Chosen design for construction-------------------------------------10 Conclusion---------------------------------------------------------------10
ABSTRACT This report analysis and calculates the forces of three of the members on six types of truss bridges when a load of 500 kilograms is applied to the centre of the bridge. The report also includes the design of the type of bridge, history and forces involved.
INTRODUCTION This report focuses on the six different types of truss bridges to find a suitable design for the destructive test for continues applied load. Truss bridges are bridges with a load-bearing structure of connected members in the shape of variant triangles. First designed by the HSC ENGINEERING TASK Page 11
Grubenmann brothers in Switzerland, they are one of the oldest types of bridge design that still exists today. The members are usually in either tension or compression and depending on the dynamic loads could be both. Since wood was widely available at the early time, this type of bridge design was widely used because it was more effective than a beam bridge. Some even exists today. The truss design dissipates the load throughout the structure which is a considerably large area and hence it is very rigid. It was also able to be constructed offsite and brought onsite for easy completion of the bridge. Some of these truss design were used as railroad bridges and some for the structure of the house roofs.
Common types of Trusses 1-
Warren Truss
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A warren truss consists of alternatively inverted equilateral triangles which are rigid. It is made up of horizontal members and equal angled cross-members that are of equal lengths. It was created by James Warren and Willoughby Theobald Monzani in 1848. This design requires fewer amounts of materials compared to other types of trusses, therefore lightweight, easy to build, great view and is economical. The members are stressed by compression and tension during the application of a live load. It is easy to repair and maintain. This design has many derivations that were later developed after the Warren truss was designed for example the Warren truss with vertical members. An example of such a bridge is the 6 th street Bridge over Savannah River on Norfolk Southern Railway in Augusta. The following force evaluation shows three members forces when a load of 500 kg is loaded at centre of the Warren truss bridge.
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2-
Warren Deck Truss
This type of truss is similar to the Warren truss but the difference is that the structure is below the roadway. So it requires a few amounts of materials, is light weight, easy to build, usually build offsite therefore quick to build, more open view on top and is also economical. The top part of the truss is in compression were as the bottom part is in tension. The interior members alternate between tension and compression to keep a stable structure. This type was first built after the design of deck bridges around 1800. An example of this type of bridge is the Admiral Way Bridge over Ravine, Fairmount Avenue on SW Admiral Way in Washington, US. The following force evaluation shows three members forces when a load of 500 kg is loaded at centre of the Warren deck truss bridge.
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Pratt Truss
The Pratt Truss was first developed by Thomas and Celeb Pratt in 1844. You 1844. You can identify identify a Pratt Pratt truss by its diagonal members, which (except for the very end ones) all slant down and in toward the center of the span. The diagonal members are subjected to the tension forces and the vertical members handle the compressive force when a live load is applied at the center of the truss. This design requires a bit of time and materials than a warren truss and can span several hundred feet but is expensive and it also spreads the load throughout its members. This design has a few derivations such as the Parker Truss. An example of such a design is the Afton Canyon Railway Bridge in California over Mojave
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River. The following force evaluation shows three members forces when a load of 500 kg is loaded at centre of the Pratt truss bridge.
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Howe Truss
The Howe truss was developed by William Howe in 1840. Although similar to the Pratt truss, the difference is that the diagonals slope up towards the centre, so the diagonal members are in compression and the verticals are in tension. This design was more used in railway bridges because of its ability to withstand a high load. This was more popular during the period where iron was expensive and wood was more available. This design requires a bit of materials but is very effective to distribute the load across
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its several members. An example of such a design is the Jefferson Rail Road Bridge in Texas. The following force evaluation shows three members forces when a load of 500 kg is loaded at centre of the Howe truss bridge.
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K – Truss
The K truss was first designed from the second variation of the Pratt Truss called the Parker Truss. It was designed by Phelps Johnson. This design allowed the vertical members to be in smaller sections which in turn resist bulking in compression. It therefore requires time to build the separate members and requires a lot more materials. It was unfavoured due to the HSC ENGINEERING TASK Page 11
members blocking the scenery even if it was able to withstand high loads. This design has the advantage of distributing its load forces throughout its many members effectively due to the smaller vertical sections and the diagonal members. An example of this type of bridge is the Bird Creek Bridge in Oklahoma. The following force evaluation shows three members forces when a load of 500 kg is loaded at the centre of the K-truss.
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Waddell “A” Truss
First designed by John Alexander Low Waddell and was built around 1898. It was first used as a railway bridge and later on started to be used as road bridges and footbridges. It is used in short
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spans, most rigid, easily built off site and constructed, low cost of materials due to fewer members. It was mostly used for small spans for road bridges or footbridges and some as railway bridges. This design was later dropped because steel became widely available and this type of truss bridge was only dependable for short spans. The shape that the members form is that of an “A” shape hence the name Waddell “A” Truss. An example of this type of bridge design is the one in Parkville, Missouri US that was relocated from a highway due to construction of a reservoir. The following force evaluation shows three members forces when a load of 500 kg is loaded at centre of the Waddell truss bridge.
Chosen design for construction: The design chosen for the construction was the Waddell “A” Truss mainly because it requires less members and hence less amount of materials. Because it has fewer members it will only require a small amount of time to build compared to the other trusses.
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From the calculation of the member forces of the “A” truss, the gaps between two forces are very small. Member AB has a compression force of 5 Kilo-Newtons whereas the member BC has a tension force of 4.33 Kilo-Newtons; therefore the load was distributed almost equally between its members. The vertical members however have lesser compressive force indicating that the external diagonal members have taken on most of the distribution of the load.
Conclusion The Waddell ‘A’ Truss bridge is effective for the short span as given in criteria for the destructive testing, a spaning of 450
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millimetre (mm). The material used to build the prototype for the destructive testing is hoop pine timber that is 8 (mm) times 6 (mm) in section. This type of timber is able to withstand high amount of compression than tension and is therefore effective with this type of truss bridge due to the long vertical member that would be in high compressive force. By test of a prototype build with popsicle sticks, it was found that it can withstand up to a load of about 7 kilograms of water which in comparison to the weight of the bridge 0.0576 kilograms it took 120 times its own weight. Therefore the prototype build for the destructive testing should be able to withstand about 120 times its own weight and higher due to the material used being thicker and stronger than Popsicle sticks.
