SLIP CRITICAL JOINTS

Joints subject to fatigue load with reversal of loading directions

Joints that utilize oversize holes Joints that utilize slotted holes Joints in which slip at the faying

surfaces would be detrimental Joints where bearing connection is not

practical

Slip Critical JointRelies on friction between faying surfaces for load bearing.

AISC 2010 Specification for Structural Steel Buildings

Class A µ = 0.30 Uncoated clean mill

scale Roughened

galvanized surfaces Class B

μ = 0.50 Unpainted blast

cleaned surfaces Class B coatings on

blast cleaned steel

EARLY TEST RESULTSCurrent provisions for slip performance of galvanized specimens are based on limited data that is not reflective of current galvanizing processes.

Grondin, Gilbert Yves, Ming Jin, and Georg Josi. Slip Critical Bolted Connections: A Reliability Analysis for Design at the Ultimate Limit State. Department of Civil & Environmental Engineering, University of Alberta, 2007.

EARLY TEST – SURFACE ROUGHENING

Early tests done on galvanized plates showed roughening improved the slip performance.

Kulak, G. L., J. W. Fisher, and J. H. Struik. "Guide to design criteria for bolted and riveted joints, 1987.“ American Institute of Steel Construction, Chicago, Il.

CURRENT SLIP FACTOR RCSC has assigned slip factor of 0.35 to

roughened galvanized surfaces AISC has assigned slip factor of 0.30 to roughened

galvanized surfaces Roughening method mentioned by both is wire

brushing No further definition of wire brushing is available All early data was based on a limited number of

samples and no generally accepted test method

AGA STUDY Determine the galvanizer with the lowest “as

received” slip factor – the worst case Determine if wire brushing changes the “as

received” slip factor Use a Class B coating to increase the slip

coefficient of galvanized steel to Class B Paint Preparation needs to be practical and

produce a Class B slip factor Application of paint by galvanizer should be

feasible in galvanizing facility

“AS RECEIVED” TESTS Six Galvanizing kettles were

chosen based on differences in chemistry

Three tests were performed for each bath for repeatability

Bath which produced lowest slip coefficient to be used in further testing – worst case

“AS RECEIVED” SLIP FACTORS

Kettle Average Slip Coefficient A 0.31B 0.33C 0.35D 0.36E 0.58F 0.20

SURFACE ROUGHNESS Surface roughness measurements made in

accordance with ASTM D7127 Rt is the distance between the highest peak and

lowest valley within a given evaluation length Surface roughness produced by wire brushing

SURFACE ROUGHNESS

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 15 20 25 30 35

Slip

Coe

ffici

ent

Average Rt (microns)

Surface Roughness and Slip Coefficient

PAINT PREP METHOD

Prep Method Slip CoefficientControl 0.29

Wire Brushing 0.25Sandpaper 0.39

Surface Etchant 0.26Picklex 0.30

CLASS B PAINT OVER HDG Nine paints were chosen for the study Mostly zinc silicates Chosen because PDS claims of achieving Class B

slip coefficient “Sandpaper Roughening” from previous testing was

chosen One test was used for each paint

PAINT TEST RESULTSPaint Slip Coefficient

Control (no paint) 0.34A 0.45B 0.48C 0.48D 0.39E 0.48F 0.53G 0.23H 0.44I 0.55

PAINT QUALIFICATION TESTS

Paint Preparation - Chemical Four paints are chosen

Highest slip coefficient Most practical

Results disappointing None of the paints gave qualifying slip

factors for Category B

FHWA SLIP TEST STUDY Perform Round Robin Test on four labs

performing slip test FHWA Lab CCC&L Lab University of Texas KTA Tator Lab

Test results showed inconsistency in test results

Recommendation that two Linear Variable Differential Transformers (LVDTs) be used in slip tests

AISC SLIP TEST STUDY Project aims to increase the experimental

database of slip performance of modern galvanized pieces. Determine the slip strength of untreated

galvanized pieces Investigate the effect of different galvanizers,

steel chemistry and other variables on the slip behavior of galvanized plates

Evaluate the effectiveness of roughening galvanized surfaces, and, if needed, recommend a more precise procedure for roughening

VARIABLES INVESTIGATED1) Coating Thickness2) Steel Chemistry (2 different steels)3) Pickling Acid4) Variation Among Galvanizers5) Bath Consistency6) Spinning of Galvanized Plates7) Surface Roughening

PREPARATION OF PLATES Holes drilled by Fabricator Plates cut by University

GALVANIZING OF PLATES

COATING TOUCH-UP

TEST SETUP AND PROCDURE Tests conducted in accordance with RCSC - App. A

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Schematic of Test SetupRCSC Figure A-3 (Recreated) for Compression Slip Test Setup

INITIAL RESULTS SLIP TEST Compared with previous slip studies, modern

galvanizing produces coatings with much higher slip coefficients.

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Tabulated slip coefficients represent a five-test average

SLIP VERSUS COATING THICKNESS

In general, coating thickness did not have a significant and consistent impact on the measured slip coefficient.

25

SLIP VERSUS CENTRIFUGING Despite the apparent change in coating structure, the

effect of spinning on slip coefficient was minimal.

29

SUMMARY OF AISC TESTS Bath chemistry and galvanizing process can vary

between galvanizers, produce significantly different slip coefficients, however all galvanizers produced coefficients higher than historically assumed.

Changes in bath chemistry over time had little effect on the slip coefficient of galvanized pieces.

Spinning of galvanized plates had limited impact on slip performance.

Surface roughening did not improve slip performance. The roughening procedures often reduced the

measured slip resistance and should probably therefore be removed.

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FUTURE TEST PLANS Qualification Tests with candidate paint systems –

Class B qualification Metallized Coatings in contact with Galvanized

Coatings Creep Tests on Galvanized and Metallized

Coatings Creep Tests on qualified paint systems over

galvanized coatings