CONNECTION DESIGN Connections must be designed at the strength limit state

Average of the factored force effect at the connection and the force effect in the member at the same point

At least 75% of the force effect in the member

End connections for diaphragms, cross-frames, lateral bracing for straight flexural members - designed for factored member loads

Connections should be symmetrical about member axis At least two bolts or equivalent weld per connection Members connected so that their gravity axes intersect at a point Eccentric connections should be avoided

End connections for floorbeams and girders Two angles with thickness > 0.375 in. Made with high strength bolts If welded account for bending moment in design

BOLTED CONNECTIONS Slip-critical and bearing type bolted connections. Connections should be designed to be slip-critical where:

stress reversal, heavy impact loads, severe vibration joint slippage would be detrimental to the serviceability of the structure

Joints that must be designed to be slip-critical include Joints subject to fatigue loading or significant load reversal. Joints with oversized holes or slotted holes Joints where welds and bolts sharing in transmitting load Joints in axial tension or combined axial tension and shear

Bearing-type bolted connections can be designed for joints subjected to compression or joints for bracing members

SLIP-CRITICAL BOLTED CONNECTION

Slip-critical bolted connections can fail in two ways: (a) slip at the connection; (b) bearing failure of the connection

Slip-critical connection must be designed to: (a) resist slip at load Service II; and (b) resist bearing / shear at strength limit states

SLIP-CRITICAL BOLTED CONNECTION Slip-critical bolted connections can be installed with such a degree

of tightness large tensile forces in the bolt clamp the connected plates together

Applied Shear force resisted by friction

Tightened

P

P

Tightened Tightened

P

P

P

P

Tb

N =Tb

N =Tb

N =Tb

P

F=N

Tb

N = Tb

F=N

N = Tb

N =Tb

P

Tb

N =Tb

Tb

N =Tb

N =Tb

N =Tb

P

F=N

N =Tb

N =Tb

P

F=N

Tb

N = Tb

Tb

N = Tb

F=N

N = Tb

N =Tb

P

F=N

N = Tb

N =Tb

N = Tb

N =Tb

P

SLIP-CRITICAL BOLTED CONNECTION Slip-critical connections can resist the shear force using friction.

If the applied shear force is less than the friction that develops between the two surfaces, then no slip will occur between them

Nominal slip resistance of a bolt in a slip-critical connection Rn = Kh Ks Ns Pt Where, Pt = minimum required bolt tension specified in Table 1

Kh = hole factor specified in Table 1Ks = surface condition factor specified in Table 3

SLIP-CRITICAL BOLTED CONNECTION

Faying surfaces Unpainted clean mill scale, and blast-cleaned surfaces with Class A coating Unpainted blast-cleaned surfaces with Class B coating Hot-dip galvanized surfaces roughened by wire brushing Class C

Bolt diameter (in.)

Required Tension (kips)

A325 A490 5/8 19 24 3/4 28 35 7/8 39 49 1 51 64

1-1/8 56 80 1-1/4 71 102 1-3/8 85 121 1-1/2 103 148

For standard holes 1.0

For oversize and short-slotted holes 0.85

For long slotted holes with the slot Perpendicular to the force direction

0.70

For long-slotted holes with the slot Parallel to the force direction

0.60

Values of Kh

Values of Pt

For Class A surface conditions 0.33

For Class B surface conditions 0.50

For Class C surface conditions 0.33

Values of Ks

SLIP-CRITICAL CONNECTION

Connection subjected to tensile force (Tu), which reduces clamping Nominal slip resistance should be reduced by (1- Tu/Pt)

Slip is not a catastrophic failure limit-state because slip-critical bolted connections behave as bearing type connections after slip.

Slip-critical bolted connections are further designed as bearing-type bolted connection for the applicable factored strength limit state.

BEARING CONNECTION In a bearing-type connection, bolts are subjected to shear and the

connecting / connected plates are subjected to bearing stresses :

Bolt in shear

Bearing stresses in plate

Bearing stresses in plate

TT

T

T

Bolt in shear

Bearing stresses in plate

Bearing stresses in plate

Bolt in shear

Bearing stresses in plate

Bearing stresses in plate

TT

T

T

BEARING CONNECTION

Bearing type connection can fail in several failure modesa) Shear failure of the bolts

b) Excessive bearing deformation at the bolt holes in the connected parts

c) Edge tearing or fracture of the connected plate

d) Tearing or fracture of the connected plate between two bolt holes

e) Failure of member being connected due to fracture or block shear or ...

BEARING CONNECTION Nominal shear resistance of a bolt

Threads excluded: Rn = 0.48 Ab Fub Ns Threads included: Rn = 0.38 Ab Fub NsWhere, Ab = area of the bolt corresponding to the nominal diameter

Fub = 120 ksi for A325 bolts with diameters 0.5 through 1.0 in.

Fub = 105 ksi for A325 bolts with diameters 1.125 through 1.5 in.

Fub = 150 ksi for A490 bolts.

Ns = number of shear planes

Resistance factor for bolts in shear = s = 0.80 Equations above - valid for joints with length < less than 50.0 in.

If the length is greater than 50 in., then the values from the equations have to be multiplied by 0.8

BEARING CONNECTION Effective bearing area of a bolt = the bolt diameter multiplied by the

thickness of the connected material on which it bears

Bearing resistance for standard, oversize, or short-slotted holes in any direction, and long-slotted holes parallel to the bearing force: For bolts spaced with clear distance between holes greater than or equal to 3.0 d

and for bolts with a clear end distance greater than or equal to 2.0 d

Rn = 2.4 d t Fu For bolts spaced with clear distance between holes less than 3.0 d

and for bolts with clear end distances less than 2.0 d

Rn = 1.2 Lc t FuWhere, d = nominal bolt diameter

Lc= clear distance between holes or between the hole and the end of the member in

the direction of applied bearing force

Fu = tensile strength of the connected material

The resistance factor bb for material in bearing due to bolts = 0.80

BEARING CONNECTION SPACING REQUIREMENTS

Minimum spacing between centers of bolts in standard holes shall not be less than three times the diameter of the bolt

For sealing against penetration of moisture in joints, the spacing on a single line adjacent to the free edge shall satisfy s (4.0 + 4.0 t) 7.0

Minimum edge distances Bolt diameter

(in.) Sheared

edge Rolled or

Gas Cut edge 5/8 1-1/8 7/8 3/4 1-1/4 1 7/8 1-1/2 1-1/8 1 1-3/4 1-1/4

1-1/8 2 1-1/2 1-1/4 2-1/4 1-5/8 1-3/8 2-3/8 1-3/4

BOLTED CONNECTION Example 1 Design a slip-critical splice for a tension member. For

the Service II load combination, the member is subjected to a tension load of 200 kips. For the strength limit state, the member is subjected to a maximum tension load of 300 kips. The tension member is a W8 x 28 section made from M270-Gr. 50

steel. Use A325 bolts to design the slip-critical splice.

Step I. Service and factored loads Service Load = 200 kips. Factored design load = 300 kips Tension member is W8 x 28 section made from M270 Gr.50. The

tension splice must be slip critical (i.e., it must not slip) at service loads.

BOLTED CONNECTIONStep II. Slip-critical splice connection

Slip resistance of one fully-tensioned slip-critical bolt = Rn = Kh Ks Ns Pt = 1.0 for slip-critical resistance evaluation Assume bolt diameter = d = in. Therefore Pt = 28 kips from Table 1 Assume standard holes. Therefore Kh = 1.0 Assume Class A surface condition. Therefore Ks = 0.33 Therefore, Rn = 1.0 x 0.33 x 1 x 28 = 9.24 kips

Therefore, number of in. diameter bolts required for splice to be slip-critical at service loads = 200 / 9.24 = 21.64.

Therefore, number of bolts required 22

BOLTED CONNECTION

Step III: Layout of flange-plate splice connection To be symmetric about centerline, need the number of bolts = multiple of 8. Therefore, choose 24 fully tensioned 3/4 in. A325 bolts with layout above.

Slip-critical strength of the connection = 24 x 9.24 kips = 221.7 kips Minimum edge distance (Le) = 1 in. from Table 4.

Design edge distance Le = 1.25 in. Minimum spacing = s = 3 x bolt diameter = 3 x = 2.25 in.

Design spacing = 2.5 in.

BOLTED CONNECTION

Step IV: Connection strength at factored loads The connection should be designed as a normal shear/bearing

connection beyond this point for the factored load of 300 kips

Shear strength of high strength bolt = Rn = 0.80 x 0.38 x Ab x Fub Ns Equation given earlier for threads included in shear plane. Ab = 3.14 x 0.752 / 4 = 0.442 in2 Fub = 120 ksi for A325 bolts with d < 1-1/8 in. Ns= 1 Therefore, Rn = 16.1 kips

The shear strength of 24 bolts = 16.1 kips/bolt x 24 = 386.9 kips

BOLTED CONNECTION Bearing strength of 3/4 in. bolts at edge holes (Le = 1.25 in.)

bb Rn = 0.80 x 1.2 Lc t FuBecause the clear edge distance = 1.25 (3/4 + 1/16)/2 = 0.84375 in. < 2 d

bb Rn = 0.80 x 1.2 x 0.84375 x 65 kips x t = 52.65 kips / in. thickness

Bearing strength of of 3/4 in. bolts at non-edge holes (s = 2.5) bb Rn = 0.80 x 2.4 d t Fu

Because the clear distance between holes = 2.5 (3/4 + 1/16) = 1.6875 in. > 2d

bb Rn = 0.80 x 2.4 x 0.75 x 65 kips x t = 93.6 kips / in. thickness

Bearing strength of bolt holes in flanges of wide flange section W8 x 28(t = 0.465 in.) 8 x 52.65 x 0.465 +16 x 93.6 x 0.465 = 892 kips

CONNECTION STRENGTH

Connection strength (Rn) > applied factored loads (Q). Therefore ok

Connection Strength

Slip-critical strength = 221.7 kips

Shear strength of bolts = 386.9 kips

Bearing strength (plate) = 892 kips

WELDED CONNECTIONS

Introduction The shielded metal arc welding (SMAW) process for field welding. Submerged metal arc welding (SAW) used for shop welding

automatic or semi-automatic process

Quality control of welded connections is particularly difficult because of defects below the surface, or even minor flaws at the surface, will escape visual detection.

Welders must be properly certified, and for critical work, special inspection techniques such as radiography or ultrasonic testing must be used.

WELDED CONNECTIONS Two most common types of welds are the fillet and the groove weld.

lap joint fillet welds placed in the corner formed by two plates Tee joint fillet welds placed at the intersection of two plates.

Groove welds deposited in a gap or groove between two parts to be connected e.g., butt, tee, and corner joints with beveled (prepared) edges Partial penetration groove welds can be made from one or both sides with or

without edge preparation.

Fillet weld

Fillet weld

Fillet weldFillet weld

Fillet weldFillet weld

WELDED CONNECTIONS Design of fillet welded connections

Fillet welds are most common and used widely Weld sizes are specified in 1/16 in. increments Fillet welds are usually fail in shear, where the shear failure occurs

along a plane through the throat of the weld

Shear stress in fillet weld of length L subjected to load P fv =

a

aThroat = a x cos45o

= 0.707 a

a

aThroat = a x cos45o

= 0.707 a

Failure Plane

L

wLa707.0P

FILLET WELDED CONNECTIONS The shear strength of the fillet weld = e2 0.60 Fexx

Where, e2 = 0.80 Fexx is the tensile strength of the weld electrode used in the welding

process. It can be 60, 70, 80, 90, 100, 110, or 120 ksi. The corresponding electrodes are specified using the nomenclature E60XX, E70XX, E80XX, and so on.

Therefore, the shear strength of the fillet weld connection Rn = e2 x 0.60 Fexx x 0.707 a Lw

Electrode strength should match the base metal strength If yield stress (y) of the base metal is 60 - 65 ksi, use E70XX electrode If yield stress (y) of the base metal is 60 - 65 ksi, use E80XX electrode

E70XX is the most popular electrode used for SMAW fillet welds For E70XX, Rn = 0.80 x 0.60 x 70 x 0.707 a Lw = 0.2375 a Lw kips

FILLET WELDED CONNECTIONS The shear strength of the base metal must be considered:

Rn = v x 0.58 Ag Fywhere, v = 1.0

Fy is the yield strength of the base metal and Ag is the gross area in shear

Strength of weld in shear Strength of base metal

= 0.80 x 0.60 x Fexx x 0.707 x a x Lw = 1.0 x 0.58 x Fy x t x Lw

Smaller governs the strength of the weld

T

Elevation Plan

T

Elevation Plan

FILLET WELDED CONNECTIONSLimitations on weld dimensions Minimum size (amin)

Weld size need not exceed the thickness of the thinner part joined. amin depends on the thickness of the thicker part joined If the thickness of the thicker part joined (T) is less than or equal to in. amin = in.

If T is greater than in. amin = 5/16 in. Maximum size (amax)

Maximum size of fillet weld along edges of connected parts for material with thickness < 0.25 in., amax = thickness of the material for plates with thickness 0.25 in., amax = thickness of material - 1/16 in.

Minimum length (Lw) Minimum effective length of fillet weld = 4 x size of fillet weld Effective length of fillet weld > 1.5 in.

FILLET WELDED CONNECTIONS

Weld terminations and end returns End returns must not be provided around transverse stiffeners Fillet welds that resist tensile forces not parallel to the weld axis or

proportioned to withstand repeated stress shall not terminate at corners of parts or members

Where end returns can be made in the same plane, they shall be returned continuously, full size around the corner, for a length equal to twice the weld size (2a)

FILLET WELD DESIGNExample 1 Design the fillet welded connection system for a double

angle tension member 2L 5 x 3 x 1/2 made from A36 steel to carry a factored ultimate load of 250 kips.

Step I. Design the welded connectionConsidering only the thickness of the angles; amin = 1/4 in.

Considering only the thickness of the angles; amax = 1/2 - 1/16 in. = 7/16 in.

Design, a = 3/8 in. = 0.375 in.

Shear strength of weld metal = Rn = 0.80 x 0.60 x FEXX x 0.707 x a x Lw= 8.9 x Lw kips

Strength of the base metal in shear = Rn = 1.0 x 0.58 x Fy x t x Lw= 10.44 Lw kips

Shear strength of weld metal governs, Rn = 8.9 Lw kips

FILLET WELD DESIGN

Design strength Rn > 250 kips Therefore, 8.9 Lw > 250 kips Therefore, Lw > 28.1 in.

Design length of 3/8 in. E70XX fillet weld = 30.0 in. Shear strength of fillet weld = 267 kips Connection layout

Connection must be designed to minimize eccentricity of loading. Therefore, the center or gravity of the welded connection must coincide with the center of gravity of the member.

Tu

f L2

f L1

L1

L2

3.4 in.Tu

f L2

f L1

L1

L2

3.4 in.

FILLET WELD DESIGN Connection layout

Connection must be designed to minimize eccentricity of loading. The c.g. of the welded connection must coincide with c.g. of the member

Total length of weld required = 30 in. Two angles assume each angle will have weld length of 15 in.

Tu

f L2

f L1

L1

L2

3.4 in.Tu

f L2

f L1

L1

L2

3.4 in.

FILLET WELD DESIGN

The tension force Tu acts along the c.g. of the member, which is 1.65 in. from the top and 3.35 in. from the bottom (AISC manual). Let, f be the strength of the fillet weld per unit length.

Therefore, fL1 + fL2 = Tu And fL2 x 3.35 - fL1 x 1.65 = 0 - taking moments about the member c.g. Therefore, L1 = 2.0 L2

But, L1 + L2 = 15.0 in.

Therefore, L1 = 10 in. and L2 = 5 in.

Design: L1 = 10.0 in. and L2 = 5.0 in.

FILLET WELD DESIGN Consider another layout

Tu

f L2

L1

L2

f L1

5f 3.4 inTu

f L2

L1

L2

f L1

5f 3.4 in

fL1 + fL2 + 5f = Tu

fL2 x 3.5 + 5f x 0.85 - fL1 x 1.65 = 0 - Moment about member c.g.

Additionally, L1 + L2 + 5 = 15.0 in.

Therefore, L1 = 7.6 in. and L2 = 2.4 in.

Design: L1 = 8.0 in. and L2 = 3.0 in.

Groove Welded Connections Connects structural members that are aligned in the same plane Basic Types:

Complete joint penetration groove weld: transmits full load of the member they join and have the same strength as the base metal.

Partial penetration groove weld: Welds do not extend completely through the thickness of the pieces being joined.

Groove WeldsComplete penetration groove welded connections Tension and compression loaded

Factored resistance = factored resistance of base metal Shear loaded on effective area lesser of

Factored resistance of weld = 0.6 x e1 x Fexx = 0.6 x 0.85 x Fexx 60% of factored resistance of base metal in tension

Partial penetration groove-welded connections Tension or compression parallel to the weld axis and compression normal

to effective area factored resistance of the base metal Tension normal to the effective area lesser of

Factored resistance of the weld = 0.6 e2 Fexx = 0.60 x 0.80 x Fexx Factored resistance of the base metal

Shear loaded lesser of Factored resistance of the weld = 0.6 e2 Fexx = 0.60 x 0.80 x Fexx Factored resistance of base metal = 0.58 Fy

Groove Welds