Steel floor joists are horizontal load-bearing structures that transfer loads from floors above them to beams. They have high strength-to-weight ratios. Steel floor joists are not common in residential construction due to the cost, but if incorporated they can provide exceptional stability to a house.
Steel floor joist framing is becoming widely popular and is now being used in residential construction as well.
This article aims to provide detailed information on steel floor joists, by highlighting their advantages, limitations, practicality, cost, etc.
What are Steel Floor Joists?
Steel floor joists are beam-like structural members that are installed beneath floors in a house.
They carry the loads from the floors and transfer them onto the beams.
Steel joists offer various benefits over traditional lumber joists in terms of quality, durability, strength, uniformity, and alignment.
Steel floor joists make excellent substructures for floors as they are quite resilient and resistant to water, mold, and pests.
Joists were traditionally made from lumber found in local woods and were used extensively to support structural frames.
Later on, engineered joists, specifically, I-joists started to gain popularity due to their efficiency and cost-effectiveness.
However, with metal becoming popular, steel was added to the mix and steel joists in the form of solid blocks and trusses were introduced.
The aim of using steel was to improve upon the previous joist types by creating a more efficient load-bearing support member.
Even though steel floor joists offer various advantages, they are still not commonly used for residential construction.
This is mainly due to their high initial and maintenance cost compared to traditional wooden joists.
However, this does not mean that steel joists should not be used in residential construction.
Steel joists are a better option for residential buildings in the long run.
Advantages of Using Steel Floor Joists
High Strength to Weight Ratio
Steel has the highest strength-to-weight ratio among all the materials used in construction.
This means that lesser amounts of steel can bear more loads compared to other materials, thus needing fewer supporting members.
This in turn reduces construction time, labor costs, and the volume of required material.
Steel having large load-bearing capacities can be used in cases where wood is not sufficient to resist heavy loads.
Resistance to Moisture, Mold, and Pests
Locations such as the basements, roofs, and attics are all particularly prone to degradation. This leads to the rotting and wearing of wooden joist members.
In comparison, steel floor joists are more resistant to moisture effects, as they do not grow mold under continuous exposure to moisture.
Steel floor joists are also resistant to rodents, termites, and other pests as they can be treated with pesticides and other chemicals.
Resistance to Noise
Incorporating steel floor joists in flooring systems makes them quieter.
The squeaking noise that is observed while walking on traditional floors is due to the wear, warping, splitting, and cracking of the floor joists which loosens the nails and bolts.
Steel joists can maintain their straight shapes and remain structurally sound for long periods.
Additionally, the bolts and nuts applied to steel floor joists are fewer and tighter than the ones used in wooden joists.
Steel floor joists once installed function effectively for a very long time.
They do not lose their strength easily and thus do not have to be replaced often.
Minimal maintenance is required when steel floor joists are used under floors.
Steel floor joists are more resistant to fire as compared to wooden joists due to steel being a non-combustible material.
A good way to better protect steel joists is to coat them with fire-resistant chemicals which allow them to be heat resistant even at very high temperatures.
High-density Spray-Applied Fire Resistive Material (SFRM) is a common chemical protection compound used to protect steel joists in the USA.
Location Specific Advantages
Steel floor joists are the only viable option in many situations in construction.
Some bridges, for example, are exposed to extreme flooding or earthquakes. It is necessary to use steel floor joists in such structures to make them structurally sound.
Some US states also have laws that desire mold-resistant construction. In such regions, steel is a good option for obtaining permit.
In terms of sustainability, steel is a material that can be recycled easily.
Most of the steel used in making steel floor joists comes from recycled materials and thus has a minimal impact on the environment.
Types of Steel Floor Joists
Steel floor joists can be classified into different types based on their shape and span.
Each type of steel joist is named under a series.
Each series is further classified depending on the chord bearing styles (top chord bearing, bottom chord bearing, etc.)
Open Web Steel Floor Joists
Open web steel floor joists are the commonly used joists in floors and roofs of buildings.
They are classified as K-series joists.
The open web design of steel floor joists offers a great advantage. It creates built-in passages/ducts for plumbing, sprinklers, and electrical wiring.
These openings help reduce the cost of the overall project
These joists generally have a depth between 8” to 30” and span up to 60’.
Long Span-Steel Joists
As the name suggests these steel floor joists can span longer than open web steel joists.
They are 18” to 48” deep and can span up to 96’.
These steel joists are called the LH-series.
Deep Long-Span Steel Joists
These steel floor joists are deeper and longer than every other series of joists.
They generally have a depth that ranges from 53” to 72” and can span a total length of 144’.
They are categorized as the DLH-series.
Disadvantages of Using Steel Floor Joists
Corrosion of Steel
Steel is a highly corrosive substance. When exposed to moisture for long periods it starts to rust.
To prevent corrosion, steel floor joists have to be painted periodically.
Steel members start to lose their thickness at the rate of 1mm/year to 1.5mm/year, if not coated properly.
A loss in thickness leads to a loss in strength which can ultimately cause the floors to collapse.
Steel floor joists are excellent conductors of heat and they can transfer the heat to other parts of buildings, which can start a fire elsewhere.
Though steel is incombustible, it loses significant strength at high temperatures (around 400ºC).
This means steel members must be applied with fire-resistant chemicals.
Additionally, steel floor joists have to be regularly painted over to avoid corrosion.
Both fireproofing and painting of steel floor joists add extra costs to the project.
Thus, maintenance of steel floor joists can be expensive.
Buckling of Steel Joists
Buckling is a failure mechanism characterized by the bending of a structural member due to a compressive load surpassing the critical limit.
Steel is good both in tension and compression, but it is susceptible to lateral torsional buckling due to applied loads.
This is because the average steel floor joist member is more slender as compared to its counterpart.
If these slender joists are applied with enough compression force, they start to buckle.
Availability and Initial Costs
Some regions in the USA do not have steel available in the required amount.
This scarcity causes steel joists to be expensive and thus the initial investment in using steel floor joists can be very high.
Are Steel Floor Joists Better than Traditional Wood Joists?
Steel joists offer a variety of advantages over traditional wood joists.
One common problem associated with wooden joists is their susceptibility to expansion and contraction due to moisture absorption.
This can cause cracks to develop in wooden joists.
In contrast, steel is resistant to moisture, which consequently makes it resistant to expansion and contraction caused by the freezing and thawing of moisture.
Thus, steel floor joists are less likely to develop cracks, warps, and other associated defects.
Nails, bolts, and screws are also more likely to retain their position and strength in the case of steel as a result of lower chances of expansion and contraction.
Thus, steel floor joists offer more structural stability and develop fewer issues as compared to wooden floor joists.
Wood is a combustible material and thus wooden floor joists are always at a risk in case of a fire.
In comparison, steel is an almost incombustible material making it more fire resistant.
In the likely event that a fire does start in a building, the overall damage delivered to the building will be less if steel floor joists are incorporated into the design.
Steel floor joists may also allow engineers to increase the flooring and ceiling space under the limits and regulations of the fire code.
Wooden floor joists are likely to develop some form of mold over time. This mold in return can reduce the air quality of a house.
However, steel does not develop mold and hence caters to providing better air quality to homes.
Wooden floor joists are also applied with pesticides and water-resisting chemicals which further reduce the air quality.
Steel floor joists do not need to be coated with such chemicals.
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Differences between Steel and Wooden Floor Joists
Cost of Steel Floor Joists
Steel floor joists or support beams come at a cost of $100 to 400$ per foot for residential construction.
This can make the total cost of installing a steel floor joist range from $1000 to $4000 depending on the size of a floor system.
The longer the span of the floor, the more expensive the steel floor joists become.
The cost of steel floor joists also depends upon the type of steel being used.
Hot-rolled steel joists are thinner and less expensive, whereas cold-formed steel joists are thicker and have more strength, but also cost more.
Another thing to note is that the $100 to $400 cost is only for the material itself.
The total cost of installing steel floor joists can range from $10,000 to $20,000, which includes the headers and webbing cost as well.
Steel floor joists are very efficient in transferring loads from floors to beams and offer a variety of advantages over wooden joists.
They are stronger, more durable, and long-lasting.
It is not common to use steel floor joists in residential construction, however it can be a good choice if you have the budget.
Though steel floor joists have high initial costs, they can be more sustainable in the long run.
Frequently Asked Questions (FAQ)
How far can steel floor joists span?
Steel floor joists can span anywhere between 60’ to 144’ depending on the type of joist used.
A k-series steel joist can span up to 60’, an LH-series steel joist can span up to 96’, while a DLH-series steel joist can span up to 144’.
Are steel floor joists used for residential construction?
Steel floor joists are increasingly being incorporated into the remodeling and redesigning of residential construction.
This is due to the qualities that steel members have which makes them more durable and rot-resistant than their alternatives.
Galvanized steel floor joists are resistant to rust, moisture, molds, and termites. Additionally, they do not undergo much twisting, warping, or cracking.
When to Use Steel Floor Joists?
Steel floor joists are widely used in buildings where large loads must be carried and transferred.
In residential construction, steel floor joists are an ideal option to be used in basements and attic spaces due to their moisture and mold resistance.
Basements and attics usually are exposed to varying environmental conditions which lead to the rotting of wooden joist members.
This can be avoided by using steel floor joists.