Assessment of Coatings for Friction-Type Connections
Mohamed Anzar1, Howard Morris1, and Thomas Smith1
1
Roads and Traffic Authority of NSW
Abstract Friction-type connections are widely used to connect steel members
where slipping is not permitted under serviceability loads. The friction developed
between the faying surfaces transfers the design shear forces at the serviceability
limit states. The bolts act in tension and are not subject to shear.As a rule, faying
surfaces are coated with inorganic zinc silicate coatings and a slip factor for these
coatings is specified in AS 5100.6. For other applied coatings and surface
conditions, AS 5100.6 specifies testing in accordance with AS 4100 Appendix
J.Epoxy zinc rich primers are generally not used for friction-type connections.
This testing program was prompted when an epoxy zinc rich primer was proposed
for friction-type connection during the course of a contract. Epoxy zinc rich
primers have some operational advantages over zinc silicate primers and a testing
program to assess the suitability of a range of zinc rich coatings for faying
surfaces was initiated. Nine different zinc rich coatings and bare metal samples
were tested in accordance with AS 4100 Appendix J and the results are presented
in this paper. The effects of variations in bolt quality and the measures taken to
modify the testing parameters to suit the bolts used for the test samples are
discussed. The effects of coating dry film thickness (DFT) on the friction
coefficient are also discussed.
Introduction
Faying surfaces of bolted connections must be either painted or galvanized to
prevent crevice corrosion developing between surfaces in contact. For faying
surfaces of painted members in friction-type bolted connections, current RTA QA
Specification B220 specifies one coat of a zinc rich primer having dry film
thickness of 75 microns and stipulates that the zinc paint coating applied to faying
surfaces of friction-type connections designed to AS 4100 must have a minimum
coefficient of friction of 0.35 measured according to AS 4100 Appendix J.
V. Ponnampalam, H. Madrio and E. Ancich
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Mohamed Anzar, Howard Morris and Thomas Smith
Inorganic zinc silicate primers have been used for steelwork and their use at
friction-type connections has posed no real issue as AS 5100.6 specifies that the
slip factor (µ) may also be taken as 0.35 for abrasive blast cleaned steel surfaces
coated with zinc silicate coatings. If any other applied finish or surface condition,
including a machined surface is desired, the slip factor needs to be evaluated based
upon adequate test evidence. According to AS 5100.6, tests performed in
accordance with the procedure specified in AS 4100 Appendix J are deemed to
provide satisfactory test evidence.
However, there has been an increasing trend over recent years for epoxy zinc rich
(also known as organic zinc) primers to be used for the priming of new steelwork
in lieu of inorganic zinc silicate primers. This is primarily the result of the
generally superior curing and over-coating times associated with epoxy coatings
compared to inorganic zinc silicate coatings particularly when ambient conditions
are cool and dry. The greater ease of mixing and applying epoxy zinc primers is
also understood to be a key reason for preferring epoxy zinc primers over
inorganic zinc silicates, particularly for on-site works.
For structures with friction-type connections, one option is to use an inorganic
zinc silicate primer for all of the steelwork. Alternatively, an epoxy zinc primer
can be used for all steelwork except at the faying surfaces of connections where an
inorganic zinc silicate is used. If epoxy zinc rich primers can be satisfactorily used
for the faying surfaces of friction-type connections, it will permit the use of one
primer only throughout the structure making the painting work simpler and more
efficient than having to use two primers. However, the slip-factors of the epoxy
coatings need to be evaluated before using them for friction-type connections.
At present, certain epoxy zinc coatings have been proposed for friction-type
connections for RTA bridgeworks, on the basis of testing commissioned by
coating manufacturers. Initially, it was thought that the epoxy zinc primers that are
certified suitable for friction-type connections achieve this by virtue of high zinc
content. However, there have been a number of instances over the years of coating
failures on RTA bridges, involving delamination of topcoats as a result of
cohesive failures through a zinc primer coating, where a zinc primer with high
zinc content has been used for the repair of an inorganic zinc silicate primer
coating or a thermal applied zinc coating. It is believed that the risk of such
cohesive failures can be reduced by using zinc primers with a lower zinc content
where to have a higher percentage of binder in the film.
The friction characteristics of epoxy zinc primers with slightly lower zinc contents
are of interest even though the lower zinc content product appeared less likely to
satisfy the current slip-factor requirements of slip factor 0.35.
Assessment of Coatings for Friction-Type Connections
257
Test Program
As part of an RTA internal evaluation program, a testing program was initiated
and three different products from each of three manufacturers were included. One
inorganic zinc silicate product and two epoxy zinc rich coatings with low and high
zinc contents were selected from each manufacturer for testing. RTA provided the
steel coupons and manufacturers applied the coatings on the samples for testing
under their supervision. The names of the manufacturers and products are
considered commercial-in-confidence and not disclosed here. Five samples of
each coating were tested. The total number of samples tested in this program was
50 comprising 5 samples each of 9 different coatings and 5 bare metal samples.
Test Method
Testing was conducted by the RTA Materials Technology Section, Auburn, to
AS 4100 Appendix J: Standard Test for evaluation of slip factor.
The test consists of bolting together a set of four painted plates as shown in
Fig.1and Fig.2 then loading this setup in tension to induce a small amount of slip.
22 dia
12
22 dia
27 dia
25
23 dia
27 dia
23 dia
12
200
250
60
120
60
250
8
74
80
40
56
56
40
80
74
Fig. 1. Dimension of samples used
The bolts compressing the plates were torqued to a range as described by the
method. Four dial gauges as in Fig.3 were placed on the setup to indicate the plate
movement (slip). The output of these gauges was recorded via a computer
program. The setup was placed in a Universal Testing Machine and tension was
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Mohamed Anzar, Howard Morris and Thomas Smith
applied to induce slippage. The point at which the plates slipped is known as the
slip load.
Fig. 2. Assembled sample for testing
Fig. 3. Sample mounted for testing
Testing Details
• Samples were blasted and painted by the respective manufacturers, prior to
arriving at the laboratory with the coating type marked.
• The only surface preparation carried out by the Auburn laboratory was on the
bare steel samples which were washed with a liquid alkaline solution (Gamlen
CA1) to remove traces of oils and greases that may have been present. Flaky
mill-scale was not removed.
• Samples were tightened by the laboratory by a pneumatic Norbar high–torque
tool.
• A specially made jig shown in Fig.4 was used to hold and align the samples
while being tightened.
• A Boltstress G5 Ultrasonic Direct Tension Monitor shown in Fig.5 was used to
measure the tension induced in the bolt while it was tightened. A probe is
placed on the end of the bolt and ultrasonic measurements are taken of the bolt
elongation. This is then instantaneously converted to a tensile load via linear
stress-strain calculations.
Assessment of Coatings for Friction-Type Connections
259
• The Monitor is calibrated using a NATA certified jack and gauge combination
with a sample bolt.
Fig. 4. Jig used while tightening
Fig. 5. Boltstress tension monitor
• The M20 bolts used in the assembly required end preparation. The probe end
(bolt head) required a suitably flat surface to allow transmittance of the
vibrations. A high speed rotary tool was used to grind the surface.
• The induced tension was measured by the monitor.
• The induced tension in the supplied bolts is lower than the minimum proof load
specified in AS5100.6. The AS 4100 Appendix J method states that the induced
tension should be at least 80% but no more than 100% of the specified proof
load. It was found with these particular bolts, non-recoverable elongation was
occurring before the minimum proof load specified in AS 5100.6 for M20 bolts
was achieved. The bolt tension induced for the series of tests was reduced to
120kN to be within the elastic limit while being more than 80% of the specified
proof load.
• Tensile load was applied to the bolted sample stepwise at a rate of 20kN/min.
The sample was tensioned at this rate for 30 seconds, then held at the load
achieved for a further 30 seconds to allow for any creep to cease (i.e.
effectively loading the sample 10kN in total per minute). This cycle was
repeated until slip of plates occurred.
Test Results
Dry Film Thickness (DFT) readings of the coated samples were taken at twenty
four locations comprising 4 readings on each face in contact before assembly
around the bolt holes of contact surfaces. The mean value of these 24 readings
was taken as the representative DFT reading of the sample and the slip factor was
compared to it.
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Mohamed Anzar, Howard Morris and Thomas Smith
The failure mode refers to the way (speed and noise) in which the samples failed,
i.e. the slipping of the painted plate surfaces. Sample graphs of a slow slip and fast
slip are provided in Fig.7 and Fig.8, respectively. Samples that underwent a slow
failure, required the slip load to be defined at a corresponding slip of 0.13mm
(refer to AS 4100 Appendix J). For those that failed suddenly, the slip load was
easily
distinguishable.
Fig. 7. Loading graph for a slow slip test
Fig. 8. Loading graph for a sudden slip test
Test results for bare metal samples are given in Table.1. The coated samples were
also tested and results were calculated. The results obtained for samples of bare
metal and 9 coatings are summarized in Table.2.
Assessment of Coatings for Friction-Type Connections
261
Table I. Test results for bare steel samples
Sample
Bolt Tension (T)
Measured Slip Load (P)
kN
kN
Slip Factor
(=(P/2)/T)
BS-1a
120.8
84.1
0.35
BS-1b
122.0
84.1
0.34
BS-2a
124.3
72.8
0.29
BS-2b
122.8
72.8
0.30
BS-3a
119.9
63.2
0.26
BS-3b
121.9
63.2
0.26
BS-4a
120.6
112.4
0.47
BS-4b
121.7
112.4
0.46
BS-5a
123.2
59.7
0.24
BS-5b
121.3
59.7
0.25
Average slip factor
0.32
Minimum slip factor
0.24
Calculated slip factor as per AS 4100 Appendix J for five samples
0.17
The slip factor to be used in design as per AS 4100 Appendix J
0.24
(Higher of above two values)
Table II. Summary test results for bare steel and coated samples
Type
BS
Slip Factor
Nature of slip
Maximum
Minimum
Average
Calculated
Design
0.47
0.24
0.32
0.17
0.24
Loud, sudden
Inorganic zinc silicate
A1
0.59
0.37
0.53
0.36
0.37
Loud, sudden
B1
0.50
0.38
0.44
0.36
0.38
Loud, sudden
C1
0.56
0.48
0.52
0.42
0.48
Loud, sudden
Epoxy zinc rich coatings (high zinc content)
A2
0.41
0.26
0.34
0.21
0.26
Loud, sudden
B2
0.40
0.28
0.35
0.25
0.28
No audible, slow
C2
0.36
0.26
0.31
0.23
0.26
No audible, slow
Epoxy zinc rich coatings (low zinc content)
A3
0.58
0.31
0.45
0.30
0.31
No audible, slow
B3
0.23
0.11
0.17
0.10
0.11
Not loud, sudden
C3*
0.20
0.14
0.16
0.11
0.14
No audible, slow
*The sample C3 originally had 5 samples but one of the test results could not be captured.
Therefore, the calculated result for this coating type was based on 4 test samples and the
statistical factor for 3 samples was used to calculate the design value for this coating shown in
Table.II.
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Mohamed Anzar, Howard Morris and Thomas Smith
160
DFT Reading
140
DFT(microns)
120
100
80
60
Inorganic zinc
40
Bare
Steel
20
Epoxy low Zn
Epoxy high Zn
0
0
5
10
15
20
25
30
35
40
45
50
Sample Number
Fig. 9. DFT Readings of Coated Samples
Fig. 9 shows the mean DFT readings from 24 readings for each sample included in
the testing program. Samples 1 to five are bare steel samples with no DFT
readings involved. Each set of five samples between two vertical lines represent
different coatings. Samples 6 to 20 are coated with inorganic zinc silicate coatings
and 21 to 50 are coated with epoxy zinc rich coatings. Coatings 21 to 35 have high
zinc contents and coatings 36 to 50 have low zinc contents.
0.7
Slip Factor
0.6
Slip Factor
0.5
0.4
0.3
Epoxy low Zn
0.2
Inorganic zinc
Bare
Steel
0.1
Epoxy high Zn
0.0
0
5
10
15
20
25
30
35
40
45
50
Sample Number
Fig. 10. Slip Factors of Coated Samples
Fig.10 shows the slip factors obtained based on slip load for each sample. The test
samples have two bolted connections and two very close slip factors for each
sample. The average of these two values is shown against each sample number in
Fig.10. The sample numbers in Fig.9 and Fig.10 correspond to identical samples.
Assessment of Coatings for Friction-Type Connections
263
The comparison of slip factors obtained for Samples 7, 17 and 37 and their DFT
readings in relation to other samples in their respective coating types between
vertical lines indicates that the slip factors do not appear to be influenced by the
variation in average DFT readings for the range of DFT thicknesses involved.
The design slip factor of 0.24 obtained for bare metal surfaces is less than the
value of 0.35 allowed by AS 5100.6. The slip factors for three out of five samples
were below 0.3 and were highly variable. The low values obtained cannot be
attributed to surface preparation which did not include blast cleaning as a previous
similar RTA testing program included three bare metal samples of blast cleaned
surfaces also showed lower than the AS 5100.6 design slip factor of 0.35. This
indicates that caution should be exercised before using 0.35 for bare metal
surfaces in design.
Slip factors obtained for three inorganic zinc silicate coatings represented by
samples 6 to 20 in Fig.10 satisfy the slip factor 0.35 allowed in AS 5100.6.
The slip factors for epoxy zinc rich samples, numbers 21 to 50, are highly
variable. For manufacturers B and C, higher zinc content values gave greater slip
factors than lower zinc content values. For manufacturer A, the opposite is the
case. From these results, no general rule can be drawn regarding slip factor versus
zinc content.
Comparison of Australian and RCSC Approaches
This project to assess the slip factors of coatings was initiated after a number of
paint manufacturers produced test certificates for various epoxy zinc rich coatings
indicating slip factors of the order of 0.50 when tested according to Specification
for Structural Joints Using ASTM A325 or A490 Bolt[3]. This specification is
issued by the United States based, Research Council for Structural Connections
(RCSC). The slip factor values reported are high compared to the values
prescribed in AS 5100.6. The key difference is that the RCSC approach utilizes
mean values of slip factor whereas the AS 5100.6 approach uses minimum values
with a statistical adjustment.
AS 5100.6 specifies a factor kh for different type holes taking 1.0 for standard
holes, 0.85 for short slotted and oversize holes and 0.70 for long slotted holes. The
RCSC specification also specifies similar factors in its equations deviating slightly
only for long slotted holes.
A case of a single bolt connection with single sliding surface and a standard hole
is used for comparison of the approaches and discussion below.
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Mohamed Anzar, Howard Morris and Thomas Smith
RCSC Approach
Specification for Structural Joints Using High-Strength Bolts[4] or its predecessor
Specification for Structural Joints Using ASTM A325 or A490 Bolts[3] is used for
testing slip coefficients. This specification defines faying surfaces into three
different classes, Class A, B or C and specifies applicable different mean slip
coefficients as follows.
0.33 for Class A faying surfaces (uncoated clean mill-scale steel surfaces or
surfaces with Class A coatings on blast-cleaned steel);
0.50 for Class B surfaces (uncoated blast-cleaned steel surfaces or surfaces
with Class B coatings on blast-cleaned steel); and
0.35 for Class C surfaces (roughened hot-dip galvanized surfaces).
The RCSC specification uses mean slip coefficient obtained from five individual
tests and specifies equations based on mean coefficient. The tests conducted
according to the specification use samples with a single bolt connection.
The RCSC specification specifies equations for design slip resistance at the
factored-load level and at the service load level. According to the commentary,
these equations are calibrated to produce essentially the same results.
The factored load equation to calculate the slip resistance for a single bolt
connection with single sliding interface subjected to shear only becomes as below:
Rn = μDuTm
where Tm is the specified minimum bolt pretension and Du is a multiplier used to
convert the specified minimum tension to the mean tension in the bolt with a
specified default value of 1.13.
The equation for the same situation at the service-load level is as follows:
Rn = μDTm
where D is a slip probability factor with the default value of 0.80 that reflects the
distribution of actual slip coefficient values about the mean installed pretension to
the specified minimum bolt pretension Tm and slip probability level.
The RCSC uses the average slip factor obtained from 5 tests and effectively
increases the specified minimum tension by 1.13 to derive the average pretension
in the bolts for factored load level designs. The slip resistance ratio obtained
between factored load case and service load case given by the ratio of the above
two equations becomes Du/D = 1.13/0.80 = 1.41, using the default values specified
in the specification. The calibration of equations may have aimed for this ratio.
Assessment of Coatings for Friction-Type Connections
265
Australian approach
The design shear capacity for a single bolt connection with single sliding interface
subjected to shear only becomes:
φVsf = φμNti
Nti is equal to Tm in RCSC. However, μ is evaluated according to AS 4100
Appendix J and is a minimum value and not a mean value as discussed above. The
design shear capacity φVsf for bolt serviceability limit state is thus obtained by
multiplying nominal shear capacity Vsf of a bolt by the capacity factor φ = 0.70 in
AS 5100.6.
The sample as per AS 4100 Appendix J has 2 bolted connections in each sample
giving two slip factors from each test. AS 4100 calculates the minimum possible
slip factor with 90% confidence level based on the test results obtained and uses a
factor of 0.90 or 0.85 depending on the number of samples to include effects of
small sample size used in the tests for evaluating the slip factor. The factor
calculated in this way is then used with the minimum tension specified as above.
AS 5100.6 specifies slip factor µ = 0.35 for clean steel surfaces and abrasive blast
cleaned steel surfaces coated with zinc silicate (inorganic) coatings.
Discussion of RCSC and Australian approaches
A comparison of the two approaches can be illustrated by considering the
serviceability resistances determined according to these methods for the following
example.
For a friction-type connection comprising uncoated blast-cleaned steel surfaces,
RCSC specifies a slip factor, μRCSC of 0.50 whilst AS 5100.6 specifies a slip
factor, μAS of 0.35.
AS 5100.6 specifies a capacity reduction factor φ = 0.70. RCSC does not specify a
corresponding reduction.
The RCSC slip resistance for service-load level, Rn=μ RCSC DTm.
The AS 5100.6 serviceability limit state design shear capacity, φVsf = φμ AS Nti.
Noting that D = 0.80 from above and that Tm and Nti are the minimum specified
bolt tensions and are equal, the ratio of the RCSC slip resistance to the AS 5100.6
design shear capacity equals,
μ RCSC D / φμ AS = 0.50*0.80/(0.70*0.35) =1.63.
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Mohamed Anzar, Howard Morris and Thomas Smith
Thus, the RCSC’s value is about 63% more than the AS 5100.6 design shear
capacity. Conversely, the AS 5100.6 design shear capacity is approximately 61%
of the RCSC slip resistance.
If a φ value of 1.0 were used instead of 0.7 in AS 5100.6 to calculate the design
shear capacity, then the above values of 63% and 61% become 14% and 88%
respectively. Thus even without a capacity reduction factor, AS 5100.6 gives
conservative values compared to the RCSC approach.
Conclusion
Nine different zinc rich coatings from three manufacturers and bare metal samples
were tested in accordance with AS 4100 Appendix J and the results are presented.
A slip factor of 0.35 or more was obtained for inorganic zinc silicate surfaces and
this validates the values specified in AS 5100.6.
No firm conclusion can be reached on the effects of high zinc content on slip
factors in view of the limited number of samples tested. However, the slip factors
for high zinc content epoxy coatings were found to be less variable than those for
low zinc content epoxy coatings.
The design factor of 0.24 was obtained for bare metal samples instead of 0.35
specified in AS 5100.6. The values from the testing are appeared to be closer to
the values expected for samples made with uncoated clean mill-scale steel
surfaces. Further studies are required to ascertain the reason for lower value than
specified including the effect of surface profile in the performance of blast clean
surfaces.
The slip factors do not appear to be influenced by the variation in average DFT
readings at least for the range of DFT thicknesses expected in practice.
It appears that AS 5100.6 can consider allowing slightly increased capacity for
friction-type bolted connections. The capacity reduction factor used in AS 5100.6
for calculating the design shear capacity may possibly be increased if the slip
factor is evaluated in accordance with AS 4100 Appendix J.
The friction coefficient tests conducted by other organizations may report mean
friction coefficients instead of the statistically adjusted values reported from tests
conducted according to AS 4100 Appendix J. These results should not be treated
equivalent.
Assessment of Coatings for Friction-Type Connections
267
Disclaimer
The opinions expressed in this paper are entirely those of the authors, and do not
necessarily represent the policy of the Roads and Traffic Authority of NSW
(RTA).
References
[1] AS 4100 – 1998: Steel Structures
[2] AS 5100.6 – 2004: Bridge design, Part 6: Steel and composite construction
[3] Specification for Structural Joints Using ASTM A325 or A490 Bolts9, Prepared by RCSC
Committee A.1—Specifications and approved by the Research Council on Structural
Connections, June 30, 2004
[4] Specification for Structural Joints Using High-Strength Bolts, Prepared by RCSC
Committee A.1—Specifications and approved by the Research Council on Structural
Connections, December 31, 2009
[5] RTA QA Specification B220, Protective Treatment of Bridge Steel Work, Edition 3
Revision 1, October 2008