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Strengthening of riveted and bolted steel constructions under fatigue loading by
preloaded fasteners - experimental and theoretical investigations
Article · January 2000
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Strengthening of riveted and bolted steel constructions
under fatigue loading by preloaded fasteners experimental and theoretical investigations
by Günther Valtinat, Ingo Hadrych and Holger Huhn,
Technical University of Hamburg-Harburg/Germany
ABSTRACT
The paper contains results on the problem: How can the fatigue resistance of riveted or bolted
steel construction members under repeated loads be improved. The main idea is to develop a
preload rectangular to the surface of the members by means of a preloaded bolt to protect the
area around the hole. The results can be used for strengthening of old railway bridges and of
masts and towers under wind loads constructed from galvanized steel members with punched
holes.
1. INTRODUCTION
We have in Germany many old railway bridges sometimes more than 100 years old. A lot of them is still
in use and many years ago the problems came up how their today resistance against repeated loads is.
These old bridges are mainly riveted. From many tests in the thirties we know, that steel members with
holes or with riveted connections have a considerable drop down in fatigue behaviour under repeated
loading.
fatigue stress in the net section in N/mm²
2 bolt and 2 rivets ‡ 17 mm
4 hs-bolts ‡ 16 mm
1 bolt and 3 rivets ‡ 17 mm
4 rivets ‡ 17 mm
joint
net section 83 %
load cycles N in 106
Figure 1: Fatigue resistance of pure and mixed riveted and bolted connections
464
Figure 2:
Stress distribution in net sections
left:
net section with a rivet
center: net section in a high strength friction grip connection
right:
net section in a high strength friction grip connection after slip
In the fifties and sixties the high strength friction grip connections with preloaded bolts have been
developed and tests have shown that the fatigue behaviour of members with these connections is very
much better than that one with rivets. The increase in capacity which means in stress range 'V or load
cycles N was so immense that sometimes the fatigue behaviour of plain bars could be reached (figure 1).
The reason for that increase was assumed to be the high pressure under the washers of the bolts around
the hole. This high pressure gives a certain protection of the hole area, so that the stress distribution in the
net section became much more favourable than for example with fitted bolts without preload (figure 2).
From this knowledge we started an investigation what advantage the preload in a hot driven rivet can
have because after the rivet is driven it cools down, the material wants to shrink which is not possible and
thus produces a preload of a certain amount. When old bridges were taken down for renewing we asked
for test pieces with riveted connections to find out which preload such a rivet has. The procedure was to
plant a strain gage in a deep hole in the center of the rivet, to machine the rivet head off and to press the
rivet out of the hole. By this procedure the rivet under tension could freely shrink and from the
measurement of the contraction we could evaluate the preload in the rivet. Test pieces of that old material
with old drilled holes got installed high strength bolts which were tightened up to that preload which we
measured in the rivets. Hereafter we did fatigue tests. All test pieces had a certain artificial crack starting
from the hole wall into the net section (see figure 3). We wanted to know the speed of the crack
propagation in connections with non preloaded bolts and in connections with bolts with a preload up to
that one of the rivets. We have found that the number of cycles between the two increased up to 4 times to
7 times as much under a certain relatively low preload (see figure 4). From these results we know that
connections with hot driven rivets have a much higher fatigue resistance than equivalent test pieces with
normal bolts show. The life time of old riveted bridges can be prolonged by this idea up to 2 times or 3
times. That gave the railway authorities enough time for planning a new bridge or for a correction of the
railway line with new bridges [1].
465
initial crack tip
position of crack tip
Figure 3: Bar with a hole and an artificial crack tip
distance of the crack tip
from the hole edge in mm
bar with a hole, FV = 0 kN, 'V = 180 N/mm²
bar with a hole, FV = 0 kN, 'V = 153 N/mm²
bar with a hole, FV = 70 kN, 'V = 180 N/mm²
bar with a hole, FV = 45 kN, 'V = 153 N/mm²
load cycles N
Figure 4: Reduction of the crack propagation speed in the net section of a bar with a
hole due to the preload of the high strength bolt
466
2. TESTS, TEST PROCEDURE AND TEST RESULTS
In figure 5 the test specimen for a steel bar with a hole and an artificial initial crack tip in the direction of
the net section is shown. The high strength bolt with the 2 washers and nut were fixed up to certain
preload. The washers covered a certain area around the hole and brought a certain pressure due to preload
in. These test pieces were tested by fatigue loading and for us it was important to see the crack
propagation. The method to make that visible was to bring in stress blocks with high and with low stress
ranges. The stress blocks with the low stress ranges produce a marking line on the rupture surface which
is shown by the thin dark lines in figure 6. The bright areas represent the stress block with the high stress
range and all the same number of cycles. We can see that the stress propagation is nearly equal within a
certain length of the crack but it speeds up towards the crack comes through [2].
The literature gives a lot of information about the stress distribution in a net section of a bar with a hole or
with a crack. But we did not find a stress distribution in such a member which is influenced by a pressure
under the washer across the thickness of the member coming from the preload of a high strength bolt. We
did computer simulations for this special stress situation (see figure 7). In order not to do a finite element
calculation every time we looked in the literature for papers with a simpler and faster method to find the
stress distribution in a crack starting from a hole wall. We found the investigation by Grandt and Kullgren
[3] which has the following basis (see figure 8):
bolt preload
high strength bolt
hard washer
oscillating load
initial fatigue crack
hard washer
high strength nut
Figure 5: Plate with a hole and an initial fatigue crack
467
stress V
Figure 6: Crack length measurement
main
markingload block
load block
crack propagation
Figure 6: Measurement of the crack propagation
'V
'V
high strength
bolt M24
bar with
a hole
washer
crack tip
preload
crack tip
Figure 7: Grid for the finite element simulation
468
1.
An infinite plate with the hole with the diameter of 2R with external load has a crack with a crack
length of a (status A) started from the edge of the hole. This was assumed to be the reality.
2.
The stress distribution in an externally loaded infinite plate with a hole along a fictive crack line
could be calculated by finite element method and could be replaced by analytical functions. If now
a crack appears this stress must become 0. This zero stress can only be reached if in the status C the
stress distribution of the status B is imposed to both sides on the crack surfaces to make the stress
0. The curve of this stress distribution can be replaced by simple analytical formulae built as a
polynom. A reduction to a non infinitive plate is possible.
external load
external load
x
2R
status A
V(x/R)
p(x/R)
status B
status C
a
KA = K C with p(x/R) = V (x/R) status without crack
Figure 8: Model and stress distribution according to Grandt and Kullgren
The stress intensity factor 'K over the crack length a depends on the preload of a bolt which is placed in
this hole. We see in figure 9 that this factor 'K drops down with an increasing preload and that is
equivalent to an increase of number of cycles under fatigue loading.
There is also an influence of the friction P between the washers and the steel member (see figure 10). If
this friction coefficient is high a small part of the load moves out of the member into the washer and back
again behind the hole and results also in a decrease of the 'K-factor [4].
469
1600
1400
0 kN
'K [N/mm3/2]
1200
30 kN
1000
800
90 kN
600
400
experiment
FE-simulation
simplified model
200
0
0
2
4
6
8
10
12
14
16
18
20
crack length a from hole edge in mm
Figure 9: Stress range intensity factor 'K depending on crack length and preload
bar with a hole d=24 mm, P=0,4, 'Vgross= 110 N/mm², R = 0,1
1,0
P = 0,1
0,9
P = 0,2
'K / 'K0
0,8
P = 0,3
0,7
0,6
P = 0,4
P = 0,5
P = 0,6
0,5
0,4
0
20
40
60
80
100
120
140
160
180
200
bolt preload in kN
Figure 10:
Related stress intensity factor 'K / 'K0, dependent on bolt
preoload and friction P
crack length a=10 mm, d=24, 'Vgross= 110 N/mm², R=0,1
We all know the diagrams with the stress propagation rate da/dN over the stress intensity factor 'K. From
the above demonstrated evaluations whose result the 'K-factor is we can go into the da/dN-diagrams and
give answer how many load cycles a steel member in a riveted or bolted connection with a certain preload
can resist until rupture.
470
3. FATIGUE BEHAVIOUR OF HOT DIP GALVANIZED STEEL MEMBERS WITH
PUNCHED HOLES
The idea from above could be transmitted to the problem of the fatigue behaviour of hot dip galvanized
steel members with punched holes which are usually taken for mast and tower constructions. Such
constructions are loaded by wind which can arrive from different directions. From that and from the
vibration of the constructions we have oscillating stresses in the members. If the members have punched
holes as usual we know that around the hole there is a certain punch affected zone with an influence on
the ductility, on the yield strength and on the ultimate strength. If the member is hot dip galvanized
afterwards there may be an additional ageing effect on these material data. Punching may induce very
small crack initiations from which obviously an earlier start of a fatigue crack can be expected. The idea
to cover this area around the hole by a high strength preloaded bolt and hence induce a certain pressure
under the washer has been transferred to this type of connection. The figure 11 shows by microscopic
photos how much the material along the hole wall changes its flow pattern when the punch is driven
through the material. Additionally to this the distribution of the hardness in three different levels with
respect to the thickness of the plate (near the entrance of the punch, in the middle plain of the plate, near
the outcome of the punch) has been worked out and we found that the values near to the hole wall
increase by about 50 %.
Only a few diagrams hereafter may show which advantage the use of a preloaded bolt in such connection
has. Figure 12 show the results of a lot of test pieces with just a hole (no connection). The open circles
demonstrate the fatigue behaviour of a member with a hole but without any bolt. The dotted line with the
full squares shows the load cycles of equivalent members where a preloaded bolt is installed in the hole.
The increase in load cycles is about 700 % [5,6].
Figure 11:
Material flow around a punched hole (calibration line 0,1 mm)
a) Upper part when punch enters the plate (first zone)
b) Cut in middle of the member (second zone)
c) Lower part when punch leaves the plate (third zone)
471
1000,0
stress range 'V in N/mm²
m = 3,0 (EC 3)
m = 5,0 (EC 3)
100,0
L, s,
L, s,
L, s,
L, s,
Detail
Detail
f, 80/10, d=18mm, N = +0,1
f, 80/10, d=18mm, N = +0,1, preloaded by 50%
nf , 80/10, d=18mm, N = +0,1
nf , 80/10, d=18mm, N = +0,1, preloaded by 50%
Category 125, EC 3 with m = 3,0
Category 125, more realistic slope with m = 5,0
10,0
10.000
100.000
1.000.000
10.000.000
number of cycles N
Figure 12: Comparison between experimental S-N curves for hot dip galvanized and non-galvanized
members with punched hole without and with 50% preloaded bolts and the S-N curve of EC 3 (detail
category 125), (L = member with a hole, s = punched hole, f = hot dip galvanized, nf = black, N =
min V/max V, m = slope of the S-N-curve of EC3).
1000,0
stress range 'V in N/mm²
m = 3,0 (EC 3)
m = 5,0 (EC 3)
100,0
V, s, f, 80/10, d=18mm, N = 0,1
V, s, f, 80/10, d=18mm, N = +0,1, preloaded by 50%
V, s, nf , 80/10, d=18mm, N = +0,1
V, s, nf 80/10, d=18mm, N = +0,1, preloaded by 50%
Detail Category 112, EC 3 with m = 3,0
Detail Category 112, more realistic slope with m = 5,0
10,0
10.000
100.000
1.000.000
10.000.000
number of cycles N
Figure 13: Comparison between experimental S-N curves for hot dip galvanized and non-galvanized
shear-bearing connections with punched hole without and with 50% preloaded bolts and the S-N
curve of EC 3 (detail category 112), (V = connection, s = punched hole, f = hot dip galvanized, nf =
black, N = min V/max V, m = slope of the S-N-curve of EC3)
472
Figure 13 shows the equivalent results for connections. The full circles and the lowest line show the load
cycles of connections with non preloaded bolts, the full triangles and the dotted line represent the results
of equivalent connections with preloaded bolts tightened up to 50 % of the required preload. The increase
can much better be expressed by the stress range 'V than by the number of cycles. We found a step from
70 N/mm2 to about 160 N/mm2 at 1 million cycles. And this S-N-curve lies for high numbers of load
cycles above the S-N-curve of the detail category 112 of EC 3 with a slope of m = 3,0.
4. CONCLUSION
The idea to protect the net section area around a hole with a rivet and a punched hole by installing a
preloaded bolt with two washers has shown, that a remarkable influence on the fatigue life can be
achieved. As well old riveted bridges can be strengthened by this idea using the preload in the rivet as
other fatigue loaded constructions can be improved by installing high strength preloaded bolts.
I highly acknowledge that the institution AiF (Arbeitsgemeinschaft industrieller Forschungsvereinigungen
in Bonn, Germany), the minister of economy of the Federal Republic of Germany, Berlin, and the GAV
(Gemeinschaftsausschuss Verzinken e.V., Düsseldorf) financially supported this research (Research
project AiF no. 11097/N1 and AiF no. 12547/N1).
REFERENCES
[1] Valtinat, G.: Restnutzungsdauer bestehender Brückenbauwerke. In Berichte aus Entwicklung,
Forschung und Normung 18/1992 - Vorträge der Fachsitzung I zum Deutschen Stahlbautag (Berlin 1992),
DASt Deutscher Ausschuss für Stahlbau.
[2] Valtinat, G. and Hadrych, I.: Stahlbau-Schrauben - Internationale Forschung und Entwicklung. Paper
presented on the Deutscher Stahlbautag 1998 in Leipzig/Germany.
[3] Grandt Jr., A. F. and Kullgren, T. E.: Tabulated stress intensity factor solutions for flawed fastener
holes. Engineering Fracture Mechanics 18, 2 (1983), S. 435-451.
[4] Hadrych, I.: Wachstum von Ermüdungsrissen an Niet- und Schraubenlöchern unter Berücksichtigung
von Vorspannkräften der Verbindungsmittel. Diss. Technical University of Hamburg-Harburg, Hamburg
2000.
[5] Valtinat, G. and Huhn, H.: Betriebsfestigkeit von SL-Verbindungen mit zugbeanspruchten,
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(Rheine, 1999), Fachhochschule Münster.
[6] Valtinat, G. and Huhn, H.: Fatigue assessment of bearing type joints (shear-bearing-connections) of
hot dip galvanized steel construction members with punched holes. Edited proceedings of the Nineteenth
International Galvanizing Conference, Berlin 2000. Published by European General Galvanizers
Association, Caterham Surrey CR3 6RE/UK, and Institut Feuerverzinken GmbH, Düsseldorf/Germany,
ISSN 0261 6599.
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