Construction and Building Materials 54 (2014) 258–269

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Construction and Building Materials

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Experimental investigations on hot-driven structural rivets in historicalFrench and Belgian wrought-iron structures (1880s–1890s)

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.12.059

⇑ Corresponding author. Tel.: +32 (0)2 629 28 46.E-mail addresses: quentin.collette@vub.ac.be (Q. Collette), stephane.sire@

univ-brest.fr (S. Sire), bvermes@jonesstuckey.com (W.J. Vermes), meslerv@lcc.edu(V.J. Mesler), ine.wouters@vub.ac.be (I. Wouters).

1 Tel.: +33 (0)2 98 01 70 05.2 Tel.: +1 330 794 7957.3 Tel.: +1 517 614 9868.4 Tel.: +32 (0)2 629 37 96.

Quentin Collette a,⇑, Stéphane Sire b,1, William J. Vermes c,2, Vernon J. Mesler d,3, Ine Wouters a,4

a Vrije Universiteit Brussel, Faculty of Engineering Sciences, Dept. of Architectural Engineering (ARCH), Pleinlaan 2, 1050 Brussels, Belgiumb University of Brest, EA 4325 LBMS, 6 Avenue Victor Le Gorgeu – CS 93837, 29238 Brest Cedex 3, Francec Jones-Stuckey Ltd, Inc., 1655 West Market Street – Suite 355, Akron, OH 44313, USAd Lansing Community College – Welding, Technical Careers Division, 5708 Cornerstone Drive, Lansing, MI 48901-7210, USA

h i g h l i g h t s

� The quality of riveting has an influence on the structural behaviour of the joints.� Geometrical and metallographic investigations are complementary and needed.� Geometrical measurements of rivet heads confirm historical literature.� The identification of the original shank diameter before driving is essential.� The grain flow and slag orientation reveals the original driving method.

a r t i c l e i n f o

Article history:Received 13 September 2013Received in revised form 16 December 2013Accepted 19 December 2013Available online 17 January 2014

Keywords:Hot-driven rivetsWrought ironHeritage structuresExperimental investigationsGeometryMetallographyRivetingCase studiesRenovation

a b s t r a c t

When assessing the structural safety of historical metal structures, the understanding of the constructionmaterials, their properties and modes of construction is a prior concern. Available information on histor-ical riveted connections, however, is very limited. Analyzing the present state of these connections to bet-ter understand their actual structural behaviour is therefore essential. Through experimental work, weinvestigated the geometry and metallography of wrought-iron rivets dismantled from four bridges andbuildings (1880s–1890s). The experimental results were then compared to historical literature. Thispaper reports the original design, manufacturing and installation of hot-driven rivets through four casestudies and discusses the impact of these parameters on the behaviour of the connections. Resultsshowed that the geometrical affinity of rivets together with their grain flow allow the original designand driving technique to be identified. The provided findings constitute supportive tools when assessingand renovating historical riveted structures.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

During the decades around the turn of the 20th century, hot-driven rivets was the most widely used joining technique in ironand steel bearing structures. Today most historical rivetedstructures need repair and/or strengthening and, if not, at the least

some maintenance. Highlighted by authors of this day such asGustafson [1], Kühn et al. [2], D’Aniello et al. [3] or Pipinato et al.[4,5], hot-driven rivets are a key factor to investigate given theirpredominant influence on the overall structural behaviour of theseconstructions. Unfortunately, today’s historic preservationists,architects and engineers are often confronted with issues – boththeoretical and practical – for which the literature may not providesatisfactory answers (e.g. missing information, parametersneglected by the Eurocodes, etc.) [3,6–11].

In particular, when assessing static and fatigue resistances of his-torical wrought-iron structures such as bridges or buildings, anin-depth knowledge of material properties is one of the necessaryfirst-stage tasks before undertaking any renovation or rehabilitationproject [1,2,12,13]. However, very limited information on material

Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269 259

properties of wrought-iron bars and plates is available in today’sliterature [6,7,14]. Moreover, given the anisotropic and inhomoge-neous features of wrought iron, results derived from microstruc-tural and mechanical analyses are, though essential, difficult tointerpret [15–17].

In addition, the bridge and building construction communitieshave steadily forgotten the know-how regarding the complex,and now largely obsolete, riveting technology (making the rivethole, heating and driving the rivet) [18–20]. Furthermore the rivet-ing technology underwent major evolutions between 1835 and1925 regarding rivet manufacture and installation [20]. Simulta-neously, the riveting theory developed and introduced a largenumber of rules of thumb, design principles and calculationassumptions [21]. Because of their direct influence on materialand mechanical properties, both the riveting theory and technol-ogy have to be taken into account.

While the structural behaviour of late-19th-century and 20th-century riveted steel connections has recently been investigatedby a limited number of authors such as D’Aniello et al. [3] or Pipi-nato et al. [4,5], the geometry, installation and metallography ofwrought-iron structural riveted connections remain undeservedlylittle-known to this day.

This research enhances the knowledge in the field of hot-drivenstructural wrought-iron rivets by identifying and revealing theiroriginal design, geometry, and manufacturing and driving tech-niques. All of these parameters directly impact on the structuralbehaviour of the connections. Our research approach follows twomain steps. First, the geometry and metallography of hot-drivenrivets and riveted connections are investigated through experi-ments with samples dismantled from four wrought-iron load-bear-ing structures – buildings and bridges – built in France andBelgium in the late 19th century. Second, the experimental resultsare analyzed and confronted with the content of historical sourcespublished between 1850 and 1966 (manuals, handbooks, treatises,etc.). Depending on the parameter discussed, a given scope of his-torical literature was used for the confrontation. General theoreti-cal and practical considerations such as manufacturing and drivingtechniques are studied through international literature, whereasparameters that are tied to the case studies (e.g. geometry of theriveted connections) are analyzed through the content of Frenchand Belgian sources.

By combining experimental, theoretical and practical consider-ations, this study increases the general knowledge on hot-drivenstructural wrought-iron rivets and supports practicing architectsand engineers in their renovation work. The main distinctive fea-ture of this study lies in the confrontation between the results de-rived from experiments and the content of historical literature. Theprovided information and tools deduced from this confrontationare relevant for the structural assessment and rehabilitation of his-torical riveted structures and contribute to the preservation oftheir historical significance.

2. Hot-driven structural wrought-iron rivets

From 1810 onwards, the hot riveting technique, which con-sists of heating the rivet before driving, had been introduced byboilermakers. From 1840 onwards, the construction of large-spaniron bridges in the UK announced the early stirrings of a new eraof joining technique on an international scale: the use of rivetedconnections within the field of iron and steel construction [22].The emblematic Conway (1848) and Britannia (1850) tubularbridges designed by William Fairbairn as well as Isambard King-dom Brunel’s Royal Albert bridge (1859) are fine examples [23].Being an important advance in fabrication, the hot riveting tech-nique developed and its use became widespread in structural iron

and steel construction. Riveted connections were used in build-ings and civil engineering applications to fabricate built-upsections and connect structural members together such as fortrusswork [20,24]. The advantages of structural riveting (reliabil-ity, affordability, stiffness, etc.) were key preconditions for thedevelopment of new girder and column shapes, and rigid connec-tions, among others.

A rivet consists of a shank and a first rivet head – called man-ufactured head or shop head – formed by crushing the end of thecut segment of a cylindrical iron bar. Once the wrought-iron rivetheated by the ‘forge boy’ (rivet stoker) in the forge took on abright orange to yellow/white-hot colour, the rivet was thrownto the ‘rivet catcher’ who caught it with tongs to insert it in therivet hole. These colours correspond to the temperature’s rangeof ca. 1000 �C (1800�F) to 1150 �C (2100�F). Then the holder-onbucked up (held in place) the rivet on the shop side with a ‘dollybar’. Finally, the riveter, as the most skilled person within the riv-eting crew, formed the second rivet head called the field head, onthe protruding shank with a hand-held hammer or a rivetingmachine equipped with a rivet snap at its end (kind of mould)[19,22,25,26]. Unlike in cold riveting, hot riveting has significantpositive influence on the slip resistance of the connection (fric-tional strength). The frictional strength is provided by the lateralcompression – the clamping force – applied by the rivet heads tothe joined plates called plies. The plies are pinched towards eachother by the heads when the rivet shank shrinks due to cooling.The magnitude of the clamping force is a function of the totaljoint thickness – the grip length, the driving technique and thedriving temperature [11] (Fig. 1).

Manual riveting was the only installation technique availableduring the first half of the 19th century. Being a tough workand incompatible with large diameters, the mechanization ofthe riveting process was needed. In 1847, Garforth, who origi-nated from Dukinfield (UK), invented the first riveting machinefitted for the erection of load-bearing structures [27]. The mech-anization of the riveting process was further developed over aperiod of 80 years through different technical evolutions: steam(1847, Garforth, UK), hydraulic (1865, Tweddell, UK), and finallypneumatic transmission (1875, Allen, USA) [28,29]. Argued bysellers of riveting machines, the better upset of the shank in therivet hole of machine-driven rivets was commonly admitted atthe time [30]. For short grips, this general statement was right,but for longer ones it needs to be nuanced. Tests carried out byFrémont [30] on 100-mm-long grips highlighted the presence ofan efficient upset and contact between the shank and the rim ofthe hole only near the field head, over a length of ca. 40 mm.Actually, clearances are more likely to occur at the centre andshop head end of the rivet [30,31]. In any case, several experi-ments implemented in countries such as France (e.g. Frémont[30]) or the US (e.g. Rumpf [32]) proved that the ultimate tensilestrength (‘UTS’) of undriven rivets was increased once mechani-cally driven (average increase of 20%). Also, the driving processinduces a reorientation of slag inclusions, called slag from nowon in the paper, at the junction of the rivet head and shank. Thiscould lead to stress concentrations, given the reduced tensilestrength and ductility [15].

The quality of riveting also depends on other parameters suchas the applied crushing pressure, the driving time and the geome-try of the rivet snap. This makes the influence of the installationprocess on the structural behaviour of wrought-iron rivetedconnections difficult to appraise. However, when assessing thestate of a riveted connection in a built structure, the identificationof visible and non-visible defects allows to retrace original installa-tion errors that affect the quality of riveting. A non-exhaustive listof visible and non-visible defective rivet characteristics is providedin Table 1 [3,6,7,15,18,19,24,26,30].

Fig. 1. Adaptation from Edwin Clark’s engraving of hot-driven rivets (1847): longitudinal cross-section showing the grain flow of machine-driven (top-left, barrel shape) andhand-driven (bottom-left, tulip shape) shop and field rivet heads. Geometrical parameters defining a rivet (top-right): shank diameter d, plate thickness e, head diameter D,head depth h and radius of curvature R [27].

260 Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269

3. Rivet samples

In order to investigate the geometry and metallography of hot-driven rivets, four wrought-iron load-bearing structures were se-lected based on their building date, typology and material avail-ability. These case studies cover a narrow time period andgeographical zone. They were built between the 1880s and 1890sand are located in north-western France (Brittany), eastern France(Bourgogne) and Belgium. These structures vary in typology(bridges, pier and hall) and status (see Table 2). The rivet sampleswere taken from structural members dismantled during either arenovation or demolition project. Depending on the studied case,the structural function of these rivets was either to fabricatebuilt-up sections (e.g. vertical post, girder, column, etc.) or to con-nect structural members together (e.g. diagonal with chord mem-ber, cover plate with flanges, etc.). The rivet samples of the FR-1883 case study were taken from the bottom-chord member ofthe primary trusses of the Louhans bridge. The samples of FR-1893 were extracted from the bearing pillars of a former railwaybridge in Brest. They connected stiffening angles to the chordmembers of the triangular legs of the pillars. With regard to FR-1897, the structural function of the rivets was to fabricate thecross-shaped built-up sections supporting Cancale’s pier by con-necting angles to a central plate. The samples of BE-1888 were ta-ken from horizontal trusses stiffening the main frontage of thenorth hall of the Brussels Cinquantenaire Park. They connected

diagonals or vertical posts to the chord members of the trussesthrough gusset plates (Table 2) (see Sire et al. [17] and Colletteet al. [33] for more information).

The rivets required for the experimental work were extractedfrom these structural members in two ways, namely with or with-out the plies they clamp. In the first case, the entire rivets togetherwith their plies were removed from assemblies with a cooledbandsaw to reduce the volume surrounding the rivet. To every en-tire rivet belong a rivet shank, a shop head and a field head thatclamp several plies. In the second case, only half-rivets could be ex-tracted since one head had already been removed during the resto-ration procedure. A half-rivet consists of one head and (a part of)the rivet shank. Then, every rivet was precisely machined alongthe longitudinal axis of its shank. Finally, the rivet samples werepolished with silicon carbide paper from grit 120 to grit 2400 tobe ready for the experimental investigations and microscopicobservations.

4. Geometry, the rivet head

Analyzing the geometry of the rivet head can directly and indi-rectly reveal the original design choices (shank diameter d, platethickness e) and driving process (shop vs. field head, quality ofriveting). As broached in Section 2, the issue of the uniform contactof the rivet shank with the rivet hole along the grip leads to the

Table 1Visible and non-visible (to the naked eye) defective rivet characteristics based on literature and know-how of experts in structural riveting.

Visible defects

Defect Origin(s) Influence(s)/Potential failure

Badly shaped rivet head Manufacturing or driving error Reduced ultimate tensile strength of the head; Rivet head poppedoff

Pitting on the rivet head Overheated/Burned rivet BrittlenessUnfilled rivet heada Driving temperature too low; Protruding shank end

too shortReduced clamping force & frictional strength; (Field) headpopped off

Unsymmetrical head Driving error of the riveter /Lip around the rivet head Excessive shank length; Snap diameter too small Insufficient upset of the shank (for lips too large)Loose rivets Improperly drawn up plates before riveting Movement of the rivet; No frictional strengthRivet head degradation Rivet corrosion Reduced clamping force & frictional strength; Rivet head popped

offPack rustb Insufficient tight connection; Inadequate

maintenance; Exposure to water/moistureRivet head popped off; Shank failure (bursting pressure)

Non-visible defects

High phosphorous content Used as strengthener for the basic metal BrittlenessExcessive d/e ratio Inappropriate design choice; Rationalization of the

different used shank diametersCrippling of the plates

Strain hardened area of the plate aroundthe hole

Punched rivet hole (without reaming/annealing) Brittleness; Microcracks (fatigue crack initiation)

Head eccentricitya Driving error of the riveter No influence c; Difficult removal (if needed)Camming effectd Misalignment of the plies before driving Reduced shear strength (cross section); Difficult removal (if

needed)Hand-driven field head Period and tradition; Available space Reduced frictional (and bearing) strength(s)Unsatisfactory contact rivet shank - rivet

holeHeating temperature too low; Long grips Reduced bearing strength

Variations in microstructural properties(shank-head intersection)

Slag reorientation during riveting Reduced ultimate tensile strength (parallel to grain) and ductilityof the head; Rivet head popped off

Rivet hole deformation Excessive driving pressure (Deep) grooves on the rivet shank

a Field head generally.b Interfacial corrosion between the plies.c According to Frémont’s experiments [30]. However, Sustainable bridges’ reports [6,7] states a maximal value of 0.15 hole diameter.d Non straight rivet shank (once driven).

Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269 261

distinction between the shop and the field head. In case of a longgrip and/or driving errors, the non-uniform contact may inducean asymmetric behaviour of the riveted connection such as bearingstress concentration or deformation. Actually, the contact betweenthe rivet shank and hole is better near the field head end [30,31].

The mechanization of rivet manufacture, namely forming theshop head, reduced the many different shapes and dimensions of riv-ets peculiar to the period before 1850, by a standardization, and thevariety was reduced [26]. The French boilermaker A. Durenne had in-vented the first rivet-manufacturing machine as early as 1836 [30].Designed to increase output in order to meet the rising demand,the rivet-making machines that were developed during the secondhalf of the 19th century were clearly aimed at solving the main tech-nical issues of the time (e.g. supply of iron bars, ejection of the forgedrivet) [20]. Both the cold and hot forming techniques were used toform the shop head, preferably without any lip, via a continuouscompressive stress. At the end of the 19th century, on-site manufac-turing had been gradually replaced by the opening of large shopsspecialized in mass-produced rivets [30].

The mechanization and standardization of rivet manufacturecan explain the almost constant proportions of the shop head men-tioned in historical literature. For a given head type such as theround head, rivets of various dimensions are geometrically affine.This means that the shop head proportions – head diameter D,head depth h, and radius of curvature R – are all expressed as afunction of one parameter: the shank diameter d [24,26]. Gener-ally, in a design, the parameter d was deduced from the plate thick-ness e (Fig. 1).

With regard to the field head, differences might be observedcompared to the shop head’s geometry. The original proportionsof the field head are basically dependent on the geometry of therivet snap used for driving as well as on the quality of riveting.

The questions to be answered by experiments were thefollowing:

� Did architects and engineers use standard shank diameters? Arethe plate thickness e and the d/e ratio reliable parameters todefine the nominal shank diameter d?� Are the rivet heads effectively geometrically affine? Can the

geometrical affinity help to identify the nominal shankdiameter d in a non-destructive way?� Do geometrical experiments allow to distinguish the shop head

from the field head?

4.1. Experimental set-up

Geometrical investigations were performed on six rivet heads percase study, which provide a sufficient statistical representation andtake potential manufacturing or driving irregularities into account.This value meets the requirement mentioned in the guidelines ofSustainable Bridges: minimum five rivet heads per inspected struc-ture [6,7]. Entire rivets as well as half-rivets were examined (see Sec-tion 3). Only one of the two heads of entire rivets was taken intoaccount if the other one was too distorted or corroded. While for en-tire rivets the presence and identification of both shop and fieldheads is obvious, the analysis of half-rivets is more unsure.

For each rivet sample, seven parameters were investigated. Onthe one hand, the plate thickness e, shank diameter d, headdiameter D, head depth h, and head eccentricity e were measuredwith a digital caliper (tolerance of ±0.02 mm). On the other hand,the head radius of curvature R and the circularity tolerance DRwere assessed with a coordinate measuring instrument (MitutoyoEuro-C 544). For each sample, 6 points were measured along thehead curvature and DR was estimated with the least square

Table 2Presentation and description of the case studies and riveted connections.

Case study tag FR-1883 FR-1893 FR-1897 BE-1888

(mm)

Typology Bridge Bridge Pier HallCity, Country Louhans, France Brest, France Cancale, France Brussels, BelgiumBuilding date 1883 1893 1897 1888Joining typologya Built-up Connecting Built-up ConnectingStatus Demolished Existing, renovated Existing, renovated Existing, renovated

a Built-up = built-up sections; connecting = connecting structural members together.

262Q

.Colletteet

al./Constructionand

BuildingM

aterials54

(2014)258–

269

Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269 263

method. For the discussion of the results, the level of accuracy con-sidered was a tenth of a millimetre, as any higher level would beirrelevant with regard to the actual state of the rivets. Surfaceirregularities (e.g. multiple paint layers) and defects, approachedwith the parameter DR for R, might have been detrimental to theaccuracy of the measurements. The defective rivet characteristicsobserved were the following: distorted rivet shank, rivet head deg-radation due to corrosion, unsymmetrical head, head eccentricity,and lip around the rivet head (see Table 1).

4.2. Experimental results and discussion

The experimental results of the geometrical analyses are summa-rized in Table 3. Samples were labelled as CO-DATE-RC-N, where:

� CO is the country where the case study is located (FR: France;BE: Belgium);� DATE is the building date of the structure;� RC is the rivet configuration (i.e. ER: entire rivet; HR: half-rivet);� N is the sample number (e.g. for ER: NA and NB rivet heads

belong to the sample N).

For each studied case, the mean value (‘Mean’) and standarddeviation (‘SD’) of the shank diameter d, head diameter D, headdepth h, and head radius of curvature R are calculated to quantifythe scatter of the results.

4.2.1. Nominal shank diameterFor every sample, the average value of the shank diameter d was

calculated based on the measurements made along the shank

Table 3Geometrical investigations: measurements of the rivets’ heads and shanks, SD and mean

Sample tag Rivet head and shank Head

dave (mm) D (mm) h (mm) R (m

FR-1883-ER-1A 22.9 37.0 14.3 19.9FR-1883-ER-1B 23.4 35.2 13.3 19.0FR-1883-ER-2A 23.5 36.3 14.1 18.7FR-1883-ER-2B 23.4 35.0 13.5 19.8FR-1883-ER-3A 22.8 37.0 14.5 20.2FR-1883-ER-4A 23.0 36.8 14.9 19.6

Mean 23.2 36.2 14.1 19.5SD 0.3 0.9 0.6 0.6

FR-1893-HR-1 21.4 29.4 11.0 15.1FR-1893-HR-2 21.3 31.3 12.0 17.0FR-1893-HR-3 21.5 31.3 11.9 16.5FR-1893-HR-4 21.5 31.9 12.8 16.9FR-1893-HR-5 20.7 30.0 12.3 15.0FR-1893-HR-6 21.6 29.2 11.0 15.7

Mean 21.3 30.5 11.8 16.0SD 0.3 1.1 0.7 0.9

FR-1897-ER-1A 21.7 35.4 14.4 19.2FR-1897-ER-1B 21.7 35.5 15.0 18.0FR-1897-ER-2A 21.6 35.6 14.0 17.3FR-1897-ER-2B 21.6 35.3 15.1 18.1FR-1897-ER-3A 21.7 35.7 14.3 17.4FR-1897-ER-4A 21.6 35.5 14.9 18.6

Mean 21.7 35.5 14.6 18.1SD 0.1 0.1 0.4 0.7

BE-1888-HR-1 21.2 35.3 14.8 18.2BE-1888-HR-2 21.6 35.0 12.5 18.7BE-1888-HR-3 21.9 37.0 15.6 17.7BE-1888-HR-4 21.0 34.7 13.0 18.4BE-1888-HR-5 20.7 34.9 13.5 17.6BE-1888-HR-6 21.1 35.0 14.5 18.5

Mean 21.3 35.3 14.0 18.2SD 0.4 0.8 1.2 0.4

a The samples for which the result is partially unsure are marked with an asterisk (⁄)

(parameter dave, Table 3). The parameter dave varies between 20.7and 23.5 mm, which is a narrow range. The mean value (Mean)of each case study is an uneven number in millimetres (e.g. about21 or 23 mm). The FR-1897 case shows results that have the lowestvariation within the sample data (small SD).

The parameter dave represents the shank configuration afterdriving, which does not reflect the nominal shank diameter, asmanufactured. In an attempt to understand the original designchoices and analyze the geometrical ratios, the nominal diameterd in millimetres has to be identified. The direct implication of thisparameter in the calculation of the allowable shear stress per rivetstrengthens the importance of this consideration. The nominalshank diameters derived were the following: 22 mm for FR-1883,and 20 mm for FR-1893, FR-1897 and BE-1888 (see column header‘Calc.’, Table 4.1). Reversing the original driving process and refer-ring to the shank diameter standards of the time allowed to findthese values. First, the following two effects were taken into ac-count to trace back to the original driving process:

(1) the tolerance of fabrication of the rivet shank: the value ofca. 0.25 mm has to be considered for shank diametersbetween 16 and 25 mm [34].

(2) the radial upset of the shank in the rivet hole while beingcrushed: on average, the hole-shank diameter differenceranges from 1 mm to 1.5 mm, and 2 mm acts as upper-bound value [34–39].

As its influence is very limited, the diametric shrinkage of theshank due to cooling was not taken into account within the calcu-lation. Next, the determination of the nominal shank diameter was

values per case study.

curvature Head eccentricity S(hop)/F(ield) heada

m) DR (mm) e/dave (%)

0.6 5 S0.1 7 F0.1 4 S⁄

0.6 1 F⁄

0.4 3 S0.5 4 S⁄

/ / // / /

0.3 3 F⁄

0.1 3 F⁄

0.2 1 F⁄

0.3 12 F0.2 16 F0.2 5 F⁄

/ / // / /

0.4 0 S⁄

0.5 0 F⁄

0.8 0 S⁄

0.8 0 F⁄

0.6 0 S⁄

0.1 2 S⁄

/ / // / /

0.6 6 F0.4 6 F0.5 8 F0.4 1 S0.4 1 S0.4 1 S

/ / // / /

.

Table 4.1Comparison between the calculated mean values of geometrical ratios and thecontent of historical French and Belgian literature: shank and plate thickness.

Case study tag e (mm)a d (mm) d/e (/)

Calc. Theory Meas. Theory

Lemaître Literature Lemaître

FR-1883 15 22 26 21–24 1.47 1.83FR-1893 6 20 13 12–16 3.33 1.88FR-1897 10 20 19 18–22 2.00 1.88BE-1888 11 20 20 18–22 1.82 1.88

a For rivet in double shear, e is the thickness of the outer plies (i.e. the thinnestply).

264 Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269

made possible by referring to standard values recommended byeducator-engineers and theoreticians contemporary with the casestudies. For load–bearing structures assembled with round headrivets, diameters ranging from 16 to 26 mm were common [26].Even and uneven diameter values as well as decimal diameter val-ues were available. However, between 1870 and 1900, the evendiameters of 18, 20, 22 and 24 mm were the most usual ones incountries using SI units, considering practical and economic con-cerns [35,37,40]. The values of 20 and 22 mm thus meet the shankdiameter standards found in historical French and Belgian litera-ture (Table 4.1).

The d/e ratio was then calculated for each case study based onthe nominal shank diameter d and the plate thickness e (see col-umn header ‘Meas.’, Table 4.1). Except for FR-1893, these ratiosare not higher than 2. The easy-to-use ratio d/e was a commonpre-design criterion used to define the shank diameter [26].

The d/e ratio played a predominant role within the morpho-genenis of the design principles of riveted connections. It condi-tioned first the empirical methods (rules of thumb, tables) andlater still influenced the analytical ones (formulas and standards).For example, the French boilermaker Lemaître proposed in 1856a design table that became a standard reference for several dec-ades. In 1857, Armengaud translated Lemaître’s design table intoa formula – ‘‘Lemaître’s empirical formula’’, which was only validfor plate thicknesses smaller than or equal to 15 mm [22,26]. Gen-erally, the plate thickness e was considered as a known parameterwithin the design process that permitted the nominal shank diam-eter d to be determined via the d/e ratio. French and Belgian liter-ature published between 1888 and 1966 provided theoreticalvalues of d and d/e based on the plate thickness e (column headers‘Lemaître’ and ‘Literature’, Table 4.1). While for FR-1883, FR-1897,and BE-1888, experimental and theoretical values match quitewell, the d values of FR-1893 are largely oversized compared withthe theory. This observation is highlighted by its high d/e ratio of3.33, exceeding the commonly recommended maximal value of 3to avoid the crippling of the plies [26]. The design of the FR-1893bridge was supervised by the acknowledged French engineer Ar-mand Considère. Original correspondence reveals that the overallstrength calculations (macro level) were not based on the 1891new French regulation for the construction of iron and steelbridges [41]. The high d values of FR-1893’s rivets, which do not

Table 4.2Comparison between the calculated mean values of geometrical ratios and the content of

Case study tag Head form Head–shank

h/D (/) D/d (/)

Meas. Theory Meas. Th

FR-1883 0.39 0.37 1.65 1.FR-1893 0.39 0.37 1.53 1.FR-1897 0.41 0.37 1.78 1.BE-1888 0.40 0.37 1.77 1.

match the published recommendations of the time, are in line withConsidère’s ‘‘freedom in design’’ approach but here at the micro le-vel of the joints.

Hence practicing engineers and architects of the time did notsystematically follow the recommendations provided by the the-ory, but without observing any major damage or failure of the con-nections. This can be explained by the inaccuracies of thesegeometrical ‘rules of thumb’, the actual stress state of the struc-tural members (i.e. amount of loads to be taken up), and againon-site practical and economic concerns. As a result, the platethickness e cannot be considered as a reliable non-destructiveassessment criterion to define the nominal shank diameter d.

4.2.2. Rivet head form and geometrical affinityTogether with a visual inspection, the rivet head form can be as-

sessed through the measurement of the h/D ratio and the head ra-dius(es) of curvature R. Various rivet head forms were available(round, button, countersunk, coned, etc.) but the round and buttonheads were the most common ones for load-carrying structures.The presence of a single R per head (small circularity toleranceDR, see Table 3) and a h/D ratio approaching the value of 0.37(see Table 4.2) give evidence for the round head for all the studiedcases. Between 1869 and 1900, historical French and Belgiansources provided a mean value of 0.37 for the h/D ratio of roundheads. Also, the button head assumption could be dismissed as ithas two radiuses of curvature [26].

Derived from measurements, the geometrical ratios D/d, h/d andR/d were calculated as the mean value of the six rivet heads percase study. The results are summarized in Table 4.2 (see columnheaders ‘Meas.’). In historical sources, the geometrical affinity ofround heads resulted in two main rough ratios: D/d and h/d,respectively equalling 5/3 and 2/3 [30,36,37]. However, the generalliterature commonly alluded to more accurate ratios – decimalnumbers instead of fractions – for which 19th-century publicationsprovide the following mean values for D/d and h/d: 1.69 and 0.62,respectively. Knowing D and h, the radius of curvature R was theneasily calculated and also expressed as a function of d (R/d = 0.86)[26]. The calculated geometrical ratios derived from measurements(‘Meas.’) show results that satisfactorily match the theoretical ra-tios for D/d, h/d and R/d. This validates the geometrical affinity onthe one hand, and the presence of round rivet heads on the otherhand. The observed slight differentials might have been causedby surface irregularities or defects. In particular, the smaller D/d ra-tio of FR-1893 could be explained by the exclusive presence of fieldheads (assumption that will be confirmed, see Section 5.2.2).

A minimal value of the h/d ratio had to be satisfied to avoid anyrivet head failure under excessive axial tensile stress (cooling/acci-dental loading). Based on Frémont’s experiments, the rough ratio ofh/d equals 2/3 proved to be safe, providing a safety margin of ca.12–15% (i.e. h/d P 0.56) [30]. For h/d < 0.56, the failure of a rivetunder tensile stress would occur at the level of the head insteadof the shank. That is the reason why rounded countersunk iron riv-et heads benefit from a higher ultimate tensile strength, given the

historical French and Belgian literature: rivet head.

h/d (/) R/d (/)

eory Meas. Theory Meas. Theory

69 0.64 0.62 0.89 0.8669 0.59 0.62 0.80 0.8669 0.73 0.62 0.90 0.8669 0.70 0.62 0.91 0.86

Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269 265

increased material and section [30,34,39]. The calculated h/d ratiosof the case studies all meet Frémont’s requirement (see Table 4.2).

From these observations, it can be concluded that D, h and R arerelevant parameters to approximate the nominal shank diameter din a non-destructive way.

4.2.3. Shop or field head?Though difficult to assess, the distinction between the shop and

the field head can reveal the localised asymmetric behaviour ofsome riveted connections within a given structural element (e.g.non-uniformly distributed bearing resistance, out-of-plane defor-mation). This phenomenon is mainly linked to the variation in con-tact conditions between the shank and the rim of the hole alongthe grip. This results from the upset of the shank during the drivingprocess.

The geometrical investigations did not allow us to reliably iden-tify the shop and the field head within the sample data. Consideredseparately, the identification is almost impossible for every sampleof a case study. However, comparing the samples within a pool ofsample data allows to potentially observe common characteristicsbetween two or more samples (e.g. d and/or D and/or h, R, e/d, etc.).The identification of the shop or field head end is even more diffi-cult for half-rivets than for entire ones (see Section 4.1.).

The complexity lies in the large amount of visible and non-vis-ible field head indicators or defects that influence the assessment(see Table 1):

� visible indicators/defects: lip around the head, circularity toler-ance DR (e.g. pitting on the head, unfilled head), unusual D/dratio (on-site choice of the rivet snap size);� non-visible indicators/defects: head eccentricity e/d, variation

of the shank diameter d along the shank, rivet hole deformation(punched hole and/or excessive driving pressure), (un)satisfac-tory contact shank-hole, grain flow.

Alone, results derived from geometrical experiments are thusnot sufficient to discuss the distinction between the shop and thefield head. Additional investigations, such as metallographic anal-yses, are needed in an attempt to draw reliable conclusions.

5. Metallography, in the footsteps of rivet driving

Analyzing and characterising the microstructure of wrought-iron rivets quantitatively highlights material inhomogeneities –even within a given case study – and provides qualitative informa-tion on the structural behaviour of the connections, especially byinvestigating the rivet driving process.

Reflecting the manufacturing process, slag is, among others, agood indicator to assess the quality of wrought iron and its ensuingmaterial properties [15,42,43]. The quality of wrought iron was in-creased thanks to repeated hot workings that refined the slag andthus improved the mechanical properties [18,43]. Previousresearchers such as O’Sullivan and Swailes [43], Bowman andPiskorowski [13] or Sparks [44] underlined the relevance to makea distinction within the structural member classes (i.e. bars, angleshapes, plates, etc.) when assessing their tensile strengths becauseof the different degrees of working they had undergone. Resultsfrom tensile test campaigns made during the second half of the19th century both in Europe (e.g. Kirkaldy, 1858–61) and northernAmerica (e.g. Watertown Arsenal, 1880–90s) reveal slightly higherUTS parallel to grain for wrought-iron bars than for plates/othershapes [13,43]. Smaller and uniformly distributed slag stringers,consequent to the greater amount of hot workings, might explainthis observation [43]. Regardless of the material origin (exceptScandinavia), toughness of wrought-iron bars was approximately

the same: mean UTS ranging from 330 MPa to 380 MPa and meanelongation at failure of ca. 22% (however together with high stan-dard deviations) [13,43]. In particular, tensile test campaigns madeat the Watertown Arsenal (Watertown, MA, USA) proved that theUTS of wrought-iron bars increased for decreasing values of bar’sdiameters ranging from 10 to 50 mm. These test records provideda mean value of 360 MPa for bars with diameters 17 and 23 mm[43]. Historical literature that was contemporary to these tests cor-roborates the above figures [10]. However, recommendationsmade by educator-engineers and theoreticians regarding the mate-rial choice for rivets and plies were heading in an opposing direc-tion. Basically, it was commonly advised to use rivets havingslightly lower or at the most the same mechanical properties asthose of the plies. This general statement can be explained bythe two following aspects: the design of riveted connections wasbased on failure modes (e.g. tearing of the plies nearby the rivetholes) and the increase of strength provided by hot-driving was aknown phenomenon. In addition, the use of ductile material forrivet manufacture was a prior concern because of the thermo-mechanical influences induced by the driving process [21].

Microstructural analyses allow to qualitatively estimate thestructural behaviour of riveted connections through the originaldriving process. According to Frémont [30], the grain flow and slagorientation reveal the forming technique of the rivet heads andshow evidence of rather manually or mechanically driven rivets(Fig. 1). Especially for short grips, machine-driven rivets benefitfrom a more efficient upset and contact between the shank andthe rim of the hole. This induces a positive influence on frictionaland bearing resistances together with an improved durability(e.g. against pack rust) [30,36]. More fundamentally, as observedby Hooper et al. [15] on the wrought-iron rivets of the RMS Titanic,the slag reorientation at the head-shank interface caused by thedriving process induces a localised decrease in toughness, that det-rimentally affects the behaviour of the connections (e.g. stressconcentration).

The issues to be dealt with by the experimental programmewere the following:

� Do the grain flow and slag orientation allow to visually identifythe original forming technique of the rivet head?� Do metallographic investigations contribute to clarify the dis-

tinction between the shop and the field head (see alsoSection 4)?� How (in)homogeneous is the microstructure of rivet samples

belonging to a given case study?

5.1. Experimental set-up

The experimental investigations regarding the metallographywere conducted on the same rivet samples as for the analysis ofthe geometry (see Section 4.1.). Two microstructural parameterswere analyzed during the experiments: slag area percentage andgrain flow. First, the polished longitudinal cross-section of each riv-et sample was photographed using a camera having a resolution of3504 � 2336 pixels. Then, each optical micrograph was treated bythe free image analysis software ImageJ that compares grey-scalevalues, which allowed the slag area percentages, present withinthe ferrite matrix, to be measured. To reveal the grain flow, the riv-et samples were chemically etched both with Nital (2%) and Oberh-offer’s etch on their polished surface during enough time for thestructure to be revealed (approximately 10 s for Oberhoffer’s etchand one minute for Nital at room temperature). Oberhoffer’s etchis usually used on wrought irons to show the distribution of phos-phorus around slag inclusions (ghost structures) [17]. We used theOberhoffer’s etch on every rivet and compared the grain flows withthose obtained with Nital. No difference in the grain flow was

Fig. 2. The original manufacturing method (shop head) and driving method (fieldhead) are clearly revealed by the grain flow and slag orientation within the heads. Aselection of manually driven rivets (FR-1893-HR-2, FR-1893-HR-4; BE-1888-HR-2)and machine driven rivets (FR-1883-ER-1A & 1B; FR-1897-ER-1A & 1B; BE-1888-HR-1) are shown.

266 Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269

expected between the two reagents. The most suitable reagent wasselected for the pictures. It appears that Nital is a softer and an eas-ier reagent to use on wrought irons. We observed that Nital’s etch-ing was particularly difficult on certain rivet samples as the grainflow was difficult to reveal. Finally, micrographs of the sampleswere taken with the same camera, to discuss the rivet drivingprocess.

5.2. Experimental results and discussion

5.2.1. Manually vs. mechanically driven rivetsThe grain flow within the rivet shank and heads is clearly visible

on the etched samples (Fig. 2, Table 3). In the shank, the presenceof an orientation always parallel to the shank axis gives evidencethat bars were used for rivet manufacture. Regarding the rivetheads, two main types of grain flows were identified: tulip shapeand barrel shape. The conducted experiments validate the distinc-tion reported by Frémont in 1906 between manually driven rivets(tulip-shaped grain flow) and mechanically driven rivets (barrel-shaped grain flow) (Figs. 1 and 2) [30]. All the rivet heads of a givencase study were formed according to one same technique: manualdriving for FR-1893 and mechanical driving for FR-1883, FR-1897,and BE-1888 (Fig. 2). As only exception disproving the rule, the riv-et sample BE-1888-HR-2 was, however, manually driven (on-siterepair, replacement, or omission). The visual comparison betweenthe BE-1888-HR-1 grain flow (barrel shape) and BE-1888-HR-2grain flow (tulip shape) makes this clear (Fig. 2). Actually, for BE-1888-HR-2, the orientation of slag reveals the original drivingprocess on its own. The use of both forming techniques withinthe BE-1888 case strengthens the need to assess a sufficient num-ber of rivet samples before drawing any general conclusion apply-ing to the whole structure. The experimental results that show acombined use of both driving techniques bring to light the actualtransitional feature of the period 1880s-1890s, the first rivetingmachine having been developed by Garforth (UK) in as early as1847 [27].

Next to the grain flow, results show a reorientation of slag with-in the rivet heads induced by the driving process (Fig. 2). The reori-entation phenomenon – orientation angle from the directionparallel to the shank – is even more pronounced for machine-dri-ven rivets, especially in the core of the rivet head (e.g. samplesFR-1897-ER-1A & 1B, Fig. 2). The mechanical behaviour of hand-driven rivets seems to be less affected by the driving process, espe-cially as the main reorientation takes place in the edges of theheads (e.g. samples FR-1893-HR-2 & 4, Fig. 2). Nevertheless, theFR-1893-HR-4 sample highlights the detrimental effect of a badlydriven field head (head eccentricity, see Table 3) on the reorienta-tion of its slag (sample FR-1893-HR-4, Fig. 2). This leads to an evenmore asymmetric behaviour of the riveted connection in service(e.g. non-uniform clamping force of the head on the ply). On theother hand, since FR-1893’s rivets were manually driven, the over-all strength and stiffness of its riveted connections are per defini-tion qualitatively lower than if they were machine driven.Accentuated by the short grip length, the high riveting quality ofthe machine-driven FR-1897 rivets is a nice example (symmetryof the grain flow, efficient upset of the shank in the rivet hole,Fig. 2). As a consequence, our investigations corroborate Hooper’sfindings on the slag reorientation phenomenon observed withinthe rivet heads [15]. The presence of regions within the rivet, i.e.the rivet heads, having lower mechanical properties is thus ob-served as well. As for wrought iron, both yield and ultimate tensilestrengths perpendicular to grain are ca. 15% lower than parallel tograin (up to 30% according to Bussell [12] and Tilly [18]), these re-gions can be considered as weaker within the rivet [15,43]. Stressconcentrations or even failure would thus be likely to occur inthe rivet heads first.

Finally, an attempt was made to identify the type of rivetingmachine (hydraulic, pneumatic) used to drive the rivets we inves-tigated. From our experience, we believe that it is almost impossi-ble to reveal this based solely on the grain flow and slagorientation. Additional information (e.g. design specifications)and lab work would be needed in an attempt to accurately definethe type of riveting machine used. In the same idea, Frémont ob-served in 1906 similar upsets for rivets driven by different typesof riveting machines [30]. Even if the bearing strength of machinedriven rivets having grip lengths exceeding 35–40 mm is not nec-essarily better uniformly distributed as for hand-driven ones [30],the frictional strength and stiffness are, however, higher. Fixed andportable hydraulic riveting machines were considered as the best

Table 5Scatter of slag area percentages within the case studies.

Case study tag Min (%) Max (%) Mean (%) SD (%)

FR-1883 1.1 1.9 1.6 0.4FR-1893 1.0 4.5 1.8 1.3FR-1897 0.05 0.4 0.2 0.2BE-1888 0.5 2.5 1.5 0.8

Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269 267

technique by past theoreticians (e.g. Leman [36]), and this is stillthe viewpoint of today’s riveting crews (e.g. Aelterman company).Widely used between 1880 and 1900, hydraulic riveting machinesinduce a higher increase in UTS of the driven rivets than pneumatichammers. Moreover, they also better improve the overall strengthof the connections [28,29]. This is particularly the case when com-paring fixed hydraulic riveting machines (field head driven in theshop) with portable pneumatic hammers (field head driven onthe building site). Actually, the riveting quality is also improvedby better working conditions (weather, installation facilities,etc.). For example, Rumpf [32] estimated the difference in UTS in-crease of driven rivets to 10% in favour of shop riveting with a riv-eting machine, compared to field riveting with a pneumatichammer.

As a result, the analysis of the grain flow and slag orientation al-lows to visually identify the original forming technique of the rivetheads, which is, in most cases, the same within one case study.

5.2.2. Shop or field head?The FR-1883-ER-1 riveted connection (heads FR-1883-ER-1A &

1B, Fig. 2) reveals major clearances between the shank and thehole. Moreover, the shank diameter d just under the head is0.5 mm smaller for the FR-1883-ER-1A head compared to the FR-1883-ER-1B head. This would indicate that FR-1883-ER-1A is theshop head. The symmetric grain flow of FR-1883-ER-1A togetherwith the absence of upset of the shank under the head confirmthe shop head assumption. The presence of these clearances andnon-uniform shank/plies contact give evidence of important bear-ing and shear stress concentrations within the plies and the shank,respectively. As no fatigue damage was observed, the frictionalstrength may have been cancelled by the slip of the plies underthe influence of the loads. Regarding the FR-1893 case, while FR-1893-HR-4 could be easily identified as a field head (asymmetry,variation of d), the better driving quality of FR-1893-HR-2 makesthe assessment more difficult (Fig. 2). However, shop heads beingpractically always machine-formed in a shop during the period1880s–1890s, the exclusive presence of manually driven headstends toward the field head assumption for each FR-1893 sample.The discussion of the FR-1897 rivets, though being entire, was dif-ficult. However, combining multiple assessment criteria allowed to

Fig. 3. Higher slag area percentage and longer slag inclusions of the

elucidate the problem. For example, the slight asymmetry of thegrain flow nearby the head edge (deviation) together with theshape of the FR-1897-ER-1B head at the head/shank interface arestrong signs of field head (Fig. 2). Indirectly, this interpretationaldifficulty is synonymous with a high riveting quality, and thus afull bearing capacity of the FR-1897 connections. For the BE-1888case, both BE-1888-HR-1 and BE-1888-HR-2 samples are fieldheads. This observation is based on the decreasing values of theshank diameter d starting from the shank/head interface, the headeccentricity and the head shape (Table 3, Fig. 2).

Provided that a large number of parameters and assessmentmethods are considered (measurements, etching), the distinctionbetween the shop and the field head appeared to be possible butcomes with uncertainties, regardless of whether the studied rivetsare half or entire. Next to the three main geometrical clues (headeccentricity e/d, variation of the shank diameter d, shape of thehead), the (a)symmetry of the grain flow and slag orientation with-in the head and in the shank nearby the head is a reliable assess-ment parameter. Paradoxically, a difficult distinction between thetwo heads of an entire rivet is a strong sign of high riveting qualityand efficient structural behaviour. More fundamentally, the rele-vance of discussing the head type depends on the total grip lengthof the connection. In general, a short grip will benefit from a betterriveting quality, as it was emphasised by the contrast between FR-1897 (short grip, symmetric behaviour) and FR-1883 (long grip,asymmetric behaviour).

5.2.3. Microstructural inhomogeneitiesSlag area percentages measured on the rivets’ cross-section are

provided in Table 5. While the amount of slag for FR-1897 is verylow, the FR-1893 case shows higher values, especially due to onepeculiar rivet sample: the FR-1893-HR-3 head (4.5%, i.e. FR-1893’s maximal value; Table 5). The visual comparison – almostto the naked eye – between FR-1893-HR-3 and FR-1893-HR-2 sam-ples makes it clear (Fig. 3). The high percentage, bad dispersion andpresence of long slag inclusions in FR-1893-HR-3 might detrimen-tally affect the ductility of the rivet [42]. This could be explained byan insufficient working of the used wrought iron bar during itsmanufacturing; repeated working caused the slag inclusions tobe refined [43]. In addition, an important scatter of the slag areapercentages was observed within each case study (Table 5).

Analyzing the microstructure of wrought iron is, among others,a valuable proxy to assess its mechanical properties and materialquality [42]. In particular, excessive and non-uniformly distributedslag within the ferrite matrix has a negative impact on themechanical properties [42,43]. Previous research showed that theductility of US wrought irons decreases as the phosphorus contentof the ferrite increases and/or in presence of excessive slag [42].

FR-1893-HR-3 sample (right) compared to FR-1893-HR-2 (left).

268 Q. Collette et al. / Construction and Building Materials 54 (2014) 258–269

As a consequence, measurements of the slag area percentage re-vealed large variations between rivet samples belonging to a givencase study, although they were generally formed through one samedriving technique (manual or mechanical). Next to the slag orien-tation discussed in Section 5.2.2., the slag area percentage of rivetsamples’ cross section is thus a convenient parameter to assesswithin a renovation project.

6. Conclusions

In 2010, professional engineer S. Patrick Sparks supported bythe expertise of Vernon J. Mesler in structural riveting pinpointedthe importance of keeping the original construction practicesregarding the restoration of the 1881/1910 Hays Street bridge inSan Antonio (TX, USA): ‘‘ [...] the team (riveting crew) used tradi-tional hot riveting methods rather than, as is more conventional,replacing the rivets with bolts.’’ [45]. The research approach andaims presented in this paper are in line with the underlying philos-ophy of this quote: the need to revive the hot-riveting technique topreserve the historical significance of wrought-iron structures. Inaddition, riveted connections have some advantages over boltedconnections. When properly done, they can guarantee a sustain-able secure fit of the connected plates, which is essential for struc-tures under cyclical loads for instance (e.g. self loosening of bolts).The clamping force induced by the hot-riveting technique is alsohigher, on average, than for prestressed bolts.

In this regard, the structural behaviour of hot-driven rivets re-moved from four French and Belgian wrought-iron structureswas qualitatively assessed (1880s–1890s). The measurementsand results of experimental investigations – geometry and metal-lography – were discussed and confronted with historical litera-ture to reveal the original design, geometry, manufacturing anddriving techniques of these rivets. Such information constitutesthe indispensable input of any static and/or fatigue assessment,or even of a finite element analysis of riveted connections. The rel-evance and usefulness of the observations relied primarily uponthe complementarity of geometrical and metallographic investiga-tions, each being insufficient on their own. With very few excep-tions (e.g. the d/e ratio via the plate thickness e), the fact that thetest results validate the content of historical literature – rivetingpractice and theory – establishes the status of this literature as abody of knowledge.

To identify the nominal shank diameter d, notably needed whenassessing the shear strength of the connections, either the geomet-rically affine features (head diameter D, head depth h, and radius ofcurvature R) or the measurements of d along the shank can beused. However, though non-destructive, the former method is lessaccurate than the latter, which is the one we recommend (only fordrilled or reamed holes though, non-reamed punched holes beingcone-shaped).

For grip lengths exceeding 35–40 mm and regardless of theused driving technique, the identification of the shop and fieldhead might testify of asymmetric clamping forces applied by thehead on the plies and asymmetric bearing strength (contactshank/rim of the hole). Here the head eccentricity e/d, variationof the shank diameter d, shape of the head (head/ply interface,form of the lip), and the (a)symmetry of the grain flow and slag ori-entation are valuable parameters to assess. A difficult or almostimpossible shop/field head distinction can indirectly be the signof a high riveting quality. Additionally, the presence of large clear-ances between the shank and the rivet hole together with plieshaving slipped reveals both shear and bearing stress concentra-tions, that is, a decrease in the overall strength of the joint.

Regarding the driving technique, the grain flow and slag orien-tation within the rivet head gives evidence of the original forming

process. Given the transitional feature of the period 1860s–1900s(i.e. use of both manual and machine driving), a destructive assess-ment appeared to be the only way to draw reliable conclusions. Inparticular, a manually-driven rivet head of a 1880s–1890s struc-ture is a strong clue of field head. Under equal conditions, the over-all strength and stiffness of machine-driven rivets is higher thanfor hand-driven ones. However, these factors must be consideredon a case-by-case basis since the grip length and upset of the shankin the hole basically depend on the driving quality.

Finally, the high scatter of slag area percentages within a givencase study stresses the need to assess a sufficient number of rivets.

To further this research, additional investigations such as chem-ical analyses, microstructural analyses (ferrite grain size andshape), surface condition of the rivet holes, parametric study ofthe driving technique, calculation of inner clamping force and fric-tional strength, mechanical characterisation, etc. would also bebeneficial to better understand the structural behaviour ofwrought-iron riveted connections.

Acknowledgments

The research presented in this paper is funded by the ResearchFoundation - Flanders (FWO Vlaanderen, Belgium). The authorswish to thank LBMS (University of Brest, France) and MEMC (VrijeUniversiteit Brussel, Belgium) departments for the use of labora-tory facilities. Additional thanks are given to Jean-Luc Brugmans(Belgian Buildings Agency, Belgium), Brest Métropole Océane(BMO, France) and the National Society of French Railways (SNCF)for the use of samples for this study, as well as Chris Aelterman(Aelterman, Belgium) for his experience in riveting practices. Theauthors also wish to thank Dario Gasparini (Case Western ReserveUniversity, OH, USA), David A. Simmons (Ohio Historical Society,OH, USA), Bill Barrow (Cleveland State University, OH, USA), SteveHowell (Ballard Forge, Seattle blacksmith, WA, USA), Jeff Dever(Michigan Pneumatic Tool Inc., MI, USA) and Christophe Cellard(University of Brest, LBMS, France) for their kind help. Finally, theauthors are grateful to Irenka Schmalzried (Nashville, TN, USA)for her review.

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