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13. BOLTED JOINTS
The calculation methods used for bolted joints
between, or to, hollow sections are basically not
different from those used for any other type of joint in
conventional steel construction.
Most details given in this chapter are presented
without (detailed) design formulae.
13.1FLANGE PLATE JOINTS
13.1.1 Flange plate joints to CHS underaxial tension load
For the flange plate joints shown in Fig. 13.1, various
investigations were carried out (Kato & Hirose, 1984;Igarashi et al., 1985; Cao & Packer, 1997).
Economical joints under tension load can be obtained
if prying force is permitted at the ultimate limit state,
with the connection proportioned on the basis of a
yielding failure mechanism of the flange plates. In
CIDECT Design Guide No. 1 (Wardenier et al., 2008a)
formulae and tables are given, based on the work of
Igarashi et al. (1985). In the context of this book, only
the failure modes are presented (Fig. 13.2). It is
preferable to design primary structural joints on the
basis of the yield resistance of the circular hollow
section.
13.1.2 Flange plate joints to RHS underaxial tension load
Research by Birkemoe & Packer (1986) and Packer et
al. (1989) on bolted RHS flange plate joints with bolts
on two sides of the RHS only, see Fig. 13.3, showed
that in principle the strength of these joints can be
analysed on the basis of the traditional prying model
developed for T-stubs by Struik & De Back (1969).
The location of the plastic hinge lines may be adjusted
for greater accuracy, i.e. the distance b in Fig. 13.4 is
adjusted to b' according to:
it2
db'b (13.1)
Detailed formulae are given by Packer & Henderson
(1997) and Packer et al. (2009a).
Many tests have been carried out on RHS flange plate
joints with bolts on 4 sides of the RHS, as shown inFig. 13.3. A thorough study of this type of bolted joint
has been undertaken by Willibald et al. (2002, 2003a).
It was revealed that RHS flange plate joints bolted on
all four sides could still be proportioned on the basis of
the two-dimensional T-stub prying model of Struik &
De Back (1969), with some minor modifications.
Following the procedure for bolted RHS flange plate
joints with bolts on two sides, the inner yield lines in
the flange plate can now be expected adjacent to the
RHS outer face and hence the term t i should be
deleted from eq. (13.1). The bolt pitch to be used is
the minimum of p from both sides. The dimension p,
the plate width or depth divided by the number of bolts
in that direction, is illustrated in Fig. 13.3. This
"minimum p" value is then used in the joint analysis
based of a two-dimensional prying model. In order for
this design model to be valid, the centres of the bolt
holes should not be positioned beyond the corners of
the RHS (as illustrated in Fig. 13.3).
Detailed information can be found in CIDECT Design
Guide No. 3 (Packer et al., 2009a).
13.1.3 Flange plate joints to CHS or RHSunder axial tension load andmoment loading
Design methods for bolted flange plate joints to date
have generally been developed for axial tension
loading. Frequently, however, hollow sections are
subjected to both axial tension load (Ni) and bendingmoment (Mi). In such cases, a hypothetical "effective"
axial load can be computed (Kurobane et al., 2004)
for use with the flange plate joint design procedures
given in Sections 13.1.1 and 13.1.2:
i
i
i
i
i AW
M
A
NaxialEffective
(13.2)
where:
Ai cross sectional area of the CHS or RHS
Wi elastic (or plastic) section modulus of the CHS orRHS
This procedure will be conservative, especially for
CHS, as it computes the maximum tensile normal
stress in the CHS or RHS and then applies this to the
whole member cross section.
13.2 END JOINTS
Some bolted end joints are shown in Fig. 13.5. The
flange of the tee in Fig. 13.5d, as well as the otherflange plates perpendicular to the CHS or RHS
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section, must be sufficiently thick to effectively
distribute the load to the cross section (Wardenier et
al., 2008a; Packer et al., 2009a), see also Section
9.7.3.
13.3 GUSSET PLATE JOINTS
For bolted gusset plate joints, the design can be
based on the various possible failure modes, e.g. for a
tension member:
- Yielding of the cross section
- Rupture of the net area
- Rupture of the effective net area reduced for shear
lag
Similar to other bolted joints, the total net area is the
sum of individual net areas along a potential critical
section of a member or gusset plate, see Fig. 13.6. If
such a critical section comprises net areas loaded in
tension and segments loaded in shear, the shear
segments should be multiplied by the shear strength
and the tension areas by the ultimate strength.
Eurocode 3 (EN 1993-1-1, 2005) specifies a Mfactor
of 1,0 for yielding and 1,25 for ultimate strength
(rupture).
A failure mode of the gusset plate which also must be
checked is yielding across an effective dispersion
width of the plate, which can be calculated using theWhitmore (1952) effective width concept illustrated in
Fig. 13.7. For this failure mode (for one gusset plate),
the strength is given by:
M
o
pyp
1p)30(tan2gtfN
Rd,i (13.3)
where the term p represents the sum of the boltpitches in a bolted connection or the length of the
weld in a welded connection, and M=1,1.
If the member is in compression, buckling of the
gusset plate must also be prevented.
Fig. 13.8 shows some examples of bolted gusset plate
joints. It must be borne in mind that fitting of these
connections is very sensitive with regard to
dimensional tolerances and to deformations of the
welded gusset due to weld-induced distortions. Thus,
care has to be taken to ensure fitting at site.
When a member is connected by some, but not all
parts of its cross section elements and if the netsection includes elements which are not connected,
the net area perpendicular to the load has to be
multiplied by a shear lag factor which depends on the
shape of the section, the number of connected faces
and the number of transverse rows of fasteners.
Such a case is illustrated in Fig. 13.8b where bolting
plates are welded to the sides of the RHS brace
member. For welds parallel to the direction of load (as
the four flare groove welds would be in Fig. 13.8b,
along the four corners of the RHS), the shear lag
factor is a function of the weld lengths and the
distance between them. For the RHS, the shear lag
reduction factors can be applied to each of the four
sides (two of width w = bi- ti, and two of width w = h i-
ti), to produce a total effective net area of the RHS
reduced by shear lag. Suggested shear lag reduction
factors for these four element areas, in terms of the
weld length Lw, are (CSA, 2009):
- 1,00 when the weld Iengths (Lw) along the RHS
corners are 2bi(or 2hi as applicable)- (0,5 + 0,25Lw/bi) when the weld lengths along the
RHS corners are biLw< 2bi, or- (0,5 + 0,25Lw/hi) when the weld lengths along the
RHS corners are hiLw< 2hi- 0,75Lw/bi when the weld lengths along the RHS
corners are Lw< bi(or hi as applicable)
13.4 SPLICE JOINTS
Fig. 13.9 shows a splice joint for circular hollowsections. This type of connection can, for example, be
executed with four, six or eight strips welded
longitudinally on the periphery of the hollow sections
and connected by double lap plates, one on each
side.
Lightly loaded splice joints in tension can be made as
shown in Fig. 13.10 and for architectural appearance
the bolts can be hidden. Using one plate on each side,
instead of the solution in Fig. 13.10, provides a more
fabrication-friendly solution. Such an eccentric joint,
however, may have little stiffness and resistance toout-of-plane flexure under compression loading, thus
the designer should be confident that such a condition
has been considered. Experimental and numerical
research on this RHS joint type, under tension loading,
has been conducted by Willibald et al. (2003b).
13.5 BEAM-TO-COLUMN JOINTS
Bolted beam-to-column joints can be designed in
various ways, mainly depending on the type of load
that has to be transmitted. In general, shear joints aresimpler to fabricate than moment joints. Typical joints
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are given in Figs. 13.11 to 13.15 without detailed
description.
13.6 BRACKET JOINTS
Some typical joints for lightly loaded beams are shownin Fig. 13.16.
13.7 BOLTED SUBASSEMBLIES
Lattice structures are often connected to columns by
bolted flanges, plates or Tee profiles. Some examples
are shown in Fig. 13.17.
13.8 PURLIN JOINTS
Fig. 13.18 shows some examples of purlin joints for
trusses with CHS or RHS chords.
13.9 BLIND BOLTING SYSTEMS
Due to the closed nature of hollow sections, in many
cases additional welded plates are used for bolted
joints. However, solutions are then not aesthetically
appealing. Nowadays, bolting systems are available
which can be used when only one side of the
connection is accessible. Blind bolting systems make
use of either special types of bolts or inserts or special
drilling systems.
13.9.1 Systems using bolts and inserts
Special types of bolts and systems allow one to bolt
from one side of a hollow section. A number of
patented blind bolting systems is available, e.g. Huck
"Ultra Twist Blind Bolt" and Lindapter "HolloFast" and
"HolloBolt". The latter, which uses a special insert and
a standard bolt, has been investigated by CIDECT
(Sidercad & British Steel, 1996; Yeomans, 1998) with
regard to its axial, shear and bending capacity (see
Fig. 13.19).
The systems are based on the principle that after
bringing them in from one side, the bolts are torqued
and a "bolt head" forms on the inside of the connected
plies.
The design rules for blind bolting systems are based
on typical failure modes, i.e.- Punching shear of the fastener through the column
face
- Yielding of the column face (yield line pattern
around the bolts)
- Bolt failure in shear, tension or a combination of
both
13.9.2 Drilling system
The Flowdrill system, see Fig. 13.20, is a special
patented method for extruded holes. CIDECT has
carried out research (Yeomans, 1994; British Steel,
1996) to assess the load bearing capacity of this type
of joint in structural hollow sections.
Flowdrilling is a thermal drilling process (Fig. 13.21) to
make a hole through the wall of a hollow section by
bringing a tungsten carbide bit into contact with the
hollow section wall and generating sufficient heat by
friction to soften the steel. As the bit moves through
the wall, the metal flows to form an internal bush. In
the next step, the bush is threaded using a roll tap.
Conventional bolts are then used in this tapped hole.
Bolting to hollow sections with wall thicknesses up to
12,5 mm can be recommended by using the Flowdrill
method, see Yeomans (1994).
13.10 NAILED JOINTS
As an alternative to bolting or welding, steel circular
hollow sections can be nailed together to form reliable
structural joints. Up to now, this method of connection
has only been verified for splice joints between two
co-axial tubes (see Fig. 13.22). In such a joint, one
tube can fit snugly inside the other, in such a way that
the outside diameter of the smaller equals the inside
diameter of the larger. Nails are then shot fired and
driven through the two wall thicknesses and arranged
symmetrically around the tube perimeter.
As an alternative, two tubes of the same outsidediameter can be joined by means of a tubular collar
over both tube ends; in this case nails are again
inserted by driving them through the two tube walls.
Research to date has covered a range of tube sizes
with various diameter-to-thickness ratios, tube wall
thickness and lack of fit (Packer, 1996). The observed
failure modes were nail shear failure, tube bearing
failure, and net section fracture of the tube. These
failure modes have been identified for both static and
fatigue loading. Simple design formulae, derived from
bolted and riveted joints, have been verified for boththese load cases.
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Fig. 13.1 Bolted CHS flange plate joint
Fig. 13.2 Failure modes for bolted CHS flange plate joints
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p p p
p
p
p p p
p
p
Fig. 13.3 Bolted RHS flange plate joints
Fig. 13.4 RHS flange plate joint with bolts at two sides of the RHS
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Fig. 13.5 Bolted end joints
Bolt hole diameter d
Shear segmentsInclined segments
Tension segment Bolt hole diameter d
Shear segmentsInclined segments
Tension segment
Total net area for critical section A-A is the sum of the individual segments:
For tension segment : An= (g1- d/2)t
For shear segment : Agv= L t
For each inclined segment : An= (g2- d)t + (s2/4g2)t
Fig. 13.6 Calculation of total net area for a gusset plate
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,,
Fig. 13.7 Whitmore criterion for gusset plate yielding
Fig. 13.8 Some examples of bolted gusset plate joints
Fig. 13.9 Bolted splice joint for CHS
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Fig. 13.10 Hidden bolted splice joint
IPE or HE cut offIPE or HE cut off
Fig. 13.11 I section beam-to-CHS column joints
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a b
c d
e fFig. 13.12 I section beam-to-RHS column simple shear joints
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a b
c d
Fig. 13.13 Moment joints between open section beams and CHS or RHS columns
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Fig. 13.14 RHS sections connected to I section columns
Fig. 13.15 Knee joint assemblies for portal frames
Fig. 13.16 Bracket joints
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a b
c d
e f
Fig. 13.17 Bolted joints for lattice girder supports
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a b
c d
e f
Fig. 13.18 Purlin joints
Fig. 13.19 Lindapter "HolloFast" connection
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Fig. 13.20 Flowdrill connection for joining end plates or angles to RHS
Fig. 13.21 Flowdrill process
Fig. 13.22 Nailed CHS joint
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