Plain and threaded bearing strengths for the design of bolted connections with pultruded FRP material

- Tests are used to characterize the pin-bearing strength of an PFRP material.
- Strengths are for steel bolting (M10, M12, M16 and M20) with and without thread.
- Characteristic strengths determined with the standard and three non-standard thread sizes.
- Lowest threaded (130 N/mm2) is < non-threaded (168 N/mm2) in longitudinal direction.
- Lowest threaded (94 N/mm2) is > non-threaded (87 N/mm2) in transverse direction.
- Reduction factor in an LRFD 'standard' of 0.6 for thread in bearing is safe.

Presented are results from testing 28 batches of 5 or 10 nominally identical specimens to characterise the laterally unrestrained pin-bearing strength when bolting is with and without thread. For the test series flange material is taken from a 254x254x9.53 mm Pultex® SuperStructural 1525 series shape. Strengths are measured with the Fibre Reinforced Polymer (FRP) material oriented at either 0° or 90° to the direction of pultrusion. Four steel bolt sizes of M10, M12, M16 and M20 are used, and when threaded there are different standard teeth (pitch) geometries. To remove this variable in a comparison with plain pin strengths a unique test series of 12 batches was carried out with three non-standard thread profiles. The effect on pin-bearing strength of having a threaded bolt is evaluated using mean and characteristic strengths, the latter determined in accordance with EN 1990. A key finding is that the proposed reduction factor of 0.6 in a forthcoming American LRFD standard to calculate a thread characteristic strength from the plain value is acceptable. Other findings are important to the determination of pin-bearing strength, and to us having knowledge and understanding to prepare a universal design procedure for resistances in bolted connections when the mode of failure is bearing.