Connections

 

Rectangular Bend development

Connections Bolted

 

A307 bolts – Unfinished, machine, plain, common or rough bolts.

 

A325 and A490 bolts – High-Strength structural bolts for all types of structures

 

Tightening:

 

HS bolts are tightened either by use of a tension indicator or by the turn-of –nut method.

In the turn-of –nut method, tensioning is obtained by tightening the bolts to a snug-tight fit and then rotating the nut (or bolt) a specified additional fraction of a turn.

 

Holes for fasteners:

 

Holes sizes – 1/16” larger than nominal dia. of bolts

 

Two basic types of connections are used to transmit shear loads:

1.      A bearing – type connection is one in which the fasteners bear against the sides of the holes in the connected material. A307 bolts shear connections are all bearing type connections. Under certain circumstances HS bolts are also used in bearing type connections.

2.      A friction slip-resistant type connection is one in which the fasteners clamp the connected parts together with an extremely high pressure, so that the shearing force is resisted by the friction b/w the connected parts and not by a shear stress on the fasteners. Only HS bolts are used in friction-type connections.

For Fv refer Table – D of Allowable Shear Design, Book -2

 

For eg: 7/8 A307 bolts, rv = Fv x pD²/4 (Ref: 5-9)

                                     = 10 x 0.6013 = 6.0 kips

 

Tension in bolts:

(Ref: 5 –11)

 

Fastener	Ft, ksi
A307	20.0
A325	44.0
A490	54.0

 

Types of Beam Connections:

 

1.      Rigid Frame – Assumes that beam-to-column connection have sufficient rigidity to hold, virtually unchanged, the original angles b/w intersecting members.

2.      Flexible simple framing – Unrestrained, free ended assumes that the beams ends are free to rotate under gravity load.

3.      Semi-rigid framing – Assumes that the connection possess a dependable and known moment capacity intermediate in degree b/w the rigidity of Type 1 and Type 2.

 

High Strength bolted connections:

 

1.      Framed  Beam connections

2.      Seated Beam connections

3.      End plate shear connection

 

Seated Beam Connections:

 

1.      Unstiffened seated connections (beam to col.web and flg. & beam to girder)

2.      Stiffened seated connections ( beams to col.webs – not used in col.flanges - because the outstanding legs of stiffener angles may protude through the fireproofong ot architectural finish)

 

Advantages of Seated beam connections over Framed beam connections:

 

1.      They permit fabrication of plain punched beams.

2.      They allow the erector a means of supporting the beam while aligning the field holes and inserting field bolts.

3.      They result in better clearance when a beam connected to col. web.

4.      In case of large beams, seated connections reduce the no. of  field bolts and result in overall economy.

 

Stresses in Fillet weld:

 

Effective area of fillet weld = effective throat thickness x eff. Length

 

Effective throat area = 0.707 w l (w = weld size, l = weld length)

 

Strength of fillet weld  = eff. throat area x allowable shear stress

                                     =  0.707 w l x Fv (Ref: 6-28)

 

 For E70XX electrode, Fv = 0.30 x 70 = 21.0 ksi

 

\ Weld strength for 1/16” of fillet weld = 0.928 kips/inch of length

           

            P = 0.928 D (where D is no. of sixteenths of an inch in the leg size of weld)

 

Max. weld size for material thickness < ¼” = material thickness

 

Max. weld size for material thickness / ¼” =  smaller material thickness – 1/16”

 

Material thickness of thicker part joined (in.)	Min.size of fillet weld (in.)
To ¼, inclusive	1/8
¼ to ½	3/16
½ to ¾	¼
Above ¾	5/16

 

Stiffened Seated Connection: (Ref: 6-45)

 

To prevent overstressing of a stiffener plate, the min.thickness of plate = 1.76 x weld size

 

Thickness of horizontal seat plate, should not be less than 3/8”.

 

Columns:

 

Location of column splice can affect the cost of a high rise structure.

The following situations are cited for consideration.

1.      Since the lower column tier is normally heavier, it is appropriate to keep the splice level as how as possible in order to reduce the weight of materials.

2.      Col. splice connection must not interfere with beam connections and bracing connection.

3.      It may appear desirable to splice col. at mid height or point of contra flexure, but this would be several feet above the steel framing and it can require additional expense in installing and tightening of bolts or welding.

 

Hole sizes for Anchor bolts:

 

Bolt Size	Hole size
¾ to 1”	Bolt dia. + 5/16”
1” to 2”	Bolt dia. + ½”
> 2”	Bolt dia. + 1”

 

Over sized hole should be covered with a flat washer.

 

Base Plates: - Should be specified in multiples of one-eighths of an inch.

 

1.      For plates 2” or less           - no machining/finishing is required, if satisfactory contact in bearing.

2.      For plates 2” to 4”             - to be straightened or finished to obtain the contact.

3.      For plates > 4”                  - must be finished.