- A framework, typically consisting of rafters, posts, and struts, supporting a roof, bridge, or other structure is known as truss.
- The main reasons for using trusses are: Long span, Lightweight, Reduced deflection (compared to plain members), Opportunity to support considerable loads.
- Truss is assembled in such a way that forces are applied only on the ends.
- The members are connected with a gusset joint that is either riveted, bolted or welded.
Component parts of trusses :
Purlin : A horizontal beam along the length of a roof, resting on principal rafter and supporting the common rafters or boards.
Principal Rafter : one of the upper diagonal members of a roof truss supporting the purlins and common rafters or those joints to which the roof boarding is secured.
Common Rafter : A rafter having no function other than to bear roofing.
Ridge Board : The horizontal timber or member at the top of a roof, to which the upper ends of the rafters are
Strut : Provided to resist longitudinal compression.
Types of Trusses :
There are variety of trusses available depending on the requirement including span length and loading condition.
Basically two types of trusses exists :-
- Bridge Truss
- Roof Truss
1) Bridge Truss
A) Pratt Truss
- A Pratt Truss has been used over the past two centuries as an effective truss method.
- The vertical members are in compression, whilst the diagonal members are in tension.
- This has a few effects – it reduces the cost of the structure due to more efficient members, reduces the self weight and eases the constructability of the structure.
- This type of truss is most appropriate for horizontal spans, where the force is predominantly in the vertical direction.
- Aware of member’s behavior – diagonal members are in tension, vertical members in compression
- The above can be used to design a cost effective structure
- Simple design
- Well accepted and used design
Not as advantageous if the load is not vertical
- Where a cost effective design is required
- Where a mix of loads are applied
- Where a simple structure is required
B) Warren Truss
- The Warren Truss is another very popular truss structure and is easily identified by its construction from isosceles triangles.
- One of the main advantages of a Warren Truss is its ability tospread the load evenly across a number of different members; this is however generally for cases when the structure is undergoing a spanned load (a distributed load)
- It’s main advantage is also the cause of it’s disadvantage – the truss structure will undergo concentrated force under a point load.
- Under these concentrated load scenarios, the structure is not as good at distributing the load evenly across its members.
- Therefore the Warren truss type is more advantageous for spanned loads, but not suitable where the load is concentrated at a single point or node.
- Spreads load fairly evenly between members
- Fairly simple design
- Poorer performance under concentrated loads
- Increased constructability due to additional members
- Long span structures
- Where an evenly distributed load is to be supported
- Where a simple structure is required
C) Howe bridge Truss
- It includes vertical members and diagonals that slope up towards the center.
- Has diagonal members that slant away from the middle.
- Howe truss were designed a long time ago when bridges needed to fill a specific role, and for the particular resources that people had available.
- The Howe truss design used a lot of wood as opposed to the Pratt which used more iron.
- This made the Howe popular earlier on when iron was expensive to produce.
Howe Bridge Truss
- The advantages of truss bridges is that it is economical, light, strong and uses short timbers.
- Greater strength with less material.
- Easier to repair and maintain.
- Disadvantage can be wasted material if not designed properly.
- Longer build time, more complex construction.
D) Bailey Bridge Truss
- It is designed for military use, the prefabricated and standardized truss elements may be easily combined in various configurations to adapt to the needs.
- The military utilizes the Bailey design, which is easily disassembled and portable.
- Most of the design has vertical members attachingto horizontal members as well as diagonal braces located between the vertical members.
- A Bailey bridge had the advantages of requiring no special tools or heavy equipment to assemble.
- The wood and steel bridge elements were small and light enough to be carried in trucks and lifted into place by hand, without requiring the use of a crane.
- The bridges were strong enough to carry tanks.
- Bailey bridges continue to be extensively used in civil engineering construction projects and to provide temporary crossings for foot and vehicle traffic
2) Roof Truss :
A) King Post Truss :
- If span length is in the range of 5 to 8 meter, then king post trusses are used.
- The trusses are spaced not more than 3m c/c.
- A vertical post is provided in the center that is called the king post.
- A king post extends vertically from a crossbeam to the apex of a triangular truss.
- It connects the apex of the truss with its base, holding up the beam (in tension) at the base of the truss.
- Its bottom chord acts as a ‘tie’
B) Queen Post Truss
- If the span length is in between 8 to 12 meter then queen post trusses are used.
- Two vertical posts are provided in 2 sides at a distance which are termed as queen posts.
- Straining beam and straining seal is used to keep the queen posts in exact position.
- Queen post is a tension member in a truss that can span longer openings than a king post truss.
- A queen-post bridge has two uprights, placed about one-third of the way from each end of the truss.
- The central square between the two verticals was either unbraced on shorter spans, or had diagonal braces from the bottom of each queen post to the cleat.
C) Fink Roof Truss
- They are used for longer spans having high pitch roof, since the web members in such truss are subdivided to obtain shorter members.
- A Fink truss is the most commonly used type of residential construction truss.
- It is composed of several boards or steel bars that connect together to form an intersection at the top angle of the roof.
- These trusses provide support for the rafters and give the roof its rigidity.
- A Fink truss is the perfect solution for many architectural designs and allows for several different types of roof lines, while providing the strength and stability needed.
D) Howe Roof Truss
- It is a roof truss with vertical web members to take tension forces and with angled braces to take compression.
- The converse of the Pratt truss is the Howe truss.
- The Howe truss can be advantageous for very lightly loaded roofs in which reversal of load due to wind will occur.
- In addition the tension chord is more heavily loaded than the compression chord at mid-span
under normal vertical loading.
- The Howe truss offers greater economy in terms of steel weight for long-span high-pitched roofs as the members are subdivided into shorter elements.
E) Warren Roof Truss
- Warren truss is a kind of design that is used in different types of construction for supporting a load.
- In warren roof truss diagonal members are alternatively in tension and compression are used in a building ranging from 20-100 m in length.
- The unique design of a Warren truss structure ensures that no strut, beam or tie bends or withstands torsional straining forces but is only subject to tension or compression.
- The use of the Warren truss design is common in prefabricated modular bridges.
- There is less material required for the construction of a Warren truss bridge.
- There is less blockage of view.
- The constituents of a Warren truss bridge can be assembled piece wise.
- The maintenance of the joints and fittings of a Warren truss bridge could be expensive
- The calculations to determine the load-bearing capacity of a Warren truss bridge can be hassling.
- There could be too much deflection for long spans