Eccentric Connecctions

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Slide 1

Module-Eccentric Connections

Unit No.II

Course-Design of Advanced Steel Structures Max. Marks. =25

Asst. Prof. Suryawanshi S.R.

In the frame of steel building, a beam may be attached to another beam or to a column. In such cases design of connections under system of loads on the elements depends on understanding of the behavior of the elementsThe beam column connections expected to resist transferred end reactions only are termed as shear connections or flexible connections ,permits free rotations of the beam end and do not have mmt restraint Other connection which do not permit any relative rotation between beam and column and expected to resist mmt in addition to end reaction are termed as mmt connections or rigid connections

Moment Diagram for Beams with different End Conditions

Types of Connections A. Shear connections (Flexible Connection)a)Riveted Shear connections 1. Bracket ConnectionsType I &II 2. Seated Connections-Stiffened and Unstiffened 3.Framed Connectionsb)Welded Shear Connections 1. Bracket ConnectionsType I &II 2. Seated Connections-Stiffened and Unstiffened 3.Framed Connections

B. Moment Resistant connections (Rigid Connections) a) Riveted mmt connection:- i.Heavy mmt connection ii. Light mmt Connectionsb) Welded mmt connections

C. Semi-Rigid Connections

WELDED SHEAR CONNECTIONS Welded connection are often subjected to shear & torsion or shear & bending Bracket type I is an example of weld subjected to shear and torsion Bracket type II is an example of weld subjected to shear and bendingSeated and framed also be implemented depends on designers choices.

Bracket Connections 1.Bracket Connection Type -IWhen twisting mmt acting is in the plane of weld i.e. C.G.of weld group lies in the plane of line of action of applied load, weld is subjected to shear and torsion.

Let ,M = Twisting mmt P = eccentric load acting over the jointe = eccentricity of load, t=effective throat thickness of the weldl = length of fillet weld (2a+d) as shown in fig. d = depth of the bracketr = the distance of extreme weld from CG of weld group, = shear stress in wled , J = polar mmt. of Inertia

Weld group subjected to 1)Direct shear stress due to load P2) Shear stress due to twisting mmt.

For Direct shear stress 1 = Load/Effective area of weld = P/(2a+d)t------------(1)For shear stress due to twisting mmt.Can be computed using torsion equation.T/J= /r 2= T.r/J= P.e.r/J-------------(2) The resultant Shear stress can be given as = < 108 N/mm2 Allowable shear stress in weld

The design of Connection is done as follows:-Assume overlap of bracket & work out the length of fillet weld.Compute C. G. Of weld group (x)Compute polar Mmt. of Inertia (J)Calculate dist. of extreme weld from C.G. Of weld group(r)Calculate 1, 2Calculate and equate it with Allowable shear stress.Weld size can be given as, t=0.7S, t is throat thk.

2.Bracket Connection Type IIWhen moment is perpendicular to the weld i.e. C. G. of the weld group lies in a plane perpendicular to the plane of line of action of the applied load, the weld is subjected to shear and bending. The eccentric load can be made concentric along with a bending moment.The weld in such a case is subjected to--- a)Direct stress due to a concentric load P and b) Bending stress due to bending moment(P.e)Either butt weld or a fillet weld can be used in such connections.

1. For Direct shear stress in weld :1 = Load/(Eff.Area of weld)

For fillet weld 1 = P/2d.t

For butt weld 1 = P/d.t

2. For bending stress in the weld:b = moment/(section modulus) For fillet Weld

Fillet weld when subjected to flexure the common relationship b = M.y/I ,used to evaluate stresses. b = {P.e(d/2)}/{2.td3/12} = P.e/(2td2/6) (Critical stresses exist on the throat of the weld at 450)The throat stresses then treated as a shear since,450 line of failure is indicative of a shear fracture2 = P.e/(2td2/6) Combined stress in fillet weld

< 108N/mm2For butt Weld

b = P.e/(td2/6)

Combined stresses and shear stress in butt weld may be checked by the interaction formula as,

< yield stress of steel used

The design of connection is done as follows:-Size of weld is assumed and the length is computed. If the length of the weld is more than twice the depth of the bracket, the size of weld is revised:-Assume the size of the weld and compute the throat thickness. Calculate depth of the bracket from either of the following appropriate equations. In case of butt weld where b =165 N/mm2 In case of fillet weld where 1 = 108 N/mm22. Direct shear stress is computed in terms of throat thk. (1)

3. the stress due to bending moment is computed I terms of throat thk. (2 or b)4. The equivalent stress is computed as appropriate

Welded Seated ConnectionsTwo types of seated connections used i.e. 1.Unstiffened Seat Connection 2. Stiffened Seat Connection In welded unstiffened seat connection two angles are used. one angle is welded to the column in the shop and forms the beam seat. A Cleat angle of nominal size (100x100x6 )mm is welded to the top of beam in the shop and to the column in the field.vertical welds are provided to connect the seat angle and these are turned at the ends.the resultant stress in the vertical weld is not uniform. resultant shear comprising of horizontal shear per mm and vertical shear per mm, is computed and should be less than allowable shear per mm of the weld.

Design steps:-A Seat angle is chosen suitably from following considerations 1. the seat angle is assumed to have a length B,equal to the width of the beam flange (B) 2. length of outstanding leg of seat angle b

The seat leg length is kept more than the calculated bearing length equation may yield negative value for large beam with small reaction therefore a minimum bearing length is specified as

where, R=end reaction in N , p=Permissible bearing stress in Mpa (0.75fy) t= thk. Of the web of beam in mm, h2=depth of the root of fillet weld from extreme fibre of flange for the beam in mm

3. thk. of seat angle is chosen such that the outstanding leg does not fail in bending on a section at the toe of the fillet (t) bending stress t2 = 6.R.e1 /B.bce1= the distance from the critical section to the reaction R acting at the centre of the bearing length =10+0.5b-t-radious of root of fillet M=mmt at critical section=R.e1B=length of seat angle equal to width of flange bc=bending stress in comp. assumed to bending stress in slab base i.e.185Mpa

B)Calculate the vertical shear per mm 1 = R/2dt(t is unity) C)Calculate the eccentricity of the reaction and compute the bending mm t.D)Calculate the horizontal Shear per mm due to the bending mmt. 2 = M/(2.d2 .16) (t is unity) d=length of weldE) Calculate resultant shear stress

F)This resultant shear per mm is equated to the strength of the weld per mm to find the size of the weld =0.7xSx1x , =Allowable shear stress =108Mpa

Q. Design an unstiffened welded seat connection for a beam I. S. M. B. 250@373N/m, transmitting 84 Kn to the I. S. H. B. 200@373N/m Soln. the relevant properties of the section from IS Handbook No.1(SP 6(1964))

ISMB 250ISHB 200B125200tw6.96.1h227.95tf12.59g6555

Q. Design an unstiffened welded seat connection for a beam I. S. M. B. 500@86.9Kg/m, transmitting 170 Kn to the I. S. H. B. 350@367.4Kg/m (SRTMUN June13)8mksSoln. the relevant properties of the section from IS Handbook No.1(SP 6(1964))

ISMB 500BeamISHB 350ColumnB180250tw10.28.3h237.9527tf17.211.6

A) Design of seat angle 1. Length of seat angle B =Width of beam flange=180mm 2. for bearing length b b=23.157 mm b=45.045mm ,providing b=100mmProvide 10 mm clearance between beam and column Provide (ISA 200x100x12) mm r1 =12mm, t=12mm 3.for thickness of seat angle (t) t2 = 6.R.e1 /B.bce1 = 10+45.045/2-12-12=8.52mm

Try another section as (200x150x18)mmr1 =13.5mm t=18mmTherefore e1 = 10+45.045/2-18-13.5=1.0225mm

Therefore provide seat angle (200x100x18)mm

B) Vertical Shear /mmd=200mm i.e. length of weld1 =R/2dt=170x103 / 2x200x1=425N/mmC)Horizontal Shear/mm2 = M/(2.d2 .16) (t is unity) =170x103x1.0225/(2x2002 6)=13.036N/mmD) Resultant shear

E)For size of weld=0.7xSx1x =Allowable shear stress =108Mpa425.2=0.7xSx108S=5.62mm, therefore providing S=6mm

2. Stiffened Seat Connection

In the stiffened seat connection ,a T-section built up of two plates is used .The bearing length of the seat plate is calculated as in case of the unstiffened seat connection.thk. Of seat plate is kept equal to the thk, of flange of beamthk. Of stiffening plate is kept equal to thk. Of web of the beam.The depth of the stiffening plate is decided depending up