Floor trusses are engineered with open web designs capable of spanning longer than traditional wooden I-Joists. Various types of floor trusses are used in a floor system, including Warren, Pratt, and Howe trusses. Trusses are made of high-grade lumber or steel.
The wooden components are arranged to maximize the efficiency of a truss system and dissipate loads properly.
Types of Floor Trusses Based on Structural Arrangement
There are three primary types of floor trusses based on the arrangement of their components.
- Warren truss
- Pratt truss
- Howe truss
The three types of trusses vary in many ways.
They differ in terms of shape, components, arrangement, and accessibility.
Warren Truss
The most common type of bridge used in bridge construction is the Warren truss.
It is easy to construct.
The steel or metal used in this truss must have high compressive strength.
It distributes loads evenly among all its components and can be used to make long spans.
Under concentrated loads, this truss system performs poorly.
Pratt Truss
Pratt trusses have a unique arrangement.
Their diagonal members slant down towards the middle.
This causes the vertical members to compress and the slanted diagonal members to resist compression.
Lighter and thinner steel is used to make the diagonal members of a Pratt truss.
This truss is mainly used in residential buildings with spans ranging from 20m to 100m, where gravity loads dominate.
Howe Truss
Howe trusses have diagonal members that slant away from the middle.
These members resist compressive forces while the vertical members tackle the tensile forces.
Howe trusses are more popular than Pratt trusses because of their superior load-dissipating capabilities.
They are mostly used in wooden construction and can span longer than Pratt trusses.
| Characteristic | Warren Truss | Pratt Truss | Howe Truss |
|---|---|---|---|
| Description | A series of isosceles or equilateral triangles are connected | Contains triangular members with diagonals slanting down towards the middle | Series of triangles with the diagonals slanting away from the middle |
| Materials | Heavy steel or iron is required to form the triangles | Thin and light steel or iron | Requires more steel or iron than Pratt Trusses |
| Shape | The triangles form a V shape | Normal N shape configuration | Reversed N shape |
| Accessibility | Easy to construct and install | Construction times are longer due to complex configurations | Complex configurations that take a long time to complete |
| Force Dissipation | Dissipates loads efficiently among various members | Under external loads, tension is induced in the diagonal member, in contrast, the vertical member resists the compressive force | Under external loads, the diagonal members tackle the compressive forces, and the vertical members resist the tensile forces |
Differences Between Various Floor Trusses
Types of Floor Trusses Based on Bearing Conditions
Floor trusses can vary based on their bearing conditions, making them more versatile.
The four types of trusses based on bearing conditions include bottom chord bearing floor trusses, bottom chord bearing trimmable end floor trusses, top chord bearing floor trusses, and mid-block bearing floor trusses.
Matching existing floor heights is easier using multiple bearing conditions than when using floor joists.
Bottom Chord Bearing Floor Trusses
This is the most common type of chord-bearing condition in floor trusses and is designed to sit on top of bearing walls.
Bottom Chord Bearing Trimmable End Floor Trusses
This truss has trimmable ends that can be shortened to 6 inches.
Top Chord Bearing Floor Trusses
This truss can be used indoors to recess beams into the floor.
They are designed so that the bearing wall is under the double-top chords.
Mid-Block Bearing Floor Trusses
These trusses are designed to match the height of existing floors and subfloors.
Types of Trusses Based on Material
Floor trusses can also be classified according to the materials used to make them.
Wooden floor trusses are cheaper and easier to install but allow for smaller spans to be built.
Steel trusses, on the other hand, are more robust and long-lasting.
They also allow for the construction of longer spans, are reusable, and require less labor to install.
But the main disadvantage of using steel floor trusses is their high cost.
Advantages of Using a Truss System
Floor trusses can span over long distances, making them ideal for large-spans.
Wooden trusses can span up to 30 feet which is much more than the span possible with traditional joists.
They have an open web design which allows easy access for utilities.
All the plumbing, electrical, and HVAC can be passed quickly and easily through the openings in floor trusses.
Floor trusses can have custom designs and strengths to suit the needs of a homeowner or a designer.
They are also easier to work with. Trusses provide a nailing surface of about 3.5”, greater than the 2.5” surface offered by most I-joists.
Trusses are generally made using exceptionally quality lumber, which minimizes the risk of shrinkage, warping, and twisting.
Thus, long-term repair costs are low.
Disadvantages of Using a Truss System
Not all trusses are adjustable once placed on site.
This depends on the planes of the spans of the trusses and their alignment.
Any replacement or change in the truss system arrangement can cause long delays in the project.
Custom designs for trusses must also be made beforehand in the plant before being shipped to the site.
Trusses are also very heavy and thus difficult to handle during installation.
This can increase labor costs as well.
Floor trusses are a lot more costly than floor joists.
A project must have heavy loads or spans for a floor truss to be included in the design.
What is the Cost of Floor Trusses?
Wooden floor trusses sell for about $4.4 per foot, whereas quality steel trusses for large projects sell for about $35 to $45 per meter.
Trusses are very cost-effective as there is not much wastage of material that occurs while using trusses.
Do Floor Trusses Need Center Support?
The value of the engineering behind floor trusses resides in their ability to span long distances without central support.
Trusses get their strength from the end metal connections that provide a strong grip on the trusses.
This means trusses can function efficiently in various loading conditions.
How do Floor Trusses and Floor I-Joists Differ?
The major difference between a floor truss and a floor I-joist is that a truss comprises triangular components throughout its horizontal span.
An I-joist is a solid I-shaped rectangular horizontal member.
Other important differences are explained in the table below.
| Characteristic | Trusses | I-Joists |
|---|---|---|
| Structural Configuration | Triangular configurations of beam-like components | A horizontal beam-like structure in I-shapes |
| Assembly | Need to be assembled before delivery | Can be installed directly on-site |
| Load Bearing Capacity | High load-bearing strength | Low load-bearing capacity |
| Frequency | A smaller number of floor or roof trusses is required for stability, increasing the spacing between each member | Increasing the number of floor and roof joist panels is necessary for stability, reducing the spacing between them |
| Holes | Several openings are already present, allowing plumbing ducts and electrical wires to pass | Holes need to be drilled into joists to allow passage for ducts and wires |
| Span | Large spans of a room are possible to construct | Spans are smaller |
| Cost | Expensive | Cheaper than trusses |
| Roof Shape | Provide a slope | Provide a much flatter roof |
| Alterations | Hard to alter after installation | Easy to alter even after installation |
Conclusion
Floor trusses are very efficient in transferring floor loads.
They make construction easy and more efficient.
Trusses are made from wood or metal, depending on the design requirements.
Wooden trusses are typically used in residential construction.
There are three types of floor trusses based on the arrangement of their triangular components.
The first is the Warren truss which dissipates loads efficiently among its members but does not perform well under concentrated loads.
It has a V shape configuration.
The second is the Pratt truss which has an N shape configuration.
Its vertical members carry the compressive forces, while the slanted members carry the tensile forces.
The third is the Howe truss which has a reverse N shape configuration.
Vertical members of the Howe truss carry the tensile forces while the slanted members transfer compressive forces.
Floor truss types also vary depending on the end chord bearing conditions.
There are four bearing conditions available for trusses.
They are bottom chord bearing, bottom chord bearing with trimmable ends, top chord bearing, and mid-block bearing.
Floor trusses offer a variety of advantages over traditional floor joists but are more expensive.
This is why floor trusses are used in large scaled projects and are less common in residential construction.
Frequently Asked Questions (FAQ)
How Long Can Floor Trusses Span?
In residential construction, a wooden floor truss can easily span up to 30 feet.
This facilitates the installation process, as no intermediate supports need to be erected.
Although steel trusses can span longer, they are rarely used in residential construction.
How Far Apart Should Floor Trusses Be?
Floor trusses can have 16”, 19.2”, and 24” spacing based on the size of the floor, the applied load, and truss strength.
The spacing requirements decrease when the number of trusses per floor is increased.
Typically, floor trusses are designed to be spaced evenly throughout a floor.
Are Floor Trusses Stronger than I-Joists?
Floor trusses offer greater strength and dissipate loads more efficiently than I joists.
They are also capable of spanning much longer than I-joists.
The strength of a truss frame is provided by its end connections, so center support is often not required.
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