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-byside 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
General format
T & Y joint
K & N gap
AWS D1.1-92
and AISC ‘97 HSS
Connection Manual
Pu sin θ = to2 Fyo [*] Qf
[*] defined below
[*] = (18.8β + 3.4) √Qβ
[*] = 32β/α + 3.4 with
α = 1.0 + 0.7 g/di ≤ 1.7
[*] = (32 β + 3.4) f 1 (q)
FEXX
f 2 (q )
Fy
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
FS Pa sin θ = T2 Fyo [*] Qf
Pn sin θ = to2 Fyo [*] Kp
Pu sin θ = t2 Fyo [*] Qf
[*] = (2.8 + 14.2β2) γ0.2
[*] = (3.1 + 15.6β2) γ0.2
[*] = 2.8 + (20 + 0.8γ) β1.6
but ≤ 2.8 + 36β1.6
[*] = (16 +1.2γ) β1.2 Qg ≤ 40 β1.2 Qg
Qg = 1+ 0.2(1-2.8g/D)3 **
⎤
⎡
0.024γ 1.2
K g = γ 0.2 ⎢1 +
⎥
⎢⎣ 1 + exp(0.5g/t 0 − 1.33) ⎥⎦
with Qg same as
CIDECT Kg and
g negative for overlap
5.2
1 − 0.81β
As for K with eff. β = ∑ βi / 3
Special combo rules
& missing cases
Check chord gap for shear + axial
(no increase over K joint)
X-X joints covered
General case not covered
Compression only
Kp = 1 – 0.3 Ū (1+ Ū)
[*] =
+ 3.2τβγ
X joint
[*] = (13.3β + 3.4) Qβ
Also can length effect
[*] = [2.8 + (12 + 0.1γ) β] Qβ
Stronger alternate for tension
K-T joint
See general
multiplanar
Use gap between
loaded diagonals
Chord load
effect
Qf or Kp
Other terms
458
See general multiplanar
(no increase over K joint)
[*] = (32β/α +3.4) Qβ0.7(α -1)
with α per Annex L
Tension or compression
Qf = 1 - 0.03 λ γ Ū2
2
2
2
Ū = [(P/Py) +(M/My) ]
Qβ =
[*] = (2.0 + 11.33β) Qg
[*] as for gap with
Qg = 0.13 + 0.65 φ √γ ∗∗
and φ = (tb/Fyb) / (toFyo)
K & N overlap
K-K joint
(delta truss)
General
multiplanar
[*] = (1.8 + 10.2β) Kg
Not covered
Commentary reference
To AWS method
Comp.(+), tens.(-) at footprint
2
P
M
Q f = 1 − C1
− C2
− C3 U
Py
My
0.3
forβ > 0.6 ** Qg not defined for
β (1 − .833β )
0.05 > g/D > -0.05
[*] =
5.7
1 − 0.81β
Not covered
Not covered
Not covered
Same as CIDECT
Ū = P/Py + M/My
Pn includes resistance factor
Pu is characteristic
ultimate
Connections in Steel Structures V - Amsterdam - June 3-4, 2004
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)
L2 = β D f2(q)
(partial footprint arc)
(lap weld ┴ to chord)
vw = 0.3 FEXX
vw = 0.8 FEXX
(allowable)
(ultimate)
with the resulting international format given by...
Ultimate Pu sin θ = to2 Fyo [*] Qf with
[*] = (32 β + 3.4 ) f1 ( q ) + 3.2τβγ
FEXX
f 2 (q )
Fy
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.
f1(q)
1.0
0.5
0.0
0.0
0.5
q
1.0
L2
L
1
1.0
f2(q)
q(βD)
0.5
0.0
0.0
0.5
q
1.0
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.
Connections in Steel Structures V - Amsterdam - June 3-4, 2004
459
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 unconservative 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.
variations for X joints
AWS & AISC Hdbk
API proposal
AISC proposal
40
Qu
30
20
10
0
0
0.2
0.4
0.6
0.8
1
1.2
β
Figure 2. Effect of β for X-joints with γ=20.
variations for X joints
AWS & AISC Hdbk
API proposal
AISC proposal
20
Qu
15
10
5
0
0
5
10
15
20
25
30
35
γ
Figure 3. Effect of γ for X-joints with β=0.5.
460
Connections in Steel Structures V - Amsterdam - June 3-4, 2004
variations for T & Y joints
AWS & AISC hDBK
API proposal
AISC proposal
40
Qu
30
20
10
0
0
0.2
0.4
0.6
0.8
1
1.2
β
Figure 4. Effect of β for T & Y joints with γ=20.
variations for T & Y joints
AWS & AISC Hdbk
API proposal
AISC proposal
20
Qu
15
10
5
0
0
5
10
15
20
25
30
35
γ
Figure 5. Effect of γ for T & Y joints with β=0.5.
Connections in Steel Structures V - Amsterdam - June 3-4, 2004
461
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.
K & N joints
AWS & AISC Hdbk
API proposal
AISC proposal
40
Qu
30
20
10
0
-1
-0.5
(overlap)
0
g/D
0.5
1
(gap)
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.
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Connections in Steel Structures V - Amsterdam - June 3-4, 2004
overlap joints
AWS & AISC Hdbk
API proposal
AISC proposal
80
Qu
60
40
20
0
0
0.2
0.4
0.6
0.8
1
1.2
β
overlap joints
Qu
AWS & AISC Hdbk
API proposal
AISC proposal
50
40
30
20
10
0
0
5
10
15
20
25
30
35
γ
overlap joints
AWS & AISC Hdbk
API proposal
AISC proposal
40
Qu
30
20
10
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
τ
Figure 7. Effects of β, γ, and τ for overlap N-joints with g/D = -0.3.
Connections in Steel Structures V - Amsterdam - June 3-4, 2004
463
gap joints
overlap joints
API proposal
AISC proposal
80
Qu
60
40
20
0
0
0.2
0.4
0.6
0.8
1
1.2
β
gap joints
AWS & AISC Hdbk
API proposal
AISC proposal
Qu
30
20
10
0
0
5
10
15
20
25
30
35
γ
gap joints
AWS & AISC Hdbk
API proposal
AISC proposal
Qu
30
20
10
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
τ
Figure 8. Effect of β, γ, and τ for gapped N-joints with g/D = 0.1.
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Connections in Steel Structures V - Amsterdam - June 3-4, 2004
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.
new API K-axial axial
new API K-axial IPB
new API T/Y axial axial
new API T/Y comp IPB
new API X β<.9 axial
new API bending axial
new API X β=1 com ax ial
new API bending IPB
AWS γ = 20 axial combined
AWS γ = 20 IPB combined
AWS γ = 20 OPB combined
new AISC all cases all cases
1.2
0.8
0.4
0
-1
-0.5
0
0.5
1
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
Connections in Steel Structures V - Amsterdam - June 3-4, 2004
465
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.
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Connections in Steel Structures V - Amsterdam - June 3-4, 2004
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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