The Victoria Falls Bridge

“I should like to have the spray of the water over the carriages” Cecil Rhodes

The weekend that the A2Z team spent in Livingstone was incredible and gave us the chance to experience more of Africa’s rich and varied wildlife as we crossed over into Botswana whilst on Safari. By sunrise the next day we were at the Victoria Falls, a site of special geological interest, taking in the enormity of its vast scale and power. However our structural senses were really spiked when we visited the Victoria Falls Bridge, spanning across the gorge from Zambia to Zimbabwe. It wouldn’t be a trip without analysing at least one structure, so in true engineering style we decided to write a special edition blog post all about the Victoria Falls Bridge.

Introduction

The Victoria Falls Bridge is strategically positioned to facilitate entry between Zambia and Zimbabwe whilst providing stunning views of the Victoria Falls.

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Becca, Emily and Zoe (stunning views 😉 ) entering Zimbabwe on the Victoria Falls Bridge.

The bridge spans between the two steep sides of the gorge, approximately 130m above the Zambezi river.

The bridge provides a crossing point for rail, car and foot passengers, and more recently is the sight of a bungee jump – as done by A2Z engineer Fraser Robinson.

Aesthetics

The bridge is located in what has become a world heritage site due to its proximity to one of the seven natural wonders of the world, the Victoria Falls.

The steel truss arch is relatively unimposing and allows for the Victoria Falls to be viewed from a variety of vantage points.The truss structure has allowed the amount of material to be reduced and helps to maintain views through the bridge section. This view may have become obstructed had the structure been more solid. This sensitive design does not detract from the area’s natural beauty, with the bridge becoming a feature in its own right.

Structure

The steel truss allows for forces to be transferred from the deck and distributed evenly along the arch.

The forces acting as a uniformly distributed load, UDL, along the arch section are transferred to the foundations by a series of compression forces.

Foundations

The bridge is founded on the hard basalt rock faces of the gorge with the foundations formed from concrete, providing an anchor point. The hard basalt rock provides excellent foundations to support the arch structure and resist the thrust generated.

The position the bridge occupies is slightly lower then originally intended as a result of an engineering survey error which suggested harder rock then found and necessitated a shift in position, 21 feet below, to achieve the required foundation capacity. The landscape has been shaped appropriately to achieve a smooth drop to the bridge deck. This achieves an aesthetically pleasing appearance as well as easing the the route for traffic.

Construction

The bridge was fabricated in the United Kingdom and shipped to Mozambique before making its way to the Victoria Falls. The construction of the bridge involved the simultaneous erection of the majority of the structure, cantilevered from either side of the gorge prior to the positioning of the central piece. 2no. 10 tonnes electric cranes, specifically designed for the construction of the Victoria Falls Bridge, travelled along each cantilever lifting and placing 1,600 tonnes of steel girders in the first 20 weeks.

The central section of the bridge did not initially fit by precisely 1 ¼ inches, but by sunrise the next day it had fallen perfectly into place due to the contraction of the steel overnight.

Sustainability

  • Durability

The Victoria Falls gorge was formed from the erosion by water of the softer rock at the base. The hard rock above eventually becames unable to support its weight and collapses. This process has occurred over a period of millions of years and the gorge is only expected to erode a few millimetres in our lifetime. Thus erosion of the gorge edge is unlikely to effect the bridge over its design life.

  • Maintenance

The arch of bridge provides access, via steps, to the foundations, as well as the underside of the deck and bridge section, enabling regular inspection, as necessary.

The spray generated from the Victoria Falls is extreme and as such it expected the bridge needs to be regularly painted to prevent corrosion of the existing steelwork. The bridge and it’s paintwork appeared in good condition for its age.

  • Future

The bridge is used infrequently for rail and road traffic – with only a few lorries passing over in the few hours we were there and approximately 170 freight trains passing over it per month. The route is unlikely to become more popular as the bridge could not support an increase which is fortunate as there is litle space to extend. The bridge capacity is likely to reduce further with time and potentially in the future will only be capable of carrying foot passengers and the tourism activities that are performed using the bridge.

Forces

The daily fluctuations of temperature in Zambia can be extreme and ranged from 5° up to around 30° during our visit, but can be much more extreme in the summer months. As such the forces caused from temperature ranges are potentially significant – demonstrated in the construction of the bridge and the positioning of the central section.

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Movement joint on the Zimbabwe side of the bridge.

The structure appears robust and potentially over-designed seen from the extreme amount of steel forming the bridge.

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However the bridge is subject to a variety of traffic forces (rail, road and passenger) and thus it is necessary for it to be robust to support the large forces imposed on the structure.

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The IDEALS girls taught us the glam squad pose

However, signage suggested that the traffic was limited to a single lorry on the bridge  at any one time and we observed significantly noticable vibrations as they passed, which may have been an indication of the bridge’s structural deficiency. Being built over 100 years ago now, the structure is likely to have been designed for significantly lower forces and will have degraded over time. Interestingly the parapets preventing road traffic accidents were surprisingly low and it is unlikely that these would be acceptable to UK standards.

The team have experienced high winds whilst in Zambia. The shape of the deck, a series of rectangles in profile, reduces the forces imposed by limiting the area exposed to winds.

This shape is likely to resist torsion imposed on the section from the extreme temperatures also. And in addition the deck section works to reduce the effective span of the deck section, and the material required to support the loads imposed on the structure.

 

Our thanks go out to the IStructE and ICE for providing us with the opportunity to travel to Zambia and the chance to see the Victoria Falls Bridge. We were inspired to share what we have learnt about this landmark structure which bridges the gap between Zambia and Zimbabwe.


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