exhaust stack holding down bolts
 

exhaust stack holding down bolts
as a mechanical and a newbie on this site i’m looking for help on exhaust stack holding down bolt information. i am being asked to specify torque settings for the 16 off m48 bs3692 grade 8.8 holding down bolts on a 2.8m diameter, 27m high exhaust stack weighing 33 tonnes.
i’ve no problem with the calcs but i’m not sure what preload to specify.
if i use the standard equation torque = 0.2 x bolt load x nom bolt dia
i get a torque setting of 1230nm.
i suspect this is too high for this application and is more suitable for bolting up a pressure flange where you need to maintain a seal and i am worried that specifying a high torque setting may be detrimental to the concrete. i have a max calculated bolt load of 128kn due to wind loading.
can anyone let me know of any relevant standards on this subject (especially din) or any reference literature thanks? sorry about the metric units!
typically a torque is only specified when you need to get a preload (residual load) in the bolts, such as for hsfg bolted joints or high pressure flanges.
most structural type joints are not preloaded.
the uk national structural steelwork specification for buildings states for non preloaded bolts:
cl 6.1.8 bolt tightening
bolts may be assembled with power tools or shall be fully tightened by hand using appropriate spanners...
holding down bolts into concrete are typically not preloaded.
take a tower on levelling nuts as a good example nut above and below the base plate and no way of pretensioning them.