In this article we will discuss about how to make connections in steel structures.
Bolted Connections:
A. Eccentric Bolted Connections:
The resistance offered by a weld was entirely to prevent a linear or translatory displacement of a plate or member connected. There are also circumstances in which the welds provided for a connection may have to offer not only a resistance to prevent translatory displacement but also a resistance to prevent rotatory displacement. A bracket connection is an example of this type of connection.
There are two types of bracket connections, viz.:
1. Bolted Connection Subjected to Moment in the Plane of the Connection:
Fig. 4.42 shows an eccentric bolted connection for a bracket. It consists of two bracket plates bolted to the flanges of a rolled steel column. If a load W be applied to the bracket, a load P = W/2 is transmitted to each bracket plate.
The line of action of the load P on the bracket plate does not pass through the centroid of the bolt group. The perpendicular distance between the centroid of the bolt group and the line of action of the load P is called the eccentricity of the load P.
The bolts connecting the bracket plate and the flange of the column have to offer the following resistances:
(i) Resistance against Translation:
ADVERTISEMENTS:
This resistance is assumed to be uniform for all the bolts.
If P be the load on one bracket plate resistance against translation per bolt = P/n
where, n = number of bolts on one bracket plate.
(ii) Resistance Offered by the Bolts against the Rotation of the Bracket Plate:
ADVERTISEMENTS:
The load P being eccentric there is a tendency for the bracket plate to rotate about the centroid G of the bolt group. The bolts therefore have to offer a resistance to prevent such rotation. Such a resisting force offered by a bolt is called the torsional shear in the bolt.
It will be assumed that the torsional shear in a bolt is directly proportion to the distance of the bolt from the centroid of the bolt group. The direction of this resisting force S is at right angles to the line joining G and the bolt.
Let S be the torsional shear for a bolt distant r from G. The torsional shear S acts at right angles to the line joining G and the bolt. As per our assumption-
It P is the factored load on one bracket plate, then the resultant resistance offered by the bolt shall be less than the design strength of the bolt.
2. Eccentric Bolted Connection Subjected to Moment in a Plane Normal to the Plane of the Connection:
ADVERTISEMENTS:
Fig. 4.63 (a) and (b) show two examples of bolted connections subjected to moment in a plane normal to the plane of connection.
Consider the bolts connecting the column flange and the bracket. These bolts are subjected to direct shear accompanied by tension as a consequence of the moment. Studying the deflected bracket under the action of the eccentric load it is observed that bolts above a certain neutral axis are subjected to tension, while below that neutral axis the bracket exerts a thrust.
The neutral axis (the axis about which the bracket tends to turn) is assumed to be at a height h/7 from the lower contacting edge of the bracket, where h is the height from the lower edge of the bracket to the upper most bolt of the bracket connection.
For a bolt above the neutral axis, the tension in the bolt is proportional to its height above the neutral axis.
Consider a bolt at a height y above the neutral axis
Tension in the bolt = Ty = Ky, where K = a constant of proportionality.
Restoring moment provided by the bolt = Ty = Ky2
Restoring moment provided by all the bolts above the neutral axis.
Design of the Bracket Connection:
The following points may be noted in the design of the connection:
1. Depending on the width of the column flange choose the number of vertical rows of bolts. In the usual cases two rows of bolts are adequate. When the bracket has to support very heavy loads, we may provide four rows of bolts.
2. Spacing of bolts in each vertical row may be chosen from 3 d to 6 d where d is the diameter of the bolt.
3. An end margin not less than 1.5 times the diameter of bolt hole may be provided.
4. Approximate number of bolts in each vertical row may be determined from the formula-
n = √(6M/n1pVsd)
where, M = moment on the connection due to factored load
n1 = Number of vertical rows of bolts
p = pitch of bolts in the vertical rows
Vsd = Design strength of the bolt
5. Provide the bolts as determined above and investigate the safety of the design.
B. Beam to Beam Connections:
A beam may be supported at each end by a masonry wall or column or by a heavier beam. When a lighter beam is supported at its end by a heavier beam.
Two types of connections are adopted namely:
1. Framed Connections:
In this case the connection is made by connecting two angles one on either side of the web of the beam to be connected. The outspread arms of the angles are connected to the web of the supporting beam. Angles of sizes, 100 mm x 75 mm, 150 mm x 75 mm, 100 mm x 100 mm, 150 mm x 150 mm may be used as connecting angles. Angles may be 8 mm, 10 mm or 12 mm.
A bolt diameter is chosen. The bolts connecting the angles and the web of the main beam are in single shear
Number of bolts required = Factored end reaction/Design strength of the bolt
These bolts are equally distributed to the outspread arms of the angles connecting to the web of the main beam.
The bolts connecting the angles and web of the secondary beam are in double shear.
Number of bolts required = Factored end reaction/Design strength of the bolt
A check may be made for the shearing strength of the angle leg.
Shearing strength of the angle legs = Vd = fy/√[3 h(2tc)]
where,
h = depth of the connecting angle
tc = thickness of the connecting angle.
2. Seated Connections:
Suppose a beam has to transmit a large reaction to a supporting beam. In such a case we provide a seated connection. In this arrangement, the bottom flange of the beam to be supported rests over the outstanding leg of a seat angle. The other leg of the seat angle is connected to the web of the supporting beam. This arrangement is possible if the depth of web of the supporting beam is deep enough to accommodate the seat angle.
C. Beam to Column Connections:
A beam can be connected either to the flange or the web of a column.
In this case also there are following two types of connections:
1. Framed Connections:
In this case, for instance if a beam has to be connected to the flange of a column the connection is made by connecting two angles one on either side of the web of the beam and the outspread arms of the angles are connected to the flange of the column.
The bolts connecting the web of the beam and the angles are in double shear while the bolts connecting the angles and the flange of the column are in single shear. The bolts are designed to resist the end reaction of the supported beam.
For design purposes the connection is assumed to act as a hinged connection offering no resistance against rotation of the beam, at the connected end. To nearly satisfy this condition the connecting angles should not be thick. Angles of thickness over 10 mm may be avoided. The depth of the connecting angles should be less than 0.6 times the depth of the beam.
2. Seated Connections:
There are two types of seated connections namely:
(a) Unstiffened seat connections, and
(b) Stiffened seat connections.
(a) Unstiffened Seated Connections:
In this arrangement a seat angle (also called a shelf angle) bolted to the column supports the end of the beam. Incidentally the seat angle also assists during the erection of the beam to its position. For additional stability a clip angle is also provided connecting the top flange of the beam and the flange of the column.
The bolts connecting the seat angle and the column flange should be able to resist the end reaction of the beam. Hence the load that can be supported by the seat angle is limited by the number of bolts that can be accommodated in the vertical connected leg of the seat angle. Usually the length of the connected leg should be such that two horizontal rows of bolts can be accommodated.
Design Considerations:
(i) First determine the web crippling length (length of web bearing) given by-
where, R = Factored end reaction of the beam
tw = web thickness of the beam
fyw = yield strength of the web
γmo = partial safety factor for material = 1.10
(ii) Allowing 45° dispersion, length of bearing on the seat angle = b1 = b – (tf + r1)
where, tf = flange thickness of the beam
r1 = radius at the root of flange of beam
(iii) Length of bearing on the seat angle beyond the root of seat angle
= b2 = b1 + g – (ta + ra)
where, g = clearance between beam end and column
ra = Radius at the root of the seat angle.
(iv) The vertical connected leg of the seat angle may be selected to accommodate two horizontal rows of bolts. On this basis the length of leg may be not less than 100 mm.
(v) The loading on the outstanding leg of the seat angle is taken to be uniformly distributed on the length b1.
∴ Intensity of distributed load = [R/b1]
The effective loaded length on the cantilevering outspread leg of seat angle is taken equal to b2.
Maximum bending moment = M = [R/b1] [b22/2]
Design bending strength of angle leg = Md = 1.2 (Ze fy)/γmo
Equating Md and M find the thickness of angle.
(vi) Design shear capacity of the outstanding leg of the seat angle
This should be greater than the factored end reaction of the beam.
(vii) Number of bolts required in the connected vertical leg of the seat angle
= Reaction of beam/Strength of bolt
Provide two nominal bolts to connect the outstanding leg of the seat angle to the beam flange. Provide also a nominal cleat angle connecting the top flange of beam and the flange of column with nominal bolts.
(b) Stiffened Seat Connections:
This type of connection is meant for resisting large beam reactions. In such cases a seat angle will not be strong enough to support the heavy load and within the size of the angle it may not be possible to accommodate the needed number of bolts. In such a case stiffened seat connection is provided.
In this arrangement the following components are provided, viz., a seat angle and stiffener angles. The stiffener angles are vertical angles connected to the flange of the column and supporting outstanding legs of the seat angle. It will be assumed that the beam reaction is fully transmitted to the stiffener angles by bearing.
The outstanding legs of the stiffener angles extend, close to the toe of the seat angle. Since the vertical stiffener angles are in compression they are liable to buckle. To prevent this condition, the ratio of the outstand length to thickness should not exceed 14. The bolts connecting the stiffener angles and the flange of the column are subjected to shear and tension.
Since the load on this connection is eccentric producing moment in a plane perpendicular to the plane of the connection. The outstanding legs of the stiffener angles are connected by tack bolts.
Design Considerations:
(i) A seat angle is selected providing sufficient bearing length.
(ii) Sufficient bearing area should be provided by the outstanding legs of the stiffener angles.
Stiffener angles may be selected with thickness more than the thickness of web of the beam supported.
(iii) The bolts connecting the stiffener angles and the flange of the column are designed considering the eccentric transmission of the load on the connection.
It will be necessary to provide a packing or filler plate to be placed in the gap between the column flange and the legs of the stiffener angles. The length of the stiffener angles shall be such as to accommodate the bolts at the selected pitch.
D. Moment Resistant Connections:
There are instances when connections are required to transmit moments in addition to shear loads. Such connections are called moment resistant connections.
Based on the magnitude of the moment acting on the joint these connections are classified as:
1. Light Moment Resisting Connections:
Light moment resisting connection is made by providing two pairs of angles to connect a beam to the column. One pair of angles consists of a seat angle connecting the bottom flange of the beam and the column flange and a clip angle connecting the top flange of the beam and the column flange. The second pair of angles (one on either side of the web of the beam) connect the web of the beam and the column flange.
If a clockwise moment is applied on the connection the bolts are subjected to a tension and the bolts 2 and 4 are subjected to shear. Similarly if an anticlockwise moment is applied on the connection the bolts 3 are subjected to a tension and the bolts 2 and 4 are subjected to shear. Suppose a clockwise moment M acts on the connection.
The bolts 1 of the clip angle will be subjected to a tension P.
The restoring couple P (g + h + g) will resist the moment M so that
M = P(g + h + g)
where, g = gauge distance for angle leg
h = Depth of the beam
The leg of the angle will be subjected to a bending moment equal to-
Ma = P[g – (t/2)]
In the expression for Ma above we have assumed that there is no initial tension in the bolts. If the initial tension exists the angle leg bends with double curvature due to pressure of column flange on the angle resulting in a reduction of bending moment and a considerable increase in the tension in the bolt.
Under this condition, for design purposes the maximum bending moment for the angle may be taken as-
where, l = length of the angle.
Equating Ma to the design bending strength we can determine the thickness of angle-
t is determined by trial and error.
Bolt 1 will be designed for the pull P
Bolts 2 and 4 are designed for a shear load
Vh = M/h
where, M = Moment applied on the connection
h = Depth of the beam
These bolts are in single shear.
Number of bolts required = Vh/strength of bolt
Bolts connecting the web clip angles to web of beam are subjected to vertical shear V = End shear of the beam.
Number of bolts required = V/strength of bolt. These bolts are in double shear.
The web clip angles must be designed to transmit the shear V.
Bolts connecting the stiffener angles to web of column are in single shear
Number of bolts required = V/strength of bolt
2. Heavy Moment Resisting Connections:
When the connection is subjected to a large moment the following arrangement called split beam connection can be used.
In this method an I-section beam is cut into two halves cutting it in the middle of the web. The two split parts are provided, one at the top and the other at the bottom of the beam in place of the cleat and seat angles of the previous type. As an alternative, instead of the split beams, tee sections of suitable dimensions may also be used. As in the previous case web clip angles are provided connecting the web of the beam and the flange of the column.
Design Considerations:
Design of Web Stiffeners:
A pair of web stiffening angles, say 90 × 90 × 8 or 100 × 100 × 8 may be selected. This section should be able to resist the end shear.
Design of Bolts:
Consider the bolts 1-1. The top or the bottom set of bolts will be subjected to a tension P depending on whether the moment on the connection is clockwise or anticlockwise.
The tension P is given by-
P = M/ (d + t)
M = Moment on the connection
d = Depth of the beam
t = Thickness of web of split section.
These bolts are designed to resist this tension.
Now consider the bolts 2-2 connecting web of the tee and flange of the beam.
The top and the bottom set of bolts are each subjected to a horizontal shear Ph given by-
Ph = M/d
These bolts must be designed to resist this shear load.
Design of Tee-section-
Let g = distance between the two rows of bolts.
Maximum bending moment for the projecting arm-
Alternative Arrangement for Heavy Moment Connection:
In this arrangement two pairs of angles with a gussel plate over the flange of the beam and two pairs of angles with a gussel plate below the beam are provided forming brackets. These brackets are connected to the column flange. The brackets function like the split beams of the previous arrangement. Web stiffener angles are provided as in the previous arrangement.
E. Pin Connections:
A pin connection is provided to allow free rotation of the end of a connected member. Some of the design considerations of a pinned connection are the same as those of bolted connections. A pin in a connection acts similar to the shank of a bolt. But since a single pin is used for the connection, the forces acting on it are much greater than those on a bolt.
Large shear forces can be resisted by pins of large diameter and of a high grade steel. A pin is relatively long and it is necessary to consider bending moment to which it is subjected. Bending stresses are affected by the arrangement of the connected plates. It is necessary to place the plates symmetrically and positioned correctly to reduce stresses.
The I.S. code has not proposed specifications for pinned connections and relevant material from B.S. code has been used in the analysis and design of these connections.
In normal practice the diameter of the pin hole should not exceed d + 0.8 mm where d is the diameter of the pin in mm. Its length should be sufficient to provide full bearing on the connected part. A pin should be well secured in position and the connected component should be restrained against lateral movement on it. A pin is generally secured by providing a cotter pin.
Pin connections offer some advantages. When provided these connections decrease the order of statical indeterminacy. Secondary stresses in the components are also minimized. A single pin is used for the connection while several bolts/revets are provided for a connection. However the pin connections have some shortcomings too.
Longitudinal tension can give rise to friction affecting the expected free rotation at the connection. The pins and the pin holes need considerable machine work. Due to non- rigidity of the connections considerable noise may occur in bridges as a consequence of vibrations due to traffic.
Pins are used in connections of braced girders, chain link cables of suspension bridges, diagonal bracings etc.
Pin connections can be made in two ways. In the type of connection shown in Fig. 4.91 an end of one of the bars is forged to form a fork with a drilled hole in it. The end of the other bar called the eye bar is forged with a drilled hole in it. The eye bar is inserted between the jaw of the fork with all the holes matching. A forged steel pin is driven through the holes of the bars. The pin secured by a cotter pin. The bar ends are made octagonal.
Fig. 4.93 shows a pin connection to connect two pairs of parallel eye bars, with an undrilled pin passing through the holes of the bars. In these cases a 3 mm spacer is provided between the inner eye bars, to allow for free rotation at the joint.
Specifications for Design:
The I.S. code has not proposed specifications for pinned connections. The specifications given below are as per B.S. code.
(i) Shear Capacity of a Pin:
(a) If rotation is not required and the pin is not intended to be removed Shear capacity = 0.6 fy A
(b) If rotation is required or the pin is intended to be removed-
Shear capacity = 0.5 fy A
where, fy = yield stress for pin
A = sectional area of the pin
(ii) Bearing Capacity:
(a) If rotation is not required and the pin is not intended to be removed.
Bearing capacity = 1.5 fy dt
(b) If rotation is required or the pin is intended to be removed.
Bearing capacity = 0.8 fy dt
where,
fy = lower of yield stresses of the pin or connected part.
t = thickness of the connected part.
(iii) Flexural Capacity:
(a) If rotation is not required and the pin is not intended to be removed-
Moment capacity Mu = 1.5 fy (πd3)/32
(b) If rotation is required and the pin is intended to be removed-
Moment capacity Mu = 1.0 fy (πd3)/32
Welded Connections:
A. Eccentric Welded Connections:
The resistance offered by a weld was entirely to prevent a linear or translatory displacement of a plate or member connected. There are also circumstances in which the welds provided for a connection may have to offer not only a resistance to prevent translatory displacement but also a resistance to prevent rotatory displacement. A bracket connection is an example of eccentric welded connection.
There are following two types of bracket connections:
1. Welded Bracket Connection Subjected to Moment in the Plane of the Weld:
Consider the bracket connection shows in Fig. 5.36. It consists of two bracket plates welded to the flanges of a steel column. If a load W be applied to the bracket a load P = W/2 is transmitted to each bracket plate. The line of action of the load P does not pass through the centroid of the weld group.
Let G be the centroid of the weld lengths
Let e = Eccentricity of the load
= Distance between G and the line of action of the load P
= x̅ + a (Fig. 5.36)
The weld has to offer the following resistances:
(a) Resistance against Translation:
This resistance is assumed to be uniform over the whole length of the weld.
... Resistance against translation per unit length of the weld = P/L
Where L = Total length of the weld on the bracket plate.
(b) Resistance against the Rotation of the Bracket Plate:
The force of resistance per unit length of the weld at any point of the weld length against rotation of the bracket plate is assumed to be proportional to the distance of the point from the centroid of the weld group.
Consider an elemental length dl of weld at any point Z of the weld line distant r from the centroid G.
Resistance offered against rotation by the elemental weld length
= Krdl acting normal to GZ
Restoring moment offered by the elemental weld length
= (Krdl) r = Kdlr2
Total restoring moment offered by the whole weld
∑Kdlr2 = K∑dlr2 = KIp
where, Ip = ∑dlr2 = Polar moment of inertia of the weld length.
But Ip = Ixx + Iyy
where, Ixx = Moment of inertia of the weld length about the axis XX in the plane of the weld through G.
Iyy = Moment of inertia of the weld length about the axis YY in the plane of the weld through G.
Total restoring moment offered by the weld length
= K(Ixx + Iyy)
External moment on the connection = Pe
Equating the restoring moment to the external moment.
From the above relation, the constant K can be determined for any given arrangement of weld length. The maximum force of resistance against rotation per unit length of the weld is offered at the point A most distant from G.
Considering unit length (say 1 mm length of weld) at A,
Resistance against translation = P/L
Resistance against rotation = S = Kra (acting at right angles to GA) (Fig. 5.38).
Let θ be the inclination of GA with the YY axis.
Total vertical force of resistance per unit length of weld at A
= V = P/L + S sin θ
Horizontal force of resistance per unit length of weld at A
= H = S cos θ
Resultant resistance offered by unit length of weld at A
= R= √(V2 + H2)
Position of centroid G of the weld group for two usual arrangement of weld lengths are given below:
(a) When the bracket plate is welded to the column flange as shown in Fig. 5.39(a). In this case the welding is done on all the four sides of the rectangle ABCD
x̅ = b/2 and e = x̅ + a
(b) When the bracket plate is welded to the column flange as shown in Fig. 5.39(b). In this case the welding is done only on the three sides AD, DC and CB of the rectangle ABCD.
x̅ = [b(b + d)]/[b + (b + d)] and e = x̅ + a
Moment of Inertia of a Weld Length (Fig. 5.40 and 5.41):
Moment of inertia of a weld line of length d about the axis XX = Ixx = d3/12
Moment of inertia of this weld line about the axis EF = lef = Ixx + d(y̅)2 parallel to the XX
Moment of inertia of this weld line about its longitudinal axis = 0
Moment of inertia of a weld line of length d about an axis parallel to the weld length at a distance x1, from the weld line = dx12
2. Welded Connection Subjected to Moment in a Plane Normal to the Plane of the Connection:
Fig. 5.52 shows an eccentric bracket connection where the weld is subjected to shear and bending stresses. Let P be the load applied on the bracket at an eccentricity e. Figure also shows the geometry of the weld group. Let all the welds be of the same size s. Let L be the total length of the weld.
Note:
Check for the combination of stresses need not be done, for fillet welds if fa + q does not exceed fu/√3γnw.
B. Beam – Column Connections:
When a beam is to be connected to a column the following types of connections can be provided:
1. When the Connection does not Provide Restraint against Rotation:
(a) Direct Web Fillet Welded Connection:
In this case the beam transmits its end reaction to the column entirely by shear. The connection is made by directly connecting the web of the beam to the flange of the column by fillet welds on both sides of the web of the beam.
The length of weld hw on each side of the web and its size are such that the necessary design to resist the factored reaction is provided. The size of the weld may be chosen about 0.7 [Web thickness of beam or flange thickness of column which ever is greater].
For this arrangement to be functional the machined end of the beam should be in full contact with the column flange for the welding to be carried out satisfactorily. It may become necessary to provide temporary erection seats to support the beam during the process of welding.
Design stress of the weld = fu/√3γmw
fu = 410 N/mm2, γmw = 1.25 for shop welding and 1.50 for field welding
Equating the design strength of the weld to the factored shear V
We can determine hw.
(b) Framed Connection to Web:
Two unequal angles 90 mm x 60 mm x 10 mm are used one on either side of the web of the beam, to connect the beam and the flange of the column. The shorter legs of the angles are shop welded to the web of the beam all around.
The outs spread longer legs of the angles are welded to the flange of the column at their sides only. The framing angles are flexible and do not transmit moment to the column. This arrangement is a better arrangement than the method described in (a) above.
Weld Connecting the Outspread Legs of Angles and the Flange of the Column:
These welds are subjected to vertical as well as horizontal shear. Vertical shear per mm height = fv = (v/2hw) [hw = height of angle = 1/2 to 2/3 depth of the beam]
Maximum horizontal shear per mm height for design purposes is taken equal to fh = 9/5. (Vx1/hw2) (x1 = length of the longer leg of the angle)
Resultant resistance per mm length fr = √(fv2 + fh2)
fn should not exceed the design strength of the weld per mm length.
Weld Connecting the Short Legs of Angles and the Beam of the Web:
Consider the weld on one side of the web (Fig. 5.63).
Consider the three sided weld
The lengths of the three welds are bw, hw and bw
x̅ = [bw (bw + hw)]/[bw + (bw + hw)]
Eccentric load on the weld P = V/2
Eccentricity = e = x̅ + 10 mm
Twisting moment = Pe
Resistance against translation per mm length of weld
= P/L, L = bw + hw + bw
Max. resistance against rotation (torsional shear) per mm length of weld
= (Pe/Ip). r
Total vertical resistance per mm length of weld
Horizontal resistance per mm length of weld
fr shall not exceed the design strength of the weld per mm length of weld.
2. Connections Providing Partial Restraint:
(a) Unstiffened Seat Connection:
In this method a seat angle is shop welded to the flange of the column. The end of the beam rests on the seat. The beam is later field welded to the seat. The seat is designed to support the beam reaction.
A cleat angle is welded to the top flange of beam and the column flange providing a normal fixture.
The outstanding legs of the angles are designed from bending consideration.
The welds are designed for vertical shear and bending moment due to eccentricity.
Design details are given below –
Length of the seat angle = Flange width of the beam.
Bearing length = b = (V/twfyw. γmo) (tw = web thickness of beam fyw = yield stress of web)
Length of the outstand of seat angle = b + c, c = clearance of 5 mm to 10 mm)
Bearing length on the seat angle = b1 = b – (tf + r1)
tf = Flange thickness of beam
r1 = Radius at root for beam.
The load V is taken to be uniformly distributed over the length b1.
Distance between the end of bearing on the seat and the root of seat angle
= b2 = b1 + c – (ta + ra)
ta = thickness of angle
ra = radius of root of angle.
Maximum bending moment for the angle leg
= M = V × (b2/b1) × (b2/2)
Equating the design bending strength to the maximum moment
1.2 (fyZe/γmo) = 1.2 fy (Bt2/6) (1/γmo) = M
[B = length of seat angle = Flange width of beam]
We can find t. This is the minimum thickness of the seat angle required.
Design shear strength of the outstand of seat angle
= (fy/√3γmo)[Bta] This should be greater than the end reaction.
Weld connection of the seat angle with the column flange
The vertical legs of the seat angle are welded to the flange of the column
Total length of weld = 2l
I = length of vertical leg of seat angle
Vertical resistance per mm length of weld
This should not exceed the strength of the weld per mm length.
(b) Stiffened Beam Scat Connection:
This type of connection is useful to transmit heavy end reaction from a beam to the flange of a column. In such cases the seat angle will not be strong enough to bear the load and will not be strong enough to resist bending.
The stiffened seat connection consists of a horizontal plate called the seat plate and a vertical plate called the stem or stiffening plate welded together to form a 7-section. This frame is welded to the flange of the column. The thickness of the seat plate should be not less than the thickness of the flange of the column.
The width of this plate should be slightly greater than the flange width of the beam. The thickness of the stem should be greater than the thickness of the web of the beam. The bearing length of the seat plate is determined as in the unstiffened seat connection.
The height of the stem shall be such as to accommodate the length of weld needed to connect it to the column flange and shall also be such as to prevent local buckling. On this consideration the ratio of the height of the stem to its thickness shall not exceed 18.9.
A nominal cleat angle is provided connecting the top flange of the beam and the flange of the column.
3. Connections Providing Full Restraint:
(a) Fully Welded End Connection:
In this connection absolute fixity is assumed. The connection is designed attain the full strength of the beam in bending as well as shear. The beam end is satisfactorily prepared so that full penetration butt weld connections are made in the flanges and the web. As an alternative the connection can be made by fillet welding on both sides. To protect the flanges of the supporting column, two backup plates (stiffening plates) are provided. When full penetration butt welding is done, there will be no need for any design calculations since such welds develop full strength of the parent metal.
(b) Moment Resistant Connections:
These connections are used in frames of multi-storeyed buildings in which full rigidity is an important requirement. However with suitably planned designs it is possible to provide connections for any desired degree of rigidity thus providing a semi rigid connection transmitting a predetermined part of the beam end moment to the supporting column. Such semi rigid connections permit redistribution of moments in the frame of the structure leading to simplifications in the structural analysis.
The many alternative arrangements for these connections depend on the degree of flexibility as well as ductility desired. For an absolutely rigid connection, the fully welded connection described earlier can be adopted.
Fig. 5.74 shows a connection where the joint is ductile enough to transmit a moment less than the ultimate moment of resistance of the section.
Design Considerations –
Let V = Factored end shear
M = Factored end beam moment
(i) Provide a top and a bottom plate over the top and under the bottom of the flange.
Tension in the plate = P = M/d
where, d = Depth of the beam.
Area required tor the plate section = A = (P/ƒy). γmo
Width of the top plate at column end = Width of flange of the beam. The width of the plate can be gradually tapered to a width equal to the width at the column end – 20 mm.
Thickness of plate required = Area of plate required/Reduced width of the plate
This plate is welded to the flange of the beam by fillet weld.
Length of fillet required = Tension in the plate/Strength of weld per mm length
This plate is also connected to the flange of the column by full penetration groove weld. This plate should be sufficiently long so that the unwelded length of the plate is not less than the reduced width of the plate.
The bottom plate is also similarly connected to the bottom flange of the beam and the flange of the column.
(ii) The end reaction of the beam is transmitted to the flange of the column by providing two angles one on either side of the web of the beam. The angles are welded to the web of beam and flange of column.
For connecting the angles to the web of the beam
Length of weld required = Beam end reaction/Strength of weld per mm length
One half of this length of weld is required for each angle. The outstanding angle arms are welded to the flange of the column to transmit the shear.
The length of the angles must be such as to accommodate these welds.
(iii) Backing plates (stiffeners) are provided to protect local bending of the column flange. These consist of a pair of plates for top back up plates and a pair of bottom back of plates.
Force in top or bottom backup plates = P = M/d.
The backup plates are welded to the flange and web of the column.
Consider the top pair of backup plates, 6 mm fillet welds may be provided for the connection.
Total length of weld required for the connection
= P/Strength of weld per mm length
Welds connecting the top back plates to the flange of the column consist of four units of length x each
x shall be less than 1/2 [Width of column flange – tw – 2r]
r = radius at root of flange of column.
After providing this (4x) length of weld, any additional length of weld to be provided can be done between the plate and the column web.