The most common steel member used for building construction is the I-beam, so-called because a horizontal cross-section of the beam resembles an upper-case letter “I.” The beam is also called a joist or girder. It has flanges attached on either side of the central core.
Another name for this type of construction is rigid frame or continuous steel frame. The beams come in various lengths and carry ratings that indicate the amount of weight it can bear as well as its size. Rigid frame construction was developed, unsurprisingly, in cities like Chicago and Cleveland in the late 19th and early 20th century.
This post will describe typical I-beam building rigid frame construction techniques and go over the benefits of using I-beam construction. We’ll also look at building applications for I-beam construction and talk about when it may not be appropriate.
What is a steel i-beam building?
In a steel I-beam building, the main frame is constructed from steel and acts as the support for the building.
- A truss is a member with four sections assembled on the ground
- There are two sidewall columns and two roof beam sections
- Each "I" beam truss is raised and bolted or formed into the concrete foundation at 20 to 30-foot intervals.
Between the trusses, steel purlins and girts are installed in a horizontal pattern up the wall and roof. After the purlins and girts are installed, workers place insulation and sheeting then create openings for doors and windows.
I-beams are rolled, welded up, or extruded by metalworking machinery that creates a standard-size beam quickly. Another method of fabrication is called "plate girder” which is created by riveting or welding together sections of steel plate. The word “rigid,” as in “rigid frame,” refers to the ability of an I-beam structure to resist deformation.
What is rigid frame construction?
Rigid frame construction provides rotational stability that enhances how the frame carries vertical loads and increases the durability of the entire structure. The joint connections in rigid frame can be fully fixed-end with fully restrained connections that cannot rotate, or they can be pin connections that allow the members to rotate freely.
- The word rigid is used because the beam-to-column connections are classified as rigid.
- The frame is designed to transmit beam end moments and shear forces into the columns without the need for bracing systems for lateral load resistance.
- Frame stability is provided solely by the rigid connections and the stiffness of the I-beams.
The rigid connections are full depth end-plate connections, typically created with a bolted end-plate beam-to-column connection. A welded connection can be used and may be required in seismically active regions. Welded connections provide full moment continuity but will be a bit more expensive than bolted connections. It is recommended that welded connections be prefabricated rather than welded on site.
Rigid frame connections transmit beam-end movements and shear forces into the columns. The joints rotate as a unit, and the members maintain the same angular relation at all times. A rigid frame is much stronger than post and beam construction and can hold heavier vertical loads.
Without rigid frame, skyscrapers would not be possible.
The benefits of metal i-beam buildings
Steel I-beam construction is highly cost effective and energy efficient. For occupants, the floors are not sensitive to vibration and so create a more comfortable environment. Other benefits include:
- Large spans
- Connections that perform better in load reversal and seismic situations
- Few limitations on building width
- Accommodate higher snow loads when truss placement is closer
- Highly available and familiar to most builders
- Walls do not support the structure of the building.
Clear span framing transfers the load to the sides of the building, allowing construction of wide open areas free of columns. Your rigid frame building can have hundreds of feet of open space, ideal for warehouses, hangars, and agriculture.
Rigid frame is quite cost effective for large non-insulated and insulated buildings such as warehouses. An airplane hangar is another structure in which insulation may or may not be needed. However, the large doors and entry and exit of airplanes make climate control for the entire building difficult.
Construction with I-beams usually requires crane work or the use of other heavy equipment. Some welding may be required depending on building size and the degree of pre-engineering by the manufacturer. Also, rigid frame is more complex than some other types of steel construction, but since it is typically used for larger buildings and special uses, the complexity is outweighed by its stability.
You may think I-beam construction limits your building to a blocky look but with the variety of finishes and dressings available for steel buildings; your structure can look like almost anything.
Types of building applications using rigid frame construction
Rigid frame buildings are used for a variety of purposes. The clear span construction provides open space without interruption. The open spans can be configured into a multitude of floor plans using interior panels. Since rigid frame is insensitive to vibration, occupants will not hear or feel people or equipment moving along the floor as much as they might with a different type of construction.
In fact, many of the buildings you see every day may be constructed of steel I-beams.
- Metal warehouses
- Retail stores
- Equipment shelters
- Manufacturing plants
- Multi-story buildings
Rigid frame is particularly applicable to buildings with special uses such as hospitals, white rooms, research facilities, and buildings storing equipment sensitive to vibration and deflection.
Rigid frame or I-beam construction is one of the strongest, most stable forms of steel building construction. It makes possible large, wide span buildings. Since steel has such a high strength-to-weight ratio, extremely tall buildings are possible. Rigid frame construction is also adaptable and versatile in design. You can dress the exterior to match almost any design, texture, or appearance and door and window placement is extremely flexible.
Rigid frame can withstand the high wind loads experienced by tall buildings because the rotational force is channeled from the beams to the columns. The joints remain properly oriented throughout any rotation caused by such loads. You also enjoy all the benefits associated with steel buildings such as a lower insurance premium.
Rigid frame is a very common form of steel construction; most contractors and construction workers are familiar with this type of construction. You shouldn’t have any trouble finding someone with the experience to erect your building properly.
Steel I-beam construction can be seen all around you. City skylines are dotted with buildings constructed with steel I-beams. Without the strength of steel, rigid frame construction would not be possible, and our world would look very different without it.