📘 Lecture: Tension Members
1. Introduction
A tension member is a structural element that carries axial tensile forces — forces that try to
stretch it along its length. Unlike compression members, they do not buckle, making them very
efficient structural components.
You see tension members everywhere:
Cables in suspension bridges
Tie rods in steel trusses
Diagonal bracing in towers and frames
Guy wires for tall masts and poles
2. Behavior of Tension Members
When loaded in tension:
The member elongates along its length.
The cross-section is subjected to uniform tensile stress, provided the load passes
through the centroid.
The member generally fails by yielding, rupture, or connection failure.
Because tensile forces produce no instability, tension members are usually lighter and more
material-efficient compared to compression members.
3. Types of Tension Members
A. Based on Shape and Construction
1. Cables and Wires
o Flexible, only carry pure tension.
o Used in suspension bridges, cable roofs, guy wires.
2. Bars and Plates
o Flat or circular cross-sections.
o Used in old trusses, bracing systems.
3. Rolled Sections
o Angles, channels, I-sections.
o Widely used in modern steel trusses.
4. Built-up Sections
o Plates + angles welded or bolted together.
o Used when a single section is not sufficient.
B. Based on Load Application
1. Concentric Tension Members
o Load passes through the centroid → uniform stress.
o More efficient.
2. Eccentric Tension Members
o Load applied away from centroid → induces bending in addition to tension.
o Less efficient, requires stronger sections.
4. Modes of Failure in Tension Members
1. Yielding of Gross Section
o When stress in the gross area reaches yield stress.
o Predictable and ductile.
Py=Ag⋅FyP_y = A_g \cdot F_yPy=Ag⋅Fy
Where:
o AgA_gAg = gross cross-sectional area
o FyF_yFy = yield stress
2. Fracture of Net Section
o Occurs across the net area (after deducting bolt holes).
o Sudden and brittle.
Pu=An⋅FuP_u = A_n \cdot F_uPu=An⋅Fu
Where:
o AnA_nAn = net area
o FuF_uFu = ultimate tensile strength
3. Block Shear Failure
o A block of material around bolt groups tears out due to combined tension and
shear.
4. Connection Failure
o Bolts shearing off, weld failure, or plate tear-out due to poor detailing.
5. Net Section and Efficiency
The net area accounts for bolt holes:
An=Ag−n⋅dh⋅tA_n = A_g - n \cdot d_h \cdot tAn=Ag−n⋅dh⋅t
Where:
nnn = number of holes
dhd_hdh = hole diameter (bolt dia. + clearance)
ttt = plate thickness
Efficiency = AnAg×100%\frac{A_n}{A_g} \times 100\%AgAn×100%
A good design has efficiency ≥ 80%.
6. Slenderness in Tension Members
Unlike compression members, slenderness does not cause buckling in tension.
However, very slender members may vibrate, deflect, or fatigue under dynamic loads.
Codes limit slenderness ratio (L/r ≤ 300–400).
7. Examples in Practice
Bridge trusses → Diagonals in Pratt/Warren trusses.
Transmission towers → Diagonal bracing under wind loads.
Suspension bridges → Main cables and suspenders.
Guyed masts → Steel wires holding tall towers.
8. Summary
Tension members carry axial tensile loads only.
They are efficient because no buckling occurs.
Design checks include:
1. Yielding of gross area
2. Rupture of net area
3. Block shear / connection failure
Net area and connection detailing are critical.
Slenderness is controlled for serviceability, not strength.
