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AD 359: Weathering steel bolts

European bolt standards do not specifically cover weathering steel bolts – i.e. bolts of material similar to that of ‘structural steels with improved atmospheric corrosion resistance’ to EN 10025-5. The execution standard, EN 1090-2, notes in Clause 5.6.6 that Type 3 Grade A fasteners to ASTM standard A325 would be suitable. There is limited availability of such fasteners in the UK; the only metric size that is (sometimes) available is M24; in imperial sizes, only the 1 inch size is readily available. The Advisory Desk has been asked on several occasions to clarify the consequences on design of using imperial size bolts to this standard as preloaded bolts.

A325 Standard specification for structural bolts, steel, heat treated, 120/105 ksi minimum tensile strength, specifies a minimum ultimate tensile strength for 1 inch size bolts that is almost exactly the same as that specified in EN ISO 898-1 for property class 8.8 fasteners and it specifies a proof strength that is 70% of the UTS. Such bolts will therefore conform to the requirements of EN 1090-2 clause 5.6.6. For design to Eurocodes, the characteristic tensile strength to be used in design is therefore the same as that for class 8.8 fasteners. However, the tensile stress area of a 1 inch bolt is 10% greater than that of an M24 bolt and thus its resistance is 10% greater, both in shear and in slip resistance (and in tension).

A 1 inch bolt requires a larger hole size and thus the minimum spacing and edge/end distances are greater; the change would affect the bolt layout and plate size if minimum values appropriate to a 26 mm hole (for an M24 bolt) had been chosen when detailing the connection. It is therefore recommended, when detailing bolted connections in a weathering steel structure, to specify M24 bolts but to ensure that the spacing and edge/end distances comply with Table 3.3 of BS EN 1993-1-8 for a hole size of 28 mm, in case 1 inch bolts have to be substituted. The resistance of the connection should nevertheless be determined on the basis of the resistance of M24 bolts in 26 mm holes (this would be conservative if 1 inch bolts were substituted). However, if it is certain that 1 inch size bolts will be used, the slip resistance and the shear resistance in bearing can be taken as the larger value, which might permit a lesser number of bolts to be used.

Contact: David Iles
Tel: 01344 636525

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