Pramin Norachan

Seismic Design of Cast-in-Place Concrete Diaphragms, Chords and Collectors

Pramin Norachan

DESIGN OF TALL BUILDINGS: TRENDS AND

ADVANCEMENTS FOR STRUCTURAL PEFORMANCE

Bangkok, Thailand

November 9, 2016

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Presentation Outline

1. Introduction

2. Diaphragm Modeling

3. Analysis

4. Design & Detailing

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Introduction

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Introduction

• Estimating the inelastic properties for a real component is not a

simple task.

• It may be tempting to argue that since the inelastic properties are so

uncertain, there is little point in using inelastic analysis, and elastic

analysis should be sufficient.

• If there is substantial inelastic behavior in an actual structure, the

results of an elastic analysis may be of uncertain value for making

design decisions, and may even be misleading.

• As a tool for obtaining information for design, even a crude inelastic

model can be more useful than an elaborate elastic model.

• Please keep in mind that the goal is to get useful information

for design, not to calculate "exact" response.

PERFORM-3D is an ideal tool

for nonlinear performance-based

analysis and design, created by

Dr. Graham H. Powell, University

of California at Berkeley Professor

Emeritus of Civil Engineering.

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PBD Guidelines

PEER-TBI 2010 LATBSDC 2014

The two mostly used guidelines for

Performance-based seismic design

are:

– PEER-TBI 2010

– LATBSDC 2014

They both refer to:

– ASCE 41

– ASCE 7 Standards

– ATC-72-1

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Design Guidelines

NEHRP

(NIST GCR 10-917-4)

IBC 2012 ASCE 7-10 ACI 318-14

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Preview

• Building structures generally comprise

structural elements to support gravity and

lateral loads.

• The seismic force-resisting system is

composed of vertical elements, horizontal

elements, and the foundation.

• The vertical elements provide a continuous

load path to transmit gravity and seismic

forces from the upper levels to the

foundation.

• The horizontal elements typically consist of

diaphragms, including collectors.

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Preview

• Diaphragms transmit inertial forces

from the floor system to the vertical

elements of the seismic force-

resisting system.

• They also tie the vertical elements

together to stabilize and transmit

forces among these elements.

• Thus, diaphragms are an essential

part of the seismic force-resisting

system and require design attention

by the structural engineer to ensure

the structural system performs

adequately during earthquake

shaking.

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The Roles of Diaphragms

Diaphragm in-plane forces:

Diaphragms span between, and transfer forces

to, vertical elements of the lateral-force resisting

system.

Diaphragm transfer forces:

Force transfers between vertical elements which

have different properties over their height, or

their planes of resistance may change from one

story to another.

A common location where planes of resistance

change is at the grade level of a building with an

enlarged subterranean plan (podium

diaphragm).

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Diaphragm in-Plane Forces

• In general, low-rise buildings and

buildings with very stiff vertical

elements such as shear walls are

more susceptible to floor

diaphragm flexibility problems

than taller structures.

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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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Diaphragm Components

• Different parts of a diaphragm include:

- Diaphragm slab

- Chords

- Collectors (Drag struts or Distributors)

- Connections to the vertical elements.

• These different parts can be identified by

considering the load path in a simple

diaphragm.

• We can idealize the diaphragm as a

simply supported beam spanning

between two supports, with reactions and

shear and moment diagrams

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Diaphragm Components

Chord (Diaphragm)

Chord (Diaphragm) Collector

(Support)

Shear Friction

(Support)

Shear (Diaphragm)

Shear Wall

Diaphragm

1

2

4

3

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Diaphragm Modeling

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Diaphragm Modeling - Stiffness

LATBSDC2014

** Nonlinear fiber elements automatically account for cracking of concrete because the

concrete fibers have zero tension stiffness.

• In cases where the analysis results are sensitive to diaphragm stiffness assumptions, it may be

prudent to “bound” the solution by analyzing the structure using both the lower and upper range

of diaphragm stiffnesses, and selecting the design values as the largest forces from the two

analyses.

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Simplified Model - Equivalent Beam Model

• This model treats the diaphragm as a horizontal beam spanning between idealized rigid supports.

The rigid supports represent vertical elements such as shear walls.

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Simplified Model - Equivalent-Beam-on-Springs Model

• The equivalent-beam-on-springs model envisions the diaphragm as a beam supported by flexible

supports. The stiffness of each spring is a representation of the stiffness of the supporting vertical

element.

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Simplified Model - Strut-and-Tie Model

• Strut-and-tie models can be used to idealize the flow of force through a diaphragm. Such models

have not been used extensively for the overall design of a diaphragm, although they can be useful

for this purpose.

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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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Analysis

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Code-Based Design

1. The first set of inertial design forces, Fx, is applied to the design of the vertical elements of the

seismic-force-resisting system.

2. The second set of inertial design forces, Fpx, is applied to the design of the diaphragms.

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Performance-Based Design

• There are alternative approaches to determine design forces in diaphragms. In performance-

based seismic design (PBD), a nonlinear response history analysis typically is used.

• Ground motions sometimes are selected and scaled with a focus on the fundamental period of

vibration.

• Diaphragm accelerations and the resulting forces can be determined directly from the analysis.

• If diaphragms are modeled as finite elements, section cuts can be used to track diaphragm

forces at each time step.

• As with any computer model, the engineer should exercise good judgment when using the

results of a nonlinear response history analysis.

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Analysis Procedures

• ASCE-41 permits four types of analyses:

1. Linear elastic static procedure (LSP)

2. Linear dynamic procedure (LDP) or response spectrum analysis

3. Non-linear static procedure (NSP) commonly referred to as the push-over analysis

4. Dynamic nonlinear response analysis (NDP).

• Tall Building Design Guidelines permit only two:

1. 3D LDP or NDP for serviceability check

2. 3D NDP for all checks

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Analysis Types

No. Procedure Analysis StructureExternal

Force

Level of

ModelingAccuracy

1Linear Static

Procedure (LSP)

Equivalent

Static AnalysisLinear Static Easy

Least

Accurate

2Linear Dynamic

Procedure (LDP)

Response

Spectrum

Analysis

LinearResponse

Spectrum

Time-History

AnalysisLinear Dynamic

3Nonlinear Static

Procedure (NSP)

Pushover

AnalysisNonlinear Static

4Nonlinear Dynamic

Procedure (NDP)

Time-History

AnalysisNonlinear Dynamic Complex

Most

Accurate

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Load Combinations

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Dynamic Response of Buildings and Diaphragms

( )gu t

Story Acceleration

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Force Scaling

Before carrying out design checks at MCE, the linear analysis results of ETABS were scaled to

match with the nonlinear time-history analysis results (NLTHA) from PERFORM-3D.

PERFORM 3D ETABS

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Diaphragm Forces

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Diaphragm Forces

Ref. CSI Knowledge Base

1. Create diaphragm model, then apply diaphragm forces

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Diaphragm Forces

Ref. CSI Knowledge Base

2. For a given load case, display any stress or shell force.

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Diaphragm Forces

Ref. CSI Knowledge Base

3. Where maximum chord forces are expected, draw or define a section cut.

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Design & Detailing

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Typical Classification of Component Actions

Source: 2014 LATBSDC

Load combination for each ground motion pair: 1.0DL+0.25LL+1.0EMCE

Actions Type Demand (D) Capacity (C) D/C

Force-Controlled Actions

Critical Actions 1) Seven or more pairs

2) Less than seven pairs

Expected strengths

Non-Critical Actions

1) Seven or more pairs

2) Less than seven pairs

Expected strengths

Deformation-Controlled Actions

1) Seven or more pairs

2) Less than seven pairs

Deformation capacity obtained from ASCE 41-13 (acceptance criteria)

1.5uF Mean 1.0u

n

FD

C F

1.0

nF

uF Mean

1.0

nF 1.0u

n

FD

C F

1.0D

C

uF Max

uF Max

Deformation Mean

Deformation Max

(Shear)

(Flexure)

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Tension & Compression Chords

C

T

Action Demand (D) Capacity (C)

Compression chord

Tension chord

uu u

MT C

d

uu u

MT C

d

u s yT A f

us

y

TA

f

'0.5u cC f

Inertia Force ( )uM Analysis SectionCut

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Shear Diaphragm

uV

Action Demand (D)

Demand (D)

Capacity (C)

( )uV Analysis SectionCut

'

'

0.17

0.66

u n

cv c t fn

cvc

V V

d d

A f fV

d d

Af

d

Inertia Force

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Collectors

uC

Action Demand (D)

Demand (D)

Capacity (C)

uT

u s yT A f us

y

TA

f

'

'

0.5min

0.85

c

u

c

fC

f A

Inertia Force

( )uT Analysis SectionCut

( )uC Analysis SectionCut

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Shear Friction

Action Demand (D)

Demand (D)

Capacity (C)

( )uV Analysis SectionCut

'0.2min

5.5

u n

n vf y

c c

c

V V

V A f

f A

A

uV

Inertia Force

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Design Detailing - Chords

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Design Detailing - Collectors

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Design Detailing - Collectors

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Design Detailing – Large Openings

• For a larger opening, the diaphragm must be designed to transfer the forces around the

opening.

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Force Transfer to Vertical Elements

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Detailing

Collector connection to shear wall boundary zone

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Detailing

A long collector with confinement

reinforcementForm savers for dowel anchorage

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