Tying Arc Springs
Aside from the frame, the springs are the most important support structure on your furniture. Don't skimp on the supplies that you use. The springs in the seat of sofa or a chair take a lot of beating, having to instantly take up to several hundred pounds, then instantly release. Don't use plastic ties or garden twine to tie the springs. When tying springs it is crucial to use a twine the will take a lot of stress and still last for many years. Use only supplies that have a proven record and that are specifically made for the job. Standard hemp/flax spring twine has a rating (depending upon the brand) of 200 to 250 lb tensile strength. Polyester Spring Twine #6 is rated at 410 lbs of tensile strength (but it has a tendency to stretch over time).
Here is an example of a modern day sofa that has arc springs in the seat. Underneath the decking fabric the springs were only covered with a felt pad.