While joists, beams, and girders are structural members used to support a building, there are significant differences between them.

Joists are the smallest of the three and are often used abundantly to act as secondary support to ceilings and floors.

Beams are relatively bigger than joists but are used in smaller numbers to provide primary support to ceilings and floors.

Lastly, girders are the biggest and the strongest of the three and are used in large-scale projects such as bridges and flyovers to transfer loads.

Despite differing in design, size, and functionality, these three members are often confused with one another.

It is crucial for a construction worker or an engineer to understand the working of these members to ensure safe designs.

The key differences between the three horizontal structural members are as follows:

Property	Joists	Beams	Girders
Size	Smallest of the three members.	Intermediate sizes; bigger than joists.	Largest of the three.
Strength	Weakest of the three members.	Relatively stronger than joists.	Strongest of the three members.
Functionality	Acts as secondary support for ceilings and floors.	Acts as primary support for ceilings and floors.	Act as support in large-scale projects.
Support	Joists rest on beams.	Beams rest on other larger beams or vertical columns.	Girders rest on vertical posts.
Quantity	Used in large numbers with small spacing.	Usually used at the ends of a frame and in the middle, with relatively larger spacing.	Usually just one or two.
Spans	Joists usually span either full or half the width of a house.	Beams span the entire width or length of a house.	Girders offer spans equal to the length of a home or the length of the structure being built. Can cross lengths of over 300ft.
Uses	They provide secondary support to floors and ceilings. They resist shear forces and bending moments.	They provide primary support to floors and ceilings. They resist shear forces and bending moments. They connect frames together. They enable uniform distribution loads.	They provide the main support in any structure that incorporates them. They resist greater values of shear forces and bending moments. With girders, long-span structures such as roads and bridges can be constructed.
End Conditions	Always fixed.	Simply supported. Continuous. Fixed. Overhanging. Cantilever.	Simply supported. Continuous. Fixed. Overhanging. Cantilever.

What is a Beam?

A beam is a horizontally load-bearing structural member used to support and transfer all the loads of the houses.

 A beam is rested either on another larger beam or a vertical column. Beams can be made from numerous different materials.

The primary function of a beam is to bear loads from the structure on top of it and transfer them to the members that support it.

Overall, beams are used to resist loads, counter shear forces and bending moments, connect frames together, and allow uniform distribution of loads.

Beams are used in houses to support floors and ceilings. Beams at mid-span are also incorporated into the design if the size of a room becomes too large.

Beams can be made from dimensional lumber, engineered wood, steel, or reinforced concrete.

On the basis of supports, beams can either be simply supported, continuous, fixed, cantilever, or overhanging.

Beams have a range of intermediate sizes. They are larger than joists but smaller than girders.

In residential construction, the standard size of a beam is usually 9” x 12” according to IS codes. The minimum size is stated to not be less than 9” x 9”.

Due to their larger sizes, there is no need to use too many beams in a design.

Mostly a single beam will be sufficient to carry loads of a large number of joists.

A single beam has enough strength to support hundreds of joists, provided that the foundation and the columns are constructed properly.

What is a Joist?

Joists are load-bearing horizontal members that are used to carry loads of the ceilings and floors above them and transfer them onto beams.

They are relatively smaller in size as compared to beams and are used in large quantities with small spacing in between.

They span along the lengths of a design and are installed in parallel positions.  The number of joists required depends on the dimensions of the design.

Joists, like beams, can be made from dimensional lumber, engineered wood, steel, or reinforced concrete.

There are many types of joists, such as band joists, rim joists, and I Joists.

I-Joists are considered to be a highly efficient form of joist due to their strength-to-weight ratio.

The most common types of I joists are 2×10 dimensional lumber joists and engineered wood joists.

Engineered I-joists are typically available in lengths of up to 48’. However, lengths of up to 60’ are also available.

The depths of I-joists range between 9 ½” to 16”, whereas the widths range from 2 ½” to 3 ½”.

Joists are preferably always fixed by nailing at the ends between beams.

Joists are lightweight, making them easy to install. They also have a very practical design which allows holes to be drilled into them.

These holes then allow passage for electrical wiring and plumbing ducts.

If you wish to avoid drilling completely, opt for a floor truss instead of an I joist system.

What is a Girder?

Girders are more rigid and larger horizontal structures with the function of having to provide the main support to the frames.

Girders are built to support significant amounts of loads encompassing structural pillars or beam responses.

There is no real limit that distinguishes between the sizes of a beam and a girder, but a girder is always larger than a typical beam.

Traditionally the Length to depth ratio of a steel girder is kept between 25 and 30.

Girders can always have all the support conditions as in the case of beams. They can be simply supported, continuous, fixed, cantilever, or overhanging.

Girders are the primary support in massive structures and act as supports to smaller beams in the design. They can bear extremely heavy loads.

Girders will carry the load from the smaller beams resting on them and then transfer it to the vertical supports below them,

Girders are incorporated into the construction of bridges, roads, flyovers, and trusses.

Girders have large spans ranging from 33 feet to over 300 feet, making the construction of the above-mentioned structures feasible.

Commonly used types of girders include plate girders and box girders.

Plate girders are used in both residential and large-scale construction projects. They come in I shapes with varying sizes.

Box girders are usually used in the construction of elevated bridges.

How do Beams Differ from Girders?

Beams are used for carrying floor and ceiling loads and connecting frames.

Beams are usually smaller than girders and support the joists under floors and ceilings in a house.

Girders, on the other hand, are the largest horizontal members in a building.

They are much fewer in number and help transfer the loads from the beams resting on top of them.

Girders being stronger and larger than beams offer larger spans to be constructed.

Hence girders are utilized in constructing structures like roads, bridges, and flyovers.

In contrast, beams are the primary load carrier in smaller-scale projects, mostly in buildings with smaller room sizes.

It is to be noted that beams and girders are often used together as beams are usually rested on girders.

The use of such configurations is dependent on the number of loads a structure has to bear.

Dimension-wise, there is no set criterion to differentiate between a beam and a girder.

But the minimum dimensions for a beam are 9” x 9”, while a girder has always to be larger than a beam in a design.

The largest beam in a design is often referred to as a girder.

How do Joists Differ from Beams?

Beams and joists go hand in hand, but as mentioned before are not the same things.

Joists bear the loads from the ceilings and floors and are rested upon beams. As a consequence, the loads from joists are transferred onto the beams.

Beams then carry these loads and pass them onto the foundations via columns.

Beams are always larger in size as compared to joists. Only a few beams are used in a frame, but in contrast, a large number of joists are utilized in the same frame.

Joists have smaller spacing in between; as such, they are used in great quantities.

Due to the size difference, beams are capable of resisting larger loads. This is why often, a single beam is used to support hundreds of joists.

Beams are usually placed at the ends or at the mid-span of a frame to provide support.

Holes can be drilled into joists without compromising the integrity of their design, but such actions are to be avoided in the case of beams.

Unlike beams, joists can be further strengthened using bridging or blocking techniques.

The span length of joists is either equal to the full width of a house or half of it.

Joists can be strengthened later if they start to sag.

 Whereas the span of a beam can be extended to either the full width or length of the house.

The support conditions of a beam can vary depending on the situation, but joists are typically always fixed at the ends.

How do Joists Differ from Girders?

Joists are relatively a lot smaller as compared to girders. Joists transfer the load onto the beams, and girders carry the load from the beams.

Joists are usually used in small-scaled buildings and houses. The maximum spans offered by engineered wood joists are typically around 60’.

Girders, on the other hand, are utilized in housing as well as bigger projects. They can carry and resist very large values of loads.

Girders offer spans of over 300ft which allows the feasible construction of bridges, roads, and other similar structures.

Girders like beams can have different end conditions, but joists remain fixed by being nailed into beams.

In construction, joists are used in large numbers, whereas typically, only one or two girders are sufficient design-wise.

Large-span projects often use trusses instead of joists.

When to use Joists, Beams, and Girders?

All three structural members are often used in unity with each other. They are designed to complement one another.

Joists are used under floors and ceilings for support. Beams are used for carrying the loads from joists and transferring them to either columns or girders.

Girders are used when there is too much load to be resisted and transferred.

When utilized, girders carry all the loads from the beams and transfer them to vertical members supporting them.

Conclusion

Joists are the smallest of the three members and are used in abundance to carry floor and roof loads. They have an easy installation process and a practical design.

Beams are used as the primary support for floors and roofs by carrying loads from the joists.

Beams are also used to connect frames properly. Usually, a single beam can carry the loads from hundreds of joists.

Girders offer the most strength out of the three members. They are usually utilized in large-scale projects such as bridges, roads, and flyovers.

Girders carry the loads from beams and transfer them to the foundations via vertical members.

In design, usually, only one or two girders are enough to carry sufficiently large amounts of loads.

All three members are important in constructing buildings. Their use depends upon the amount of load that must be resisted.

FAQ

Are Beams Stronger than Joists?

Beams are both bigger in size and stronger than joists.

Usually, a single beam is sufficient to support and carry the loads from hundreds of joists.

Beams act as the primary support for floors and ceilings, whereas joists act as the secondary support.

What are the advantages of box girder?

A box girder offers more torsional stiffness and strength as compared to normal plate girders.

They give off greater aerodynamic stability for horizontally curved bridges and make the flanges less susceptible to lateral-torsional buckling.

Box girders are usually used in the construction of elevated bridges.

Why are joists, beams, and girders given I shapes?

I-shaped members provide sufficient surface area to bear and resist large amounts of loads. They also have enough depth to resist bending.

Using an I-shape also reduces the weight of the structural member without compromising the integrity of the design strength.