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© The Wood Products Council 2013© The Wood Products Council 2013

Understanding Restraint Rod SystemsShear Wall Overturning vs. Wind Uplift

and how to design them properly

Alicia Eikenberry, P.E.Simpson Strong Tie

Northwest Territory Branch EngineerNorthwest Territory Branch Engineer

Credit Information

AIA Member Information

The “Wood Products Council” is a Registered Provider with TheAmerican Institute of Architects Continuing Education Systems(AIA/CES). Credit(s) earned on completion of this program will bereported to AIA/CES for AIA members Certificates of Completion forreported to AIA/CES for AIA members. Certificates of Completion forboth AIA members and non AIA members are available upon request.

This program is registered with AIA/CES for continuing professionaleducation. As such, it does not include content that may be deemedor construed to be an approval or endorsement by the AIA of anymaterial of construction or any method or manner of handling, using,distributing, or dealing in any material or product.distributing, or dealing in any material or product.

Questions related to specific materials, methods, and services will beaddressed at the conclusion of this presentation.pThis course offers 1LU/HSW Credit.

Learning Objectives

Upon completing this course, you will be equipped to:

• Explain and contrast the differences between wind uplift andshear wall overturning restraint rod systems.g y

• Account for AC391 acceptance criteria in restraint rod systemdesign.

• Evaluate performance differences between all floors tied offand skipped floor shear wall overturning restraint rod

tsystems.

• Evaluate 3 different design specification methods and selectan appropriate method for any projectan appropriate method for any project.

Restraint Rod Systems

A Brief History

• Over 20 year history in the marketplace.• Address both uplift and overturning

restraint.• Evolved from early system of connectors

(metal ties,brackets or screws) to today’s systemof rods andcouplers.

• Shrinkage compensation devicesintroduced in 1998introduced in 1998.

Restraint Rod Systems

A Brief History

• “Engineering judgment” was the earlydesign guidance.

• Today uplift systems: guided by ICCy p y g yAcceptance criteria AC391 and AC316(shrinkage compensation devices).

• Today overturning systems:– designed by EOR with

manufacturer’s design input.– Guided by code reports and

id li (NDS ASTM AISC St lguidelines (NDS, ASTM, AISC SteelConstruction Manual, ACI, and IBC).

Shear Wall Overturning vsShear Wall Overturning vs.Wind Uplift Forces

Overturning vs. Uplift Forces

Overturning (Lateral)When a building weight cannotWhen a building weight cannotstabilize a structure under a lateralforce, it causes the structure to rotateoff its foundation.

U lift (V ti l)Uplift (Vertical)As wind flows over the roof the windcreates a strong lifting effect, muchg g ,like that of air flowing over an airplanewing.Diff t F Diff t S tDifferent Forces = Different Systems

Two Different Types of Restraint Rod Systems

Key Differences:

• Rod location• Load direction• Framingg

Requirements• Load path• Shrinkage/

compressionlocation

Wind Uplift Shear Wall Overturning

Key Differences

R d

Shear Wall OverturningWind Uplift

RodLocation

Shear wall ends

Applied

OC spacing at bearing walls

AppliedLoad

Each levelRoof level only (typically)

Restraints

Diaphragm/blocking/sole/ /

Roof level only

Diaphragm/truss( f )/ l /b

Each level

Load Path plate/sheathing/compressionstud/bearing plate/rod

Sh i k &

(rafter)/plate/bearingplate/rod

Shrinkage &Compression

Segmented per floorOccurs at a single location

Design and Industry Practices,Building Codes, Research Reports, and Acceptance Criteria for Windand Acceptance Criteria for Wind

Uplift Restraint Rod Systems

Design Guidance for Wind Systems

Is there a prescriptive design guide for wind uplift restraint rod systems?wind uplift restraint rod systems?

Currently, there is no code referencedstandard for continuous rod tie down systems.

So, where can building designers turn to whendesigning these wind uplift restraint systems?

Manufacturers?fEngineering Judgment?Acceptance Criteria alone?

What have Manufacturers Provided up until now?

Evaluation Reports, issued prior toA C i i b iany Acceptance Criteria being

developed for rod systems.

Newer Evaluation Reports include anew “design” section requiring thedesigner to evaluate top plateb di d fl i d h dbending, deflection, and other woodcomponent performance.

Caveat Emptor… trust but verify….Be sure your manufacturer hasevaluated the performance of thewood system, not just the rod system.

And!!

There is some additional help….

In June 2010 ICC ES passed and made effectiveIn June 2010, ICC ES passed and made effectiveAcceptance Criteria 391 for continuous rod tie down

systems for uplift.

This standard establishes the minimum levels ofperformance to meet the code’s intent.

ICC-ES AC391: CRTR and CRTS

ICC ES AC391 establishes guidelines for the evaluation of either:

ContinuousRod Tie-Down System (CRTS)

[includes•The steel componentscomprising continuous rod tiedown runs (CRTR) only.

[includes

CRTR]

•The entire continuous rod tiedown systems (CRTS), whichincludes CRTR and the light

ContinuousRod Tie-includes CRTR and the light

framed wood structure used toresist wind uplift.

Rod Tie-Down Run

(CRTR)

Specific Design Requirements of AC391

Section 1 2Section 1.2Limited to resisting roof

wind uplift in woodlight framedlight framedconstruction.

Excluded from AC391:Excluded from AC391:shear wall overturningsystems or use in coldformed steel framing.g

What’s NOT in the Code?

The building code does not define the requirements forthe wood structural system the rod system is installedwithin such as:within, such as:

• Deflection limit for top plate flat bending

• Top plate rotation

• Rod elongation

• Wood plate crushing

• Wood shrinkage• Wood shrinkage

• Bearing plates

• Anchorage capacity

But…. AC391 does establish guidelines forl i f h d f h devaluation of the wood frame system the rod

system must perform in.

Design Guidance for Wind Systems

Some manufacturers define the winduplift restraint rod components (CRTR)uplift restraint rod components (CRTR),but leave the “system” components(CRTS) design to the engineer.

Who is responsible for the design ofthe wood structural system…..

CRTR

?CRTR

CRTS? CRTS

Restraint Rod Systems forRestraint Rod Systems for Shear Wall Overturning

Overturning Restraint Rod System – System Load Path

Observe how forcesl h h denter, travel through, and

exit the shear walloverturning restraintg

system.

Overturning Restraint Rod System – System Load Path

Lateral forces are appliedt h llto shear walls.

Overturning Restraint Rod System – System Load Path

Sheathing fastenerstransfer loads into

posts, creating upwardcompression on woodcompression on woodplates and blocking as

overturning forcesoccur Overturningoccur. Overturning

forces are incremental,per floor.

Overturning Restraint Rod System – System Load Path

At each level, posts transfercompression forces to woodmembers, causing bearing

and bending on steel plates.and bending on steel plates.

Overturning Restraint Rod System – System Load Path

Rods transfer cumulativeoverturning forces,

translated into tensionforces, down level to,

level.

Overturning Restraint Rod System – System Load Path

Anchorage transfersl ti t icumulative overturning

forces into foundation.

Overturning Restraint Rod System – System Load Path

This side of the structureis in TENSIONis in TENSION.

Overturning Restraint Rod System – System Load Path

This side of the structure isi COMPRESSION hin COMPRESSION, wherethe forces are transferreddown to the foundation by

the wood members.

Overturning Restraint Rod System

The uplift load is delivered toThe uplift load is delivered toposts through sheathingfasteners.

Posts then deliver uplift loaddirectly to the bearing plate.

Compression posts are closetogether to minimize top platebending

Spacing limited tocontrol top plate

bending.

p pbending

Skipped-Floor System

Three-story skipped-floor lexample

• Second and third floor Skipped• Second- and third-floorrestraints are not provided (skipped).

pp

• Single bearing plate at the top level is the sole point of

Skipped

level is the sole point ofrestraint.

All-Floors-Tied-Off System

All-floors-tied-off examplep

• Each floor is restrained with a bearing plate and nut.

• Loads are evenly distributed• Loads are evenly distributedthrough the system at each floor.

Skipped-Floor vs. All Floors Tied Off

Skipped Floor System All Floors Tied Off System(Not Recommended) (Recommended)

Pointof

Restraint

Pointsof

Why are skippedfloor systems notrecommended?

Restraint Restraint

A single point of restraintfor multiple levels

Overturning restraintprovided at each level

Cost Complications

Skipped FloorOverturning forces transfer up theb ildi il i ibuilding until a restraint isreached..

RESULT:Elements at restrained point must

2- 2x46- 2x4

Elements at restrained point mustresist uplift forces for nonrestrained stories below.

RESULT: 4- 2x46- 2x4RESULT:Increased lumber, threaded rod,and bearing plate sizes.

Tied offIncremental uplift at each level istransferred directly into the

6- 2x46- 2x4

tiedown system at that level,reducing lumber/rod/plate sizes. Skipped Tied off

Rod Elongation

Skipped system: elongation accumulates at the base, damaging shearfastening, weakening system.Tied off system: elongation distributed between floors, lessening impact.

Rodl ti

divided

elongationaccumulatesat base

Tied Off SystemSkipped Floor System

Design Example

Skipped Floor System 27 k

F3=4kT3=27k

C3=13+10+4=27k 23 k

F2=10kT =27kT2=27k

C2=13+10=23k

13.5 k 13.5 k

13 k

F1=13kT1=27kC1=13k

27 k

Post axial load uplift side

Detail A

Testing Tells No Lies

Tyrell Gilb Lab Test3 Story Skipped Floory pp

Click to playvideo

Notice stud andsheathing shear failure

Design Example

All Floors Tied Off System4 k

Uplift forces

F3=4kT3=4kC =4k

14 k

10 kC3=4k

F2=10kT =10+4=14k

2 k

13 k

2 k

13 k

T2=10+4=14kC2=10k

2 k

4 k27 k

2 kF1=13k

T1=13+10+4=27kC1=13k

Detail C1

Post axial load uplift side

Detail B

Testing Tells No Lies

Tyrell Gilb Lab Test3 Story All Tied Off3 Story All Tied Off

Click to playvideo

Wood Solutions Fair Field Test

Today’s Audience Participation Question???“Are the principles of ‘All Tied Off’ support systems even loadAre the principles of All Tied Off support systems, even load

distribution, multiple points of restraint, and lateral load restraintsufficient to prevent overturning in today’s world?”

We’re going to demonstrate this principle before your very eyesg g p p y y yand let you be the judge.

Construction Stability

Skipped floor system: unstable until allSkipped floor system: unstable until allfloors are complete. Susceptible to midconstruction collapse in seismic or high

i d twind event.

Tied off system: restraint installed as eachTied off system: restraint installed as eachlevel complete, minimizing risk.

Frequency of Restraints

When no redundancy ….. One fails, all fail.

RedundancyNo Redundancy

All-Floors-Tied-Off

Accommodatesshrinkage at eachfloor independently: Lower overall cost:

Greater systemredundancy and isconsistent with typical

Reduced drift:consistent with typicaldesign load path:

Forces in the elements getsmaller as they move up thestructure:

Skipped-Floor vs. All Floors Tied-Off Summary Where does design guidance come from?

What is in the Building Code for Overturning Systems?

All parts of an overturning restraint rod except the optional take up devices.

T l t

Part of System Defined By:

Top platecapacity

NDS Supp. Tables 4A & 4B

ASTM A36 bearing stress of the plate in bendingSteel bearingplates/nuts

ASTM A36 – bearing stress of the plate in bending.AISC Steel Construction Manual Table 7 20 for heavy hexnut sizes

Steel rodcapacity

AISC Steel Construction manual, 13th Ed.

What is in the Building Code for Overturning Systems?

All parts of an overturning restraint rod except the optional take up devices.

Part of System Defined By:

T k U

Coupler nuts ACI 318 – 21.13.3 Mechanical reinforcement

Take UpDevices

(Not defined by code – see ICC ES AC316)

Anchorage ACI 318 – Appendix D

Shear wall driftSection 4.1.3 of the AWC Special Design Provisions forWind and Seismic.

Designing and Specifying Sh W ll O t i R t i tShear Wall Overturning Restraint

Rod Systems

Manufacturer Solutions

• Multiple manufacturers design shearwall overturning restraint rod systems.

• Product solutions should addressoverturning as well as shrinkage thatoccurs at each level of a multi storystructure.

• Shrinkage compensation (take up)g p ( p)devices are one way to addressshrinkage.

• Manufacturers differentiate themselvesManufacturers differentiate themselveswith design services or product andinstallation enhancements such as colorcoded rod ends and bearing plates

Take up devicescoded rod ends and bearing plates.

Take up devices

Specification Methods

Design Professional Responsibilities

• Determine project loading and performance requirements.

• Determine best solution to meet requirements (overturning restraintrod vs. conventional holdowns and strapping.

• Stipulate (specify) these requirements on the design documents.• Review proposed solutions and shop drawings for conformance top p p g

specs and design intent.

Specification Methods

Design Review Checklist for Overturning Restraint Rod Systems

• Run type design vs. structural requirementsyp g q

• Rod manufacturer component capacity check

– Rods/steel strength and mill certs

– Bearing plate size and capacity

– Shrinkage compensation devices for compression and tension

• AC 316 compliance• AC 316 compliance

• Movement/shrinkage check including maximum rod elongation andmaximum deflection per floor

• Tension rod location check

• Drawing and calculation review

(See Design Review Checklist Handout)(See Design Review Checklist Handout)

Specification Methods

Design Considerations

Areas of concern????

– Performance of the wood structural system, not just the rodsystem

– Cost based decisions that effect product qualityProduct substitution…Grade stamps and mill certificationsplease?

– Deferred submittal…..Friend of Foe?

Specification Methods

Specification methods for overturning restraint rod systems

1. Send loads to manufacturers who optimize the runs.

a) Software analysis

b) Engineering calculations

2. Provide loads and generic specifications on drawings, thenf fsubmit to manufacturer for optimization.

3. Specify catalog runs from a product catalog.

Manufacturer can optimize runs during design review processManufacturer can optimize runs during design review process

Installation drawings and calculations typically provided for EORreview. (See sample submission set)

Conclusion

Summary

• There are two types of restraint rod systems; one for wind uplift and onefor shear wall overturning.

• Significant performance differences exist between overturning skippedfloor systems and all floors tied off systemsfloor systems, and all floors tied off systems.

• Testing rod systems for uplift has shown that by not accounting for thewood structure in designs, system performance is compromised.

• For overturning systems, manufacturer’s have a number of ways to assistthe EOR in the design and selection of the system.

Course Completion

Simpson Strong TieSimpson Strong Tie

What are your Questions?y

This concludes the American Institute of ArchitectsContinuing Education Systems course

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