Gusset Plate8 05

Forell / Elsesser Engineers

SEAONC MINI SEMINARSEAONC MINI SEMINAR

Gusset Plate Design

Russell BerkowitzForell / Elsesser Engineers, Inc.


What We Will CoverWhat We Will Cover Overview of prominent research and

experiments to date

Current gusset plate design requirements

Limitations of current gusset plate design requirements

Recommendations for future research to develop gusset plate design guidance


Gusset Plate Design ReferencesGusset Plate Design References Seismic Behavior and Design of Gusset Plates

Abolhassan Astaneh-AslSteel Tips December 1998

Brace Frame Gusset Plate Research Literature Review Janice Chambers and Christopher Ernst University of Utah February 2005

On the Analysis and Design of Bracing Connections W.A. Thornton (1991)

Proceedings, National Steel Construction Conference


Gusset Plate Design ReferencesGusset Plate Design References

Handbook of Structural Steel Connection Design & Details Tamboli, 1997

Handbook of Structural Steel Connection Design & Details Thornton & Kane 1999

AISC Manual of Steel Construction, 3rd Edition Seismic Provisions (2002, 2005)


Brace / Gusset ConfigurationsBrace / Gusset Configurations

Astaneh, 1998


Whitmore (1952)Whitmore (1952)

Tested aluminum joints

Iso-stress lines obtained by strain gages mounted on gusset plate

Plots showed stress trajectories to be along 30 lines with the connected member


Whitmores SectionWhitmores Section

Whitmore, 1952


Whitmores SectionWhitmores Section

Astaneh, 1998


Whitmore (1952)Whitmore (1952) Distribution of normal and shear stresses

along critical sections of gusset do not match beam formulas:

Maximum normal and shear stresses measured matched beam theory values

Location of maximums is different

= VQ It = Mc I


Bjorhovde Bjorhovde & & Chakrabarti Chakrabarti 19831983--8888 Six full size steel assemblages

30, 45, 60 angle braces

Monotonic

No frame action

Not applicable to determining interface loads

Used to validate FEM


BjorhovdeBjorhovde && ChakrabartiChakrabarti 19831983--8888


BjorhovdeBjorhovde && ChakrabartiChakrabarti 19831983--8888

Rabern and Chakrabarti, 1983


Gross & Gross & Cheok Cheok (1988)(1988) Used regular frame subassemblages

Moment and forces in members showed all members resist lateral loads

Gusset failed by buckling when brace was in compression

Not monitored for interface forces

Predicted prying action failure but frame forces precluded development


Gross &Gross & CheokCheok (1988)(1988)

Gross & Cheok, 1988


Cheng et al.Cheng et al. Experiments included frame action

Buckling capacity of gusset 4% - 107% higher with frame action

Experimental buckling capacity 63% higher than calculated capacity (using K = 0.65)

Cyclic tests with / without edge stiffeners Slight increase in compressive capacity with stiffeners Tapered plate dramatically reduced compressive and

energy absorption of gusset plates (46%) Flexibility of tapered gusset caused weld fracture at the

boundaries with increasing deformation


Richards et el. , Williams 1986Richards et el. , Williams 1986

Most rigorous analytical research to date

Used FEA INELAS and NASTRAN

51 configurations

Frame action considered

Measured fastener behavior modeled into nonlinear FEA to determine gusset interface forces


Richards et el. , Williams 1986Richards et el. , Williams 1986 Interface forces largely dependent on:

Plate aspect ratio Brace load Brace angle

Interface forces less dependent on: Direction of force (tension vs. compression) Bracing configuration Beam and column properties Gusset fasteners (bolted vs. welded) Brace eccentricity


Richards et el. , Williams 1986Richards et el. , Williams 1986 Frame action

beam and column load the gusset, equally as much as the brace

Pinching occurs , frame angle changes Brace in tension buckles gusset

Direction of forces align with brace with increased loading

1.4 connection factor


Richards et el. , Williams 1986Richards et el. , Williams 1986

Williams, 1986


Berkeley BRB Tests, 2002Berkeley BRB Tests, 2002

Lopez et al. 2002


Berkeley BRB Tests, 2002Berkeley BRB Tests, 2002 Test 1

Yielding at brace-to-column gusset plates Yielding at column base Yielding at beam-column moment connection

Test 2 CP welds at gusset - col. initiated crack at 1.7% , 2

long at 2.6% drift Free edge of gusset buckled at 2.6% drift when brace

was in tension



Aiken et al. 2002



Lopez et al. 2002


Observed Seismic Performance Observed Seismic Performance of Gusset Platesof Gusset Plates Satisfactory performance in general A few cases of gusset failure have been

reported: Mexico City, Northridge, Kobe Earthquakes Observed failure modes

Fracture of welds Buckling of gusset plate Net section fracture of gusset plate or brace Most of these failures are related to non-ductile

design and poor detailing


Observed Seismic PerformanceObserved Seismic Performance

Astaneh, 1998


Current Gusset Design (SCBF)Current Gusset Design (SCBF) AISC Seismic Provisions (2002) Tensile strength of bracing connection

RyAgFy Maximum force that can be delivered by structure

Flexural strength of bracing connection In-Plane Buckling = 1.1RyMp Out-of-Plane Buckling

Connection must be able to accommodate inelastic rotations associated with post-buckling deformations

Design compressive strength at least FcrAg


Current Gusset DesignCurrent Gusset Design Astaneh recommends the following

hierarchy for gusset design failure modes

Astaneh, 1998


OutOut--ofof--Plane Brace BucklingPlane Brace Buckling

Astaneh, 1998


OutOut--ofof--Plane Brace BucklingPlane Brace Buckling Hinges at brace midpoint and in gussets Provide min. 2t to allow rotation in gusset

max 4t

Astaneh, 1986


OutOut--ofof--Plane Brace BucklingPlane Brace Buckling

Astaneh, 1998


Limit States at Brace Limit States at Brace Gusset Gusset ConnectionConnection

Astaneh, 1998


Limit States at Brace Limit States at Brace Gusset Gusset ConnectionConnection Block shear failure Calculate using AISC Eq. J4-3

Bolt tear through on the gusset Calculated using AISC Eq. J3-2

Strength of Bolts or Welds


Limit States at Brace Limit States at Brace Gusset Gusset ConnectionConnection

Astaneh, 1991


Tension Yielding and Net Section Tension Yielding and Net Section Fracture of Whitmores AreaFracture of Whitmores Area

Tension Yielding is the most desirable mode of gusset failure Py = AgwFy

Net Section Fracture is the least desirable Astaneh suggests:

=n y y

n nw u

P (1.1R P )

P A F


Buckling of Gusset PlateBuckling of Gusset Plate

Astaneh, 1998



Yamamoto et al. 1988



Pseudo-Column Buckling Approach

Equivalent Strip or Thornton Method

Applies buckling compressive stress over Whitmores area



Astaneh, 1998



Use AISC column equations for Fcr

== =

2c

yc

cr y

cr y2c

FKlEr

F (0.658 )F

.877F F

>

c

c

1.5

1.5


Buckling of Gusset PlateBuckling of Gusset Plate L =

Average of l1, l2, l3 Longest one-inch wide strip Longest of l1, l2, l3



What K value to use for buckling length? Values from 0.5 1.2 have been proposed

K = 0.65 (0.45 for double) often used Consistently conservative

K = 1.2 proposed by Brown (1988) and Astaneh (1998) Tests indicating possibility of end of bracing

member moving out of plane


Gusset Plate Buckling Limit StateGusset Plate Buckling Limit State Not been accurately modeled by pseudo-column

buckling approach

Highly variable compared to test results

Consistently conservative

Buckling capacity strongly dependent on frame action effects

Local gusset plate research needed to produce more accurate methods of predicting buckling


Gusset Plate Edge BucklingGusset Plate Edge Buckling

Astaneh, 1998


Gusset Plate Edge BucklingGusset Plate Edge Buckling

Astaneh, 1991


Edge StiffenersEdge Stiffeners AASHTO (1997)

This has been around for years for steel bridge trusses

Brown (1988) Formula proposed to prevent edge buckling prior to gusset

yielding

Adequate for monotonic loading


Edge StiffenersEdge Stiffeners Astaneh 1998

Gussets showed edge buckling when Brown criteria satisfied during cyclic tests

Limit Lfg / t to the point where Fcr / Fmax is reduced significantly

Proposed criteria to prevent cycling free edge buckling prior to reaching maximum compression capacity

fgy

L E0.75t F


Edge StiffenersEdge Stiffeners Little experimental research published on the

effects of stiffeners

Four tests with 3/8 and 1/4 plates 3/8 plate showed 15% - 19% increase in buckling

capacity, only 2% for plate Strain measurements showed more force going

through stiffeners than gusset plate Energy absorption increased in compression

FEA shows no increase in peak capacity, but post-buckling capacity was increased


Gusset Plate Interface ForcesGusset Plate Interface Forces

Astaneh, 1998


Interface Connection ModelsInterface Connection Models

Astaneh, 1998


Gusset Plate Interface LoadsGusset Plate Interface Loads

Models are based on load paths dictated by the designer

Lower Bound Theorem Limit Analysis

Determine force distribution in equilibrium with applied load

If no forces in structure exceed yield criteria, loads will not likely lead to collapse



KISS Model (Thornton 1991)

Thornton, 1991


Interface Connection ModelsInterface Connection Models AISC Model (AISC 1984)

Thornton, 1991



Ricker Model

Thornton, 1991



Modified Richard Method (Williams 1986)

Thornton, 1991


Interface Connection ModelsInterface Connection Models Thornton Model Uniform Force Method

Thornton, 1991


Interface Connection ModelsInterface Connection Models Thornton UFM

Comprehensive Offers approximate value to capture frame action

effects and a way to incorporate into design

Richard Method Captures frame action effects Based on empirical evidence Not applicable for column web connections

AISC-LRFD 3rd ed. Manual Recommends use of UFM


AISC Uniform Force MethodAISC Uniform Force Method


AISC UFM Special Case 1AISC UFM Special Case 1


Design Criteria for Gusset Plates Design Criteria for Gusset Plates at Interface with Beam / Columnat Interface with Beam / Column Astaneh check for critical sections

Chambers and Ernst Determine von Mises and the maximum principal

stresses considering shear and normal stresses Von Mises stress < 0.9Fy

Maximum principal stress < 0.75 Fu

( ) + + 2 4Y P YN/ N M/ M (V / V ) 1.0

2 2e x y x y xy3 = + +


Gusset Connection to Beam / ColGusset Connection to Beam / Col The 1.4 Ductility Factor in AISC 3rd Ed.

Connection must be designed for the larger of the peak stress or 1.4 x average stress

Originated from figures by Williams and Richards

FEA showed ratio max / ave fastener force and the ratio min / ave fastener force

Handbook of Structural Steel Connections (1997)

Hewitt and Thornton (2004) reviewed plots and suggest ductility factor should be 1.25


Gusset Connection to Beam / ColGusset Connection to Beam / Col

Hewitt & Thornton, 2004


Gusset Connection to Beam / ColGusset Connection to Beam / Col

FEA shows resultant connector forces on welds are not longitudinal

Resistance of weldements up to 50% stronger when not loaded longitudinally

Consider vector direction of forces on welds for design

Use eq. A-J2-1 of AISC 3rd ed.


Frame ActionFrame Action

Traditional approach assumes lateral loads resisted by diagonal braces

Large rotational restraint provided by gusset connection Frame providing bending resistance Braces loaded in bending Semi-rigid, forces at joint strongly dependent on

connection rigidity Welded connections approach fixed condition



Richards uses F- relationships to approximate M- PRCONN program uses results of nonlinear FEA to

develop M- relationships

Research needed to develop M-equations for braced frame connections


Detailing to Reduce Frame Action Detailing to Reduce Frame Action EffectsEffects


Research RecommendationsResearch Recommendations



Development of moment-rotation curves for semi-rigid strong and weak axis connection

Local response of connections must incorporate realistic rigidity of connection

Shears, axial forces and moments on local connection determined from global gusset research results

Local gusset plate connection research to determine load distribution through connections



Local gusset plate research to track peak stress values and locations at connections

This will help with determining and designing for individual connector design loads


SEAONC MINI SEMINARSEAONC MINI SEMINAR

Gusset Plate Design

Russell BerkowitzForell / Elsesser Engineers, Inc.

SEAONC MINI SEMINARWhat We Will CoverGusset Plate Design ReferencesGusset Plate Design ReferencesBrace / Gusset ConfigurationsWhitmore (1952)Whitmores SectionWhitmores SectionWhitmore (1952)Bjorhovde & Chakrabarti 1983-88Bjorhovde & Chakrabarti 1983-88Bjorhovde & Chakrabarti 1983-88Gross & Cheok (1988)Gross & Cheok (1988)Cheng et al.Richards et el. , Williams 1986Richards et el. , Williams 1986Richards et el. , Williams 1986Richards et el. , Williams 1986Richards et el. , Williams 1986Richards et el. , Williams 1986Richards et el. , Williams 1986Berkeley BRB Tests, 2002Berkeley BRB Tests, 2002Berkeley BRB Tests, 2002Berkeley BRB Tests, 2002Observed Seismic Performance of Gusset PlatesObserved Seismic PerformanceCurrent Gusset Design (SCBF)Current Gusset DesignOut-of-Plane Brace BucklingOut-of-Plane Brace BucklingOut-of-Plane Brace BucklingLimit States at Brace Gusset ConnectionLimit States at Brace Gusset ConnectionLimit States at Brace Gusset ConnectionTension Yielding and Net Section Fracture of Whitmores AreaBuckling of Gusset PlateBuckling of Gusset PlateBuckling of Gusset PlateBuckling of Gusset PlateBuckling of Gusset PlateBuckling of Gusset PlateBuckling of Gusset PlateGusset Plate Buckling Limit StateGusset Plate Edge BucklingGusset Plate Edge BucklingEdge StiffenersEdge StiffenersEdge StiffenersGusset Plate Interface ForcesInterface Connection ModelsGusset Plate Interface LoadsInterface Connection ModelsInterface Connection ModelsInterface Connection ModelsInterface Connection ModelsInterface Connection ModelsInterface Connection ModelsAISC Uniform Force MethodAISC UFM Special Case 1AISC UFM Special Case 2AISC UFM Special Case 3Design Criteria for Gusset Plates at Interface with Beam / ColumnGusset Connection to Beam / ColGusset Connection to Beam / ColGusset Connection to Beam / ColFrame ActionFrame ActionFrame ActionDetailing to Reduce Frame Action EffectsDetailing to Reduce Frame Action EffectsResearch RecommendationsResearch RecommendationsResearch RecommendationsSEAONC MINI SEMINAR

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Gusset Plate8 05