Column and beam frames have been around since the Egyptians put up the pyramids. If you look at ancient architecture, you will see everyone from the Greeks forward used this construction method for almost everything they built.
The success of the column and beam structure has continued to this day when we use durable, recyclable steel inside nearly all of our non-residential construction.
Column and beam steel frames
Column and beam steel frame structures have a skeleton of steel connected by bolts or welds that act as the support for the rest of the building. A column and beam frame can be braced or continuous.
A braced frame has a web of smaller members acting to stabilize and provide rigidity to the frame, which is generally a simple pin connection. Bracing is added horizontally or vertically to the main frame, often in shaped patterns to facilitate transferring load stresses.
A continuous frame is not braced. Bolts or welds create rigid connections capable of withstanding the design loads without additional frame members. The key advantage to a continuous frame is the ability to minimize the depth of the beams. In addition, a continuous frame has aesthetic advantages and promotes accessibility without the need for triangulated bracing systems or solid wall systems.
Continuous framing is a more durable but more costly choice. A combination of continuous and braced frame may offer an economical solution.
More about continuous frames
A continuous frame is defined as a moment-resisting frame where the beam to column connection is classified as rigid. This type of frame does not take into account local beam to column connection rotation in global frame analysis.
Continuous frame is recommended for special use buildings such as those for medical uses, research, white rooms, and housing for sensitive equipment.
The connections are designed to transmit resulting end beam moments and shear forces into the column without the need for bracing to resist lateral load. Frame stability comes from the rigidity of the connections and stiffness of the members.
Continuous frames have a lower beam deflection than a simply supported beam, and the floors are less sensitive to vibration. The rigid connections of a continuous frame perform better than a braced frame in load reversal situations due to the added robustness and during seismic activity at the construction site.
Connections are important
The connections between beams and columns, joists to beam, and column to foundation are the most important, and the weakest, part of the frame. Most structural failures occur at the connections.
Construction supervisors and contractors must understand this concept. They are responsible for the correct selection and installation of the appropriate connections within the frame because the structural engineer rarely designs the connections. At most, guidelines will be provided with the building plans.
Over time, steel fabricators have developed connection systems that were cost effective for their own products but that were not appropriate for another fabricator’s frame. A rather complicated process resulted:
- The structural engineer sizes the members.
- The steel fabricator designs the connections.
- The structural engineer reviews and approves the connections.
Lots of room for error. It is critical for the construction supervisor to be well-educated and vigilant about connections in a column and beam steel frame.
Beam end types
Select beam ends according to the building design. You have three basic types to choose from:
- Partial depth end plate
- Full depth end plate
- Fin plate
The partial depth end plate is easy to fabricate and takes surface treatment well. It provides 75% of the shear resistance calculated as a percentage of beam resistance and has a moderate degree of tying resistance. It is only fair in ease of erection, onsite adjustment, and temporary stability. Partial depth end plate is not the best to use for skewed joint or eccentric beam designs.
A full depth end plate provides 100% shear resistance and better tying resistance than partial depth end plate. It also provides better connections for the column web. Also easily fabricated and surface treated, it can be readily adjusted as needed at the jobsite.
It is not the easiest to erect nor does it give the best temporary stability. This is also not the best choice for skewed joint or eccentric beam designs.
Fin plates are the easiest to erect and adjust onsite while providing better temporary stability than a partial or full depth end plate. It is also the best for a skewed or eccentric beam connection. It also has good tying resistance.
However, fin plates have only 50% shear resistance and have a weaker connection to the column web than end plates. A long fin plate may require additional stiffening upon fabrication.
Connections are pinned or fixed. This is a crucial concept for the structural supervisor to understand.
A pinned connection is also known as a simple connection. It is designed to transmit end shear only; resistance to rotation is negligible. This means the beam should not move in or out nor up and down. It may rotate at the point of connection, creating a weakness in the frame.
The principle forms of pinned connections use flexible end plates and fin plates. Stability is added with braces or a concrete core. If the building design includes skewed joints, beams placed eccentrically to columns or a connection to column webs, a special pinned connection is appropriate.
Pinned connections are classified by stiffness and strength.
A fixed connection is moment-resisting and resists the rotation seen in pinned connections. You can have plates welded to the top and bottom flanges of the column. Welding stops the rotation at the connection.
Spliced connections are required for strength and continuity of stiffness around both axes of the column. Splices hold members in line and are typically added every two to three stories. The additional material needed increases the cost too much to splice at every level.
Starting about 600 mm above ground, where the moment due to strut action is insignificant, spliced connections allow for easy access from the adjacent floor for bolting up onsite. Spliced connections also provide more convenient member lengths for fabrication, transport, and erection.
Column and beam steel continuous frame structures, which are built with fixed connections, are highly moment-resisting, making them good choices for areas with high seismic activity or regions with the tendency for high winds.
For calmer environments or lower rise buildings, a pinned connection is an appropriate and economical choice to use with a braced column and beam frame.
The construction supervisor is responsible for selecting the right type of connection and ensuring correct installation, keeping in mind that most structural failures occur at the connection.