Remove the cushions of the sofa chair.
Measure the exact base from beam to beam.
Cut a 1/4 inch piece of plywood to the exact size of the base.
Set the plywood over the base beams and then put the cushions back into position. The plywood will evenly distribute the weight, preventing sagging.
Remove the cushions of the sofa chair, and carefully peel away any fabric covering the springs and beams.
Measure the broken beam, and cut a beam to the same size. Most sofas use either a 1-by-2-inch beam or a 1-by-1-inch beam.
Remove the nails holding the hold beam in place, and carefully slide the beam out from the spring configuration.
Thread the new beam through the springs, and attach to the frame with two wood screws on each side.