

• Indeterminate• Rigid, or• Semi-rigid

o Analyze building• Relative frame stiffness• Diaphragm rigidity• Frame location

o Forces to frames = diaphragm collector forces

51

Analysis of Flexible Diaphragms

52




Typical diaphragm analysis

53

Fp

V

Fcoll

Fchord

33%

17%

17%

17%

17%

33%

33%

33%

Typical diaphragm analysisChordTension

ChordCompression

Collector Collector

Uniform shear

54

Alternate diaphragm analysisChordTension

ChordCompression

Collector Collector

Non-uniform

shear

55

Alternate diaphragm analysis

56

ChordTension

ChordCompression

Collector Collector

Non-uniform shearLocal chords




Alternate diaphragm analysis

57

ChordTension

ChordCompression

Collector Collector

Non-uniform shearLocal chordsInternal collectors

Alternate diaphragm analysisChordTension

ChordCompression

Collector Collector

Non-uniform shearLocal chordsInternal collectorsCritical for

design

58

Shear

Analysis of Non-flexible Diaphragms

59

Non-flexible diaphragms activate the perpendicular system to help resist torsion (due to eccentricity between center of mass and center of rigidity)

Analysis of Non-flexible Diaphragms

ShearMoment

MomentCorrectionCorrected Moment

60

A 3-dimensional analysis captures this effectCombination of orthogonal load effects is necessary




61

Using the results of 3-D analysis

61

Critical for design:Collectors and chords

Loading distribution adjusted to satisfy statics

62

Using the results of 3-D analysis

62

Critical for design:Collectors and chords

63

Collectors

Collectors• Protected element

• Reinforcement in composite deck

• Steel framing

64




Protected element

65

Late

ral l

oad

Deformation,

Fuses designed for this load

(E: design base shear)

Critical elements designed for this load

(oE)

ColumnsCollector beams

Collector and frame loads: Case 1

66

Force Colors

Capacity

Statics

33%F1(i) 33%F1(i)33%F1(i)

Tmax(i+1)

Fmid(i)

Cmax(i)

Fleft (i) Fright(i)

Cmax(i+1)

Tmax(i)

F1(i)

V(i)

V(i+1)F1(i) =Fleft (i)

+Fmid(i)+Fright(i)

V(i+1) =(Tmax(i+1) +Cmax(i+1))cos((i+1))

V(i) =(Tmax(i) +Cmax(i))cos((i))

F1(i) =V(i) −V(i+1)

Tmax(i+1)

Cmax(i)

Cmax(i+1)

Tmax(i)

Shear enteringframe line

Collector and frame loads: Case 2

67

Force Colors

Diaphragm

Capacity

StaticsFp(i)

V(i)

V(i+1) T (i+1)

Fmid(i)

Cmax(i)

Fleft (i) Fright(i)

C (i+1)

Tmax(i)

F2(i) = (i) Fp= Fleft (i)+Fmid(i)+Fright(i)

V’(i+1) = (i) V(i+1)

V(i) =(Tmax(i) +Cmax(i))cos((i))

33%F2(i)33%F2(i) 33%F2(i)

T’ (i+1)

Cmax(i)

C’ (i+1)

Tmax(i)

Shear enteringframe line

(i) =V(i) / [V(i+1)+ Fp]

F2(i)

V(i)

V’(i+1)

From analysisFor static equilibrium

Reinforcement in deck• Wide section of deck

o Low stresso Stability not critical

• Eccentricity from frameo Local chords

• Concentrated shear transfer

68




Reinforcement in deck

69

Reinforcement used for collector forces

Braced frame

Reinforcement as collector

oE / A= 0.5 fc’(unconfined concrete)

oE / (wt)= 0.5 fc’w ≥ oE / (0.5 fc’ t)

e = w/2

Local chord force:C = e (oE)/L

w

e

oE

oEL

C

C

Beam-columns• Compressive

strengtho Wide-flange with discreet

lateral and torsional bracing

• Major axis flexural buckling• Minor-axis flexural buckling• Torsional buckling

o Higher strength than minor-axis FB for same unbraced length

71


strengtho Wide-flange with

continuous lateral bracing

• Major axis flexural buckling

• Constrained-axis flexural-torsional bucklingo Strength between

minor-axis FB and torsional buckling

72




Constrained-axis flexural-torsional buckling

Minor axis flexural buckling(no restraint)

Torsional buckling (restraint at

centroidal axis)

Constrained-axis Flexural-torsional buckling

(restraint at top flange)

73

Beam-columns• Constrained-axis

flexural-torsional bucklingo Use 0.9 PE to calculate

Fcr

74

2222

22 1π

arrGJ

LK

aICEP

yxz

ywe

a


strengtho Wide-flange with

continuous torsional bracing

• Major axis flexural buckling

• Required torsional stiffness TBDo Slab stiffnesso Web stiffness

75




Beam-columns• Flexural strength

o Composite deck• Composite strength

o Steel deck only• Lateral bracing with

flutes perpendicular• Unbraced with flutes

parallel

77

Collector connections• Gravity

o Shear forces

• Seismico Axial forces (horizontal)o Rotation

78

Limit StatesPlate Yield & RuptureBolt shear Bearing & SplittingBlock ShearWeld Rupture

Vu

Hu

Collector connections

Rn (y)from Manual

Rn (x)

Hu

Rn (x)

2 Vu

Rn (y)

2

+ 1






• Rotationo Single-plate connection

• Follow Manual ruleso Plate thicknesso Bolt sizeo Spacing

o Double column of bolts• Extended plate method• Proportioning rules


• Rotationo Welded top flange

• Introduces some eccentricity

o Moment connection• Attracts moments• May have ductility

demands• Detail for ductility

Deformation compatibility

• Shear distortion adjacent to tall frameso Due to

• Lateral drift• Column axial deformation

o May result in large rotation demands

83

Amplified rotation

84

Deformation compatibility




Deformation compatibility• Necessity• Connections• Flexible diaphragms• Stairs• Pounding• Critical conditions

85

Necessity

• Inelastic responseo Large drifts

• Lateral system• Gravity system

• Performance goalo Prevent collapse

• Global• Local

86

Gravity connections• Connection rotation angle

~ drift angle• Simple connections in the

Manual provide inelastic rotation capacityo 3% (minimum) for design rangeo Seismic drift assumed to be

accommodated

87

Flexible diaphragms• Diaphragm

deformation adds to story drift

• Columns and connections at diaphragm mid-spano Increased rotationso Increased P-

88

Gravity column




Stairs• Act as braces

o Stiff• Not ductile• Continued function

necessary• Detail to allow

movemento Maintain gravity support

89

Pounding• Dynamic effects• Column damage

o At offset levels

90

Pounding

Column damage

Critical conditions• High consequence

o Loss of gravity supporto Loss of egress

• Treat with extra careo Estimate upper-bound

displacementso Absolute sum, not SRSS

91

Support on bracket

Member spanning seismic separation

Critical conditions

92

1.5Cd/R1.5Cd/R




93

Summary

Summary

94

• Structures require a complete load path to maintain stability

• Foundations and diaphragms are an integral part of the load path

• The entire structure must be capable of deforming along with the seismic load resisting system

95

Parting thought

How are these issues treated by building codes?

96

End of session 7

Next:Session 8:Building codes




Additional resources

97

http://www.nehrp.gov/pdf/nistgcr11-917-11.pdf

98

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asic webinar session 7 - building configuration slides

Documents

building configurationapril

steel behavior5

steel frame design

building configuration

material of construction

todays audio

earthquake effects2

discussion of issues