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DESIGN CONCEPT OF NAILED SHEAR CONNECTIONSIN COMPOSITE TUBE COLUMNS

Gerhard Hanswille*, Hermann Beck** and Till Neubauer**Institute of Steel and Composite Structures, University of Wuppertal, Germany**Hilti Corp., Direct Fastening Development, Principality of Liechtenstein

Nailed shear connections are a new alternative type of shear connection in compositecolumns with concrete filled circular and rectangular tubes. The main advantage of anailed shear connection is, that it can be applied from the outside of the tube without anyprefabrication efforts. Based on the behaviour derived experimentally from push-outtests, proposals for the design resistance of nails for ultimate and serviceability limitstates are made, providing provisions with regards to minimum steel thickness, concretegrade and nail spacing are fulfilled. In order to get a more realistic knowledge of thedeformation behaviour in serviceability limit states, the combined shear resistance of thenails and the shear strength due to bond and friction and including long-term effects hasto be considered. The background of a corresponding test program will be discussed.

1. Introduction

Nailed shear connection in composite tube columns was introduced in practice in 1998and 1999 in buildings in Austria and Germany [1], [2]. The design of the nailed shearconnection for these projects was based on experimental work performed within theVHF-Research Project [3], [4]. Additional tests were also performed by HiltiCorporation [5] in order to investigate the load-slip behaviour of the nailed shearconnections for general conditions.

The objective of this paper is to introduce the design method for nailed shearconnections in buildings for a limited area of application. A design resistance for theultimate limit state will be proposed. All the push-out tests described in [3] to [5] wereperformed to evaluate conservative resistances for the ultimate limit state. Therefore, theinside surface of the tube specimens was always lubricated prior to concreting, in orderto eliminate load contributions of chemical bond and friction in the interface between

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concrete and steel. However, neglecting chemical bond and friction completelyoverestimates deformations in the serviceability limit state, leading to inefficientutilisation of the nailed shear connections. To gain a more realistic knowledge of thebehaviour in the serviceability limit state, an additional test program was worked out.Backgrounds of this new program will be introduced, in addition results of ongoingcreep tests will be presented. Finally the paper will provide an example of aconstructional detail with the corresponding design equations for the ultimate and theserviceability limit state.

2. Nailing method

High strength nails with smooth shanks are driven through the tube wall from the outsideusing a powder-actuated tool. No predrilling of the tube is required to enable thepenetration process. The front sections of the nails protrude into the inside of the hollowtube. After the tube has been filled with concrete, the nail shank acts as shear connectorbetween the concrete core and the hollow tube. The method is applicable for both pipesand rectangular hollow sections.

Embedment Depth

Powder-actuated fastener:Hilti X-DSH32 P10fu = 2200 N/mm²Diameter = 4.5 mm

Powder-actuated tool:Hilti DX750G,Power loads and toolenergy settings dependenton strength and thickness ofthe tube to finally achieveflush installation.

Fig. 1. Principle of method

3. Push-out test results

Fig. 2 shows examples of typical load-slip behaviour of nailed shear connections derivedfrom push-out tests. With regards to details of the performed test program, it is referredto [4] and [5]. At great slips a high load-bearing capacity per fastener is attainablereflecting excellent ductile behaviour. Due to concrete confinement inside the tube, theconcrete develops high local compressive stresses amounting a multiple of the uniaxialconcrete compressive strength. As the overall behaviour is significantly affected by localconcrete deformations, the load-slip curves typically exhibit a parabolic shape. From theexamples in Fig. 2 the effect of the concrete strength can be clearly observed. Withincreasing concrete strength, the ultimate capacity of the nailed connection increases aswell as a greater initial stiffness combined with smaller initial deformation is given.

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Additional tests with specimens without nails proved that the lubrication of the innerpipe surface was completely effective as only negligible small ultimate values (in total16.7 kN, 0.6 kN per nail) developed in these control tests. Nevertheless, the ultimateloads generally exceed significantly the total shear load capacity of all nails installed, asindicated in Fig. 2. Caused by the lateral restraint of the local concrete deformations [5],a significant local compression between the pipe and the concrete develops (see Fig. 3),which is finally responsible for the additional friction load contribution.

Pipe: d = 508 mm, wall thickness = 8.8 mm

0100200300400500600700800900

10001100

0 2 4 6 8 10 12 14 16 18 20

Slip [mm]

P [k

N]

C20/25

C45/55

Level of nail shear strength

24 nails per specimen

Fig. 2: Examples of load-slip curves from push-out tests [5]

Fig. 3. Inside view of push-out test specimens: Right: Detail of a bent nail;Left: Bent nails and visible dark areas of lateral compression at the location of each nail.

Dark areas caused bylateral compression

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4. Design concept

The basic idea of the design concept is to provide constant values for the design shearstrength for the ultimate and the serviceability limit states in combination with thefollowing scope and detailing provisions:

• circular and rectangular tubes with no limits in the outside dimensions,• wall thickness: 5.6 to 12.5 mm. For higher strength steel grades, the application

restrictions with regards to installability of the nails have to be consideredaccording to manufacturers provisions,

• limitation of diameter/thickness ratios [6] to avoid local buckling of steel tubes,• concrete strength classes not lower than C30/37 and maximum aggregate size

not exceeding 16 mm,• minimum spacing between the fasteners not less than 50 mm in vertical and

lateral direction,• minimum distance of 20 cm to a possible concrete joint,• installation of the X-DSH32P10 according to manufacturers specifications.

4.1 Ultimate limit stateConsidering the restrictions above, the design resistance for the ultimate limit state canbe based on the steel shear strength of the nails. For the X-DSH32P10 a designresistance was determined from test results in accordance with Annex Z of Eurocode 3.A characteristic value Rk = 21 kN and a design value Rd = Rk / γv = 16.8 kN per nail wasevaluated [6], where the partial safety factor is given by γv = 1.25. This pragmaticconservative approach is justified as follows:

• Contribution of chemical bond and large area friction between tube and concretewas virtually completely excluded in the push-out test.

• In all tests in the defined scope, the ultimate loads in the push-out tests weregenerally significantly greater than the ultimate shear capacities of the nails. Thisbeneficial effect is explained with local frictional forces developing owing to therestriction of the local concrete deformations in the compression area to the nail,comp. Fig. 3.

• In [4] tests have also been performed with very low strength concrete (fc = 15.7N/mm²) utilising recycled aggregates. Also in such low strength concrete, theultimate load was in the range and beyond the total shear capacity of all installednails. However, with regards to serviceability and also practical relevance, concretestrength Class C30/37 (fc = 30 N/mm²) was selected as minimum grade.

Utilising the high ultimate loads evaluated in the tests for practical design, would requirean accurate theoretical knowledge of the combined influence of the shape anddimensions of the hollow sections and the concrete strength. However, due to thesignificant slip, these higher values could not be utilised at the end, becauseserviceability requirements would govern design. Furthermore the design methods forcolumns are based on the assumption that the composite section remains plane.

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Therefore in the load introduction area of columns excessive slip at the interfacebetween steel and concrete must be avoided.

4.2 Serviceability limit stateWith regard to serviceability limit state requirements, the slip at the interface betweenthe steel tube and concrete must be limited. Based on the experience with other types ofshear connection like headed studs or gusset plates trough the profile, slip between steeland concrete should be limited to values of approximately δ = 0.5 mm in order to avoiduncontrolled redistribution of the sectional forces of the column. The load slip curveaccording to Fig. 2 demonstrates that this serviceability criterion can govern the designof nailed shear connectors.

In [5] a proposal was made to limit the design resistance for the serviceability limit statewith 8.5 kN per nail. However, that value results in an inefficient utilisation of the nails,because it was evaluated from push-out tests in which chemical bond and large areafriction was excluded due to lubrication of the inner tube surface. However, with regardto the serviceability limit state neglecting effects of bond strength and friction in theinterface between steel and concrete seems to be a too conservative approach.

5. Ongoing experimental investigations on serviceability limit state

5.1 Slip at the serviceability limit stateBond and adhesion in combination with friction between steel and concrete wasextensively investigated in the past (for example [7]), resulting in corresponding designprovisions. Push-out tests typically show a peak load corresponding to the load at whichbond and adhesion is destroyed. With increasing deformations the load drops to theremaining level of effects of friction. Because the ultimate capacity of bond andadhesion shows a great scatter, it is not considered explicitly in design. Only effects offriction can be utilised. Literature [7] further indicates, that deformations beforedebonding are very small. Basically no slip in the interface was measured for loads up to70 percent of the ultimate load.

Therefore, an additional test program was worked out to investigate the combinedbehaviour of effects of friction with the nailed shear connection. The principle set-up ofseries I (Fig. 4) corresponds with those in the former tests. The load will be introducedinto the concrete and has to be transferred into the uniformly supported pipe. For thepurpose of this investigation, the symmetric test set-up I with an axial load introductionrepresents the most critical practical situation. To measure realistic initial deformations,no lubrication of the inner tube surface will be made. The introduction length is selectedwith 2.5-times the pipe diameter following the provisions given in [8]. The loadsequence is selected in such a way to describe a practical situation realistically. Theprimary objective of these tests is to verify a design load PSLS for serviceability limitstate verifications of approximately 12 kN per nail. Therefore, the specimen will beloaded in the first step up the target working load PSLS (compare Fig. 4) and relieved to

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zero to measure remaining deformations. Afterwards 100 load cycles varying between0.5 and 0.8-times of PSLS will be performed to simulate live loads. Then the specimenwill be relieved again followed with the loading up to failure.

323,9

50

Pipe 323.9/6.3, S235

X-DSH32 P10

Air gap

810Concrete:

C30/37

Series I Series II(schematic)

Type A: 16 nails: PSLS = 16 . 12 = 192 kNType B: 32 nails: PSLS = 32 . 12 = 384 kN

Fig. 4: SLS-Tests: Series I and II: Set-up and load sequence

Tests with two different numbers of nails (16 and 32) will be performed. It is knownfrom [7], that the peak strength of bond and adhesion varies in a range between 0.45 and1.4 N/mm². That means that for the given geometry the load before debonding may varybetween 350 and 1100 kN. Test series I comprises also control tests without any nails.With the results from the control tests and those from the two different nail patterns, thecombined behaviour will be known for this specific configuration. Therefore, the testsserve as the experimental basis for a numerical generalisation of the combined behaviourin the service state, in which just the allowable frictional bond will be utilised.

Series II differs from series I in the load introduction and the specimen support. The loadintroduction corresponds to the practical situation in which to load is introduced into thetube by a fillet welded vertical gusset plate. The objective of these tests is to investigatethe plastic load bearing behaviour of the composite column. Specifically shall beclarified, if the actual pipe deformations are high enough to activate all installed nails.The number of nails was selected in such a way, that the plastic resistance of thecomposite section can be achieved.

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5.2 Long-term effectsTests have been performed in order to investigate the long-term behaviour and influenceof creep of concrete of the nailed shear connections. The test set-up of the creep testsperformed at Hilti Corporation is shown in Fig. 5. Additional tests have been performedat the University of Innsbruck [9] using basically the same test set-up and specimenconfiguration. Only four nails were used per specimen, as the specimen size had to bekept smaller with regards to test execution. In order to eliminate potential effects ofbond, adhesion and friction, the inner pipe surface was again lubricated. The load wasapplied at an age of concrete of eight days.

Spring tensionedpressure device

M16 thread

2 displacement gages

Centric threadedrod M16

ca. 2 cm air gap

300

4 nailsX-DSH32 P10

3030

Lubricated innerpipe surface

Plastic pipewith gap to theanchor rod

Pipe 219 / 6,3

240

Circular plate d = 195

180

100

20

240

Fig. 5: Test set-up for long-term tests

Table 1: Parameter and results of long-term testsTest at # fc at

loading (8days)

[N/mm²]

fc at 28days

[N/mm²]

Load Pper nail

[kN]

Initialslip

[mm]

deformationafter 90

days [mm]

deformationafter 395

days [mm]

Hilti 1 27.8 44.4 10 1.04 0.76 0.882 27.8 44.4 10 1.28 0.75 0.853 27.8 44.4 5 0.56 0.24 0.29

Innsbruck 4 na 47.2 15 na 0.03 -5 na 47.2 15 na 0.03 -

In comparison with the measurements from the push-out tests [4], [5], the initial slips aregreater. Reasons for that behaviour might be the small concrete age of 8 days at loadingand the small number of just four nails. It is assumed that with increasing nail number

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the stiffness per fastener also would increase caused by the fact, that the contact areabetween nail and concrete is of the same order as the concrete aggregates themselves.Fig. 6 shows the development of the creep deformations. Up to approximately 10 daysthe deformation exhibits a parabolic shape, beyond which the development of thedeformation flattens significantly. The creep deformation after 10 days amounts toapproximately 75 percent of the value measured after one year of loading. Consideringthe range of permanent dead loads, creep deformations derived from test #3 amount toapproximately 50 percent of the initial slip.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

1,1

1,2

0 50 100 150 200 250 300 350 400

Duration of loading [days]

Cre

ep d

efor

mat

ion

[mm

]

Test #1 Test #2 Test #3Test #4 Test #5

Fig. 6: Deformations derived from long-term tests #1 to #5

Interestingly the tests performed at the University of Innsbruck led to complete differentresults. As basically the same test equipment was used as in the tests 1 to 3, it wasspeculated that bond, adhesion and friction was not adequately prevented by thelubrication measurement of the inner tube surface. Therefore, the tests were stopped after90 days to perform push-out tests with these specimens, followed by a visualinvestigation of the inner tube surface. From these visual comparison it was clearly seen,that bond and adhesion and large area friction was partially active. That incompleteprevention of natural bond explains that even with the greater load of 15 kN per nailbasically no creep deformations were recorded.

This observation is a further indication, that natural bond effects shall be considered fora realistic and economical assessment of the slip characteristics in the service state. Theresults of the push-out tests summarised in Table 2 further support that assumption. Asnatural bond was partially active the achieved loads at a slip of 0.5 mm are significantlygreater than the target value of 12 kN per nail for the serviceability limit state. In thesepush-out tests no real load maximum was observed within the recorded slips up to 35mm, indicating that already a few number of nails will change the characteristics ofbond, adhesion and large area friction completely.

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Table 2: Result of push-out tests of creep specimen #4 and #5Test load P [kN] per nail at slip of

Test at # fc [N/mm²] 0.5 mm 30 mmInnsbruck 4 47,2 30.7 170.0

5 47,2 20.0 172.5

6. Worked design example

Fig. 7 shows a typical example of nailed shear connection as part of a composite joint.The load is introduced into the column by a fillet-welded gusset plate connected on theoutside of the pipe. Generally the nails can be uniformly distributed along thecircumference of the pipe.

Fig. 7: Example of composite beam column joint

Table 3: Summary of design equations, load introduction into steel tubeUltimate limit state (ULS) Serviceability limit state (SLS)

ηULS FSd ≤ n RDSH,Rd,ULS

Rd,pl

sdscdcULS N

fAfA +=η

Npl,Rd = Aa fyd + Ac fcd + As fsd

ηSLS Fk ≤ n RDSH,Rd,SLS

i

socSLS A

An/A +=η

Ai = Aa + Ac/n0 + As

Composite tube columns ∅ 508 x 6,3 S235; C45/55; 8∅28 S 500Nails X-DSH 32 P10; Rk = 21,0 kN, RDSH,Rd, ULS = 21,0 / 1,25 = 16,8 kNAa = 99,3 cm² As = 49,3 cm² Ac = 1878,2 cm²Npla,Rd = 2121,4 kN Npls,Rd = 2143,5 cm² Nplc,Rd = 5634,6 kNNpl,Rd = 9899,5 kN no = 21000 / 3600 = 5,8Two composite beams with ΣFG = 530 kN (permanent load), ΣFQ = 320 kN (variableload)

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FSd =1,35 ⋅530 + 1,50 ⋅320 = 1195,5kN

79,05,9899

6,56345,2143ULS =+=η

required number of nails:nULS = 0,79 ⋅ 1195,5 / 16,8 = 56,2

Fk = 530 + 320 = 850,0 kN

79,03,993,498,5/2,1878

3,498,5/2,1878SLS =

+++=η

required number of nails:nSLS = 0,79 ⋅ 850 / 12 = 56,0

ref. 3 ⋅ 20 = 60 X-DSH 32 P10withFk ... Characteristic value of the total support reactionFSd .. Design value of the total support reactionRDSH,Rd,ULS =16,8 kN Design resistance per X-DSH32P10 for the ULSRDSH,Rd, SLS =12,0 kN Design resistance per X-DSH32P10 for the SLSn .... total number of nails per jointNpl,Rd ... Plastic resistance to compression of a composite cross-section according to [8]Ai ... Cross-sectional area of the composite cross-section based on steel propertiesn0 ... Modular ratio for short term loading

7. References

1. Tschemmernegg, F., ´Innsbrucker Mischbautechnologie im Wiener MillenniumTower´, Stahlbau 68, Heft 8, (1999), 606-611

2. Angerer, T., Rubin, D., Taus, M., ´Verbundstützen und Querkraftanschlüsse der Ver-bundflachdecken beim Millennium Tower´, Stahlbau 68, Heft 8, (1999), 641-646.

3. Tschemmernegg, F., Beck, H. ´Nailed shear connection in composite tube columns´,(1998) March 1998 ACI Convention, Houston, Session on Performance of Systemswith Steel-Concrete Columns

4. Larcher, T.Z., ´Versuche zur Krafteinleitung der Trägerauflagerkräfte beiHohlprofilstützen mit Setznägeln´, (1997), Diplomawork at the Institute of Timberand Mixed Building Technology, Leopold-Franzens-University of Innsbruck

5. Beck, H., ´Nailed shear connection in composite tube columns´, (1999) Proceedingsof the Conference Eurosteel ´99, Prague, 26-29 May 1999

6. Hanswille, G., Neubauer, T., ´Zulassungsantrag beim DIBt. Stellungnahme für dieNägel X-DSH32P10 zum Einsatz bei betongefüllten Hohlprofilen alsVerbundmittel´.

7. Roik, K., Breit, M., Schwalbenhofer, K., ´Untersuchung der Verbundwirkungzwischen Stahlprofil und Beton bei Stützenkonstruktion´. (1984) Projekt 51, Institutfür konstruktiven Ingenieurbau, Ruhr-Universität Bochum.

8. DIN 18800-5, ´Stahlbauten: Teil 5: Verbundtragwerke aus Stahl und Beton,Bemessung und Konstruktion´. (1999), Draft, January 1999

9. Frischhut, M., Michl, T., ´Versuchsbericht: Hilti-Nägel im Verbundbau,Langzeitverformungen und Late-Push-Out´, (2000), Test report 13.09.2000, Instituteof Timber and Mixed Build. Technology, Leopold-Franzens-University of Innsbruck