Lateral Design Considerations for Mid-Rise Structures

7/14/2016

1

Lateral Design Considerations

for Mid-Rise Structures

Marselle Condominiums Trochalakis, PLLC Photographer: Matt Todd Photographer

5 stories of wood over 6 stories concrete (podium) 2 above grade

By: R. Terry Malone, PE, SESenior Technical Director

Architectural & Engineering Solutions

www.woodworks.org

E-mail: [email protected]

120 Union, San Diego, CATogawa Smith Martin

This presentation is copyrighted by Woodworks

Mid-rise Codes and Standards

Crescent Terminus building‐ credit: Richard Lubrant.

The analysis techniques provided in this presentation are intended to demonstrate one method of analysis, but not the only means of analysis. The techniques and examples shown here are provided as information for designers to consider to refine their own techniques.

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Course Description

Research, full scale testing and code requirements continue to evolve on mid-rise construction*. This presentation will focus on important engineering considerations related to the lateral design of mid-rise wood buildings in particular. Implementation of a well-considered design requires the understanding of diaphragm and shear wall flexibility and their effects on the horizontal distribution of forces through the structure. In mid-rise, multi-family buildings, corridor only shear walls are becoming very popular as a way to eliminate exterior shear walls. The special design issues this situation creates will be addressed, as will the unique load distribution and flexibility considerations associated with tall shear walls.

*Definition:A mid-rise building can be described as something between a high-rise and low-rise structure —that is, between four and ten stories with heights ranging between 35 to 75 feet tall.

Learning Objectives• Design Considerations

Discuss important lateral engineering issues that need to be considered to successfully design a mid-rise structure.

• Diaphragm and Shear Wall Flexibility

Understand the relationship between diaphragm and shear Wall flexibility on the horizontal distribution of shears within a mid-rise structure.

• Tall Shear Walls

Discover when shear walls should be designed as tall shear walls and discuss the effect of tall shear walls on the distribution of shear forces.

• Overview-Open Front and Non-Open Front StructuresReview key points in the analysis of open front diaphragms and

non-open front designs.

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Presentation Topics

• Design Considerations

• Complete Load Paths

• Diaphragm Flexibility

• Tall Shear Walls-Flexibility

• Horizontal Distribution of Shears

• Overview-Mid-rise: Exterior shear Walls

• Overview-Mid-rise: Open Front


Design Considerations

Method of analysis for open-front or tall shear wall mid-rise structures is currently evolving as more complex building geometries are becoming more prevalent. Building shapes and footprints are driving the design procedures for all lateral resisting systems and materials.

Design requirements:

• IBC1604.4- Analysis:o Method of analysis shall take into account equilibrium, general stability,

geometric compatibility, and both short-and long term material properties.

o Shall be based on rational analysis in accordance with well established principals of mechanics. Analysis shall provide complete load paths.

Considerations:

• Make sure that you have complete continuous lateral load paths.

• Address vertical / horizontal irregularities.

• Investigate diaphragm flexibility.

• Check shear wall stiffnesses and the effects of tall shear walls.

• Check the effects of offset diaphragms and shear walls.

• Verify the horizontal distribution of forces within the diaphragm and to the shear walls.

Note: Possible changes in diaphragm flexibility can occur from floor to floor: non-open front (flexible) vs. non-open front or open front (rigid or semi-rigid).

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76’

35’

6’

35’

Open Front & Non-open Front Floor Plan w/ and w/o offsets

7654321

Typical Unit

F

C

B

A

D

E

Open Front

Non-Open Front

8

SWTypical

SWTypical

• ASCE 7-10 Section 12.3.1.1- (c), Light framed construction, meeting all conditions:

Longitudinal-Typically permitted to be idealized as flexible, provided exterior shear walls exist. However, diaphragm can be semi-rigid or open front, even if exterior walls exist.

Transverse-Traditionally assumed to be flexible diaphragm.

If token or questionable wall stiffness at exterior, use semi-rigid analysis using the envelope method-(conservative), or semi-rigid, or idealize as rigid.

Non-open front

Open Front Structures:

• SDPWS 4.2.5.2 :

Loading parallel to open side - Model as semi-rigid (min.), shall include shear and bending deformation of the diaphragm, or idealized as rigid.

Loading perpendicular to open side-traditionally assumed to be flexible.

Drift at edges of the structure ≤ the ASCE 7 allowable story drift ratio whensubject to seismic design forces including torsion, and accidental torsion(strength, multiplied by Cd). No set drift requirements required for wind (Drift can be tolerated).

Includes flexible and rigid analysis

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Presentation Topics









Strut/chord

Open

3

4

5

21

F

E

D

C

B

6 9 107 8

Strut/chord

Str

ut

Str

ut

(typ

.)

Strutchord

Strut chord

Strut /chord

Str

ut

Strut/chord

Strut/chord

SW

1

SW5

SW2

SW3

SW6

SW4

Str

ut

MR

F1

Multiple offset diaphragm

Offset strut

Support Support

Co

llect

or

Collector

Collector(typ.)

Collector(typ.)

Collector (typ.)

Co

llect

or

(typ

.)

A

Complete Continuous Lateral Load Paths

Analysis: ASCE7-10 Sections:• 1.3.1.3.1-Design shall be based on a rational analysis• 12.10.1-At diaphragm discontinuities such as openings and re-entrant

corners, the design shall assure that the dissipation or transfer of edge (chord) forces combined with other forces in the diaphragm is within shear and tension capacity of the diaphragm.What does

this mean?

Complete Load Paths

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Strut/chord

Open

3

4

5

21

F

E

D

C

B

6 9 107 8

Strut/chord

Str

ut

Str

ut

(typ

.)

Strutchord

Strut chord

Strut /chord

Str

ut

Strut/chord

Strut/chord

SW

1

SW5

SW2

SW3

SW6

SW4

Str

ut

MR

F1

Support Support

Co

llect

or

Collector

Collector(typ.)

Collector(typ.)

Collector (typ.)

Discontinuousdiaphragm chord/strut

Discontinuousdiaphragm chord

Discont.diaph. chord

Discont.diaphragm chord

Discont.diaphragm chord

Co

llect

or

(typ

.)

A


ASCE7-10 Section 1.4-Complete load paths are required including members and their splice connections

Strut/chord

Open

3

4

5

21

F

E

D

C

6 9 107 8

Strut/chord

Str

ut

Str

ut

(typ

.)

Strutchord

Strut chord

Strut /chord

Str

ut

Strut/chord

Strut/chord

SW

1

SW5

SW2SW3

SW6

SW4

Str

ut

MR

F1

Offset shear wallsand struts

Support Support

Co

llect

or

Collector

Collector(typ.)

Collector(typ.)

Collector (typ.)

Offset shear walls

Co

llect

or

(typ

.)

A


ASCE7-10 Section 1.4-Complete load paths are required including members and their splice connections

B

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Strut/chord

Open

3

4

5

21

F

E

D

C

B

6 9 107 8

Strut/chord

Str

ut

Str

ut

(typ

.)

Strutchord

Strut chord

Strut /chord

Str

ut

Strut/chord

Strut/chord

SW

1

SW5

SW2

SW3

SW6

SW4

Str

ut

MR

F1

Openingin diaph.

Support Support

Co

llect

or

Collector

Collector(typ.)

Collector(typ.)

Collector (typ.)

Co

llect

or

(typ

.) Vertical

offset indiaphragm

A


Design: • IBC 2305.1.1-Openings in shear panels that materially effect their strength shall be fully

detailed on the plans and shall have their edges adequately reinforced to transfer all shear stresses.

Dis

con

tin

uo

us

cho

rds

Tra

ns

vers

e

Cant.

Mid-rise Multi-family

SWSWSWSW

SWSWSWSWSW

SW

Lds. Discontinuous strutsLongitudinal

Lds.

No exteriorShear walls

Flexible, semi-rigid, or rigid???

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TD3

TD1

TD2

SW4

SW3

SW5

SW2

SW1

Diaphragm stiffness changes

Multi Story, Multi-family Wood Structure

1

2

3

Loads

I1 I2 I3

Higher shears and nailing requirements

ASCE7-101.3.5 - Cont. ld. Paths12.1.3 - Cont. ld. paths-inter-conn. Ties12.10.1-Openings, re-entrant. –transfer of dis-cont. forces

combined with other forces12.10.2-Collector elements

I1 I2

SW4

SW3

SW5

SW1

1

2

3Transfer AreaHigher shears and nailing Reqmts.

Collector(typ.)

Higher shearsand nailing Reqmts.

SW2

Co

llect

or

Str

ut/

ch

ord

Be

am

TD2

TD1

Str

ut

Main diaphragm becomes TD3

Optional Framing Layouts

Collector(typ.)

Out-of-Plane Offset Shear WallsAssumed to act in the Same Line of Resistance

Loads

Co

llect

or

Collector

Collector

TD1

SW1

SW2

Transfer area

Offset

Discont.drag strut

Dra

g

stru

t

Dra

g

stru

t

SW3

Co

llect

or

Collector

Collector

TD2

• Offset walls are often assumed to act in the same line of lateral-force-resistance.

• Calculations are seldom provided showing how the walls are interconnected to act as a unit, or to verify that a complete lateral load path has been provided.

• Collectors are required to be installed to transfer the disrupted forces across the offsets.

Discont.drag strut

Typical mid-rise multi-family structure at exterior wall line

Where offset walls occur in the wall line, the shear walls on each side of the offset should be considered as separate shear walls and should be tied together by force transfer around the offset (in the plane of the diaphragm).

Check for Type 2 horizontal irregularityRe-entrant corner irregularity

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SW

SW (Typ.)

SW

SW (Typ.)

Transverse walls typical

Longitudinal exterior wallstypical

Corridor wallstypical

Transfer Area

Tying Shear Walls Across the Corridor

Typical Rectangular plans

collector

Co

llec

tor

collector

Co

llec

tor

Tying Shear Walls Across the Corridor or a Large Diaphragm

Tying Shear Walls Across a Large Diaphragm

SW

SW

3. Collectors for shear transfer to individual full-height wall segments shall be provided.

SDPWS 4.3.5.1

Presentation Topics









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Average drift of walls

Is maximum diaphragm deflection (MDD) >2x average story drift of vertical elements,using the Equivalent Force Procedure of Section 12.8?

Maximum diaphragm deflection

Semi-rigid (Envelope Method)

Is diaphragm untopped steel decking or Wood Structural Panels

Idealize as

flexible

Is span to depth ratio ≤ 3and having no horizontal irregularities ?

Is diaphragm concrete slab or concrete filled steel deck ?

Structural analysis must explicitly include consideration of the stiffness of the diaphragm (i.e. semi-rigid modeling).

Is any of the following true?

1 & 2 family Vertical elements one Light framed constructionDwelling of the following : where all of the following are

met:1. Steel braced frames 1. Topping of concrete or2. Composite steel and similar material is not

concrete braced frames placed over wood structural3. Concrete, masonry, steel SW panel diaphragms except

or composite concrete and for non-structural topping steel shear walls. not greater than 1 ½” thick.

2. Each line of vertical elements of the seismic force-resisting system complies with the allowablestory drift of Table 12.12-1.

Idealizeas rigid

Idealize asflexible

Yes

YesYes

No

No

No

No

No

Sta

rt

ASCE7-10 Section 12.3 Diaphragm Flexibility Seismic

Yes

Yes

Section 12.3.1- The structural analysis shall consider the relative stiffnesses of diaphragms and the vertical elements of the lateral force resisting system.

(Could apply to CLT or Heavy timber diaphragms)

Is diaphragm untopped steel decking , concrete filled steel decks or concrete slabs, each having a span-to-depth ratio of two or less?

Yes

Yes

No

Diaphragm canbe idealized asflexible

Is diaphragm ofWood Structural Panels ?

Diaphragm canbe idealized asrigid

No

Is diaphragm untopped steel decking , concrete filled steel decks or concrete slabs, each having a span-to-depth ratio greater than two ?

Yes Calculate as flexible or semi-rigid

ASCE7-10, Section 26.2 Diaphragm Flexibility Wind

Start

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Presentation Topics









∆ + ∆

A/R h/d ≤ 2:1

Tall Shear Walls

+

+

Traditional Shear Walls: Max. A/R=3.5:1

• Traditional floor to floor method is good for 3 stories or less (w/ exception)

VR

V3

V2 Tra

dit

ion

al

Me

tho

dTa

ll W

all

Me

tho

d

∆ ∆ +∆ ∆

Testing shows that the traditional deflection equation is less accurate for walls with aspect ratios higher than 2:1.(Dolan)

Walls assumed to be pinned at top and bottomat each floor, rigid wall sections and rigid out-of-plane diaphragm stiffness

Tall Shear Wall

Floor to floor A/R’s and Stiffness of Shear Walls

Doesn’t account for multi-story shear wall effects

Tall Shear Walls

• Tall walls greater than 3 stories should consider flexure and wall rotation.

o Rotation and moment from walls aboveand wall rotation effects from walls below.

Tall

∆

• Allowable story drift for traditional and tall shear walls is checked floor to floor.

Total displ.of Tall Wall. More flexible.

A/R=3.5:1flr.-flr.

Total displ. Traditional walls

A/R=2:1flr.-flr.

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Rim joist

Should consider as flexible because it is unknown where rim joist splices will occur

Platform framed

Semi-balloon framed(Very flexible)

Flexibility does not allow dead load to aid in resisting wall rotation

Stiffness might allow dead load to aid in resisting wall rotation, provided joints are strategically placed. (justify by calculation)

If diaphragm out-of-plane stiffness=FlexibleAnalyze entire wall as a tall wall

If diaphragm out-of-plane stiffness=Rigid (steel beam, conc. beam) Analyze entire wall as traditional floor to floor

Compression blocking

V

CT

M

Diaphragm out-of-plane Flexibility

Current Examples of Mid-rise Analysis

• APEGBC Technical & Practice Bulletin � Revised April 8, 2015 “5 and 6 Storey Wood Frame Residential Building Projects (Mid-Rise)”-Based on FPInnovations Mechanics Based Approach

• FPInnovations-Website ”A Mechanics-Based Approach for Determining Deflections of Stacked Multi-StoreyWood-Based Shear Walls”, Newfield

• Shiotani/Hohbach Method-Woodworks Slide archive

http://www.woodworks.org/education/online‐seminars/

• Thompson Method-Woodworks Website

http://www.woodworks.org/wp‐content/uploads/HOHBACH‐Mid‐Rise‐Shear‐Wall‐and‐Diaphragm‐Design‐WSF‐151209.pdf

Design Example: ”Design of Stacked Multi-Storey Wood-Based Shear Walls Using a Mechanics-Based Approach ”, Canadian Wood Council

Webinar

http://www.woodworks.org/wp‐content/uploads/5‐over‐1‐Design‐Example.pdf

Paper

Current Examples of Tall Shear Wall Analysis

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Deflections of Stacked Multi-story Shear Walls-U.S. StandardsBased on FPInnovations ”A Mechanics-Based Approach for Determining Deflections of Stacked Multi-Storey Wood-Based Shear Walls”, Newfield-(Metric)

∆

Lwall

Lc

xtr

∆Where

∆ , = +

∆ , = , ,

∆ , = 0.75 ,

∆ , = , (Includes affects of rod

elongation and crushing)

∆ , ∆ , +∆ ,

∆ ,

Rotational deflection

∆ ,

α

…

⋯

Level i

Level i‐1

= , ( +

Transformed composite section

ATS - cont. hold down usedC.L.Brg

∆∑

+ ∑

, ,0.75 ,

, ∑ ∆ , +∆ ,

Shear Defl.

Nail Slip Rod Elong.

crushing

Bending deflection Sumrotation

below

∆ ,

,

Alt.

∆ , = ,

∆ , ∆ , ∆ , ∆ ,

∑+ ∑

. .

Tall Wall Deflection

∑V5

VR

∑V4

∑V2

∑V3

=510.5’k

. ′

. ′

. ′

5.195

11.192

15.761

18.902

20.659

α1

θ

∆1

α2

θ2

∆2

+

θ3

∆3

+

α3

θ4

∆4

+α4

∆5

+

α

α1

α

α

Deflection-Bending Deflection-Wall rotation)

∆ ∆ + ) α + ∆ α +∆Ex. 3rd flr.

∆

translates to top translates to top

(Wall rotation)

+ +

+Rotation

α

α

α

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Presentation Topics









Distribution of shear to vertical resisting elements shall be based on:

• Analysis where the diaphragm is modeled as :o Idealized as flexible-based on tributary area.

o Idealized as rigid-Distribution based on relative lateral stiffnesses of vertical-resisting elements of the story below.

o Modelled as semi-rigid. Not idealized as rigid or flexible Distributed to the vertical resisting elements based on the relative stiffnesses of the

diaphragm and the vertical resisting elements accounting for both shear and flexural deformations.

In lieu of a semi-rigid diaphragm analysis, it shall be permitted to use an enveloped analysis (Minimum analysis).

Average drift of walls

Maximum diaphragm deflection (MDD) >2x average story drift of vertical elements, using the ELF Procedure of Section 12.8?

Maximum diaphragm deflection

Calculated as Flexible

• More conservatively distributes lateral forces to corridor, exterior and party walls

• Allows easier determination of building drift• Can over-estimate torsional drift• Can also inaccurately estimate diaphragm

shear forces

• Can under-estimate forces distributed to the corridor walls (long walls) and over-estimate forces distributed to the exterior walls (short walls)

• Can inaccurately estimate diaphragm shear forces

Note:Offsets in diaphragms can also affect the distribution of shear in the diaphragm due to changes in the diaphragm stiffness.

2015 SDPWS 4.2.5 Horizontal Distribution of Shear (Revised)

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∆ . .

I1 I2 I3Rigid or spring Support ??

Ext

erio

r sh

ear

wal

ls

Typical Multi-residential Mid-rise unit

Rigid support orPartial support

Support Support

TD1

TD2 NoSupport

TD1

TD2

I1I2I3

No support(Full cantilever)

Adjust terms of the equation for support condition, stiffness and loading conditions

Wind Loads as applicableSeismic Loads

Support Support

Full support(SW rigid)

Partial support(DecreasingSW stiffness)

No SW support

If flexible diaphragm

Corridor only SWCondition B

Condition ACondition C

• Flexible• Semi-rigid• Rigid

• Semi-rigid• Rigid

Effects of Tall Shear Walls

Note that possible changes in diaphragm flexibility can occur from floor to floor: non-open front (flexible) vs. non-open front or open front (rigid or semi-rigid).

open front?

Semi-rigid?

Flexible?

Effects of openings?

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Presentation Topics









The following information is from an on-going example calculation and is subject to further revisions and validation. The information provided is project specific, and is for informational purposes only. It is not intended to serve as recommendations or as the only method of analysis available. As such, most of the information is based on the authors opinions.

76’

156’

35’

6’

35’

Case Study-Non-Open Front Floor Plan With Offsets

26’ typ.

Typical Unit

7654321

F

C

B

A

1.33 1.67

D

E

A.5

E.5

SW5SW4

SW3SW2

SW1

11.5’ 23’

27’ 35’

Varies’

• Flexible analysis• Rigid analysis• Semi-rigid (Envelope)

• ASCE 7-10 Section 12.3.1.1- (c), Light framed construction, meeting all conditions: All Light framed construction Non-structural concrete topping ≤ 1 ½” Each elements of the seismic line of vertical force-resisting system complies with the

allowable story drift of Table 12.12-1 Longitudinal-Allowed to be idealized as flexible, provided exterior shear walls exist and

is in compliance with ASCE 7-10 Section 12.3.1.1 (c). If calculations show that the story drift at a line of lateral force resistance exceeds the allowable limit, flexible diaphragm behavior cannot be used.

Open Front

Non-Open FrontMax. A/R=4:1

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ATS isolator nut take-up deviceand bearing plate

Coupler

Steel rod

Semi-balloon framing

Platform framing

Automatic Tensioning Systems/Devises

• Restraints are required at roof and each floor level to get best results

• Software programs are available for design

• Must be installed per manufacturers recommendations

Number and size of studs as required by design

Anchorage into foundation as required by calculation and per manufacturers recommendations

Ove

rtu

rnin

g

Res

ista

nce

Calculate vertical distribution of forces

Determine initial forces to SW using envelope method (flexible / rigid)

Brief Over-view-Calculations

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Resulting Shears & Bending Moments

∆

∑

=165.88’k

=102.9’k

=51.32’k

=16.32’k

∆

∑

=537.83’k

. ′

. ′

.

∆

∑

. ′

. ′

. ′

.

SW3SW2SW1A

1.632

1.632

1.8683.5

1.6585.158

1.14

6.298

0.637

6.935

5.235.23

6.183

11.41

5.327

16.737

3.66

20.397

2.04922.45

7.667.66

8.7516.41

6.26422.674

4.30726.977

2.40929.39

=249.1’k =807.22’k =1089.93’k

1.142

1.142

1.308

2.45

1.1613.611

0.798

4.409

0.446

4.855

3.6613.661

4.33

7.991

3.729

11.718

2.562

14.28

1.43415.714

5.3625.362

6.12311.487

4.38515.87

3.015

18.885

1.68620.571

ASD STR ASD STR ASD STR

11.42 ‘k

35.92 ‘k

72.03 ‘k

116.12 ‘k

174.37 ‘k

36.61 ‘k

116.5 ‘k

233.68 ‘k

376.48 ‘k

565.05 ‘k 762.95 ‘k

516.08 ‘k

327.21 ‘k

168.49‘k

53.62 ‘k

ASDSTR

∑ ∑

Determine Wall Nailing

Determine Initial Boundary Elements Sizes

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Determine SW Composite Section Properties

Determine Wall Rotation

Determine Tall Wall Stiffness and Drift

Determine Final forces to SW’s

Drift check

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Determine Diaphragm and TD Nailing, and Chord ForcesSym.C.L.

TD1TD2

TD1TD2I1I2

I3

26’ 6’

8’

12’

4’

4’

27’

11.5

’3’

11.5

’

35’

3’

23’

156’

Chord Slip at TD

TD1

TD2

Longitudinal Loading

Collector 2

Collector 1

TD1 Shear

TD2 Shear

Basic Diaphragm Shear

Sym.C.L.

Bending Stiffness =

"

"

"

26’ 6’

8’

12’

4’

4’

27’

11.5

’3’

11.5

’

35’

3’

23’

156’

35’

, =66825

, =216513 x 6

, =28726137

" "

Longitudinal Loading

Determination of Diaphragm Bending Stiffness

TD1TD2

TD1TD2I1I2

I3

7/14/2016

21

26’

Co

rrid

or

∆

.

6’

8’

12’

4’

4’

27’

11.5

’3’

11.5

’Spring Support

(SW stiff.)

I1I2I3

Computer Model

Tall or narrow SW

Full depth

M

V

35’

156’

Sym.C.L.

3’

23’

Spring Support

(SW stiff.)K1 K2

Seismic

Moderately stiff. support

.Positive moment

∑ Kdiaph.

Shear Deflection per USDA Research Note FPL-0210

∆

∆ .

∆∆∆∆

∆∆∆∆

9’ 9’ 9’ 4’ 4’

Co

rrid

or Ext.

SW

∆ .

∆

∆

Acc. torsion

Bending

Check Total deflection-Diaphragm flexibility

Check exterior SW stiffness effects

Off

se

t

Off

se

t

∑ Vsw ∑ Vsw

Shear DeflectionNail slip deflection

Determination of Non-Open Front Diaphragm Flexibility

∆ .

Bending increases at start of offset

Roof Plot- Longitudinal Loads- A/R*=1.5:1 (8’ wall)

Base Line=0

Base Line=0

Base Line=0

En

d

Off

set

Off

set

51.13 k46.4 k

43.21 k

80.48

414.2’k

526.55’k

692.1’k

Full cantilever

A/R 1.5:1 wallA/R 2:1 wall

A/R 3.5:1 wall

Not fullsupport

Full cantilever

Partialsupport

*Aspect ratios Floor to Floor

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Drift by semi-rigid or rigid analysis only

Spring support If Shear walls

+

Same as

=

Check Story Drift-3D model or 2D plane Grid

+Same

as

=

Collectors

Story Drift Check Longitudinal + Torsion (8’ ext. walls)

Significant shear distributed to corridor walls (cantilever action)

Notice curvature (bending) of offset sections

Maximum drift

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Story Drift Check Transverse + Torsion

Maximum driftMaximum driftTorsional irregularity????

Presentation Topics






• Mid-rise: Exterior shear Walls



The following information is from an on-going example calculation and is subject to further revisions and validation. The information provided is project specific, and is for informational purposes only. It is not intended to serve as recommendations or as the only method of analysis available. As such, most of the information is based on the authors opinions.

7/14/2016

24

Relevant 2015 SPDWS Sections

New definitions added:

• Open front structures• Notation for L’ and W’ for

cantilever Diaphragms• Collectors

Relevant Revised sections:• 4.2.5- Horizontal Distribution

of Shears• 4.2.5.1-Torsional Irregularity• 4.2.5.2- Open Front Structures

(a)

L’

W’

Force

Cantilever DiaphragmPlan

Open front

SW

SW

L’

Cantilever DiaphragmPlan

W’

Open front

SW

SW

SWForce

L’

W’

Open front

SW

SWForce

Cantilever Diaphragm

Figure 4A Examples of Open Front Structures

(d)

L’

W’

Open front

SW

SW

Force


SW

SWL’


(c)(b)

Open front

4.2.5.2 Open Front Structures: (Figure 4A)

For resistance to seismic loads, wood-frame diaphragms in open front structures shall comply with all of the following requirements:

1. The diaphragm conforms to:a. WSP-L’/W’ ratio ≤ 1.5:1 4.2.7.1b. Single layer-Diag. sht. Lumber- L’/W’ ratio ≤ 1:1 4.2.7.2c. Double layer-Diag. sht. Lumber- L’/W’ ratio ≤ 1:1 4.2.7.3

2. The drift at edges shall not exceed the ASCE 7 allowable story drift ratio when subject to seismic design forces including torsion, and accidental torsion (Deflection-strength level amplified by Cd. ).

3. For open-front-structures that are also torsionally irregular as defined in 4.2.5.1, the L’/W’ ratio shall not exceed 0.67:1 for structures over one story in height, and 1:1 for structures one story in height.

4. For loading parallel to open side:a. Model as semi-rigid (min.), shall include shear and bending deformation of the diaphragm,

or idealized as rigid.

5. The diaphragm length, L’, (normal to the open side) does not exceed 35 feet. Exception: Where the diaph. edge cantilevers no more than 6’ beyond the nearest line of vertical elements of the LRFS.

4.2.5.2.1 For open front structures of 1 story, where L’≤25’ and L’/W’ ≤1:1, the diaphragm shall be permitted to be idealized as rigid for the purposes of distributing shear forces through torsion.

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76’

156’

35’

6’

35’

Case Study-Open Front Floor Plan With Offsets

26’ typ.

Typical Unit

7654321

F

C

B

A

1.33 1.67

D

E

A.5

E.5

SW5SW4

SW3SW2

SW1

11.5’ 23’

27’ 35’

Varies’

• Rigid analysis• Semi-rigid (Envelope)

Open Front

Non-Open Front

Determination of Open Front Diaphragm Flexibility

26’

Co

rrid

or

Corridor only shear walls

.

∆

∆

I1I2I3

Computer Model

6’

8’

12’

4’

4’

27’

11.5

’3’

M

V

23’

156’

Sym.C.L.

3’

Spring Support

(SW stiff.) K1

Full depth

Seismic

Bending ∆

∑ Kdiaph.

Shear Deflection per USDA Research Note FPL-0210

Acc. torsion

∑ Vsw

∆

Shear DeflectionNail slip deflection

L’ Limit= 35’

• Check drift at edges (extreme corners) including torsion plus accidental torsion (Deflection-strength level x Cd. )

• Seismic-meet ASCE7 drift• Wind-deflections can be tolerated.

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Final Diaphragm (Total) Deflections

∆∆∆∆

9.5’ 9.5’ 8’ 4’ 4’

Co

rrid

or

∆ .

Roof

5th Floor

4th Floor

Co

rrid

or

. ∆

6’

8’

12’

4’

4’

27’

11.5

’3’

23’

∆ . 35’

∆∆∆∆

9.5’ 9.5’ 8’ 4’ 4’

Co

rrid

or

∆ .

∆∆∆∆

9.5’ 9.5’ 8’ 4’ 4’

Co

rrid

or

∆ .I1I2I3

Seismic∑ Vsw

Check for diaphragm flexibility

Verify Diaphragm Flexibility:• If flexible, add blocking• If still flexible, decrease nail spacing

to stiffen up

Reference Materials

http://www.woodworks.org/wp-content/uploads/Irregular-Diaphragms_Paper1.pdf

• The Analysis of Irregular Shaped Structures: Diaphragms and Shear Walls-Malone, Rice-Book published by McGraw-Hill, ICC

• Woodworks Presentation Slide Archives-Workshop-Advanced Diaphragm Analysis

• NEHRP (NIST) Seismic Design Technical Brief No. 10-Seismic Design of Wood Light-Frame Structural Diaphragm Systems: A Guide for Practicing Engineers

• SEAOC Seismic Design Manual, Volume 2

• Woodworks-The Analysis of Irregular Shaped Diaphragms (paper). Complete Example with narrative and calculations.

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• Slide Archive-Offset Diaphragms and Shear Walls• Webinar Archive- Offset Diaphragms -Part 1

Information on Website

Horizontally Offset Diaphragms

Example Offset Diaphragms

• Slide Archive-Offset Diaphragms and Shear Walls• Webinar Archive- Offset Shear Walls-Part 2

Information on Website

Horizontally Offset Shear Walls

Example Offset Shear Walls

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Questions?

R. Terry Malone, P.E., S.E.Senior Technical DirectorWoodWorks.org

Contact Information:[email protected]

This concludes The National Council of Structural Engineers Association Webinar on:

Lateral Design Considerations for Mid-rise StructuresPresentation-July 19, 2016

Lateral Design Considerations for Mid-Rise Structures

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