Review of Tubular Joint Criteria

REVIEW OF TUBULAR JOINT CRITERIA

P.W. Marshall, Moonshine Hill Proprietary, Texas

ABSTRACT This note reviews and compares four sets of tubular connection design criteria for axially loaded circular tubes. The four criteria are AWS D1.1, API RP2A, ISO/WD 15-1.2, and ANSI/AISC 360-05.

INTRODUCTION The existing American design codes for welded tubular connections are AWS D1.1 Structural Welding Code (1990 thru 2002) and the substantially identical AISC Specification for the Design of Steel HSS (1997), the basis of which is documented in the author’s book (1). Three new sets of design criteria are in the works. They are: (1) Proposed update to API RP2A, Design... of Fixed Offshore Platforms, based on research conducted by Prof. Pecknold at the University of Illinois, and sponsored the API Offshore Tubular Joint Research Consortium. A lengthy Commentary is included to self-document these criteria. Extensive nonlinear finite element analyses were used to extend the experimental data base, particularly in the areas of overlapped K-joints, a moment-free baseline for T-joints, and chord stress interaction for a wide variety of joint types and loadings. In view of reduced scatter compared to existing criteria, a reduced WSD safety factor of 1.6 is proposed. This update has been approved for publication in the 22nd edition of RP2A, and is in the final stages of editing. (2) Static Strength Procedure for Welded Hollow Section Joints, IIW doc XV-E-03-279, based on CIDECT research. This is also on the fast track to becoming an international standard as ISO/WD 15-1.2, with IIW commission XV as secretariat. The immediate purpose of this note was to provide comments for a Sept. 2003 meeting of IIW s/c XV-E. (3) ANSI/AISC 360-05, Standard Specification for Structural Steel Buildings (draft of August 20, 2003), Chapter K, HSS Connections, being prepared by an ad hoc task group under the direction of Larry Kloiber. This is essentially the same as the CIDECT-based IIW document, except that it gives the characteristic ultimate strength without hiding a partial safety factor therein. Separate safety factors are then given for LRFD and ASD. COMPARISON OF THE DESIGN EQUATIONS Principal results of this review are shown in the Tables and Figures. Table 1 gives a side-by-side tabulation of the design criteria for different types of circular joints. The square bracket term is Qu in API, and simply written out in IIW and AISC. AWS criteria have been converted to this format for comparison.

Connections in Steel Structures V - Amsterdam - June 3-4, 2004 457

Table 1. Design equations.

CIRCULAR TUBE JOINTS -- AXIAL LOAD

AWS D1.1-92 and AISC ‘97 HSS Connection Manual

API RP 2A-WSD Offshore Platforms 22nd edition

CIDECT IIW-XV-E-03-279 ISO/WD 15-1.2

AISC 360-05 Chapter K Kloiber draft 8/03

General format Pu sin θ = to2 Fyo [*] Qf [*] defined below FS Pa sin θ = T2 Fyo [*] Qf Pn sin θ = to2 Fyo [*] Kp Pu sin θ = t2 Fyo [*] Qf

T & Y joint [*] = (18.8β + 3.4) √Qβ [*] = 2.8 + (20 + 0.8γ) β1.6

but ≤ 2.8 + 36β1.6 [*] = (2.8 + 14.2β2) γ0.2 [*] = (3.1 + 15.6β2) γ0.2

K & N gap [*] = 32β/α + 3.4 with α = 1.0 + 0.7 g/di ≤ 1.7

[*] = (16 +1.2γ) β1.2 Qg ≤ 40 β1.2 Qg Qg = 1+ 0.2(1-2.8g/D)3 **

K & N overlap ( )

)(2.3

)(4.332[*]

2

1

qfF

Fqf

y

EXXτβγ

β

+

+=

[*] as for gap with Qg = 0.13 + 0.65 φ √γ ∗∗ and φ = (tb/Fyb) / (toFyo)

[*] = (1.8 + 10.2β) Kg

( )⎥⎥⎦⎤

⎢⎢⎣

⎡

−++=

33.1g/t5.0exp1024.01

0

2.12.0 γγgK

[*] = (2.0 + 11.33β) Qg with Qg same as CIDECT Kg and g negative for overlap

X joint [*] = (13.3β + 3.4) Qβ

Also can length effect [*] = [2.8 + (12 + 0.1γ) β] Qβ

Stronger alternate for tension [ ]

β81.012.5*

−= [ ]

β81.017.5*

−=

K-T joint See general multiplanar

Use gap between loaded diagonals

As for K with eff. β = ∑ βi / 3 Special combo rules & missing cases

Not covered

K-K joint (delta truss)

See general multiplanar (no increase over K joint) Not covered Check chord gap for shear + axial

(no increase over K joint) Not covered

General multiplanar

[*] = (32β/α +3.4) Qβ0.7(α -1)

with α per Annex L Commentary reference To AWS method

X-X joints covered General case not covered Not covered

Chord load effect Qf or Kp

Tension or compression Qf = 1 - 0.03 λ γ Ū2

Ū2 = [(P/Py)2 +(M/My)

2]

Comp.(+), tens.(-) at footprint 2

yyf U

MM

PPQ 3211 CCC −−−=

Compression only Kp = 1 – 0.3 Ū (1+ Ū)

Ū = P/Py + M/My

Same as CIDECT

Other terms ( ) 6.0833.13.0

>−

= βββ

forQβ

** Qg not defined for 0.05 > g/D > -0.05 Pn includes resistance factor Pu is characteristic

ultimate

458 Connections in Steel Structures V - Amsterdam - June 3-4, 2004

0.0 0.5 1.0q

0.5

1.0

0.0

f 2(q)

Details of the AWS conversion to international format for overlap K&N joints are given below, where q = percent overlap as a fraction. In the existing Code, vp is the allowable punching shear and the allowable capacity normal to the chord is given by... Allowable Pn sin θ = vp to L1 + 2 vw tw L2 For conversion, the following substitutions are made...

L1 = π β D f1(q) (partial footprint arc) vw = 0.3 FEXX (allowable) L2 = β D f2(q) (lap weld ┴ to chord) vw = 0.8 FEXX (ultimate) with the resulting international format given by...

Ultimate Pu sin θ = to2 Fyo [*] Qf with ( ) )(2.3)(4.332[*] 21 qfF

Fqfy

EXXτβγβ ++=

In Figure 1, f1 and f2 shown for 45º N-joint with β < 0.5. These values were obtained graphically from a scale layout of joints with varying degrees of overlap.

Figure 1. Layout and functions f1 and f2 for overlapping 45º N-joints with β=0.5. It may be noted from Table 1 that the AWS criteria cover a wider variety of design situations than the others, with particular reference to the general multi-planar case. The proposed AISC criteria cover the least, in a deliberate effort to minimize complexity.

0.0 0.5 1.0

0.5

1.0

0.0

q

f 1(q)

q(βD)

L2

L1


RESULTS Figures 2-5 show parametric comparison of the criteria for T&Y joints and X joints, which was performed on an Excel spreadsheet. The base case is beta of 0.5, tau of 0.5, and gamma of 20. The upper plots show the effect of varying beta, with the other parameters kept at the base case. The author’s 1969 and 1975 criteria were subsequently shown to be un-conservative for X-joints, but they are very close to the latest OTJRC results for T&Y joints. The lower plots show the effect of varying gamma; there is no effect if the T2Fy format tells the whole story. There was no effect of varying tau in any of these cases.

Figure 2. Effect of β for X-joints with γ=20.

Figure 3. Effect of γ for X-joints with β=0.5.

variations for X joints

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2

β

Qu

AWS & AISC Hdbk API proposal AISC proposal

variations for X joints

0

5

10

15

20

0 5 10 15 20 25 30 35

γ

Qu



Figure 4. Effect of β for T & Y joints with γ=20.

Figure 5. Effect of γ for T & Y joints with β=0.5.

variations for T & Y joints

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1 1.2

β

Qu

AWS & AISC hDBK API proposal AISC proposal

variations for T & Y joints

0

5

10

15

20

0 5 10 15 20 25 30 35

γ

Qu



Figures 6-8 plot plots results for K&N joints. The large-scale plot in Figure 6 shows the effect of gap or overlap for base case beta, tau, and gamma. The single expression in the CIDECT-based AISC proposal, covering both gap and overlap, matches AWS in the gap range, and matches API finite element results in the overlap range, at least for the base case parameters.

Figure 6. Effect of gap/overlap for 45º N-joint with β=0.5, τ=0.5, and γ=20. Figures 7 and 8 show the effect of varying the parameters β, γ, and τ at 60% overlap and 0.1D gap, respectively. Here we see that the proposed AISC (and IIW/CIDECT) criteria completely miss the strong effect of tau in the overlap region, as predicted by the AWS strength-of-materials approach and confirmed by the API finite element studies. They also appear to under-predict the beneficial effect of large beta.

K & N joints

0

10

20

30

40

-1 -0.5 0 0.5 1

(overlap) g/D (gap)

Qu



Figure 7. Effects of β, γ, and τ for overlap N-joints with g/D = -0.3.

overlap joints

0

20

40

60

80

0 0.2 0.4 0.6 0.8 1 1.2

β

Qu


overlap joints

01020304050

0 5 10 15 20 25 30 35

γ

Qu


overlap joints

0

10

20

30

40

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

τ

Qu



Figure 8. Effect of β, γ, and τ for gapped N-joints with g/D = 0.1.

gap joints

0

20

40

60

80

0 0.2 0.4 0.6 0.8 1 1.2

β

Qu

overlap joints API proposal AISC proposal

gap joints

0

10

20

30

0 5 10 15 20 25 30 35

γ

Qu


gap joints

0

10

20

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

τ

Qu



Finally, Figure 9 shows the chord load effect, with the reduction factor called Qf in US specs and kp internationally. All criteria pass through unity at zero chord load. Based on Yura’s tests, AWS criteria show the effect of chord buckling tendencies at large gamma, a feature which is missing from the other criteria. The API-OTJRC finite element parameter studies show quite different results for different brace types and loadings, and are represented in the criteria by a quadratic expression with tabulated coefficients; linear terms tilt the parabolas. AISC and IIW proposals simplify this to a single line, which is reasonable representation of the effect on joint strength at chord service loads, for most joint types. The notable exception is equal diameter X joints with compression in the branches, where biaxial membrane stresses at the saddle position make tensile chord loads detrimental and compressive chord loads slightly beneficial.

Figure 9. Reduction factor (kp or Qf) versus chord utilization. DISCUSSION Q. What are conclusions of the comparison? A. When comparing existing AWS-AISC criteria for circular tubular connections to CIDECT, both in 1992 and today, neither criteria appear to have significantly different errors on the unsafe side. Thus, one may ask the following questions: “Why churn the Code by adopting essentially similar criteria but in a different format?” “Why not look at new API results having a more significant impact on reliability?” The issue is not simply whether or not to maintain the American status quo. It is important to keep the Codes evergreen in the sense that they reflect the latest data, with researchers still

0

0.4

0.8

1.2

-1 -0.5 0 0.5 1

new API K-axial axial new API K-axial IPBnew API T/Y axial axial new API T/Y comp IPBnew API X <.9 axial new API X =1 com ax ialnew API bending axial new API bending IPBAWS = 20 axial combined AWS = 20 IPB combinedAWS = 20 OPB combined new AISC all cases all cases

0

0.4

0.8

1.2

-1 -0.5 0 0.5 1

new API K-axial axial new API K-axial IPBnew API T/Y axial axial new API T/Y comp IPBnew API X <.9 axial new API X =1 com ax ialnew API bending axial new API bending IPBAWS = 20 axial combined AWS = 20 IPB combinedAWS = 20 OPB combined new AISC all cases all cases

γ

β

γ

γ

β


active into the future who remember where it all comes from. Q. There are obviously parameters that are treated very differently (tau overlapped and overlap) and others that vary much less between the standards. Why? A. Tau: All the criteria capture the effect of tau (t/T) in the same way, but without explicit expression in point load criteria for T, Y, and X connections. This is not to say that tau is unimportant; indeed, its primary importance was more obvious to the user in the old AWS punching shear format. When one gets called in after the fact on structural failures and tubular projects in trouble, one of the first things to look for is excessive tau ratios. Tau effect in overlapped connections: In the old AWS-API-AISC criteria, this is captured by mechanistic consideration of both punching at the partial footprint (L1) and shear in the overlap weld (L2). In Pecknold’s new API criteria, this is based on an extensive inelastic finite element parameter study, with totally separate Qg expressions for gap and overlap joints. Pecknold’s indicated higher strength for large beta and large tau is also consistent with the unexpectedly good performance of same-size overlapped K-bracing in Hurricane Andrew. In CIDECT-IIW-ISO criteria, overlap is treated as an extension of the behaviour of gap connections in which tau has no effect; a single Kg expression for both is curve-fit to the smaller empirical test data base. This makes computerized design easier, but gives the designer no insight into the physical mechanism of load transfer. Effect of overlap amount: Both Pecknold (new API) and CIDECT agree that there is a significant, but nearly constant, beneficial effect beyond g/D of -0.1. The old AWS-API approach (dating back to 1975) was apparently intuitively appealing but wrong, while remaining on the safe side for moderate amounts of overlap. Pecknold gives no equations for |g/D| smaller than 0.05, and suggests interpolation in this region. In older codes, this was a prohibited zone, due to concern over creating a weak hard spot and awkward welding conditions. The smooth transition shown in the CIDECT strength criteria (and in Efthymiou’s SCF criteria) may simply be an artefact of curve-fitting. This issue needs to be re-examined, seeking data in the prohibited zone. Welded connections with tensile strains over 2% in inelastic finite element solutions may be considered vulnerable to fracture. Effect of chord loading: The 1972 AWS Code included a modest Qf penalty for compressive chord loads, based on Japanese data. Existing AWS-AISC criteria reflect further effects of gamma and chord load type, based on Yura’s X-joint data. CIDECT criteria are simpler, with Kp close to being on the safe side of Yura. Pecknold’s criteria, based on extended finite element results, reflect effects of both chord and brace load type (but not gamma). This is reasonably consistent with CIDECT for K and N connections, but its better prediction for other types of connections has a significant effect on reliability, prompting a modest reduction in working stress design safety factor in API. One common design case in which both CIDECT and AWS-AISC are significantly on the unsafe side is equal size X-braces with equal but opposite loads and no joint can. A caveat for this case is urgently needed. Q. Do we need more data to choose the right direction for the parameters that vary widely? A. Pecknold’s API data base includes both the CIDECT physical tests and his extended finite element results. It could be readily compared to CIDECT-IIW-ISO criteria, to quantify the reliability consequences of adopting these into AISC. Gathering additional research data must be left to the future. In order to meet its ambitious publication schedule, AISC should select one set of criteria and stick with it – no mix-and-match. For circular connections at this point in time, Pecknold’s criteria have not been as widely vetted as CIDECT, while CIDECT does not have the extensive set of worked examples and familiarity to American designers as the existing AWS-AISC criteria.


Q. Are the difference due to the wide difference in the type of structures the standards are based on and do these variances really show variances due to exceeding testing limits i.e. do the API people test 48 dia 1 inch connections while the CIDECT people test 4 in x 1.4 tube? A. The differences are not always that great. D/t of 48 is quite typical for offshore structures, so punching at the material shear strength rarely governs. Most CIDECT tests are in the D/t range of standard weight structural tubing, which ranges from 6 to 34. European bridge designers seem to favor the 3 to 13 D/t range of double extra strong. American designers use D/t up to 120 for fabricated large diameter chord members, with much thicker joint cans at the nodes. None of the design criteria show an adverse size or thickness effect on static strength, although fracture mechanics and some of the tension test data suggest one. AWS criteria give no bonus for tension. Q. What is the effect on joints designed and built routinely? A. People generally want to know the impact on cost and safety before they change a design code. AISC should get a feel for this while re-working all the example problems and tabulated results in the HSS manual. Bridges and offshore structures are also influenced by fatigue, so the impact of a static strength change is muted. CONCLUSION Although this paper may be regarded as a “stream of consciousness” examination of ongoing design code developments, it is already drawing worthwhile discussion. ACKNOWLEDGEMENTS The foregoing Q&A discussion was prompted by thoughtful review of an earlier draft of this note from Tom Schlafly at AISC. Drafting of the figures has been performed by Dakang Liu at TU Delft. The author is grateful to the conference organizers for accommodating this paper. REFERENCE (1) P. W. Marshall, Design of Welded Tubular Connections, Developments in Civil

Engineering #37, Elsevier Science Publishers, Amsterdam, 1992 (limited availability at civilbooks.com)



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Review of Tubular Joint Criteria