Steel Connection Design-I



Steel 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 floor beams 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 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.

  • 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

Where, Pt = minimum required bolt tension specified in Table 1

Kh = hole factor specified in Table 1

Ks = 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

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:

Bearing type connection can fail in several failure modes:

a) 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

Nominal shear resistance of a bolt

  • Threads excluded: Rn = 0.48 Ab Fub Ns

  • Threads included: Rn = 0.38 Ab Fub Ns

Where, 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 <>

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

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 3d and for bolts with clear end distances less than 2d

Rn = 1.2 Lc t Fu

Where, 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

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

  • Minimum edge distances

Part-II will be uploaded soon..