CONNECTIONS

Design assumptions

In general, the designer of a structure will either adopt Simple design (in which it is assumed the connections are nominally pinned, i.e. no significant moments are transferred across the joint) or Continuous design (in which it is assumed the connections are rigid and transfer moment between members). In some cases the designer may use Semi-continuous design and assume that the connections are semi-rigid, but this is relatively unusual. Typical beam-to-column nominally pinned connections are shown in Figure and rigid connections are shown in second Figure. The assumptions made about connection behaviour during the frame analysis must be consistent with the final connection details.

Typical beam-to-column pinned connections

Wednesday, October 28, 2015 9:10 AM

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Typical beam-to-column rigid connections

The code requires that joints used in simple design should be capable of transmitting the calculated forces and should also be capable of accepting the resulting rotation. The connections must not transmit significant moments.

In continuous design the connection must be capable of transmitting the forces and moments calculated in the global analysis.

BS 5950-1 does not contain a design method for connections but presents information on the detailing requirements and the calculation of strength for individual components within a connection. This has resulted in a wide variety of acceptable methods of design and details to transfer shear, axial and bending forces from one structural member to another.

Current techniques include the use of black bolts, high-strength friction-grip (H.S.F.G.) bolts, fillet welds, butt welds and, more recently, the use of flow-drill techniques for rolled hollow sections. In addition there are numerous proprietary types of fastener available. Since fabrication and erection costs are a significant proportion of the overall cost of a steel framework, the specification and detailing of connections is also an important element in the design process.

The basis of the design of connections must reflect the identified load pathsthroughout a framework, assuming a realistic distribution of internal forces, and must have regard to local effects on flanges and webs. If necessary, localised stiffening must be provided to assist load transfer.

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stiffening must be provided to assist load transfer.

All buildings behave as complex three-dimensional systems exhibiting interaction between principal elements such as beams, columns, roof and wall cladding, floors and connections. BS 5950-1:2000 Clause 2.1.2 specifies four methods of design which may be used in the design of steel frames:

(i) Simple Design (Clause 2.1.2.2)In simple design it is assumed that lateral stability of the framework is provided by separate identified elements such as shear walls, portal action, or bracing. The beams are designed assuming them to be pinned at the ends, and any moments due to eccentricities of connections are considered as nominal moments when designing the columns. It is important that sufficient flexibility exists in the connection detail (i.e. the flexible end-plates, fin-plates, web cleats etc.,) to permit rotation at the joint as assumed in the design.

(ii) Continuous Design (Clause 2.1.2.3)In continuous design, full continuity is assumed at connections transferring shear, axial and moment forces between members. In addition it is assumed that adequate stiffness exists at the joints to ensure minimum relative deformation of members and hence maintaining the integrity of the angles between them.

(iii) Semi-Continuous Design (Clause 2.1.2.4)In this technique partial continuity is assumed between members. The moment -rotation characteristics of the connection details are used both in the analysis of the framework and the design of the connections. The complexity and lack of readily available data renders this method impractical at the present time.

(iv) Experimental Verification (Clause 2.1.2.5)Loading tests may be carried out to determine the suitability of a structure with respect to strength, stability and stiffness if any of the methods (i) to (iii) are deemed inappropriate.

The uses of bolts and welds as used in simple and rigid connections are illustrated in Figure

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The design of connections requires analysis to determine the magnitude and nature of the forces which are to be transmitted between members. In both bolted and welded connections this generally requires the evaluation of a resultant shear force and, in the case of moment connections, may include combined tension and shear forces.

Simple ConnectionsThese are most frequently used in pin-jointed frames and braced structures in which lateral stability is provided by diagonal bracing or other alternative structural elements. Typical examples of the use of simple connections, such as in single or multi-storey braced frames, or the flange cover plates in beam splices, are shown in Figure

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Moment ConnectionsThese are used in locations where, in addition to shear and axial forces, moment forces must be transferred between members to ensure continuity of the structure. Typical examples of this occur in web cover plates in beam splices, unbraced single or multistory continuous frames, support brackets with the moment either in the plane of or perpendicular to the plane of the connection, and as shown in Figures

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Applied Moment in the Plane of the Connection

(i) Bolted connectionThe bolts with the maximum force induced in them are those most distant from the centre of rotation of the bolt group and with the greatest resultant shear force when combined with the vertical shear force, i.e. the bolts in the top and bottom right-hand corners. Consider a group of six bolts which are subjected to an eccentric axial load as illustrated in Figure

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Each of the bolts has the same vertical shear due to the force F. The bolts with the maximum rotational shear force due to the moment induced by the

eccentric force (i.e. F e) are numbers 1, 3, 4 and 6 which are most distant

from the centre-of-rotation. In general terms, considering n number of bolts, the resultant maximum shear force on a bolt is given by the vector summation of the direct shear component and the rotational shear component. Bolts F1 and F3 have the maximum resultant as follows:

whereF is the applied vertical force,e is the eccentricity of the applied force about the centre-of-rotation,Z is the perpendicular distance from the centre-of-rotation to the line of action of the rotational shear force in a bolt,Z1 is the maximum value of Z,

is the angle between the horizontal axis and Z1,n is the number of bolts.

(ii) Welded connectionIn a welded connection the extreme fibres of the weld most distant from the centre-ofgravity of the fillet weld group are subjected to the maximum stress as indicated in Figure.

The resultant maximum shear force/mm length of fillet weld is found in a similar manneras for bolts.

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Applied Moment Perpendicular to the Plane of the Connection

(i) Bolted connectionIn this type of connection it is often assumed that the bracket will rotate about the bottom row of bolts. While this assumption is not necessarily true, it is adequate for design purposes. In this case the bolts will be subjected to

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adequate for design purposes. In this case the bolts will be subjected to combined tension and shear forces, as shown in Figure above.

In general terms, considering n the number of bolts and m the number of vertical columns of bolts, the maximum tensile force on a bolt is found by the consideration of the assumed rotation of the bracket about the bottom line of bolts, as indicated in Figure above. Bolts which are furthest from the line of rotation, i.e. distance y3, have the maximum tension force, which can be determined from:

The bolts must be designed for the combined effects of the vertical shear and horizontal tension.

(ii) Welded connectionIn welded connections the maximum stress in the weld is determined by vectorial summation of the shear and bending forces/mm, as shown in Figure

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Bolted Connections

The dimensions and strength characteristics of bolts commonly used in the U.K. are specified in BS 4190 (black hexagon bolts) and BS 4395 (High-Strength-Friction-Grip bolts). Washer details are specified in BS 4320. BS 5950 mentions several strength grades: e.g. Grade 4.6 which is mild steel, Grade 8.8 and Grade 10.9 which are high-strength steel.

The most commonly used bolt diameters are 16, 20, 24 and 30 mm; 22 mm and 27 mm diameter are also available, but are not preferred. Combinations of bolts, washers and nuts should match those specified in Table 2 of BS 5950-2:2001.

The usual method of forming site connections is to use bolts in clearance holes which are 2 mm larger than the bolt diameter for bolts less than or equal to 24 mm dia., and 3mm larger for bolts of greater diameter. Such bolts are untensioned and called non-preloaded bolts; referred to in this text as black bolts. In circumstances where slip is not permissible, such as when full continuity is assumed (e.g. rigid-design), vibration, impact or fatigue is likely, or connections are subject to stress reversal (other than that due to wind loading), preloaded bolts, referred to in this text as high strength friction grip bolts, should be used.

Black Bolts

Black bolts transfer shear at the connection by bolt shear at the interface and bearing on the bolts and plates as shown in Figure below

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Bolt Spacing and Edge Distances

In Clauses 6.2.1, 6.2.2 and Table 29, BS 5950-1:2000 specifies minimum and maximum distances for the spacing of bolts, in addition to end and edge distances from the centreline of the holes to the plate edges.

Lifting of the edges between the bolts is prevented by specifying a maximum edge distance. The values specified are illustrated in Figure.

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edge distance. The values specified are illustrated in Figure.

Bolt Shear CapacityThe shear capacity of a black bolt is given in Clause 6.3.2.1 by: Ps = ps As where:As is the cross-section resisting shear: normally this is based on the root of the thread (tensile area At); if the shear surface coincides with the full bolt shank then the shank area (A) based on the nominal bolt diameter can be used ps is

given in Table 30 as 160 N/mm2 for Grade 4.6 bolts and 375 N/mm2 for Grade 8.8 bolts.

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The shear capacity Ps is reduced in the following circumstances:

♦ when using steel packing (note: total thickness of packing ≤ 4d/3),

♦ when requiring large grip lengths (i.e. > 5d),

♦ when using long joints such as in splices (with > 2 rows of bolts),

♦ when using kidney-shaped holes,

to allow for effects such as possible bolt bending and/or an unequal distribution of force within the bolts. In each case Ps is equal to the minimum value determined from the following equations as appropriate:

In the case of connections which have two bolts, one in a standard clearance hole and theother in a kidney-shaped slot, the shear capacity is reduced by 20% such that: Ps = 0.8ps As as indicated in Clause 6.3.2.4.

Plate Shear CapacityThe effect of bolt holes on the shear capacity of a plate may be ignored provided that:

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Block Shear Capacity

The possibility of failure by block shear as shown in Figure below (i.e. by combined shearing of the plate through the bolt holes parallel to the applied load and tensile failure in a plane perpendicular to the applied load) is considered by ensuring that the applied force Fr is less than the block shear capacity given by:

Bolt Bearing CapacityThe bearing capacity of a black bolt is given in Clause 6.3.3.2 as:

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Plate Bearing CapacityIn Clause 6.3.3.3 the bearing capacity of a connected plate is:

Bolt Tension CapacityConnections in which the bolts are subjected to tension as shown in Figure induce secondary tensile forces in the bolts due to prying action as indicated. The magnitude of the additional force is dependent on the stiffness and the details of the parts being connected.

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If the prying forces are accounted for in the total applied axial load (Ftot), then as indicated inClause 6.3.4.3 of the code using ‘…the more exact method…’, the tension capacity of the boltcan be determined from:

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