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A Tension-Controlled Open Web Steel Joist

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No joist will withstand sudden and catastrophic

impact forces that exceed system capability.

Flex-Joist design offers probability of high

ductility and time delay under static gravityoverload conditions.

DISCLAIMER:

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Improved Ductility andReliability under Static

Gravity Overload

Purpose:

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Flex-JoistEngineered Limit States

Intentionally imbalanced member strength ratios

Weaker components serve as ductile fuse

Initial limit state of ductile yielding in primary tension members

Other limit states inhibited until advanced state of collapse

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Increased Probability of Safe Evacuation

Slower Collapse Mechanism

Sensory Warning via Large Inelastic Deflections

What is so great about ductility?

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Improved Structural Reliability: Reduced Variance in Strength

Influence of Variance on Reliability

Which population has the greatest probability of a value below 1.0?

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Idealized parallel system sketch

Load shared equally between

components


Improved Structural Reliability: Load Sharing

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Sudden Strength Loss (lack of

ductile behavior)

Load dumps to remainingcomponents (progressive

collapse)

System strength limited by

weakest component

System variance equals

variance of individual

components population



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Idealized parallel system sketch

Load shared equally between

components

Elasto-Ductile system



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Ductile behavior

Weakest member continues to

support plastic capacity afterexceeding elastic limit

System strength a function of

average component strength

System Variance:

=



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Compressive Buckling

Design Strength

Compression

Element Buckling

Ultimate Strength


Slower Collapse Mechanism with Sensory Warning

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CompressionElement Buckling

Tension Element Yield

Design Strength

Ultimate Strength

Ductile Tensile Yielding


Slower Collapse Mechanism with Sensory Warning

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Flex-Joist Load/Deflection Data Plot

When Loads Exceed Capacity of a Flex-Joist

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Flex-Joist Design Reliability StudyRatio of Plastic Strength /

Experimental Design Load

From Villanova Data

Series Sample

LRFD

Design

Load (plf)

Fy Experi-

mental

(ksi)

Adjusted

Design

Critical

Load (plf)

Plastic

Strength

(plf)

Ratio

Plastic /

Adj Crit

Load

J1-1 568 1.01

J1-2 574 1.02

J1-3 567 1.01J1-4 589 1.05

J1-5 592 1.06

J1-6 582 1.04

J2-1 1878 1.07

J2-2 1882 1.07

J2-3 1886 1.07

J2-4 1852 1.06

J2-5 1868 1.06

J2-6 1855 1.06

J3-1 582 1.01

J3-2 589 1.03

J3-3 567 0.99

J3-4 568 0.99

J3-5 572 1.00

J3-6 566 0.99

K-Series 418 60.3 560

LH-Series 1303 60.6 1755

Rod-Web-

Series 420 61.5 574

Average 1.033

Std Dev 0.030

COV 0.029

Qty 18

All

Plastic Strength Ratio

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Flex-Joist Design Reliability StudySteel Dynamics Roanoke Bar

Division A529-50 merchant bar

May 2008 to October 2012

11546 samples / 4337 batches

Stat's

Yield

Stress

(psi)

Ratio

Yield

Stress /

50 ksi min

Average 56764 1.1353

Minimum 50000 1.0000

Maximum 76570 1.5314

Std Dev 3415.6 0.0683

COV 0.0602 0.0602

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Flex-Joist Design Reliability StudyStructural Reliability Analysis:

= 0.90

Live / Dead Load Ratio = 3

= 3.2

=ln

2+

2+

2+

2

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Summary of Flex-JoistDesign Characteristics

System based on N = 4 statistically unlinked joists working in parallel

Criteria Std Joist Flex-Joist % Diff

Joist Strength Reliability 2.6 3.2 22%System Strength Reliability 2.6 3.4 31%

Average ASD Test Strength Ratio 1.8 2.3 29%

Average Test Ductility Ratio 1.4 3.2 129%

Tension Limit State Probability Low High

Electronic Monitoring Suitable Okay Excellent

Average Relative Weight 100% 107%

Joist Performance Comparison

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Approximately 30% higher Reliability Index ().

Approximately 7% heavier, on average.

Clearly room for potentially reducing weightwhile retaining superior reliability.

Subject to justification being provided to support a

higher y

value and/or lower y

value, in an ICC

Engineering Services Report submittal.

Limited applications until fire testing has been

performed

Summary of Flex-JoistDesign Characteristics

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Tension-Controlled Joist Limiting Design Factors

Conditions preventing the Bottom Chord and End Web from developing theirtensile capacity:

Unusually high material Fy

High compression under net uplift loads, axial loads, or end moments

Unusually strict deflection criteria

Minimum material size criteria

Unnecessarily strict tension member slenderness criteria

Uniformly distributed loading on a 20K7 steel joist with a base length of 33

Lowest Stress Highest Stress

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Tension Slenderness Ratio

Remnants of the 1946 slenderness requirement carried over

as far as the 8thedition (1980) AISC:

The slenderness ratio, Kl/r, of compression members shall not

exceed 200.

The slenderness ratio, l/r, of tension members, other than

rods, preferably should not exceed:

For main members.......240

For lateral bracing members and other secondary members300

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Tension Slenderness Ratio

Current (14thedition, 2010) AISC states in Section D1:

User Note: For members designed on the basis of

tension, the slenderness ratio L/r preferably should not

exceed 300. This suggestion does not apply to rods orhangers in tension.

There is no slenderness limit for members in tension.

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When safe and reliable is not enough

Increased reliability

Increased probability of time for safe evacuation

www.newmill.com/flex

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1

Experimental Investigation of Open WebSteel Joists Designed for Tension-

Controlled Strength Limit State

Joseph Robert Yost, Ph.D., PEAssociate Professor, Structural EngineeringDepartment of Civil and Environmental Engineering

Villanova University

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2

Presentation Overview

1. Introduction and Methodology

2. Experimental Matrix

3. Load and Support Details

4. Test Results

5. Conclusions

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3

Research Program

Experimental investigation of simply supported uniformlyloaded open web steel joists subjected to gravity loading.

Top chord in combined compression and bending.

Bottom chord and end webs in axial tension.

Interior webs alternating tensionand compression.

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4

Member Limit States and ExperimentalObjective

Member strength limit states Top chord compression buckling

Bottom chord and end webs tensile yield

Interior webs alternating tensionand compression

Load

Displacement

Compression buckling

Tension yielding

Experimental Objective

Design and test series ofOWSJ for tensioncontrolled failure limitstate.

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5

Methodology

Design individual members so that tension yield of BC or EW occursbefore compression buckling of TC or webs. Call tension-controlleddesign methodology.

Over size compression members relative to strength demand.

Define member Demand Capacity Ratio (DCR) as:

Tension-Controlled Design Methodology

All compression members DCR < 1.0 (reserve strength)

Critical tension member DCR = 1.0 (at failure)

Other tension members DCR 1.0 (close to failure)

Increase slenderness limit on tension members to 300

CR =Required Strength

Provided Strength

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6

Tension-Controlled Design Term andMember Selection

n =

DCRn

DCRmax tension =1.0Introduce relative strength term, r:

Relative Strength Ratios Used for Member Selection of Experimental Joists

Bottom C. and/or End Webs r= 1.0 (failure)

Interior Tension Webs r 0.95 (5% reserve strength)

Top Chord r 0.90 (10% reserve strength)

Compression Webs r 0.80 (20% reserve strength)

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7





4. Test Results

5. Conclusions

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8

K-Series x 6 identical samples

LH-Series x 6 identical samples

K-Series Rod Web x 6 identical samples

Sample Count

33 ft.

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P P P P

2P

4.5' 8' 8' 8' 4.5'

Cylinder

#1

Cylinder

#2

Cylinder

#3

Cylinder

#4

1 ft

(typ.)

10

Uniform Load Condition

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11

2P

Cylinder

#1

Cylinder

#2

Cylinder

#3

Cylinder

#4

1 ft

(typ.)

Load Distribution Unit Detail

DistributionUnit

Load Distribution Unit

HydraulicCylinder

DistributionBeam

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12





4. Test Results

5. Conclusions

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2250l

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0

250

500

750

1000

1250

1500

1750

2000

2250

0 1 2 3 4 5 6 7 8 9 10 11 12

Load(lb

/ft)

Midspan Displacement (in)

J2-1

J2-2

J2-3

J2-4

J2-5

J2-6

Unloading to adjusttest apparatus

DL = 77 lb/ft

Yield in

BC

Strain Hardening

LRFD Design Capacity= 1303 lb/ft

14

LH-Series Results

800 Rod-Web Series Results

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0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12

Load

(lb/ft)

Midspan Displacement (in)

J3-1

J3-2

J3-3

J3-4

J3-5

J3-6DL = 45 lb/ft

Unloaded to adjusttest apparatus

Yield of BC

and End Web

Apparent strain hardening

LRFD Design Capacity = 420 lb/ft

15

Rod-Web Series Results

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16

D = design strength

Y = yield strength

P = plastic strength

U = ult. strength

Strength Ratios

1.29 1.281.26

1.39

1.44

1.37

1.491.52

1.63

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

K (J1) LH (J2) Rod Web (J3)

AverageStrengthR

atio(-)

Joist Series

Y/D P/D U/D

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17

Deflection Ratios (U/Y)

2.83

3.79

3.15

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1 2 3 4 5 6 Average

DisplacementRatioU

/Y(-)

Sample

K-SeriesLH-Series

Rod-Web-Series

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18




4. Test Results

5. Conclusions


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The tension-controlled yield limit state was successfullyachieved with all 18 test samples.

Relative strength factors of 0.80 for compression web, and

0.90 for top chord was sufficient to prevent primary limitstate compression failure.

Reserve strength relative to design capacity. Y-to-Dstrength ratios = 1.30, P-to-D strength ratio = 1.40, and

U-to-D strength ratio = 1.50. Significant ductility with average deflection ratios of U-to-Y

= 2.8, 3.8 and 3.2 for K-, LH-, and RW-Series.

Conclusions

Documents

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