Reinforcing bar placing drawings are the instructions from the engineers to the contractors on how to build the structure to resist the anticipated loads. They provide the details and placing instructions so that the ironworkers can set the reinforcing bars into place at the jobsite.
Placing drawings are used to show where the reinforcing bars to be placed. Sometimes the structural and placing drawings are very specific about exactly where reinforcing bar is placed. The ironworkers need to figure out how those reinforcing bars relate to the overall dimensions of the structure.
The amount of concrete cover over the reinforcing bar is very important. If it's constructed properly, the concrete protects the reinforcing bar from corrosion. Bar supports are used to hold the reinforcing bars in place to attain the proper depth of cover. Types of bar supports range from as simple as plain concrete blocks to all-plastic chairs, to wire bar supports.
Rebar is tied together, using wire, to hold it in place.
To the right is a diagram of typical ties:
Detail A: "Snap Tie" (diagram upper left) is the simplest and is usually used for rebar in a flat horizontal position.
Detail B: "Wrap and Snap Tie" (diagram upper right) is normally used when tying vertical wall reinforcement to hold the bars securely into place.
Detail C: "Saddle Tie" (diagram bottom left) is more complicated than snap ties or wrap and snap ties. They are commonly used for securing ties to column corner bars and strirrups to beam corner bars..
Detail D: "Wrap and Saddle Tie" (diagram bottom middle) is similar to the saddle tie except that the wire is wrapped 1-1/2 times around the first bar, then completed like Detail C.
Detail E: "Figure Eight Tie" (diagram bottom right) can be used on walls in lieu of the saddle or wrap and snap ties. This type of tie is used to secure heavy mats.