injection bolts by prof dr tihomir nikolovski

FAKOM AD - Skopje TI 01-1/15

LEADING COMPANY FOR PRODUCTION AND ERECTIONO F STEEL STRUCTURES IN THE SO UTH-EAST EURO PE

TECHNICAL INFORMATION 01 Amended version 2: April 2009

INJECTION BOLTS Prof. d-r Tihomir Nikolovski, Development adviser

1. Introduction According to definition given in EUROCODE 3, Part 1-8 (EN 1993-1-8:2005) Design of

steel structures, Part 1-8: Design of connections [1], and European pre-standard prEN 1090-2: 2006 Execution of steel structures and aluminium structures, Part 2: Technical requirements for steel structures [2], injection bolts are bolts in which the cavity between the bolt and the wall of the hole is completely filled up with a two component epoxy resin (Figure 1.). Filling (injection) of the clearance is carried out through a small hole in the head of the bolt. After injection and complete curing of the resin, the connection is slip resistant. The injection bolts may be applied as non-preloaded and preloaded ones. For the connections category A (non-pre-loaded bolts) according to EN 1090-2:2006 the transfer of the force is carried out through bearing pressure (on the resin), while for the connections category B and C the load bearing capacity is the sum of the slip resistance and bearing pressure on the resin.

injection hole

Air escape groovein the washer

chamferedwasher

resin

chamferedwasher

Figure 1. Injection bolt in a double lap joint

2. Field of application The first application of injection bolts dates from 1970, in The Netherlands. In time,

the use of injection bolts became standard practice for the repair of old riveted bridge struc-tures, especially old railway bridges, but also for new bridges and other types of dynamically loaded structures (cranes, heavy loaded crane girders, civil engineering machines, earth excavators, mine equipment etc, as well as vertical steel structures – radio and TV antennas, guyed masts and towers where horizontal displacements due to slip in connections are limited) as a significantly more economical alternative of fitted bolts.

Great step forward has been made with the experimental investigations carried out in the Stevin Laboratory of the Delft University of Technology, through which the initial (prac-tically more than one decade lasting) skepticism of the designers and investors in relation to their reliability has been eliminated (most likely regarding the bearing resistance of epoxy resin), and the results of these investigations were subsequently introduced in the ECCS Recom-

Technical information 01: Injection bolts


mendations No 79 [3] (ECCS = European Convention for Constructional Steelwork) issued 1994 and European standards. In addition, it should be pointed out that from the fatigue tests it is obvious that the fatigue resistance of connections with injection bolts is close to the resistance of riveted connections and better than the resistance of connections with non-preloaded bolts and even of connections with fitted bolts.

One of the first applications of injection bolts in Germany is in 1996 (old riveted bridge at Oranienburg near Berlin) [4].

3. Advantages Repair and strengthening of existing structures

• Application in connections with a low slip factor. Such are the riveted connections (due to completely filled in hole hammering) and fitted bolt connections (permitted slip 0,3 mm). The replacement of rivets will become more and more virtually impossible because of the lack of equipment and skilled labour. In addition, there is a danger of damaging the holes during the cutting of the heads and removing the old rivets. The alternative solution with fitted bolts may be, due to reaming, expensive and inappro-priate compared to injection bolts which may be selected with higher class (for example 8.8 as a standard in most European countries) and placed in holes 3 mm (2 mm) bigger than their nominal diameter.

• Good resistance in bearing. With a reasonable ratio between the thickness of the plates and the diameter of the hole [1], the design resistance in bearing is usually sufficient to replace faulty rivets.

• No internal corrosion. Since the resin completely fills the cavity, internal corrosion is avoided (which is also important for new structures).

Application in new structures • No slip in case of overloading. In connections with high strength friction grip bolts, slip

due to overloading is possible. It is usual practice, in connections subjected to bend-ing at least the central part of bolts to be carried out as fitted bolts. In connections with injection bolts, such sudden slip is impossible.

• Good resistance in bearing. Assuming a reasonable ratio between the thickness of the plates and the diameter of the hole [1], the design resistance in bearing is of the same magnitude as the slip resistance of high strength friction (preloaded) bolts.

• No special requirements for the contact surfaces and for the control of tightening. For high strength friction bolts special requirements are prescribed for the contact surfaces to achieve the necessary slip factor. If corrosion protection is required, the paint to be used has to accomplish the necessary slip factor. For non-preloaded injection bolts no any special preparation is necessary to avoid the slip. Consequently, the possibility to avoid necessary calibration of the tightening tools and/or tightening procedures is an advantage in application of non-preloaded injection bolts.

Costs The costs of injection bolts consist of:

• The purchase of the bolts, nuts and washers themselves; • The costs for preparation of bolts and washers (drilling a hole in the bolt heads and

preparation of the special washers, Figure 2., 3. and 4); • The costs of the resin and the injection.

Because the equipment for the injection is cheap and the amount of resin per bolt is small, the material costs for the injection are low. The labour costs for injection per bolt depend on:

• The total number of bolts to be injected; • The number of bolts per connection;



• The accessibility of bolts; • The size (diameter) and the length of the bolts; • Possible delay since the holes have to be dry during injection (depends on weather con-

ditions). Roughly, the necessary time for injection of a bolt is 1 -2 minutes. When injection bolts

are applied, the total number of bolts in the connection will be less(1), thereby the costs for drilling holes and purchase and placement of bolts will be reduced. In addition, the bigger clearance between the bolt shank and the hole will make the placement easier and con-sequently reduce the labour costs.

According to Dutch sources [5], the replacement of a faulty rivet with injection bolt during repairing of old riveted structures (dismantling, eventually grinding of the surface, placement, tightening and injection) takes about 25 minutes with two workers.

After curing of the resin, the injection bolts can not be untightened or removed, which is additional advantage for permanent connections. In a case when dismantling of a connection would be necessary, special measures should be made in advance, e.g. use of special liquid to prevent bonding of the resin to the bolt and the walls of the hole. According to [3] possible solution is to heat the bolt (e.g. with a blow lamp).

4. Production of injection bolts According to some information, the injection bolts are factory produced in The Nether-

lands only (FABORY)(2), but of Grade 10.9 only and of diameter M20 and larger. Wider practice is the injection bolts to be produced (adapted) by the contractors themselves using normal standard bolts for structures, with geometrical dimensions and quality class according to EN.

nozzle of theinjection device

k

k/2k/2

1,5

Ф5,5 mmФ ,3 2 mm

Figure 2. Preparation of the head of the bolt

The top part of the hole in the head of the bolt (Ф5,5 mm) is sufficient to enter the nozzle of the injection device, while the bottom part enables the injection of the resin. According to the detail (Figure 2.) the internal edge of the hole in the bottom part is in contact with the bolt shank.

d1 d1 d1

d +31 d +31

.25 s

.25 s600

s

(a) Drilled (b) Chamfered Figure 3. Preparation of the washer for use under the bolt head

Under the bolt head a special washer should be installed, with the machined rebate towards the bolt head (Figure 3.). This additional space facilitates the flow of the resin around (1) See the comparative calculation in Annex A (2) According to FABORY web-site, the company has its representative office in Sofia, Bulgaria.



the bolt which completely seals the cavity between the bolt shank and the walls of the hole. The inner diameter of this washer should be at least 0,5 mm larger than the diameter of the bolt.

groove Figure 4. Preparation of the washer for use under the nut

Under the nut a special washer provided with an air escape groove should be installed (Figure 4.). The washer should be positioned with the groove towards the nut. This position prevents the groove to be filled in with paint.

5. Two component epoxy resin for injection Although the epoxy resins possess high strength characteristics, they are susceptible

to creep (displacement during the time at constant loading)(3). Therefore, the bearing pressure should be kept within certain limits. According to [3], design stress of 130 N/mm2 is recom-mended. For the renewal of Oranienburg bridge [4], following the testing performed in Stevin Laboratory in Delft, a design bearing stress of 150 N/mm2 has been adopted on the basis of the criterion that at this level of bearing stresses the maximum slip should not exceed 300 μm (0,3 mm) (4) as a maximum during the referent life period of structure (e.g. 50 years), that is the same limit value as for fitted bolts. The tests were carried out on joints assembled with М24 bolts and clearances of 0+, 1 and 2 mm as well as with slotted holes, and before injection the test samples were tightened with the maximum clearance (i.e. maximum thickness of the resin) in the direction of bearing stresses (Figure 5. and 6.). The injection was carried out using two com-ponent epoxy resin Araldite (SW404 and non-toxic hardener HY2404) produced by Huntsman (formerly Ciba-Geigy).

Figure 5. Injection bolt in double lap joints prepared for testing [4]

More detailed data on how to determine the design bearing stress of the injection resin are given in the new prEN 1090-2:2007 [2] Execution of steel structures and aluminium (3) Creep tests at constant loading may take several years. Except for referent (laboratory) temperature of 200С,

the ЕССЅ Recommendations No 79 [3], prescribe creep tests at constant temperature of 700С, as a maximum (summer) temperature for exposed structures in open space.

(4) According to test results [4], it could be concluded that the recommended values are underestimated. Namely, maximum measured creep displacements at the end of tests lasting several years amount to about 200 μm, while to about 250μm at the temperature of 700С, with a declaration that the recommended values may be increased to 175N/mm2 and even to 200 N/mm2. The new ЕN1090-2:2007 does not define these values and foresees testing of the resin (producer).



structures, Part 2: Technical requirement for steel structures. The test samples are to be manu-factured with the same dimensions as for tests to determine the slip factor in connections with preloaded bolts.

Figure 6. Laboratory testing of joints [4]

Figure 7. Injection of bolts using hand driven device (gun) [4]

The injection of the bolts should be carried out in accordance with the recommenda-tions by the manufacturer, while the resin should have particular characteristics.

• After mixing of two components, the mixture should have such a viscosity at ambient temperature which allows the narrow space be injected with ease. The flow out of the mixture should stop after ceasing of injection pressure. Modelling clay may be used to close the hole in the head and the groove in the washer immediately after injection.

• The potlife of the resin (time of using the mixture) at ambient temperature should be at least 15 minutes. If there are no data available, procedure tests should be carried out to determine the relation between ambient temperature and curing time.

• The temperature of the resin should be between 15 и 25оС. In cold weather the resin and if necessary the steel components may also be preheated. If the temperature is too high and the mixture flows out modelling clay may be used.

• The resin has to be cured before any loading of the structure (24 hours). If it is neces-sary in some cases to reduce the curing time (for instance to about 5 hours during renewal of structures in service), the connections may be heated to a maximum of 50оС after the potlife has passed.



• The connections should be free from water at the time of injection. It may be consid-ered that one day of dry weather is normally required before starting the injection procedure.

6. Installation of injection bolts Before injection, the bolts should be tightened and the connection in its final position.

Two workers carried out the injection: one for injection and the other for checking on the appearance of the resin from the groove of the washer at the nut side. Including the pre-paration of the mixture, the injection of a group of 20 bolts takes about 30 minutes. For injection of short bolts hand driven device (gun) is appropriate (Figure 7.) The whole proce-dure is exclusively simple and easy to perform.

7. Conclusion Our interest for application of this kind of bolts can be evaluated through following two

aspects: • To adopt a relatively new, reliable and both technologically simple and cheap procedure

for fabrication of connections for construction of new and repairing of old statically and dynamically loaded steel structures;

• The most of the old steel structures in Macedonia (practically all steel structures in Skopje Steel Mill and all railway bridges constructed by seventies of the past century) are riveted ones. Taking into consideration their significant age (many of them con-structed even about 1920), the influences they suffered during their use and practically absence of any maintenance and protection during the last 20 years, these old steel structures may ask for repairing very soon.

References [1] EN 1993-1-8:2005 EUROCODE 3 Design of steel structures, Part 1-8 Design of joints

Section 3.6.2, (Translation in Serbian, 2006), Yugoslav Association of Structural Engineers (YuASE) and Faculty of Civil Engineering, Belgrade

[2] prEN 1090-2:2007-08 Execution of steel structures and aluminium structures, Part 2: Technical requirements for steel structures (Also ENV 1090-1 General rules and rules for buildings, Annex F, Internal translation in Macedonian, Reprotrust -Skopje)

[3] ECCS (1994). European recommendations for bolted connections with injection bolts, ECCS publication No.79, Brussels

[4] Gresnigt, A.M., Sedlacek, G. & Paschen, M.: Injection bolts to repair old bridges (2000) [5] Personal communication with prof. Nol Gresnigt, Delft University, The Netherlands.



Annex А COMPARATIVE CALCULATIONS

OF THE DESIGN RESISTANCE OF INJECTION BOLTS

А.1 DESIGN RESISTANCE OF PRELOADED INJECTION BOLTS (NEW STRUCTURES)

According to ЕUROCODE 3 Part 1-8 (EN 1993-1-8:2005): Design of steel structures, Part 1-8: Design of joints [1], Article 3.6.2.4 (4) “The design shear load of any bolt in a category B(1) connection and the design shear load of any bolt in a category C connection should not exceed the design slip resistance of the resin as obtained from 3.9 at the relevant limit state plus(2) the design bearing resistance of the resin as obtained from 3.6.2.2(5) at the relevant limit state. In addition, the design ultimate shear load of a bolt in a category B or C connection should not exceed either the design shear resistance of the bolt as obtained from 3.6, nor design bearing resistance of the bolt as obtained from 3.6 and 3.7.“

А.1.1 Design resistance of preloaded bolts (ЕN 1993-1-8:2005, Article 3.9)

, ,3

ss Rd p C

M

k nF Fμγ⋅ ⋅

= ⋅ (3.6)

1.0sk = for holes with normal clearances 0 2 (3d d mm mm= + for 27 )M mm≥ n = number of contact surfaces in friction. For double sided connections 2n = μ = slip factor. According to Table 18, EN 1090-2:2007(3) Class А 0,5μ→ = : surfaces blasted with shot or grit with loose rust removed, no pitting Class В 0,4μ→ = : surfaces blasted with shot or grit а) spray-metallized with an aluminium or zinc based product b) with alkali-zinc silicate paint with a thickness of 50-80 μm Class C 0,3μ→ = : surfaces cleaned by wire-brushing or flame cleaning, loose rust removed Class D 0,2μ→ = : surfaces as rolled 0,1μ→ = : surfaces hot deep galvanized

, 0,7p C ub sF f A= ⋅ ⋅ → preloading force in the bolt

ubf = nominal ultimate tensile strength of the bolt material sA = tensile stress area of the bolt

3Mγ = partial safety factor: 3 1.25Mγ = for ULS and 3 1.10Mγ = for SLS (1) To category B belong the connections resistant to slip at serviceability limit state, while in category C the

connections resistant to slip at ultimate limit state, both of them made using preloaded bolts Class 8.8 or 10.9. To category A belong non-preloaded connections made using bolts Class 4.6 to 10,9. (Table 3.2 of EN 1993-1-8:2005)

(2) In the translation of EN 1993-1-8:2005 an error is made, most likely non-intentionally. The word “plus” from the English original is translated with “као и“ (“and also”) which may be understood as “as well as”. Consequently, the meaning of the Article may be essentially changed thus degrading the design resistance of preloaded injection bolts.

(3) The Norm defines that the cited values of the slip factors are “referent”. This means that the Designer may choose the corresponding value, but the responsibility of the Contractor should be to provide and also to prove it (when requested by the Engineer). According to Annex E (normative) of ЕN 1090-2:2007, the determination of the slip factor is carried out through experimental (laboratory) testing following strictly established procedure.



Table А.1 Design resistance of preloaded bolts Fs,Rd n = 2; μ = 0,4 (Class В surfaces)

Ultimate limit state (ULS) γM3 = 1,25

Serviceability limit state (SLS) γM3 = 1,10 Bo

lt

Tens

ile

stre

ss a

rea

(mm

2 )

Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9

М16 М20 М22 М24 М27

157 245 303 353 459

56.3 87.8 108.6 126.5 164.5

70.3 109.8 135.7 158.1 205.6

63.9 99.8 123.4 143.8 186.9

79.9 124.7 154.3 179.7 233.7

А.1.2 Design bearing resistance (of the resin) (ЕN 1993-1-8:2005, Article 3.6.2.2(5))

, ,, ,

4

t s b resin b resinb Rd resin

M

k k d t fF

βγ

⋅ ⋅ ⋅ ⋅ ⋅= (3.4)

tk = coefficient depending on the nature of loading(1) 1.0tk = → for serviceability limit state SLS (long duration) 1.2tk = → for ultimate limit state ULS

1.0sk = for normal hole clearances 0 2 (3d d mm mm= + for 27 )M mm≥ , or 1.0 0,1sk m= − for oversized holes.

m is the difference (in mm) between the normal and oversized hole dimensions. 1

2

1,66 0,33 tt

β = − , but also 1,0 1,33β≤ ≤

coefficient depending of the thickness ratio of the connected plates. For t1/t2 = 0,6 1,10β→ ≈ d = diameter of the bolt М. It is taken for comparative calculation 1t M=

, 1,5b resint d≤ is the effective bearing thickness of the resin. In this case ,b resint d M= =

,b resinf bearing strength of the resin. According to EN 1993-1-8, the bearing strength should be determined by testing following the provisions of EN 1090-2. According to ECCS Recommendations No79, for epoxy resin Huntsman (CibaGeigy) RenGel SW404 with hardener HY2404, it is taken in our case 2

, 150 /b resinf N mm= (2)

4 1,0Mγ = → partial safety factor.

With these values and parameters: 2

, , 1,0 1,10 15,0 16,5b Rd resin t tF k M M M k= ⋅ ⋅ ⋅ ⋅ ⋅ = ⋅ ⋅ (1) According to ECCS No79: European recommendations for bolted connections with injections bolts, the bearing

strength of the resin fb,resin is defined as a long duration process at which the creep displacement is not greater than 0,3mm. Sonce the short duration loadings have small influence on the creep, it is reasonable to adopt higher bearing strength value for ultimate limit state than for serviceabiliti limit state: kt,ULS = 1.2; kt,SLS = 1.0. In the translation in Serbian of ЕN 1993-1-8, Article 3.6.2.2(5) it should be added: kt,SLS = 1.0 is related to long duration loading (as in English original).

(2) Long term investigations (1996-2001) of the creep phenomena of this resin in the Stevin Laboratory, Delft University, The Netherlands (Prof. Nol Gresnigt) show that the bearing strength of the resin may be taken much higher, fb,resin = 175 N/mm2 or even 200 N/mm2. For the repair of the Oranienburg bridge near Berlin (1996), a value of fb,resin = 130 N/mm2 was adopted.



Table А.2 Design bearing resistance of the resin Fb,Rd,resin (Huntsman RenGel SW404 + HY2404)

Bolt

Ultimate limit state

(ULS) kt = 1,2

Serviceabilitylimit state

(ЅLS) kt = 1,0

М16 М20 М22 М24 М27

50.7 79.2 95.8 114.0 144.3

42.2 66.0 79.9 95.0 120.3

А.1.3 Design resistance of injection bolts (Sum of design bearing resistance Fb,Rd,resin + slip resistance Fs,Rd)

Ultimate limit state (ULS) Serviceability limit state (SLS)

Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9 Bolt

Resin Fb,Rd,resin Fs,Rd Total Fs,Rd Total

СмолаFb,Rd,resin Fs,Rd Total Fs,Rd Total

М16 М20 М22 М24 М27

50.7 79.2 95.8 114.0 144.3

56.3 87.8

108.6 126.5 164.5

107.0 167.0 204.4 240.5 308.8

70.3 109.8135.7158.1205.6

121.0189.0231.5272.1349.9

42.2 66.0 79.9 95.0 120.3

63.9 99.8 123.4143.8186.9

106.1 165.8 203.3 238.8 307.2

79.9 124.7 154.3 179.7 233.7

122.1190.7234.2274.7354.0

Annotation: The design bearing resistance of the resin for serviceability limit state (SLS) is less than corresponding design bearing resistance for ultimate limit state (ULS), while the design slip resistance of the preloaded bolts for SLS is bigger than corresponding slip resist-ance for ULS. Obviously, such values are a consequence of multiplying nominal resistances for ULS with partial factors bigger than 1.0.

А.1.4 Check up of design shear resistance and design bearing resistance of preloaded injection bolts

With the Clause 3.6.2.2 (4) of EN 1993-1-8:2005, check up of design shear resistance and design bearing resistance of preloaded injection bolts(1),(2) according to Clauses 3.6 and 3.7. According to Table 3.4:

• Design shear resistance, for single shear plane

,2

v ubv Rd

M

f AF αγ⋅ ⋅

=

0.6vα = → in a case when the shear plane does not pass through the part of the bolt with thread

ubf → nominal ultimate tensile strength of the bolt material A → gross area of the bolt shank

2 1.25Mγ = → partial safety factor

(1) See the quotation of the clause in the preamble of Annex A. (2) The second sentence of the clause 3.6.2.2(4), although could be estimated as conservative, practically means

that any bolt should have the necessary design resistance even in a case when, due to any reason, the con-nection slipped (loosed the preloading resistance) and/or the resin is damaged (loosed the injection resistance). In such a case, the bolts Grade 8.8 or 10.9 should continue to transfer the loads as ordinary, non-preloaded bolts and to prevent both the connection and the structure as a whole from any (more) serious consequences.



• Design bearing resistance

1,

2

b ub Rd

M

k f d tF αγ

⋅ ⋅ ⋅ ⋅=

bα → smallest value of , /d ub uf fα (1) or 1,0 Using minimal distances(2)

1 0 2 0 1 2 02,0 ; 1,5 ; 3,0e d e d p p d= = = = :

01

0 0

2,0 0,673 3d

ded d

α = = = → for end bolts in the direction of force,

01

0 0

31 1 0,753 4 3 4d

dpd d

α = − = − = → for intermediate bolts in the direction of force,

21

0

2,8 1,7ekd

= − or 2,5 for end bolts perpendicular to force 1 2,5k→ =

21

0

1,4 1,7pkd

= − or 2,5 for intermediate bolts perpendicular to force 1 2,5k→ =

uf → tensile strength of the base material. For checkup calculations the “weakest” material is taken inro consideration: S235 according to EN 10025, fu = 360 N/mm2 d → diameter of the bolt М. For checkup calculations it is adopted d M= t → thickness of the steel plate. For checkup calculations t M=

Table А.4-а Design shear resistance Fv,Rd and design bearing resistance Fb,Rd of single plane bolts Grade 8.8 and 10.9

Non-preloaded bolts Preloaded injection bolts

Shear Fv,Rd

Bearing Fb,Rd

Ultimate limit state(ULS)

Serviceability limit state (SLS) Bo

lt

Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9

М16 М20 М22 М24 М27

77.2 120.6 146.0 173.8 219.9

96.5 150.8 182.5 217.1 274.8

122.9 192.0 232.3 276.5 349.9

74.2 115.9 141.4 167.0 213.8

81.3 126.9 155.0 182.8 234.0

70.4 109.9 134.3 158.3 202.9

78.4 122.4 149.8 176.3 226.2

Annotation: Whenever the “traditional” distances between the bolts are applied, the design shear resistance and the design bearing resistance of non-preloaded single plane bolts Grade 8.8 and 10.9 are bigger than corresponding resistances of preloaded injection bolts. In case of failure of single plane preloaded injection bolts, whether as a consequence of slip in the connection or due to defects in the resin, they will safely transfer the force as non-preloaded bolts, without any consequence on ultimate bearing capacity of both, the connexcion and the structure as a whole, but obviously with increased deformations.

(1) For bolts Grade 8.8 and 10.9, the ratio fub / fu is always less than 1. (2) The reduced distanced between the bolts according to Table 3.3 of EN 1993-1-8:2005 strongly influence the

design bearing resistance and critical (absolutely smallest) becomes the resistance Fb,Rd. In this case it is better to adopt “traditional” distances between the bolts since using reduced distances the design resistance is decreased for about 2,5 times in relation to “traditional” distances.



Table А.4-b Design shear resistance Fv,Rd and design bearing resistance Fb,Rd of bolts Grade 8.8 и 10.9 in double lap joints

Non-preloaded bolts Preloaded injection bolts

Shear Fv,Rd

Bearing Fb,Rd

Ultimate limit state(ULS)

Serviceability limit state (SLS) Bo

lt

Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9 Grade 8.8 Grade 8.8

М16 М20 М22 М24 М27

154.4 241.3 291.9 347.4 439.7

193.0 301.6 364.9 434.3 549.7

122.9 192.0 232.3 276.5 349.9

107.0 167.0 204.4 240.5 308.8

121.0 189.0 231.5 272.1 349.9

106.1 165.8 203.3 238.8 307.2

122.1 190.7 234.2 274.7 354.0

Annotation: Whenever the “traditional” distances between the bolts are applied, the design shear resistance and the design bearing resistance of non-preloaded bolts Grade 8.8 and 10.9 in double lap joints are bigger than corresponding resistances of preloaded injection bolts. In case of failure of preloaded injection bolts in double lap joints, whether as a con-sequence of slip in the connection or due to defects in the resin, they will safely transfer the force as non-preloaded bolts, without any consequence on ultimate bearing capacity of both, the connexcion and the structure as a whole, but obviously with increased deformations.

А.1.5 About the reduction of necessary preloaded bolts when injection is applied

When using injection, it is obvious that the number of preloaded bolts in a connection may be significantly reduced. If the necessary number of preloaded (non-injection) bolts in a con-nection is 100%, then the necessary number of preloaded injection bolts (in %) in the same connection would be(1):

Ultimate limit state (ULS) Serviceability limit state (SLS) Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9 Bolts

n = 1 n = 2 n = 1 n = 2 n = 1 n = 2 n = 1 n = 2

М16 - М27 ∼38% ∼53% ∼43% ∼58% ∼46% ∼60% ∼51% ∼65%

Annotation: Applying injection, the necessary number of preloaded bolts in a connection is significantly reduced. In double lap connections (n = 2) the reduction amounts to 1,5 to 1,9 times, while in single plane connections (n = 1) even to 2 to 2.6 times. In addition, in EN 1993-1-8 reduced minimal end distances are foreseen than according to “traditional” practice (1,2d0 instead of 1,5d0 or 2,0d0), as well as reduced intermediate distances between the bolts (2,2d0 and 2,4d0 instead of 3,0d0).

CONCLUSION: The preloaded injection bolts are modern, reliable and safe fasteners for steel struc-tures. It could be considered that after 40 years of their first application in The Netherlands, the initial skepticism – first of all due to use of epoxy resins - is finally removed. Applying preloaded injection bolts number of bolts, length of connections and dimensions of connection plates and, in general, the scope and the price of works for production and assembling of connections are reduced to TWICE in average.

(1) The necessary number of preloaded injection bolts in double lap connections (two faces of contact / friction:

n=2) could also be obtained directly from the ratio:ηr = Fs,Rd / (Fs,Rd + Fb,Rd,resin) from Table A.3



А.2 DESIGN RESISTANCE OF NON-PRELOADED INJECTION BOLTS (RECONSTRUCTION AND RENEWAL OF OLD STRUCTURES)

For reconstruction and renewal of riveted steel structures (as most, practically all of old structures in Skopje Steel Mill and all railway bridges in RM constructed by seventies of the pas century), preloaded bolts may be applied conditionally or do not be applied at all. The reason is, in general, the unknown slip factor (friction) in contact surfaces of the connection due to possible internal corrosion as well as due to previously applied painting layers. However, it would be to much conservative to consider that friction in these connections does not exist at all(1). At he other side, the riveted structures have connection with no slip. No slip connection could be provided with non-preloaded bolts due to the gap between the shank of the bolt and the hole itself (2 to 3mm according to EN). Very good alternative for rivets (for which it is obvious that as now as in the future there will be shortage of appropriate materials, tools and skilled riveters, do not talk about economic parameters), can be injection bolts.

А.2.1 Design resistance of rivets (ЕN 1993-1-8:2005, Table 3.3)

0,

2

0,6 urv Rd

M

f AFγ⋅ ⋅

=

urf → strength of the rivet material. According to ЕN 1993-1-8:2005, Clause 3.6.1(15) for steel Grade Ѕ235 (equivalent to Č.0361 acording to MKS C.B0.500 or Fe37) may be adopted 2400 /urf N mm=

20 0 / 4A d π= ⋅ → cross section of the rivet,

2 1.25Mγ = → partial safety factor.

А.2.2 Design resistance of injection bolts (resin) (ЕN 1993-1-8:2005, Clause 3.6.2)

Following rules will be applied for substitution of rivets with injection bolts: • 0 17d mm= rivet is substituted with М16 bolt; • 0 20d mm= rivet is substituted with М20 bolt, but the hole should previously be enlarged

by drilling to 0 22d mm= ; • 0 23d mm= rivet is substituted with М22 bolt; • 0 26d mm= rivet is substituted with М24 bolt.

Characteristic values of design resistances of injection bolts Fb,Rd,resin may be undertaken from Table А.3.

А.2.3 Design resistance of non-preloaded bolts Grade 5.6 or 5.8 (ЕN 1993-1-8:2005, Clause 3.6 and 3.7)

Introducing fub = 500 N/mm2 for nominal ultimate tensile strength for the bolt material, the calculation is performed using the same procedure as in Item A.4.

А.2.4 Design resistance of preloaded bolts Grade 8.8 and 10.9 (ЕN 1993-1-8:2005, Clause 3.6.2, slip factor μ = 0,2)

Characteristic values of design resistances of preloaded injection bolts Fs,Rd calculated for slip factor μ = 0,2 may be obtained from characteristic values as given in Table А.3 using a reduction factor 0,5.

(1) According to EN 1090-2:2007, Table 18, to Class С having slip (friction) factor μ = 0.3 belong the surfaces cleaned

by wire brushing or flame cleaning and loose rust removed, while in Class D having μ = 0.2 belong as rolled surfaces. Thus, it could bi considered that non-corroded contact surfaces or surfaces with loose rust removed possess a slip (friction) factor 0.2 < μ < 0.3.



Table А.5 Comparison of design resistances of rivets, non-preloaded bolts and non-preloaded injection bolts Grade 5.5 or 5.8 in single lap joints

and double lap joints

Rivets Non-preloaded bolts 5.6 or 5.8 Injection bolts 5.6 or 5.8 (ULS) (1) Bolt

/ rivet Fv,Rd,1 Fv,Rd,2 Fv,Rd,1 Fv,Rd,2 Fb,Rd Fb,Rd,r1 Fb,Rd,r2

d0=17/ М16 d0=20/ М20 d0=23/ М22 d0=26/ М24

43.6 60.3 79.7 102.0

87.2 120.6 159.4 204.0

48.2 75.4 91.2 108.5

96.5 150.8 182.5 217.1

122.9 192.0 232.3 276.5

46.1 72.0 87.1 103.7

50.7 79.2 95.8 114.0

Annotation: The design resistance of non-preloaded injection bolts (resin) does not depend on the grade of bolt material, but depends on the characteristics of the resin and the connection itself. Thus, bolts of lower grade may be applied (5.6 or 5.8) instead of Grade 8.8 or 10.9 bolts. According to above results, the non-preloaded injection bolts in Category A connections can sub-stitute (have higher resistance than) the single plane rivets. The non-preloaded injection bolts in Category A connections (excluding d0=20mm) cannot substitute the rivets in double lap joints, unless if it is proved through a checkup calculation that the design resistance of such bolted joint is higher than applied forces. The rivets in double lap joints which do not transfer any load or serve only to connect the elements of the cross section, can be substituted with non-preloaded injection bolts.

Table А.6-а Design resistance of preloaded injection bolts in double lap joints (Sum of design bearing resistance (resin) Fb,Rd,resin +

design slip rsistance Fs,Rd for n =2 and μ = 0,2)

Гранична состојба на носивост (ULS) Гранична состојба на употребл. (SLS)

Класа 8.8 Класа 10.9 Класа 8.8 Класа 10.9

Завртка

Смола Fb,Rd,resin Fs,Rd Вкупно Fs,Rd Вкупно

СмолаFb,Rd,resin Fs,Rd Вкупно Fs,Rd Вкупно

М16 М20 М22 М24

50.7 79.2 95.8 114.0

28.1 43.9 54.3 63.3

78.8 123.1 150.1 177.3

35.2 54.9 67.9 79.1

85.9 134.1167.3193.1

42.2 66.0 79.9 95.0

32.0 49.9 61.7 71.9

74.2 115.9 141.6 166.9

40.0 62.4 77.1 89.9

82.2 128.4157.0184.9

3 (1) The analyzed connection (with no preloading, using Grade 4.6 to 10.9 bolts, and where the loads are transferred

through bearing) belongs to Category А according to EN 1993-1-8, Table 3.2. According to Clause 3.6.2.2(2), for this category of connections only the conditions for ultimate limit state (ULS) should be satisfied:

Fv,Ed ≤ Fv,Rd and Fv,Ed ≤ Fb,Rd

where: Fv,Ed - design value of load (action) on the connection (rivet, bolt) for ultimate limit state (ULS), which is obtained as a sum of nominal partial actions (permanent action G, variable actions Qi, etc.), each of them multiplied by corresponding partial load factor γG, γQ,i, etc. As a rule, for permanent action G, γG=1.35; for unique and dominant variable action Q1, γQ,1=1.50; but for simultaneous influence of more (more than one) variable actions Qi, γQ,i=1.35. Fv,Rd и Fb,Rd - design resistance of the connection (bolt, rivet), i.e. shear and bearing resistance correspondingly.

3 In addition, in this particular case and under assumption that the rivets which are to be substituted already have the necessary design resistance to the design loads (actions) on the connection, the following conditions should be satisfied:

Fv,Rd,rivet ≤ Fv,Rd,bolt and Fv,Rd,rivet ≤ Fb,Rd,bolt as well as Fv,Rd,rivet ≤ Fb,Rd,resin

or in other words, design shear resistance of the bolt, design bearing resistance of the bolt, as well as design bearing resistance of the injection bolts should be at least equal to the design resistance of the substituted rivet.



Table A.6-b Comparison of design resistances of rivets in double lap joints and preloaded injection bolts Grade 8.8 and 10.9 in double lap joints

(slip – friction factor μ = 0,2)

Ultimate limit state (ULS) Serviceability limit state (SLS)(2) Grade 8.8 Grade 10.9 Grade 8.8 Grade 10.9

Rivet / bolt

Rivets Fv,Rd,2

Fs+r,Rd Ratio Fs+r,Rd Ratio Fs+r,Rd Ratio Fs+r,Rd Ratio

d0=17/ М16 d0=20/ М20 d0=23/ М22 d0=26/ М24

87.2 120.6 159.4 204.0

78.8 123.1 150.1 177.3

0.9041.0210.9410.869

85.9 134.1 167.3 193.1

0.9851.1121.0500.947

74.2 115.9 141.6 166.9

1.1491.2971.1991.104

82.2 128.4 157.0 184.9

1.2731.4371.3301.223

Annotation: It could be considered that, at ultimate limit state (ULS), the design resistances of preloaded injection bolts in double lap joints (two contact surfaces) is similar as for rivets in double lap joints. The highest deviation appear (as expected) for Grade 8.8 M24 bolt and d0=26mm rivet amounting to 0.869 (=–13.1%), while for Grade 10.9 the deviation could be accepted (– 5.3%). The comparison with the results from Table А.3 shows that the condition of contact surfaces is of substantial significance: the friction faction should, in no way fall down below μ = 0.2. For instance, if μ = 0.1 (presence of loosed rust), the design resistance of M24 bolt falls down to 0.714 (= – 28.6%) which is unacceptable, while the portion of preloading in the total design resistance of the preloaded injection bolt amounts to about 22% only. However, if for instance the slip (friction) factor is increased from μ = 0,2 to μ = 0,3 (wire brush cleaning and loose rust removing), the design resistance satisfies in all cases(3),(4).

(2) In the calculation of ratios for serviceability limit state (SLS) in the above Table A-6.b no error is made, and the

design resistance of preloaded injection bolts is at the first site only less than the design resistance of rivets. Namely, the design loads (for rivets) for ultimate limit state are obtained by multiplying the nominal actions with corresponding partial load factors, while the design loads (for bolts) for serviceability limit state are, as a rule, equal to nominal actions. In this case, in order to make the resistances compatible for comparison, the ratios given in Table A6-b are obtained by multiplying the design resistance of the bolts with γG= γQ,i =1.35, or by dividing the design resistances of the rivets with the same partial factor.

(3) In order to check the condition of contact surfaces and/or to undertake measures to increase the slip (friction) factor, the connections have to be completely un-riveted. Partial un-riveting, with precisely defined number of rivets to be un-riveted and strictly defined procedure how to do this have to be prescribed in the Design for renewal (reconstruction, maintenance), having always in mind that the connection (element, structure as a whole) must possess appropriate bearing capacity for loadings which, at that period of time, act (or may act, even accidentally) on the connection (element, structure). In many cases, these are the permanent loads (dead loads) only which, for instance, for crane girders and railway bridges of small and middle spans with open railway may amount to 10% of the total loads as most. If the condition of connections at the top and bottom plates of a riveted girder is good (or they have previously been reconstructed), an experienced structural engineer would estimate that, without any danger, the riveted double lap connection of the web could be completely un-riveted and, after that, to undertake necessary measures to increase the friction factor even to the design level as for new structures (blasting, than spray metallization or application of corresponding layer).

(4) In any particular case, it should be taken into account that regarding the condition of contact surfaces and possible internal corrosion, substantial difference may exist between structures in open space (bridges, wet environment and/or exposed to wheather conditions, without ventilation) and structures in protected, closed space (workshops, dry environment). For more details see ESDEP, Section 4A: Corrosion protection.



CONCLUSION: Preloaded injection bolts can be applied to substitute rivets in double lap connections for

renewal, reconstruction or maintenance of old steel structures and industrial equipment, but with previous measure of attention first of all regarding the condition of contact surfaces and existence of internal corrosion.

To substitute rivets in double lap connections which transfer forces, in all cases where the condition of contact surfaces in old riveted structures is unknown or unclear, such condition is dif-ficult or impossible to be find out in any different way, and even smallest doubt exists regarding internal corrosion in the connection, one of following measures can be applied(1):

• Complete un-riveting of the connection (if the loadings at the time of renewal are of ap-propriate level to do this) and substitution of rivets with preloaded injection bolts of corres-ponding diameter. The estimation and decision whether the connection may be un-riveted or not should be given in the design for renewal or could be made, in writing, by a com-petent and authorized structural engineer. Immediately after un-riveting, the garter plates should be marked regarding their location and orientation, all contact surfaces (on the element and garter plates) should be cleaned up to necessary level (blast cleaning, spray metallization or application of corresponding layer, or wire brush cleaning only, as the case may be), installation of preloaded bolts, tightening and finally injection.

• By consecutive un-riveting and substitution of individual rivets or previously defined groups of rivets with preloaded injection bolts having one caliber bigger diameter: - d0=17mm with М20 bolt with enlarged hole d0=22mm; - d0=20mm with М20 bolt with enlarged hole d0=22mm; - d0=23mm with М24 bolt with enlarged hole d0=26mm; - d0=26mm with М27 bolt with enlarged hole d0=30mm. The sequence of un-riveting of individual rivets or groups of rivets and their substitution with preloaded injection bolts should be defined in the design for renewal or by competent and authorized structural engineer in writing.(2),(3)

• In general and in all cases, by increasing the strength of the resin fb,resin from 150 N/mm2 to 175N/mm2 or even to 200N/mm2, on the basis of our own experimental investigations, as suggested in the investigations of Prof. Nol Gresnigt [4].

(1) All previous analyses and calculations are performed with direct comparison between nominal design

resistances of the rivets and nominal design resistances of non-preloaded injection bolts or preloaded injection bolts. That means that the principle of so called “nominal resistance covering” is applied, according to which the new fasteners possess at least same or bigger resistance than the existing rivets. In any particular case when the original calculations and drawings are not available, this approach is unique and appropriate for renewal and reconstruction of structures and industrial equipment since it solely provides necessary resistance and reliability.

(2) The number of rivets in a group and location of that group of rivets which are to be un-riveted in one step directly depends on the ratio of magnitude of actions (loadings) on the structure (connection) at the time of renewal and magnitude of actions (loadings) as in design: as bigger are the actions (loadings) at the time of renewal, as smaller should be the number of rivets to be un-riveted. This practically means that at any moment of the renewal the structure (equipment) must be safe and stable. When doing renewal, those rivets which are (already) disrupted or damaged should be substituted at first. When substituting complete connections, the next group of rivets should not be substituted until the bolts of the previous group would be tightened and injected and the resin cured (24 hours at normal temperature of the ambient).

(3) For accidental conditions (loadings having very low probability of appearance, for instance once or few times during the expected life of the structure) significantly lower safety factors are foreseen in the regulations (γf,А = 1.10 instead of γf,Q = 1,35). Consequently, if the renewal is considered as a controlled accidental situation regarding the ultimate limit state, the maximum number of rivets whish may be un-riveted in one step would be 1/6 of the total number of rivets in the connection, as most.

injection bolts by prof dr tihomir nikolovski

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