7 Top Howe Truss Advantages and Disadvantages | Types of Trusses | Howe Truss Vs. Pratt Truss
Howe Truss Advantages and Disadvantages | Types of Trusses | Howe Truss Vs. Pratt Truss |Howe Truss Bridges in Use
What is Howe Truss?
A Howe truss is a type of truss bridge made up of chords, verticals, and diagonals, with the vertical members in tension and the diagonal elements in compression. William Howe created the Howe truss in 1840, and it was widely utilized as a bridge in the mid to late 1800s.
The Howe truss bridge is made up of an upper and lower “chord,” each of which is made up of two parallel beams and is parallel to the other. Verticals, braces, and counter-braces make up the web[b].
Vertical posts link the upper and lower chords, resulting in “panels.” A diagonal brace in each panel strengthens the bridge, and a diagonal counter-brace in each panel reinforces it even more.
Howe truss bridges can be all wood, all iron, or a combination of the two. Wooden timbers should have square ends with no mortise and tenons, regardless of design.
The design of an all-metal Howe truss is similar to that of a hardwood truss.
Howe Truss Bridges in Use
Because of its ease of construction, the Howe truss was very cost effective. The timber parts can be created with nothing more than a steel square and a scratch awl, and the truss can be framed with just an adze, auger, and saw.
Panels may be prefabricated and delivered to the construction site, and in certain cases, whole trusses could be constructed and assembled off-site before being carried by rail to the intended location. To build the bridge, falsework, usually in the shape of a trestle, is necessary.
History Of Howe Truss and Pratt Truss Bridge Designs
The invention of the Pratt and Howe trusses accelerated the development of metal bridges in the United States.
Until 1850, there were few iron bridges in the country that were more than 50 feet long (15 m). The Pratt and Howe trusses’ simple design, ease of manufacture, and ease of construction prompted Benjamin Henry Latrobe II, chief engineer of the Baltimore and Ohio Railroad, to build a huge number of iron bridges.
Due to the collapse of two prominent steel bridges (one in the United States, the other in the United Kingdom), small number of them were built in the North.
As a result, the majority of iron bridges built before to the American Civil War were in the South. Around 1867, there was a spike in the construction of metal bridges across the United States.
The Howe truss, Pratt truss, Bollman truss, Fink truss, and Warren truss were the most typically adopted designs. Because they used fewer members, the Howe and Pratt trusses became preferred.
The only maintenance required for a Howe truss is to adjust the nuts on the vertical posts to equalize strain.
Since these diagonals in a wooden Pratt truss proved difficult to maintain in good alignment, the Howe truss became the favored design for a wooden bridge or a “transitional” bridge of wood with iron verticals.
In 1913, engineering professor Horace R. Thayer recognized the Howe truss to be the best type of wooden truss bridge, and it was the most often used truss bridge in the United States at the time.
Around 1845, the first all-iron Howe trusses were built. For the Boston and Providence Railroad, a 50-foot (15-meter) long iron Howe truss was built, as was a 30-foot (9.1-meter) long railroad bridge over the Ohio and Erie Canal near Cleveland.
Metal, on the other hand, was the popular bridge material for automotive and rail traffic, and the Howe truss did not lend itself well to all-iron construction.
The single diagonal bracing method of the Pratt truss meant cheaper expense, and its ability to use wrought-iron stringers under railroad rails and ties prompted bridge builders to adopt the Pratt over the Howe.
Bridge builders recommended plate girder and Towne lattice bridges for spans less than 60 feet (18 m), and Warren girder bridges for all greater spans, due to heavier live loads, particularly by railroads.
Types of Trusses
A truss is a structure made up of components or elements that only take tension or compression but no bending is caused.
The members are joined by a gusset joint, which is riveted, bolted, or welded in such a way that only axial forces are created in the structure.
rationale for axial forces is that external loads are imposed in such a way that their effects are solely in the form of forces acting on joints.
A truss is composed of three basic parts, one is top chord, the beam at the top which is usually in compression, bottom chord, beam at the bottom which is usually in tension, webs are interior beams.
What are some types of Trusses?
There are variety of trusses available depending on the requirement including span length and loading condition.
Basically, two types of trusses exist: Bridge Truss and Roof Truss.
The Bridge Truss and Roof Truss include;
- Pratt Bridge Truss
- Howe bridge Truss
- Baltimore Bridge Truss
- Bailey Bridge Truss
- Fink Roof Truss
- Warren Roof Truss
- Kingpost Truss
- Queenpost Truss
- Gambrel truss
- Hip Truss etc.
- Dual Pitch Truss
- Inverted Truss
- Scissors Truss
- Flat Truss
Howe Truss Vs. Pratt Truss
The key distinctions are that the Pratt truss has no end force and the compression members are vertical. The Howe truss has no central force and the tension members are vertical.
The diagonal members of a Pratt truss (excluding the end diagonals) are slanted down towards the middle of the bridge span.
When subjected to external loads, tension is created in diagonal members, whereas compressive pressures are dealt with by vertical members.
As a result, thinner and lighter steel or iron can be utilized for diagonal members, resulting in a more efficient structure.
The Howe truss design is the inverse of that of the Pratt truss in that the diagonal members are tilted in the opposite direction of the Pratt truss, generating compressive forces in the diagonal members.
Consequently, using steel members to handle compressive load is not cost effective.
Howe Truss Advantages and Disadvantages
Howe Truss Advantage
- Economical, light and strong. The main advantages of truss bridges are that it is economical, light, strong and uses short timbers. The middle is highest above the water surface.
- Easier to repair and maintain. This is why they are still extensively used today.
The longer the timber span, the cheaper it becomes
- Faster Construction. Truss bridges could be built rapidly compared to other bridge constructions using wooden girder bridges
- Greater strength with less material. Howe truss design increases the strength of the bridge, it is much stronger compare to other truss design.
The main reason of this is that the vertical members are short and are often made from solid wood. The diagonals are also shorter which reduces the area they have to carry weight through.
Howe Truss Disadvantages
- Disadvantage can be wasted material if not designed properly. These materials are discarded and are thrown away because they are too heavy to be used in the bridge.
Also, the cost for replacing it is without a doubt expensive.
- Strong wind can bring the bridge down, and this is why they are not used more often than the Pratt trusses
- The main disadvantage of Howe trusses is that if it is not designed properly, it will be weak at supporting live load/people walking on it.
Howe Bridge Truss FAQs
1. How much strength does the truss add?
Truss bridges are more efficient than beam bridges because of the geometry. The visual effect is achieved by using a large size of wood to span and a smaller number of vertical and diagonal members for longer spans. This results in greater resistance to horizontal forces.
The truss frame shapes the shell into a box-like shape which increases the resistance of a bridge’s load to horizontal forces.
2. How do truss bridges deal with traffic?
The capacity of the bridge is determined by the number and size of its structural elements, not by the amount of traffic it carries.
A bridge can be made to carry more weight, dead or live or more load by increasing its structure, but it will still have the same dimension on its surface regardless of increased load capacity
3. What are the advantages of truss bridges over beam bridges?
Truss bridges are lighter than beam bridges. The cost is reduced by using few materials to construct.
They are more efficient, flexible and quick in construction. They can carry more load than Beam Bridges if designed properly for that particular application.
4. What is a Truss bridge?
A truss bridge is made up of a main structure (truss) containing vertical, horizontal and diagonal members which rest on a foundation.
The main strength of the truss is in the compression members of the diagonals which are short and strong. This allows them to carry load more efficiently.
Truss bridges are ideal for traffic use because they can carry heavier loads by using more material and less labor.
5. What are the advantages of truss bridges over beam ones?
Truss bridges are characterized by their interconnecting triangular structures, which give them the strength to withstand more heavy and dynamic loads.
Truss bridges can carry heavier traffic than beam bridge, they are strong and efficient while beam bridge usually works with a column anchored into a foundation.
6. What are the advantages of truss bridges?
Truss bridges can be produced quickly and inexpensively. Construction costs are reduced because most components can be fabricated at the site.
7. How much weight can a truss bridge carry?
The amount of weight a truss is able to carry is dependent on the size and shape of the individual members. This is why the maximum loads for each member must be calculated.
8. Are bracing diagonals required in Howe trusses?
The main structure of the truss consists of vertical and horizontal members with some diagonal members connecting them all together.
If the structure and the diagonal members are strong enough, they can bear all the traffic without any need for additional bracing but sometimes due to unevenness or to reduce the effects of wind, additional diagonals are placed outside or inside of a truss
9. What is the difference between a Howe truss and a Pratt truss?
The Pratt truss was traditionally constructed using timber vertical members in compression and diagonal iron braces.
The Howe truss was built with iron vertical poles and hardwood diagonal braces. Both trusses employed counter-bracing, which was becoming increasingly important as heavier railroad trains used bridges.
8. Which truss is better, Pratt or Howe?
Although both truss bridges dissipated force far more effectively than the beam bridge, the Pratt truss diffused the load more efficiently than the Howe truss.
Furthermore, on average, the Pratt truss deflected the least and retained the most, whereas the beam bridge deflected the most and held the least.