Scissor Trusses | Suitable Uses & Applications of Scissor Truss | Types of Trusses in Construction
What is a Scissor Truss?| Suitable Uses & Applications of Scissor Truss | Types of Trusses in Construction |Advantages and Disadvantages of Scissor Truss
What is a Scissor Truss?
A scissor truss is a type of truss used mostly in buildings in which the bottom chord members cross each other, connecting to the angled top chords at a point halfway along the length of the top chords, giving the appearance of an opened pair of scissors
Scissor trusses are almost exclusively employed in building construction to support a pitched roof with a sloping or elevated ceiling surface.
A scissor truss is, in simplest terms, an A-frame structure made of two triangular shapes, or “legs,” joined at the top with a beam, or “pitch.”
Scissor trusses are used to support the weight of the roof of a building, and are usually located along the roof’s edge. Special attention is needed to determine the appropriate size, or “spans,” for the supporting beams.
A scissor truss is a building component primarily used in the construction of the truss roof. They are not always the most efficient way of building a truss roof, but they are quick and easy to construct.
Scissor trusses are quick and easy to construct, which is why they are sometimes the building components chosen for constructing the roof.
Suitable Uses & Applications of Scissor Truss
Scissor trusses are used under sloping roofs. The use of scissors trusses is appropriate for spanning between two support points (columns, walls). It may also be used as a horizontal roof truss.
The scissors truss is one of the most common roof truss types in use today, used in a wide range of buildings from those built for agricultural purposes to those used for industrial purposes. It is also very commonly seen on residential buildings.
Common Scissor Trusses Problems
The main problem with this particular design is with the common purlins, which are suspended from the upper chords at their midpoint.
These are prone to sagging under load, which then pulls the inner trusses outwards. When this happens, the trusses can be forced out far enough that the outer chords become damaged.
Functions of Scissor Trusses
A scissor truss is a structural system consisting of a series of triangular units in the shape of a scissor, with the connecting members either parallel, or perpendicular, to each other.
The triangular units are positioned with one point in the ground, and the other two points from the top of the triangle with a horizontal connection.
The purpose of a scissor truss is to provide the upper triangle with lateral stability. The triangular shape helps reduce the effects of swaying due to wind or traffic.
They are often used for rooftop structures or for pedestrian bridges.
Types of Trusses in Construction
1. Queen Post Truss
A queen post truss is a vertical upright with two triangles on either side. These are no longer commonly use because the Fink truss has taken their place.
2. Fink Truss
A fink truss is a very common roof truss type with a ‘W’ shape between the chords. It is a very simple but efficient shape and can be used in all manner of projects.
In its simplest form and contains web members that follow a V-pattern that can be repeated numerous times.
The V pattern grows considerably thinner when the top chords slant downward from the center.
Fink trusses can be particularly efficient at transmitting loads to the support because they rely more on diagonal members.
3. King Post Truss
A king post truss is a variation on the queen post and is used in roughly the same way.
4. Gambrel Truss
A gambrel truss features two different slopes on the outside, with the slope getting steeper from the center.
Because of their outward protrusion, gambrel trusses can be effectively equipped with a hollow center that can be used as a storage room.
As a result, the upper section of a barn is typically constructed in a gambrel shape. In the case of a barn, because the members are typically made of wood, the structure functions more like a frame than a truss.
5. Pratt Truss
A Pratt Truss has been used as a useful truss technique for over two centuries. The vertical members are compressed, whereas the diagonal members are tensed.
Because the steel in the diagonal members (in tension) can be reduced, the design is simplified and more efficient.
This has a few effects: it reduces the cost of the structure owing to more efficient members, it reduces self-weight, and it makes the structure easier to build. This style of truss is best suited for horizontal spans when the force is mostly vertical.
6. Scissor Truss (also known as Vaulted Truss)
A truss roof is made up of two triangular or compound triangles which meet each other at a point. These are similar to the design of scissors trusses however; they are not constructed with rafters.
A scissor truss is a variation of the standard form with each member offset at its mid-point. Both vertical members and horizontal members are compressed.
7. K Truss
The K Truss is a more complex variation of the Pratt Truss. The key difference is that the vertical members have been reduced, which improves its buckling resistance.
It does, however, offer similar advantages and disadvantages to the Pratt Truss, and while it is not often used, it is a sturdy design.
One of its major drawbacks is that its members do not always behave as intended. Under one load scenario, a member may be in compression, whereas in another, it may be in tension. This may imply that the structure cannot be designed optimally.
8. Mono Pitch Truss
A mono pitch truss has a single rafter and is a right-angle truss used to form lean-to roofs or single pitched roofs.
9. Raised Tie Truss
Raised tie trusses are used to create vaulted ceilings. The lower chord is raised to create a flatter profile for higher ceilings with angled edges.
Scissors truss FAQs
1. Are scissor trusses more expensive?
Scissor trusses are often more costly than hangars and open web members. This is because scissor trusses are more difficult to manufacture. These trusses also need to be transported in smaller pieces before they can be erected
Compared to hangars, scissor trusses are more durable. Designs that use scissor trusses do not need to be combined with other types of trusses because they are more than capable of bearing the load.
Scissor trusses can be used in a variety of different building types, but are usually reserved for large buildings like industrial sites or apartment buildings.
2. What is a scissor truss in construction?
Scissors trusses are a type of framing truss. They are a very common form of roof framing and are used extensively in both residential and commercial construction.
The term ‘scissors truss’ comes from the design of the truss resembling the two blades of a pair of scissors.
A scissors truss consists of two rafters that intersect in the center, and are attached at the top and bottom by a beam and two diagonal members that meet at the center.
3. What is a truss roof?
A truss roof is made up of two triangular or compound triangles which meet each other at a point. These are similar to the design of scissors trusses however; they are not constructed with rafters.
A truss roof does not use any horizontal beams and is often used in industrial buildings and places that require more energy efficient roofs.
4. What is the difference between a truss and a scissors truss?
Scissor trusses are made by joining two triangles together, thus forming an ‘A- shape. This is different from other trusses which have a horizontal beam
A scissors truss is made of two rafters and a diagonal brace that crosses at the top and bottom of the truss. The horizontal beam then rests on the diagonal members.
5. What is a scissor truss used for?
Scissor trusses are used mainly in residential homes. They are also known as hip rafters and consist of two framing triangles with a beam placed over them. The beams can be made out of wood, metal, or any other material.
6. Is a scissor truss strong?
Scissor trusses are very strong and one of the best ways to roof a building. However, they do require strict construction standards and need to be inspected before being used in a building.
7. How are scissor trusses built?
Scissor trusses are easily built. They are made from two framing triangles that meet at the top with a beam placed on top.
The diagonal members must be attached to one side of the triangles and then the other side attaches at the bottom to form an H shape.
8. Where are scissors trusses used?
Scissor trusses can be used for any type of roofing structure. They are very popular in homes, warehouses, and in buildings that require a strong roof support.
9. What are the three common types of trusses?
There are three different types of trusses: The open web truss, the compression truss, and the king post truss.
10. Are scissors trusses better than a traditional roof?
Scissors trusses are just as good as a traditional roof. They are supported by structural members at the top and bottom of each rafter, which means they can be used without an engineer’s approval.
Advantages of Scissor Trusses
One of the main advantages of using scissors trusses, as opposed to using a traditional roof truss, is that they make the roof much stronger and more stable.
While the roof of a traditional truss is made up of individual pieces, a scissors truss is designed so that it can withstand and support more weight than a similarly sized traditional truss.
In that case, there is less risk for it to collapse or have components fall out of place.
Beyond that, scissors trusses have the added advantage of being lighter than traditional trusses which means they are easier to transport, faster to assemble, and are also easier and cheaper to ship.
Another advantage of scissors trusses is that they’re able to properly distribute the weight load of a roof.
The scissors truss is made of two perpendicular triangular beams that are connected with a rod or a beam. It also has a saddle board at each end.
The saddle board is a horizontal beam that crosses the two perpendicular triangular beams, and it distributes the weight load, as the name suggests.
The weight load that would have been on the beams also shifts to the saddle board, which is much stronger. The rods and beams form an upside-down ‘Y’ shape.
Scissors trusses have the ability to span longer distances and reach higher heights without sacrificing strength.
Disadvantages of Scissors Trusses
When the roof of the scissors truss is steep, blown-in insulation materials such as cellulose or blown-in fiberglass may fall downhill or collapse.
Because scissors trusses allow very narrow access at the eaves as compared to standard roof trusses, construction workers are frequently hesitant to crawl to the edge of the attic to undertake work.
It’s difficult to insulate scissors trusses with the flash-and-batt technique, as it is with many truss types, because it’s difficult to figure out how to maintain the fiberglass batts in physical touch with the cured spray foam.
