ASIC Webinar Session 7 - Building Configuration Slides

AISC Night Night School School Session 7: Building Configuration April 21, 21, 2014

Fundamentals of Earthquake Engineering for Building Structures

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AISC Night Night School School Session 7: Building Configuration April 21, 21, 2014


Course Description

Copyright Materials

Building Configuration April 21, 2014

This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written permission of AISC is prohibited. © The American Institute of Steel Construction 2014

This Building Configuration lecture will focus on load path and the role and components of diaphragms. There will be a discussion 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 compatibility will be presented. This lecture will also include a discussion of issues related to fixity and rotation demand.

Learning Objectives • Gain an understand understanding ing of load load path for the design design of of steel framed structures • Become Become familiar familiar with with diaphragm diaphragm behav behavior ior and design design principles. • Learn and and understand understand about about foundat foundation ion design design concepts concepts for steel framed structures. • Learn and understand understand about deformation deformation compatiblity compatiblity in steel framed structures.

Fundamentals of Earthquake Engineering for Building Structures Rafael Sabelli, SE

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AISC Night School Session 7: Building Configuration April 21, 2014


Course outline Session 7:

1. Seismology and earthquake effects 2. Dynamics and response 3. Building dynamics and response

Building configuration

4. Steel behavior 5. System ductility and seismic design 6. Steel systems 7. Building configuration 8. Building codes 9

10

Session topics • Load path

Load path

• Foundations • Diaphragms • Collectors • Deformation compatibility

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Load path

Load path

• Connects “point of application” to “point of resistance”

Lateral framing

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

Gravity framing

• Foundation is considered point of resistance 13

Wind vs. seismic loads

14

Wind load path

• Wind loads o

External • Exposed areas participate

• Seismic loads o

Inertial • All mass participates

• Load path required between mass and foundation 15

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Seismic load path

Seismic load path

Mass without connection to structure: No load path

• All masses must have positive connection to seismic-load-resisting system • Magnitude of connection force due to o

Ground motion

o

Mass of item

o

Building dynamics (local acceleration)

• Diaphragms contain the majority of typical building mass 17

18

Seismic-load-resisting system

Load path issues

• Vertical frames o

Beams

o

Columns

o

Braces (if any)

• Continuity o

• Diaphragms o

Deck

o

Chords

o

Collectors

Load path must be continuous between mass and foundation

• Foundations 19

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Load path issues • Eccentricity o

o

Load path issues Compression Tension

Horizontal eccentricity between mass and frame causes flexure in diaphragm

• Change in direction At a change in direction load path there is an additional force

o

• Vertical

Vertical eccentricity between mass and foundation causes overturning in frame

o

Overturning

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Foundations Foundations

• Shallow foundations

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• Deep foundations

o

Support

o

Support

o

Lateral resistance

o

Lateral resistance

o

Stability

o

Stability

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Shallow foundations: lateral resistance

Shallow foundations: support • Overturning at frames o

• Lateral resistance

Bearing pressure • Short-duration increase in resistance

o

“Sliding”

o

Friction

o

• Idealized as triangular

o

• Or modeled with soil springs o

Bearing (passive pressure) Engagement of multiple footings

No tension!

25

Shallow foundations: lateral resistance

Shallow foundations: stability

• Lateral resistance o

“Sliding”

o

Friction

o

o

Grade beam

H

Bearing (passive pressure) Engagement of multiple footings • Relative lateral movement of footings can be problematic

H

H

H

H

H

H

H

• May be governing consideration for foundation • Nonlinear

H

H

H

H

H

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o

May be stable under ASD and unstable under LRFD loads • Minimum requirement: Evaluate under ASD • Design footings for soil capacity (amplified)

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Shallow foundations: stability

Deep foundations: support

• Implications of designing for stability with reduced loads o

o

o

• Overturning at frames o

Compression • End bearing

Rocking may be governing mode

• Friction

System above may have lower ductility demand

o

Displacements may be larger than anticipated

o

Tension • Friction

Short-duration increases

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Deep foundations: lateral resistance

Deep foundations: lateral resistance

• Lateral resistance o o

o

o

• Lateral resistance

Pile shear and bending

o

Pile-cap bearing (passive pressure)

o

Engagement of multiple footings

o

Batter piles

o

Pile-cap bearing (passive pressure) Engagement of multiple footings

• Prevent relative movement

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H

H

H

H

H

H

H

H

H

H

H

H

Buildings tied together • Engage all piles

31

Grade beam

Pile shear and bending

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Deep foundations: stability • Stability o

o

o

Diaphragms

Addressed by strength design of piles Upper-bound soil strength difficult to establish Rocking mechanism not applicable

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Steel Deck (AKA “Metal Deck”)

Deck and Fill

Shear load path through steel deck and fasteners.

Shear load path through steel deck and fasteners.

Steel chords and collectors.

Concrete stiffens deck and prevents buckling. Steel chords and collectors.

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Steel deck with reinforced concrete fill

Horizontal truss diaphragm

Shear load path through reinforced concrete and shear studs. Chords and collectors: Steel members, or Reinforcement in deck

Shear load path through steel diagonals and framing. Steel chords and collectors.

Shear studs.

Deck is for gravity only.

Reinforcement

Truss

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Diaphragms

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Roles of diaphragms

• Roles of Diaphragms

• Support gravity

• Diaphragm Components

• Deliver forces to frames

• Diaphragm Behavior and Design Principles

• Brace columns for stability

• Building Analysis and Diaphragm Forces

• Transfer forces between frames

• Diaphragm Analysis and Internal Component Forces

• Resist P- thrust

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Distribute inertial forces

Lateral bracing of columns

KL (K=1)

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Resist P- thrust

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Transfer forces between frames

Vertical beam reaction Sloped column axial force

Horizontal thrust

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Transfer diaphragms

Transfer diaphragms Podium

Setbacks

45

Backstay Effect

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Diaphragm Components Collector

Demand at backstay diaphragm

Deck (“diaphragm”) Stiff “plaza level” diaphragm

Shear reversal at plaza level Horizontal Force Couple Vertical Force Couple

V

M

Chord

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Diaphragm Components

Diaphragm rigidity

Collector

Flexible

Deck

Rigid

(“diaphragm”)

Chord

Semi-Rigid

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Diaphragm types and analysis • Determinate

• Rigid, or

• 3 lines of resistance

o

Analyze diaphragm

Analysis of Flexible Diaphragms

• Indeterminate

• Flexible, or o

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• Semi-rigid o

Analyze building • Relative frame stiffness

Diaphragm reactions load frames

• Diaphragm rigidity • Frame location o

Forces to frames = diaphragm collector forces

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Typical diaphragm analysis

Typical diaphragm analysis Chord Tension

F coll 33%

33% 17%

17% 17%

Collector

Collector

17%

33%

33%

V F chord

F p

Uniform shear

Chord Compression

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Alternate diaphragm analysis

54


Chord Tension

Collector

Chord Tension

Collector

Collector

Collector

Non-uniform shear Local chords Nonuniform shear

Chord Compression

Chord Compression

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Alternate diaphragm analysis Chord Tension

Collector

Chord Tension

Collector

Collector

Chord Compression

Non-uniform shear Local chords Internal collectors

Collector

Chord Compression

Critical for design

Non-uniform shear Local chords Internal collectors

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Analysis of Non-flexible Diaphragms

Analysis of Non-flexible Diaphragms

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

Moment

Shear Moment Correction Corrected Moment

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

Shear

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Using the results of 3-D analysis

Critical for design: Collectors and chords

Using the results of 3-D analysis

Loading distribution adjusted to satisfy statics

Critical for design: Collectors and chords

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Collectors • Protected element

Collectors

• Reinforcement in composite deck • Steel framing

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Collector and frame loads: Case 1

Protected element

Force Colors

V (i+1) =(T max (i+1) +C max (i+1))cos((i+1)) Critical elements designed for this load (

d a o l l a r e t a L

E )

Shear entering frame line

Columns Collector beams

F1(i) =F left (i) +F mid (i) +F right (i)

o

Fuses designed for this load (E : design base shear)

V (i+1) F left (i) 33%F1(i) V (i)

T max (i+1) F1(i) F mid 33%F1 (i)(i) C max (i)

C max (i+1)

Capacity Statics

F right (i) F1(i) =V (i) −V (i+1) 33%F1(i) T max (i)

V (i) =(T max (i) +C max (i))cos((i))

Deformation,

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Collector and frame loads: Case 2 (i)

/ [V

=V (i)

(i+1)+

F p

]

Force

F2 (i) = (i) F p = F left (i) +F mid (i) +F right (i)

For static From analysis equilibriumV’ V (i+1) (i+1)

F left (i)

T T’ (i+1) (i+1) F F2 p(i) (i)

• Wide section of deck

Colors

V’ (i+1) = (i) V (i+1) Shear entering frame line

Reinforcement in deck

C’ C (i+1) (i+1) F right (i)

Diaphragm

o

Low stress

Capacity

o

Stability not critical

• Eccentricity from frame

Statics

o

33%F2 (i) V (i)

F mid ((i)i) 33%F2 C max (i)

33%F2 (i)

Local chords

• Concentrated shear transfer

T max (i)

V (i) =(T max (i) +C max (i))cos((i))

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Reinforcement in deck

Reinforcement as collector

Reinforcement used for collector forces

e

oE / A=  0.5 f c ’ (unconfined concrete)

oE / (wt )=  0.5 f c ’ w ≥ oE / (0.5 f c ’ t )

L

oE

C C

e = w /2 Braced frame

w

Local chord force: C = e (oE )/L oE 69

Beam-columns

Beam-columns

• Compressive strength o

• Compressive strength

Wide-flange with discreet lateral and torsional bracing

o

Wide-flange with continuous lateral bracing

• Major axis flexural buckling • Minor-axis flexural buckling

• Major axis flexural buckling

• Torsional buckling

• Constrained-axis flexuraltorsional buckling

o

Higher strength than minor-axis FB for same unbraced length

o

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Strength between minor-axis FB and torsional buckling 72



Constrained-axis flexural-torsional buckling

Beam-columns • Constrained-axis flexural-torsional buckling o

Use 0.9 P E to calculate F cr



 π 2 E C w  I y a 2

P e  



Minor axis flexural buckling Constrained-axis (no restraint) Flexural-torsional buckling (restraint at top flange)

Torsional buckling (restraint at centroidal axis)

Beam-columns • Compressive strength Wide-flange with continuous torsional bracing • Major axis flexural buckling • Required torsional stiffness TBD o

Slab stiffness

o

Web stiffness

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  GJ 

1

 r x2  r y2  a 2 a

73

o

 K z L 2

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Beam-columns

Collector connections • Gravity

• Flexural strength o

Composite deck

o

• Composite strength o

Shear forces

• Seismic

Steel deck only

o

• Lateral bracing with flutes perpendicular

o

Axial forces (horizontal) Rotation

• Unbraced with flutes parallel

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Collector connections Limit States Plate Yield & Rupture Bolt shear Bearing & Splitting Block Shear Weld Rupture

Collector connections Rn (y) from Manual

Vu Hu

Hu Rn ( x)

2

+

Vu Rn (y)

2

1

Rn (x)

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Collector connections

Collector connections

• Rotation o

• Rotation

Single-plate connection

o

• Follow Manual rules

o

o

Plate thickness

o

Bolt size

o

Spacing

Welded top flange • Introduces some eccentricity

o

Moment connection • Attracts moments

Double column of bolts • Extended plate method

• May have ductility demands

• Proportioning rules

• Detail for ductility

Deformation compatibility • Shear distortion adjacent to tall frames o

Deformation compatibility Amplified rotation

Due to • Lateral drift • Column axial deformation

o

May result in large rotation demands

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Deformation compatibility

Necessity

• Necessity

• Inelastic response

• Connections

o

Large drifts • Lateral system

• Flexible diaphragms

• Gravity system

• Stairs

• Performance goal

• Pounding

o

Prevent collapse • Global

• Critical conditions

• Local

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Gravity connections

Flexible diaphragms

• Connection rotation angle ~ drift angle

• Diaphragm deformation adds to story drift

• Simple connections in the Manual provide inelastic rotation capacity o o

• Columns and connections at diaphragm mid-span

3% (minimum) for design range Seismic drift assumed to be accommodated

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o

Increased rotations

o

Increased P-

Gravity column

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Stairs

Pounding

• Act as braces o

• Dynamic effects

Stiff

Column damage

• Column damage

• Not ductile

o

At offset levels

Pounding

• Continued function necessary • Detail to allow movement o

Maintain gravity support

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Critical conditions

Critical conditions 1.5C d /R 

• High consequence o

Loss of gravity support

o

Loss of egress

• Treat with extra care o

o

Estimate upper-bound displacements Absolute sum, not SRSS

1.5C d /R 

Member spanning seismic separation Support on bracket

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Summary Summary

• 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

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Parting thought

End of session 7

How are these issues treated by building codes?

Next:

Session 8: Building codes

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Additional resources Question time

http://www.nehrp.gov/pdf/nistgcr11-917-11.pdf 97

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