The steel frame shown is to be bolted to steel beams on the

The steel frame shown is to be bolted to steel beams on the ceiling of a room using metric grade 8.8 bolts. If the total bolt grip is 48 mm, what tightening torque should be used if the threads are lubricated? What portion of the external load is taken by the bolts? By the members?

Solution

Data given: Grade of bolts = ISO grade 8.8 Load applied W = 40000 N Total bolt grip Lg = 48 mm Size of bolts = M20 x 2.5 (so, d=20mm) No. Of bolts = 2 Evaluation: Properties of ISO grade 8.8 bolts: Stress area: 245 mm2 Proof stress: 600 N/mm2 Tensile stress: 830 N/mm2 Load carried by each bolt, P = 40000 / 2 = 20000 N (a) Tightening torque: Connection: Permanent Lubricated: Hence, torque factor K = 0.18 Area, Ad = (PI/4)*20^2 = 314.2 mm2 Proof load, Fp = Stress area*Proof stress = 245*600 = 147000 N Fi is Preload For permanent connections Fi = 0.9*Fp Fi = 0.9*Fp = 0.9*147000 = 132300 N Torque = K*Fi*d = 0.18*132300*20 = 476280 N-mm = 476.28 N-m So, tightening torque needed is476.28 N-m (b) portion of the external load is taken by the bolts, By the members. Length of the bolt, L = length of total grip + height of nut + two threads beyond the nut = 48 + 20 + 5 = 73mm. The next preferred size is 80mm. So L = 80mm Length of grip, Lg = 48 mm For bolt length <125mm and diameter < 48mm, Threaded length Lt = 2*diameter + 6 = 2(20)+6 = 46 mm