Previous|Next|ContentsESDEP WG 11CONNECTION DESIGN: STATIC
LOADINGLecture 11.7: Partial Strength Connectionsfor
Semi-Continuous FramingOBJECTIVE/SCOPETo explain the attributes
necessary in connections for semi-continuous framing, and how
suitable connections can be selected in practice.RELATED
LECTURESLecture 11.1.1: Connections in BuildingsLecture 11.1.2:
Introduction to Connection DesignLecture 14.13: Design of
Multi-storey Frames with Partial Strength and Semi-rigid
ConnectionsSUMMARYThe fundamental importance of the ductile partial
strength connection in semi-continuous design practice is reviewed.
The mechanics of such connections are discussed and a suitable
candidate, the bolted end plate connection with appropriate choice
of plate thickness, is introduced. Calculation methods for
strength, rotational stiffness and rotation capacity according to
Eurocode 3 Annex J [1] are briefly discussed.1. INTRODUCTIONFor
beam-to-column connections of typical building frames, which may be
braced or unbraced, there have traditionally been two choices:
'simple' (nominally pinned connections) or 'continuous'
(moment-resisting connections). Simple construction requires that
the frame is braced, either by triangulation or by something like a
reinforced concrete core to which it is connected at each level. In
practice, continuous construction is associated with unbraced
frames; it is rarely used in braced frames except in hybrid
high-rise designs and locally in other frames.Why semi-continuous
frame design?The use of semi-continuous frame design is a matter of
economics. Continuous framing implies either rigid or full strength
connections. Both are expensive to fabricate.While 'simple'
connections are cheap, beams designed as simply supported are
bigger than they would otherwise need to be.The semi-continuous
approach offers a middle course. It is based ondesigner choiceof a
convenient beam end moment, as illustrated in Figure 1. This end
moment is usually set equal to the resistance of a suitable
not-too-elaborate connection detail. The beam is then sized for
midspan M = MFREE- MCONN. The connection is, therefore, the key to
semi-continuous frame design.
2. DEFINING TERMSFigure 2 demonstrates the definition of moment
connections by strength, rigidity and ductility.
The important features of a connection in semi-continuous
framing are that it isductileandpartial strength.Ductile: capable
of acting as a plastic hinge.Ductility of a connection is
synonymous withrotation capacity(the term used in Eurocode 3), and
should not be confused with ductility of a material such as
steel.Partial Strength: Able to resist less than the plastic moment
of the beam.Since, in continuous frames, maximum moments occur at
the beam ends, it is virtually inevitable that a partial strength
connection in this location will be 'overloaded'. It must be
capable of rotating plastically to the extent necessary for
compatibility with beam end (and possibly column) rotations under
design load. The beam may not remain elastic. Around midspan, there
is liable to be an almost fully developed plastic hinge. The
rotation which the connection must accommodate therefore varies
with the circumstances but may be as much as 0,02 to 0,04
radians.In practice, the chosen moment resistance of the connection
is often in the range 30% to 50% of the plastic moment resistance
of the beam.3. PLASTIC AND ELASTIC GLOBAL ANALYSISDesign of
semi-continuous frames, as outlined above, is based onplasticglobal
analysis.In principle, semi-continuous elastic analysis could also
be performed. This implies that rotational springs of appropriate
stiffness (rigidity) are used to model the connections.Elastic
global analysis is a relatively unattractive proposition to the
designer of a semi-continuous frame. Reliable prediction of
rotational stiffness is difficult, and the bending moment
distribution depends on this. The interaction between element and
connection stiffness and the distribution of moments in the frame
makes it very difficult for the designer to control the design and
achieve overall economy.The main reason for mentioning elastic
analysis is to introduce the term:-Semi-rigid: too flexible to
qualify as Rigid, but not a pin.A Rigid connection is stiff enough
for the assumptions made in conventional elastic analysis to be
valid. The perfectly rigid connection does not exist, but practical
connections can approach this ideal sufficiently closely for their
flexibility to be neglected in the analysis. In other words the
bending moment distribution remains acceptably close to the
theoretical one which results from elastic analysis. Codes vary in
their definition of where to 'draw the line' for this purpose. The
distinction is only relevant to elastic analysis of hyperstatic
frames.This special meaning of the word 'Rigid' is emphasized, in
this lecture, by the use of the capital 'R'.It is important to
understand that a connection can be rigid enough to perform its
function in the structure without qualifying as Rigid according to
the code definition. Semi-rigid connections can be adequately
rigid.It is necessary to be aware that just as the term rigid is
sometimes used loosely to mean nothing more than
'rotation-resistant', the term semi-rigid is sometimes used to
describe semi-continuous construction in general. This is
unfortunate. Although the connections which are the subject of this
lecturewilloften be semi-rigid, what matters is that they
arePartial StrengthandDuctile.The neutral term 'Partial Restraint',
meaning 'Partial Strength and/or Semi-rigid', will also be
encountered.4. WHAT MAKES A CONNECTION SUITABLE?In order to deliver
the necessary rotation capacity, some component of the connection
must yield in a controlled way. Plates in bending and column webs
in shear are suitable candidates.Most importantly, other parts of
the connection must be prevented from failing, because they would
do so abruptly. Welds, and bolts in tension, are in this
category.For this reason all-welded connections, apart from some
unconventional ones, are not generally compatible with the
semi-continuous approach.To protect the brittle components, e.g.
welds and bolts in tension, it is necessary for at least one other
component of the connection to be designed as a deliberate 'weak
link'. Unusually in structural design, the maximum strength as well
as the minimum strength of this component must be limited.A
frequently chosen connection is the bolted end plate, either
'flush' or 'extended', see Figure 3. Other styles of connection may
be suitable, but this one is unique in that it is supported by
authoritative design rules in Eurocode 3 Annex J [1].
Not all end plate connections are ductile. Figures 4 and 5
demonstrate the behaviour that is required. Generally only Mode 1
behaviour achieves the required ductility.
It is usually found necessary to restrict the end plate
thickness to approximately 60% of the bolt diameter (assuming bolts
not weaker than 8.8). Thicker end plates would transmit more moment
but with the risk of premature failure due to the bolts snapping
before the required degree of rotation has taken place.End plate
connections designed with regard for strength alone usually have
end plates whose thickness equals or exceeds the bolt diameter.
They are non-ductile. For example, as shown in Figure 5, a 25 mm
thick end plate is necessary to develop the full strength of M24
8.8 bolts.However, in all respects other than thickness, the
ductile end plate can look identical to full strength end
plates.Apart from the inevitable relative inefficiency in bolt
utilisation, it should be recognised that the thinner end plate
makes the ductile connection less rigid than its orthodox
counterpart.5. THE NEED FOR RIGIDITYRigidity, used here as a
synonym for rotational stiffness, is significantly more important
in unbraced frames than in braced ones. In the former, it
contributes to frame stability and sway resistance. In braced
frames, its contribution is less crucial; it helps limit beam
deflection and restrains column rotation.The rigidity required to
maintain stability and/or serviceability of an unbraced frame
varies according to the circumstances - a multibay low rise frame
obviously requires less than a slender one, other things remaining
equal. However, it is generally less, maybe much less, than that
required (according to code rules) for the connection to be
designated 'Rigid' for the purpose of elastic analysis.It is hard
to imagine circumstances in which too much rigidity would be an
embarrassment, irrespective of whether the frame is braced.
Standard details can, therefore, be designed to maximise it. For
this purpose, 'compact' bolt arrangements, in which the bolts are
placed as close to the flange and web as is practical, are
preferable.In relation to choice of end plate thickness, rigidity
and ductility are in direct opposition.Stiffness of the end plate,
which tends to be the most flexible component of the connection
and, therefore, dominant, is proportional to its thickness to the
power of two if not three. Ductility must not be compromised, so
larger and/or stronger bolts, which permit a thicker end plate, are
advantageous.The combination of 15mm thick end plates with M24
bolts (8.8 or 10.9) is often found suitable.6. STANDARDIZED
CONNECTION DESIGNSRecalling that the connections in semi-continuous
design are the subject ofdesigner choice, it can be seen that a
standardized approach holds a special attraction.It is frustrating
for the designer to select a particular trial connection moment,
say 30% of the free moment, only to discover after pages of
calculation that a connection using two tensile bolts just fails to
achieve it.A relatively small range of standard details, based on
preferred geometries, can be presented with tabulated moment
resistances for each beam size. This shortcuts the process of trial
and error choice of beam size and connection style, and retains
much of the simplicity of traditional 'simple' design. Figure 6
shows an example of standard details from the United Kingdom [2]
for ductile partial strength connections.DETAIL NUMBER: 5 (15
M24)
Figure 6. Example of a standard detailMOMENT RESISTANCEM.R. (in
Nm) = 193 x [h - 0,5 tf+ 40]+ 315 x [h - 0,5 tf- 60]+ 287 x [h -
0,5 tf- 150]Dimensions for detailing (mm)Serial size/Mass per
metreMoment Resistance (kNm)
Relative to top of steeldtfh
a1a2a3a4a5a6c
40404040406060606060150150150150150395390387383378485480477473468585580577573568889092939672021,318,817,415,613,2544,6539,5536,7533,1528,3533
x 210122 109 1019282371368366364361
40404040406060606060150150150150150317314310307304407404400397394507504500497494919395969865019,617,716,014,512,7467,4463,6460,2457,2453,6457
x 19198 89827467310308306304302
404040404060606060601501501501501503153113073053004054013973953905055014974954909294969810065018,917,015,013,310,9465,1461,3457,2454,7449,8457
x 15282 74676052308306304302299
40404040606060601501501501502632592562533533493463434534494464438990929459016,014,312,810,9412,8409,4406,4402,6406
x 17874676054268266264262
40406060150150252247342337442437899158011,28,6402,3397,3406 x
1404639261258
- Where tf > 18 use EFPTBW to flange- If beam is S275 use
EFPTBW to flangeCOLUMN LIMITATIONSFb809 kNS275GradeS355
viviiiiiiZoneiiiiiiivv
Web TensionWeb CrushingWeb BucklingWeb ShearFlange BendingSerial
size/ Mass per metreFlange BendingWeb ShearWeb BucklingWeb
CrushingWeb Tension
__________1089940814691*____356368202177153129____140612131051892____________
___________________13641124905791681*565*______30530528324019815813711897________145111681021879730*___________________
__________935746*613*497*406*___2542541671321078973____1207963791*642*525*___________
____497*395*349*300*273*2032038671605246_642*510*451*387*353*_______
* Less thanFbReinforcement required
SHEAR RESISTANCESee Note 4
1020 kN
7. CALCULATION OF CONNECTION PROPERTIESThe strength (moment
resistance) of the connection is calculated exactly as for any
other end plate moment connection, seeLecture 11.1.2.The same is
true of the rigidity (rotational stiffness) for which Eurocode 3,
Annex J gives a formula [1].The reliability of the predictions of
rotational stiffness which are not based on tests is, however,
limited. While serviceability calculations may reasonably be based
on predictions, the present state of the art is not such as to
encourage their use to determine the design ('ultimate') bending
moment distribution.Verification of connection ductility (rotation
capacity) is outlined in the next section. It should be understood
that in practice these checks are normally made using
purpose-designed software or by reference to tables of standard
details.Verification of DuctilityAs shown in Figure 7, Eurocode 3,
Annex J prescribes that a connection may be regarded as ductile,
i.e. it will possess sufficient rotation capacity to act as a
plastic hinge, where subject to one of the following
conditions:-(i) Shear zone of the column limits the moment
resistance(ii) Column flange (in bending) limits the moment
resistance with Mode 1 failure(iii) End plate (in bending) limits
the moment resistance with Mode 1 failureMode 1 failure is the
'double bending' mode which governs if the plate is relatively
thin.Only in the special case of one-sided connections, e.g.
perimeter columns, is it realistic to design on the basis of option
(i). Where there is a beam on each side of the column, the moments
can oppose one another, reducing the shear in the web panel -
perhaps to zero.
Generally, the column is already sized; any scope that the
connection designer has to alter it is in the upwards direction
only. Frequently, therefore, (ii) will not be an available option
either.Option (iii), designing for Mode 1 failure in the end plate,
is the only universally available route to satisfying the
requirement; standard details can be based on this.As illustrated
in Figure 8, it should be noted that Eurocode 3 Annex J offers a
formula by which rotation capacity may be calculated and compared
with the designer's assessment of what the situation requires. This
formula is applicable to connections in which Mode 2 prevails, i.e.
the end plate is somewhat thicker than the limit for Mode1. In
practice, it is rather unproductive of rotation capacity, except
for the shallowest of beams. In any case, the designer generally
prefers to avoid quantifying the required rotation capacity.
Satisfying condition (i), (ii) or (iii) above means that the
connection is 'ductile' - its rotation capacity will be ample for
all normal circumstances.
8. ECONOMICAL CONNECTION DESIGNIt is fundamental to the ethos of
the semi-continuous approach that, where it is in competition with
'simple' framing, the connections are little, if any, more costly
than their 'nominally pinned' counterparts, see Figure 9.
A larger end plate, a slightly larger weld or an extra pair of
bolts can be accepted.However, if stiffeners become necessary in
the column, or a haunch in the beam, it is likely that the point
has been missed. A saving in the beam size is unlikely to be
substantial enough to compensate for these labour-intensive
additions. Indeed, it is almost always preferable to increase the
weight of a column rather than to weld in stiffeners.One form of
column reinforcement, loose flange backing plates, Figure 10 can,
however, be justified as a means of upgrading the resistance of a
thin-flanged column at modest cost.
The message is that the designer should exercise his/her freedom
to choose the connection moment with due regard for the cost
effects of this decision.9. UNBRACED FRAMESUnbraced frames designed
according to the wind-moment method which is traditional in some
countries, i.e. connections proportioned to resist wind moment
only) are, whether their designers recognised it or not, precursors
of the semi-continuous unbraced frame. The satisfactory service
performance of numerous structures of this type, not all of whose
connections would be judged ductile according to the application
rules of Eurocode 3, encourages confidence in the acceptability of
semi-continuous unbraced frames.Nevertheless, unbraced frames with
ductile partial strength connections should be approached with some
caution. It is necessary to ensure that connection rigidity is not
unacceptably low for serviceability or stability of the
frame.Ideally, the connection stiffness is predicted and the frame
is analysed with the connections modelled as rotational springs.
(Formulae are available to modify beam bending stiffness so that
this can be performed with programs which do not offer rotational
spring elements). This analysis gives sway predictions which can be
compared directly with code limits, provided second order effects
are negligible, i.e. the frame is non-sway. If they are not
negligible, second order analysis is required.This approach demands
knowledge of the rotational stiffness of the connections, which may
not be reliably available. It amounts to semi-continuous elastic
global analysis.Provided that the structure is low rise and of
reasonably normal proportions, a simpler approach can be followed,
Figure 11. Parametric studies [3] have shown that it is acceptably
accurate to apply an arbitrary multiplier of 1,5 to the sway
predicted by a conventional continuous elastic global analysis,
subject to certain conditions.
These points are covered in earlier Lectures. They are repeated
here as a reminder that rigidity could influence connection design
in certain cases, leading perhaps to extended or stiffened end
plates where (for strength alone) less elaborate configurations
would suffice.In the case of an unbraced frame, the objective is to
avoid bracing rather than to make savings in the beams. The
economic comparison is with the full strength and/or rigid
connections of the competing 'continuous' design.The emphasis on
avoiding costly welded stiffeners and other labour-intensive
fabrication remains valid.10. CONCLUDING SUMMARY What makes the
semi-continuous approach worth pursuing is the freedom to use
relatively uncomplicated and low cost moment connections. The
connections arepartial strengthand are required to beductile. They
are likely to be semi-rigid but this is of little consequence in a
braced frame. In an unbraced frame, connection rigidity is
important in relation to stability and serviceability. The
semi-continuous approach to design, as recognised in this lecture,
is based on semi-continuousplasticanalysis. It offersdesigner
controlof the bending moment diagram to optimise overall
economy.11. REFERENCESREFERENCES[1] Eurocode 3: "Design of Steel
Structures: Part 1.1: General Rules and Rules for Buildings", ENV
1993-1-1, 1992.[2] Hughes, A. F. et al, "Ductile Connections for
Wind-Moment Frames" Steel Construction Institute, Ascot, UK (to be
published).[3] Anderson, D. A. et al, "Wind-Moment Design for
Unbraced Frames, Steel Construction Institute, Publication P082,
Ascot, UK,
1991.Previous|Next|Contentshttp://www.fgg.uni-lj.si/~/pmoze/ESDEP/master/wg11/l0700.htm