DESIGN OF STRUCTURE  3  :  TENSION MEMBERS

TENSION MEMBERS

A tension member is the structural member which subjected to two pulling  or tensile forces applied at its ends.

The strength of the tension member is governed by the various forces such as the length of the connection , size of the member , net area of cross-section , fabrication type etc.

Fallowing are the various types of the tension members :-

Ø  WIRES AND CABLES

Wires cables are mainly used for hoisting purpose and as a guy wires in steel stack sand towers.

Cables are  generally used as floor suspenders in the suspension, bridge are made from individual strands wound together in rope like fashion.

They have good strength and flexibility but cannot resist compression.

Ø  BARS AND RODS

Rods and bars have less flexural stiffness resulting in the sag under their own weight especially during the erection.

Bolted rods are often work loose and rattle , if not pretensioned.

Ø  PLATES AND FLATS BARS

Plates and flat bars are also used as the tension members in the open transmission towers , foot bridges , etc

They are also used in the columns to keep the component members in the proper position.

Ø  SINGLE STRUCTURAL SHAPES

Mainly the tension members are made up of flat bars , but in the modern practice it used mainly the fallowing  sections for tension members where possible :-

Open section

Closed section

Compound or build-up section

Ø  BUILD-UP MEMBERS

Ø  Build-up members are generally made up of two or more shapes and connected to act as single members. They are connected to get the required area which can not be provided by the single rolled shape. These members are more rigid and sufficiently stiff to carry the compression as well as tension members.

 

 

TYPES OF FAILURE TENSION MEMBERS

Fallowing are the various modes for the failure of tension member :-

1.     GROSS SECTION YIELDING

In this failure considerable deformation of the member in longitudinal direction may takes place before it fracture, making the structure unserviceable.

2.     NET SECTION RUPTURE

Net section rupture occurs in the member when the net cross-section of the member reaches the ultimate stress.

3.     BLOCK SHEAR FAILURE

In this failure the segment of the block of the material shears out  due to the possible use of the high bearing strength of the steel and high-strength bolts resulting in the smaller connection length.

 
SOME IMPORTANT DEFINATIONS RELATED TO THE TENSION MEMBERS :-

 

Ø  NET SECTIONAL AREA

The difference between the gross sectional area of the member and sectional areas of the maximum number of holes is called the net sectional area of the tension member.

 

Ø  EFFECTIVE NET AREA

Effective net area is the modified net area it may be defined as the fallowing equation:-

       Ane = k1k2k3k4An

K1 = ductility factor

K2 = factor for method of fabrication

K3 = geometry factor

K4 = shear lag factor.

 

 

Ø  LUG ANGLES

It is an additional angles used along the tension member to decrease the length of joints and also the size of the gusset plates.The lug angle can be removed by providing  the unequal angle section with wider legs and the bolts are provided in two rows and staggered.

 

Ø  SLENDERNESS RATIO OF TENION MEMBER

Slenderness ratio is defined as the ratio of the effective length to corresponding radius of gyration of the section.

The slenderness ratio is equal to = Le/r

 

Ø  DISPLACEMENT OF THE TENSION MEMBER

The increase in the length of the tension member under service load is called displacement of the tension member.

The displacement is a serviceability limit state criterion and it is checked under service load only.

                  Δ = PL/EAg

P = Unfactored axial load in N

L = Length of the member in mm

E = Modulus of elasticity

Ag = Gross cross-sectional area of the member in mm square.

 

Ø  GUSSET PLATE

The plate which provided to make connection at the place where more than one member is to be jointed is called the gusset plate. For example joint of truss , truss girder etc.

The size and shape of the gusset plate is decided from the direction of the member meeting at the joints.

A gusset plate is capable to bear the shear stresses, direct stresses, bending stresses and therefore it have sufficient thickness to resist all these forces.

The thickness of the gusset plate should not be less than 12 mm.

 

Ø  TENSION MEMBER SPLICES

Splices are used to join two section when a joint is to be provided that is these replace the member at the joint where it is cut. A tension member is spliced when available length is smaller than the required length of the tension member and it is also spliced when the member of the different thickness are join together, In this case packing is required to fill the gap.