Design of prefabricated reinforced concrete structural Frames

Prefabricated structural members are of three types. They can normally be reinforced, prestressed or post-tensioned. Reinforced precast members are normally designed in accordance with common reinforced concrete practice. However, as a result of factory control, high-strength concrete is produced whether it is necessary or not, since it is actually more economical to design reinforced concrete for higher strengths in the range of 5,000 psi (35 MPa).

The members of a precast structural frame are often prestressed; that is, the tensile reinforcement has been placed under tension before the member is erected. Prestressing is a technique in which reinforcement is placed in forms and stretched against fixed columns; Concrete is then placed around the wires or cables. The prestressed members are made several at a time in a long bed, and the main requirements are therefore simplicity and quantity.

Post-tensioning involves first forming and molding the member with ducts along its length. After the concrete cures, the reinforcing cable is placed in the ducts and anchored at one end. It is stretched against the concrete by means of jacks. The tendons are gripped and tension is maintained by several patented gripping devices that fit onto the end of the limb. Post-tensioning is economical if the member is long or large.

Column erection

The first step in the construction of a prefabricated structural frame is the preparation of the column foundation. Because the columns must be mechanically attached to them, the anchor bolts must be precisely positioned in the footing, each column foot has anchor plates cast so that the column can be bolted to the foot. Large columns require special steel pads to transfer the load from the column to the foundation. Note the large union nuts on the top of each anchor bolt.

A screw is turned on each anchor bolt before the column is placed on them. After erection, the column is plumbed, using guy wires if necessary, and the nuts are tightened until the column rests firmly on them. The top nuts are then fitted and tightened to hold the column in position. Finally, the space between the top of the footing and the column is filled with grout, often made with metallic aggregate, to provide uniform pressure over the entire area.

Column to base connections

Actualmente se conocen cinco disposiciones comunes para conectar columnas prefabricadas a un muelle de cimentación, una tapa de pilote, una base de pared o una base extendida. En cada uno, se utiliza el sistema de doble tuerca con un espacio de 2 a 2-1/2 pulg. (50 a 65 mm) entre la parte superior de la base y la parte inferior de la placa base para la lechada sin contracción. En las ubicaciones de las columnas, es necesario colocar suficientes lazos en la parte superior del muelle o la pared para asegurar los pernos de anclaje.

Beam erection

The beams are lifted into position by crane, and some form of temporary connection is usually provided until the permanent ones have been completed. In the structure shown below, pins cast in the column seats hold the beams in place during erection. Angle iron clips screwed through holes in the columns restrain the beams until final connections can be made.

Connections can be moment resistant or pin type. Moment-resistant connections prevent rotation between the column and beam and contribute to the lateral stability of the frame. For moment connections to resist the tensile force resulting from the developing couple, some method of producing continuity in the connection is required. Welding, shear anchors, inserts, additional rebar, post-tensioning, or some combination of these may be used to provide this continuity. Care should be taken to provide temporary reinforcement during construction of beams with these types of connections until the connections have been completed.

Beam to column connections

There are currently seven different methods for connecting beams to columns. These are not the only types of connections used for this purpose, but some of the basic techniques. Although most beams are rectangular in cross section, any other commonly used type of beam could be connected by the same methods. The seven variations of beam-to-column connections are described below:

• This is a welded connection that hides a column. Make the hitch without using a damped end beam is for a single span condition, but can be used as a moment connection by using non-shrinkage grout between the end of the beam and the column and by providing stress transfer in the top of the beam.
• This is a welded connection, with the beam supported on a bracket projecting from the side of the column. It is also for a simple span condition, but can be converted to a moment connection.
• This is called a cushioned end connection, also welded. Its development for a moment connection requires grouting at two interfaces.
• This welded connection is often used when it is required to be hidden for architectural reasons. An H-shaped section is shown projecting from the column, but other forms of structural steel can be used instead.
• This is a pinned connection, with bars projecting from the column into steel tubes cast in the beam. The tubes are then filled with grout.
• This is a welded connection in which the tension bars are welded at small angles. If future extension of the column is required, anchor bolts can be placed in the cast concrete between the ends of the beam.
• In this type of connection, a post-tension bar is tensioned after non-shrinkage grout has been placed, between the column and the end of the beam.

Erection of additional joints

Back splices are added to a prefabricated frame building by joining the upper story columns to those already erected. In many ways, they are similar to column-to-beam connections. Most column splices require non-shrink grouting between sections to provide variations in their dimensions. There are five basic methods for splicing columns and they are described below:

• The main column reinforcement is welded to the base plate. Pockets can be placed on the sides of the column or in the corners.
• This connection also uses a plate inserted between the angles. The angular recess is grouped together after erection is complete.
• Four small corner base plates are used here instead of one full size one under the angles.
• Reinforcing bars projecting from the upper column extend into cast steel tubes in the lower one.
• The column is spliced through a continuous beam, and reinforcement is required within the beam to transfer the load from one column to another.

Erection of floor and ceiling slabs

Finally, in most precast structural frame designs, prestressed floor and roof slabs extend from beam to beam, beam to wall, or wall to wall to complete the structure. In most cases, the slabs will be laid edge to edge, but they may be spaced and the space between them filled with cast-in-place reinforced concrete.