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. These trusses are made of high-grade lumber or steel.
Types of Floor Trusses Based on Structural Arrangement
There are three types of trusses, and they differ in terms of shape, components, arrangement, and accessibility.
| Characteristic | Warren Truss | Pratt Truss | Howe Truss |
|---|---|---|---|
| Description | A series of isosceles or equilateral triangles are connected | The triangles form a V-shape | 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 |
Different Types of Floor Trusses
Warren Truss
The Warren truss is the most common type of bridge used in construction because it is easy to construct and efficiently distributes loads among its components.
It requires steel or metal with high compressive strength, making it suitable for long spans.
However, it performs poorly under concentrated loads.
Pratt Truss
Pratt trusses have a unique arrangement, and their diagonal members slant towards the middle.
In this design, the vertical members bear compression forces while the diagonal members resist tension.
Therefore, diagonal members in Pratt trusses are made of lighter and thinner steel.
This truss is common in residential buildings with spans ranging from 65 to 330 feet, 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.
Types of Floor Trusses Based on Bearing Conditions
Floor trusses can vary based on their bearing conditions, making them more versatile.
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.
However, 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.
For instance, a wooden truss can span up to 30 feet, which is much greater than a traditional joist.
Additionally, they have an open web design which allows easy access for utilities.
Floor trusses can also have custom designs and strengths to suit the needs of a homeowner or a designer.
Moreover, they are easier to work with and provide a nailing surface of about 3.5”, greater than the 2.5” surface offered by most I-joists.
Furthermore, trusses are made using exceptionally quality lumber, which minimizes the risk of shrinkage, warping, and twisting, reducing maintenance costs.
Disadvantages of Using a Truss System
Not all trusses are adjustable once placed on site.
This depends on the spans of the trusses and their alignment.
Any replacement or change in the truss system arrangement can cause long delays in the project.
It is also crucial to include any custom designs in trusses before transporting them to the site.
Additionally, floor trusses are a lot more costly than floor joists.
Trusses are also very heavy and thus difficult to handle during installation, which increases the labor costs.
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?
Trusses are cost-effective due to minimal material wastage.
Wooden floor trusses cost around $4 to 5 per square foot, whereas steel trusses for large projects cost $10 to $15 per square foot.
Frequently Asked Questions (FAQ)
Do Floor Trusses Need Central Support?
Floor trusses can span long distances without the need for central support.
The end metal connections provide structural stability to the truss systems and can function efficiently under various loading conditions.
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.
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.
