WHAT IS A RIGID FRAME BUILDING?

You can think of a rigid frame building like a table. An ordinary dier table Gauge Steel Framing Machine held together and made stable by very tight junctions of the tabletop and legs is built along the same principles as a rigid frame building.

The structure of a steel rigid frame building is characterized by its rigid coections of straight or tapered columns and beams. These structures look similar to that of a post and beam system; the difference is its considerable ability to hold vertical loads without additional bracing, due to the rigid coections of its members.

Rigid frame structures were developed in cities like Chicago and Cleveland in the late 19th and early 20th century. This type of frame is preferred for its quick erection, open span, and versatile design use.
FRAME TYPES
Rigid frame, sometimes called continuous frame, offers a rotational stability that enhances how it carries vertical loads, increasing the longevity of the entire structure. Besides rigid frame, the same framing principles are used in the simple frame and partially restrained frame buildings.

The joint coections can be fully fixed-end with fully restrained coections that caot rotate, or they can be "pin coections," in which the members can freely rotate.

DEFINING RIGID FRAME CONSTRUCTION
In this type of frame, the beam to column coections are classified as rigid, hence the name. The frame is designed to transmit beam end moments and shear forces into the columns without bracing systems to resist lateral loads. The members can be straight or tapered.

The frame stability if only provided by the rigid coections and member stiffness. It looks similar to post and beam but is significantly stronger and able to hold vertical loads.

Local beam-column coection rotations are not considered in global frame analysis; the coections are designed to transmit the resulting beam end movements and shear forces into the columns.

The joints are not always fully fixed to either used c purlin making forming machine horizontal or vertical members; when the beam rotates from a vertical load, the columns rotate with it. This allows the joint to rotate as a unit and members maintain the same angular relationships during the rotation.

Rigid frame buildings are highly adaptable and flexible in design. Doors and windows can be placed anywhere, and HVAC units can be placed on the roof or the side. The exterior can easily be dressed to look like any envelope type including stone, brick, or wood.

DIFFERENCES BETWEEN POST AND BEAM AND RIGID FRAME
Post and Beam: When a vertical load weighs on a common post and beam structure, it is carried by a horizontal member and then shifted by bending to columns or vertical members. The beam is simply supported by its columns, sitting on top of them so that the ends of the beams can rotate on top of the columns with no restraint. The result is that the horizontal members of the structure then only carries axial forces.
Rigid Frame: When a rigid frame structure is subjected to a vertical load, it is also picked up by the beams and eventually transferred through the columns to the ground. However, the joints are strongly coected, preventing any free rotation from occurring at the beams ends. This slight difference changes everything about the behavior of the beams, which is now the same as a fix-ended beam.
Rigid frame construction provides many benefits, such as decreased deflections, decreased internal bending moments, and increased rigidity. However, the columns are experiencing some degree of internal bending themselves as the beams stay rigid.

HOW IS IT DESIGNED?
The coections of the joints in rigid frame construction vary between being fully fixed-end, in which the coection offers full restraint and no ability to rotate and pin coection, where the members are completely free to rotate. This relationship is referred to the relative stiffness of the horizontal and vertical members.

From a designer’s perspective, a rigid frame structure can be designed smaller than other post and beam systems. This is because internal bending moments are reduced by the rigidity. On the other hand, the columns or vertical members should be designed to be a bit larger, since they are carrying both axial loads and internal bending moments.

MOMENT RESISTING CONNECTIONS
The so-called rigid coections are typically full depth end-plate coections and extended end-plate coections. The most common of these is the bolted end-plate beam-to-column coection. The selection depends on the budget.

Welded coections can be used in place of the bolted end-plate, especially in seismically active regions. Welded coections can provide full moment continuity but tend to be on the expensive side. If used, the welded coections should be prefabricated rather than welded on-site.

Site-bolting takes less time and minimizes labor costs.