---
title: "Tower structures, wind cables and ties"
subject: "Theory of Structures I"
chapter: 7
key: "semester_4/theory_of_structures_i:tower-structures-wind-cables-ties"
source: https://www.bctengineer.com/q/semester_4/theory_of_structures_i/tower-structures-wind-cables-ties
site: BCT Engineering
---

# Tower structures, wind cables and ties

Theory of Structures I · Chapter 7: Suspension Cable Systems

- Asked in: 2070 Bhadra, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/theory_of_structures_i/tower-structures-wind-cables-ties
- Whole chapter: https://www.bctengineer.com/md/semester_4/theory_of_structures_i/chapter-7

## Question

Explain with neat sketches tower structures as well as wind cables and ties.

## Short answer

Towers are typically portal frames, single-column, A-frame or rocker types, designed for axial compression; cables ride on saddles with rollers to avoid bending moments. Wind cables and ties provide lateral stability: wind ties form horizontal trusses between stiffening girders, and wind cables anchored at abutments limit lateral sway of the deck.

## Answer

#### Tower structures
Towers carry the main cables over a saddle and transfer the vertical component of the cable pull to the foundation. They may be of steel or reinforced concrete and are built as:

- **Portal frame towers:** two legs connected by cross beams (struts) with diagonal bracing; used for road bridges, and stiff against wind across the bridge.
- **Single-column or A-frame towers:** for narrow bridges or where the cables are in one plane.
- **Rocker towers (hinged at the base):** allow the tower top to move with the cable.

Design points: the tower is designed mainly for axial compression. If the cable is clamped to the tower top, any difference between the horizontal pulls of the main span and the side span causes bending in the tower. To avoid this, the cable is usually seated on a saddle on rollers (or a sliding plate) at the top, so that the horizontal pulls on the two sides are equal and the tower carries essentially only vertical load.

```text
     saddle
   ___|___|___        top strut
  |    X    |        cross bracing
  |   / \   |
  |  /   \  |        legs (compression)
  |_/_____\_|
   foundation
```

#### Wind cables and ties
Suspension bridges are light and flexible, so they tend to sway sideways and vibrate in the wind. Their lateral stability is improved by:

- **Wind ties (lateral bracing):** horizontal bracing between the two stiffening girders, or between the hangers in the plane of the deck, forming a horizontal truss that carries the wind load to the towers and abutments.
- **Wind cables:** additional cables placed horizontally on each side of the deck (or inclined from deck level to the piers or the ground) and anchored at the abutments; they limit the lateral sway of the deck.
- **Inclined suspenders and stays:** hangers arranged in a triangular pattern, or cross ties, to damp vertical oscillation.
- **Streamlined box girders:** reduce wind forces and flutter.

```text
 plan view:   =====girder=====        wind
              \   /  \   /  \          ->
              wind ties (horizontal truss)
              =====girder=====
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. For what is a suspension bridge tower mainly designed?
   A. Axial compression, with some bending due to unequal horizontal pull of main and side spans
   B. Pure bending
   C. Pure tension
   D. Torsion only
   Answer: A. Towers carry the vertical component of the cable pull as compression, with some bending when the pulls are unequal. (Key point: Tower design)

2. Why is the cable placed on a sliding or roller saddle at the tower top?
   A. To increase the tower height
   B. To make the horizontal pull equal on both sides so the tower is not bent
   C. To reduce the cable length
   D. To transfer the deck load to the girder
   Answer: B. The saddle equalises the horizontal pull across the tower, leaving it essentially in axial compression. (Key point: Role of the saddle on the tower)

3. What is the purpose of wind ties and wind cables?
   A. To carry vertical traffic load
   B. To support the main cable at the towers
   C. To increase lateral stability by resisting wind forces and limiting sideways sway of the light flexible deck
   D. To anchor the stiffening girder to the towers vertically
   Answer: C. Horizontal bracing between girders and horizontal or inclined wind cables carry wind load to the towers and abutments and limit lateral sway. (Key point: Wind ties and wind cables)

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