Bolted Connections Introduction | Design of Steel Structures

Design of Bolted Connections

* Assumption in design of bearing bolted connections:

1. The friction between the plates is negligible.

2. Shear is uniform over the cross section of the bolt

3. The stress distribution between bolt holes is uniform

4. Bolts in a group subjected to direct loads share the load equally.

Principles observed in design:

1. The centre of gravity of bolts should coincide with the centre of gravity of connected members.

2. The length to connection should be as small as possible.

* Design tensile strength of a joint is least of the following:

1. Tensile strength of plate

where gmc = partial safety factor = 1.25

fu = ultimate stress

An = net area of the plate

where h = width of plate

t = thickness of thinner plate

do = diameter of bolt hole

g = gauge length

Ps = staggered pitch

n = number of bolt holes at critical section

2. Shear capacity of bolts

where fub = ultimate tensile strength of bolt

nn = number of shear planes with threads

ns = number of shear planes with threads intercepting shear planes (in shank)

Asb = nominal shank area of the bolts

Anb = net area of the bolt at threads

= , where p = pitch of thread

= 0.78 for ISO threads.

Reduction factors for shear strength:

(a) For long joints: If the distance between first and least bolt exceed 15 d, the reduction factor

Subjected to 0.75 £ bij £ 1.0

(b) Reduction factor, if grip length is larger, i.e., total thickness for connected plates is more than 5 d

(c) Reduction factor if packing plates used are more than 6 mm thick:

bpk = 1 – 0.0125 tpk

where tpk = thickness of thicker packing in mm

Thus, the bearing capacity of the bolts in shear is

3. Bearing capacity of bolts

where gmb = partial safety factor of material = 1.25

and kb

is smaller of

where e = end distance

p = pitch distance

do = diameter of hole

d = nominal diameter of bolt

t = thickness of the connected plates experiencing bearing stress in the same direction.

Nominal diameter of bolts (d): 12 14 16 20 22 24 30 36

Diameter of bolt hole: 13 15 18 22 24 26 33 39

* Efficiency of Joint :

• 
  where strength of solid plate =
• Eccentric Connection

(a) Line of action of load is in the plane of group of bolts

If F2 acts at q to line joining CG of bolts and the point,

This value should be within the limit.

Fig. 8.1 Line of action of load in the plane of group of bolts

(b) Load causing moment in the plane perpendicular to the plane of group of bolts

Fig. 8.2 Load causing moment in the plane perpendicular to the plane of group of bolt

On compression side the connecting angle assists in transferring load to column while on tension side

only bolts have to resist. Hence, N – A is assumed at a distance th depth of the bracket.

\ Total moment resisted by bolts on tension side

where M at joint and V is design shear strength of bolt and p is pitch.

Try the above number and check with interaction formula.

HSFG Bolts

Nominal shear capacity of HSFG bolts

Vnsf = mf ne kh Fo

where mf = coefficient of friction, as given in Table 20 in is 800–2007 for different surfaces.

ne = number of effective interfaces offering frictional resistance to the slip

(ne = 1 for lap joint, 2 for double cover butt joint)

kh = 1.0 for fasteners in clearance holes.

= 0.85 for fasteners in oversized and short slotted holes and long slotted holes loaded perpendicular

to the slot

= 0.70 for fasteners in long slotted holes parallel to the slot

Fo = Anb Fo

where Anb = net area of the bolt in threads

= 0.78

and fo = proof stress = 0.70 fub

in which gmf = 1.10, if slip resistance is designed at service load

= 1.25 if designed at ultimate load

Tension capacity of HSFG bolts

fub

for holts of grade 8.8 i.e., 800 MPa

and fyh = 640 MPa

* Interaction formula for combined shear and tension

Prying Forces

* If HSFG bolts are used, additional forces develop at connected point due to the flexibility of

connected plates.

where lv = distance from bolt centre line to the toe of the fillet weld or half the root radius for rolled

sections.

lc = distance between the prying forces and bolt centre line is the minimum of either the end distance

on the value given by

where b = 2 for non-pre-tensioned bolts and 1 for pre-tensioned bolts

h = 1.5, be = effective width of flange per pair of bolts

fo = proof stress, t = thickness of end plate