Bolts and Nuts

Most steel structures are fastened together on site by nuts and bolts. These come in a variety of sizes, from 6mm to 100mm in diameter. They can be soft (grade 4.6, 8.8, 10.9, 12.9): the figures refer to the ultimate strength and yield strength of bolts. For example a 4.6 has an ultimate strength of 400N/mm2 but yields at 60% of this, 240N/mm2; an 8.8 bolt has an ultimate strength of 800N/mm2, and yields at 80% of this, 640N/mm2. Bolts can act in tension (pulling) and in shear (being sliced by two sliding plates). Bolts can also be designed to work in friction, by squeezing plates so hard together that they cannot slip (these are called friction grip bolts, and sound good: but it is extremely hard to get them all to the right tension, and they should normally be avoided). In general, structural bolts are 8.8