Bolted Joints-examples

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.

13.1 FLANGE PLATE JOINTS 13.1.1 Flange plate joints to CHS CHS under axial 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 RHS under axial 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: b'  b 

d 2

 ti  

(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 in Fig. 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 RHS under axial tension load and moment 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 (N i) and bending moment (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:

 N Effective axial   i   A i



Mi    A i   Wi  

(13.2)

where:  Ai cross sectional area of the CHS or RHS Wi elastic (or plastic) section modulus of the CHS or RHS 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 other flange 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 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 γM factor 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 the Whitmore (1952) effective width concept illustrated in Fig. 13.7. For this failure mode (for one gusset plate), the strength is given by:

N i,Rd

 f yp t p g  2 (tan 30 o )  p

1

M

 

(13.3)

where the term p represents the sum of the bolt pitches 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 net section 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 - t i, 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 L w, are (CSA, 2009): - 1,00 when the weld Iengths (L w) along the RHS corners are  2bi (or 2hi as applicable) - (0,5 + 0,25L w/bi) when the weld lengths along the RHS corners are b i  Lw < 2bi, or - (0,5 + 0,25L w/hi) when the weld lengths along the RHS corners are h i  Lw < 2hi - 0,75Lw/bi  when the weld lengths along the RHS corners are L w < bi (or hi as applicable)

13.4 SPLICE JOINTS Fig. 13.9 shows a splice joint for circular hollow sections. 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 to out-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 are simpler to fabricate than moment joints. Typical joints

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are given in Figs. 13.11 to 13.15 without detailed description.

- Yielding of the column face f ace (yield line pattern around the bolts) - Bolt failure in shear, tension or a combination of both

13.6 BRACKET JOINTS Some typical joints for lightly loaded beams are shown in 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, However, solutions solutions are then not aesthetically 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

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 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, alternative, two tubes of the same outside diameter 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 both these 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

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

Tension segment

Bolt hole diameter diameter d’

Inclined segments

Shear segments

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 : A gv = L t 2 For each inclined segment : A n = (g2 - d’) t + (s /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 IPE or HE cut cut off  off 

Fig. 13.11 I section beam-to-CHS column joints 168

a

b

c

d

e

f

Fig. 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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